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Comparison of the radial effects of burnup on fast reactor MOX fuel microstructure and solid fission products
Journal of Nuclear Materials 531 (2020) 152003
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Comparison of the radial effects of burnup on fast reactor MOX fuel microstructure and solid fission products Riley J. Parrish a, Fabiola Cappia b, Assel Aitkaliyeva a, * a b
Department of Materials Science and Engineering, University of Florida, Gainesville, FL, 32611, USA Idaho National Laboratory, Idaho Falls, ID, 83415, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2019 Received in revised form 7 January 2020 Accepted 13 January 2020 Available online 20 January 2020
This work presents a comparison between the microstructural evolution of three annular fast-reactor mixed-oxide (MOX) fuel pellets irradiated to varying burnups at the Fast Flux Test Facility (FFTF). Fuel pellets irradiated to 3.4%, 13.7%, and 21.3% fissions per initial metal atom (FIMA) were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. The cross-section of the low burnup pellet displayed minor structural changes, but the central annulus of the pellets at 13.7% and 21.3% FIMA shrank from their starting size. The high burnup fuel pellet featured streaking and porosity migration associated with columnar grain growth. The radial fission product distribution in each of the pellets had a higher number density of metallic particles >5 mm in diameter near the fuel centerline. Solid fission products in the fuel-cladding gap were observed in the low and intermediate burnup pellets. The low burnup sample showed minor accumulation of Ba in the gap, while the volatile Cs was primarily observed at the pellet surface. The intermediate burnup pellet displayed a porous mixture of fission products, consistent with the joint-oxide gain (JOG) that has been previously observed in fast-reactor MOX fuel pellets. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Advanced nuclear reactor concepts are pushing fuels to reach burnups much higher than traditional Light Water Reactors (LWRs) [1]. A primary challenge of implementing fuel systems is predicting the evolution of the fuel microstructure during extended exposure to harsh reactor operating conditions. Changes in grain structure, fission product accumulation, and other alterations have significant effects on the overall performance of the fuel. Pu-bearing mixed oxide (MOX) fuels used in fast-reactors typically have much higher enrichments (20e30 wt% Pu) than LWR counterparts (3e5% U-235) and have the benefit of eliminating Pu from long-term storage. There are many features that differentiate the microstructures of LWR oxide fuels from UePu fast-reactors MOX fuels. The peak operating temperatures of fast-reactor MOX fuels are commonly much higher than LWR UO2 (~2000 C for MOX vs. ~1200 C for UO2), leading to dramatic microstructural modification over the
* Corresponding author. 176 Rhines Hall, Department of Materials Science and Engineering, University of Florida, PO Box 116400, Gainesville, FL, 32611-6400, USA. E-mail address: [email protected]fl.edu (A. Aitkaliyeva). https://doi.org/10.1016/j.jnucmat.2020.152003 0022-3115/© 2020 Elsevier B.V. All rights reserved.
lifetime of the fuel [2]. Fast-reactor MOX fuels operating at high power experience pore migration by a vapor-transport mechanism that leads to the formation of a central void. The trails of the migrated pores and fission gas bubbles can form the radial boundaries of the columnar grains observed across the pellet radius [3]. The formation of secondary fission product phases varies between the two fuel types due to the differences in fission product yields of splitting U-235 or Pu-239, the temperature profiles, and burnup of the fuels [4]. The authors have previously performed extensive characterization on the annular fast-reactor MOX fuel pellets to analyze the microstructure and fission product distribution at low [5], intermediate [6], and high burnups [7]. The microstructure of the low burnup fuel pellet showed minimal changes with respect to the grain structure of the fuel pellet and did not reach the threshold conditions needed to initiate restructuring. At the intermediate burnup, the peak centerline temperature was considerably lower than the other pellets examined, leading to a significant reduction in the size of the pellet annulus due to fission product accumulation. Only the high burnup pellet experience significant microstructural modification during operation, leading to the formation of columnar grains near the pellet center and a cauliflower-like
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high burnup structure on the pellet rim. Solid fission products characterized were sorted into three distinct categories. The five metal precipitates (FMPs), otherwise known as the five-metal epsilon (ε) phase, white inclusions, and noble metal precipitates, are composed of the group of metallic fission products including Mo, Ru, Rh, Pd, and Tc. The crystal structure of the FMPs is primarily the ε-Ru hexagonal structure, and their composition has been shown to vary across burnups. An insoluble oxide perovskite phase, called Grey Phase (GP), rich in Ba, Zr, and other oxide forming rare earth elements typically assumes the perovskite BaZrO3 structure [4]. A less common Pd-rich metallic precipitate (PdP) has been shown to form at higher burnups in fast reactor MOX fuels, alloying with Te and other trace metallic species [6,8e10]. The crystal structure is believed to be a-Pd type face-centered cubic (FCC) structure [11], though no crystallographic characterization of the phase has been performed to confirm this hypothesis. While each manuscript provides an in-depth analysis of the features specific to those fuel pellets, the focus of this work is to provide a side-by-side examination of the microstructural features in each of the fuel burnups to provide direct observation of the MOX fuel evolution. Discussion of the previously published microstructural changes and solid fission product phases will focus on the evolution with respect to the operating lifetimes. Both qualitative and quantitative analysis of the secondary phases is discussed with the intent of determining trends based on fuel burnup, radial position, and operating temperature. Finally, the solid fission products accumulated in the fuel-cladding gap will be examined to provide a complete radial analysis of the fuel pellet from center to cladding edge. 2. Experimental methods 2.1. Sample background The fuel pellets examined in this work were irradiated as part of a core demonstration experiment at the Fast Flux Test Facility (FFTF) in the late 1980s [12]. Following irradiation, pins from the sub-assemblies FO-2 and ACO-3 were retrieved and sectioned at varying heights along the fuel column. Sections were extracted from positions at 40.1, 68.9, and 89.1 cm from the bottom of the fuel column to obtain a spread of approximately mid-height, ¾ height, and near the top of the pin, respectively [13]. This was done to obtain a broad range of burnup and heating rate conditions of the samples. Information about the specifics of the samples used in this analysis is listed in Table 1. The samples obtained were irradiated to burnups of ~3.4%, 13.7%, and 21.3% FIMA. The low burnup sample was collected from near the top of a medium-burnup pin from the FO-2 sub-assembly, while the pellets at 13.7% and 21.3% FIMA were taken from the top and ¾ height of a pin from sub-assembly ACO-3, respectively. Linear Heat Generation Rates (LHGRs) were estimated based on the fuel pellet position within the pins [12]. The fresh fuel pellets were annular and clad with HT-9 stainless-steel. Peak
Table 1 Comparison of the experimental parameters for the MOX fuel pellets examined. Sample (Subassembly)
A (FO-2) B (ACO-3) C (ACO-3)
End Burnup (% FIMA) Peak Linear Heat Generation Rate (kW m1) Peak Fuel Center Temp. ( C) Peak Fuel Surface Temp. ( C) Starting Composition (wt%) (Pu/U þ Pu) Starting Oxygen-to-metal Ratio Distance from Bottom of Fuel Column (cm)
3.4 26.7 1675 979 26 1.96 89.1
13.7 23.8 1265 792 29 1.95 89.1
21.3 35.6 1866 873 29 1.95 68.9
