Monolithic fuel plates are being developed for application in research reactors throughout the world. These fuel plates are comprised of a U-Mo alloy foil encased in aluminum alloy cladding. Three different fabrication techniques have been looked at for producing monolithic fuel plates: hot isostatic pressing (HIP), transient liquid phase bonding (TLPB), and friction stir welding (FSW). Of these three techniques, HIP and FSW are currently being emphasized. As part of the development of these fabrication techniques, fuel plates are characterized and tested to determine properties like hardness and the bond strength at the interface between the fuel and cladding. Testing of HIPed samples indicates that the foil/cladding interaction behavior depends on the Mo content in the U-Mo foil, the measured hardness values are quite different for the fuel, cladding, and interaction zone phase and Ti, Zr and Nb are the most effective diffusion barriers. For FSW samples, there is a dependence of the bond strength at the foil/cladding interface on the type of tool that is employed for performing the actual FSW process

Burkes, D. E.

Jue, J. F.

Keiser, D. D.

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This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint should not be cited or reproduced without permission of the author. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, or any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party’s use, or the results of such use, of any information, apparatus, product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. The views expressed in this paper are not necessarily those of the United States Government or the sponsoring agency. INL/CON-07-12224PREPRINTCharacterization and Testing of Monolithic RERTR Fuel Plates Research Reactor Fuel Management ConferenceD. D. Keiser, Jr.J. F. Jue D. E. Burkes March 2007 Characterization and Testing of Monolithic RERTR Fuel Plates 1byD. D. Keiser, Jr., J. F. Jue, and D. E. BurkesIdaho National LaboratoryP. O. Box 1625Idaho Falls, ID 83403-6188Paper to be presented at the Research Reactor Fuel Management ConferenceLyon, FranceMarch 11-15, 20071 Work supported by the U.S. Department of Energy, Office of Nuclear Materials ThreatReduction (NA-212), National Nuclear Security Administration, under DOE-NE IdahoOperations Office Contract DE-AC07-05ID14517.CHARACTERIZATION AND TESTING OF MONOLITHICRERTR FUEL PLATESD.D. KEISER, JR., J. F. JUE, and D. E. BURKESIdaho National Laboratory, P.O. Box 1625, MS 6188, Idaho Falls, ID 83403-6188, USAABSTRACTMonolithic fuel plates are being developed for application in research reactors throughoutthe world. These fuel plates are comprised of a U-Mo alloy foil encased in aluminumalloy cladding. Three different fabrication techniques have been looked at for producingmonolithic fuel plates: hot isostatic pressing (HIP), transient liquid phase bonding(TLPB), and friction stir welding (FSW). Of these three techniques, HIP and FSW arecurrently being emphasized. As part of the development of these fabrication techniques,fuel plates are characterized and tested to determine properties like hardness and the bondstrength at the interface between the fuel and cladding. Testing of HIPed samplesindicates that the foil/cladding interaction behavior depends on the Mo content in the U-Mo foil, the measured hardness values are quite different for the fuel, cladding, andinteraction zone phase and Ti, Zr and Nb are the most effective diffusion barriers. ForFSW samples, there is a dependence of the bond strength at the foil/cladding interface onthe type of tool that is employed for performing the actual FSW process.1. IntroductionMonolithic fuel plates are being developed as an LEU fuel for application in research reactors.[1]. To fabricate these fuels, three techniques have been evaluated: friction stir welding (FSW),hot isostatic pressing (HIP), and transient liquid phase bonding (TLPB), and the current emphasisis on FSW and HIP. As part of this evaluation, samples have been generated from fuel plates thatwere fabricated using the emphasized techniques, which were then characterized and tested. Thecharacterization was performed using both scanning electron microscopy and optical microscopy.The testing was conducted using either hardness testing, which was performed on FSW and HIPsamples, or pull testing, which was conducted only on FSW samples. The pull test was employedto investigate the bond strength at the foil/cladding interface in a monolithic fuel plate. This paperwill discuss the results from the characterization and testing that was performed on varioussamples that were produced using FSW and HIP.2. Results and Discussion2.1 Hot Isostatic PressingThe HIP samples that were characterized were HIPed at 580˚C for 90 minutes using a pressure of15,000 psi. Figure 1 shows SEM micrographs for samples with 6061 Al cladding and U-x wt%Mo (x=7, 8, 10, and 12) fuel alloys. For the U-7Mo sample, the original fuel alloy is completelyconsumed. Another U-7Mo sample was run for 90 minutes at 580˚C that was not