Comprehensive nuclear materials 5 11 material performance in helium cooled systems

Comprehensive nuclear materials 5 11 material performance in helium cooled systems Comprehensive nuclear materials 5 11 material performance in helium cooled systems Comprehensive nuclear materials 5 11 material performance in helium cooled systems Comprehensive nuclear materials 5 11 material performance in helium cooled systems Comprehensive nuclear materials 5 11 material performance in helium cooled systems

R Wright and J Wright

Idaho National Laboratory, Idaho Falls, ID, USA

C Cabet

Commissariat a l’Energie Atomique, Gif-sur-Yvette, France

ß 2012 Elsevier Ltd All rights reserved.

5.11.8 Environmental Effects of VHTR Atmospheres on Materials 269

Abbreviations

AGCNR Advanced gas-cooled nuclear reactor

AVR Arbeitsgemeinschaft Versuchsreaktor

DLOC Depressurized loss of coolant

GE General Electric

GMAW Gas-metal-arc welding

GTAW Gas-tungsten-arc welding

Division Ltd.

PCS Power conversion system

251

PFHE Plate and fin heat exchanger

PSHE Plate stamped heat exchanger

RCS Reactivity control system

RCSS Reactor control and shutdown system

RSS Reserve shutdown system

SMAW Shielded metal arc welding

THTR Thorium Hochtemperatur Reaktor

VHTR Very high-temperature reactor

5.11.1 Introduction

Over the past decade, there has been renewed

inter-est in very high-temperature reactor (VHTR)

tech-nology This type of reactor is of interest because

of a number of unique characteristics, including

pas-sive safety, electricity production on a more modest

scale compared to light water plants that might be

more compatible with the electrical distribution

sys-tem in developing countries, and very high outlet

temperature that can be used for process heat or

hydrogen production The relative value of

electric-ity production or process heat applications varies

considerably with world economic conditions

Cur-rently, it appears that steam for process heat and

hydrogen production will drive development of this

technology rather than electricity production

There are currently two operating VHTR

proto-types, the high-temperature engineering test reactor

(HTTR) in Japan and the high-temperature reactor

(HTR-10) in China The HTTR is a 30 MWt

(mega-watts thermal output) prismatic core reactor and the

HTR-10 is a 10 MWt pebble bed prototype reactor

Both the operating reactors are designed to investigate

electricity production with the VHTR technology;

however, each program has parallel activities to

develop process heat and hydrogen production as well

The challenges for high-temperature materials are

not significantly different for either prismatic or pebble

bed reactor designs Interest in specific applications for

VHTR technology is evolving rapidly It appears that

the most significant immediate interest is in a reactor

with an outlet temperature on the order of 750
C with

a steam generator for either electricity generation or

process heat This technology would use a relatively

mature conventional steam generator technology and

is expected to present lower technical risk Higher

outlet temperatures using a heat exchanger between

the primary helium coolant and a secondary gas are

viewed to be higher risk development projects that

offer the opportunity for outlet temperatures from

850 to 950
C for hydrogen production by chemical processes or higher temperature processheat for industrial applications The material issuesassociated with reactor internals are not affected sig-nificantly by the reactor outlet temperature; however,heat exchangers operating at the higher outlet tem-peratures represent significantly different issues com-pared to steam generators The focus of this chapter is

thermo-on higher outlet temperature systems because of thedevelopment challenges

The next generation nuclear plant (NGNP) beingdeveloped in the United States is one particularVHTR concept that is under very active developmentand is typical of the development around the world.This reactor is being developed to produce hydrogen

as well as electricity Conceptual designs call for a cooled reactor with an outlet temperature greater thanthe 850
C required to efficiently operate the hydrogengeneration plant, with a maximum of 950
C While thedesign concepts are not yet final, it is highly probablethat helium will be the primary coolant in the reactor.The primary material in the core will be graphite, andthe prime candidates for high-temperature metalliccomponents are the nickel-based alloys Alloy 617 orAlloy 230 An artist’s representation of one conceptfor the reactor and power conversion vessel and theassociated hydrogen generation plants is shown in

gas-Figure 1 In this representation, a heat exchangercarries most of the reactor thermal output to a second-ary circuit that powers a turbine for electricity genera-tion An additional heat exchanger takes10% of thethermal energy of the reactor and diverts it as processheat to the hydrogen production plant

The most critical metallic component in theVHTR system is the intermediate heat exchanger(IHX) This heat exchanger will operate at a reactoroutlet temperature of up to 950
C In addition, thereactor system is intended to have a license period of

