Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo)

Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo)Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo) Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo) Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo) Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo) Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo) Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo) Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo) Comprehensive nuclear materials 3 14 uranium intermetallic fuels (u–al, u–si, u–mo)

Yeon Soo Kim

Argonne National Laboratory, Argonne, IL, USA

ß 2012 Elsevier Ltd All rights reserved.

3.14.2.2 Thermal Conductivity of U–Al Alloy and UAlx–Al Dispersions 396

3.14.3.2 Thermal Conductivity of (U–Si Intermetallic)–Al Dispersions 401

3.14.4.2 Thermal Conductivity of (U–Mo Alloy)–Al Dispersions 413

EFPD Effective full power days

EOL End of life EPMA Electron probe microanalysis ETR Engineering test reactor (at INL)

FD Fission density in fuel phase HEU High-enrichment uranium (usually
93

wt%235U)

391

IL Interaction layer (reaction layer)

INL Idaho National Laboratory (Idaho Falls, ID)

LEU Low-enrichment uranium ( <20 wt% 235

U) MEU Medium-enrichment uranium (35–45

ORR Oak Ridge research reactor (at ORNL)

PIE Postirradiation examination

RERTR Reduced enrichment for research and

test reactors (program)

SEM Scanning electron microscopy

SEM secondary electron image

TEM Transmission electron microscopy

3.14.1 Introduction

3.14.1.1 Background

Uranium intermetallic fuels such as U–Al, U–Si, and

U–Mo are chiefly meant for research and test

reac-tors in which neutron production, instead of power

generation, is the main purpose The operation

tem-peratures of these fuels are lower than those of

ura-nium ceramic fuels used for power generation such as

UO2 In general, the U intermetallic fuels can achieve

much higher fission densities than can the oxide fuel

Currently available research reactor fuels are

pre-dominantly in a dispersion form that is composed of

fuel particles dispersed in an inert matrix.Figure 1

illustrates the cross section of a dispersion fuel plate

The fueled zone in a dispersion fuel plate, that is,

the fuel particles–matrix mixture zone, is frequently

called the ‘fuel meat’ or ‘fuel core’ and is

metallurgi-cally bonded to the cladding Throughout this

chap-ter, ‘fueled zone’ is used instead of fuel meat

Aluminum is the most popular choice for the

matrix because of its low neutron absorption cross

section, low cost, and good fabricability It also has

adequate mechanical, physical, thermal, and chemicalproperties for cladding material Aluminum alloy ascladding material also has good corrosion resistance toany slightly acidic coolant Another useful attribute ofaluminum is its relative compatibility to reprocessing.1

3.14.1.2 Historical Evolution of

U Intermetallic FuelsThe U intermetallic fuels for use in research and testreactors are, in the order of earliest to latest, U–Al,U–Si, and U–Mo The basic driving force for thedevelopment of a new fuel is to obtain a higheruranium density in the fuel phase Uranium metalhas the highest uranium density, but it is not usablebecause of poor irradiation stability The uraniumintermetallics were introduced to achieve stable irra-diation performance of uranium metal The uraniumdensities for the candidate fuels are given inTable 1.Because of its structural similarity to matrix alu-minum, the first uranium intermetallic fuel chosenfor research and test reactor purposes was U–Al alloy.U–Al alloy has a well-established performance his-tory as the fuel for the materials testing reactor(MTR) and engineering test reactor (ETR)

Fabrication of U–Al alloys with high uranium tents poses difficulties during the rolling process, anduranium inhomogeneity increases proportionally withuranium content The typical picture-frame method offuel assembly and the related rolling fabrication methodare illustrated inFigure 2 When alloys of greater than

con-25 wt% uranium contents are needed, dispersions of

UO2instead are used.2(Full use of UO2–Al dispersionhas been limited in the United States, chiefly because ofswelling problems due to a reaction between UO2and

Al encountered early in its development, although this

is not the case for Russian-built reactors Instead, a littleless dense U3O8–Al dispersion has been used.)Table 1gives the basic properties of fuels currently used (orcandidate fuels) for research and test reactors

The application of monolithic U–Al alloy in higherpower reactors such as the advanced test reactor(ATR) at INL and high-flux isotope reactor (HFIR)

at ORNL was deemed limited because of the tion limitations inherent for a high U-density fuel,

fabrica-Cladding Fueled zone (or fuel meat)

Figure 1 Schematic of the cross section of a dispersion fuel plate frequently adopted in research and test reactors The fueled zone (or fuel meat) is composed of fuel particles (darker phase) dispersed in an Al matrix (brighter phase).

high fuel swelling, and the need for adding B-10

burnable absorber The use of U3O8–Al dispersion,

that is, U3O8 particles dispersed in an Al matrix in

high-power reactors, was first considered and

aban-doned due to concern about the exothermic reaction

between U3O8 and Al3 and the interdiffusional

reac-tion growth between U3O8and Al, although this fuel

form is still used in some other reactors

The fuel form of U–Al alloy with a U density high

enough to satisfy the need for high-power rectors

is a mixture of UAl2, UAl3, and UAl4, known as

UAlx It develops when the U weight fraction is

pushed beyond
62 wt% The exact fractions of the

compounds included in UAlxdepend on the

fabrica-tion process Whitacre4reported that typical powder

lots used in the ATR contained phase fractions of

7.6 wt% UAl2, 78.6 wt% UAl3, and 13.8 wt% UAl4

UAlxhas several positive features that enable its

superior performance in high-power reactors Fuel

swelling can be reduced by accommodating fission

product swelling in the powder dispersions, which

include pores left during fabrication It also has

ex-ceptional resistance to fission gas bubble formation

In addition, fabrication with a uniform distribution

of burnable absorbers is possible.5

To achieve a higher U-density fuel, UAlx

com-posed of dominantly UAl instead of UAl was

tested.6,7 When handled in air, however, UAl2 ismore pyrophoric than UAl3, and this leads to com-plications in fabrication and the potential for oxygenimpurities in the fuel