centerline and surface temperatures for the fuel pellets were initially calculated using the SAFE simulation code [13], but the high burnup fuel pellet temperature profile was later modelled using finite element analysis [14]. Note that the starting composition and oxygen-to-metal ratio of the fuels are slightly different. The authors recognize that the ideal scenario would be to directly compare samples with identical starting conditions, temperature profiles, and cover more incremental increases in the burnup. However, due to the nature of the samples and differences in irradiation campaigns such task cannot be completed. Thus, the authors elected to compare the selected samples to provide unique and valuable insight related to the long-term behavior of MOX fuels. 2.2. SEM/EDS analysis After sectioning, each fuel pellet was reduced to 1 mm in thickness, mounted in 32 mm stainless-steel metallography mounts, and back potted with epoxy under vacuum to preserve the structural integrity. Surface preparation for the samples have been previously described in their respective manuscripts [5e7]. Backscattered electron (BSE) SEM micrographs were collected using a JEOL JSM 7000F field emission microscope equipped with an Oxford X-Max 50 EDS system. Each pellet cross-section was imaged using an electron beam accelerating voltage of 20 kV and micrographs at 100 magnification stitched together using the Adobe Photoshop® software package. Higher magnification micrographs (>1000 ) were collected at several locations along the pellet radius to observe the microstructure and solid fission products in the region. Radial positions are described as normalized relative radii, being denoted by the term r/r0 with a value of r/r0 ¼ 0 corresponding to the pellet center and r/r0 ¼ 1 being the pellet surface. EDS maps were collected from each of the radial positions imaged for SEM analysis to observe the formation behavior of solid fission product phases. Each map consists of a minimum of 100 frames with a dwell time of 150 ms Wavelength dispersive X-ray spectroscopy (WDS) was used to verify the identities of elements identified with EDS and mitigate issues related to X-ray energy peak overlap. Composite elemental maps were produced by combining individual EDS maps using the NIH ImageJ software package [15]. Secondary phases were measured using the ImageJ Particle Analysis tool to determine the phase area, area fraction, and number density within each region. Average diameter calculations were performed by using the area of the FMPs and approximating them as perfect circles. Though not an ideal estimation, it provided the authors with a standardized means to measure and discuss the particle diameters. In instances where the particle shapes were complex and elongated, the average diameters were not calculated. 3. Results and discussion 3.1. Pellet cross-sections When comparing the cross-sections of the three fuel pellets shown in Fig. 1, the most noticeable difference is the size of the central void. The as-fabricated diameter of the central annulus was measured at ~1.40 mm for the pellets from sub-assembly FO-2 and ~1.47 mm for ACO-3. Fig. 1 shows the side-by-side comparison of the three fuel pellet cross-sections stitched together from BSE SEM micrographs. The low burnup fuel pellet in Fig. 1a was irradiated to 3.4% FIMA with a calculated peak centerline temperature of 1675 C and surface temperature of 979 C, showing very little change in the overall structure of the pellet. Several radial cracks have formed, and the central annulus is close to as-fabricated fuel pellet at ~1.40 mm. The pellet irradiated to 13.7% FIMA in Fig. 1b has
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Fig. 1. Stitched BSE-SEM cross-section overviews for the fuel pellets irradiated to a) 3.4%, b) 13.7% and c) 21.3% FIMA.
undergone more notable changes, with the cracking along the pellet edge being more extensive and the central void shrinking to ~0.53 mm in diameter. At the highest burnup of 21.3%FIMA in Fig. 1c, the structural integrity of the fuel pellet has significantly deteriorated. Fragments of the fuel in the top-left and bottom-right quadrants of the image have fallen away during sample preparation. The central void is significantly larger than that of Fig. 1b, measuring at 1.05 mm in diameter, though it is smaller than the starting diameter of 1.47 mm. In Fig. 1a and b, the darker ring around the outside of the fuel pellet corresponds to the HT-9 stainless-steel cladding. The ring is absent from Fig. 1c because of pellet separation from the cladding during sample preparation steps. There is very little published literature describing the central void behavior for annular fuel pellets [16,17], and it is difficult to make definitive comparisons of the observed behavior in these experiments to the previously examined pellets. Fuel swelling is directly impacted by the release of volatile fission products at high temperatures. The fuel pellet irradiated to 13.7% FIMA has a significantly lower peak centerline temperature and LHGR (1265 C and 23.8 kW m1, respectively) than the sample at 21.3% FIMA (1866 C and 35.6 kW m1). The higher temperatures lead to a higher fractional release of volatile fission products and fission gases in the high burnup fuel, therefore limiting the shrinkage of the central hole for the high burnup sample. The high temperatures of the high burnup fuel also facilitate the porosity-vapor migration mechanism in which porosity collects near the fuel centerline and causes the central hole to increase in diameter. To highlight radial changes of the fuel microstructures, Fig. 2 offers a close-up view of the pellets. The low burnup fuel pellet in Fig. 2a resembles the microstructure of the fresh, unirradiated pellet. No signs of porosity migration or columnar grain formation are apparent, indicating that restructuring did not occur. Grain boundaries near the fuel centerline have become more pronounced, while the outer half of the pellet radius displays porosity primarily resulting from the sintering process and particle pullout during sample preparation. Fig. 2b is the radial profile of the sample irradiated to 13.7% FIMA. Pores several micrometers in diameter have started to form near the central void along grain boundaries, highlighting that the grains are equiaxed and indicating that no restructuring has taken place. The high burnup fuel pellet in Fig. 2c shows clear porosity migration and accumulation near the central void, with the streaking being characteristic of the columnar grain formation. Columnar grain growth in fast-reactor MOX fuels is primarily a function of LHGR. The threshold power output for the restructuring process to occur is approximately 35 kW m1 [2], so it is within
expectations that only the high burnup fuel pellet displays the porosity migration and columnar grain formation characteristics. Near the fuel centerline in the pellets irradiated to 3.4% and 13.7% FIMA, the grains are equiaxed with fission gas bubbles collecting on the boundaries. This behavior does not occur near the centerline of the high burnup fuel pellet due to fission gas release and columnar grain formation. 3.2. SEM/EDS solid fission product analysis Higher magnification micrographs from several radial positions were acquired to further characterize the microstructure and solid fission products in the fuel. The approximate locations for each of the radial positions are outlined in black on Fig. 2. Note that the boxes are not meant to identify the exact location of the images collected, but instead to illustrate an equivalent radial position along the fuel. Fig. 3 shows the BSE SEM micrographs and combined EDS maps for the three fission product phases across the different burnups. The authors would again like to emphasize that the results are not directly comparable across the fuel pellets due to varying axial positions and temperature profiles, but important structural differences can be gleaned from the gathered data. At the lowest burnup of 3.4% FIMA, only the metallic FMPs are observed at the magnifications used to collect the EDS maps in Fig. 3a. Porosity near the central void is relatively fine and exists primarily along the grain boundaries. As burnup increases to 13.7% (Figs. 3b) and 21.3% FIMA (Fig. 3c), the perovskite GP forms near the fuel centerline, while the PdPs become visible in the outer half of the fuel pellets. Aside from the introduction of the GP and PdPs, the most clearly observable changes resulting from the elevated burnup pertains to the size and number density of FMPs and pores. Qualitatively, the FMPs are much finer and appear in larger quantities in the EDS maps of Fig. 3a. As burnup increases and more solid fission products are accumulated, the FMP forming elements start to agglomerate to form larger precipitates in addition to nucleating new particles. The formation of porosity on the outer rim of the pellets also appears to be burnup and temperature dependent. On the outer rim of the low burnup pellet (r/r0 ¼ 0.80) in Fig. 3a, the structure appears to be fully dense with the existing pores likely formed during pellet fabrication. The shading difference in the composite map can be attributed to the topography of the specimen but can be also a result of insufficient contrast of the image. At r/ r0 ¼ 0.90 in Fig. 3b, the porosity is very fine but more evident than the lower burnup pellet. The pellet irradiated to 21.3% FIMA in Fig. 3c has developed the highest porosity of the pellets as demonstrated by the wide size distribution of pores in the micrograph.
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Fig. 2. Radial SEM micrograph traverses for the MOX pellets irradiated to a) 3.4%, b) 13.7% and c) 21.3% FIMA. The black boxes outline the regions designated for closer SEM examination.
Fig. 3. BSE-SEM micrographs and composite EDS maps for fission product phases observed in the fuel pellet irradiated to a) 3.4% b) 13.7%, and c) 21.3% FIMA at several positions along the fuel pellet radius.