60 years The combination of very high-temperatureoperation and long duration of service restricts mate-rial choices for the heat exchanger to a small number

of coarse-grained solid-solution strengthened alloysthat provide stability and creep resistance and havehigh chromium content for environmental resistance

5.11.2 Experience with VHTR Systems

Very early in the development of nuclear power forelectricity generation or process heat, the concept of aninert gas-cooled, high-temperature reactor was explored

The Peach Bottom reactor in the United States and

the European Dragon project were among the first

to seriously address the technical issues associated

with high-temperature environmental interaction

between the cooling gas and metallic components.1–3

Proposals for a VHTR with an outlet temperature of

1000
C or above were put forward in the late 1970s

The Arbeitsgemeinschaft Versuchsreaktor (AVR) wasthe first experimental pebble bed reactor A commer-cial demonstration scale pebble bed, the ThoriumHochtemperatur Reaktor (THTR), was developedbased on AVR experience A summary of importantdesign characteristics for gas-cooled VHTRs thathave been operated to date are given inTable 1.1–7

Table 1 Design characteristics of VHTRs that have been built and operated

a Prestressed concrete reactor vessel.

Source: Simon, R A.; Capp, P D Operating experience with the dragon high temperature reactor experiment In Proceedings of the Conference on High Temperature Reactors, Petten, NL, Apr 22–24, 2002; pp 1–6.

Burnette, R D.; Baldwin, N L Specialists Meeting on Coolant Chemistry, Plate-Out and Decontamination in Gas Cooled Reactors, Juelich, FRG, Dec 1980; International Atomic Energy Agency, 1980; pp 132–137.

Shaw, E N History of the Dragon Project – Europe’s Nuclear Power Experiment; Pergamon: New York, 1983.

Ba¨umer, B.; et al AVR – Experimental High-Temperature Reactor; 21 Years of Successful Operation for a Future Energy Technology; Association of German Engineers (VDI), The Society for Energy Technologies: Du¨sseldorf, Germany, 1990.

Baumer, R.; Kalinowski, I Energy 1991, 16(1/2), 59–70.

Brey, H L Energy 1991, 16(1/2), 47–58.

Fuller, C H Design Requirements, Operation and Maintenance of Gas-Cooled Reactors, San Diego, CA, Sept 21–23, 1988;

International Atomic Energy Agency, 1989; pp 55–61.

Hydrogen

Hydrogen

Power for electrolysis

Generator Turbine

exchanger Pebble-bed or

prismatic reactor

Hydrogen production (thermochemical)

Commercial power

Hydrogen production (electrolysis)

Figure 1 An artist’s conception of a very high-temperature gas-cooled reactor and associated hydrogen production plants.

The HTTR in Japan is the only one of the reactors

listed in the table that is still in operation The HTR-10

is not included in the table since there is no extensive

operating experience with this reactor as yet

Operating experience with these reactors has

shown that the primary helium coolant tends to

con-tain H2O, H2, N2as well as carbon-containing

com-pounds CO, CO2, and CH4at concentrations of a few

parts per million Impurities are introduced through

adsorption on the fuel, leaks into the coolant, and

lubricants from components like the helium

circula-tors In the reactors that are currently under

consid-eration, the gas pressure is typically between 5 and

7 MPa The coolant is circulated at high velocity,

reaching velocities over 100 m s1in some designs

5.11.3 Comparison of IHX Concepts

The IHX design for the VHTR will be influenced

by a number of interrelated considerations, including

the required separation distance between the reactor

and the hydrogen production or other process heat

plant, the heat losses from the intermediate loop

pip-ing, the operating pressure, the working fluid in the

secondary loop, and the target efficiency of the

hydro-gen or process heat plant The required separation

distance will affect the intermediate loop piping size,

the intermediate loop pumping requirements, and the

piping heat losses to the environment The

intermedi-ate loop pressure is critical; a low pressure will produce

a high pressure differential between the primary and

secondary sides of the IHX and high stress on the IHX

A high intermediate loop pressure will produce a high

pressure differential across the intermediate loop pipe

walls and within the hydrogen production or process

heat equipment Pressure drops within the IHX affect

the pumping power requirements, which also depend

on the intermediate loop working fluid, and the fluid

temperature and pressure, and will have an effect on

the overall VHTR cycle efficiency

The IHX may be arranged in parallel or in series

with the VHTR power conversion system (PCS) In a

serial arrangement, the total primary system flow

(reactor outlet gas) passes through the IHX The

IHX receives gas of the highest possible temperature

for delivery to the hydrogen production process (with

slightly cooler gas going to the PCS), and must be

large enough to handle the full primary flow

A parallel configuration splits the reactor outlet gas

flow, with only a portion entering the IHX for the

hydrogen or process heat plant, and the remainder of

primary flow going to a direct cycle power generationturbine This results in the smallest possible IHX andthe highest overall electrical power efficiency butlower process heat efficiency because of the coolergas reaching that process