completelyconsumed. Composition analysis of the completely consumed samples showed that the phasesthat formed were (U,Mo)Al3 and (U,Mo)0.9Al4. Only a small region of original U-7Mo alloyremained at the center of the sample. For a U-7Mo sample HIPed at 580˚C for half the time, theamount of reaction was less. This sample showed that the main phase to develop in theinteraction zone is (U,Mo)Al3, along with some minor phases at the reaction zone/6061 Alinterface that contain component like Mg and Fe (see Figure 2). Nearest the unreacted U-7Mo isa multiphase zone that is commonly seen in diffusion couple experiments (see Figure 3) [2]. The(a) (b)(c) (d)Figure 1. SEM micrographs of the (a) U-7Mo (580˚C; 3 hrs), (b) U-8Mo (580˚C; 1.5 hrs), (c)U-10Mo (580˚C; 3 hrs), and (d) U-12Mo (580˚C; 1.5 hrs) foils after HIPing. The black areas are6061 Al cladding and the medium-contrast areas are interaction product.U-7MoU-8MoU-10Mo U-12MoFuel/Cladding ReactionFuel/Cladding Reaction(a) (b)Figure 2. (a) SEM micrograph of interdiffusion zone nearest the unreacted 6061 Al for U-7Moplate HIPed at 580˚C for 90 minutes at 15,000 psi. (b) Table showing compositions at variouspoints in (a) going from 1 to 15.(a) (b)Figure 3. (a) SEM micrograph of interdiffusion zone nearest the unreacted fuel for U-7Mo plateHIPed at 580˚C for 90 minutes at 15,000 psi. (b) Table showing compositions at various points in(a) going from 1 to 15.015ppt Appt B6061 aluminumFuel/Claddingreaction layerU (at %) Mo Al Mg Si Fe0 20.44 2.46 75.69 2.58 - -1 19.14 2.57 74.71 2.22 2.09 -2 17.33 2.30 75.95 3.42 1.57 -3 18.79 2.86 75.66 2.09 0.68 -4 16.50 1.86 73.66 3.35 4.95 -5 15.33 3.90 74.53 2.46 4.32 -6 8.19 2.94 71.54 13.55 0.44 3.337 6.58 0.41 79.13 2.06 - 12.618 3.73 0.25 90.41 4.23 - 1.849 5.48 - 78.86 2.23 0.29 13.5110 - - 97.23 3.20 - -11 - 0.33 92.44 3.74 - 2.4212 - 0.35 96.62 2.00 0.82 0.2513 - 0.11 98.09 1.6 0.34 -14 - 0.03 98.88 1.29 0.07 -15 0.03 - 97.31 1.3 1.39 0.13ppt A 28.18 6.95 47.07 18.95 0.82 -ppt B - 0.18 92.59 1.01 2.78 3.67U (at%)Mo Al1 80.42 13.73 5.842 80.35 15.17 4.493 86.33 11.51 2.174 82.92 10.95 6.135 81.88 15.02 3.106 51.69 5.79 42.527 49.35 8.13 42.528 48.52 9.08 42.399 43.11 8.45 48.4410 29.80 4.82 65.3811 19.79 4.63 75.5312 19.75 3.28 76.9613 20.53 2.11 77.3614 20.31 4.04 75.6515 21.44 2.72 75.84115U-7MoFuel/Claddingreaction layerU-8-Mo sample displays less interaction between the fuel and the cladding, but a significantamount of interaction product still forms, and a significant quantity of U-rich precipitates aredistributed throughout the interaction zone. For the U-10Mo and U-12Mo samples, only a few-micron-thick layer forms at the foil/cladding interface. From looking at these different samples, itis clear that at 580˚C there is a significant correlation between the Mo content of the fuel and theamount of foil/cladding interaction that occurs during HIPing. By looking at the U-Mo phasediagram [3], it can be seen that at around 562˚C and at about 10.5 wt% Mo, a eutectoid is present.For the hypo-eutectoid U-7 and 8 wt% Mo alloy compositions, the alloys are in a two-phaseregion consisting of -U and -U at 580˚C. Yet, the U-10 wt% Mo alloy composition is verynear the / +  phase field boundary and is probably -phase. The U-12 wt% Mo alloy is in the +  two-phase region. When HIPing is performed at 560˚C, all the alloys will be in the  +  two-phase region and how quickly the various alloys transform to these two phases will affect howquickly the alloys react with the 6061Al alloy cladding. Once -U is present in the alloymicrostructure, the amount of interaction should increase, since it has been shown that -U reactsmuch more quickly with Al than does -U [2].To reduce reaction between the fuel and cladding a variety of diffusion barriers (Zr, Nb, Ta, C, Si,and Ti) have been evaluated. Figure 4 shows a cross section of a sample where a Zr foil wasemployed as a diffusion barrier. Bonding between the Zr and the Al was good. Based onultrasonic testing (UT), there was also bonding between the Zr and the fuel foil. Yet, during thedestructive examination process the Zr de-bonded from the fuel foil. This indicates that the bondquality was relatively poor. Ti and Nb exhibited behaviors similar to that of Zr. Ni was tried as adiffusion barrier, but extensive interaction with Al to produce nickel-aluminides precludes usingNi as a diffusion barrier. For C and Mo, UT analysis indicates poor bonding between thesematerials and the foil and 6061 Al. Initial UT analysis suggests that Ta is a good diffusionbarrier, but ongoing SEM examinations must be completed to confirm this result.Figure 4. SEM micrograph of a U-7Mo fuel plate with a Zr liner used as a diffusion barrier thatwas HIPed at 580˚C for 90 minutes. There is a gap between the Zr and the U-7Mo foil. Nb andTi exhibited behavior similar to that of Zr.U-7MoZrFuel/CladdingReaction6061 AlHardness values were measured for the fuel, cladding, and interaction phases for HIPed samples.For the fuel and cladding, measurements were made before and after HIPing The initial Vickershardness values before HIPing at 