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Three-dimensional bubble reconstruction in high burnup UO2
Journal Pre-proof Three-dimensional bubble reconstruction in high burnup UO2 Casey McKinney, Rachel Seibert, Grant Helmreich, Assel Aitkaliyeva, Kurt Terrani PII:
S0022-3115(19)31407-2
DOI:
https://doi.org/10.1016/j.jnucmat.2020.152053
Reference:
NUMA 152053
To appear in:
Journal of Nuclear Materials
Received Date: 1 November 2019 Revised Date:
6 January 2020
Accepted Date: 11 February 2020
Please cite this article as: C. McKinney, R. Seibert, G. Helmreich, A. Aitkaliyeva, K. Terrani, Threedimensional bubble reconstruction in high burnup UO2, Journal of Nuclear Materials (2020), doi: https:// doi.org/10.1016/j.jnucmat.2020.152053. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Three-Dimensional Bubble Reconstruction in High Burnup UO2* Casey McKinney a,b, Rachel Seibert a, Grant Helmreich a, Assel Aitkaliyeva b†, Kurt Terrani a a Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA b University of Florida, Gainesville, FL, 32611, USA Keywords: uranium dioxide, high burnup fuel, FIB tomography, fission gas release, 3D reconstruction. Abstract In light water reactor (LWR) UO2 fuels, the evolution of volatile fission products is one of the critical areas of fuel behavior that is yet to be fully understood. In UO2 irradiated to high burnups, it is well known that most released fission gases come from the central region of the fuel as opposed to the highly porous high burnup structure (HBS) on the periphery of the pellets. However, fuels with and without interconnected bubble networks at the fuel center showed high to moderate release fractions, which conceals the mechanisms responsible for the gas release in the latter scenario. In this work, focused ion beam tomography was used to investigate the threedimensional bubble structure in an irradiated LWR UO2 fuel pellet with high degree of fission gas retention so that the degree of bubble interconnection could be assessed. Six radial locations with different burnups and temperatures were serially sectioned and imaged to reconstruct the three-dimensional bubble structure. As expected, the highest porosity was observed at the periphery of the fuel (HBS). The porosity then decreased towards the pellet center, except for the centermost location. This location had a slightly higher porosity than its adjacent mid-radial location, which was attributed to the temperature difference between the two locations. This study provides a first-time volumetric evaluation of the porosity at different radial locations on a UO2 fuel pellet. During this investigation, no significant bubble interconnection was noted at any of the six radial locations. 1. Introduction Since UO2 has been the main fuel for light water reactors (LWRs) for decades, numerous investigations into its performance under different irradiation conditions have been conducted [1]. One of the critical areas of fuel behavior is the evolution of noble gas fission products, which tend to have low solubility in UO2. As a result of this low thermodynamic solubility, fission gas atoms diffuse through the matrix to the grain boundaries and pre-existing pores, where they can *
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-publicaccess-plan). † Corresponding author: [email protected]; Address: 176 Rhines Hall, Department of Materials Science and Engineering, University of Florida, PO Box 116400, Gainesville, FL, 32611-6400.
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nucleate intra- and inter-granular bubbles. When the porosity interconnects, fission gas can leave the fuel to the fuel-cladding gap and plenum. The gas atoms within the grains, referred to as intra-granular bubbles, can be trapped by irradiation-induced defects and/or smaller bubbles, and are subject to re-solution. Gas atoms along the grain boundaries, or inter-granular bubbles, can interconnect to form a tunnel network, through which the content can be released. The process of gas transport, bubble nucleation, growth, coalescence, and release is highly complex [2]. Early on, simple models describing diffusive fission gas release (FGR) were formulated [3] and continuously refined [4, 5] to capture the processes expected to occur in the fuel during reactor operation. Improvements in the understanding of fission gas transport and release in UO2 is often initiated by experimental observations [6, 7, 8] or analytical examinations of the efficacy of various governing processes [5, 9]. Many of the fundamental processes remain elusive, and much of the complexity remains uncaptured by the current computational framework. One such instance is the “conundrum” described in Section 3 of Ref. [2] that discusses FGR from high burnup fuel. It is now well known that the high burnup structure (HBS) [10] that develops at high burnups and low temperatures [11], does not result in significant FGR [12] due to the closed bubble structure up ~25% porosity [13]. On the other hand, fuel pellets that were operated at high linear heat ratings and experienced high temperatures at their centerline, form an interconnected network of bubbles. This interconnected network is considered to be the main source of FGR [14]. Recent examination of the microstructure of high burnup fuels, which operated at relatively low powers and showed modest FGR, did not show evidence for bubble interconnection at the center [15]. Therefore, the mechanism of FGR in this scenario is not well understood. This paper applies large-scale focused ion beam (FIB) tomography to study the distribution of bubbles inside high burnup UO2. The goal is to complement our current understanding by providing additional high-quality experimental observations to decipher the mechanism of microstructure evolution and FGR. Although FIB tomography does not offer sufficient resolution to observe 1–10 nm intragranular bubbles, it is very useful for observing large inter- and intragranular bubbles and therefore is well suited for the study of bubbles in HBS and central regions of the fuel, where typical average bubble diameters are in the range of 1–2 µm and 0.5–1 µm, respectively [2, 16]. Three-dimensional (3D) characterization methods offer unique means to gather data on the true structure of a material, as opposed to two-dimensional (2D) methods, which merely provide approximations. Synchrotron X-ray tomography has challenges with larger volumes of uraniumbased materials (which have a high scattering cross section due to high z), and neutron tomography lacks the spatial resolution needed for the study of fission gas bubbles. The availability of such facilities that can handle irradiated fuels is another challenge. Instead, FIB tomography, a destructive technique, offers the resolution required for this study. The micrographs gathered from the FIB tomography were used to reconstruct the bubble structure at different locations on a UO2 fuel pellet so that the existence of bubble interconnection could be determined. A unique fuel pin with high burnup and high degree of fission gas retention was chosen for this study. In addition to the interconnectivity of the bubbles, the reconstructions were used to assess the bubble volume fraction (porosity) as well as bubble shape and spacing. The investigations across the fuel pellet radius provided a way to characterize the bubble structure of
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regions experiencing different burnups and temperatures. While previous studies with relevant scales were only conducted in 2D [17], utilizing FIB tomography for this process provides a first-time volumetric evaluation of the porosity evolution as a function of burnup and temperature in a UO2 fuel pellet. It is expected that the data presented in this study and other data sets—to be generated on fuels with different microstructure, power, and temperature histories— are needed to provide a more complete picture of fission gas behavior and release in nuclear fuels. 2. Materials and Methods 2.1 Fuel Specimen Pedigree In this study, a segment of a fuel pin (E02) from the H. B. Robinson Unit 2 commercial pressurized water reactor irradiated to an average burnup of 72 MWd/kgU was analyzed. The UO2 fuel pellet encapsulated in Zircaloy-4 cladding was irradiated for seven cycles; its thorough irradiation history can be found in Ref. [18]. The end-of-life (EOL) measured FGR for the E02 fuel pin is reported at 2.1%. The power history of the central 2 m section of the E02 rod was used to estimate the fuel temperature profile in a previous study [19]. The estimated fuel centerline temperature alongside the Vitanza threshold [20] for 1% FGR is plotted in Figure 1. If fuel centerline conditions exceed those outlined by this threshold, the fuel would be considered to have high FGR. Also shown in the plot are the empirical modifications to the Vitanza threshold based on more recent high burnup fuel data that mark the onset of small amounts of FGR [21, 22]. The low fuel temperatures throughout the irradiation are expected to result in low FGR, making this a unique specimen to study bubble evolution in high burnup UO2. 1600
Fuel Centerline Temperature [°C]
1500 1400 1300 1200 Vitanza 1% FGR threshold
1100
empirical small FGR threshold
1000
H.B Robinson fuel segment from E02 pin
900 800 700 600
0
10
20
30
40
50
60
Burnup [MWd/kgUO 2]
Figure 1: Estimated fuel centerline temperature for central section of the E02 fuel pin from ref. [19] compared to Vitanza and the empirical small FGR thresholds.
