Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance

Comprehensive Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance Comprehensive nuclear materials 3 07 TRISO coated particle fuel performance nuclear materials 3 07 TRISO coated particle fuel performance

D A Petti, P A Demkowicz, and J T Maki

Idaho National Laboratory, Idaho Falls, ID, USA

3.07.2.1.7 TRISO-coated particle fuel irradiation testing 156

151

ATR Advanced Test Reactor

AVR Arbeitsgemeinschaft

Versuchsreaktor

BAF Bacon anisotropy factor

BISO Bi-structural isotropic

BOL Beginning of life

BR-2 Belgian Reactor 2

CCCTF Core Conduction Cooldown Test

Facility

CVD Chemical vapor deposition

DOE Department of Energy

EFPD Effective full-power day

FACS Fuel accident condition simulator

FIMA Fissions per initial metal atom

FPMS Fission product monitoring system

FRJ Research Reactor Juelich

GETR General Electric Test Reactor

HEU Highly enriched uranium

HFEF Hot Fuel Examination Facility

HFIR High-Flux Isotope Reactor

HFR High-Flux Reactor

HRB HFIR Removable Beryllium

HTGRs High-temperature gas-cooled

reactors

HTR-10 High Temperature Reactor 10

HTTR High-temperature test reactor

IFEL Irradiated fuel examination laboratory

IMGA Irradiated microsphere gamma

analyzer

INET Institute of Nuclear and New Energy

Technology INL Idaho National Laboratory IPyC Inner pyrolytic carbon ITU Institute for Transuranium

Elements JMTR Japan Material Test Reactor KuFA Cold finger apparatus (in German) LEU Low-enriched uranium

LHTGR Large High Temperature Gas

Reactor LTI Low temperature isotropic MOL Middle of life

NE-MHTGR Commercial version of NP-MHTGR NGNP Next Generation Nuclear Plant NP-MHTGR New Production Modular

High-temperature Gas-Cooled Reactor

ORNL Oak Ridge National Laboratory ORR Oak Ridge Research Reactor PIE Postirradiation examination R&D Research and development R/B Release to birth ratio SiC Silicon carbide TRIGA Training research and isotope

production, General Atomics TRISO Tristructural isotropic

UO 2 Uranium dioxide VHTR Very-high-temperature reactors VXF Vertical experimental facility

3.07.1 Introduction

For all high temperature gas reactors (HTGRs),

tris-tructural isotropic (TRISO)-coated particle fuel

forms the heart of the concept Such fuels have

been studied extensively over the past four decades

around the world, for example, in countries including

the United Kingdom, Germany, Japan, United States,

Russia, China, and more recently, South Africa In

early gas-cooled reactors, the coated particle fuel

form consisted of layers of carbon surrounding the

fissile kernels Highly enriched uranium (HEU) and

thorium carbides and oxides were used as fissile and

fertile kernels Ultimately, the carbon layer coating

system (termed BISO for bistructural isotropic) was

abandoned because it did not sufficiently retain fission

products, leading to the development of the current

three-layer coating system (termed TRISO for

tris-tructural isotropic) In TRISO-coated fuel, a layer of

silicon carbide (SiC) is sandwiched between pyrolytic

carbon layers This three-layer system is used to both

provide thermomechanical strength to the fuel and

contain fission products In addition, for operational

and economic reasons, the fuel kernel of choice today

is low-enriched uranium (LEU) uranium dioxide

(UO2) for the pebble bed design and uranium

oxycar-bide (UCO) for the prismatic design

In both pebble bed and prismatic gas reactors,

the fuel consists of billions of multilayered

TRISO-coated particles (
750–830 mm in diameter) distributed

within fuel elements in the form of circular cylinders

(12.5 mm in diameter and 50 mm long) called ‘compacts’

or spheres called ‘pebbles’ (6 cm in diameter) The

active fuel kernel is surrounded by a layer of porous

carbon, termed ‘the buffer’; a layer of dense carbon,

termed ‘the inner pyrolytic carbon layer’; a layer of

SiC; and another dense carbon layer, termed ‘the outer

pyrolytic carbon layer.’ These collectively provide for

accommodation and containment of fission products

generated during operation The buffer layer is designed

to accommodate fission recoils, volumetric swelling of

the kernel, and fission gas released under normal

opera-tion The inner pyrolytic carbon layer protects the

ker-nel from reactive chlorine compounds produced during

SiC deposition in the chemical vapor deposition (CVD)

coater The SiC layer provides structural strength to the

particle The outer pyrolytic carbon layer protects the

particles during formation of the fuel element Under

normal operation, radiation damage causes shrinkage of

the pyrolytic carbon layers, which induces compressive

stresses in the SiC layer to counteract tensile stresses

associated with fission gas release All three layers of

the TRISO coating system exhibit low permeability.These fuel constituents are extremely stable and aredesigned not to fail under normal operation or antici-pated accident conditions, thereby providing effectivebarriers to the release of fission products.Figure 1is amontage of TRISO fuel used in both prismatic andpebble bed high-temperature gas reactors

Rigorous control is applied at every step of thefabrication process to produce high-quality, very low-defect fuel Defect levels are typically on the order

of one defect per 100 000 particles Specificationsare placed on the diameters, thicknesses, and densities

of the kernel and layers; the sphericity of the particle;the stoichiometry of the kernel; the isotropy of thecarbon; and the acceptable defect levels for eachlayer Statistical sampling techniques are used to dem-onstrate compliance with the specifications usually atthe 95% confidence level For example, fuel produc-tion for German reactors in the 1980s yielded onlyapproximately 100 defects in 3.3 million particles pro-duced This remains the standard for gas-cooled-reactor fuel production today.1,2

Irradiation performance of high-quality, defect coated particle fuels has been excellent In

art in irradiation testing, capabilities of existing sion reactors worldwide to irradiate TRISO fuel, andthe irradiation behavior of modern TRISO-coatedparticle fuel around the world will be discussed.Testing of German fuel under simulated accidentconditions in the 1980s has demonstrated excellentperformance Section 3.07.3 describes the accidentbehavior of TRISO-coated particle fuel largely on thebasis of the German database and the plans to performsimilar testing for the current generation of TRISO-coated fuels Additional limited testing of TRISO-coated particle fuel performed under air and wateringress events and under reactivity pulses has beenreported elsewhere3and will not be repeated here.The outstanding irradiation and accident simula-tion testing results obtained by German researchersform the basis for fuel performance specificationsused in gas-cooled-reactor designs today Specifica-tions for in-service failure rates under irradiation andaccident conditions are very stringent, typically onthe order of 104and 5 104, respectively.Significant research and development (R&D)related to TRISO-coated fuels is underway worldwide

fis-as part of the activities of the Generation IV tional Forum on Very-High-Temperature Reactors(VHTRs) The focus is largely on extending the cap-abilities of the TRISO-coated fuel system for higher

Interna-burnups (10–20%) and higher operating temperatures

(1250C) to improve the attractiveness of

high-temperature gas-cooled reactors as a heat source for

large industrial complexes where gas outlet

tempera-tures of the reactor would approach 950C.4Of

great-est concern is the influence of higher fuel temperatures

and burnups on fission product interactions with the

SiC layer leading to degradation of the fuel and the

release of fission products Activities are also underway

around the world to examine modern recycling

tech-niques for this fuel and to understand the ability of gas

reactors to burn minor actinides.5,6

3.07.2 Irradiation Performance

3.07.2.1 Overview of Irradiation Facilities

and Testing

This section provides a brief overview of irradiation

facilities that are available today to perform

TRISO-coated particle irradiations

3.07.2.1.1 BR-2The Belgian Reactor 2 (BR-2) reactor is a materials testreactor in Mol, Belgium7 that produces very fast(3.5 1014

neutrons cm2s1[E > 1 MeV]) and mal neutron fluxes (1012neutrons cm2s1) The facil-ities have irradiation test rigs (
15 mm ID and 400 mmlong) that can be used to irradiate coated-particle gasreactor fuel forms They have adequate flux, fluence,and temperature characterization for the capsule,and have the infrastructure needed for capsule disas-sembly and postirradiation examination (PIE) Thecapsule size precludes irradiation of pebbles; how-ever, it could handle approximately six to eight fuelcompacts

ther-3.07.2.1.2 IVV-2MThe IVV-2M is a 15-MW water-cooled reactor thathas been used in Russia for a variety of coated-particle testing.8 Four different test rigs have beenused to test specimens ranging from particles, tocompacts, to spheres The coated particle ampoule

Matrix

Fueled zoneFuel-free shell

TRISO-coated fuel particles are formed

into fuel spheres for pebble bed reactor

Fuel sphere Dia 60 mm

Half section

5 mm graphite layer

Coated particles imbedded

in graphite matrix

Inner PyC-layer SiC-layer Outer PyC-layer

Kernel

Prismatic

Pyrolytic carbon Silicon carbide Uranium dioxide or oxycarbide kernel

ParticlesTRISO-coated fuel particles (left) are formed into fuel compacts (center) and inserted into graphite fuel elements (right) for the prismatic reactor

Buffer layer

Figure 1 TRISO-coated particle fuel and compacts and fuel spheres used in high temperature gas reactors.

is a noninstrumented rig that can hold 10–13 graphite

disks (15 mm in diameter and 2 mm thick), each of

which can hold 50 particles The rig can also hold

coated particles in axial holes, 1.2 mm in diameter,

and a uniform volume of coated particles, 12–18 mm

in diameter and 20–255 mm high, in a graphite

matrix Another rig, termed a ‘CP hole,’ is 27 mm in

diameter and that can handle six to eight capsules

A third rig, identified as ASU-8, is a 60-mm hole that

can handle three compacts The largest channel

avail-able is Vostok, which is 120 mm in diameter and

contains four cells All of these rigs can irradiate

fuel at representative temperatures, burnups, and

fluences for HTGRs There is a large degree of

flexibility in the testing options at IVV-2M Their

rigs can handle particles, compacts, and spheres

3.07.2.1.3 HFR Petten

The High Flux Reactor (HFR) in Petten, Netherlands,

is a multipurpose research reactor with many

irradi-ation locirradi-ations for materials testing.9 The HFR has

two different types of irradiation rigs/locations in the

facility: one that can accommodate compacts and

another that can accommodate spheres Rigs for

spheres are multicell capsules, 63–72 mm in diameter

that can handle 4–5 spheres in up to 4 separate cells

For compacts rigs/locations are
32 mm in diameter

and 600 mm in useful length They can handle three

or four parallel channels of compacts For the

three-channel configuration, approximately 30 compacts

could be irradiated in the rig There is a large axial

flux gradient across the useable length (40% spread

maximum to minimum) that must be considered in

the design of any experiment

3.07.2.1.4 HFIR

The High Flux Isotope Reactor (HFIR) at Oak Ridge

National Laboratory (ORNL) is a light-water-cooled,

beryllium-reflected reactor that produces high

neu-tron fluxes for materials testing and isotope

produc-tion.10 Two specific materials irradiation facilities

locations are available for gas reactor fuel testing:

(a) the large RB positions (eight total) that are

46 mm in diameter and 500 mm long, and can

accom-modate capsules holding up to 24 compacts (three in

each graphite body, eight bodies axially) in a single

swept cell; and (b) the small vertical experimental

facility (VXF) positions (16 total) that are 40 mm in

diameter and 500 mm long, and can accommodate

capsules holding up to 16 compacts (eight in each

graphite body, two bodies axially) in a single swept

cell Capsules can be irradiated in the lower flux small

VXF positions and then moved to the higher fluxremovable beryllium positions Neither of these posi-tions can accommodate pebbles A third facility, thelarge VXF positions (six total), are farther out inthe reflector (and therefore have lower fluxes), butare 72 mm in diameter and also 500 mm long As withthe HFR, there is a large axial flux gradient that must

be considered in the design of any experiment in any

of these facilities

3.07.2.1.5 ATRThe Advanced Test Reactor (ATR) at Idaho NationalLaboratory (INL) is a light-water-cooled, beryllium-reflected reactor fuel in a four-leaf clover configura-tion to produce high neutron fluxes for materialstesting and isotope production.11 The clover leafconfiguration results in nine very high flux positions,termed ‘flux traps.’ In addition, numerous other holes

of varying size are available for testing Several tions can be used to irradiate coated-particle fuel.The 89-mm-diameter medium I position (16 total)and the 100–125-mm-diameter flux traps can accom-modate pebbles Specifically, the use of a medium

posi-I position early in the irradiation, required because

of the enrichment of the fuel, followed by transfer ofthe test train to the northeast flux trap can provideirradiation conditions representative of a pebble bedreactor Approximately 10–12 pebbles in five or sixindividually swept cells can be envisioned in the testtrain The large B positions in ATR (four total) are

38 mm in diameter and 760 mm in length They canaccommodate six individually swept cells, with twographite bodies per cell, containing up to three 2-in.long compacts per body Thus, 36 full-size US com-pacts can be irradiated in this location Of specialnote, here is the very flat burnup and fluence profileavailable axially in the ATR over the 760 mm length.This allows for nearly identical irradiation of largequantities of fuel

3.07.2.1.6 SAFARIThe SAFARI Reactor in Pelindaba, Republic ofSouth Africa, is an isotope production and researchreactor.12The core lattice is an 8 9 array, consisting

of 28 fuel assemblies, 6 control rods, and a number

of aluminum and beryllium reflector assemblies.The reactor is cooled and moderated by light waterand operates at a maximum power level of 20 MW.In-core irradiation positions include six high-fluxisotope production positions: two hydraulic, twopneumatic, and two fast transfer systems that areaccessible during operation Several other irradiation

positions can also be accessed when the reactor

is shut down A large poolside facility allows for a

variety of radiation applications An intermediate

storage pool and a transfer canal allow for easy and

safe transport of activated materials to a hot cell

3.07.2.1.7 TRISO-coated particle fuel

irradiation testing

The historical experience in irradiation testing of

coated particle fuels suggests that multicell capsules

wherein fuel can be tested in separate compartments

under different temperature, burnup, and fluence

con-ditions allow for tremendous flexibility and can actually

save time and money in an overall fuel qualification

program Although there are differences in details of

the test trains used in each of the reactors, they share a

number of important similarities in the state of the art

with irradiation testing of this fuel form In this section,

these important similarities are presented to highlight

the technical considerations in executing this type of

testing

Because of the differences in neutron flux

spec-trum between a gas reactor and a light-water

materi-als test reactor, simultaneous matching of both the

rate of burnup and the rate of accumulation of fast

neutron fluence is difficult to achieve In addition, the

traditional 3-year fuel cycle of high-temperature gas

reactors makes real-time irradiation testing both

time-consuming and an expensive part of an overall fuel

development effort To overcome these shortcomings,

irradiations in material test reactors have historically

been accelerated relative to those in the actual reactor

Usually, the time acceleration is focused on achieving

the required burnup in a shorter time than would be

accomplished in the actual reactor, with the value of

the fast fluence left as a secondary variable that must

fall between a minimum and maximum value

The level of acceleration can also impact the

potential for fuel failure during irradiation The

level of acceleration at a given test reactor power,

coupled with fuel loading in the experiment, results

in a power density for the fuel specimen in the

experiment The power density peaks at the

begin-ning of the irradiation when the fissile content is

highest and decreases as the fissile material is burned

out of the fuel As the level of acceleration increases,

the temperatures in the fuel kernels increase above

that in the fuel matrix because of the thermal

resis-tances associated with the coatings of the particle,13

and the potential for high temperature, thermally

driven failure mechanisms to play a deleterious role

in fuel performance becomes more important

As discussed in Section 3.07.2.7, the irradiationperformance database suggests that modest levels ofacceleration (1.5–3) appear to be acceptable with-out jeopardizing fuel performance in the irradiation,and should be a baseline requirement for future gasreactor irradiations This acceleration level can betranslated into a maximum power per fuel body orpower per particle that can be used by experimenters

in the design of the irradiation capsule

Given the limitations of materials test reactorsaround the world, the TRISO-coated particle irradi-ation database contains results from tests conductedunder a range of accelerations Successful GermanTRISO-coated particle fuel irradiations in theEuropean HFR-Petten reactor were conducted using

an acceleration of less than a factor of three By parison, other German irradiations in the Forschung-zentrum Reaktor Juelich (FRJ) reactor at Ju¨lich had

com-a neutron spectrum thcom-at wcom-as too thermcom-alized Thisresulted in the fuel receiving too little fast fluence

to be prototypic of a high-temperature gas reactor.Similarly, historic US irradiations in ORNL’s HFIRreactor had too high a thermal flux resulting in signifi-cant burnup acceleration of the irradiation On the basis

of these considerations, the large B positions (38 mmdiameter) in the ATR (seeFigure 2) were chosen forthe US Department of Energy’s (DOE) Advanced GasReactor (AGR) Program fuel irradiations because therate of fuel burnup and fast neutron fluence accumula-tion in these positions provide an acceleration factor

of less than three times that expected in the temperature gas reactor

high-3.07.2.1.8 Thermal and physics analysisconsiderations

Given the complexity of the capsules currently beingdesigned, the extensive review by safety authorities ofthe thermo-mechanical stresses, and the importance ofeach capsule in terms of irradiation data for fuel quali-fication, three-dimensional physics and thermal ana-lyses are essential in irradiation capsule design Theseanalyses are critical to ensure that the fuel reaches theintended burnup, fluence, and temperature conditions

To achieve high burnups with these fuels requiresdetailed physics calculations to determine the time toreach full burnup Given the concerns about severelyaccelerated irradiations, it is not uncommon for suchirradiations to take approximately 2 years to reach fullburnup in LEU TRISO-coated particles In addition,because thermocouples should not be attached directly

to the fuel, thermal analysis is used to calculate the fueltemperature during the irradiation

Examples of a test train for fuel compacts used in

INL’s ATR and the pebbles used in HFR-Petten are

shown inFigures 3 and 4 respectively

These irradiation capsules have extensive

instru-mentation to measure temperature, burnup, and fast

fluence at multiple locations in the test train

Tradi-tional commercial thermocouples have been used

extensively in past irradiations, but thermocouples

can suffer from drift and/or de-calibration in the

reactor Redundancy in thermocouple measurements

is another consideration in light of the low reliability

of thermocouples at high temperatures and long

times in neutron fields typical of TRISO-coated

par-ticle fuel irradiations Melt wires are inexpensive and

have been used as a backup to thermocouples where

space was available in the capsule However, melt

wires only indicate that a certain peak temperature

has been reached, and not the time of that peak

Direct temperature measurements of the coatedparticles are problematic because direct metal con-tact (e.g., thermocouple wires or sheaths) with thefuel element is not recommended as the metals canattack the TRISO fuel coatings Thus, temperaturesmust be calculated on the basis of thermocoupleslocated elsewhere in the capsule Thermocouplesare generally located as close as possible to thefuel body to minimize the uncertainties on thecalculated fuel temperatures related to irradiation-induced dimensional change and thermal con-ductivity changes of the materials in the capsule.Encapsulating the fuel element in a graphite sleeve

or cup and inserting thermocouples into the graphitehas been used successfully in many designs Thehigh conductivity of graphite minimizes the effect

of irradiation-induced dimensional changes on thecalculated fuel temperature

I-1 I-2 I-3

I-4

I-5

I-6

I-7 I-8

I-9 I-10 I-12 I-11I-13

I-14 I-15 I-16 I-17 I-18

I-19 I-20

OS-1 OS-3 OS-4 OS-5 OS-6 OS-7

OS-8 OS-9 OS-10 OS-11 OS-12

OS-13 OS-14 OS-15 OS-16 OS-17

OS-18 OS-19 OS-20 OS-21 OS-22

OS-2

ON-8

ON-3 ON-9 ON-10 ON-11 ON-12

ON-1 ON-4 ON-5 ON-6 ON-7

ON-2

Fuel elements

East large B position location for AGR-1

North

H positions

In-pile tube

I positions

Small B position

Control drum

Figure 2 Schematic of ATR showing fuel and select irradiation positions.