Following qualitative analysis of the solid fission products, quantitative measurements were collected to analyze the size distribution of the particles. Histograms for the particle sizes are shown in Fig. 4 for each of the regions examined in Fig. 3. The provided data was measured using the micrographs shown Fig. 3, so it is important to state these results are not necessarily
representative of the entire fuel pellet due to the limited sampling area. The FMPs analyzed in the pellet irradiated to 3.4% FIMA are shown in Fig. 4a and b. Near the pellet center (Fig. 4a), most of the particles have an area of less than 5 mm2 with the highest number of counts being between 1 and 2 mm2. Moving toward the intermediate radius, the precipitates are still primarily smaller than 5 mm2,
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Fig. 4. Histograms of the particle size distributions for the five metal precipitates (FMPs), perovskite grey phase, (GP) and Pd-rich precipitates (PdPs) for the MOX fuel pellets analyzed. The particles sizes are tabulated and compared for the pellets irradiated to 3.4% (a, b), 13.7% (cee), and 21.3% FIMA (feh).
though the curve has a wider distribution and the frequency of larger particles increases. Because no solid fission product phases are visible in the outermost region examined in the low burnup fuel pellet, no histogram is provided. Each of the three solid fission product phases were observed in the fuel pellet irradiated to 13.7% FIMA. Fig. 4c shows the measurements for the GP and FMPs analyzed near the pellet center. Both the FMPs and GP particles are primarily smaller than 10 mm2, though the size distribution for the GP has a more gradual decrease in the frequency of larger particles. The overall shape of the FMPs and GP distribution curve in the intermediate region (Fig. 4d) is similar to the observations made near the fuel centerline (Fig. 4c), though the total counts for both phases are significantly lower. The PdPs also start to appear in the intermediate region of the fuel, though the few particles present are all less than 10 mm2. On the outer rim of the pellet in Fig. 4e, the GP is no longer measurable in the region which leaves the FMPs and PdPs as the only observed solid fission products. The counts for the FMPs smaller than 10 mm2 in the region are higher than the other two analyzed positions, and the overall number of PdPs in the micrograph are slightly higher as well. Near the central void in the pellet irradiated to 21.3% FIMA (Fig. 4f), the GP particles are the most prominent solid fission
product phase in the region. The phase is primarily less than 10 mm2 in area, but several larger particles between 20 and 150 mm2 have formed. FMPs in the area exist in fewer numbers than those observed in the lower burnup fuels. The histogram in Fig. 4g highlights the changes that occurred near the intermediate radius of the fuel. The GP has transitioned to a small collection of particles less than 10 mm2, while the fine FMPs are the most frequently measured phase in the region. Particles between 30 mm2 and 100 mm2 are also present, though typically less than 5 counts are observed for each size. On the outer rim of the fuel pellet in Fig. 4h, the number density of FMPs decreases while the PdPs continue to form. The PdPs observed in each region of the high burnup fuel pellet and across the different burnups are typically less than 10 mm2 in area, with no particles greater than 20 mm2 observed. Overall precipitate area fractions, average areas, average diameters, and peak diameters can be seen in Table 2. The ε-phase FMPs are larger at the higher burnups as compared to the sample evaluated at 3.4% FIMA. Average particle diameters are on the order of 1e2 mm with relatively low standard deviations of approximately ±0.6 mm in the low burnup pellet, while the FMPs in the samples at 13.7% and 21.3% FIMA measure between 3 and 4 mm with deviations commonly greater than 60% of the average diameter. The differences in FMP diameter between the
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Table 2 Measurements of precipitates from each of the regions examined in Fig. 3.
intermediate and high burnup fuel samples is minimal when compared to the low burnup pellet. Metallic precipitate formation behavior is expected to change as a function of radial position along the thermal gradient [10,18,19], decreasing in size and area fraction at lower fuel temperatures. However, the analysis performed in this work cannot definitively come to that conclusion. In the case of all three fuel pellets, the spread of average area and diameters for the particles are well within a single standard deviation of each other. When looking at the histograms in Fig. 4, the high number of counts from fine precipitates skews the average towards much smaller particle diameters while the larger precipitates create a broad spread. While it cannot be definitively stated that particle size decreases with decreasing temperature, it is reasonable to conclude that elevated temperatures produce a higher number density of particles greater than 5 mm in diameter. 3.3. Pellet-cladding gap fission product analysis The pellet edge near the fuel-cladding gap was examined for the fuel pellets irradiated to 3.4% and 13.7% FIMA to analyze the diffusion behavior of fission products in the region. Because the cladding was not retained in the high burnup fuel sample at 21.3% FIMA, any fission products that accumulated in the region were lost during sample preparation. Since the fuels were subjected to physical and chemical processes, such as self-irradiation damage and He build-up from alpha decay of the actinides, microstructure of the fuels was likely altered by ageing. At the time of the examination, the fuels have been in storage for 40 years, which is sufficiently long to dismiss effects from short-lived fission products (e.g. 140Ba, 134Ce, 134Cs, etc.). The medium-lived fission products such as 103Ru and 137Cs, on the other hand, can contribute to the observed EDS signal and contribute to any of the observed signals, which is discussed next.
Fig. 5a shows the BSE SEM micrograph of the fuel-cladding gap of the pellet irradiated to 3.4% FIMA. The low burnup fuel does not appear to contain significant concentrations of FMP forming elements, as shown by the EDS maps in Fig. 5b through 5f. The GP forming elements Ba and Zr show contrasting behavior, with the fuel displaying a prominent Ba layer between the fuel and cladding (Fig. 5g), alongside a map with very little Zr (Fig. 5h). The observed behavior can be explained by the decay of Cs into Ba, which would explain their proximity. The EDS map for Cs in Fig. 5i shows a concentrated layer located on the pellet surface. The product Te (Fig. 5j) does not appear to be enriched beyond the background signal of the fuel. Fig. 5k through 5m show the constituent elements of the HT-9 stainless-steel cladding. Fig. 5k shows that the Fe has very limited interaction with the fuel in terms of bulk diffusion, while the oxide forming elements Cr and Mn (Fig. 5l and m, respectively) have segregated to the pellet surface. At the burnup of 13.7% FIMA, the fission product layer near the fuel-cladding gap becomes well developed. The region has undergone significant microstructural modification as demonstrated by the micrograph in Fig. 6a. The FMP forming elements begin to demonstrate a very different behavior when compared to the lower burnup fuel. Ru is mostly absent from the region (Fig. 6b) while Rh and Pd (Fig. 6c and d) have formed a dense layer on the surface of the cladding with smaller particles in the solid layer that fills the fuel-cladding gap. This solid layer is a complex mix of volatile and oxide-forming fission products. Fig. 6e shows that Mo has alloyed with Rh and Pd on the cladding surface, while also incorporating into the mixed layer of Ba, Cs, and Te (Fig. 6g, i and j, respectively). The cladding elements in Fig. 6k through 6h show similar behavior to that of the low burnup fuel, though the Cr appears to have depleted near the surface of the cladding. The layer of fission product compounds found in the fuelcladding gap is commonly referred to as the joint oxide-gain
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Fig. 5. Examination of the fuel-cladding gap in the MOX fuel pellet irradiated to 3.4% FIMA. A) BSE SEM micrgraph of the gap, along with the EDS maps of the fission products b) Ru, c) Rh, d) Pd, e) Mo, f) Tc, g.) Ba, h) Zr, i) Cs, and j) Te, along with HT-9 cladding components k) Fe, l) Cr, and m) Mn.
Fig. 6. Examination of the solid fission products in the fuel-cladding gap from the MOX fuel pellet irradiated to 13.7% FIMA. A) BSE SEM micrgraph of the gap, along with the EDS maps of the fission products b) Ru, c) Rh, d) Pd, e) Mo, f) Tc, g.) Ba, h) Zr, i) Cs, and j) Te, along with HT-9 cladding components k) Fe, l) Cr, and m) Mn.