Specific IHX designs under consideration includecountercurrent tube and shell, plate and fin, involuteheat exchangers, microchannel heat exchangers, andthe printed circuit heat exchanger (PCHE) Thedesign has a significant influence on the requiredmaterial properties Tube-and-shell designs have theadvantage of technological maturity, use heavy gaugematerials, and are fabricated using conventional fusionwelding methods For the most simple tube-and-shellconfiguration, it has been estimated that 13 tons ofhigh-temperature alloy is required per megawatt ofheat transfer capability; helical designs can reducethis value to about 1.2 tons MW1 Compact heatexchanger designs have the potential for greater heattransfer efficiency; it is estimated that some of thesedesigns will require only 0.2 tons of alloy per MW Thecompact designs are much less technologically matureand increase the demands on material performance.Some compact designs have wall thicknesses of lessthat 1mm which places a premium on corrosion resis-tance and have significant stress concentrations thatwill lead to increased demand for creep resistance

In addition, several of these design concepts requirediffusion bonding of multiple sheets of material orbrazing in complex geometries Neither of these join-ing methods has been used yet in nuclear applications,and nondestructive inspection methods have not beenwell developed

5.11.3.1 Shell-and-Tube

A shell-and-tube heat exchanger is the most commontype of heat exchanger It consists of a number oftubes (often finned) placed inside a volume (shell).One of the fluids runs through the tubes while thesecond fluid runs across and along the tubes to beheated In one variation of this concept, the heattransport fluid will flow on the shell side, allowingthe tubes to contain the catalysts necessary for hydro-gen production In the simple configurations, the tubeaxis is parallel to that of the shell The VHTR IHX-proposed design features the tubes arranged in ahelical configuration This type of arrangementincreases efficiency because of increased surfacearea and reduces the size, providing the potential todecrease the cost of materials Tube-and-shell heatexchangers represent relatively mature technology

that has been widely commercialized in both nuclear

and fossil energy systems A helical design was

exten-sively tested for the AVR reactor program and a

similar system is in use in the HTTR in Japan

5.11.3.2 Plate and Fin

The plate and fin heat exchanger (PFHE) transfers

heat between two fluids by directing flow through

baffles so that the fluids are separated by metal

plates with very large surface areas The fluids spread

out over the plate, which facilitates the fastest

possi-ble transfer of heat This design has a major advantage

over a conventional heat exchanger because the size

of the heat exchanger is less for a comparable heattransfer capability However, the candidate heatexchanger materials have relatively low thermal con-ductivities and will reduce the efficiency of a finnedstructure Brazing is typically used to join the fins tothe plate Brazed plate heat exchangers are used inmany industrial applications, although usually at low

or even cryogenic temperatures Although brazedproducts have been developed for high-temperatureaerospace applications, the strength and creepproperties of brazed joints in an IHX for a high-temperature reactor are of great concern

The unit cell heat exchanger is a typical modularplate-fin design that is being developed by BraytonEnergy An example is shown inFigure 2 Many ofthese individual unit cells would be grouped intolarger heat exchanger assemblies Integration of themodules within the vessel and with the interfacingpiping is critical Offset fin plate heat exchangershave very large heat transfer area density and effec-tive countercurrent flow

5.11.3.3 Etched PlateEtched plate heat exchangers are diffusion-bonded,highly compact heat exchangers that can achieve athermal effectiveness of over 98% in a single unit.Compact heat exchangers are four to six times smallerand lighter than conventional shell-and-tube heatexchangers of the equivalent heat transfer capability(Figure 3) The small size gives the compact diffusion-bonded heat exchangers significant benefits over con-ventional heat exchangers across a range of industries.They are well established in the oil production, petro-chemical, and refining industries In addition, theyare suitable for a range of corrosive and high-purity

(a)

(b)

Figure 2 Unit cell heat exchanger (a) primary side plate,

(b) the unit cell showing countercurrent flow.