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Since the central section of the rod is expected to operate at the highest power and temperature, the local FGR is expected to be higher than the integral value for the pin (>2.1%). Yet, the fuel centerline temperature in this section stays below the 1% threshold and barely goes above the empirical threshold for small (<1%) FGR. 2.2 Experiment Details When preparing the sample, a rectangular section of the fuel and cladding was removed, as can be seen in Figure 2 (for a detailed description of the sample and its preparation, see Ref. [19]). After removal from the hot cell, the sample was transferred to Oak Ridge National Laboratory’s (ORNL) Low Activation Materials Development and Analysis (LAMDA) Laboratory for microscopy [23]. The facility’s shielded FEI Quanta dual-beam FIB/scanning electron microscope (SEM), equipped with a tungsten filament, was used to section and image the fuel.
Figure 2: Micrograph of the six AOI locations on the axial cross section of the UO2 specimen. Superimposed on the image is the calculated EOL radial burnup profile. The fuel segment is separated into four different regions starting with the HBS on the left moving to the central region on the far right. See Table 1 for r/r0 and local burnup values.
In a previous work, Gerczak et al. designated the different regions of this fuel segment as HBS (r/r0 ~ 1.0–0.97), transition (r/r0 ~ 0.97–0.93), mid-radial (r/r0 ~ 0.93–0.45), and central (r/r0 ~ 0.45–0.25) [19]. Areas of interest (AOIs) were prepared at six separate radial locations based on Gerczak’s defined regions and accompanying electron backscatter diffraction (EBSD) data, the calculated burnup profile, and the average temperature profile [19, 24]. These locations with respect to the burnup profile can be seen overlaid on a micrograph of the fuel segment in Figure
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2. Since one of the main objectives was to investigate the probability of bubble interconnection in the HBS region, three areas within the HBS (locations 1–3 in Figure 2) were reconstructed to examine the bubble structure. These areas were positioned equidistantly to ensure that all relevant phenomena had been captured. Outside of the high burnup region, three additional locations were chosen for reconstruction, as illustrated by the white boxes labeled as locations 4– 6 in Figure 2. The location farthest from the HBS (location 6 in Figure 2) was chosen to assess possible interconnection at the hottest central region of the fuel pellet, where fission gas is expected to be released [25]. The other locations outside of the high burnup region (locations 4 and 5) were chosen to examine mid-radial regions that experienced varied conditions. Utilizing FIB tomography for this investigation provided a first-time volumetric evaluation of the bubble structure that showed the fuel’s response to different burnups and temperatures. Before sectioning could begin, the AOI had to be pre-milled to a size of 12×12×12 µm3 for investigation. After each AOI had been pre-milled (Figure 3), FEI’s Auto Slice and View software was used to automate the sectioning process. The Slice and View parameters were set to mill a 16×14×6 µm3 volume (denoted as x, y, and z in Figure 3) with 100 nm thick layers. The overestimation of the x and y parameters was set to compensate for instrument drift, and the underestimation of z was due to the manner in which the UO2 milled. Previous milling of the sample had shown that an underestimated z parameter would still mill the desired amount; therefore, the z parameter could be cut down to minimize milling time while still milling the desired volume. For the sectioning process, the ion beam was set at 30 kV and 0.5 nA to reduce curtaining effects, and the electron beam was set at 5 kV and 0.41 nA to get greater contrast between the bubbles and the fuel. This process produced stacks of micrographs that progressed sequentially through the volume of material that then needed to be reconstructed into the 3D bubble structure.
Figure 3: SEM micrograph of the AOI at location 3, outlined in white, after initial milling with Slice & View x,y,z notation shown.
2.3 MATLAB Reconstruction Construction of 3D bubble images from the series of 2D SEM images from each location was complicated by minor changes in position and imaging angle from image to image. A 5
customized transformation routine was developed using MATLAB in which the operator selected a set of fiducial marks (easily identifiable features or bubbles around the trenches that are unique to the area) in each micrograph. These points were then used to generate a unique transformation matrix for each micrograph including translation, rotation, and projection operations to align the corresponding fiducial marks for all micrographs acquired at each location. The transformed micrograph sets were then filtered to reduce noise (using Wiener filter) and to smooth minor variations in intensity (using Local Laplacian filter). The resolution of the SEM micrographs was on the order of 4.56 nm/pixel with a spacing between micrographs of 55–63 nm, where the slight discrepancy between spacing was caused by the instrument drift and thus accounted for in each reconstruction. Since the depth between images was greater than the image pixel size, composite intermediate images were created to produce an image stack with cubic voxels with edge lengths identical to the original SEM micrograph. These composite images were formed using cubic interpolation within the original images. Finally, 2-binning was applied to the image stack to reduce the computational burden of segmentation. Bubbles were segregated from the image stack using an adaptive threshold that automatically corrected for local variations in image brightness (MATLAB function “adaptthresh”). Morphological operations were then used to fill gaps (MATLAB function “imfill”) within bubbles and morphologically dilate them slightly (MATLAB function “imdilate”) so that the following edge-based active contour algorithm with a preference toward shrinking (MATLAB function “activecontour”) could find the bubble edges. In Figure 4, a filtered micrograph can be seen before and after this segmentation process was applied. This approach was preferred as the edges of bubbles tended to be well-defined even when their interior featured non-uniform intensity. Properties were then tabulated for each segregated bubble with 26-pixel connectivity, including volume, centroid position, and the principal axes of the best-fit ellipsoid.