(JOG) [20,21]. The JOG is a complex structure that is not well understood at this time but is found to contain solid Cs and Mo-based compounds (Cs2MoO4, Cs2Mo2O7), volatile fission products, oxide forming elements, and Pd-bearing species. The presence of the JOG has important impact on the thermal behavior of the pin, as modelling has demonstrated its presence lowering the peak centerline temperature by 200e300 C [14,22,23]. It is important to note that the pins were cut and polished using water-based solutions. The JOG layer is highly soluble in water, so some or all oxide layer may have been washed in the examined samples as a result. Therefore, the results presented here are not meant to be entirely representative of JOG formation behavior in fast-reactor MOX fuels. In the low burnup fuel pellet, the gap is primarily filled with the oxide forming species. Gamma scans results from the FO-2 fuel pin indicated redistribution of Cs towards the lower half of the pin, which is not relevant to the examined fuel specimen (the fuel was prepared from the top of the FO-2 pin). Though Cs was present on the surface of the fuel pellet, there is no enrichment of Mo in the area. Molybdenum primarily travels to the fuel-cladding gap in the form of gaseous oxides that are produced when the fuel burnup is greater than ~7% FIMA [20]. At 13.7% FIMA, the JOG layer more closely resembles what has been previously described in the literature. The dense layer formed
between the fuel and cladding is rich in Cs, Te, and oxide forming elements (Mo, Ba, and Ce). The gamma scan of ACO-3 pin indicated significant axial migration of Cs towards both depleted stacks. Thus, presence of Ba in the region is likely due to the decay of gaseous Cs that migrated to the fuel-cladding gap. However, Ba oxides have been shown to demonstrate volatile behavior due to thermal decomposition. The compound BaO2 behaves similarly to that of MoOx type compounds, in that the compound is volatile at reactor operating temperatures. The boiling point of BaO2 is ~800 C, but it decomposes into BaO þ O2 when the temperature exceeds 820 C [24,25]. The metallic Pd coated on the cladding surface was likely transported to the fuel-cladding gap via gaseous diffusion from the high temperature fuel regions [26]. It is possible that the JOG formation contributed to the overall failure of the cladding in the sample irradiated to 21.3% FIMA. The continued accumulation of volatile fission products and cladding oxidation in conjunction with the high dose received would likely lead to brittle fracture after extended irradiation. 4. Conclusions The work presented here characterizing fast-reactor MOX fuel pellets across a range of burnups demonstrates the clear evolution
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that the fuel undergoes during its operating lifetime. The grain structure in the low burnup fuel showed little alteration outside of fission gas bubbles collecting along grain boundaries, while the pellet irradiated to 21.3% FIMA displayed features consistent with columnar grain growth. Solid fission product phases in the intermediate and high burnup fuel pellets were on average larger than the low burnup fuel, while the number density of precipitates greater than 5 mm in diameter formed more frequently near the fuel centerline at all burnups. In the fuel cladding gap, the low burnup fuel shows the early accumulation of volatile and oxide forming fission products, with the JOG formation in the intermediate burnup displayed a complex mix of fission products consistent with previous findings. These results will continue to grow the body of knowledge associated with fast-reactor MOX fuels, improving the understanding of the fuel for implementation into future fuel performance models. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Riley J. Parrish: Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Fabiola Cappia: Writing - review & editing. Assel Aitkaliyeva: Conceptualization, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgements This work was supported by the U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07- 051D14517 as part of a Nuclear Science User Facilities experiment. The authors would also like to acknowledge Dr. Jason Harp of Oak Ridge National Laboratory for the valuable feedback in the preparation of this manuscript. The authors would also like to thank Alexander Winston, Dr. Lingfeng He, Dr. Brandon Miller, Karen Wright, Miles Cook, Jeffrey Bailey, James Madden, Alexander Pomo, Nicholas Bolender, Mark Taylor, and JoAnn Grimmett of the Idaho National Laboratory for their assistance at various stages of this project. References [1] T.R. Allen, K. Sridharan, L. Tan, W.E. Windes, J.I. Cole, D.C. Crawford, Materials challenges for generation IV nuclear energy systems, Nucl. Technol. 162 (2008) 342e357, accessed, https://www.tandfonline.com/doi/pdf/10.13182/ NT08-A3961?needAccess¼true. (Accessed 30 April 2018). [2] Y. Guerin, Fuel performance of fast spectrum oxide fuel, in: R.J.M. Konings, T.R. Allen, R.E. Stoller, S. Yamanaka (Eds.), Compr. Nucl. Mater., first ed., Elsevier Science, 2012, pp. 547e578, https://doi.org/10.1016/B978-0-08056033-5.00043-4. [3] D.R. Olander, Fundamental Aspects of Nuclear Reactor Fuel Elements, 1976, https://doi.org/10.2172/7343826. [4] H. Kleykamp, The chemical state of the fission products in oxide fuels, J. Nucl. Mater. 131 (1985) 221e246, https://doi.org/10.1016/0022-3115(85)90460-X.
[5] R. Parrish, X. Liu, A. Winston, J. Harp, A. Aitkaliyeva, Radial microstructural evolution in low burnup fast reactor MOX fuel, J. Nucl. Mater. 523 (2019) 182e188, https://doi.org/10.1016/j.jnucmat.2019.06.009. [6] R. Parrish, K. Wright, A. Winston, C. McKinney, J. Harp, A. Aitkaliyeva, Characterization of solid fission products in 13.7% FIMA MOX fuel using electron microscopy techniques, J. Nucl. Mater. 524 (2019) 67e79, https://doi.org/ 10.1016/j.jnucmat.2019.06.042. [7] R.J. Parrish, A.J. Winston, J.M. Harp, A. Aitkaliyeva, TEM characterization of high burnup fast-reactor MOX fuel, J. Nucl. Mater. 527 (2019), https://doi.org/ 10.1016/j.jnucmat.2019.151794. [8] J.I. Bramman, R.M. Sharpe, D. Thom, G. Yates Ukaea, Metallic fission-product inclusions in irradiated oxide fuels, J. Nucl. Mater. (1968) 201e215, https:// doi.org/10.1016/0022-3115(68)90045-7. [9] D. Cui, V.V. Rondinella, J.A. Fortner, A.J. Kropf, L. Eriksson, D.J. Wronkiewicz, K. Spahiu, Characterization of alloy particles extracted from spent nuclear fuel, J. Nucl. Mater. 420 (2012) 328e333, https://doi.org/10.1016/ j.jnucmat.2011.10.015. [10] H. Kleykamp, The chemical state of fission products in oxide fuels at different stages of the nuclear fuel cylcle, Nucl. Technol. 