Historically, metal sleeves have not been allowed

to touch fuel elements because of past experiences

in which SiC was attacked by transition metals (Fe,

Cr, and Ni) Although quantitative data on transport

rates of such metals into the fuel element and

corro-sion rates of the SiC are unknown, 2 or 3 mm

thick-ness of graphite between the fuel element and the

metallic components (e.g., graphite sleeve) has been

found to be effective in minimizing the potential for

interaction

These irradiation experiments typically include

both thermal and fast fluence wires A number of

different fluence wires have been used successfully

to measure thermal and fast neutron fluences in coated

particle fuel irradiations The specific type of wire to

be used will depend on the measurement need (fast

or thermal), the temperature it will experience

dur-ing the irradiation, and compatibility with the

mate-rial of encapsulation Quartz encapsulation is not

recommended for high-temperature, high-fluence

applications Neutronically, transparent refractories

(e.g., vanadium) are a good alternative material of

encapsulation Inert gas filling of the flux wire

encapsulation is recommended to ensure no oxygeninteraction with the flux wire Although fissionchambers and self-powered neutron detectors havebeen used extensively in other reactor irradiations,they may not be practical in the space-constrainedcapsules expected for TRISO-coated particle fuelqualification tests

3.07.2.1.9 Gas control system considerationsAutomated gas control systems – designed to changethe gas mixture in the experiment to compensate forthe reduction in fission heat and changes in thermalconductivity with burnup – minimize human opera-tor error and have proven to be a reliable method ofthermal control during these long fuel irradiations.The temperature of each experiment capsule is con-trolled by varying the mixture of two gases withdiffering thermal conductivities in a small insulatinggas jacket between the specimens and the experimentcontainment A mixture of helium and argon has beenused in the past and provides a wide temperaturecontrol band for the experiments Unfortunately,

Figure 4 Schematic of pebble irradiation experiment used by the Germans.

argon cannot be used in fuel experiments where

online fission product monitoring is used because

the activated argon will reduce detectability of the

system Therefore, helium and neon are used instead

Computer-controlled mass flow controllers are

typi-cally used to automatitypi-cally blend the gases (on the

basis of feedback from the thermocouples) to control

temperature The gas blending approach allows for a

very broad range of control Automatic gas

verifica-tion (e.g., by a thermal conductivity analyzer) has

been implemented in some experiments to prevent

the inadvertent connection of a wrong gas bottle Gas

purity is important and an impurity cleanup system

should be implemented during each irradiation Flow

rates and gas tubing should be sized to minimize

transit times between the mass-flow controllers and

the experiment, as well as between the experiment

and the fission product monitors

3.07.2.1.10 FPMS considerations

In addition to thermal control, sweep gas is used to

transport any fission gases released from the fuel to a

fission product monitoring system (FPMS) A

num-ber of techniques have been used historically to

quantify the release of fission gases from the fuel in

these irradiation capsules Techniques include gross

gamma monitoring, online gamma spectroscopy, and

offline gamma spectroscopy of grab samples Online

gross gamma monitoring of the effluent gas in the

experiment using ion chambers and sodium iodidedetectors is an excellent means to capture anydynamic failures of the coated particles associatedwith the instantaneous release upon failure Grabsamples can provide excellent noble gas isotopicinformation The temporal resolution and the number

of isotopes that can be measured depend on the quency of the grab samples and the delay timebetween acquisition of the grab sample and offlineanalysis Weekly grab samples are typical in mostirradiations, although daily or even hourly samplesare possible if failure has occurred, assuming opera-tion and associated analysis costs are not too high.Typical isotopes to be measured include85mKr,87Kr,88

fre-Kr,131mXe,133Xe, and135Xe Measurement of long-lived isotopes (e.g., 85Kr) would be useful inelucidating fission product release mechanisms fromthe kernel, but would also require waiting for thedecay of the shorter lived isotopes in the sample.Online gamma spectroscopy, although the mostexpensive in terms of hardware costs, can providethe most detailed real-time information with detailedisotopic spectrums as often as necessary subject todata storage limitations of the system An example ofthe system used for the US AGR program is shown in

experiment to the detector should be minimized toallow measurement of short- and medium-lived iso-topes, but must remain long enough to allow decay of

Grab sample

Fission product monitoring system

Capsules in-core

Particulate filters

Vessel wall

Ne He

Filter

Temperature control gas mixing system

Figure 5 Integrated fission product monitoring system used in US AGR program irradiations.

any short-lived isotopes associated with the sweep

gases (
2–3 min) With this delay time, 89

Kr, 90Kr,135m

Xe,137Xe,138Xe, and139Xe should also be

capa-ble of being measured online Measurements of

xenon gas-release during reactor outages are

recom-mended to provide information on iodine release

behavior from the decay of xenon precursors Multiple

options for fission gas-release measurements should be

considered for long irradiations where reliability of the

overall fission gas measurement system can be a

con-cern Redundancy is also recommended for online

systems so that failure of a spectrometer does not

jeopardize the entire experiment

On the basis of the online concentration data, a

release-to-birth ratio (R/B), a key parameter used in

reactor fuel behavior studies,14can be calculated and

provide some insight into the nature of any particle

failures Because these instruments are online during

the entire irradiation, a complete time history of gas

release is available Gas release early in the

irradia-tion (i.e., from the start of the irradiairradia-tion) is indicative

of initially failed particles or contamination outside

of the SiC layer Release later during the irradiation is

indicative ofin situ particle failure The timing of the

failure data can then be correlated to temperature,

burnup, and/or fluence that can be used when

cou-pled with PIE to determine the mechanisms

respon-sible for the fuel failure

3.07.2.2 German Experience

Previously, particle fuel development was conducted

by German researchers in support of various HTGR

designs that employed a pebble bed core These

reactors were intended to produce process heat or

electricity The sequence of fuel development used

by German researchers followed improvement in

particle quality and performance and was largely

independent of developments in reactor technology

German fuel development can be categorized

according to the sequence of fuels tested as provided

German irradiation test conditions generally

covered projected fuel operating conditions, where

fuel was to reach full burnup at fast fluences

of 2.4 1025

n m2 and operate at temperatures

up to 1095C for process-heat applications and

up to 830C for electrical production applications

With the exception of irradiation duration, the

vari-ous experiments performed bounded expected

nom-inal conditions or were purposely varied to meet

other test objectives In order to obtain results in a

timely manner, tests conducted by German ers were generally accelerated by factors of 2–3.The following sections present irradiation experi-ment summaries for fuels of ‘modern’ German design.1For these experiments, this definition extends tohigh-enriched (Th, U)O2 TRISO-coated particlesfabricated since 1977, and low-enriched UO2TRISO-coated particles fabricated since 1981.Table 2pro-vides the physical attributes of the fuel used in thesetests Mixed oxide fuel test summaries are presentedfirst, followed by the UO2tests

research-3.07.2.2.1 R2-K12 and R2-K13The R2-K12 and R2-K13 cells were irradiated in theR2 reactor at Studsvik, Sweden The main objective

of the R2-K12 experiment was to test mixed oxide(Th, U)O2 and fissile UC2/fertile ThO2 fuel ele-ments, whereas for R2-K13, the main objective was

to test mixed oxide (Th, U)O2 fuel elements andsupply fuel for subsequent safety tests

In R2-K12, four full-size spherical fuel elementswere irradiated in four independently gas-swept cells.Two cells contained mixed oxide fuel spheres, whilethe other two contained fissile/fertile fuel spheres Asthe German researchers did not develop the two-particle fissile/fertile system further, only the mixedoxide results were reported R2-K13 was a combinedexperiment with the United States Four indepen-dently gas-swept cells were positioned vertically ontop of one another The top and bottom cells eachcontained a full-size German fuel sphere The middletwo cells contained US fuel and will be discussed in

from both experiments are given inTables 3 and 4.Cold gas tests on each fuel sphere during PIEindicated that all the particles had remained intact

in both R2-K12 and R2-K13 These tests are ducted after the fuel has been stored (for
14 days) atroom temperature and a quasi-steady-state release offission gas has been reached The fuel is then sweptwith a carrier gas that is monitored for various fission

con-Table 1 German particle fuel development sequence Date of design

consideration

Fuel form

1972 BISO coated (Th, U)O 2

1977 Improved BISO coated (Th, U)O 2

TRISO-coated UCO fissile particles with ThO 2 fertile particles

TRISO-coated (Th, U)O 2

1981 LEU TRISO-coated UO 2

gases (usually85mKr) and heated to
60C Sudden

increases in the amount of fission gas detected

indi-cate failed particles The amount of increase is

proportional to the gas source, and in a calibratedsystem, indicates the number of failed particles.The fuel sphere from R2-K12 Cell 1 was partiallydeconsolidated and visual inspection revealed twokernels ‘without coating.’ Segments from each of thetwo fuel spheres were also metallographically exam-ined; those examinations revealed a reaction zone onthe inner side of the buffer layer, as well as tangentialcracks between the buffer and the inner pyrocarbonlayer Only one particle exhibited a radial crack in thebuffer layer beyond the reaction zone All of the SiCand PyC layers examined remained intact

3.07.2.2.2 BR2-P25The BR2-P25 capsule was irradiated in the BR2 reac-tor at Mol, Belgium The primary objective of thisexperiment was to test (Th, U)O2 mixed oxide fuel.One independently gas-swept cell contained 12 com-pacts Each compact was cylindrical in shape andcontained a small fuel sphere Configuration and irra-diation data are given inTables 5 and 6, respectively.During PIE, Compacts 3 and 7 were electrolyticallydeconsolidated with no particle failures being evident.Ceramographic examination of cross-sections fromCompacts 4 and 8 revealed some radial cracks inthe buffer layers; however, no defective particleswere found

Table 2 Characteristics of modern German TRISO fuel particles

BR2-P25 HFR-P4

FRJ2-P27 HFR-P4 HFR-K3 SL-P1

Notes: The  entries are one standard deviation Entries in square brackets, [ ], are estimated values.

BAF is the Bacon anisotropy factor for the layer, where values closer to one are more isotropic.

Table 3 R2-K12 and R2-K13 configuration

Spherical fuel element

diameter

59.9 mm 59.77 mm

Th per fuel element 4.961 g 10.125 g

Heavy metal per fuel

element

6.076 g 11.27 g Number of particles per

spherical fuel element

Defective SiC layers a

(U/U-total)

<1  10 5 <5  10 6

a Defective SiC layer fractions reported for German fuel are per

pebble with the exception of loose particle experiments that are

per particle batch.

Source: Gontard, R.; Nabielek, H Performance Evaluation of

Modern HTR TRISO Fuels; Tech Rep HTA-IB-05/90;

Forschungszentrum Ju¨lich GmbH: Ju¨lich, Germany, 1990.

3.07.2.2.3 HFR-P4

The HFR-P4 capsule was irradiated at the HFR

in Petten The main objective of this experiment

was to compare the fuel performance of particles

with 36- and 51-mm-thick SiC layers irradiated at

1000C, beyond burnups of 12% fissions per initialmetal atom (FIMA), and beyond fast fluences of

6 1025

n m2(E > 0.10 MeV) The performance ofthe 36mm SiC layer fuel was also to be evaluated at anirradiation temperature of 1200C Three indepen-dently gas-swept cells each contained 12 compactsthat were cylindrical in shape and contained a smallfuel sphere in each Configuration and irradiationdata are given inTables 7 and 8, respectively Notethat the burnup and fast fluence goals were met, whilethe irradiation temperature goals were not PIErevealed that the test articles remained intact How-ever, some failures caused by the thermocouples andgas inlet tubes were found on the upper compacts

3.07.2.2.4 SL-P1The SL-P1 experiment was irradiated at the Siloe¨Reactor in Grenoble, France The objective of theexperiment was to test reference LEU fuel up to thepotential limits for burnup and fast fluence at 800C

Table 4 R2-K12 irradiation data

EOL (report date) 85m Kr R/B 3.0  10 7 2.0  10 7 7.0  10 8 5.0  10 8

Source: Gontard, R.; Nabielek, H Performance Evaluation of Modern HTR TRISO Fuels; Tech Rep HTA-IB-05/90; Forschungszentrum Ju¨lich GmbH: Ju¨lich, Germany, 1990.