Figure 4: SEM micrograph of location 3 after filtering (left) and segmentation (right).
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2.4 Bubble Shape Qualification Figure 5 illustrates the notation and categories used to sort the bubble shapes within the AOIs. Each bubble had three principal axis lengths (x, y, z) that were used to define its size and shape. The relationships between those axes were used to determine the bubble shape. The more spherical bubbles were defined as having three similar principal axis lengths (x ≅ y ≅ z). Diskshaped bubbles were defined as having two similar principal axis lengths that were greater than the third axis (x ≅ y > z). Footballs were also defined as having two similar principal axis lengths, but with a third axis longer than the others (x ≅ y < z). If the similar axes of these shape types were within ±50% of each other, they were then classified as either spheroid, football, or disk, contingent on the specified criterion. In the case of the football-shaped bubbles, an additional step was implemented to ensure the accuracy of the shape identification. If the initial similarity threshold of 50% was not met in football-shaped bubbles, the bubble could still appear as a football if the third axis were significantly larger than the two similar axes. To avoid confusion, such bubbles were defined by relating the difference between the two smaller axes to the larger axis with the following equation: 20%
(1)
If the criterion in Equation 1 was met, the bubble was identified as a football. Shapes that failed to meet any of these criteria were denoted “other”.
Figure 5: Illustration of the different bubble shape categories.
3. Results 3.1 Volumetric Bubble Reconstructions Figure 6 shows the bubble reconstructions of the six radial locations progressing from location 1 at the fuel/cladding interface to location 6 at the centermost region of the fuel segment. As can be seen in the figure, the reconstructed volume is noticeably smaller than the volume prepared for serial sectioning. The difference in x and z is due to image cropping to avoid overly bright or shadowed regions from edge effects. The difference in the y depth analyzed is due to instrument drift during the Slice & View process. For convenience, the locations selected for reconstruction, their approximate local burnup range, and the relative bubble information are tabulated in Table 1. Location distances were measured from the cladding to the edge of the AOI closest to the
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cladding. The local burnup range was calculated from this edge, closest to the cladding, to the edge of the AOI furthest from the cladding using the burnup profile previously calculated by Terrani et al. [24]. The mean spacing between the bubbles depicts the average surface to surface distance between each bubble and its nearest neighbor. It was assumed that the nearest neighbor would be the most likely to interconnect with the bubble due to the close proximity, which is why this method was chosen to assess the spacing at each location.
Loc. 1
Loc. 2
Loc. 3
Loc. 4
Loc. 5
Loc. 6
Figure 6: Bubble reconstructions of the six radial locations progressing from location 1 – location 6 with bubble size color coded by relative volume and axes scaled in microns. Please note that the reconstruction for location 2 does not include the bubbles missed by the MATLAB reconstruction.
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Location
Distance from cladding (µm)
r/r0
Calculated local burnup range* (MWd/kgU)
Bubbles per µm3
Average bubble volume (µm3)
Volume occupied by bubbles
Mean spacing between bubbles (nm)
1
~0
~1.0
128.7–124.8
1.9
0.034
6.3%
290
2
44
0.99
114.3–110.3
2.6
0.015
3.9%
215
3
88
0.98
99.8–95.8
4.6
0.009
4.2%
200
4
880
0.81
68.3–68.3
1.8
0.003
0.6%
352
5
2000
0.56
64.0–64.0
3.4
0.001
0.4%
299
6
3500
0.23
61.9–61.9
2.1
0.005
1.0%
315
Table 1: Location distances, burnup, and porosity. Please note that the bubble characteristics tabulated for location 2 do not include bubbles missed by the MATLAB reconstruction. *Interpolated from the burnup profile in Ref. [24].
3.2 Porosity The HBS is characterized by an increased bubble volume fraction, hereafter referred to as porosity, and a decreased grain size [10]. The restructuring that results in decreased grain size was previously determined via EBSD [19]. The characteristic increased porosity of the HBS (locations 1–3) with respect to the porosity outside of the high burnup region (locations 4–6) was apparent when comparing the reconstructions. An assessment of the porosities of each location, given in relative percentages in Table 1, showed that the expected trend of porosity increasing with burnup was followed, except for locations 3 and 6. The trend interruption at location 3 was due to the error in the reconstruction process for location 2. The micrographs gathered from the serial sectioning of location 2 were the hardest to reconstruct because of the instrument instability during data acquisition process. After the threshold was applied to the micrograph stacks, the images were then analyzed to determine the accuracy of the segmentation. At location 2, it was determined that the reconstruction process did not account for some smaller (~200 nm) bubbles. The missed bubbles were manually counted by progressing through the image stack and an average volume was calculated for them by approximating them as spheres. Using this method, it was estimated that approximately 8% of that region’s porosity was dropped, which when restored, would give location 2 approximately 4.2% porosity. Doing so corrected the expected trend of porosity increasing with burnup, except for location 6. From location 5 to 6, the porosity increased toward the pellet center, opposite the direction of burnup. This can be elucidated by understanding the role that burnup and temperature play in fission gas generation and mobility. The explanation that porosity depends on burnup held true for locations 1–5, but it does not explain the relative increase in porosity from location 5 to location 6, which can be explained by considering the high temperatures seen by the central region of the fuel pellet. The high centerline temperatures, as high as 900°C near the pellet’s EOL, resulted in greater mobility of the fission gases via the increased diffusion [26]. The increase in mobility allowed more of the fission gas atoms retained in the lattice to migrate to the bubbles via thermal diffusion at the centermost location, and thus increased the region’s porosity. At location 5, nanoscale intragranular bubbles were likely present, similar to those reported by Gerczak et al. in
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unrestructured grains in the transition region [19]. It is postulated that these nanoscale bubbles, which would be undetectable by the technique used in this study, held most of the local inventory of fission gas at location 5. At the pellet rim the increased grain boundary density caused by the formation of the HBS promoted gas mobility and precipitation. In the central region of the pellet, increased mobility is favored by the higher temperatures. The mid-radial location lacked these two mechanisms; therefore, the bubbles in the mid-radial region were not able to coarsen, which caused it to have a lower porosity than the fuel center despite its higher burnup. Authors would like to acknowledge the limited statistical significance of the provided results, seeing that FIB tomography is limited by the size of analyzed volumes and random sampling, which could lead to inconsistencies in the reported size distribution. Our observations of the uniform bubble structures in the sampled regions allowed us to reach the conclusions shown here. We would like to note that to improve statistical significance of the results, further reconstructions are required to obtain a comprehensive understanding of the relationship between bubble volumes and porosity, which is beyond the scope of this work. 3.3 Bubble Shape Comparing the percentages of different bubble shapes given in Figure 7 shows that the regions outside of the HBS contained predominantly ellipsoidal bubbles, similar to the findings of Gerczak et al. in their 2D survey of the surface of this fuel segment [19]. The addition of a third dimension allowed for the characterization of the disk, football, and other volumes, which combined made up at least half of the bubble shapes found in each location. Due to the anisotropic nature of these shapes, they could not be properly assessed unless a volumetric approach was used.
