The starting microstructure of a dispersion fuel plate can have a dramatic impact on the overall performance of the plate during irradiation. To improve the understanding of the as-fabricated microstructures of dispersion fuel plates, SEM and TEM analysis have been performed on RERTR-9A archive fuel plates, which went through an additional hot isostatic procsssing (HIP) step during fabrication. The fuel plates had depleted U-7Mo fuel particles dispersed in either Al-2Si or 4043 Al alloy matrix. For the characterized samples, it was observed that a large fraction of the ?-phase U-7Mo alloy particles had decomposed during fabrication, and in areas near the fuel/matrix interface where the transformation products were present significant fuel/matrix interaction had occurred. Relatively thin Si-rich interaction layers were also observed around the U-7Mo particles. In the thick interaction layers, (U)(Al,Si)3 and U6Mo4Al43 were identified, and in the thin interaction layers U(Al,Si)3, U3Si3Al2, U3Si5, and USi1.88-type phases were observed. The U3Si3Al2 phase contained some Mo. Based on the results of this work, exposure of dispersion fuel plates to relatively high temperatures during fabrication impacts the overall microstructure, particularly the nature of the interaction layers around the fuel particles. The time and temperature of fabrication should be carefully controlled in order to produce the most uniform Si-rich layers around the U-7Mo particles

Keiser, D. D. Jr.

Perez, E.

Sohn, Y. H.

Yao, B.

