The combination of metallic fuel and stainless steel cladding in a fuel element forms a complex multicomponent diffusion couple at elevated temperatures. Interdiffusion of fuel and cladding components can in principle lead to several phenomena that could affect the reliable performance of a fuel element. These phenomena include the formation of strength reducing diffusion zones in the cladding, intergranular penetration of fuel components into the cladding, and the formation of compositional zones with melting points below the anticipated operating temperatures. A series of ex-reactor tests were performed to assess and study this potential problem in fuel elements consisting of U-Zr, U-Pu-Zr fuel clad in Ti stabilized austenitic stainless steel (D9) and ferritic stainless steel (HT-9). Diffusion couples of various combinations of fuel and the different steels were annealed at temperatures ranging from 650/sup 0/C to 800/sup 0/C for up to 3000 h in an argon atmosphere. The post-test analysis of the diffusion couples included: metallographic examinations, scanning electron microscopy, and scanning Auger microscopy

Hins, A. G.

Hofman, G. L.

Leibowitz, L.

Porter, D. L.

Wood, E. L.

UNT Digital Library

CONP-860931—6 -.7.DE87 004758CHEMICAL INTERACTION OF METALLIC FUEL WITHAUSTENITIC AND FERRITIC STAINLESS STEEL CLADDING*G. L. Hofman, A. G. Hins, D. L. Por ter ,L. Leibowitz, and E. L. WoodArgonne National Laboratory9700 South Cass AvenueArgonne, I l l ino is - Idaho Falls, IdahoDISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.Submitted for Presentationat theInternational Conference onReliable Fuels for Liquid Metal Reactors,Tucson, ArizonaSeptember 7-11, 1986*Work Supported by the U.S. Department of Energy, Reactor Systems,Development, and Technology, under Contract W-31-109-Eng-38.DISTRIBUTION OF THIS DOCUMENT IS UNLIMITEDCHEMICAL INTERACTION OF METALLIC FUEL WITHAUSTENITIC AND FERRITIC STAINLESS STEEL CLADDINGG. L. Hofman, A. G. Hins, D. L. Porter,L. Leibowitz, and E. L. WoodArgonne National Laboratory9700 South Cass AvenueArgonne, Illinois 60439ABSTRACTThe combination of metall ic fuel and stainless steel cladding in a fuelelement forms a complex multi-component dif fusion couple at elevatedtemperatures. Interdi f fusion of fuel and cladding components can in pr inciplelead to several phenomena that could affect the rel iable performance of a fuelelement. These phenomena include; the formation of strength reducingdiffusion zones in the cladding, intergranular penetration of fuel componentsinto the cladding, and the formation of compositional zones with meltingpoints below the anticipated operating temperatures.A series of ex-reactor tests were performed to assess and study th is potentialproblem in fuel elements consisting of U-Zr, U-Pu-Zr fuel clad in Tistabi l ized austenitic stainless steel (D9) and f e r r i t i c stainless steel(HT-9). Diffusion couples of various combinations of fuel and the di f ferentsteels were annealed at temperatures ranging from 650°C to 800° for up to3000 h in an argon atmosphere. The post-test analysis of the di f fusioncouples included: metallographic examinations, scanning electron microscopy,and scanning Auger microscopy.The i n i t i a l results of th is study may be summarized as fol lows: at 725°C andbelow the interdi f fusion at the fuel cladding interface primari ly involveszirconium and nitrogen. A zirconium layer, a few m in thickness containingup to 20 at.% nitrogen is formed at the interface of a l l fuel-cladding com-binations as well as a thinner layer of uranium-plutonium between the z i r -conium and the cladding. A large chemical potential gradient for zirconiumPlutonium and nitrogen in fuel and cladding is the l i ke ly driving force forthe dif fusion of these elements. The primary source of nitrogen appears to bethe cladding, but experimental work to veri fy th is is s t i l l in progress atth is time.I t is proposed that the formation of the zirconium-nitrogen layer controls theinterdi f fusion of uranium-pljtoniurn and iron and thus prevents or retards theformation of low melting alloys that exists in th is ternary system. Diffusionanneals are in progress to determine the kinetics of the interface layerformation. . U-10wt%Zr fuel was found to not s ign i f icant ly interact chemicallywith the cladding al loys. Diffusion couples tested at 650°C for 2880 hr and750°C for 720 hr indicated that essentially no fuel/cladding interdi f fusionoccurred. At 650°C the bulk interdi f fusion was l imited to several microns orless, and at 75O°C the interdi f fusion was l imited to 5 microns or less.