Table 5 BR2-P25 configuration

Cylindrical compact diameter 26.58–27.74 mm

Cylindrical compact height 29.87–30.03 mm

Diameter of spherical fuel zone 20 mm

Th per fuel compact 0.6744 g

Heavy metal per fuel compact 0.8264 g

Number of particles per compact 1490

Number of particles per cell 17 880

Defective SiC layers (U/U-total) <1  10 5

Source: Gontard, R.; Nabielek, H Performance Evaluation of

Modern HTR TRISO Fuels; Tech Rep HTA-IB-05/90;

Forschungszentrum Ju¨lich GmbH: Ju¨lich, Germany, 1990.

Table 6 BR2-P25 irradiation data

Duration (full power days) 350

Source: Gontard, R.; Nabielek, H Performance Evaluation of

Modern HTR TRISO Fuels; Tech Rep HTA-IB-05/90;

Forschungszentrum Ju¨lich GmbH: Ju¨lich, Germany, 1990.

Table 7 HFR-P4 configuration

Number of compacts per cell 12 Cylindrical compact diameter 23–29 mm Cylindrical compact height 32 mm Diameter of spherical fuel zone 20 mm

Particle batch – cells 1 and 3 EUO 2308 Particle batch – cell 2 EUO 2309

235

Number of particles per compact 1630 Number of particles per capsule 19 600 Defective SiC layers (U/U-total) <1  10 6

One gas-swept cell contained 12 compacts Each

cylindrical compact contained one small fuel sphere

Configuration and irradiation data are provided in

Tables 9 and 10, respectively The operational

objec-tives for this experiment were met PIE revealed that

none of the compacts showed mechanical failure

3.07.2.2.5 HFR-K3

The HFR-K3 capsule was irradiated at the HFR in

Petten The primary objective of this experiment was

to determine the performance of reference LEU fuel

from an accelerated test Four full-size spherical fuel

elements were irradiated in three independently

gas-swept cells The cells were vertically positioned on

top of one another, with the middle cell containing

two fuel spheres To minimize flux gradient effects

on the test fuel, the entire test rig was rotated 90

several times during the irradiation Configuration

and irradiation data are given in Tables 11 and 12,respectively Subsequent PIE reported no failures.3.07.2.2.6 FRJ2-K13

FRJ2-K13 cells were irradiated at the DIDO reactor inJu¨lich, Germany The main objective of this test was tosupply irradiated reference fuel for subsequent safetytests Fuel performance was also to be examined underthe controlled irradiation conditions of significantburnup with negligible fast neutron fluence Four full-size spherical fuel elements were irradiated in two

Table 9 SL-P1 configuration

Cylindrical compact diameter 30.1 mm

Cylindrical compact height 30.8 mm

Diameter of spherical fuel zone 20 mm

Number of particles per

compact

1634 Number of particles per cell 19 600

Defective SiC layers (U/U-total) <1  10 6

Table 10 SL-P1 irradiation data

Duration (full power days) 330

Table 8 HFR-P4 irradiation data

Start date 10 June 1982

End date 28 November 1983

power days)

359

Burnup (% FIMA) 7.5 10.0 10.6 9.0 Fast fluence

(10 25 n m2,

E > 0.10 MeV)

Center temperature (C)

Surface temperature (C)

BOL 85m Kr R/B 1  10 9 9 10 10 9 10 10 2 10 9

EOL 85m Kr R/B 2  10 7 1 10 7 1 10 7 3 10 7

independently gas-swept cells The cells were vertically

positioned on top of each other, with the fuel spheres

similarly positioned within the cells Configuration and

irradiation data are given inTables 13 and 14,

respec-tively Subsequent PIE reported no failures

3.07.2.2.7 FRJ2-K15

FRJ2-K15 cells were irradiated at the DIDO reactor

in Ju¨lich, Germany The main objectives of this test

were to demonstrate the high burnup potential of

reference fuel used in AVR reload 21-1 and to

per-form in-core temperature transient tests Fuel

perfor-mance was also to be examined under the controlled

irradiation conditions of significant burnup with

neg-ligible fast neutron fluence Three full-size spherical

fuel elements were irradiated in three independently

gas-swept cells Configuration and irradiation data

are given inTables 15 and 16, respectively

Capsules 2 and 3 underwent a temperature transient

test at a burnup of
10% FIMA The temperature of

the sphere surfaces was raised to 1100C and held for

11 h The85mKr R/B ratio from each capsule increased

to a maximum of
108at the start of the transient andthen dropped back to the pretransient levels after thetemperature was returned to the nominal test condition.3.07.2.2.8 FRJ2-P27

FRJ2-P27 cells were irradiated at the DIDO reactor

in Ju¨lich, Germany The main objectives of this testwere to investigate fission product release at variouscyclic temperatures and to determine the effective-ness of thicker SiC layers on the retention of110mAg.Each of the three independently gas-swept cellscontained three compacts and two coupons (trays).The compacts were cylindrical in shape and contained

an (unspecified) outer fuel-free zone The couponswere graphite disks with holes, annularly spaced, forthe insertion of 34 particles Of the two coupons thatcontained the thicker SiC particles (51mm vs 36 mm),one was placed in Cell 1, and the other in Cell 3.Configuration and irradiation data are provided inTables 17 and 18, respectively

Table 14 FRJ2-K13 irradiation data

Start date 24 June 1982

End date 12 February 1984

Table 16 FRJ2-K15 irradiation data Start date 4 September 1986 End date 20 May 1990 Duration (full

E > 0.10 MeV)

Center temperature (C)

Surface temperature (C)

Number of fuel spheres 4

Spherical fuel element diameter 59.98 mm

Number of particles per spherical

fuel element

16 400 Defective SiC layers (U/U-total) 4  10 5

PIE revealed that all specimens and components

were in excellent condition Cold gas tests of all

compacts and coupons determined that there was

only one defective/failed particle present This

par-ticle was from a Capsule 2 coupon (with nominal SiC

thickness) Ceramographic examination revealed that

the particle was inserted in the coupon ‘without

coating’ and that kernel interactions led to a

com-pression of the inner side of the buffer to a thickness

of
10 mm

3.07.2.2.9 HFR-K6 and HFR-K5

The HFR-K6 and HFR-K5 capsules were irradiated

at the HFR in Petten.1,9These experiments were a

proof test for HTR MODUL reference fuel In each

experiment, four full-size spherical fuel elements

were irradiated in four independently gas-sweptcells A typical reactor temperature history wassimulated in the test with 17 temperature cycles(corresponding to 17 passes through the core) Forone-third of a cycle, the fuel sphere center tempera-ture was held at 800C; for the other two-thirds

of the cycle, the center temperature was 1000C

In addition, three temperature transients (sphere ter temperature held at 1200C for 5 h) were per-formed at beginning of life (BOL), middle of life(MOL), and end of life (EOL) Limited configurationand irradiation data are given in Tables 19 and 20,respectively There were no particle failures reported

cen-as a result of the irradiations

3.07.2.3 US ExperienceHistorical US particle fuel development effort(through the mid 1990s), which included design andtesting, coincided with the development of variousHTGRs This sequence of development is listed

under consideration at that time US gas reactorswere designed to use prismatic graphite blockscontaining fuel compacts, and were primarilyintended to produce electricity with the exception

of the New Production Modular High-temperatureGas-Cooled Reactor, which was designed to producetritium Over the years, the design has also supportedsteam cycle, direct cycle, process heat, and weaponsmaterial disposition applications More recently, DOEestablished the AGR Fuel Development and Qualifi-cation Program to provide a baseline fuel qualificationdata set at a peak fuel centerline temperature of

1250C15,16in support of the licensing and operation

of the Next Generation Nuclear Plant (NGNP).Irradiation test conditions employed by the UnitedStates generally covered projected fuel operating con-ditions US fuel was to operate at temperatures as

Table 18 FRJ2-P27 irradiation data

Start date 17 February 1984

End date 10 February 1985

Spherical fuel element diameter

TRISO

LEU UO 2 – TRISO

Number of compacts per cell 3

Number of coupons per cell 2

Cylindrical compact diameter 27.9–28.03 mm

Cylindrical compact height 29 mm

Diameter of coupon fuel annulus 23 mm

LTI – TRISO Particle batch for compacts and four

coupons

EUO 2308 Particle batch for two coupons (thick SiC) EUO 2309

235

Number of particles per compact 2424

Number of particles per coupon 34

Number of particles per cell 7340

Defective SiC layers (U/U-total) <3  10 6

high as 1400C and reach full burnup (commensurate

with235U enrichment and kernel composition) at fast

fluences of 4 1025

n m2 With the exception of diation duration, the various experiments performed

irra-either bounded expected nominal conditions or were

purposely varied to meet other test objectives In order

to obtain results in a timely manner, US tests were

accelerated by factors of 3–10

The particle fuel irradiation experiments and PIE

results summarized below consider only selected

tests of key US fuel types These fuel types include

TRISO fissile/BISO fertile particles, weak acid resin

(WAR) TRISO fissile/BISO fertile particles, TRISO

fissile/TRISO fertile particles, and TRISO-P fissile

particles (conventional TRISO-coated particles with

an additional ‘protective’ pyrolytic carbon layer above

the outer pyrolytic carbon layer) as well as TRISO

fissile particles General Atomics and Babcock &

Wilcox manufactured the majority of the kernel and

coating batches However, some of the batches were

manufactured by ORNL The following US

experiment summaries are listed in chronologicalorder and are not grouped by fuel type Listed config-uration and irradiation data are actual values, notspecification values or ranges Interpretations of PIEresults are from the original sources and no overtattempt has been made to reinterpret the results.3.07.2.3.1 F-30

The F-30 experiment was irradiated in the eral Electric Test Reactor (GETR) at Pleasanton,California.17 The primary objective of this experi-ment was to demonstrate the irradiation performance

Gen-of Fort St Vrain production fuel Five independentlygas-swept cells contained the fuel Cells 1, 3, and 4contained only fuel compacts, Cell 2 contained onlyloose particles, and Cell 5 contained both fuel com-pacts and loose particles Configuration and irradia-tion data are given inTables 22 and 23, respectively.Postirradiation metallographic examination ofseven fuel compacts containing fissile and fertileparticles was performed In addition, five sets of