Figure 7: Bubble shape percentages at each location.
3.4 Bubble Interconnection As can be seen from Table 1 and Figure 7, within HBS regions (corresponding to locations 1–3), bubble size and sphericity was highest as compared to other regions. This was likely due to large microscale bubbles growing at the expense of the smaller nanoscale bubbles or bubbles growing into each other, similar to previous observation by Spino et al. [27]. The enhanced mobility of atomic gas can favor enhanced precipitation and growth in the HBS bubbles, which could
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incorporate adjacent bubbles through static impingement. The reduced total number of bubbles in the same volume of material would provide additional space, as observed in this study. A similar trend was observed at location 6, which was attributed to the interconnection of the bubbles. The fission gas atoms at the center of the fuel pellet would be more mobile due to the higher local temperature, which would allow transport processes to facilitate bubble interconnection and reduce the number of bubbles while still increasing porosity. Another potential explanation for the observed interconnection is the influence of grain morphology, which was not considered in this work. Bubble accumulation can be impacted by the grain morphology. Our study did not include EBSD and we were not able to identify the nature of the grain boundaries and ascertain the presence of specific grain boundaries based on the collected secondary electron micrographs. The cube lift-out locations were selected randomly and were likely located inside an original grain. Regarding location 6, some bubbles can be assumed to be on grain boundaries based on their shape. However, given their proximity to other large bubbles in the same volume, combined with overall size of grain boundaries in this region, the likelihood of these bubbles being intergranular is low. 4. Discussion 4.1 HBS Bubble Interconnection In the HBS, where almost all of the fission gases are retained in the bubbles, there was no evidence of the interconnected bubble networks. It was postulated that as burnup increased, the large bubbles grew at the expense of the small bubbles or the bubbles coalesced, and thus increased the mean spacing between bubbles. If the bubbles were beginning to interconnect or on the verge of interconnecting, the mean spacing between bubbles should have decreased as they came closer together, but that was not observed in this investigation. Additionally, they did not create the oddly shaped bubbles (shown in Figure 8) that would indicate the formation of small isolated interconnected networks, as seen at the center of the pellet. Because of the limitations of the approach that were discussed previously, the authors cannot exclude the possibility of local bubble interconnection. If HBS experiences creep, matrix mobility is allowed, and bubble overpressurization accounted for, oddly shaped bubbles can relax into spheres to minimize the stress early on during irradiation. However, authors are unable to ascertain if this was the case based on the current analysis. For interconnection to occur, the porosity of the HBS would have to increase enough that the bubbles would grow into one another rather than migrating and interconnecting. This threshold porosity for bubble interconnection was originally analyzed by Barnes [28] and more recently refined by others using 3D simulations and experimental observations [13, 27, 29]. The more recent studies first stated that above 24% porosity, the fraction of open bubbles from the formation of interconnected networks would increase rapidly. Spino et al. challenged this statement after reconstructing the 3D bubble structure in a high burnup LWR fuel sample with maximum rim porosity of 25.3% [13]. This 25% threshold was believed to be conservative and it was later refined to state that the HBS porosity would remain closed up to 29% porosity [27]. Below that threshold limit, the bubbles of the HBS could not form the interconnected networks that would cause substantial FGR.
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In the fuel segment examined in this study, the maximum porosity was found to be around 6.3%, which is significantly lower than the threshold limit. The H. B. Robinson rod in this study was run in the reactor to 72 MWd/kgU, which is significantly less (by ~ 26 MWd/kgU) than the high burnup fuel examined by Spino [27]. The higher burnup of the Spino fuel rod is not standard, but in attempts to maximize the time that these rods can be run in the reactor, it may be approached. Future attempts to increase fuel rod burnups should consider Spino’s studies, and the limit predicted (~29%) before the fraction of open bubbles increases rapidly. 4.2 Central Bubble Interconnection The greatest evidence of interconnection was found at the centermost location on the fuel segment where the temperatures were the highest (location 6). In this region, there were isolated interconnected bubbles (shown in Figure 8) that made up less than 1% of the total number of bubbles. The small interconnected fraction of bubbles is not sufficient to cause substantial FGR at the pellet’s EOL, which is supported by the low fractional release of fission gas (~2.1%) from this fuel. Even with the limited resolution of the tungsten filament SEM, and the large step size used for the serial sectioning, if the bubbles had been forming large interconnected networks such as those seen by White, it would have been evident since the features are clearly visible at the microscale [7]. The oddly shaped bubbles at the center of the pellet were also seen previously by Noirot et al. in an examination of a section of UO2 with an average burnup of 73 MWd/kgU, similar to the one examined in this study [15]. Their study also concluded that there were no significant interconnected bubble networks at the center of the pellet [15]. In an effort to compare both results with White’s investigation, the temperatures in these studies need to be considered. In White’s examination, the interconnected networks were seen only after the fuel was held at 1800°C for 30 minutes [7]. This temperature is over 800°C higher than the highest temperature that the fuels in this study (900–925oC) and in Noirot’s study (800–900oC) experienced during their last few cycles in the reactor. High centerline temperatures promote mobility of fission gas atoms, which can allow them to interconnect if the temperature is high enough.
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Figure 8: Isolated interconnected bubbles from location 6 (axes are scaled in nanometers).