The US Department of Energy (US DOE) initiatedthe RERTR (Reduced Enrichment for Research andTest Reactor) program in 1978 to convert the world’sresearch and test reactors using high-enrichment ura-nium (HEU) to those using low-enrichment uranium(LEU) An enrichment in235U of 20 at.% is the thresh-old between HEU and LEU To use a fuel with reducedenrichment, keeping the fuel phase volume the same inthe fueled zone (seeFigure 1), requires using a fuelhaving a higher uranium density to compensate for thereduced fissile fraction in LEU

In the RERTR program, the fuel form developed

to accomplish this is U3Si2, which allows the highestpossible uranium loading among the qualified fueltypes This fuel showed excellent stability duringirradiation Fission gas bubble swelling is of no con-cern for fuel dimension expansion at typical researchand test reactor applications This fuel enabled LEUcore conversion of
60% of the research and testreactors worldwide Another U–Si intermetallic fuel,

U3Si, can achieve even a higher U loading than U3Si2,but in plate-type designs, it shows unstable growth offission gas bubbles at high temperatures and burn-ups.8In addition, the interaction layer (IL) between

U3Si and Al grows faster than that of U3Si2–Al In

a rod-type design, U3Si–Al dispersion, however, isknown to have acceptable performance due to theability of the pin to constrain fission gas bubbleswelling in a more stable manner.9

Failure to convert high-power research reactorsusing HEU to LEU U3Si2 called for fuels of evenhigher uranium density Given the unstable irradiationbehavior of the high-uranium-content compounds(e.g., U3Si and U6Fe), the fuel development effort hasshifted to uranium–molybdenum alloys with Mo con-tent ranging 6–10 wt%, in both monolithic and disper-sion fuel forms Since 1997, the U–Mo alloys havebeen irradiation-tested under the auspices of the U.S.RERTR program and other programs in Argentina,Canada, France, South Korea, and Russia These testshave shown that U–Mo alloy has stable irradiationbehavior

A major complication in U–Mo alloy dispersion

in Al is the reaction between U–Mo and Al Undercertain irradiation conditions, fission gas bubblegrowth in this reaction product is sufficient to causefuel plate failure A small amount of silicon added

to the matrix aluminum has been found to be a

Table 1 Basic properties of uranium intermetallic fuels

Fuel Melting point

(C)

Physical density (g cm3)

Uranium loading (g cm3)

c This case is for the mixture of 60 wt% UAl 3 + 40 wt% U 0.9 Al 4

d Al is included for reference.

promising remedy to this problem The U–Mo

monolithic fuel, in which a U–Mo thin foil is

sand-wiched between cladding and directly bonded to

cladding, is currently under development and has

the advantage of providing higher U density than

the dispersion form while essentially eliminating the

problem related to reaction products between the

fuel and matrix However, the problem of gap

forma-tion between fuel and cladding must be solved before

this fuel form is usable

3.14.1.3 Performance Topics of

U Intermetallic FuelsThe performance of U intermetallic fuels is closelyrelated to whether they are crystalline or amorphousduring irradiation The U intermetallic fuels tend to

be amorphized by damage in the crystal structurecaused by highly energetic fission fragments Theviscosity of an amorphized material is lower thanwhen it is crystalline: in other words, the fluidity

Cover plate

Fuel bearing alloy or meat compact Frame

Cover plate (a)

Al powder

Fuel powder

Fuel compact Al-alloy frame and

cover for cladding

Cold roll

X-ray fuel geometry

Hot roll*

Assemble and weld

Shear excess material to desired plate dimensions

* The hot roll procedure is typically composed of several passes of rolling.

Before the first roll pass, annealing for about 1 h is performed.

Between passes, additional annealing

is performed for ~15 min.

The annealing temperature is determined by the hardness of cladding material.

For Al6061 cladding, it is ~485⬚C.

The softer the cladding material, the lower the annealing temperature

is used.

Preirradiation blister test#

# The rolled plate is annealed at ~485 ⬚C for

1 h for blister test.

If blisters form, the plate is disqualified.

(b)

Figure 2 Illustration of plate fabrication (a) Exploded view of the dispersion fuel compact assembly by the

picture-frame method (b) Flow diagram of the hot-rolling fabrication method.