Figure 3 The diffusion-bonded heat exchanger in the foreground undertakes the same thermal duty, at the same pressure drop, as the stack of three shell-and-tube exchangers behind.

streams and are particularly advantageous when

space is limited and weight is critical

The most widely commercialized etched plate

heat exchanger is a PCHE developed by Heatric

Division of Meggitt (UK) Ltd PCHE consists of

metal plates on the surface of which millimeter-scale

semicircular fluid-flow channels are photochemically

milled, using a process analogous to that used for the

manufacture of electronic printed circuit boards

The plates are then stacked and diffusion-bonded

together to fabricate a heat exchanger core shown

schematically in Figure 4 Heatric reports pressure

capability in excess of 70 MPa and the ability to

withstand temperatures ranging up to 900
C

Note that the channels are straight in this

sche-matic, but in reality they have a zigzag configuration

Flow distributors can be integrated into plates or

welded outside the core, depending on the design

The channel diameter, plate thickness, channel

angles, and other attributes can be varied, so each

PCHE is custom-built to fit a specified task Channel

dimensions are generally between 3 and 0.2 mm and

the thickness of the web of material left after milling

is typically less than 1 mm.8The current fabrication

limits are 1.5 m 0.6 m plates and 0.6 m stack height

The diffusion-bonded blocks made from several

hun-dred individual sheets are modular and multiple

blocks can be welded together to form larger units

5.11.3.4 Microchannel Heat Exchangers

Microchannel heat exchangers, produced, for

exam-ple, by Velocys, also feature a compact design similar

to the etched plate design; however, the manufacturing

process is somewhat different They are constructed

from diffusion-bonded corrugated sheets rather than

etched plates The layers of corrugated sheet formmany small-diameter channels that result in a highsurface area/volume ratio and a high heat transfercoefficient

5.11.3.5 Plate-Stamped Heat ExchangerThe plate-stamped heat exchanger (PSHE) conceptconsists of a set of modules, each being composed of astacking of plates stamped with corrugated channels.The plates are stacked in such a way as to cross thechannels of two consecutive plates and therefore toallow the different channels to communicate throughthe width of the plate as shown on the left in thefigure A general view of a plate is shown inFigure 5.Assembly of the plates into an IHX module isaccomplished by welding only on the edges of theplates No joining is performed in the active part ofthe plates, which gives the module relatively goodflexibility Therefore, this concept is thought toaccommodate the thermal stresses better than theother concepts of plate IHXs The location of thewelded joints is also favorable to inspection, even if

this remains a difficult question The joining

pro-cesses which seem to be the most relevant are laser

or electron beam welding due to the capability to

perform narrow-gap joints and to avoid the

overlap-ping of the welds of two consecutive plates It should

also be noted that the thickness of the PSHE plates

is the largest among the metallic plate types IHX

(1.5 mm), which means that it is the most favorable

concept with respect to corrosion life These reasons

suggest that the PSHE concept may be the most

promising among the plate IHXs

5.11.3.6 Foam IHX

The foam IHX concept is based on stacking plates

separated by metallic foam The barrier between the

fluids is constituted by the separated plates and the

fluids flow through the foam (seeFigure 6) It is a

new technology for heat exchanger application for

which very high efficiency has been claimed Several

concerns have been identified regarding this type of

IHX concept The pressure losses induced by the

foam are particularly high Loss of small fragments

of the foam is hardly avoidable and the geometry of

the foam leads to an increased risk of clogging by

graphite dust

5.11.3.7 Capillary IHX

A concept with thread tubes between two tube-plates

with external shell including bellows has been

investi-gated The diameter of the tubes is 2–3 mm This kind

of heat exchangers is currently being developed on an

industrial scale The small size of the tubes allows a

sharp reduction in size and mass, but some difficulties

arise at the same time, including the concern that

the vibration risk is increased so that the supporting

system needs to be very robust The number of tubes

reaches very high values, which increases the plexity of manufacturing, notably as assembly bynarrow gap welding is required Demonstration ofthe elements necessary for successful implementa-tion of the technology is mainly based on technologi-cal feasibility tests like demonstration of individualtube to tube-plate welds by laser techniques Theresults confirm the feasibility for limited thickness ofthe plate (a small mock-up is shown in Figure 7).5.11.3.8 Ceramic IHX

com-The development of IHXs made of ceramics is still atthe research stage Ceramic heat exchangers underdevelopment are either tubular or plate IHXs (mostlyPFHE for the ceramic plate IHXs) Tube-and-shell heatexchangers based on SiC composite tubes have beendeveloped for fossil energy applications for example.Joining the fiber-reinforced composite tubes to tubesheets and accommodating thermal expansion arethe dominant technical challenges Their resistance toaggressive environment is remarkable and they canoperate at very high temperatures,>1000
C Small

monolithic compact designs have been developedfrom silicon carbide and silicon nitride through con-ventional ceramic forming and firing routes In addi-tion to technical issues, the cost of ceramic tubes ofsufficient size for a VHTR IHX remains problematic