The formation of interconnected bubble networks that could release the significant amounts of fission gases that accumulate in the HBS bubbles by the pellet’s EOL is unlikely to occur under normal reactor operating conditions due to the current low operational lifetimes for standard fuel rods. The region most susceptible to bubble interconnection is at the center of the pellet where the temperatures are the highest. However, the fuel pellet in this study did not experience the temperatures needed for full bubble mobility and interconnection. 5. Conclusions A unique LWR fuel pellet irradiated to high burnup at relatively low temperatures with limited FGR was examined in this work. Six separate locations on the UO2 fuel pellet were serially sectioned using a FIB/SEM so that the 3D bubble structure at each location could be reconstructed from the micrographs. The bubbles were analyzed based on their size, shape, number density, and percentage of the total volume they occupied. This analysis was also used to determine the possibility of bubble interconnection that would result in the release of fission gas. Based on this investigation, a relationship between burnup, temperature, and the resulting bubble characteristics was determined. As expected, the porosity increased with burnup, but in central regions where burnup was uniform, increased temperature also caused the porosity to increase. Consistent with prior literature, in the high burnup region, where bubble volume fraction was the largest (6.3%), no interconnection that could facilitate FGR was observed. The centermost bubble reconstruction, the location of highest temperature, showed the most evidence of 13
interconnection, but there were still no observed large interconnected networks like those needed for FGR pathways. Acknowledgments The authors of this work are grateful to the many people involved in the specimen preparation and handling at the hot-cell and radiological facilities at Oak Ridge National Laboratory. Authors acknowledge Joseph Burns for burnup calculations, and Andrew Nelson, Tyler Gerczak, and Jason Harp for their support and critical review of the manuscript prior to publication. This research was supported in part by an appointment to the Oak Ridge National Laboratory Higher Education Research Experience Program, sponsored by the US Department of Energy and administered by the Oak Ridge Institute for Science and Education.
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References [1] D. R. Olander, "Fundamental Aspects of Nuclear Reactor Fuel Elements," Technical Information Center, Springfield, VA, 1976. [2] J. Rest, M. Cooper, J. Spino, J. Turnbull, P. Van Uffelen and C. Walker, "Fission gas release from UO2 nuclear fuel: A review," Journal of Nuclear Materials, vol. 513, pp. 310-345, 2019. [3] A. H. Booth, "A suggested method for calculating the diffusion of radioactive rare gas fission products from UO*sub 2/ fuel elements and a discussion of roposed in-reactor experiments that may be used to test its validity," Atomic Energy of Canada Ltd. Chalk River Project, Chalk River, Ont., 1957. [4] M. V. Speight, "A calculation on the migration of fission gas in material exhibiting precipitation and re-solution of gas atoms under irradiation," Nuclear Science and Engineering, vol. 37, pp. 180-185, 1969. [5] D. R. Olander and D. Wongsawaeng, "Re-solution of fission gas-A review: Part I. Intragranular bubbles," Journal of Nuclear Materials, vol. 354, pp. 94-109, 2006. [6] J. A. Turnbull, "The distribution of intragranular fission gas bubbles in UO2 during irradiation," Journal of Nuclear Materials, vol. 38, pp. 203-212, 1971. [7] R. J. White, "The development of grain-face porosity in irradiated oxide fuel," Journal of Nuclear Materials, vol. 325, pp. 61-77, 2004. [8] J. -P. Hiernaut, T. Wiss, J. -Y. Colle, H. Thiele, C. T. Walker, W. Goll and R. M. Konings, "Fission product release and microstructure changes during laboratory annealing of a very high burn-up fuel specimen," Journal of Nuclear Materials, vol. 377, pp. 313-324, 2008. [9] K. Govers, C. L. Bishop, D. C. Parfitt, S. E. Lemehov, M. Verwerft and R. W. Grimes, "Molecular dynamics study of Xe bubble re-solution in UO2," Journal of Nuclear Materials, vol. 420, pp. 282290, 2012. [10] V. V. Rondinella and T. Wiss, "The high burn-up structure in nuclear fuel," Materials Today, vol. 13, no. 12, pp. 24-32, December 2010. [11] M. Kinoshita, T. Kameyama, S. Kitajima and H. Matzke, "Temperature and fission rate effects on the rim structure formation in a UO2 fuel with a burnup of 7.9% FIMA," Journal of Nuclear Materials, vol. 252, pp. 71-78, 1998. [12] C. T. Walker, S. Bremier, S. Portier, R. Hasnaoui and W. Goll, "SIMS analysis of an UO 2 fuel irradiated at low temperature to 65MWd/kgHM," Journal of Nuclear Materials, vol. 393, pp. 212223, 2009. [13] J. Spino, D. Papaioannou and J. -P. Glatz, "Comments on the threshold porosity for fission gas release in high burn-up fuels," Journal of Nuclear Materials, vol. 328, pp. 67-70, 2004. [14] W. Beere and G. L. Reynolds, "The morphology and growth rate of interlinked porosity in irradiated UO2," Journal of Nuclear Materials, vol. 47, pp. 51-57, 1973. [15] J. Noirot, I. Zacharie-Aubrun and T. Blay, "Focused ion beam-scanning electron microscope examination of high burn-up UO2 in the center of a pellet," Nuclear Engineering and Technology, vol. 50, pp. 259-267, 2018. [16] R. Restani, M. Horvath, W. Goll, J. Bertsch, D. Gavillet, A. Hermann and M. Martin, "On the condition of UO2 nuclear fuel irradiated in a PWR to a burn-up in excess of 110 MWd/kgHM," Journal of nuclear materials, vol. 481, pp. 88-100, 2016. [17] F. Cappia, D. Pizzocri, A. Schubert, P. Van Uffelen, G. Paperini, D. Pellottiero, R. Macian-Juan and V. V. Rondinella, "Critical assessment of the pore size distribution in the rim region of high burnup UO2 fuels," Journal of Nuclear Materials, vol. 480, pp. 138-149, 2016.