UNT Digital Library

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-09-17107PREPRINTSEM and TEM Characterization of As-Fabricated U-7Mo Dispersion Fuel Plates RERTR 2009 D. D. Keiser, Jr. B. Yao E. Perez Y. H. Sohn November 2009 RERTR 2009 _ 31st INTERNATIONAL MEETING ONREDUCED ENRICHMENT FOR RESEARCH AND TEST REACTORSNovember 1-5, 2009Kempinski Hotel Beijing Lufthansa CenterBeijing, ChinaSEM and TEM CHARACTERIZATION OF AS-FABRICATED U-7MO DISPERSION FUEL PLATESD.D. KEISER, JR.1, B. YAO2, E. PEREZ2 and Y.H. SOHN21 Idaho National Laboratory, P.O. Box 1625, MS 6188, Idaho Falls, ID 83403-6188, USA2 University of Central Florida, P.O Box 162450, Orlando, FL 32816 USAABSTRACTThe starting microstructure of a dispersion fuel plate can have a dramaticimpact on the overall performance of the plate during irradiation.  Toimprove the understanding of the as-fabricated microstructures ofdispersion fuel plates, SEM and TEM analysis have been performed onRERTR-9A archive fuel plates, which went through an additional hotisostatic procsssing (HIP) step during fabrication. The fuel plates haddepleted U-7Mo fuel particles dispersed in either Al-2Si or 4043 Al alloymatrix.  For the characterized samples, it was observed that a largefraction of the -phase U-7Mo alloy particles had decomposed duringfabrication, and in areas near the fuel/matrix interface where thetransformation products were present significant fuel/matrix interactionhad occurred.  Relatively thin Si-rich interaction layers were also observedaround the U-7Mo particles.  In the thick interaction layers, (U)(Al,Si)3and U6Mo4Al43 were identified, and in the thin interaction layersU(Al,Si)3, U3Si3Al2, U3Si5, and USi1.88-type phases were observed.  TheU3Si3Al2 phase contained some Mo.  Based on the results of this work,exposure of dispersion fuel plates to relatively high temperatures duringfabrication impacts the overall microstructure, particularly the nature ofthe interaction layers around the fuel particles. The time and temperatureof fabrication should be carefully controlled in order to produce the mostuniform Si-rich layers around the U-7Mo particles.1. IntroductionIn order to improve the irradiation performance of U-Mo dispersion and monolithic fuels,approaches to mitigate the affect of fuel/matrix and fuel/cladding interactions are beingpursued.  For the dispersion fuels, Si is being added to the matrix so that a Si-richinteraction phase develops at the U-Mo/matrix interface that exhibits good irradiationperformance.  For the monolithic fuels, a Zr barrier is inserted between the U-Mo foil andthe cladding.  During the fabrication of each fuel type, it has been demonstrated that thetime at high temperature will impact the final as-fabricated microstructure. [1,2].  Thesemicrostructures can contain a variety of different phases depending on the fabricationhistory.For U-Mo dispersion fuel plates with Al matrices that contained Si, an investigation hasbeen performed using scanning electron microscopy (SEM) and transmission electronmicroscopy (TEM) to identify the exact phases that are contained in the interaction zonesthat develop around the U-7Mo particles during fuel plate fabrication.  The dispersionfuel plates that were analyzed were exposed to significant times at high temperatures andexhibited thin Si-rich layers along with relatively thick Si-depleted interaction layers.This allowed for the opportunity to investigate how Si-rich layers change duringfabrication to form the Si-depleted layers.This paper will describe the specific phases that were found in the different archive U-Modispersion fuel plates. Comments will be made about how the development of differentphases may affect the overall irradiation performance of a fuel plate.2. ExperimentalThe production of fuel plates for the RERTR-9A experiment involved the standarddispersion fuel fabrication steps.  As a result, these fuel plates were exposed to 500˚C forup to 1 hour during rolling, and to 485˚C for 30 minutes during the blister-annealing step.In addition, an extra hot isostatic press (HIP) step (30 minutes at 500˚C) was employedon the fuel plates to improve the bonding at the interface between the fuel meat and the6061 Al cladding for these high-density (8 gU/cc) dispersion plates.  