Table 2provides a summary-level comparison ofthe significant attributes of the different IHX conceptalternatives

5.11.4 Heat Exchanger Alloys

The desire for higher temperature operation resulted

in the evolution of the materials under consideration,from stainless steels to iron-based high-temperature

Figure 6 Foam heat exchanger concept.

alloys to nickel-based alloys (seeChapter2.08, Nickel

Alloys: Properties and Characteristics) An extensive

German program in the 1980s carried out exhaustive

studies of the corrosion behavior of the iron-based

Alloy 800H for control rods and nickel-based Alloy 617

for structural applications.9–12 The Japanese HTTR

program extensively studied Alloy X and developed a

variation known as XR with improved properties for

some applications, while retaining Alloy 800H for the

control rods.13Compositions of these candidate alloys

are given in Table 3.13–16 Based on creep resistance

above 850
C, the leading candidate alloys for VHTRs

are Alloy 617 and Alloy 230

A common characteristic of the alloys that have

been put in service in high-temperature gas-cooled

reactors is that they rely primarily on the formation

of a tenacious chromia scale for long-term protectionfrom environmental interaction with the gas-coolingenvironment.9,10,12,17 The alloys are also primarilysolid-solution strengthened with carbides on thegrain boundaries to stabilize the microstructure andenhance the creep resistance Sustaining such a pro-tective surface oxide requires sufficient oxygen par-tial pressure The primary coolant gas of choice forVHTRs is helium Although the helium is nominallypure and thus considered to be inert, there are inevi-tably impurities at the parts per million by volume (ppm)levels in the coolant in operating high-temperaturereactors Although at low levels, the impurities cansignificantly affect the performance of materials,

Figure 7 Capillary heat exchanger mock-up.

Table 2 Comparison of IHX concept alternatives

in conventional industry

Challenging but best stress accommodation among the plate IHXs

Tubular IHX Industrial components in

operation

Limit of state of the art Better than plates

but still sensitive

0.4 MW m3

of fragments risk)

Comparable to other plate IHXs Capillary IHX Industrial developments No results Very sensitive Better than

classical tubular IHX

Ceramic IHX R&D Difficult design because of

fragile behavior

Resistant Comparable to

other plate IHXs

depending on the chemistry of the particular alloy,

the concentration of impurities, and the temperature

at which the alloy can be oxidized, carburized, or

decarburized Several reviews of the behavior of

metallic alloys for control rods, core internals, and

heat exchangers in the reactor helium environment

are available.9–12,17

Regardless of the IHX design, material selection

for this component is critical The material must be

available in the appropriate product forms – both

plate and sheet, weldable and suitable for use at

800
C or above The majority of materials research

and development programs in support of

high-temperature gas reactors (HTGRs) were conducted

from the 1960s to the early 1980s The thrust of these

programs was to develop a database on materials for

application in steam-cycle and

process-nuclear-heat-based HTGRs Less work has been done on materials

with emphasis on direct and/or indirect

gas-turbine-based HTGRs The available material property data

were reviewed in detail, and an assessment of

rele-vant factors was made including thermal expansion,

thermal conductivity, tensile, creep, fatigue, creep–

fatigue, and toughness properties for the candidate

alloys Thermal aging effects on the mechanical

properties and performance of the alloys in helium

containing a wide range of impurity concentrations

are also considered.17 The assessment includes four

primary candidate alloys for the IHX: Alloy 617,

Alloy 230, Alloy 800H, and Alloy X

5.11.4.1 Regulatory Issues

The IHX will form part of the pressure boundary for

the VHTR and material selection and design will be

subject to regulatory requirements In the United

States, the design will be guided by Section III of

the ASME Boiler and Pressure Vessel (B&PV) Code.This section specifies materials and design data forcomponents in nuclear systems Subsection NH ofthe Code, which specifies materials and design para-meters for materials that will undergo inelastic defor-mation, includes only a very few materials Thetemperature limits for Subsection NH Code materials,other than bolting, at 300 000 h are listed inTable 4.The maximum temperatures at which fatigue curvesare provided are also listed Note that of the materialsunder consideration for VHTR service, only Alloy800H is currently contained in the appropriate sec-tion of the ASME Code and the service temperature