15
[18] E. J. Ruzauskas and K. N. Fardell, "Design, Operation, and Performance Data for High Burnup PWR Fuel from HB Robinson Plant for Use in the NRC Experimental Program at Argonne National Laboratory," 2001. [19] T. J. Gerczak, C. M. Parish, P. D. Edmondson, C. A. Baldwin and K. A. Terrani, "Restructuring in high burnup UO2 studied using modern electron microscopy," Journal of Nuclear Materials, vol. 509, pp. 245-259, 2018. [20] C. Vitanza, "Fission gas release from UO2 pellet at high burnup," in American Nuclear Society Topical Meeting on Light Water Reactor Fuel Performance, Portland, OR, USA, 1979. [21] T. Tverberg, "In-pile results from the FGR tests on MOX fuel disks in IFAs 629.7 and 629.8," HWR1159, 2016. [22] T. Ikonen, "Summary of observed fission gas release behaviour in selected experiments performed at the Halden Reactor Project," HWR-1197, 2016. [23] C. M. Parish, N. P. Kiran Kumar, L. L. Snead, P. D. Edmondson, K. G. Field, C. Silva, A. M. Williams, K. Linton and K. J. Leonard, "LAMDA: Irradiated-Materials Microscopy at Oak Ridge National Laboratory," Microscopy and Microanalysis, vol. 21, no. 3, pp. 1003-1004, 2015. [24] K. A. Terrani, M. Balooch, J. R. Burns and Q. B. Smith, "Young's modulus evaluation of high burnup structure in UO2 with nanometer resolution," Journal of Nuclear Materials, vol. 508, pp. 3339, 2018. [25] R. Manzel and C. T. Walker, "EPMA and SEM of fuel samples from PWR rods with an average burn-up of around 100 MWd/kgHM," Journal of Nuclear Materials, vol. 301, pp. 170-182, 2002. [26] J. A. Turnbull, C. A. Friskney, J. R. Findlay, F. A. Johnson and A. J. Walter, "The Diffusion Coefficients of Gaseous and Volatile Species During the Irradiation of Uranium Dioxide," Journal of Nuclear Materials, vol. 107, pp. 168-184, 1982. [27] J. Spino, A. D. Stalios, H. Santa Cruz and D. Baron, "Stereological evolution of the rim structure in PWR-fuels at prolonged irradiation: Dependencies with burn-up and temperature," Journal of Nuclear Materials, vol. 354, pp. 66-84, 2006. [28] R. S. Barnes, "A Theory of Swelling and Gas Release for Reactor Materials," Journal of Nuclear Materials, vol. 11, pp. 135-148, 1964. [29] Y.-H. Koo, J.-Y. Oh, B.-H. Lee and D.-S. Sohn, "Three-dimensional simulation of threshold porosity for fission gas release in the rim region of LWR UO2 fuel," Journal of Nuclear Materials, vol. 321, pp. 249-255, 2003.
16
Three-Dimensional Bubble Reconstruction in High Burnup UO2 • • • •
The evolution of fission gas bubbles was investigated in light water reactor fuel. The high burnup structure (HBS) showed no interconnected bubble networks. The central region of the fuel showed little evidence of bubble interconnection. Interconnection did not cause substantial fission gas release.
Casey McKinney: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing, Visualization Rachel Seibert: Writing – review & editing, Supervision Grant Helmreich: Software, Formal analysis, Data curation, Writing – original draft, Writing – review & editing, Visualization Assel Aitkaliyeva: Writing – original draft, Writing – review & editing Kurt Terrani: Conceptualization, Writing, - original draft, Writing – review & editing, Project administration, Funding acquisition
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-3115(19)31407-2
DOI:
https://doi.org/10.1016/j.jnucmat.2020.152053
Reference:
NUMA 152053
To appear in:
Journal of Nuclear Materials
Received Date: 1 November 2019 Revised Date:
6 January 2020
Accepted Date: 11 February 2020
Please cite this article as: C. McKinney, R. Seibert, G. Helmreich, A. Aitkaliyeva, K. Terrani, Threedimensional bubble reconstruction in high burnup UO2, Journal of Nuclear Materials (2020), doi: https:// doi.org/10.1016/j.jnucmat.2020.152053. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Three-Dimensional Bubble Reconstruction in High Burnup UO2* Casey McKinney a,b, Rachel Seibert a, Grant Helmreich a, Assel Aitkaliyeva b†, Kurt Terrani a a Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA b University of Florida, Gainesville, FL, 32611, USA Keywords: uranium dioxide, high burnup fuel, FIB tomography, fission gas release, 3D reconstruction. Abstract In light water reactor (LWR) UO2 fuels, the evolution of volatile fission products is one of the critical areas of fuel behavior that is yet to be fully understood. In UO2 irradiated to high burnups, it is well known that most released fission gases come from the central region of the fuel as opposed to the highly porous high burnup structure (HBS) on the periphery of the pellets. However, fuels with and without interconnected bubble networks at the fuel center showed high to moderate release fractions, which conceals the mechanisms responsible for the gas release in the latter scenario. In this work, focused ion beam tomography was used to investigate the threedimensional bubble structure in an irradiated LWR UO2 fuel pellet with high degree of fission gas retention so that the degree of bubble interconnection could be assessed. Six radial locations with different burnups and temperatures were serially sectioned and imaged to reconstruct the three-dimensional bubble structure. As expected, the highest porosity was observed at the periphery of the fuel (HBS). The porosity then decreased towards the pellet center, except for the centermost location. This location had a slightly higher porosity than its adjacent mid-radial location, which was attributed to the temperature difference between the two locations. This study provides a first-time volumetric evaluation of the porosity at different radial locations on a UO2 fuel pellet. During this investigation, no significant bubble interconnection was noted at any of the six radial locations. 1. Introduction Since UO2 has been the main fuel for light water reactors (LWRs) for decades, numerous investigations into its performance under different irradiation conditions have been conducted [1]. One of the critical areas of fuel behavior is the evolution of noble gas fission products, which tend to have low solubility in UO2. As a result of this low thermodynamic solubility, fission gas atoms diffuse through the matrix to the grain boundaries and pre-existing pores, where they can *
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-publicaccess-plan). † Corresponding author: [email protected]; Address: 176 Rhines Hall, Department of Materials Science and Engineering, University of Florida, PO Box 116400, Gainesville, FL, 32611-6400.