The HIP processconstituted a 75 minute heat-up to 500˚C for a 30 minute soak, followed by a 75 minutecool-down to room temperature.SEM analysis was performed on archive dispersion RERTR-9A fuel plates using a ZeissUltra-55 field emission SEM with X-ray energy dispersive spectroscopy (XEDS).Specimens for TEM were prepared with a focused ion beam (FIB) in-situ lift-out (INLO)technique using a FEITM 200 TEM. Initially, a Pt layer was deposited onto the selectedarea of interest, to protect the surface of the sample from the accelerated Ga+ ion beam.The high energy Ga+ beam was utilized to mill material creating a trench on both sides.The edges of the sample were then milled leaving only a small bridge of material so thatthe sample remained attached to the bulk alloy. A W omniprobe was then lowered in andPt welded to the bridge connecting the sample and the bulk alloy. The partially attachededge of the specimen was then milled completely to release the sample. The Womniprobe, with the TEM specimen still welded to it, was then lifted away from the stageand lowered toward a slotted copper TEM grid. The sample was then Pt welded to thecopper grid. Each specimen was thinned further, milled to about 100 nm in thickness inorder to obtain electron transparency. The FEI/Tecnai F30 300keV TEM/STEM equippedwith a Fischione high angle annular dark field (HAADF) detector was used for the TEManalysis. For each specimen, both low and high magnification HAADF STEM imageswere collected for bulk and detailed analysis. Selected area and convergent beamdiffraction patterns (SAED and CBED) were collected and indexed to identify the phaseconstituents.3. Results3.1 SEM AnalysisThe results from an earlier SEM analysis of the RERTR-9A archive samples [1] aresummarized in Table. 1.Table 1.  Results of SEM Compositional Analysis of As-Fabricated RERTR-9A U-7MoDispersion Plates [1].Matrix Fabrication Details Si-rich LayerDescriptionNotes4043 Al(4.81wt%Si)Roll bonded andHIPed (500˚C/15ksi/30 min)Up to 7 μm thick.Uniform layer.Al: 30.2 (14.1)*Si: 24.5 (8.5)Mo: 5.8 (3.2)U: 39.6 (16.7)Bulk interaction(~10% area).**Al: 56.9 (7.2)***Si: 0.8 (0.5)Mo: 5.6 (1.1)U: 36.8 (6.6)Al+2Si(mixtureof pureAl andpure Sipowders)Roll bonded andHIPed (500˚C/15ksi/30 min)Varies from 2 to 5μm thick. Notuniform.Al: 28.0 (23.1)*Si: 10.7 (10.3)Mo: 7.9 (3.5)U: 53.4 (22.4)Bulk interaction(~20% area).Al: 53.2 (5.2)*Si: 1.8 (0.6)Mo: 5.9 (1.3)U: 39.7 (4.0)Al-2Si(alloypowders)Roll bonded andHIPed (500˚C/15ksi/30 min)From 2 to 5 μmthick. Not uniform.Al: 36.5 (25.0)*Si: 11.4 (11.0)Mo: 6.6 (3.6)U: 45.6 (23.7)Bulk interaction(~20% area).Al: 53.2(5.2)***Si: 1.2 (0.7)Mo: 5.9 (1.3)U: 39.8 (4.0)* layer composition in at% (standard deviation), based on 10 measurements.** Bulk interaction refers to the formation of the Si-deficient layer.***  layer composition in at% (standard deviation), based on 7 measurements.From Table 1, it can be seen that the samples contained Si-rich layers around the fuelparticles, along with some layers that contained little or no Si.  Si X-ray maps generatedfor the different samples have confirmed the presence of Si-rich layers around the U-7Moparticles in the areas where the Si-deficient interaction layers were not present.Fig. 1 shows backscattered electron images of the Si-deficient layers in the three RERTR-9A fuel plates.  Up to 20% of the microstructural area consisted of the Si-deficient phasefor the samples with Al+2Si mixture as the matrix or Al-2Si alloy as the matrix.  Thesample with 4043 Al alloy matrix had approximately 10% of the area that contained theSi-deficient layer.Fig. 1.Backscattered electron images of the microstructures observed for the as-fabricated RERTR-9A fuel plates with (a) 4043 Al, (b) Al+2Si, and (c) Al-2Si alloymatrices, respectively. The bright phase is U-7Mo alloy, the dark phase around the U-7Mo particles is the matrix, and the medium-gray phase is the Si-deficient interactionlayer.  Not observable are the thin, Si-rich interaction layers around the U-7Mo particles.In Figure 2 are representative backscattered electron images of the microstructureobserved for each of the three samples in areas where the interaction layer was relativelythin.  Figure 3 shows a higher magnification image of areas in the U-7Mo fuel particleswhere islands of transformed U-7Mo could be observed and areas where the decomposedU-7Mo alloy was in contact with the matrix, which resulted in increased interaction.Some fuel particles were totally consumed by the interaction product (see Figure 4).Fig. 2.Backscattered electron images of the microstructures observed for the as-fabricated RERTR-9A fuel plates with (a) 4043 Al, (b) Al+2Si, and (c) Al-2Si alloymatrices, respectively.  Most of the interaction layers in these areas of the samples arerelatively thin and Si-rich.Fig. 3.Backscattered electron images for the U-7Mo/Al-2Si sample showing (a)decomposition in the U-7Mo alloy and (b) the morphologies of the interaction layersaround different fuel particles.Fig. 4.Backscattered electron images of the microstructures observed for the as-fabricated RERTR 9A fuel plates with (a) 4043 Al, (b) Al+2Si, and (c) Al-2Si alloymatrices, respectively, where some fuel particles were completely consumed byinteraction products.3.2 TEM AnalysisTEM analysis was performed in two different regions of the U-7Mo/4043 Al sample, oneat a location where there was thick interaction layer and another where the interactionlayer was relatively thin.  