80 (1988) 412e422, https:// doi.org/10.13182/NT88-A34065. [11] H. Kleykamp, Constitution and thermodynamics of the Mo-Ru, Mo-Pd, Ru-Pd and Mo-Ru-Pd systems, J. Nucl. Mater. 167 (1989) 49e63, https://doi.org/ 10.1016/0022-3115(89)90424-8. [12] A.E. Bridges, A.E. Waltar, R.D. Leggett, R.B. Baker, J.L. Ethridge, A liquid-metal reactor core demonstration experiment using HT-9, Nucl. Technol. 102 (1993) 353e366, https://doi.org/10.13182/NT93-A17034. [13] M. Teague, B. Gorman, J. King, D. Porter, S. Hayes, Microstructural characterization of high burn-up mixed oxide fast reactor fuel, J. Nucl. Mater. 441 (2013) 267e273, https://doi.org/10.1016/j.jnucmat.2013.05.067. [14] M. Teague, M. Tonks, S. Novascone, S. Hayes, Microstructural modeling of thermal conductivity of high burn-up mixed oxide fuel, J. Nucl. Mater. 444 (2014) 161e169, https://doi.org/10.1016/j.jnucmat.2013.09.035. [15] W.S. Rasband, ImageJ, 1997-2018, https://imagej.nih.gov/ij/, 2019. [16] C.N. Venkiteswaran, V.V. Jayaraj, B.K. Ojha, V. Anandaraj, M. Padalakshmi, S. Vinodkumar, V. Karthik, R. Vijaykumar, A. Vijayaraghavan, R. Divakar, T. Johny, J. Joseph, S. Thirunavakkarasu, T. Saravanan, J. Philip, B.P.C. Rao, K.V. Kasiviswanathan, T. Jayakumar, Irradiation performance of PFBR MOX fuel after 112 GWd/t burn-up, J. Nucl. Mater. 449 (2014) 31e38, https:// doi.org/10.1016/j.jnucmat.2014.01.045. [17] A. Ishimi, K. Katsuyama, Y. Kihara, H. Furuya, Fuel performance of hollow pellets for fast breeder reactors, J. Nucl. Sci. Technol. 53 (2016) 951e956, https://doi.org/10.1080/00223131.2015.1085337. [18] M. Teague, B. Gorman, Utilization of dual-column focused ion beam and scanning electron microscope for three dimensional characterization of high burn-up mixed oxide fuel, Prog. Nucl. Energy 72 (2014) 67e71, https:// doi.org/10.1016/j.pnucene.2013.08.006. [19] M. Teague, B. Gorman, B. Miller, J. King, EBSD and TEM characterization of high burn-up mixed oxide fuel, J. Nucl. Mater. 444 (2014) 475e480, https:// doi.org/10.1016/j.jnucmat.2013.10.037. [20] M. Tourasse, M. Boidron, B. Pasquet, Fission product behaviour in phenix fuel pins at high burnup, J. Nucl. Mater. 188 (1992) 49e57, https://doi.org/ 10.1016/0022-3115(92)90453-R. [21] M. Naganuma, J. Noirot, D. Lespiaux, S. Koyama, T. Asaga, J. Rouault, G. Crittenden, C. Brown, J. Noirot, D. Lespiaux, J. Rouault, G. Crittenden, C. Brown, High Burnup Irradiation Performance of Annular Fuel Pins Irradiated in Fast Reactor PFR, International Atomic Energy Agency (IAEA), 2000. http://inis.iaea.org/search/search.aspx?orig_q¼RN:31062353. [22] T. Uwaba, T. Mizuno, J. Nemoto, I. Ishitani, M. Ito, Development of a mixed oxide fuel pin performance analysis code “cEDAR”: models and analyses of fuel pin irradiation behavior, Nucl. Eng. Des. 280 (2014) 27e36, https:// doi.org/10.1016/j.nucengdes.2014.08.032. [23] T. Ozawa, T. Abe, Development and verifications of fast reactor fuel design code CEPTAR, Nucl. Technol. 156 (2006) 39e55, https://doi.org/10.13182/ NT156-39. [24] I. Sato, H. Furuya, T. Arima, K. Idemitsu, K. Yamamoto, Behavior of fission products zirconium and barium in fast reactor fuel irradiated to high burnup, J. Nucl. Sci. Technol. 36 (1999) 775e780, https://doi.org/10.1080/ 18811248.1999.9726267. [25] C. Sari, Solubility and migration of fission product barium in oxide fuel, J. Nucl. Mater. 79 (1979) 255e259, https://doi.org/10.1016/0022-3115(79)90455-0. [26] I. Sato, H. Furuya, K. Amira, K. Idemitsu, K. Yamamoto, Behavior of metallic fission products in uranium- plutonium mixed oxide fuel, J. Nucl. Mater. 273 (1999) 239e247, https://doi.org/10.1016/S0022-3115(99)00071-9.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Comparison of the radial effects of burnup on fast reactor MOX fuel microstructure and solid fission products Riley J. Parrish a, Fabiola Cappia b, Assel Aitkaliyeva a, * a b
Department of Materials Science and Engineering, University of Florida, Gainesville, FL, 32611, USA Idaho National Laboratory, Idaho Falls, ID, 83415, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2019 Received in revised form 7 January 2020 Accepted 13 January 2020 Available online 20 January 2020
This work presents a comparison between the microstructural evolution of three annular fast-reactor mixed-oxide (MOX) fuel pellets irradiated to varying burnups at the Fast Flux Test Facility (FFTF). Fuel pellets irradiated to 3.4%, 13.7%, and 21.3% fissions per initial metal atom (FIMA) were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. The cross-section of the low burnup pellet displayed minor structural changes, but the central annulus of the pellets at 13.7% and 21.3% FIMA shrank from their starting size. The high burnup fuel pellet featured streaking and porosity migration associated with columnar grain growth. The radial fission product distribution in each of the pellets had a higher number density of metallic particles >5 mm in diameter near the fuel centerline. Solid fission products in the fuel-cladding gap were observed in the low and intermediate burnup pellets. The low burnup sample showed minor accumulation of Ba in the gap, while the volatile Cs was primarily observed at the pellet surface. The intermediate burnup pellet displayed a porous mixture of fission products, consistent with the joint-oxide gain (JOG) that has been previously observed in fast-reactor MOX fuel pellets. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Advanced nuclear reactor concepts are pushing fuels to reach burnups much higher than traditional Light Water Reactors (LWRs) [1]. A primary challenge of implementing fuel systems is predicting the evolution of the fuel microstructure during extended exposure to harsh reactor operating conditions. Changes in grain structure, fission product accumulation, and other alterations have significant effects on the overall performance of the fuel. Pu-bearing mixed oxide (MOX) fuels used in fast-reactors typically have much higher enrichments (20e30 wt% Pu) than LWR counterparts (3e5% U-235) and have the benefit of eliminating Pu from long-term storage. There are many features that differentiate the microstructures of LWR oxide fuels from UePu fast-reactors MOX fuels. The peak operating temperatures of fast-reactor MOX fuels are commonly much higher than LWR UO2 (~2000 C for MOX vs. ~1200 C for UO2), leading to dramatic microstructural modification over the
* Corresponding author. 176 Rhines Hall, Department of Materials Science and Engineering, University of Florida, PO Box 116400, Gainesville, FL, 32611-6400, USA. E-mail address: [email protected]fl.edu (A. Aitkaliyeva). https://doi.org/10.1016/j.jnucmat.2020.152003 0022-3115/© 2020 Elsevier B.V. All rights reserved.