Table 21 Historical US particle fuel development and testing sequence

Date of design

conception

1964 Fort St Vrain built TRISO-coated (Th, U)C 2 fissile

TRISO-coated ThC 2 fertile

BISO and TRISO-coated ThO 2 fertile

1984 NE-MHTGR commercial design only TRISO-P coated UCO fissile

TRISO-P coated ThO 2 fertile

1989 NP-MHTGR government design only TRISO-P coated UCO

1995 GT-MHR commercial design only TRISO-coated UCO fissile

TRISO-coated UCO and/or UO 2 fertile fuel not yet tested

Table 20 HFR-K6 and HFR-K5 irradiation data

loose fissile particles and five sets of loose fertile

particles were examined Fissile particle failure,

defined as a crack completely through the SiC layer,

ranged between 0% and 6.1%, while fertile particle

failure ranged between 0% and 15.1% A typical

photomicrograph of SiC failure in an F-30 fissile

particle is presented in Figure 6 Metallography

revealed that inner pyrolytic carbon layers had

remained bonded to the SiC layer throughout

irradi-ation.Figure 7displays a typical photomicrograph of

a fissile particle with an IPyC layer crack and a

densified buffer

3.07.2.3.2 HRB-4 and HRB-5

The HRB-4 and HRB-5 capsules were irradiated in

HFIR at ORNL.18 The main objective of these

experiments was to test candidate fuel materials

and manufacturing processes for the proposed large

HTGR Each test involved a single gas-swept cell

containing six fuel compacts vertically positioned

Table 23 F30 irradiation data

Duration (full power days) 269

Table 22 F30 configuration

Total number of fuel compacts 13

Cylindrical fuel compact diameter 12.45 mm

Cylindrical fuel compact lengths 18.54 and 49.28 mm

Fissile fuel type HEU (Th, U)C 2 TRISO

Fissile particle diameter 429–560 mm

Fertile particle diameter 648–771 mm

Number of fissile particle batches 7

Number of fertile particle batches 9

Defective SiC layer

fraction – fissile particles

<5  10 4 –1.5  10 3

Defective SiC layer

fraction – fertile particles

3  10 4 –1.0  10 3

Figure 6 A typical SiC layer crack in an F-30 fissile fuel particle Reproduced from Scott, C B.; Harmon, D P Post Irradiation Examination of Capsule F-30; GA-A13208, UC-77; General Atomics Report, 1975.

Figure 7 A typical IPyC layer crack in a fissile F-30 fuel particle Reproduced from Scott, C B.; Harmon, D P Post Irradiation Examination of Capsule F-30; GA-A13208, UC-77; General Atomics Report, 1975.

Configuration and irradiation data are given in

Tables 24 and 25

Metallographic examinations were performed on

each fuel compact A typical photomicrograph of

an irradiated HRB-4 fissile particle is presented in

in the kernel and the densification of the buffer IPyC

layers of the examined fissile particles had remained

bonded to the SiC The examination indicated that the

fissile particles had failed between 0% and 6% of the

SiC layers These failures consisted primarily of radial

cracks through the SiC layer Between 4% and 73% of

the OPyC layers failed during irradiation There were

no tabulations of IPyC layer failures reported.Several of the fissile particles examined displayedevidence of fission product attack This attack mostlyoccurred in large concentrations at the IPyC–SiCinterface and where fission products in smaller con-centrations had diffused up to 25mm into the SiC

fis-sion product attack in HRB-4 fissile particles

In HRB-5, IPyC layers of the examined fissileparticles had remained bonded to the SiC There

Table 24 HRB-4 and HRB-5 configurations

Cylindrical fuel compact

diameter

12.4 mm 12.4 mm Cylindrical fuel compact

lengths

25.4 mm 25.4 mm Fissile fuel type WAR UC 2

TRISO

WAR UC 2

TRISO Fertile fuel type ThO 2 BISO ThO 2 BISO

Fissile particle diameter 639 mm 639 mm

Fertile particle diameter 805 mm 805 mm

Fissile particle batch OR52A OR52A

Fertile particle batch T01424BIL T01424BIL

Total number of fissile

Start date 8 October 1972 8 October 1972

End date 26 June 1973 3 February 1973

Duration (full power

Figure 9 Photomicrographs of typical fission product attack in irradiated HRB-4 fissile particles Reproduced from Scott, C B.; Harmon, D P Post Irradiation Examination of Capsules HRB-4, HRB-5, and HRB-6; GA-A13267, UC-77; General Atomics Report, 1975.

were no tabulations of IPyC layer failures reported.

There was no evidence of fission product attack as

seen in the HRB-4 fissile particles However, the

examination indicated that between 0.4% and

17% of the SiC layers in fissile particles had failed

These failures consisted primarily of radial cracks

through the SiC layer A typical photomicrograph

of irradiated HRB-5 fissile particles with cracked

SiC layers is presented inFigure 10 This

photomi-crograph is also representative of HRB-4 fissile

par-ticles with cracked SiC layers It was reported that a

large fraction of these cracked SiC layers were due to

metallographic preparation and not a result of fast

neutron exposure or fuel burnup effects

3.07.2.3.3 HRB-6

The HRB-6 capsule was irradiated in HFIR at

ORNL.18Fissile fuel particles used in HRB-6 came

from the same production batch as used in the first core

of Fort St Vrain and were one of the batches previously

irradiated in the F-30 experiment This test involved a

single gas-swept cell containing six fuel compacts

ver-tically positioned During operation, the sweep gas flow

rate was reduced because of high activity in the sweep

lines Because of this gas flow reduction, in-pile fission

gas-release data were not obtained The irradiation of

HRB-6 in HFIR coincided with part of the HRB-4

irradiation Configuration and irradiation data are

given inTables 26 and 27

PIE included gas-release measurements of eachfuel compact performed in the Training Researchand Isotope Production, General Atomics (GA)(TRIGA) reactor However, during the unloading ofthe HRB-6 capsule, fuel compacts 2A, 2B, and 2Cwere damaged and as many as 30 broken fuel parti-cles were observed Therefore, the TRIGA gas-release measurements at EOL for these compactswould be higher than in-pile sweep line measure-ments had they been performed

A typical photomicrograph of an irradiatedHRB-6 fissile particle is presented in Figure 11,which shows the formation of gas bubbles in thekernel and densification of the buffer The photomi-crograph also shows an incipient crack in the IPyClayer No tabulations of IPyC layer failures werereported IPyC layers of the examined fissile particleshad remained bonded to the SiC, and there was noevidence of fission product attack However, theexamination indicated that the fissile particles hadfailed between 0% and 2% of the SiC layers Thesefailures do not include the fissile particles brokenduring capsule unloading It was reported that a

Table 26 HRB-6 configuration

Number of fuel compacts 6 Cylindrical fuel compact diameter 12.4 mm Cylindrical fuel compact length 25.4 mm Fissile fuel type HEU (Th, U)C 2 TRISO

235

Fissile particle diameter 556 mm

Fertile particle diameter 888 mm Fissile particle batch CU6B-2427 Fertile particle batch T01451BIL-W Defective SiC layer

fraction – fissile particles

<5  10 4

Table 27 HRB-6 irradiation data

Duration (full power days) 183 Peak fissile burnup (% FIMA) 26.6 Peak fertile burnup (% FIMA) 9.3 Peak fast fluence (1025n m2,

E > 0.18 MeV)

7.9 Peak temperature (C) 1100 Minimum TRIGA BOL85mKr R/B 5.0  10 7

Maximum TRIGA EOL85mKr R/B 2.7  10 4

Figure 10 Typical HRB-4 fissile particle irradiated to

27.7% FIMA and 10.5  10 25 n m2fast fluence.

Reproduced from Scott, C B.; Harmon, D P Post

Irradiation Examination of Capsules HRB-4, HRB-5,

and HRB-6; GA-A13267, UC-77; General Atomics Report,

1975.

large fraction of these failures were due to

metallo-graphic preparation

3.07.2.3.4 OF-2

The OF-2 capsule was irradiated in the Oak Ridge

Research Reactor (ORR).19 The main objectives of

the test were to investigate the irradiation

perfor-mance of various particle fuel forms (mostly WAR

UCO with different stoichiometries) and to compare

the performance of fuel particles fabricated from

different coaters OF-2 consisted of 88 fuel compacts

(and several sets of loose inert particles) contained in

a single capsule that was divided into two

indepen-dently gas-swept cells Various combinations from 15

fissile batches, 16 fertile batches, and 4 compact

matrix compositions comprised the fuel compacts

(each compact contained fuel from only one fissile

batch and one fertile batch) Configuration and

irra-diation data are given inTables 28 and 29

Postirradiation metallography was performed

on three fuel compacts from Cell 1 and on 27 fuel

compacts from Cell 2 A significant level of OPyC

layer failures was observed in the fissile

TRISO-coated particles from Cell 1 However, there were

no observed SiC layer failures or any layer failures

in the BISO-coated fertile and inert particles in these

compacts Examination of 11 fuel compacts from

Cell 2, containing the same three fissile particle

batches as in Cell 1, also indicated significant levels

of OPyC layer failures The fissile particle batch

with the highest OPyC anisotropy (optical Bacon

anisotropy factor (BAF)¼ 1.069) had 100% OPyClayer failure, while the other two batches with loweranisotropy (optical BAF of 1.035 and 1.030) had0–33% OPyC layer failures

Of the 30 fuel compacts metallographically ined, only one compact (that contained WAR UCOfissile particles) displayed cracked SiC layers Amongthe 27 fissile particles observed in this compact,

exam-16 displayed cracked SiC layers These cracks wereidentified as artifacts of polishing However, nophotomicrographs of these cracks were presented tosupport this conclusion The metallographic exami-nations also revealed typical WAR UCO behavior ofkernel and buffer densification This densificationwas also accompanied by varying degrees of kernelmigration

photomi-crograph that displays kernel and buffer tion, and OPyC layer failure Examination of OF-2particles also indicated several incidences of fission

densifica-Table 28 OF-2 configuration

dimensions (48 compacts)

15.75 mm OD, 3.30 mm ID, 12.70 mm long Cell 2 cylindrical fuel compact

dimensions (24 compacts)

15.75 mm diameter, 50.8 mm long Fissile fuel type WAR UC x O y TRISO

(Th, U)O 2 TRISO

UC 2 TRISO

235

Fissile particle diameter 600–753 mm

Fertile particle diameter 806–889 mm Number of fissile particle batches 15

Number of fertile particle batches 16

Table 29 OF-2 irradiation data

Duration (full power days) 352

Burnup (% FIMA) 75.9–79.6 50.0–79.5 Fast fluence (1025n m2,

E > 0.18 MeV)

5.86–8.91 1.94–8.36 Maximum temperature (C) 1350 1350 BOL85mKr R/B 2  10 5 1  10 4

Examination of Capsules HRB-4, HRB-5, and HRB-6;

GA-A13267, UC-77; General Atomics Report, 1975.

product accumulation at the IPyC and SiC interface.