of the material increases when it becomes

amor-phous Amorphization of a crystalline material to

metallic glass is usually accompanied by an increase

in volume – a quantity called ‘free volume’ – which

facilitates atomic mobility, enhancing diffusion.10

Fission gas mobility is also high in amorphous

mate-rial and the fuel matemate-rial is more readily deformed

by the growing gas bubbles Hence, overall, fission

gas bubble growth in an amorphous material is faster

The three U–Al intermetallics undergo

amorphi-zation depending on the fission rate and

tempera-ture The lower the irradiation temperature and the

higher the fission rate, the more readily the fuel

becomes amorphous Among the three, UAl4

amor-phizes most readily and UAl2the least The reaction

products between the fuel and matrix are also

ura-nium aluminides and undergo amorphization The

U–Si intermetallics (U3Si and U3Si2) also become

amorphous during irradiation, and the reaction

prod-uct between the fuel and matrix, U(Si, Al)3, also

undergoes amorphization The U–Mo alloy is not

amorphized during irradiation, but the reaction

prod-uct between the fuel and the matrix, (U, Mo)Alx,

becomes amorphous

In the following sections on U–Al, U–Si, and U–Mo

fuels, the areas of (1) physical properties, (2) fabrication

methods, and (3) irradiation performance are

dis-cussed Each of the three review areas are described

in detail The physical properties section discusses the

phase diagram, lattice structure of important

composi-tions, and density The section on fabrication methods

discusses relevant fuel particle fabrication processes

The section on irradiation performance includes fuel

swelling, IL growth between fuel particles and matrix

aluminum, and blister threshold temperature

Fuel particle swelling and IL growth are two major

fuel performance topics in research reactor fuel plates

Both work to increase the fueled zone volume In plate

geometry, fueled zone volume expansion is transferred

directly to plate thickness increase because the

restraint in this direction is the weakest Monitoring

plate thickness is an effective method of tracking the

fueled zone swelling An excessive plate thickness

increase is the indicator for potential fuel plate failure

A unique measure for sound fuel performance

considered in research and test reactors is ‘blister

threshold temperature’ testing with irradiated plates

In typical research and test reactor fuel designs,

because there is neither a gap between fuel and

cladding nor a plenum, no fission gas release and

collection is possible outside of the fueled zone

This is another advantage of using dispersion fuel

Fission gas and any gas included during fabricationremain in the fueled zone; in particular, fission gasesare contained in pores or fission gas bubbles Gaspressure in large pores and fission gas bubbles, whichmay be insufficient to cause detrimental creep or yield-ing of fuel, could instead result in blistering of a fuelplate when the plate is heated to a certain temperature.Two types of mechanisms can be considered for blis-tering One is pore (or void) connection, and the other

is pressure rupture of fission gas bubbles Figure 3shows the images of a typical blister-tested plate Inthe typical blister test, the sample plate is held at aspecified temperature for 30–60 min during eachannealing step

The temperature at which blisters form is termedthe ‘blister temperature.’ The higher the blister tem-perature, the more resistant the fuel is to blistering

As a design requirement, the minimum blister peratures, also called ‘blister threshold temperatures,’are typically tested with irradiated plates for theanticipated power excursions

tem-3.14.2 U–Al

3.14.2.1 U–Al Fuel PropertiesThe U–Al phase diagram is shown inFigure 4 Thereare three intermetallic compounds in the U–Al sys-tem: UAl2, UAl3, and UAl4 UAl2forms directly fromthe liquid, but UAl3 and UAl4 form by peritectoidreactions with aluminum as follows:

(a)

(b)

Figure 3 Images of a U 3 Si 2 –Al dispersion fuel plate after a postirradiation blister test at 450C (a) Blistered plate surface morphology (b) Cross section of a blistered plate in the plate thickness direction.

UAl2has a face-centered cubic structure witha ¼ b ¼

c ¼ 0.776 nm (the MgCu2-type crystal structure) UAl3

has a simple cubic structure witha ¼ b ¼ c ¼ 0.426 nm

(the Cu3Au-type structure).11 UAl4 has a

body-centered orthorhombic structure with a ¼ 0.441,

b ¼ 0.627, c ¼ 1.371 nm.12

UAl4 is found as U-latticedeficient, and therefore U0.9Al4 is frequently used to

designate this compound This compound is, however,

expressed stoichiometrically as UAl4.9

The densities of the compounds are 8.14 g cm3

for UAl2, 6.80 g cm3 for UAl3, and 6.06 g cm3for

UAl4 However, it is reasonable to assume the density

of UAl4 as 5.7 g cm3 considering its U-deficiency

structure

3.14.2.2 Thermal Conductivity of U–Al

Alloy and UAlx–Al Dispersions

The thermal conductivity of U–Al alloy depends on

the uranium composition in the alloy and on the

temperature At 65C, the thermal conductivities of

the as-cast U–Al alloys are a linearly decreasing

function of the uranium concentration13:

kTðU  Al alloyÞ ¼ 225  2:9CU ½1

where kTis the U–Al alloy thermal conductivity in

W m1K1 and CU is the uranium content in thealloy in wt%

Thermal conductivity of UAl2 is not available,though it can be reasonably estimated as a valuelower than that of UAl3 In general, the higher the Alcontent in a U–Al intermetallic, the higher the ther-mal conductivity because Al is more thermally con-ducting than is U Among the U–Al compounds, UAl4has the lowest thermal conductivity due to its defec-tive structure Because of peritectoid reactions given ineqns [I] and [II], during heating a UAlx–Al dispersionthe UAl4fraction increases while that of UAl3remainsnearly unchanged and that of UAl2decreases Hence,the overall thermal conductivity of a UAlx–Al disper-sion decreases during heating, but the thermal con-ductivity of UAlx–Al is determined chiefly by the Almatrix For a UAlx (60 wt% UAl3 and 40 wt% UAl4)volume fraction of 35% in the fueled zone, the ther-mal conductivity of the fueled zone is
62 W m1K1,considering a porosity of
6% in the fueled zone,which is typical for this fuel This is similar to that of

U3O8–Al but slightly lower than that of U3Si2–Al withthe same fuel volume fraction and porosity

Atomic percent Al

600 800 1000 1200 1400 1600

400

1800

641 731

The recommended data for the thermal

con-ductivities of UAlx–Al dispersions obtained from

Hofman and Snelgrove14are used to fit an equation

wherekTis in W m1K1andvUAlx is the UAlxvol%

(for the 60 wt% UAl3 and 40 wt% UAl4 mixture)

including typical fabrication porosity in the fueled

zone

3.14.2.3 U–Al Fabrication

This section deals with the fabrication methods used

for the fuel phase made of U–Al intermetallics The

U–Al alloy is in the form of a single slab, whereas

UAlxis in the form of a particle powder The fueled

zone of the fuel plate containing U–Al alloy is

there-fore monolithic in that no distinct aluminum matrix

is involved, whereas that of UAlxis composed of fuel

particles in a matrix of aluminum In the overall fuel

plate fabrication, both fuels have the same procedure

shown inFigure 2, except for the difference that lies

in whether the fueled zone is a monolithic alloy or a

dispersion compact

3.14.2.3.1 U–Al alloy

U–Al alloy is produced directly by melting and

cast-ing proper amounts of U and Al metal together,

which determines the U density in the alloy For a

plate-type fuel, the alloy melt is poured into graphite

molds to produce cast alloy slabs Each monolithic

slab of U–Al alloy is then sandwiched between Al

alloy cladding slabs and hot-rolled into dimension to

form a plate-type fuel element

This method can be used for fabrication of U–Al

alloys with up to
25 wt% uranium Up to 40 wt%

uranium, U–Al alloy can be fabricated in the same

way with a small amount of UAl4precipitates in the

alloy Alloys with such content ratios are liable to

contain small fractions of metastable UAl3

3.14.2.3.2 UAlx

UAlxdesignates a mixture of UAl2, UAl3, and UAl4:

The exact composition varies depending on the

pow-der fabricator Fabrication of UAlxis determined by

its composition

The first step of the powder fabrication of U–Al

compounds with the high U weight fraction is

arc-melting of the mixture of U and Al metals

Since U aluminides are brittle, the typical methodfor powder fabrication is mechanical pulverization bythe use of jaw crushers and hammer mills U–Alintermetallics are pyrophoric In particular, UAl2 ishighly pyrophoric; hence fabrication of this com-pound is more difficult, although it is the highest in

U density among the three U–Al intermetallics Thedesired particle size is controlled by the use of metal-lic sieves The undersized or oversized particles arerecycled The comminuted particles are irregular.They have sharp corners, cracks, and high surface-to-volume ratio The fuel particles are also brittle, whichcauses higher porosity in the fueled zone after platefabrication Since the comminution is performed in air,oxygen is absorbed into the fuel particles The oxygeninclusion has an effect on fuel performance (to bediscussed later)

It is difficult to fabricate pure U aluminide pounds Typically the product powder is a mixture ofUAl2, UAl3, and UAl4 If necessary, pure compoundscan be prepared by the use of uranium hydride,15butthis method is not a commercially viable optionbecause of high cost, the hazardous process involved,and difficulty of reprocessing the scrap materials.Porosity is common in as-fabricated UAlx–Al and

com-U3Si2–Al dispersions, with the amount depending

on the fabrication method For all fabrication methods,however, it typically increases with the fuel phase vol-ume fraction in the fueled zone For example, in UAlxcomposed of 60 wt% UAl3 and 40 wt% U0.9Al4, thedensity is rUAlx ¼ 0:6ð6:83Þ þ 0:4ð5:7Þ ¼ 6:4 gcm3and the average stoichiometry x ¼ 0.6(3) þ 0.4(4.9) ¼3.8 For this case, as-fabricated porosity as a function

of UAlxvolume fraction can be estimated using mation given by Beestonet al.,5

infor-as shown inFigure 5along with the data for U3Si2–Al

3.14.2.4 U–Al Irradiation Performance3.14.2.4.1 Fuel swelling by fission productsFuel swelling by fission products is convenientlydivided into two distinct parts One is fuel swelling

by solid and liquid fission products due to the ence between the volume of a uranium atom and solidfission products, and the other is by gaseous fissionproducts that form bubbles The former takes place inthe matrix of fuel particles and is due to the atomicvolume difference between the uranium atom andsolid fission product atoms (Fuel matrix here standsfor the solid part in the fuel particles, excluding thefission gas bubbles.) This also includes liquid fissionproduct atoms Most fission gas atoms remain in the

differ-matrix of fuel particles particularly at low burnup.

This also contributes to fuel matrix swelling, with

the rate proportional only to burnup or fission

den-sity, independent of the fabrication method, fuel type,

and fuel temperatures Hence, it is commonly

appli-cable to other U intermetallic fuels also The latter

is due to the fission gas bubble growth As the gas

bubble swelling increases at higher burnup, however,

more gas atoms move into and collect in gas bubbles

The resulting solid swelling rate is therefore

propor-tionately lower Modeling this effect in detail has not

been tried due to the lack of accuracy in relevant

measurements

3.14.2.4.1.1 Fuel swelling by solid fission

products

Fuel swelling by solid (including liquid fission

pro-ducts also) fission propro-ducts is a result of difference in

atomic volumes between uranium atoms destroyed by

fission and solid fission products and how the fission

products stay in the alloy Hofman and Walters17

esti-mated burnup-dependent fuel swelling by solid

fis-sion products for U–Zr alloy Their estimation is

1.2% per at.% burnup in U–10Zr, which can be

trans-formed in terms of fission density of U–10Mo as

where fd is the fission density in 1027fissions m3

However, this estimation is applicable only for the

situation in which alkali and alkaline-earth fission

products release and dissolve in the liquid sodium

gap material in fast reactor fuel pins However, in theU–Mo particle cases, this loss is largely negligiblebecause no sink for gas release is available, althoughsome occur by fission recoils to matrix aluminum.Counting in this portion changes the value ineqn [3]

by
0.5% per 1027

fissions m3 Consequently, thefuel swelling by the solid (plus liquid) fission productscan be expressed by

Fuel swelling by fission gas bubbles is more difficult

to quantify The fission gas bubbles that form in UAlxare so small that they were hardly observable byauthors in the literature Francis20observed fission gasbubbles in UAl3 with burnup of 60%, or 6.51027fissions m3(in fuel particles) He also reported that

no gas bubbles were observed in reaction layers,which were UAl4 He suggested that this was due tothe lower uranium density in UAl4and the lower fissiondensity of
4.61027

fissions m3 than in UAl3 andalso suggested that the defect structure in UAl4mightaccommodate extra fission gas, delaying bubble forma-tion Miller and Beeston21found no visible fission gasbubbles in UAl2up to 4.6 1027

fissions m3 Hofman,22too, found no fission gas bubbles in UAlxup to 7 1027fissions m3 Miller and Beeston21and Hofman22com-monly observed fission gas bubbles only in uraniumoxide clusters, but none in UAl2or UAlx The source ofoxygen in the clusters is the air included in the fueledzone from both the fuel powder and the later platefabrication performed in an air environment.Figure 6shows a scanning electron microscopy (SEM) image

of the fracture surface of UAlxirradiated at
130C

to 56% burnup of 93% enriched fuel (or 7 1027

sions m3) It is unclear whether the oxide clusters acted

fis-as reservoirs absorbing fission gfis-as, or whether UAl4

in UAlx helps retard bubble formation More recenthigh-resolution transmission electron microscopy(HR-TEM) investigation of irradiated uranium inter-metallic fuels has revealed the presence of small fissiongas bubbles in the IL.23,24Thus, it may be possible thatvery small micro bubbles are present in irradiated UAlx,but were undetected due to limits on the resolution ofmicroscopy techniques at the time