lifetime of the fuel [2]. Fast-reactor MOX fuels operating at high power experience pore migration by a vapor-transport mechanism that leads to the formation of a central void. The trails of the migrated pores and fission gas bubbles can form the radial boundaries of the columnar grains observed across the pellet radius [3]. The formation of secondary fission product phases varies between the two fuel types due to the differences in fission product yields of splitting U-235 or Pu-239, the temperature profiles, and burnup of the fuels [4]. The authors have previously performed extensive characterization on the annular fast-reactor MOX fuel pellets to analyze the microstructure and fission product distribution at low [5], intermediate [6], and high burnups [7]. The microstructure of the low burnup fuel pellet showed minimal changes with respect to the grain structure of the fuel pellet and did not reach the threshold conditions needed to initiate restructuring. At the intermediate burnup, the peak centerline temperature was considerably lower than the other pellets examined, leading to a significant reduction in the size of the pellet annulus due to fission product accumulation. Only the high burnup pellet experience significant microstructural modification during operation, leading to the formation of columnar grains near the pellet center and a cauliflower-like
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high burnup structure on the pellet rim. Solid fission products characterized were sorted into three distinct categories. The five metal precipitates (FMPs), otherwise known as the five-metal epsilon (ε) phase, white inclusions, and noble metal precipitates, are composed of the group of metallic fission products including Mo, Ru, Rh, Pd, and Tc. The crystal structure of the FMPs is primarily the ε-Ru hexagonal structure, and their composition has been shown to vary across burnups. An insoluble oxide perovskite phase, called Grey Phase (GP), rich in Ba, Zr, and other oxide forming rare earth elements typically assumes the perovskite BaZrO3 structure [4]. A less common Pd-rich metallic precipitate (PdP) has been shown to form at higher burnups in fast reactor MOX fuels, alloying with Te and other trace metallic species [6,8e10]. The crystal structure is believed to be a-Pd type face-centered cubic (FCC) structure [11], though no crystallographic characterization of the phase has been performed to confirm this hypothesis. While each manuscript provides an in-depth analysis of the features specific to those fuel pellets, the focus of this work is to provide a side-by-side examination of the microstructural features in each of the fuel burnups to provide direct observation of the MOX fuel evolution. Discussion of the previously published microstructural changes and solid fission product phases will focus on the evolution with respect to the operating lifetimes. Both qualitative and quantitative analysis of the secondary phases is discussed with the intent of determining trends based on fuel burnup, radial position, and operating temperature. Finally, the solid fission products accumulated in the fuel-cladding gap will be examined to provide a complete radial analysis of the fuel pellet from center to cladding edge. 2. Experimental methods 2.1. Sample background The fuel pellets examined in this work were irradiated as part of a core demonstration experiment at the Fast Flux Test Facility (FFTF) in the late 1980s [12]. Following irradiation, pins from the sub-assemblies FO-2 and ACO-3 were retrieved and sectioned at varying heights along the fuel column. Sections were extracted from positions at 40.1, 68.9, and 89.1 cm from the bottom of the fuel column to obtain a spread of approximately mid-height, ¾ height, and near the top of the pin, respectively [13]. This was done to obtain a broad range of burnup and heating rate conditions of the samples. Information about the specifics of the samples used in this analysis is listed in Table 1. The samples obtained were irradiated to burnups of ~3.4%, 13.7%, and 21.3% FIMA. The low burnup sample was collected from near the top of a medium-burnup pin from the FO-2 sub-assembly, while the pellets at 13.7% and 21.3% FIMA were taken from the top and ¾ height of a pin from sub-assembly ACO-3, respectively. Linear Heat Generation Rates (LHGRs) were estimated based on the fuel pellet position within the pins [12]. The fresh fuel pellets were annular and clad with HT-9 stainless-steel. Peak
Table 1 Comparison of the experimental parameters for the MOX fuel pellets examined. Sample (Subassembly)
A (FO-2) B (ACO-3) C (ACO-3)
End Burnup (% FIMA) Peak Linear Heat Generation Rate (kW m1) Peak Fuel Center Temp. ( C) Peak Fuel Surface Temp. ( C) Starting Composition (wt%) (Pu/U þ Pu) Starting Oxygen-to-metal Ratio Distance from Bottom of Fuel Column (cm)
3.4 26.7 1675 979 26 1.96 89.1
13.7 23.8 1265 792 29 1.95 89.1
21.3 35.6 1866 873 29 1.95 68.9
centerline and surface temperatures for the fuel pellets were initially calculated using the SAFE simulation code [13], but the high burnup fuel pellet temperature profile was later modelled using finite element analysis [14]. Note that the starting composition and oxygen-to-metal ratio of the fuels are slightly different. The authors recognize that the ideal scenario would be to directly compare samples with identical starting conditions, temperature profiles, and cover more incremental increases in the burnup. However, due to the nature of the samples and differences in irradiation campaigns such task cannot be completed. Thus, the authors elected to compare the selected samples to provide unique and valuable insight related to the long-term behavior of MOX fuels. 2.2. SEM/EDS analysis After sectioning, each fuel pellet was reduced to 1 mm in thickness, mounted in 32 mm stainless-steel metallography mounts, and back potted with epoxy under vacuum to preserve the structural integrity. Surface preparation for the samples have been previously described in their respective manuscripts [5e7]. Backscattered electron (BSE) SEM micrographs were collected using a JEOL JSM 7000F field emission microscope equipped with an Oxford X-Max 50 EDS system. Each pellet cross-section was imaged using an electron beam accelerating voltage of 20 kV and micrographs at 100 magnification stitched together using the Adobe Photoshop® software package. Higher magnification micrographs (>1000 ) were collected at several locations along the pellet radius to observe the microstructure and solid fission products in the region. Radial positions are described as normalized relative radii, being denoted by the term r/r0 with a value of r/r0 ¼ 0 corresponding to the pellet center and r/r0 ¼ 1 being the pellet surface. EDS maps were collected from each of the radial positions imaged for SEM analysis to observe the formation behavior of solid fission product phases. Each map consists of a minimum of 100 frames with a dwell time of 150 ms Wavelength dispersive X-ray spectroscopy (WDS) was used to verify the identities of elements identified with EDS and mitigate issues related to X-ray energy peak overlap. Composite elemental maps were produced by combining individual EDS maps using the NIH ImageJ software package [15]. Secondary phases were measured using the ImageJ Particle Analysis tool to determine the phase area, area fraction, and number density within each region. Average diameter calculations were performed by using the area of the FMPs and approximating them as perfect circles. Though not an ideal estimation, it provided the authors with a standardized means to measure and discuss the particle diameters. In instances where the particle shapes were complex and elongated, the average diameters were not calculated. 3. Results and discussion 3.1. Pellet cross-sections When comparing the cross-sections of the three fuel pellets shown in Fig. 1, the most noticeable difference is the size of the central void. The as-fabricated diameter of the central annulus was measured at ~1.40 mm for the pellets from sub-assembly FO-2 and ~1.47 mm for ACO-3. Fig. 1 shows the side-by-side comparison of the three fuel pellet cross-sections stitched together from BSE SEM micrographs. The low burnup fuel pellet in Fig. 1a was irradiated to 3.4% FIMA with a calculated peak centerline temperature of 1675 C and surface temperature of 979 C, showing very little change in the overall structure of the pellet. Several radial cracks have formed, and the central annulus is close to as-fabricated fuel pellet at ~1.40 mm. The pellet irradiated to 13.7% FIMA in Fig. 1b has
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Fig. 1. Stitched BSE-SEM cross-section overviews for the fuel pellets irradiated to a) 3.4%, b) 13.7% and c) 21.3% FIMA.