One region of the U-7Mo/Al-2Si sample where the interactionlayer was relatively thin was also analyzed.Figure 5 shows HAADF TEM micrographs from an area along a decomposed U-7Moparticle/matrix interface for the U-7Mo/4043 Al sample. A relatively large interactionlayer developed in this location.  CBED patterns were examined at specific locations inthe interaction layer that confirmed the existence of cubic U(Al,Si)3, orthorhombicU6Mo4Al43, and orthorhombic -U phases. The U(Al,Si)3 phase contained small amountsof Mo, based on XEDS analysis via TEM.Fig 5. HAADF TEM micrographs and corresponding CBED patterns from the U-7Mo/4043 Al sample at a location along a decomposed U-7Mo particle/matrix interfacewhere the particle had reacted with the matrix to form reaction product (medium gray):(a) a region consisting of U(Al,Si)3 phase; (b) a region where U6Mo4Al43 was identified;and (c) a fuel region where -U was identified.Figure 6 shows an area where a narrower interaction layer developed at the interfacebetween U-7Mo and 4043 Al.  CBED analysis indicated the presence of tetragonalU3Si3Al2 in the layer.  XEDS analysis via TEM indicated up to 3 at% Mo was present inthis phase. Near the thin layer (Fig. 6b), an area with increased interaction was observed.This appeared to be at a location of the sample where the original -(U,Mo) phase haddecomposed, and in this localized area where the U was present, a relatively high rate ofinterdiffusion with Al had taken place.  This accelerated interdiffusion has been observedin other studies looking at heat-treated U-Mo dispersion samples, where the -U in theU-Mo powders displayed a relatively high rate of interdiffusion with Al. [3,4].Fig 6.  HAADF TEM micrographs and the corresponding CBED pattern from the U-7Mo/4043 Al sample at the U-7Mo (bright)/matrix (black) interface where a relativelythin interaction layer (medium gray) was observed: (a) CBED of region 1 corresponds toU3Si3Al2; (b) onset of transformation and accelerated interaction.Figure 7 shows an area where a thicker Si-rich interaction layer developed at the interfacebetween U-7Mo and 4043 Al.  Four different phases were identified in this layer:hexagonal U3Si5, U(Al,Si)3, Al, and another very minor uranium-silicon phase (maybeUSi).Fig 7.  HAADF TEM micrograph showing four locations in a relatively thick Si-richinteraction layer where separate phases were identified.  The dark phase to the right andwhite phase to the left are Al and U-7Mo, respectively.For the sample with U-7Mo particles dispersed in Al-2Si matrix, TEM analysis wasperformed at an area where relatively thin interaction layer was present. HAADF TEMmicrographs showing the locations in the interaction layer where diffraction analysis wasperformed are presented in Figure 8. A tetragonal U1.88-type of phase  was present nearestthe unreacted U-7Mo, and a cubic U(Al,Si)3-type phase was present adjacent to theunreacted matrix as presented in Fig. 7.  Negligible Mo was present in these phases.Fig 8.  HAADF TEM micrographs from the U-7Mo/Al-2Si sample (a-c) at the U-7Mo(bright)/matrix (black) interface where a relatively thin interaction layer (medium gray) ispresent.  Circles indicate regions where diffraction analysis identified the presence of  (1)U1.88 and (2) U(Al,Si)3.4. DiscussionBased on the SEM and TEM analysis that was performed on RERTR-9A archivedispersion fuel plates, decomposition of the U-7Mo alloys during fabrication can have adramatic impact on the thickness of the fuel/matrix interaction layers that can developand the types of phases that are present in the layers for fuel plates with 4043Al and Al-2Si matrices. The markedly thicker interaction zones can be associated with locationswhere a decomposed fuel region was in contact with the matrix. The narrower Si-richinteraction layers did not appear to have areas of decomposition in close proximity.These zones were observed to contain ternary U-Al-Si phases (viz. U(Al,Si)3 andU3Si3Al2) and binary U-Si phases (USi1.88 and U3Si5). It was found that within the thin Si-rich interaction layer, the tetragonal USi1.88-type phase was present in a zone nearest theunreacted U-7Mo, and the cubic U(Al,Si)3-type phase was present in a zone adjacent tothe unreacted matrix.  Only U(Al,Si)3 was found both in the thin Si-rich interaction layeralong with the thicker Si-deficient interaction layer. U6Mo4Al43 was found only in thethick Si-deficient interaction layer.TEM results have also been reported for two other samples taken from the RERTR-9Afuel plate with Al-2Si matrix [1].  This previous analysis focused on the interfacebetween decomposed U-7Mo particles and the matrix.  An ordered fcc U(Al,Si)3 phasewas observed in the interaction layer that contained some Mo.  No evidence of aU6Mo4Al43 phase was observed.  