Recent TEM work by Ganet al.23

shows that smallbubbles, visible only in TEM, are in fact formed in

Figure 5 Fabrication porosity in a UAl x –Al dispersion

plate as a function of the volume of UAl x Information

on U 3 Si 2 –Al dispersion was obtained from Matos and

Snelgrove,16and the fabrication data for UAl x –Al were

from Beeston et al.5

interaction products of U–7 wt% Mo dispersion in Al(see Figure 7) Since the interaction products areprobably similar to those in UAlx–Al (even though

Mo is involved), this TEM work suggests that fissiongas bubbles also form in UAlxbut would be too small

to be seen in an SEM, particularly in older, lowerresolution devices For example, the image inFigure 8

is a recent SEM work by Jue,24which indeed showsfission gas bubbles with a maximum size of
0.2 mm inthe IL between U–7Mo and Al after irradiation to4.1 1027

fissions m3in fuel particles To summarize,fission gas bubbles form in UAlx but have not beenconsistently observed

A direct quantification of the gas bubble swellingrate is currently impossible Instead, the gas bubbleswelling rate is estimated by subtraction of solidfission product swelling from the total swelling.The total swelling rate, that is, contribution by solidfission products and fission gas bubbles, obtained forATR fuel tests is

fissions m3, approximates the tion by fission gas bubbles The contribution of fissiongas bubbles to total fuel swelling is considerable, evenwhen nearly invisible In addition, fission gas bubbles

contribu-in the oxide clusters also contribute

In general, the overall plate thickness increase

in UAlx–Al dispersions because fueled zone swelling

UAlx

U-oxide phase including

fission gas bubbles

5 μm

Figure 6 Scanning electron microscopy image of a 93%

235 U-enriched UAl x –Al dispersion irradiated to 60%

235 U burnup Fission gas bubbles are visible in U oxide

phase and no gas bubbles are visible in UAl x

Reproduced from Hofman, G L Nucl Technol 1987,

Figure 7 Transmission electron microscopy (TEM) image

showing bubbles formed in interaction layer between

U–7Mo and Al Fission gas bubbles are visible as gray

spheres in the interaction product with a size range of

0.01–0.2 mm Some conspicuous bubbles are marked with

arrows The bright upper left corner and a bright hole in the

interaction product were formed during preparation of the

TEM sample Reproduced from Gan, J.; et al J Nucl Mater.

2009, 396, 234.

Al U–7 wt%Mo

Interaction zone

1μm

Figure 8 Scanning electron microscopy image showing fission gas bubbles formed in interaction zones between U–7Mo and Al irradiated to 4.1  10 27 fissions m3 Reproduced from Jue, J F Private communication; Idaho National Laboratory; 2009.

is lower than in any other fuel dispersions This

advantage can be attributed to several factors,

includ-ing higher as-fabricated porosity, the defect structure

of UAl4, and the high resistance of UAlx to large

bubble formation

3.14.2.4.2 Interaction between U–Al and Al

UAlxand Al react during irradiation even at low

tem-peratures due to irradiation-enhanced interdiffusion

The U–Al phase diagram shown inFigure 4 shows

that UAl2and UAl3react with matrix Al according to

the peritectoid reactions given in eqns [I] and [II]

Because there is no higher compound in terms of Al

content than UAl4, only UAl4 stays stable with Al

Reaction products on U aluminides are discernable

in optical micrographs due to color difference, as

shown inFigure 9 The volume fraction analyses of

UAlx–Al before and after irradiation showed that the

volume fraction of UAl4increases while that of UAl3is

nearly unchanged and that of UAl2decreases

The U–Al diffusion-couple tests in the

tempera-ture range of 100–600C showed that UAl3 is the

dominant phase created UAl4is less prevalent, and

UAl2 is present the least.12 At lower temperatures,

there was a greater tendency to form UAl2

In U–Al diffusion-couple tests at 400–600C,

how-ever, Castleman25 did not observe UAl4 and rarely

observed UAl2 This is because UAl2is unstable around

Al as thermodynamic data indicate (see Table 2).The diffusion layers for both works by Kiessling andCastleman showed a considerable amount of pores.Out-of-pile measurements by Nazare et al.28yielded activation energies of 220 kJ mol1 for thereaction between UAl3 and Al, and 180 kJ mol1for the reaction between UAl2 and Al The slightlyhigher value for UAl3–Al dispersion will result in aslower reaction rate than UAl2–Al dispersion.Measured reaction data of UAlx–Al from in-piletests are scarce because reactions between the fueland matrix are not an irradiation performance issuewith this fuel Relevant changes in volume fractionsdue to fuel–matrix reactions are also generally small.All three uranium aluminides undergo amorphiza-tion, depending on the fission rate and temperature.Therefore, the reaction products of these fuels with Alare also subject to amorphization The lower the irra-diation temperature and the higher the fission rate,the more the tendency of amorphization Among thethree, UAl4is the easiest and UAl2is the hardest foramorphization Crystalline material is more stableduring irradiation than amorphous material

3.14.2.4.3 U–Al blister threshold temperature

No blister threshold temperature data were foundfor U–Al alloy fuel, which is probably because thisfuel is more prone to failures by breakaway swelling.UAlx–Al dispersion fuels have high blister thresh-old temperatures due to their high resistance againstformation of fission gas bubbles Beeston et al.5

posed that the blister threshold temperature could bemeasured as a function of fueled zone fission density,and Nazare29fitted Beeston’s data, adding more data

pro-to give the following correlation:

For a fuel plate without B4C,

10 mm

D

Figure 9 Scanning electron microscopy image of a 93%

235 U-enriched UAl x –Al dispersion after a 60% 235 U burnup.

A is UAl 2 , B is UAl 3 , C is the interaction product (UAl 4 ), and

D is U oxide Adapted from Hofman, G L Nucl Technol.