undergone more notable changes, with the cracking along the pellet edge being more extensive and the central void shrinking to ~0.53 mm in diameter. At the highest burnup of 21.3%FIMA in Fig. 1c, the structural integrity of the fuel pellet has significantly deteriorated. Fragments of the fuel in the top-left and bottom-right quadrants of the image have fallen away during sample preparation. The central void is significantly larger than that of Fig. 1b, measuring at 1.05 mm in diameter, though it is smaller than the starting diameter of 1.47 mm. In Fig. 1a and b, the darker ring around the outside of the fuel pellet corresponds to the HT-9 stainless-steel cladding. The ring is absent from Fig. 1c because of pellet separation from the cladding during sample preparation steps. There is very little published literature describing the central void behavior for annular fuel pellets [16,17], and it is difficult to make definitive comparisons of the observed behavior in these experiments to the previously examined pellets. Fuel swelling is directly impacted by the release of volatile fission products at high temperatures. The fuel pellet irradiated to 13.7% FIMA has a significantly lower peak centerline temperature and LHGR (1265 C and 23.8 kW m1, respectively) than the sample at 21.3% FIMA (1866 C and 35.6 kW m1). The higher temperatures lead to a higher fractional release of volatile fission products and fission gases in the high burnup fuel, therefore limiting the shrinkage of the central hole for the high burnup sample. The high temperatures of the high burnup fuel also facilitate the porosity-vapor migration mechanism in which porosity collects near the fuel centerline and causes the central hole to increase in diameter. To highlight radial changes of the fuel microstructures, Fig. 2 offers a close-up view of the pellets. The low burnup fuel pellet in Fig. 2a resembles the microstructure of the fresh, unirradiated pellet. No signs of porosity migration or columnar grain formation are apparent, indicating that restructuring did not occur. Grain boundaries near the fuel centerline have become more pronounced, while the outer half of the pellet radius displays porosity primarily resulting from the sintering process and particle pullout during sample preparation. Fig. 2b is the radial profile of the sample irradiated to 13.7% FIMA. Pores several micrometers in diameter have started to form near the central void along grain boundaries, highlighting that the grains are equiaxed and indicating that no restructuring has taken place. The high burnup fuel pellet in Fig. 2c shows clear porosity migration and accumulation near the central void, with the streaking being characteristic of the columnar grain formation. Columnar grain growth in fast-reactor MOX fuels is primarily a function of LHGR. The threshold power output for the restructuring process to occur is approximately 35 kW m1 [2], so it is within
expectations that only the high burnup fuel pellet displays the porosity migration and columnar grain formation characteristics. Near the fuel centerline in the pellets irradiated to 3.4% and 13.7% FIMA, the grains are equiaxed with fission gas bubbles collecting on the boundaries. This behavior does not occur near the centerline of the high burnup fuel pellet due to fission gas release and columnar grain formation. 3.2. SEM/EDS solid fission product analysis Higher magnification micrographs from several radial positions were acquired to further characterize the microstructure and solid fission products in the fuel. The approximate locations for each of the radial positions are outlined in black on Fig. 2. Note that the boxes are not meant to identify the exact location of the images collected, but instead to illustrate an equivalent radial position along the fuel. Fig. 3 shows the BSE SEM micrographs and combined EDS maps for the three fission product phases across the different burnups. The authors would again like to emphasize that the results are not directly comparable across the fuel pellets due to varying axial positions and temperature profiles, but important structural differences can be gleaned from the gathered data. At the lowest burnup of 3.4% FIMA, only the metallic FMPs are observed at the magnifications used to collect the EDS maps in Fig. 3a. Porosity near the central void is relatively fine and exists primarily along the grain boundaries. As burnup increases to 13.7% (Figs. 3b) and 21.3% FIMA (Fig. 3c), the perovskite GP forms near the fuel centerline, while the PdPs become visible in the outer half of the fuel pellets. Aside from the introduction of the GP and PdPs, the most clearly observable changes resulting from the elevated burnup pertains to the size and number density of FMPs and pores. Qualitatively, the FMPs are much finer and appear in larger quantities in the EDS maps of Fig. 3a. As burnup increases and more solid fission products are accumulated, the FMP forming elements start to agglomerate to form larger precipitates in addition to nucleating new particles. The formation of porosity on the outer rim of the pellets also appears to be burnup and temperature dependent. On the outer rim of the low burnup pellet (r/r0 ¼ 0.80) in Fig. 3a, the structure appears to be fully dense with the existing pores likely formed during pellet fabrication. The shading difference in the composite map can be attributed to the topography of the specimen but can be also a result of insufficient contrast of the image. At r/ r0 ¼ 0.90 in Fig. 3b, the porosity is very fine but more evident than the lower burnup pellet. The pellet irradiated to 21.3% FIMA in Fig. 3c has developed the highest porosity of the pellets as demonstrated by the wide size distribution of pores in the micrograph.
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Fig. 2. Radial SEM micrograph traverses for the MOX pellets irradiated to a) 3.4%, b) 13.7% and c) 21.3% FIMA. The black boxes outline the regions designated for closer SEM examination.
Fig. 3. BSE-SEM micrographs and composite EDS maps for fission product phases observed in the fuel pellet irradiated to a) 3.4% b) 13.7%, and c) 21.3% FIMA at several positions along the fuel pellet radius.
Following qualitative analysis of the solid fission products, quantitative measurements were collected to analyze the size distribution of the particles. Histograms for the particle sizes are shown in Fig. 4 for each of the regions examined in Fig. 3. The provided data was measured using the micrographs shown Fig. 3, so it is important to state these results are not necessarily
representative of the entire fuel pellet due to the limited sampling area. The FMPs analyzed in the pellet irradiated to 3.4% FIMA are shown in Fig. 4a and b. Near the pellet center (Fig. 4a), most of the particles have an area of less than 5 mm2 with the highest number of counts being between 1 and 2 mm2. Moving toward the intermediate radius, the precipitates are still primarily smaller than 5 mm2,
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Fig. 4. Histograms of the particle size distributions for the five metal precipitates (FMPs), perovskite grey phase, (GP) and Pd-rich precipitates (PdPs) for the MOX fuel pellets analyzed. The particles sizes are tabulated and compared for the pellets irradiated to 3.4% (a, b), 13.7% (cee), and 21.3% FIMA (feh).
though the curve has a wider distribution and the frequency of larger particles increases. Because no solid fission product phases are visible in the outermost region examined in the low burnup fuel pellet, no histogram is provided. Each of the three solid fission product phases were observed in the fuel pellet irradiated to 13.7% FIMA. Fig. 4c shows the measurements for the GP and FMPs analyzed near the pellet center. Both the FMPs and GP particles are primarily smaller than 10 mm2, though the size distribution for the GP has a more gradual decrease in the frequency of larger particles. The overall shape of the FMPs and GP distribution curve in the intermediate region (Fig. 4d) is similar to the observations made near the fuel centerline (Fig. 4c), though the total counts for both phases are significantly lower. The PdPs also start to appear in the intermediate region of the fuel, though the few particles present are all less than 10 mm2. On the outer rim of the pellet in Fig. 4e, the GP is no longer measurable in the region which leaves the FMPs and PdPs as the only observed solid fission products. The counts for the FMPs smaller than 10 mm2 in the region are higher than the other two analyzed positions, and the overall number of PdPs in the micrograph are slightly higher as well. Near the central void in the pellet irradiated to 21.3% FIMA (Fig. 4f), the GP particles are the most prominent solid fission
product phase in the region. The phase is primarily less than 10 mm2 in area, but several larger particles between 20 and 150 mm2 have formed. FMPs in the area exist in fewer numbers than those observed in the lower burnup fuels. The histogram in Fig. 4g highlights the changes that occurred near the intermediate radius of the fuel. The GP has transitioned to a small collection of particles less than 10 mm2, while the fine FMPs are the most frequently measured phase in the region. Particles between 30 mm2 and 100 mm2 are also present, though typically less than 5 counts are observed for each size. On the outer rim of the fuel pellet in Fig. 4h, the number density of FMPs decreases while the PdPs continue to form. The PdPs observed in each region of the high burnup fuel pellet and across the different burnups are typically less than 10 mm2 in area, with no particles greater than 20 mm2 observed. Overall precipitate area fractions, average areas, average diameters, and peak diameters can be seen in Table 2. The ε-phase FMPs are larger at the higher burnups as compared to the sample evaluated at 3.4% FIMA. Average particle diameters are on the order of 1e2 mm with relatively low standard deviations of approximately ±0.6 mm in the low burnup pellet, while the FMPs in the samples at 13.7% and 21.3% FIMA measure between 3 and 4 mm with deviations commonly greater than 60% of the average diameter. The differences in FMP diameter between the
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Table 2 Measurements of precipitates from each of the regions examined in Fig. 3.
intermediate and high burnup fuel samples is minimal when compared to the low burnup pellet. Metallic precipitate formation behavior is expected to change as a function of radial position along the thermal gradient [10,18,19], decreasing in size and area fraction at lower fuel temperatures. However, the analysis performed in this work cannot definitively come to that conclusion. In the case of all three fuel pellets, the spread of average area and diameters for the particles are well within a single standard deviation of each other. When looking at the histograms in Fig. 4, the high number of counts from fine precipitates skews the average towards much smaller particle diameters while the larger precipitates create a broad spread. While it cannot be definitively stated that particle size decreases with decreasing temperature, it is reasonable to conclude that elevated temperatures produce a higher number density of particles greater than 5 mm in diameter. 3.3. Pellet-cladding gap fission product analysis The pellet edge near the fuel-cladding gap was examined for the fuel pellets irradiated to 3.4% and 13.7% FIMA to analyze the diffusion behavior of fission products in the region. Because the cladding was not retained in the high burnup fuel sample at 21.3% FIMA, any fission products that accumulated in the region were lost during sample preparation. Since the fuels were subjected to physical and chemical processes, such as self-irradiation damage and He build-up from alpha decay of the actinides, microstructure of the fuels was likely altered by ageing. At the time of the examination, the fuels have been in storage for 40 years, which is sufficiently long to dismiss effects from short-lived fission products (e.g. 140Ba, 134Ce, 134Cs, etc.). The medium-lived fission products such as 103Ru and 137Cs, on the other hand, can contribute to the observed EDS signal and contribute to any of the observed signals, which is discussed next.