A typical photomicrograph of fission product

accu-mulation is presented inFigure 13

3.07.2.3.5 HRB-14The HRB-14 capsule was irradiated in HFIR atORNL.20 The main objectives of this experimentwere to test LEU particles and to demonstratereduced matrix–OPyC layer interactions by usingcure-in-place fuel compacts This test involved asingle gas-swept cell equally divided among 20 fuelcompacts vertically positioned and molded planchets(wafers) containing BISO-coated ThO2fertile parti-cles Online fission gas-release measurements werenot reported Also, irradiation results from the BISO-coated fertile particles were reported separately andare not included in this summary Configuration andirradiation data are given inTables 30 and 31.Disassembly of the HRB-14 capsule after irradia-tion produced five fuel compacts with no remainingstructure; in essence, there were five collections ofloose particles, four compacts that were partiallyintact, nine compacts that were intact but displayedsignificant amounts of debonding, and only two com-pacts in relatively good shape

Table 30 Lower half of HRB-14 configuration

Total number of fuel compacts 20 Cylindrical fuel compact diameter 12.50 mm Cylindrical fuel compact length 9.52 mm

(Th, U)O 2 TRISO

UO 2 TRISO

235

Fissile particle diameter 760–813 mm

Fertile particle diameter 786–882 mm Number of fissile particle batches 5

Number of fertile particle batches 8 Defective SiC layer

fraction – fissile particles

7.0  10 7 – 1.3  10 4

Table 31 Lower half of HRB-14 irradiation data

Duration (full power days) 214 Peak fissile burnup (% FIMA) 28.6 Peak fertile burnup (% FIMA) 8.5 Peak fast fluence (10 25 n m2,

E > 0.18 MeV)

8.3 Maximum temperature (C) 1190 Minimum temperature (C) 895 Minimum TRIGA BOL85mKr R/B 3.8  10 7

Maximum TRIGA EOL85mKr R/B 3.0  10 4

Figure 13 Photomicrograph of irradiated OF-2 fissile

fuel particles displaying fission product accumulation

at IPyC–SiC interface Reproduced from Tiegs, T N.;

Thoms, K R Operation and Post Irradiation Examination of

ORR Capsule OF-2: Accelerated Testing of HTGR Fuel;

ORNL-5428; 1979 Courtesy of Oak Ridge National

Laboratory, U.S Department of Energy.

Figure 12 Photomicrograph of irradiated OF-2 fissile

WAR UCO particle Reproduced from Tiegs, T N.;

Thoms, K R Operation and Post Irradiation Examination

of ORR Capsule OF-2: Accelerated Testing of HTGR

Fuel; ORNL-5428; 1979 Courtesy of Oak Ridge National

Laboratory, U.S Department of Energy.

Metallographic examination was performed on 15

fuel compacts, and 8 of them contained fissile

parti-cles A few fissile particles were reported to have SiC

layer cracks but these cracks were attributed to

metallographic preparation It should be noted that

visual inspection of each compact during capsule

disassembly indicated that between 0% and 9% of

the visible particles (from compact surfaces and loose

particles that had fallen off) had failed SiC layers

However, this visual inspection did not distinguish

between fissile and fertile particles

The metallographic examination of fissile

parti-cles revealed that between 0% and 3% of the IPyC

layers had failed (cracked) and that the IPyC layers

had debonded from the SiC in 0% to 7.7% of the

particles Buffer layers did not crack in the UO2or

(Th, U)O2fuel but did crack in 10–71% of the UCO

fuel particles Kernel extrusion was reported only in

UCO fuel.Figure 14 displays typical kernel

extru-sion, and Figure 15presents a typical

photomicro-graph of kernel migration

In several particles of each fuel form, high

con-centrations of fission products were observed in

small, localized regions at the SiC–IPyC layer

inter-face In addition to fission product accumulation,

localized chemical attack was also observed in the

SiC layers of several (Th, U)O2and UO2fuel particles

This localized attack, which had penetrated
2 mm

into the SiC, was attributed to palladium, and was

observed in 8% of the particles UCO fuel particlesthat did not display localized chemical attack, haduniform attack along the inner SiC layer (usually onone side of the particles) This uniform attack wasattributed to rare earth fission products Figure 16displays typical uniform fission product attack in aUCO fuel particle It should be noted that withoptimized UCO stoichiometry, the kernel retains rareearth fission products and does not display kernelmigration as found here with non-optimized UCOkernels containing excess UC2 leading to rare earthmigration

Metallographic examination of fertile particlesindicated that between 0% and 2.4% of the particles

in each compact had total coating failure, defined ascracked OPyC and SiC layers These failures wereattributed to pressure vessel failure Figure 17dis-plays a typical failed fertile particle Separate tallies

of particles where only the SiC layer had failed werenot reported Other fertile particle observationsinclude the following:

 1.5–29.1% of the particles had failed OPyC layers

 8–70% of the particles had failed IPyC layers

 11–85% of the particles had IPyC layers debondedfrom the SiC

 6–26% of the particles had cracked buffers

 no kernel migration was observed

 a few kernels had extruded into buffer cracks

Figure 14 Photomicrograph of a UCO particle (batch

6157-08-020) from Compact 10 irradiated at 1040C to

27.8% FIMA and to a fast fluence (E > 0.18 MeV) of

7.1  10 25 n m2displaying kernel extrusion Reproduced

from Young, C A Pre- and Post Irradiation Evaluation of

Fuel Capsule HRB-14; GA-A15969, UC-77; General

Atomics Report, 1980.

Figure 15 Photomicrograph of a UCO particle (batch 6157-08-020) from Compact 10 irradiated at 1040C to 27.8% FIMA and to a fast fluence (E > 0.18 MeV) of 7.1  10 25

n m2 Reproduced from Young, C A Pre- and Post Irradiation Evaluation of Fuel Capsule HRB-14; GA-A15969, UC-77; General Atomics Report, 1980.

3.07.2.3.6 HRB-15B

The primary objective of the HRB-15B experiment

irradiated in HFIR at ORNL21was to test a variety

of LEU fissile fuel designs and ThO2fertile particle

designs This test involved a single gas-swept cell

containing 184 thin graphite trays Each tray couldaccommodate up to a maximum of 116 individual,unbonded fuel particles The loose fissile fuel parti-cles included UC2, UCO with four different stoichio-metries, (Th, U)O2, UO2, and two types of UO2*(one type had ZrC dispersed throughout the bufferlayer and the other had a pure ZrC coating aroundthe kernel) Each fissile fuel type was tested withboth TRISO coating and silicon–BISO coatingwhich consisted of the kernel surrounded by a bufferlayer, an IPyC layer, and finally a silicon dopedOPyC layer The loose fertile particles testedincluded TRISO-, BISO-, and silicon–BISO-coatedThO2 Configuration and irradiation data areprovided inTables 32 and 33

silicon-BISO (Th, U)O 2 TRISO and silicon-BISO

UC 2 TRISO and silicon-BISO

UO 2 TRISO and silicon-BISO

UO 2 * TRISO and silicon-BISO

Fissile particle diameter 742–951 mm Fertile fuel type ThO 2 TRISO, BISO

and silicon-BISO Fertile particle diameter 773–836 mm Number of fissile particle batches 19

Number of fertile particle batches 22 Note: Two types of UO2* fuel were tested, one with ZrC dispersed

in the buffer and the other with pure ZrC layer around the kernel.

Figure 16 Photomicrograph of a UCO particle (batch

6157-08-020) from Compact 10 irradiated at 1040C to

27.8% FIMA and to a fast fluence (E > 0.18 MeV) of

7.1  10 25 n m2displaying fission product attack of the

SiC layer Reproduced from Young, C A Pre- and Post

Irradiation Evaluation of Fuel Capsule HRB-14; GA-A15969,

UC-77; General Atomics Report, 1980.

Figure 17 Photomicrograph of a ThO 2 fertile particle

(batch 6252-17-010) irradiated at 1130C to 8.5% FIMA

and to a fast fluence (E > 0.18 MeV) of 8.3  10 25 n m2

displaying pressure vessel failure Reproduced from

Young, C A Pre- and Post Irradiation Evaluation of Fuel

Capsule HRB-14; GA-A15969, UC-77; General Atomics

Report, 1980.

Table 33 HRB-15B irradiation data

Duration (full power days) 169 Peak fissile burnup (% FIMA) 26.7 Peak fertile burnup (% FIMA) 6.0 Peak fast fluence (1025n m2,

E > 0.18 MeV)

6.6 Time average temperature (C) 815–915

Postirradiation metallography was performed on

20 different particle types, each consisting of

approx-imately 20 particles These examinations revealed

considerable gas bubble formation in UC2and UCO

kernels, and buffer densification in TRISO-coated

particles Some SiC layer cracking was observed in

each TRISO-coated fuel type, but mostly in the

UCO particles These cracks were reported to have

occurred during mount preparation because of the

crack orientation and because the visual examination

detected no OPyC cracking No further tabulation of

layer failures was reported

3.07.2.3.7 R2-K13

The R2-K13 capsule was irradiated in the R2 reactor

at Studsvik, Sweden.22 The main objective of this

experiment was to test reference UCO fissile

parti-cles and ThO2 fertile particles Four independently

gas-swept cells were positioned vertically on top of

one another The middle two cells contained US fuel

The top and bottom cells each contained a full-size

German fuel sphere (discussed in the section on

German irradiation results) Configuration and

irra-diation data are given inTables 34 and 35

Postirradiation metallographic examination was

performed on two fuel compacts All of the 99

fissile particles examined displayed debonding

between the buffer and IPyC layers In some

cases, debonding between the buffer, IPyC, and

SiC layers was also observed Likewise, all of the

68 fertile particles examined displayed debonding

between the buffer, IPyC, and SiC layers The SiC

layers of all the particles examined were observed

to be intact

3.07.2.3.8 HRB-15AThe main objective of the HRB-15A experimentirradiated in HFIR at ORNL23 was to test severalcandidate fuel designs for the proposed Large HighTemperature Gas Reactor (LHTGR) This testinvolved a single gas-swept cell containing 20 cylin-drical fuel compacts positioned vertically on top ofone another Interspersed between the fuel compactswere 17 tray assemblies Each assembly had a graph-ite tray holding 54 unbonded particles in separateholes, and serving as a lid, a graphite wafer containing

54 particles bonded in separate holes with ceous matrix material Configuration and irradiationdata are given inTables 36 and 37

carbona-Table 36 HRB-15A configuration

Total number of fuel compacts 20 Cylindrical fuel compact diameter 12.54 mm Number of short fuel compacts/length 3/9.53 mm Number of long fuel compacts/length 17/19.05 mm Number of bonded wafer/unbonded