1987, 77, 110–115; Ryu, H J.; Kim, Y S.; Hofman, G L.

a Chiotti and Kateley 26

b Kubaschewski and Alcock 27

where TB is the blister temperature in K, fd is

the fueled zone fission density in 1027fissions m3,

andB is the B4C content in wt%

B4C is added for reactivity control by absorbing

neutrons in some plates Neutron absorption in

boron atoms results in generation of helium gas,

which additionally lowers the blister temperature

as ineqn [7]

3.14.2.5 Summary for U–Al

U–Al alloys and uranium aluminides clad with

alu-minum alloy have advantages for water-cooled

ther-mal research and test reactors operating at low

temperatures primarily due to the favorable

charac-teristics of aluminum U–Al alloys are monolithic

fuels, while uranium aluminides are in dispersion

form in an aluminum matrix

The three uranium aluminides undergo

amorphi-zation depending on the fission rate and temperature

The lower the irradiation temperature and the higher

the fission rate, the faster the amorphization Among

the three, UAl4 amorphizes most readily and UAl2

least readily Crystalline material is more stable

dur-ing irradiation than amorphous material

UAlx–Al dispersions, where UAlxdenotes a

mix-ture of UAl2, UAl3, and UAl4, have lower fueled zone

swelling than any other type fuel dispersions due to

low fission gas bubble swelling Large fission gas

bub-bles greater than the resolution limit of an SEM do not

form in UAlxbecause of the defective structure in UAl4

and high resistance to large bubble formation

Measured interaction data of UAlx–Al from in-pile

tests are scarce because reactions between the fuel and

matrix have not been an irradiation performance issue

for this fuel Relevant changes in volume fractions

due to fuel–matrix reactions are also generally small

because the interaction products between the uranium

aluminides and the aluminum matrix are also uranium

aluminides The volume fractions of UAl4 increases,

while that of UAl2 decreases, and the change in the

volume fraction of UAl3 depends on the kinetics of

other compounds Because UAl3has the highest

ther-mal conductivity of the three, and UAl2and UAl4have

similar thermal conductivities, the evolution of

vol-ume fractions in UAlxpartly counteracts the decrease

in overall thermal conductivity

UAlx–Al dispersion fuels have high blister

tempera-tures due to their high resistance against formation of

fission gas bubbles The blister threshold temperature

is a function of fission density

3.14.3 U–Si

3.14.3.1 U–Si Fuel Properties

In the U–Si system, U3Si, U3Si2, and USi are thecompounds of interest for candidate fuels chieflybecause of their high uranium density (seeTable 1).The physical densities of U3Si, U3Si2, and USi are15.3, 12.2, and 10.96 g cm3, respectively Clearly,

U3Si is the most favorable fuel among U–Si tallic fuels for the same reason The U–Si phase dia-gram is shown in Figure 10 U3Si2 and USi formdirectly from liquid, but U3Si forms only by the fol-lowing peritectoid reaction at 925C:

interme-U3Si2þ U ! U3Si ½III

At room temperature, U3Si has a body-centeredtetragonal structure It undergoes transformation to aface-centered cubic crystal structure at 765C, andthis structure is maintained up to 925C Hence, atusual reactor operation temperatures, U3Si is in atetragonal structure The U3Si structure is classified

as a distorted variant of Cu3Au-type No close Si–Sibonding occurs; only U–U and U–Si bonds are pres-ent This is the reason for the unusual ductility ofthe intermetallic compound.11The lattice parameters

of the tetragonal structure are a ¼ 6.029 A˚ and

c ¼ 8.696 A˚.11,30

U3Si2 has a congruent melting point at 1665Cand has no transformations in the solid state It has aprimitive tetragonal structure witha0¼ 7.3299  4 A˚,

c0¼ 3.9004  5 A˚, c/a ¼ 0.532, and ten atoms per unitcell The crystal structure is a deformed Cu3Au-typewith pairs of Si atoms replacing a single Si atom.These close Si–Si bonds impart brittleness to thecompound.11

The crystal structure of USi is controversial in theliterature The most recent data suggest that thestructure is tetragonal with the lattice parameters of

a ¼ 10.5873 A˚ and c ¼ 24.3105 A˚.31

3.14.3.2 Thermal Conductivity of(U–Si Intermetallic)–Al DispersionsThermal conductivities of U3Si–Al and U3Si2–Aldispersions are similar to each other Consideringthe uncertainties related to the fueled zone porosityand nonstoichiometric nature of these fuels, that is,miscibility of U3Si and U3Si2, it is reasonable that thesame thermal conductivity is used The difficulty

of fabricating a stoichiometric U–Si compound is

discussed inSection 3.14.3.3 The thermal

conduc-tivities of U3Si–Al and U3Si2–Al dispersions are

fitted on the basis of the data given in Matos and

wherevU 3 Si 2 is the U3Si2or U3Si volume fraction in

% including typical fueled zone porosity

3.14.3.3 U–Si Fabrication

U3Si2 is used for plate-type fuels, whereas U3Si is

currently applied only for rod-type fuels In practice,

however, it is almost impossible to fabricate the exact

stoichiometric form of one of these compounds This

is why fuel manufacturers instead start with slightly

Si-rich alloys, which lead to final products more

abundant in higher Si content compounds For

example, the stoichiometric U3Si2 requires an Si

composition of 7.3 wt% To suppress the formation

of U solid solution and U3Si, typically 7.5 wt% Si

is added to the U–Si alloy A prolonged heat ment at 925C has shown to remove the presence of

treat-U solid solution.32 The secondary phases typicallyreside inhomogeneously in a fuel particle, whichcauses inhomogeneous size distributions of fissiongas bubbles inside the fuel particles In some cases,

U3Si occupies whole fuel particles in a nominal U3Si2fuel As a result, postirradiation images show anomaly

in fission gas bubble size between fuel particles.Alloy ingots of U–Si are made by mixing andmelting of uranium and silicon with a desired Si/Uratio The ingots are sometimes annealed in an inertatmosphere to complete compound formation Theseingots are then broken into smaller particles by apowder fabrication process