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nucleate intra- and inter-granular bubbles. When the porosity interconnects, fission gas can leave the fuel to the fuel-cladding gap and plenum. The gas atoms within the grains, referred to as intra-granular bubbles, can be trapped by irradiation-induced defects and/or smaller bubbles, and are subject to re-solution. Gas atoms along the grain boundaries, or inter-granular bubbles, can interconnect to form a tunnel network, through which the content can be released. The process of gas transport, bubble nucleation, growth, coalescence, and release is highly complex [2]. Early on, simple models describing diffusive fission gas release (FGR) were formulated [3] and continuously refined [4, 5] to capture the processes expected to occur in the fuel during reactor operation. Improvements in the understanding of fission gas transport and release in UO2 is often initiated by experimental observations [6, 7, 8] or analytical examinations of the efficacy of various governing processes [5, 9]. Many of the fundamental processes remain elusive, and much of the complexity remains uncaptured by the current computational framework. One such instance is the “conundrum” described in Section 3 of Ref. [2] that discusses FGR from high burnup fuel. It is now well known that the high burnup structure (HBS) [10] that develops at high burnups and low temperatures [11], does not result in significant FGR [12] due to the closed bubble structure up ~25% porosity [13]. On the other hand, fuel pellets that were operated at high linear heat ratings and experienced high temperatures at their centerline, form an interconnected network of bubbles. This interconnected network is considered to be the main source of FGR [14]. Recent examination of the microstructure of high burnup fuels, which operated at relatively low powers and showed modest FGR, did not show evidence for bubble interconnection at the center [15]. Therefore, the mechanism of FGR in this scenario is not well understood. This paper applies large-scale focused ion beam (FIB) tomography to study the distribution of bubbles inside high burnup UO2. The goal is to complement our current understanding by providing additional high-quality experimental observations to decipher the mechanism of microstructure evolution and FGR. Although FIB tomography does not offer sufficient resolution to observe 1–10 nm intragranular bubbles, it is very useful for observing large inter- and intragranular bubbles and therefore is well suited for the study of bubbles in HBS and central regions of the fuel, where typical average bubble diameters are in the range of 1–2 µm and 0.5–1 µm, respectively [2, 16]. Three-dimensional (3D) characterization methods offer unique means to gather data on the true structure of a material, as opposed to two-dimensional (2D) methods, which merely provide approximations. Synchrotron X-ray tomography has challenges with larger volumes of uraniumbased materials (which have a high scattering cross section due to high z), and neutron tomography lacks the spatial resolution needed for the study of fission gas bubbles. The availability of such facilities that can handle irradiated fuels is another challenge. Instead, FIB tomography, a destructive technique, offers the resolution required for this study. The micrographs gathered from the FIB tomography were used to reconstruct the bubble structure at different locations on a UO2 fuel pellet so that the existence of bubble interconnection could be determined. A unique fuel pin with high burnup and high degree of fission gas retention was chosen for this study. In addition to the interconnectivity of the bubbles, the reconstructions were used to assess the bubble volume fraction (porosity) as well as bubble shape and spacing. The investigations across the fuel pellet radius provided a way to characterize the bubble structure of
2
regions experiencing different burnups and temperatures. While previous studies with relevant scales were only conducted in 2D [17], utilizing FIB tomography for this process provides a first-time volumetric evaluation of the porosity evolution as a function of burnup and temperature in a UO2 fuel pellet. It is expected that the data presented in this study and other data sets—to be generated on fuels with different microstructure, power, and temperature histories— are needed to provide a more complete picture of fission gas behavior and release in nuclear fuels. 2. Materials and Methods 2.1 Fuel Specimen Pedigree In this study, a segment of a fuel pin (E02) from the H. B. Robinson Unit 2 commercial pressurized water reactor irradiated to an average burnup of 72 MWd/kgU was analyzed. The UO2 fuel pellet encapsulated in Zircaloy-4 cladding was irradiated for seven cycles; its thorough irradiation history can be found in Ref. [18]. The end-of-life (EOL) measured FGR for the E02 fuel pin is reported at 2.1%. The power history of the central 2 m section of the E02 rod was used to estimate the fuel temperature profile in a previous study [19]. The estimated fuel centerline temperature alongside the Vitanza threshold [20] for 1% FGR is plotted in Figure 1. If fuel centerline conditions exceed those outlined by this threshold, the fuel would be considered to have high FGR. Also shown in the plot are the empirical modifications to the Vitanza threshold based on more recent high burnup fuel data that mark the onset of small amounts of FGR [21, 22]. The low fuel temperatures throughout the irradiation are expected to result in low FGR, making this a unique specimen to study bubble evolution in high burnup UO2. 1600
Fuel Centerline Temperature [°C]
1500 1400 1300 1200 Vitanza 1% FGR threshold
1100
empirical small FGR threshold
1000
H.B Robinson fuel segment from E02 pin
900 800 700 600
0
10
20
30
40
50
60
Burnup [MWd/kgUO 2]
Figure 1: Estimated fuel centerline temperature for central section of the E02 fuel pin from ref. [19] compared to Vitanza and the empirical small FGR thresholds.
3
Since the central section of the rod is expected to operate at the highest power and temperature, the local FGR is expected to be higher than the integral value for the pin (>2.1%). Yet, the fuel centerline temperature in this section stays below the 1% threshold and barely goes above the empirical threshold for small (<1%) FGR. 2.2 Experiment Details When preparing the sample, a rectangular section of the fuel and cladding was removed, as can be seen in Figure 2 (for a detailed description of the sample and its preparation, see Ref. [19]). After removal from the hot cell, the sample was transferred to Oak Ridge National Laboratory’s (ORNL) Low Activation Materials Development and Analysis (LAMDA) Laboratory for microscopy [23]. The facility’s shielded FEI Quanta dual-beam FIB/scanning electron microscope (SEM), equipped with a tungsten filament, was used to section and image the fuel.
Figure 2: Micrograph of the six AOI locations on the axial cross section of the UO2 specimen. Superimposed on the image is the calculated EOL radial burnup profile. The fuel segment is separated into four different regions starting with the HBS on the left moving to the central region on the far right. See Table 1 for r/r0 and local burnup values.
In a previous work, Gerczak et al. designated the different regions of this fuel segment as HBS (r/r0 ~ 1.0–0.97), transition (r/r0 ~ 0.97–0.93), mid-radial (r/r0 ~ 0.93–0.45), and central (r/r0 ~ 0.45–0.25) [19]. Areas of interest (AOIs) were prepared at six separate radial locations based on Gerczak’s defined regions and accompanying electron backscatter diffraction (EBSD) data, the calculated burnup profile, and the average temperature profile [19, 24]. These locations with respect to the burnup profile can be seen overlaid on a micrograph of the fuel segment in Figure
4
2. Since one of the main objectives was to investigate the probability of bubble interconnection in the HBS region, three areas within the HBS (locations 1–3 in Figure 2) were reconstructed to examine the bubble structure. These areas were positioned equidistantly to ensure that all relevant phenomena had been captured. Outside of the high burnup region, three additional locations were chosen for reconstruction, as illustrated by the white boxes labeled as locations 4– 6 in Figure 2. The location farthest from the HBS (location 6 in Figure 2) was chosen to assess possible interconnection at the hottest central region of the fuel pellet, where fission gas is expected to be released [25]. The other locations outside of the high burnup region (locations 4 and 5) were chosen to examine mid-radial regions that experienced varied conditions. Utilizing FIB tomography for this investigation provided a first-time volumetric evaluation of the bubble structure that showed the fuel’s response to different burnups and temperatures. Before sectioning could begin, the AOI had to be pre-milled to a size of 12×12×12 µm3 for investigation. After each AOI had been pre-milled (Figure 3), FEI’s Auto Slice and View software was used to automate the sectioning process. The Slice and View parameters were set to mill a 16×14×6 µm3 volume (denoted as x, y, and z in Figure 3) with 100 nm thick layers. The overestimation of the x and y parameters was set to compensate for instrument drift, and the underestimation of z was due to the manner in which the UO2 milled. Previous milling of the sample had shown that an underestimated z parameter would still mill the desired amount; therefore, the z parameter could be cut down to minimize milling time while still milling the desired volume. For the sectioning process, the ion beam was set at 30 kV and 0.5 nA to reduce curtaining effects, and the electron beam was set at 5 kV and 0.41 nA to get greater contrast between the bubbles and the fuel. This process produced stacks of micrographs that progressed sequentially through the volume of material that then needed to be reconstructed into the 3D bubble structure.