Fertile particle batches 5 Defective SiC layer fraction – fissile particles

1.4  10 5 – 7.4  10 2

Defective SiC layer fraction – fertile particles

6.7  10 5 – 1.4  10 3

Note: Two types of UO 2 * fuel were tested, one with ZrC dispersed

Table 34 R2-K13 US configuration

Total number of fuel compacts 12

Cylindrical fuel compact diameter 12.52 mm

Cylindrical fuel compact length 25.4 mm

Total number of piggyback sample

sets

31

Fissile particle diameter 803 and 824 mm

Fertile particle diameter 781–805 mm

Fissile particle batches 2

Fertile particle batches 3

Defective SiC layer fraction – fissile

particles

1.9  10 4 and 4.4  10 4

Defective SiC layer fraction – fertile

particles

<2  10 6 – 1.6  10 5

Table 35 R2-K13 US irradiation data

Duration (full power days) 517

Peak fissile burnup (% FIMA) 22.5 22.1 Peak fertile burnup (% FIMA) 4.6 4.5 Peak fast fluence (10 25 n m2,

Postirradiation metallographic examination was

performed on five fuel compacts Between 0% and

5.6% SiC (and OPyC) layer failures were reported

for the UO2particles but were attributed to sample

preparation In contrast, the ZrC layer failures

observed in the UO2 ZrC–TRISO-coated particles

were also attributed to sample preparation but were

not tabulated A photomicrograph of a UO2 ZrC–

TRISO-coated particle displaying a cracked ZrC

layer is presented inFigure 18 No SiC layer failures

were reported for the UCO fuel

Between 0% and 12.5% of the SiC layers and

between 83% and 92% of the IPyC layers were

reported to have failed in the fertile particles Thesehigh layer failures for the fertile ThO2particles wereattributed to the high IPyC BAF values for these parti-cles The high BAF was a result of intentionally deposit-ing the IPyC layer at low coating rates in an attempt toproduce layers that were impermeable to chlorine(chlorine trapped in the particle during SiC depositionmay enhance SiC degradation during irradiation).3.07.2.3.9 HRB-16

The main objective of the HRB-16 experiment ducted in the HFIR at ORNL24was to test a variety ofLEU fissile particle fuel designs This test involved asingle gas-swept cell containing 18 fuel compactsstacked vertically and interspersed with 27 trays ofunbonded particles and several encapsulated fissionproduct piggyback transport specimens Configurationand irradiation data are given inTables 38 and 39.Postirradiation metallographic examination wasperformed on seven fuel compacts that containedparticles from six different fissile batches and onefertile batch For fuel compacts containing multiplefissile batches, the following visual criteria were used

con-to identify fuel forms:

 UO2* had the conspicuous, bright ZrC layer next

ThC 2 BISO

Fissile particle diameter 742–884 mm Fertile particle diameter 756 and 786 mm Fissile particle batches 9

Fertile particle batches 2 Defective SiC layer fraction – fissile particles

4.6  10 7 – 4.4  10 4

Defective SiC layer fraction – fertile particles

1.6  10 5 and 5.0  10 4

Note: Two types of UO2* fuel were tested, one with ZrC dispersed

in the buffer and the other with pure ZrC layer around the kernel.

Table 37 HRB-15A irradiation data

Duration (full power days) 174

Peak fissile burnup (% FIMA) 29.0

Peak fertile burnup (% FIMA) 6.4

Peak fast fluence (10 25 n m2,

E > 0.18 MeV)

6.5 Average center temperature (C) 1150

Figure 18 Photomicrograph of a UO 2 ZrC–TRISO-coated

particle (batch 6162-00-010) irradiated at 1075C to

27.2% FIMA and to a fast fluence of 6.0  10 25

n m2(E > 0.18 MeV) displaying ZrC layer cracks Reproduced

from Ketterer, J.; et al Capsule HRB-15A Post Irradiation

Examination Report; GA-A16758, UC-77; General Atomics

Report, 1984.

 UC2 had very small gas bubbles (voids) in the

kernel, or if present in larger form were very

irregular in shape

 UCO had medium size, mostly circular voids in

the center of the kernel and small voids at the

periphery of the kernel

 UO2 had large, mostly circular voids evenly

distributed throughout the kernel

The metallographic examinations revealed that only

the UO2particles displayed kernel migration Kernel

migration was observed in approximately 28% of the

UO2particles in fuel compacts 2 and 13 and in
60%

of the UO2 particles in compact 14 A

photomicro-graph of a UO2 particle from compact 14 displaying

kernel migration is presented inFigure 19

All of the UC2 particles examined (eight total)

showed extensive buffer and IPyC layer failure and

significant amounts of fission product accumulation.Two of the UC2particles, or 25% of those examined,had SiC layer failures These SiC failures occurrednext to areas of the IPyC where high concentrations

of fission products were present

Examination of the UCO particles revealed icant amounts of fission product attack of the SiC.The extent of this attack ranged from slight to severe.Although not directly measured from examinations

signif-of a similar batch signif-of UCO particles irradiated inHRB-15A, it was surmised that this fission productattack was also due to palladium

Of the total 315 fertile ThO2particles examined,over half displayed IPyC layer failure and nearly 2%displayed SiC layer failure

3.07.2.3.10 HRB-21The objective of the HRB-21 capsule irradiated

in HFIR at ORNL25was to demonstrate the tion performance of reference NE-MHTGR fuel

irradia-A single gas-swept cell contained eight graphitebodies, each of which held three fuel compacts Eachgraphite body also contained three sets of encapsulated(piggyback) specimens These samples were sealed inniobium tubes of up to 52 mm length and 2.2 mmdiameter, and each sample contained either absorptiv-ity specimens or loose fuel particles The test wasoriginally scheduled to be irradiated for six reactorcycles; however, because of difficulty in maintainingcontrol of test temperature, the experiment was termi-nated after five reactor cycles Configuration and irra-diation data are given inTables 40 and 41

Table 40 HRB-21 configuration

Number of fuel compacts 24 Number of encapsulated piggyback specimens

24 Cylindrical fuel compact diameter 12.27–12.51 mm Cylindrical fuel compact lengths 49.13–49.35 mm

Fissile particle diameter 904 mm Fertile particle diameter 988 mm Fissile particle batch 8876-70-0 Fertile particle batch 8876-58-0 Total number of fissile particles 42 540 Total number of fertile particles 106 240 Defective SiC layer fraction – fissile

Table 39 HRB-16 irradiation data

Duration (full power days) 170

Peak fissile burnup (% FIMA) 28.7

Peak fertile burnup (% FIMA) 6.1

Peak fast fluence (10 25 n m2,

E > 0.18 MeV)

6.3 Average center temperature (C) 1150

Figure 19 Photomicrograph of a UO 2 particle (batch

6152-04-010) irradiated at 1100C to 26.9% FIMA and to

a fast fluence of 5.61  10 25

n m2(E > 0.18 MeV) displaying kernel migration Reproduced from Ketterer, J W.;

Myers, B F Capsule HRB-16 Post Irradiation Examination

Report; HTGR-85-053, 1985.

Postirradiation metallographic examination of

three fuel compacts was performed SiC layer failure

for both fissile and fertile particles ranged between

0% and 5% During irradiation, the online ionization

chambers recorded several spikes that indicated the

failure of approximately 130 particles

The metallographic examinations also revealed

that the IPyC layer was in contact with the SiC

layer However, in some cases where the IPyC was

cracked radially, the IPyC layer was debonded from

the SiC Fission product attack of the SiC layer was

also observed The chemical attack took place at the

tips of cracks in the IPyC layer where fission product

transport was not likely to be enhanced However,

scanning electron microscopy did not detect

loca-lized high concentrations of fission products in the

SiC but did detect low levels of palladium extending

5–10mm uniformly into the SiC

3.07.2.3.11 NPR-1 and NPR-2

The NPR-1 and NPR-2 capsules were irradiated

in HFIR at ORNL26 to demonstrate the irradiation

performance of reference NP-MHTGR fuel at the

upper bounds of burnup, temperature, and fast fluence

NPR-1 was irradiated one month before and then

concurrently with the NPR-2 capsule in HFIR

NPR-1 consisted of a single gas-swept cell containing

16 fuel compacts in addition to 12 sets of loose particles

The loose specimens were sealed in niobium tubes,

29 mm long and 2.2 mm in diameter NPR-2 consisted

of a single gas-swept cell containing 16 fuel compacts,

in addition to 16 sets of loose particles The loose

specimens were sealed in niobium tubes, 29 mm long

and 2.2 mm in diameter Configuration and irradiation

data for both capsules are given inTables 42 and 43

Postirradiation metallographic examination of two

NPR-1 fuel compacts was performed The

examina-tion indicated that
0.6% of the SiC layers had failed

in one compact and that 0% had failed in the other

compact The online gas measurements recorded 526

spikes from the ionization chamber Assuming thateach spike corresponds to a particle failure, 0.7% ofthe total number of particles had failure of all coatings.Postirradiation metallographic examination ofone NPR-2 fuel compact was performed This exam-ination indicated that
3% of the SiC layers hadfailed The online gas measurements recorded 135spikes from the Geiger–Mu¨ller tube This detector isless sensitive than ionization chambers, and may havemissed some transient spikes However, assumingeach spike corresponds to a particle failure, a lowerbound of 0.2% can be set for the total number ofparticles that failed

The metallographic examinations also revealedthat the IPyC layer had remained bonded to theSiC except in the vicinity of SiC cracks where

Table 41 HRB-21 irradiation data

Duration (full power days) 105

Peak fast fluence (10 25 n m2,

E > 0.18 MeV)

3.5 Average temperature (C) 950

particles

Defective SiC layer fraction 3  10 6 3  10 6

Table 43 NPR-1 and NPR-2 irradiation data

Start date 25 July 1991 28 August 1991

Duration (full power days)

Peak fast fluence (10 25 n m2,

E > 0.18 MeV)

Average temperature (C)

Peak compact temperature (C)

BOL85mKr R/B 1  10 8 5  10 9

EOL85mKr R/B 3  10 4 6  10 5

debonding was observed It was also observed that

between 10% and 30% of the particles with failed

IPyC layers also displayed cracked SiC layers

3.07.2.3.12 NPR-1A

The NPR-1A capsule was irradiated in the ATR at

the INL.27 The primary objective of the test was to

demonstrate the irradiation performance of referenceNP-MHTGR fuel at the upper bounds of nominaloperating conditions The same reference fuel wasalso irradiated in the NPR-1 and NPR-2 tests ForNPR-1A, 20 fuel compacts were placed vertically in asingle, gas-swept cell Originally, the test was sched-uled for 104 days of irradiation, but was terminatedafter 64 days because of indications of a significantnumber of fuel particle failures Configuration andirradiation data are given inTables 44 and 45.Postirradiation metallographic examination of onefuel compact was performed This examination indi-cated that
1% of the SiC layers had failed On thebasis of the online gas measurements, it was estimatedthat approximately 48 particles had failed, whichcorrespond to 0.06% of the total particle population.3.07.2.3.13 AGR-1

The AGR-1 experiment involves six separate sules, each containing approximately 50 000 particles

cap-in the form of fuel compacts It is an cap-instrumentedlead experiment, irradiated in an inert sweep-gasatmosphere with individual online temperature mon-itoring and control of each capsule A horizontal cap-sule cross-section at the top of the test train is shown

shown previously in Figure 3, and the experimentflow path was shown previously in Figure 5 Thesweep gas also has online fission product monitoring

on its effluent to track performance of the fuel in eachindividual capsule during irradiation (seeFigure 5).The first of eight planned experiments, AGR-1, had

Stack 1

Stack 2

Stack 3

ATR core center

Hf shroud

SST shroud Fuel compact

Gas lines

Insulating

Graphite

Thermocouples

Figure 20 Horizontal cross-section of an AGR experimental capsule.