- 1 -The diffusion zones observed in the ternary combinations at higher tem-peratures are much more complex. For example, the ferritic steel (HT-9) andthe 19% plutonium fuel diffusion couple has an 80 ym, multi phase, zone aftera 300 hour anneal at 75O°C. An interchange of the cladding and fuelcomponents is evident with high uranium and plutonium phases at the claddingside of the diffusion zone and high iron and zirconium-rich phases at the fuelside. The zirconium-nitrogen band seen at 700°C is not present and arelatively large amount of iron and fuel component diffusion has takenplace. The interdiffusion of iron and uranium-plutonium has now created alayer of about 15 pm at the cladding side of the diffusion zone which appearsto be once-molten alloy. Evidence of melting is observed in all fuel-claddingcombinations at various temperatures between 725°C and 800°C after 300 houranneals. Melting occurs at lower temperatures in this range for highplutonium fuel in general and at the lowest temperature for low chromiumferritic steel and high plutonium fuel combinations.It can be stated that based on these ex-reactor tests thus far conducted, nodeleterious effects stemming from fuel-cladding interdiffusion are apparent upto temperatures of 725°C, which is well in excess of the maximum anticipatedoperating temperature of a fuel element and the formation of molten phases inthe diffusion layers occurs at temperatures that lie in the off-normaloperating range of the fuel. The kinetics and extent of zone formation andmolten phase formation are being studied. A comparison of the ex-reactor datawith fuel element irradiation results that are now becoming available is inprogress.I. INTRODUCTIONChemical interaction between fuel and cladding is a concern in any fuelelement design. This phenomena takes very different forms depending on thetype of fuel. In the case of IFR metallic U-Pu-Zr alloy fuel, FCCI manifestsitself as a multi-component metallic diffusion problem. Because Pu and stain-less steel components such as Fe and Ni can form relatively low melting pointcompositions, the interdiffusion of these elements has to be either avoided oraccompanied by third species that raise the reactivity of these Pu com-positions well above the cladding operating temperatures.Early diffusion tests [1] of Pu containing fuels and austenitic stainlesssteel identified Zr as an effective alloy addition to avoid low melting phaseformation in the diffusion zone. In fact, the selection of the present U-Pu-Zr fuel composition is based on that work and subsequent irradiation tests.The present work does not at this time address the kinetics of inter-diffusion, but is limited to the determination of the temperature at whichmolten phase formation first takes place between heretofore untestedcombinations of U-Pu-Zr and austenitic and ferritic cladding materials.- 2 -II. EXPERIMENTAL RESULTSThe composition of the materials used is given in Table 1. Fuel andcladding samples were cut from cylindrical bars to ~2.8 mm thick slices.These slices were ground flat on 30 pm AI2O3 paper. Fuel and cladding sampleswere alternatingly stacked, in columns of up to 12 samples. The sample stackswere wrapped in Zr getter foil and placed in a Mo holder, which in turn wasplaced in a welded, Ar containing, Type 304 stainless steel capsule. The•thermal expansion difference between the Mo holder and the sample stackensured axial compressive loading of the samples at test temperature. Thenominal test temperatures were 650°C-800°C and were controlled within 10°C.Annealing times of the majority of the tests was 300 h with a few tests endingat 3000 h. After annealing, the samples were prepared for opticalmetallography, Scanning Electron Microscopy (SEM) and Scanning Auger Micro-scopy (SAM) in a conventional manner.FeCrNiMoTiVWMnSiCN0UPuZrTABLE 1.U-8Pu-10Zr/EFL02982.398..419.80Fuel and Clad Compositions"MaterialU-19Pu-10Zr/EFL03671.7519.2510.34HeatHT9/91353(Ferritic)Bal.12.020.571.030.0020.340.510.500.220.210.00370.0058D983510(Aus.)Bal.13.4515.611.650.32<0.01<0.012.050.850.0340.00510.0062aIn wt.% except as notedNo sign of formation of liquid alloys was observed in any of the diffusioncouples that were annealed at 650°C and 725°C. However, narrow bands at thecladding-fuel interface were present at all fuel-cladding combinations.Results of examinations of these bands with the SEM and SAM are summarized inFigs. 1 and 2. The data shown in Fig. 1 are representative of fuel-ferriticsteel combinations. There is a typically 1 to 3 y Zr layer on the fuel side- 3 -63 UTTTu20~Z775 Zr25 NU NTu - "0"Zr CFig. 1 . Compositional Analysis of Interaction Zone in(U-19Pu-10Zr)-(HT-9) Diffusion Couple After 300 h@ 700°C, Using X-ray and Auger Electron AnalysisZr Phase "2",Cr NiFig. 