is limited to 760
C Any VHTR design that has anintended IHX service above this temperature willrequire extension of the Code to include additionalmaterials, or at a minimum, extension of the use ofAlloy 800H to higher temperatures Other regulatorysystems are in use internationally; however, it is gen-erally true that additional materials and expandeddatabases are required before a new VHTR designcan be finalized and licensed

5.11.4.2 Alloy 617 (52Ni–22Cr–13Co–9Mo)Alloy 617, also designated as Inconel 617, UNSN06617, or W Nr 2.4663a, was initially developedfor high-temperature applications above 800
C It isoften considered for use in aircraft and land-basedgas turbines, chemical manufacturing components,metallurgical processing facilities, and power gener-ation structures The alloy was also considered andinvestigated for the HTGR programs in the UnitedStates and Germany in the late 1970s and early 1980s.The high Ni and Cr contents provide the alloywith high resistance to a variety of reducing andoxidizing environments In addition, the Al also

Table 3 Compositions of potential high-temperature alloys for VHTR (compositions in wt%)

Source: Incoloy Alloy 800H & 800HT, product sheet, Special Metals, 2004.

Inconel 230, UNS N06230, product sheet, Special Metals, 2004.

Inconel 617, UNS N06617, product sheet, Special Metals, 2005.

Tanaka, R.; Kondo, T Nucl Technol 1984, 66(1), 75–87.

forms the intermetallic compound g0-(Ni3Al) over a

range of temperatures, which results in precipitation

strengthening on top of the solid-solution

strength-ening imparted by the Co and Mo Strengthstrength-ening is

also derived from M23C6, M6C, Ti(C, N), and other

precipitates when in appropriate sizes, distributions,

and volume fractions

Observations and predictions of which precipitatesform in Alloy 617 at given temperature ranges have notbeen consistent A comprehensive review of the pre-cipitates in Alloy 617 has been performed by Ren.18,19Additional reviews can be found in Natesan et al.20

However, it is clear from the reviews that the kinetics

of the precipitation and coarsening processes areimportant in determining the effects of aging onproperties The g0intermetallic is generally too fine

to be observed in optical microscopy

Other phases that have been identified includeCrMo(C, N) and TiN,21M12C and a possible Lavesphase,22and a Ni2(MoCr).23A summary of observa-tions is given inTable 5 The apparent trend is that

in the temperature range of interest to the VHTRIHX, precipitates may form at initial exposure andthe alloy may become stronger, but most of the pre-cipitates will be dissolved after long-term exposure,and the alloy will depend on solid-solution strength-ening in the long run The most recent information

on precipitation in Alloy 617 upon aging is shown inthe T–T–T diagram inFigure 8 The influence ofaging on the mechanical properties of the alloysunder consideration for IHX applications is discussed

in the section on environmental effects

The grain size also plays an important role inthe strength of the alloy For general applications, agrain size of45 mm or coarser is typically preferred,but it has been shown that creep strength increaseswith increasing grain size, so microstructures of100–200 mm grain size are often produced A trade-off exists, however, when fatigue is an issue, sincefiner grain sizes are preferred for fatigue resistance

In addition, for compact IHX, the thin sheet form

Table 4 Materials specified in NH for elevated

temper-ature service in nuclear applications

NH code materials

(other than bolting)

Maximum temperature (
C) For stress allowables S0, Smt, St, Sr up to

300 000 ha

For fatigue curves

a The primary stress limits are very low at 300 000 h and the

maximum temperature limit.

b

Temperatures up to 649
C (1200
F) are allowed up to 1000 h.