Table 44 NPR-1A configuration

Number of fuel compacts 20

Cylindrical fuel compact diameter 12.37–12.50 mm

Cylindrical fuel compact lengths 49.33 mm

Fuel particle diameter 758 mm

Fuel particle batch FM19-00001 composite

Total number of fuel particles 75 360

Defective SiC layer fraction 3  10 6

Table 45 NPR-1A irradiation data

Duration (full power days) 64

Peak fast fluence

(10 25 n m2, E > 0.18 MeV)

2.1 Average temperature (C) 977

Peak temperature (C) 1220

been under irradiation at the INL ATR and was

completed in November, 2009; PIE is scheduled to

begin in April, 2010.28,29Table 46presents pertinent

attributes of the fuel that is being irradiated in

AGR-1.30 Configuration data are presented in Table 47

Irradiation of the experiment began on 24 December

2006, and will continue for approximately 2.5 years to

reach a peak burnup of 19% FIMA for the fuel

compacts

status of the AGR-1 experiment Detailed as-run

physics and thermal analyses are performed cycle

by cycle to track fuel burnup, fast neutron fluence

damage, and fuel temperatures during the irradiation

Peak burnups ranged from 16 to 19% FIMA and

fast fluences were between 3.0 and 4.0 1025

n m2(E > 0.18 MeV)

On the basis of the fuel temperature

distribut-ions during each cycle, time-averaged peak and

time-averaged volume-averaged temperatures arecalculated as the irradiation progresses After 514effective full power days, the time-averaged peakfuel temperatures ranged between 1120 and 1180C

Table 46 Fuel attributes for AGR-1

mean value

Actual mean value  population standard deviation Baseline Variant 1 Variant 2 Variant 3 Kernel diameter ( mm) 350  10 349.7  9.0

Kernel density (Mg M3) 10.4 10.924  0.015

Buffer thickness ( mm) 100  15 103.5  8.2 102.5  7.1 102.9  7.3 104.2  7.8 IPyC thickness ( mm) 40  4 39.4  2.3 40.5  2.4 40.1  2.8 38.8  2.1 SiC thickness ( mm) 35  3 35.3  1.3 35.7  1.2 35.0  1.0 35.9  2.1 OPyC thickness ( mm) 40  4 41.0  2.1 41.1  2.4 39.8  2.1 39.3  2.1 Buffer density (Mg M3) 0.95  0.15 1.10  0.04 1.10  0.04 1.10  0.04 1.10  0.04 IPyC density (Mg M3) 1.90  0.05 1.904  0.014 1.853  0.012 1.912  0.015 1.904  0.013 SiC density (Mg M3) 3.19 3.208  0.003 3.206  0.002 3.207  0.002 3.205  0.001 OPyC density (Mg M3) 1.90  0.05 1.907  0.008 1.898  0.009 1.901  0.008 1.911  0.008 IPyC anisotropy a (BAF) 1.035 1.022  0.002 1.014  0.001 1.023  0.002 1.029  0.002 OPyC anisotropy (BAF) 1.035 1.019  0.003 1.013  0.002 1.018  0.001 1.021  0.003 IPyC anisotropy

postcompact anneal (BAF)

Not specified 1.003  0.004 1.021  0.002 1.036  0.001 1.034  0.003 OPyC anisotropy

postcompact anneal (BAF)

Not specified 1.003  0.003 1.030  0.003 1.029  0.004 1.036  0.002 Sphericity (aspect ratio) 1% of the particles

shall have an aspect ratio 1.14

a Specification does not apply to variants 1 and 2.

b Value is an estimate of an attribute property, not the mean of a variable property.

Table 47 AGR-1 configuration data

Number of compacts per cell 12 Cylindrical compact diameter 12.34–12.36 mm Cylindrical compact height 25.0–25.3 mm

Particle batch – capsule 3 and 6 Baseline Particle batch – capsule 1 and 4 Variant 3 Particle batch – capsule 2 Variant 2 Particle batch – capsule 5 Variant 1

235

Number of particles per compact 4 150 Number of particles per capsule 49 800 Defective SiC layers <4  10 5

and time-averaged volume-averaged temperatures

were
100–150C lower, depending on the capsule.

R/B rate ratios have been calculated for many of the

short-lived fission gases.31In all cases, the R/B is less

than
107, indicative of release from heavy metal

contamination (A failure of one particle in a capsule

would result in an R/B of
3.5  106based on 4150

particles per capsule and a release of
1.5% from the

kernel, which is a typical value at these temperatures

and burnups.)32

3.07.2.4 European Experience

The European Commission’s 7th Framework

Programme has conducted recent TRISO-coated

particle fuel irradiations termed ‘HFR-EU1’ and

‘HFR-EU1bis.’ The experiments share the objective

of exploring the potential for high performance

and high burnup of the existing German UO2

TRISO-coated particle fuel pebbles for advanced

applications, such as the conceptual Generation IV

very-high-temperature gas-cooled reactor As

dis-cussed in Section 3.07.2.2, during extensive

irra-diation tests at and above nominal power-plant

conditions in the 1980s and 1990s, not a single coated

particle of ‘near-to-production’ fuel elements

pro-duced by German researchers with

LEU–TRISO-coated particles failed Irradiating this fuel under

defined conditions to extremely high burnups and

higher temperature would allow a better

under-standing of the ultimate irradiation performance of

German UO2TRISO-coated particle fuel

The goal of the HFR-EU1 was to obtain

particu-larly high burnup (
20% FIMA) at a peak

tempera-ture of 1150C, typical of pebble bed operation,

whereas HFR-EU1bis was dedicated to a particularly

high central pebble temperature of up to 1250C and

up to typical pebble bed burnups (
10% FIMA).33,34

The irradiated pebbles were 60 mm in diameter with

LEU-TRISO-coated UO Details of the German

(Arbeitgemeinschaft Versuchsreaktor) and Chinese(Institute of Nuclear and New Energy Technology)fuel attributes are found inTable 49

The design of HFR-EU1 and HFR-EU1bis is

on the basis of previous experience of HTR fuelirradiations within the European Union HFR-EU1contained five pebbles and six mini samples (tencoated particles each, packed in graphite powderand contained in a niobium tube) Five pebbles in

a full-size standard high-temperature gas reactorfuel element rig were used in HFR-EU1bis Sche-matic drawings for each arrangement are shown inFigures 21 and 22 Configuration and irradiationdata are inTables 50 and 51

In HFR-EU1, the upper sample holder containingthe two Chinese INET fuel pebbles is equippedwith 14 thermocouples, while the lower holdercontaining the German AVR fuel pebbles has 20

In HFR-EU1bis, the central temperature was heldconstant to control the experiment, whereas controlwas achieved in HFR-EU1 by holding the surfacetemperature of the pebble constant Measured tem-peratures during the irradiation (without correctionfor thermal drift and neutron induced decalibration)ranged between 800 and 1000C for HFR-EU1 andbetween 900 and 1200C for HFR-EU1bis, consis-tent with the peak fuel temperature targets set for theirradiations

In HFR-EU1bis, neutronic calculations indicatethat the peak pebble burnup varied between
9%and 11% FIMA and neutron fluence varied between

3.0 and 4.0  1025

n m2(E > 0.1 MeV) depending

on the axial location of the pebble After 12 cycles inHFR-EU1, neutronic calculations indicate that thepeak pebble burnup varied between
9% and 11%FIMA and neutron fluence varied between
2.7 and3.7 1025

n m2 (E > 0.1 MeV) depending on theaxial location of the pebble

Fission product monitoring in HFR-EU1 wasaccomplished using gas grab samples At the end of

Table 48 AGR-1 irradiation data

Time-averaged volume-averaged temperature (C) 1029 991 980 1041 1005 980

Peak fast fluence (10 25 n m2, E > 0.18 MeV) 3.2 3.8 4.1 4.0 3.7 3.0 BOL 85m Kr R/B 8  10 8 1  10 8 6  10 9 9.0  10 9 1  10 8 1  10 8

EOL 85m Kr R/B 9  10 8 4  10 8 1  10 8 5  10 8 2  10 7 1  10 7

the HFR-EU1bis irradiation, the R/B was
4  106.

In the earlier experiments, HFR-K5 and HFR-K6,

R/B values of 5 107had been measured on fresh

fuel If it is assumed that when particle coatings fail,

the particle releases
1% of the measured lived Kr and Xe isotopes (which is not an unreason-able estimate on the basis of other irradiations), thenHFR-EU1bis would have contained a few initiallydefective particles at the beginning of irradiationand additional tens of particle failures at the end.The particle failures may have been related to over-heating of the fuel early in the irradiation when animproper gas mixture was inadvertently introducedinto the capsule

short-After 12 irradiation cycles in HFR-EU1, R/B

of
4  108 and 1.4 107 were measured forChinese INET and German AVR fuel, respectively.These low values suggest that in HFR-EU1, no fail-ures have been detected Instead, the measuredfission gas-release probably originates, again, fromuranium and thorium impurities in the matrix graph-ite of the pebbles and in the graphite cups used tohold the pebbles in place

3.07.2.5 Chinese ExperienceThe Chinese fuel development effort has been per-formed in part to support the HTR-10 reactor.35,36HTR-10 is a 10 MW modular high-temperature gas-cooled test reactor fueled with 60 mm diameterspherical fuel elements, each containing
8300 low-enriched UO2 TRISO-coated fuel particles Over

20 000 spherical fuel elements have been tured for the HTR-10 in 2000 and 2001

manufac-Table 49 German and Chinese fuel attributes for EU1 and EU1bis