2. Interface Between U-19Pu-10Zr andD-9 after 300 h at 73O°Cof the interface that contains N up to the solubility limit in the o Zrphase. Between this layer and the steel exists a thin, 0.5-2 y, layer of fuelthat is markedly enriched in Pu and, in the case of U-Pu-Zr fuel, containssome Fe, Cr and Zr as well as a few percent N, 0, and C. The situation withfuel-austenitic steel combinations is essentially similar except that Ni andCr are segregated in the Zr layer as shown in Fig. 2. The evidently morerapid diffusion of Ni results in a depletion of Ni in the steel to a depth ofa few microns, in effect forming a ferritic layer at the steel-fuel interface.At temperatures above 725°C, several degrees of interaction were observedranging from incipient molten phase formation to complete melting of fuel andpart of the steel in some' diffusion couples at temperatures above 800°C. Aschematic presentation of the test results in simple terms of observed moltenphase formation is given in Fig. 3. Little can be said about the samples thatexperience gross melting during 300 h anneals to ellucidate the interdiffusionother than that severe intermixing of steel and fuel components at these hightemperatures, and the attendant formation of low melting eutectic compos-itions, obliterated the original diffusion couples. We will therefore con-centrate on the temperature range of approximately 725°C to 775°C wherevarious stages of incipient molten phase formations were observed.- 5 -650 -JU-8PU-10ZR0-9 CLADDINGU-19PU-10ZR0-9 CLADDINGLEGENDNO MELTINGINIT. MELTGROSS MELTU-8PU-10ZRHT-9 CLADDINGIM9PU-10ZRHT-9 CLADDINGFUEL-CLAD COMBINATIONFig. 3. Schematic Presentation of Fuel-CladdingCompatibility Test ResultsSome examples of early stages of molten phase formation are shown inFig. 4. The example shows a U-Pu-Zr/D9 couple annealed for 700 h at 750°C.Figure 4-A shows the transition of the zone of incipient molten phase for-mation and the Zr fuel band structure similar to that found at lower tem-peratures. This molten phase formation always starts at the center of the dif-fusion couple and appears to s-pfead outward to the periphery of the sample(downward in Fig. 4-A). Figure 4-B represents the microstructure of theinteraction zone. Most of the Zr layer has fallen out during grinding of thesample but it consisted of a two-phase structure as shown preserved inFig. 4-A. This two phase structure is similar to that found in lower tem-perature samples and is a result of Ni segregation. The light grey dendriticphase (labeled 1), is high in Cr and Fe and is probably the equivalent ofl)2Fe. These dendrites reach to the boundary of the Ni depleted zone that wasmentioned in a preceding paragraph. The light phase (labeled 2) was judged,from its composition and morphology, to have been molten at the testtemperature. The phase (labeled 3), contains a large amount of Zr as does themixture between phases (2) and (3). It seems that the liquid (U-Pu-Fe) phaseis dissolving Zr and thereby solidifying on the Zr side of the band.- 6 -Fig. 4. Interaction Between U-19Pu-10Zr Fuel andAustenitic Cladding (D-9) 700 h @ 750°C- 7 -The example of a U-Pu-Zr/HT-9 couple, annealed at 725°C for 300 h, inFig. 5 serves to point out similarities as well as the difference between theinteraction of U-Pu-Zr and austenitic and ferritic steel As in the case of D-9, the enhanced interaction associated with the formation of molten phasesstarts at the center of the sample and progresses outward to the periphery(see Fig. 5-A). The microstructure of the interaction zone shows some veryunique features, in particular the distinct band structure and the graingrowth in the steel. The tip of the meltfront shown enlarged in Fig. 5 isessentially U-Pu-Fe.Increasing amounts of Zr and Fe are present behind the meltfront (see Fig.6) resulting in solidification and band formation. The narrow layer of blockyprecipitates separating two fuel-rich bands contains primarily Cr, Fe and Cand N. The dark intergranular phase in the fuel-rich zones is high in Zr.Both features are shown in detail in Fig. 5.III. DISCUSSIONIn the present tests of D9 and HT9 stainless steel and U-Pu-Zr fuelcombinations, no evidence of molten phase formation was found up to atemperature of 725°C. This is approximately the same temperature limit foundin previous experiments with 304 and 316 Type stainless steel and U-5Fs fuel[2], The previous work also reported molten phase formation at approximately800°C for several combinations of 304 and 316 Type stainless steel and U-Pu-Zrfuel, with one combination, i.e., 304 SST + U-17-Pu-6Zr fuel showing meltingat 725°C after an 168 h anneal. These results taken together point out thatthe initial formation