Fig. 5a shows the BSE SEM micrograph of the fuel-cladding gap of the pellet irradiated to 3.4% FIMA. The low burnup fuel does not appear to contain significant concentrations of FMP forming elements, as shown by the EDS maps in Fig. 5b through 5f. The GP forming elements Ba and Zr show contrasting behavior, with the fuel displaying a prominent Ba layer between the fuel and cladding (Fig. 5g), alongside a map with very little Zr (Fig. 5h). The observed behavior can be explained by the decay of Cs into Ba, which would explain their proximity. The EDS map for Cs in Fig. 5i shows a concentrated layer located on the pellet surface. The product Te (Fig. 5j) does not appear to be enriched beyond the background signal of the fuel. Fig. 5k through 5m show the constituent elements of the HT-9 stainless-steel cladding. Fig. 5k shows that the Fe has very limited interaction with the fuel in terms of bulk diffusion, while the oxide forming elements Cr and Mn (Fig. 5l and m, respectively) have segregated to the pellet surface. At the burnup of 13.7% FIMA, the fission product layer near the fuel-cladding gap becomes well developed. The region has undergone significant microstructural modification as demonstrated by the micrograph in Fig. 6a. The FMP forming elements begin to demonstrate a very different behavior when compared to the lower burnup fuel. Ru is mostly absent from the region (Fig. 6b) while Rh and Pd (Fig. 6c and d) have formed a dense layer on the surface of the cladding with smaller particles in the solid layer that fills the fuel-cladding gap. This solid layer is a complex mix of volatile and oxide-forming fission products. Fig. 6e shows that Mo has alloyed with Rh and Pd on the cladding surface, while also incorporating into the mixed layer of Ba, Cs, and Te (Fig. 6g, i and j, respectively). The cladding elements in Fig. 6k through 6h show similar behavior to that of the low burnup fuel, though the Cr appears to have depleted near the surface of the cladding. The layer of fission product compounds found in the fuelcladding gap is commonly referred to as the joint oxide-gain
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Fig. 5. Examination of the fuel-cladding gap in the MOX fuel pellet irradiated to 3.4% FIMA. A) BSE SEM micrgraph of the gap, along with the EDS maps of the fission products b) Ru, c) Rh, d) Pd, e) Mo, f) Tc, g.) Ba, h) Zr, i) Cs, and j) Te, along with HT-9 cladding components k) Fe, l) Cr, and m) Mn.
Fig. 6. Examination of the solid fission products in the fuel-cladding gap from the MOX fuel pellet irradiated to 13.7% FIMA. A) BSE SEM micrgraph of the gap, along with the EDS maps of the fission products b) Ru, c) Rh, d) Pd, e) Mo, f) Tc, g.) Ba, h) Zr, i) Cs, and j) Te, along with HT-9 cladding components k) Fe, l) Cr, and m) Mn.
(JOG) [20,21]. The JOG is a complex structure that is not well understood at this time but is found to contain solid Cs and Mo-based compounds (Cs2MoO4, Cs2Mo2O7), volatile fission products, oxide forming elements, and Pd-bearing species. The presence of the JOG has important impact on the thermal behavior of the pin, as modelling has demonstrated its presence lowering the peak centerline temperature by 200e300 C [14,22,23]. It is important to note that the pins were cut and polished using water-based solutions. The JOG layer is highly soluble in water, so some or all oxide layer may have been washed in the examined samples as a result. Therefore, the results presented here are not meant to be entirely representative of JOG formation behavior in fast-reactor MOX fuels. In the low burnup fuel pellet, the gap is primarily filled with the oxide forming species. Gamma scans results from the FO-2 fuel pin indicated redistribution of Cs towards the lower half of the pin, which is not relevant to the examined fuel specimen (the fuel was prepared from the top of the FO-2 pin). Though Cs was present on the surface of the fuel pellet, there is no enrichment of Mo in the area. Molybdenum primarily travels to the fuel-cladding gap in the form of gaseous oxides that are produced when the fuel burnup is greater than ~7% FIMA [20]. At 13.7% FIMA, the JOG layer more closely resembles what has been previously described in the literature. The dense layer formed
between the fuel and cladding is rich in Cs, Te, and oxide forming elements (Mo, Ba, and Ce). The gamma scan of ACO-3 pin indicated significant axial migration of Cs towards both depleted stacks. Thus, presence of Ba in the region is likely due to the decay of gaseous Cs that migrated to the fuel-cladding gap. However, Ba oxides have been shown to demonstrate volatile behavior due to thermal decomposition. The compound BaO2 behaves similarly to that of MoOx type compounds, in that the compound is volatile at reactor operating temperatures. The boiling point of BaO2 is ~800 C, but it decomposes into BaO þ O2 when the temperature exceeds 820 C [24,25]. The metallic Pd coated on the cladding surface was likely transported to the fuel-cladding gap via gaseous diffusion from the high temperature fuel regions [26]. It is possible that the JOG formation contributed to the overall failure of the cladding in the sample irradiated to 21.3% FIMA. The continued accumulation of volatile fission products and cladding oxidation in conjunction with the high dose received would likely lead to brittle fracture after extended irradiation. 4. Conclusions The work presented here characterizing fast-reactor MOX fuel pellets across a range of burnups demonstrates the clear evolution
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that the fuel undergoes during its operating lifetime. The grain structure in the low burnup fuel showed little alteration outside of fission gas bubbles collecting along grain boundaries, while the pellet irradiated to 21.3% FIMA displayed features consistent with columnar grain growth. Solid fission product phases in the intermediate and high burnup fuel pellets were on average larger than the low burnup fuel, while the number density of precipitates greater than 5 mm in diameter formed more frequently near the fuel centerline at all burnups. In the fuel cladding gap, the low burnup fuel shows the early accumulation of volatile and oxide forming fission products, with the JOG formation in the intermediate burnup displayed a complex mix of fission products consistent with previous findings. These results will continue to grow the body of knowledge associated with fast-reactor MOX fuels, improving the understanding of the fuel for implementation into future fuel performance models. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Riley J. Parrish: Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Fabiola Cappia: Writing - review & editing. Assel Aitkaliyeva: Conceptualization, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding acquisition. Acknowledgements This work was supported by the U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07- 051D14517 as part of a Nuclear Science User Facilities experiment. The authors would also like to acknowledge Dr. Jason Harp of Oak Ridge National Laboratory for the valuable feedback in the preparation of this manuscript. The authors would also like to thank Alexander Winston, Dr. Lingfeng He, Dr. Brandon Miller, Karen Wright, Miles Cook, Jeffrey Bailey, James Madden, Alexander Pomo, Nicholas Bolender, Mark Taylor, and JoAnn Grimmett of the Idaho National Laboratory for their assistance at various stages of this project. References [1] T.R. Allen, K. Sridharan, L. Tan, W.E. Windes, J.I. Cole, D.C. Crawford, Materials challenges for generation IV nuclear energy systems, Nucl. Technol. 162 (2008) 342e357, accessed, https://www.tandfonline.com/doi/pdf/10.13182/ NT08-A3961?needAccess¼true. (Accessed 30 April 2018). [2] Y. Guerin, Fuel performance of fast spectrum oxide fuel, in: R.J.M. Konings, T.R. Allen, R.E. Stoller, S. Yamanaka (Eds.), Compr. Nucl. Mater., first ed., Elsevier Science, 2012, pp. 547e578, https://doi.org/10.1016/B978-0-08056033-5.00043-4. [3] D.R. Olander, Fundamental Aspects of Nuclear Reactor Fuel Elements, 1976, https://doi.org/10.2172/7343826. [4] H. Kleykamp, The chemical state of the fission products in oxide fuels, J. Nucl. Mater. 131 (1985) 221e246, https://doi.org/10.1016/0022-3115(85)90460-X.
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