Two types of powder fabrication methods forU-silicide fuels are most commonly used The rela-tively brittle compounds of U3Si2 and USi areobtained by comminution of the alloy in a glove box

in a nitrogen atmosphere with a hardened steel tar and pestle U3Si powder fabrication, because U3Sihas greater toughness than U Si and USi, is typically

600 800 1000 1200 1400 1600

450 668

carried out by grinding in a ‘shatter box.’ This

method calls for laborious processes because of the

toughness of U3Si Typical particle sizes range

between 40 and 150 mm.33

An atomization technology widely used in powder

metallurgy is applied to fabricate spherical powders

of U3Si2 and U3Si.9 This method typically uses a

rotating disk in a vacuum chamber When a liquid

fuel melt is poured on the disk, the centrifugal force

of the disk produces liquid fuel droplets that are

cooled in the process of flying away The size of fuel

particles is determined by the disk rotation speed

Figure 11shows a comparison between

atomiza-tion and comminuatomiza-tion powders Atomized powder has

several advantages over comminuted powder First, the

surface-to-volume ratio of the atomized particles is

smaller, so reaction product volume between fuel

par-ticles and matrix aluminum is smaller Second,

ato-mized particles have higher homogeneity in silicon

content and fewer impurities because they are rapidly

solidified from the liquid and are uncontaminated by

mechanical pulverization Finally, atomized particles

have lower residual stresses and defects, which is an

advantage from the view point of fuel swelling

The typical plate fabrication method shown in

Figure 2 is also used for the fabrication of U–Si

powder dispersion plates

3.14.3.4 U–Si Irradiation Performance

3.14.3.4.1 Fuel swelling by fission products

Fuel swelling by solid fission products discussed in

Section 3.14.2.4.1is also applicable for U–Si in that

this kind of fuel swelling is dependent only on

burnup, regardless of the fuel kind The swelling

rate given by eqn [4] can be used However, fuel

swelling kinetics by fission gases, that is, fission gas

bubble growth, is different and discussed more in

Fuel swelling kinetics of U–Si fuel particles is well

documented in the literature.14InFigure 12, the fuel

swelling kinetics of U3Si and U3Si2 are plotted

together with the fuel swelling by solid fission

pro-ducts calculated witheqn [4] For each fuel type, the

fuel swelling by fission gas bubble growth can be

estimated by subtracting the fuel swelling by solidfission products from the total fuel swelling Thedata included in the graph were obtained from tests

at temperatures below 110C, where fission gas ble growth, and therefore fuel swelling, have beenconsidered athermal and dependent only upon theburnup

bub-U3Si and U3Si2are known to become amorphousunder irradiation34,35 due to fission damages Theprimary damage to the crystal structure is caused byhighly energetic fission fragments In the amorphous

(a)

(b)

Figure 11 Examples of comminuted and atomized powders of U 3 Si 2 (a) Scanning electron microscopy (SEM) image of U 3 Si 2 powder fabricated by the comminution method (b) SEM image of U 3 Si 2 powder fabricated by the atomization method Reproduced from Kim, C K.; Kim,

K H.; Jang, S J.; Jo, H D.; Kuk, I H In Proceedings of the

1992 International Meeting Reduced Enrichment for Research and Test Reactors (RERTR), ANL/RERTR/TM-19, CONF-9209266, Sept 27–Oct 1, 1992; Argonne National Laboratory: Argonne, IL, 1992.

fuel, fuel swelling depends on the viscosity of fuel The

viscosity of an amorphized material is lower than when

it is crystalline: in other words, the fluidity of the

material increases when it becomes amorphous Fission

gas mobility is also high in amorphous material and the

fuel material is more readily deformed by the growing

gas bubbles Hence, overall fission gas bubble growth in

an amorphous material is faster.Figure 13shows

fis-sion gas bubble morphology of amorphous fuels

Bub-bles are large and interconnected, which is observable

in breakaway swelling

Amorphization is clearly a low-temperature

phenomenon, as amorphized materials devitrify

(recrystallize) at the so-called glass transition

tem-perature Above this temperature, amorphization is

not possible and the fuel in question exhibits the

familiar crystalline irradiation behavior U–Si fuels

are normally amorphous during irradiation because

the glass transition temperature for U-silicides is

much higher than typical fuel operation

tempera-tures (
120C).

These fuels show that they preserve their

preirra-diation hardness and brittleness The observed

fluid-like behavior thus only exists during irradiation

Figure 14 shows fuel microstructures and the

fission gas bubble morphology of irradiated U3Si

and U3Si2 Although both are amorphous during

irra-diation, there are inherent differences: notably, fission

gas bubble growth in U3Si is high and unstable,

whereas that of U3Si2 is generally low and stable

For this difference in fission gas bubble growth,

Hofman and Kim8offered an explanation by evoking

the correlation between free volume and viscosity,

which was first developed by Doolittle36:

 ¼ 0exp C

DVR

½9where C is a constant and DVR is the part of thequenched-in free volume associated with structuralrelaxation that is recovered during annealing of theglass prior to recrystallization Hofman and Kim8noted that U3Si has larger free volume than U3Si2

It has been shown in the literature that the freevolume of a glassy metal is strongly affected bycomposition, since the short-range bonding character

of an alloy is maintained in the glass state.37Apparently, the additional Si bonds in U3Si2have

a large effect on the amount of free volume in theglassy state, and therefore also on the fluidity of

(b)

Figure 13 Optical microscopy images showing unstable fission gas bubble growth shown in amorphous fuels at fission density of 4.5 10 27 fissions m3 (a) U 3 Si (b) U 6 Fe.