of molten phases between stainless steel and U-Pu-Zrfuel probably occurs in the 725°C-800°C temperature range depending on minordifferences in chemistry and local conditions at the fuel-steel interface.The role of Zr in raising the molten phase temperatures well above thoseof the U-Pu-Fe (Ni) eutectic compositions is clear, but it is the minorcompositional differences and local conditions at the diffusion interface thatdetermine the efficacy of Zr. Diffusion of Fe and Ni into the fuel does notpresent a problem because the presence of Zr in the fuel prevents the for-mation of low melting eutectic compositions. The extent of diffusion of Puand U into the steel without accompanying diffusion of Zr is the key problemin fuel-cladding capability. This process appears to be controlled by Zr aswell, but in a more complex manner involving the formation of a Zr compoundlayer at the fuel-steel interface. Previous annealing experiments with U-Pu-Zr alloys in NaK and He filled stainless steel capsules also showed formationof Zr and Pu rich bands at surfaces that were not in contact with steel [4].As was suggested in [1,4] and is clearly confirmed by the present results, theefficiency of this layer formation depends on the availability of interstitialelements. The main source of interstitial elments in the fuel element is thecladding, but in the present diffusion tests the atmosphere in the testcapsule may have contributed.- 8 -Fig. 5. Interaction Between Fuel and Ferritic Cladding HT-9300 h 9 725°C- 9 -U-PuFig. 6. X-ray Dot-Map Showing Distr ibut ion ofFe, Zr and U-Pu in HT-9/Fuel Interaction Zone- 10 -The thickness of the Zr layer at the diffusion interface is determined bythe available Zr and N in its vicinity. Fe and Ni appear to diffuse readilyinto the Zr layer with the Ni diffusing faster as expected [3]. The rapiddiffusion of Ni results in the formation of two phases in the Zr layer, aswell as the formation of a ferritic layer in the adjacent D9 steel.. Thisfeature and the difference in available N explains the difference in structureof the fuel-steel interaction bands between 09 and HT9.The extent of initial interdiffusion of Pu and U with cladding elements,or the diffusion of U and Pu through the Zr layer determines the eventualformation of a U-Pu-Fe molten phase. At very high temperatures this occurs ina short time, but in the range of 725°C-800°C, The thickness, uniformity, andrate of formation of the Zr layer appears to be the controlling factor.Almost all diffusion interfaces had a dish-shaped Zr layer, i.e., the Zr layerwas thicker toward the periphery cf the sample and indeed molten phaseformation always started at the sample center where the Zr layer formation wasleast. Note that in fuel elements, the Zr layer on the fuel surface issubstantial because reaction with the ZrC^ mold wash establishes a high Zrconcentration on the surface of the fuel pin even vefore irradiation [5].It is not clear at this time whether sample preparation or capsuleatmosphere is the cause for the dish shaped Zr layer. Upon initial formationof a U-Pu-Fe molten phase, diffusivities of all species involved is en-hanced. The molten phase appears to dissolve Zr and solidify. This resultsin a rapid lateral motion of the meltfrcnt v/hile the overall penetration intothe steel is slowed down. The great affinity of Zr for N results in adecomposition of precipitates in HT9 and pronounced recrystalization of thesteel. This also occurs before a molten phase is formed but on a much smallerscale.It seems that a thick and uniform Zr layer retards the formation of moltenphases, and that the ample availability of Zr, and N in particular, can assuresuch a layer.REFERENCES1 . S. T. Zegler and C. M. Walter, Compatibility Between Metall ic U-Pu-BaseFuels and Potential Cladding Materials, Pub. AIME Nuclear MetallurgySymposium on Plutonium Fuels Technology, AIME 1_3, pp. 335-344 (October1967).2. C. M. Walter and J . A. Laht i , Nuclear Applications, Vol. 2 (August 1966)3. Steven J . Rothman, Diffusion in Uranium, i ts Al loys, and Compounds, ANL-5700, Part C Metals", Ceramics and Materials (TID-4500, 16th Eds.T'AECResearch and Development Report (May 1961),4. D. R. O'Boyle, Argonne National Laboratory, private communication.5. R. G. Pahl, C. E. Lahm, G. L. Hofman, W. N. Beck, Recent I rradiat ion Testsof Uranium-Plutonium-Zirconium Metal Fuel Elements, ( this confe'renceT!- 11 -

Chemical interaction of metallic fuel with austenitic and ferritic stainless steel cladding

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Chemical interaction of metallic fuel with austenitic and ferritic stainless steel cladding

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