c The specifications for Grade 91 steel covered by subsection

NH are SA-182 (forgings), SA-213 (small tube), SA-335

(small pipe), and SA-387 (plate) The forging size for SA-182 is not

T  1093
C Not observed Small wt% persist to

Observation in material aged for much longer than 10 000 h at 482–871
C

also observed eta-MC

Observed at 482, 538, &

593
C, not at 704
C for 43 000 h and longer, nor 870
C after long time

Not observed TiN observed

restricts development of large grain size Whether the

grains will significantly coarsen after the dissolution

of certain grain boundary precipitates at long-term

exposure is not clear

The existing mechanical property database for

Alloy 617 is extensive (Table 6) This alloy has

ade-quate creep strength at temperatures above 870
C,

good cyclic oxidation and carburization resistance,

and good weldability It also has lower thermal

expansion than most austenitic stainless steels and

high thermal conductivity relative to the other

can-didates It retains toughness after long-time exposure

at elevated temperatures and does not form

interme-tallic or Laves phases that can cause embrittlement

Preliminary testing described later indicates that

Alloy 617 has the best carburization resistance of

the four alloys

During early development, Alloy 617 was

sys-tematically studied by Huntington Alloys, Inc for

applications in gas turbines, nitric acid production

catalyst-grids, heat-treating baskets, Mo refinement

reduction boats, etc When Alloy 617 was considered

for the HTGR, it was extensively investigated

by Huntington, Oak Ridge National Laboratory

(ORNL), and General Electric (GE) The Huntington

data were used to develop ASME B&PV Code

quali-fication, including the 1980s draft Code Case for the

HTGR and applications covered by (nonnuclear)

Section I and Section VIII Division 1 Alloy 617 is

not currently qualified for use in ASME CodeSection III, although it is allowed in Section I andSection VIII, Division 1 (nonnuclear service) Efforts

to gain the approval from the ASME Code committeesfor nuclear service were stopped when interest inVHTR technology waned in the 1990s

Both the ORNL-HTGR and GE-HTGR studiesgenerated data from Alloy 617 that had been agedand/or tested in simulated HTGR helium Thehelium impurities were the same as those consideredfor the VHTR system but the concentrations weredifferent Unfortunately, only processed data stillexist; all original test curves needed for certain mod-eling efforts are irretrievable Alloy 617 was alsoextensively investigated in Germany for its HTGRand other programs The data generated were col-lected in the Online Data & Information Network(ODIN) Original test curves, if not all, are stored inthe ODIN However, the strain measurements of creeptest curves were not all conducted with fine resolutionand may not all be ideal for constitutive equationdevelopment The aging effects on Alloy 617 are sum-marized in Ren and Swindeman.18The development

in modeling creep behavior of Alloy 617 is ized in Swindemanet al.24

summar-It is believed that creep–fatigue will be the mostsignificant failure mechanism for materials in theIHX Creep–fatigue damage results from cyclic loadssuperimposed on materials subjected to temperatures

and loads that will induce creep damage under

mono-tonic loading Recent creep–fatigue data for Alloy 617

and Alloy 230 are shown in Figures 9 and 10 for

tests that involved fully reversed cyclic loading at

total strain ranges of 0.3% and 1% with varying

hold time during the tensile portion of the cycle at

800 and 1000
C The plots show the reduction in

cycles to failure with increasing tensile hold time

under creep loading conditions It can be seen that

in general, increased hold time results in decreased

cycles to failure At 800
C, the two alloys have

similar behavior; however, at 1000
C, Alloy 617

appears to have somewhat higher cycles to failure

compared to Alloy 230 A limited number of tests

have been carried out on specimens that contain

weldments and it has been found that the cycles to

failure in specimens containing a fusion weld is

reduced In these specimens, the cracking is

typi-cally in the weld metal and not in the heat-affected

zone or at the weld–base metal interface

5.11.4.3 Alloy 230 (57Ni–22Cr–14W–2Mo–La)

Alloy 230, also designated as Haynes 230, UNS

N06230, or W Nr 2.4733, is a newer alloy than

Alloy 617 In addition to outstanding resistance to

oxidizing environments, Alloy 230 has good

weld-ability and fabricweld-ability It also has a lower thermal

expansion coefficient than Alloy 617; it appears that

thermal expansion has an inverse correlation with Ni

content Alloy 230 has a higher tensile strength than

Alloy 617 up to 800
C, but above that the difference is

insignificant It appears that Alloy 617 has slightlybetter creep properties than Alloy 230 Alloy 230has a better thermal fatigue crack initiation resistancebut a worse thermal cycling resistance compared toAlloy 617

The Ni base and high Cr content impart resistance

to high-temperature corrosion in various ments, and oxidation resistance is further enhanced

environ-by the microaddition of the rare earth element La.Compared to Alloy 617, Alloy 230 has a high Wconcentration which replaces much of the Co in

Table 6 Summary of testing done on Alloy 617

Research organization Number of heats Number of samples Test type Temperature (
C)

a Some tests exposed to HTGR environment.

b Air, pure helium and vacuum environments.