the fuel – the fission gas diffusivity – and the resulting

swelling behavior Although amorphization is a

pre-requisite for low-temperature high-swelling behavior,

it needs to be accompanied by an increase in free

volume

The bubble morphology from higher temperaturetests is available in the literature.38,39 Figure 15shows the fission gas bubbles at different tempera-tures Compared toFigure 14(a), Figure 15(c)sug-gests that bubble growth in U3Si2 can be enhanced

to the level of U3Si if the temperature is increased

by
60C U

3Si2 appears to experience a bubblegrowth phenomenon at high temperatures similar tothat of U3Si at low temperatures; the low bubblegrowth advantage of U3Si2 provided by the highSi/U ratio is negated if the temperature is increased

A mechanistic rate-theory model demonstrates thatthe bubble coarsening process in irradiated amorphousmaterials such as U3Si and U3Si2 depends on their

(a)

(b)

Figure 14 Fission gas bubble morphology of U-silicide

fuels (19.5% 235 U enriched) irradiated at temperatures

(
100 C) in the Oak Ridge research reactor (a) U

3 Si irradiated to 15 at.% BU, (b) U 3 Si 2 irradiated to 19 at.% BU.

Notice the difference in magnification Notice that the scale

in (b) is a factor of 10 greater than that in (a).

Figure 15 Optical microscopy images of U 3 Si 2

(75% 235 U enriched) (a) T ¼ 105 C and FD¼ 3.2 

10 27 fissions m3(13 at.% U total BU), (b) T ¼ 136 C and

FD ¼ 5.4  10 27 fissions m3(22 at.% U total BU), (c)

T ¼ 160 C and FD¼ 6.1  10 27 fissions m3(25 at.% U total BU) Adapted from Kim, Y S.; Hofman, G L.; Rest, J.; Robinson, A B J Nucl Mater 2009, 389, 443; Kim, Y S.; Hofman, G L.; Yacout, A M J Nucl Mater 2009, 392, 164.

viscosity.40,41 Estimated irradiation-induced viscosity

values were obtained by comparing the calculated

bubble size distribution with the observed average

bub-ble size and density as a function of fission rate and

burnup This model predicts the viscosity values

determined from the bubble size distribution analysis

The calculated temperature dependence of the

viscosity depends on the assumption that the rate of

change of the calculated formation enthalpy with

respect to temperature is symmetric with respect to

the uranium concentration In addition, the

tempera-ture independence of certain material properties

(such as thermal expansion coefficient) has also been

assumed Thus, only the trend of the calculations

should be meaningful It is important to note that, as

U3Si2is irradiated, the Si/U ratio shifts to the right In

any event, the calculations show that a
30 K increase

in temperature results in a viscosity for U3Si2that is

similar to that of U3Si irradiated at the lower

temper-ature (seeFigure 16) In addition, the calculated

vis-cosity of U3Si2is much more sensitive to temperature

than that of U3Si

3.14.3.4.2 Interaction between U–Si and Al

U3Si, U3Si2, and USi react with Al to form a single

intermetallic compound, U(AlSi)3 The solubilities of

Al in U3Si, U3Si2, and USi are very small (1 at.%).11

U(AlSi)3has a composition intermediate between UAl3

and USi3, both of which are mutually soluble The

Al/Si ratio in the IL is the highest for U3Si–Al, lowerfor U3Si2–Al, and the lowest for USi–Al The composi-tions of the compounds lie on the tielines between theuranium silicides and Al, as shown in the isothermalsection of the ternary phase diagram (seeFigure 17).For all cases, the reaction products U(AlSi)3 have adensity of
7.1 g cm3, and approximately equalvolumes of uranium silicide and Al are used Only asmall volume change,
4%, is involved in the reaction.The compositions of the IL from in-pile tests

of U3Si2–Al are also shown inFigure 17 The positions deviate from the exact stoichiometry, that

com-is, (Alþ Si)/U ¼ 3 This indicates that the reactionproducts become amorphous during irradiation, ashas been observed in in-pile tests reported previ-ously.45,46 Since the IL is amorphous, U, Al, and Siatoms exist in a mixture without crystalline restric-tion of stoichiometry

When matrix Al exists, the IL of U3Si2–Al is rich

in Al with the Al/Si ratio
3.5 and the (Al þ Si)/Uratio is
5.3.47

Leenaers et al.45,44

reported a larger(
5.0) Al/Si ratio and a smaller (
4.6) (Al þ Si)/Uratio than those by Kim Kim’s (Alþ Si)/U ratio islarger than Leenaers’ due to higher burnup Usingthe IL physical density of 7.1 g cm3and assumption

of a linear time-dependent burnup profile in the IL,the (Alþ Si)/U ratio increases to
4.5 at a fissiondensity of 1.43 1028

fissions m3 This suggests thatthe (Alþ Si)/U ratios from both the present testand Leenaers’ test are higher than the theoreticalassessment

The (Alþ Si)/U ratio was seen to decrease to
3.3with Al/Si
0.29 with depletion of the matrix Alaround the U3Si2particles during irradiation.47A two-phase mixture of USi2and U(AlSi)2was also observed.This is consistent with the findings of Nazare,48whoobserved the formation of U(Al, Si)2in annealing tests

of U3Si/Al and U3Si2/Al dispersion fuels at 600C,when the Al matrix was completely consumed Thisprobably occurs when Al atoms continue to diffuse intothe fuel from the IL while no further Al flux exists fromthe Al matrix Subsequently, the Al/Si ratio continu-ously decreases, and eventually Si becomes more prev-alent than Al The compositions move toward the Sicorner and U–Si side of the ternary diagram

The formation of gas bubbles in the ILs is tant because of its potential effects on the IL growthrate The gas bubbles in the IL, on one hand, reducethe effective diffusion area and thereby reduce the ILgrowth rate On the other hand, they increase the

impor-IL volume itself, which results in a higher measured

Figure 16 Viscosity of U–Si intermetallic fuels during

irradiation versus Si/U ratio for three temperatures Adapted

from Kim, Y S.; Hofman, G L.; Rest, J.; Robinson, A B.

J Nucl Mater 2009, 389, 443; Kim, Y S.; Hofman, G L.;

Yacout, A M J Nucl Mater 2009, 392, 164.