The presence of the U(Al,Si)3 phase agrees with theresults of this paper.Comparing the results of this study with those reported for diffusion couple testsconducted at 500˚C or 550˚C using U-7Mo and other Si-containing matrices, viz. 6061Al, there is good agreement in terms of the development of relatively thick interactionlayers near decomposed U-7Mo and narrower interaction layers where decomposed U-7Mo was not apparent [5,6]. Composition analysis showed that both the thick and narrowinteraction layers contained a Si enriched phase and a phase more enriched in Al.  Resultsfrom an X-ray diffraction analysis of a U-7Mo/6061 Al couple annealed at 550˚C for 3hours [5] showed results for regions near the thicker interaction zone where U(Al,Si)3 andU6Mo4Al43 phases were identified.  This agrees with the results of this paper where bothof these phases were also identified near the thicker interaction zone.  In a diffusioncouple between U-7Mo and an Al A356 alloy (7.1 wt% Si) annealed at 550˚C, U(Al,Si)3,UMo2Al20, and U3Si5 phases were observed in the diffusion zone [7].  In [8], diffusioncouples were annealed between U-7Mo and Al-Si alloys and then characterized using μ-XRD. In the diffusion couples that contained either an Al-2Si alloy or 4043 Al, two typesof zones made up the formed interaction layer.  The zone nearest the U-7Mo containedUAl3 and U6Mo4Al43, and the other zone contained U(Al,Si)3 and UMo2Al20.  For thecouples with alloys that contained more than 5 wt% Si, it was observed that both U3Si5and USi2 developed in the interaction zone.  Similar U-Si phases were observed in somefuel plate samples characterized in this study.As discussed in [1], the fuel plates in the RERTR-9B experiment never went through theextra HIP step, as did the RERTR-9A plates.  The result was interaction layers around theU-7Mo particle that were much thinner and more uniform after fabrication.Consequently, when the RERTR-9B fuel plates went into reactor very little, if any, of theU6Mo4Al43 phase was present at the interface of the fuel particles.  Instead, U(Al,Si)3,U3Si3Al2, U3Si5, and USi2 were most likely present with possibly some other yet to beidentified minor phases. The significance of the RERTR-9A having significant amountsU6Mo4Al43 in the diffusion zone is that the presence of a phase with a composition likeUAl4 in an irradiated fuel plate has been linked to poor fuel performance [9].  Whereas,fuel plates with interaction layers that are relatively thin and Si-rich have been found toexhibit good irradiation performance [10].5. ConclusionsBased on the TEM characterization of RERTR-9A archive fuel plates with 4043Al andAl-2Si matrices that were exposed to high temperatures for relatively long times,comments can be made about how decomposition of the U-7Mo fuel affects interactionlayer development.  When fuel plates are exposed to temperatures near 500˚C forrelatively long times, large interaction layers can develop adjacent to areas where the U-7Mo fuel has decomposed, and these layers can contain U(Al,Si)3 and U6Mo4Al43 phases.In areas along the U-7Mo/matrix interface where fuel decomposition is not apparent,relatively narrow, Si-rich layers develop that can contain U(Al,Si)3, U3Si3Al2, U3Si5, andUSi1.88.  As a result, during fabrication of dispersion fuel plates the time at hightemperatures should be controlled in order to produce interaction layers that are narrowand Si-rich.AcknowledgmentsThis work was supported by the U.S. Department of Energy, Office of Nuclear MaterialsThreat Reduction (NA-212), National Nuclear Security Administration, under DOE-NEIdaho Operations Office Contract DE-AC07-05ID14517. Personnel in the Hot FuelExamination Facility are recognized for their contributions in destructively examiningfuel plates. Pavel Medvedev is acknowledged for calculating the irradiation conditionsfor the various fuel plates.References[1] D. D. Keiser, Jr. et al., RERTR 2008, Washington , D. C.,  October 5-9, 2008.[2] D. D. Keiser, Jr. et al., RRFM 2007, Lyon, France, March 11-15, 2007.[3] K. H. Kim et al., Nucl. Engr. and Design, 178 (1997) 111-117.[4] D. B. Lee et at., J. Nucl. Mater. 250 (1997) 79-82.[5] D. D. Keiser, Jr., Defect and Diffusion, Vol. 266 (2007) pp. 131-148.[6] C. Komar Varela et al., RRFM 2007, Lyon, France, March 11-15, 2007.[7] M. Mirandou et al., J. Nucl. Mater. 394 (2009) 268-273.[8] J. Allenou et al., RRFM 2009, Vienna, Austria, March 22-25, 2009.[9] A. Leenaers et al., J. Nucl. Mater. 335 (2004) 39-47.[10] D. D. Keiser, Jr. et al., GLOBAL 2009,  Paris, France, Sep. 6-10, 2009.

SEM and TEM Characterization of As-Fabricated U-7Mo Disperson Fuel Plates

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SEM and TEM Characterization of As-Fabricated U-7Mo Disperson Fuel Plates

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