104

10 5

Alloy 617CCA – 1.0% total strain

Alloy 230 – 1.0% total strain

Alloy 617 – 1.0% total strain

Alloy 617CCA – 0.3% total strain

Alloy 230 – 0.3% total strain

Alloy 617 – 0.3% total strain

Figure 9 Effect of hold time on cycles to failure in creep–fatigue at 800
C.

Alloy 617 The W and Mo in conjunction with C are

largely responsible for the strength of the alloy, and its

relatively high B content in comparison to that in Alloy

617 can be controlled to achieve optimized creep

resis-tance Usually, B acts as an electron donor; it can affect

the grain boundary energy and help improve ductility

In Ni-based alloys, B also segregates to grain

bound-aries and helps to slow grain boundary diffusion, thus

reducing the creep process On the other hand, excess

boron in a neutron field could also lead to

embrittle-ment due to transmuted He, although irradiation is not

a factor for IHX applications

In the solution-annealed condition in which this

alloy is typically supplied, the grain size is typically

>45 mm with large carbide precipitates rich in W,

presumably of the M6C type After aging, Alloy 230

typically exhibits M6C and M23C6precipitates After

aging for 1000 h at 850
C, very small carbide

pre-cipitates rich in Cr and M23C6were observed along

the grain boundaries No grain coarsening was

observed.25Creep strength is believed to be brought

about by solid-solution strengthening, low stacking

fault energy, and precipitation of M23C6carbides on

glide dislocations.26,27 However, a negative impact

of M23C6 on room temperature ductility was also

reported After aging at 871
C for 8000 h, the room

temperature tensile elongation of Alloy 230

de-creased from50% to 35%, with a precipitation of

M23C6observed in microstructural examination, but

an additional 8000 h of aging did not further decrease

ductility.26 Significant microstructural changes werealso observed after thermal aging in air for 10 000 h

at temperatures ranging from 750 to 1050
C Afterthe 750
C aging, coarser intergranular precipitation

of M23C6 and coarse and blocky intra- and granular precipitates of M6C were observed Afterthe 850–1050
C aging, the M6C carbides were irregu-lar in shape After aging at 1050
C, the secondaryintragranular M23C6 appeared to have dissolved

inter-A decrease in toughness and ductility coincided withthe appearance of the intragranular M23C6and reached

a minimum after the aging at 850
C The toughnessand ductility recovered after the aging at 1050
C.28There is less characterization of Alloy 230 com-pared to Alloy 617 The major known large-scalestudy was tensile and creep tests by Haynes Interna-tional Creep times ranged from 15.3 to 28 391 h LikeAlloy 617, Alloy 230 is not currently qualified for use

in ASME Code Section III, although it is allowed inSection VIII, Division 1 (for nonnuclear service) Atpresent, the database for Alloy 230 is significantlysmaller than that for Alloy 617 and a much largereffort is required to develop an Alloy 230 Code Casefor elevated temperature application Some recentdata on environmental effects of exposure to proto-typical VHTR chemistries are given in the followingsections and creep–fatigue properties are included

inFigures 9 and 10

5.11.4.4 Alloy 800H (42Fe–33Ni–21Cr)This alloy is the only iron-based alloy under consid-eration, although it has a solid-solution strengthenedaustenitic structure like the other three alloys Uponaging, precipitates can form and somewhat reducethe tensile and creep ductility Alloy 800H has thelowest creep rupture strength and the lowest resis-tance to oxidation of the four alloys There is anadditional variant of this alloy, 800HT, that has acomposition similar to that of 800H, but has an addi-tional specification for coarse grain size The majority

of material that is currently available in this alloyseries is Alloy 800HT, which also meets the specifi-cation for Alloy 800H

Among the four candidate materials, Alloy 800H

is the only one that is Code qualified for use innuclear systems, but only for temperatures up to

760
C and a maximum service time of 300 000 h.Alloy 800H was the primary high-temperature alloyused in the German HTGR programs and an enor-mous amount of data were obtained However, onlyvery limited data from the German HTGR programs

Alloy 617CCA – 1.0% total strain

Alloy 230 – 1.0% total strain

Alloy 617–1.0% total strain Alloy 617CCA – 0.3% total strain

Alloy 230 – 0.3% total strain

Alloy 617– 0.3% total strain

Figure 10 Effect of hold time on cycles to failure in

creep–fatigue at 1000
C.