580˚C for 3 hours for a 6061 T6 Al alloy and U-10Mo alloywere 107 ± 2 and 263 ± 4, respectively. After HIPing, the 6061 Al alloy and the U-10Mo alloyregistered Vickers hardness values of 47 ± 1, and 288 ± 4, respectively. The Vickers hardnessvalues of the two uranium-aluminide layers that formed during HIPing were also measured. The(U,Mo)Al3 phase had a value of 617 ± 3, and the (U,Mo)0.9Al4 had a value of 653 ± 32. Based onthese results, it can be seen that after HIPing the 6061 Al alloy cladding is softer, while there islittle change in the hardness of the U-10Mo alloy. The uranium-aluminide layers that are presentafter HIPing are much harder than the U-10Mo and 6061 Al cladding.2.2 Friction Stir WeldingThe strength of the bond at the foil/cladding interface is of particular interest in FSW samples.To investigate this parameter, pull tests of friction stir weld (FSW) monolithic fuel platespecimens were conducted with samples fabricated using two different materials as the tool face.A description of the pull test method and procedure may be found in [4], while a description ofthe FSW process may be found in [5]. The two tool facing materials are defined as Alloy A andAlloy B. In particular, Alloy A has a thermal conductivity of x, while Alloy B has a thermalconductivity of 2.5x. Thermal conductivity is an important parameter in the FSWing ofmonolithic fuel plates since increased heat removal away from the weld surface allows higherloads to be applied, resulting in enhanced bonding. For example, if a low thermal conductivitymaterial is used as the tool face, less heat will be removed from the weld face, and in order toavoid a corresponding increase in process temperature and void formation, the applied load mustbe decreased. Diffusion of atoms across the fuel-cladding interface that results in adequatebonding is driven by two mechanisms: 1) temperature to drive the kinetic reaction and 2) appliedload to keep the interface in intimate contact with one another allowing diffusion to occur.Applied load is especially important with the FSW process, since the aluminum cladding is in aplastic state and stirs on top of the fuel foil. Lower applied loads do not result in the intimatecontact across the interface therefore limiting the diffusion of atoms, even if the temperature issufficiently high. This hypothesis is shown graphically in Figure 5, where stress-time plots forAlloy A and Alloy B are provided. Analysis of Figure 5 shows that Alloy B, the higher thermallyconductive material, has superior bond strength (greater than 7 times) than that of Alloy A.Reasoning behind this observation is Alloy A, as discussed above, produces mainly a mechanicalbond, while Alloy B produces a diffusion enhanced bond, similar to observations made for theHIPing process. It is important to note that the pull test was terminated for the Alloy B specimennot because the foil-clad interface failed, but because the epoxy used to bond the test specimen tothe aluminum platens for pull testing failed.4. ConclusionsBased on characterization and testing described above for monolithic fuel plates fabricated byFSW and HIP, the following conclusions can be drawn:1. The amount of interaction that will occur during HIPing of monolithic fuel plates will dependon the Mo content of the fuel foil, the temperature and time that is used, and whether or not adiffusion barrier is applied.2. The hardness of the 6061 Al alloy cladding will decrease during HIPing, while the U-10Mowill maintain about the same hardness. Any uranium-aluminide layers that develop will berelatively hard.3. Based on pull tests performed on FSW samples, there is a dependence of the bond strength ofthe fuel plate on the type of FSW tool that is used to fabricate the plate.010203040500 100 200 300 400 500 600 700 800 900Time (seconds)Stress (MPa)Approximatelimit of test rigAlloy AAlloy BFigure 5. Stress-time plots obtained from pull test specimens fabricated employing FSW toolface materials Alloy A and Alloy B.AcknowledgmentsThis work was supported by the U.S. Department of Energy, Office of Nuclear Materials ThreatReduction (NA-212), National Nuclear Security Administration, under DOE-NE IdahoOperations Office Contract DE-AC07-05ID14517.References[1] C.R. Clark, J.F. Jue, G.A. Moore, N.P. Hallinan, B. H. Park, and D.E. Burkes, 26thInternational Meeting of Reduced Enrichment for Research and Test Reactors (RERTR),Cape Town, South Africa, October 29-November 2, 2006.[2] D.D. Keiser, Argonne National Laboratory Report, ANL-05/14 (July, 2005).[3] T. B. Massalski, Ed., Binary Alloy Phase Diagrams, Vol. III, ASM International, MaterialsPark, OH (1990).[4] D. E. Burkes, D. D. Keiser, D. M. Wachs, J. S. Larson and M. D. Chapple, “Characterizationof Monolithic Fuel Foil Properties and Bond Strength,” these proceedings (2007).[5] C. R. Clark, et al., This meeting.

Characterization and Testing of Monolithic RERTR Fuel Plates

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Characterization and Testing of Monolithic RERTR Fuel Plates

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