Figure 3: SEM micrograph of the AOI at location 3, outlined in white, after initial milling with Slice & View x,y,z notation shown.
2.3 MATLAB Reconstruction Construction of 3D bubble images from the series of 2D SEM images from each location was complicated by minor changes in position and imaging angle from image to image. A 5
customized transformation routine was developed using MATLAB in which the operator selected a set of fiducial marks (easily identifiable features or bubbles around the trenches that are unique to the area) in each micrograph. These points were then used to generate a unique transformation matrix for each micrograph including translation, rotation, and projection operations to align the corresponding fiducial marks for all micrographs acquired at each location. The transformed micrograph sets were then filtered to reduce noise (using Wiener filter) and to smooth minor variations in intensity (using Local Laplacian filter). The resolution of the SEM micrographs was on the order of 4.56 nm/pixel with a spacing between micrographs of 55–63 nm, where the slight discrepancy between spacing was caused by the instrument drift and thus accounted for in each reconstruction. Since the depth between images was greater than the image pixel size, composite intermediate images were created to produce an image stack with cubic voxels with edge lengths identical to the original SEM micrograph. These composite images were formed using cubic interpolation within the original images. Finally, 2-binning was applied to the image stack to reduce the computational burden of segmentation. Bubbles were segregated from the image stack using an adaptive threshold that automatically corrected for local variations in image brightness (MATLAB function “adaptthresh”). Morphological operations were then used to fill gaps (MATLAB function “imfill”) within bubbles and morphologically dilate them slightly (MATLAB function “imdilate”) so that the following edge-based active contour algorithm with a preference toward shrinking (MATLAB function “activecontour”) could find the bubble edges. In Figure 4, a filtered micrograph can be seen before and after this segmentation process was applied. This approach was preferred as the edges of bubbles tended to be well-defined even when their interior featured non-uniform intensity. Properties were then tabulated for each segregated bubble with 26-pixel connectivity, including volume, centroid position, and the principal axes of the best-fit ellipsoid.
Figure 4: SEM micrograph of location 3 after filtering (left) and segmentation (right).
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2.4 Bubble Shape Qualification Figure 5 illustrates the notation and categories used to sort the bubble shapes within the AOIs. Each bubble had three principal axis lengths (x, y, z) that were used to define its size and shape. The relationships between those axes were used to determine the bubble shape. The more spherical bubbles were defined as having three similar principal axis lengths (x ≅ y ≅ z). Diskshaped bubbles were defined as having two similar principal axis lengths that were greater than the third axis (x ≅ y > z). Footballs were also defined as having two similar principal axis lengths, but with a third axis longer than the others (x ≅ y < z). If the similar axes of these shape types were within ±50% of each other, they were then classified as either spheroid, football, or disk, contingent on the specified criterion. In the case of the football-shaped bubbles, an additional step was implemented to ensure the accuracy of the shape identification. If the initial similarity threshold of 50% was not met in football-shaped bubbles, the bubble could still appear as a football if the third axis were significantly larger than the two similar axes. To avoid confusion, such bubbles were defined by relating the difference between the two smaller axes to the larger axis with the following equation: 20%
(1)
If the criterion in Equation 1 was met, the bubble was identified as a football. Shapes that failed to meet any of these criteria were denoted “other”.
Figure 5: Illustration of the different bubble shape categories.
3. Results 3.1 Volumetric Bubble Reconstructions Figure 6 shows the bubble reconstructions of the six radial locations progressing from location 1 at the fuel/cladding interface to location 6 at the centermost region of the fuel segment. As can be seen in the figure, the reconstructed volume is noticeably smaller than the volume prepared for serial sectioning. The difference in x and z is due to image cropping to avoid overly bright or shadowed regions from edge effects. The difference in the y depth analyzed is due to instrument drift during the Slice & View process. For convenience, the locations selected for reconstruction, their approximate local burnup range, and the relative bubble information are tabulated in Table 1. Location distances were measured from the cladding to the edge of the AOI closest to the
7
cladding. The local burnup range was calculated from this edge, closest to the cladding, to the edge of the AOI furthest from the cladding using the burnup profile previously calculated by Terrani et al. [24]. The mean spacing between the bubbles depicts the average surface to surface distance between each bubble and its nearest neighbor. It was assumed that the nearest neighbor would be the most likely to interconnect with the bubble due to the close proximity, which is why this method was chosen to assess the spacing at each location.
Loc. 1
Loc. 2
Loc. 3
Loc. 4
Loc. 5
Loc. 6
Figure 6: Bubble reconstructions of the six radial locations progressing from location 1 – location 6 with bubble size color coded by relative volume and axes scaled in microns. Please note that the reconstruction for location 2 does not include the bubbles missed by the MATLAB reconstruction.
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Location
Distance from cladding (µm)
r/r0
Calculated local burnup range* (MWd/kgU)
Bubbles per µm3
Average bubble volume (µm3)
Volume occupied by bubbles
Mean spacing between bubbles (nm)
1
~0
~1.0
128.7–124.8
1.9
0.034
6.3%
290
2
44
0.99
114.3–110.3
2.6
0.015
3.9%
215
3
88
0.98
99.8–95.8
4.6
0.009
4.2%
200
4
880
0.81
68.3–68.3
1.8
0.003
0.6%
352
5
2000
0.56
64.0–64.0
3.4
0.001
0.4%
299
6
3500
0.23
61.9–61.9
2.1
0.005
1.0%
315
Table 1: Location distances, burnup, and porosity. Please note that the bubble characteristics tabulated for location 2 do not include bubbles missed by the MATLAB reconstruction. *Interpolated from the burnup profile in Ref. [24].
3.2 Porosity The HBS is characterized by an increased bubble volume fraction, hereafter referred to as porosity, and a decreased grain size [10]. The restructuring that results in decreased grain size was previously determined via EBSD [19]. The characteristic increased porosity of the HBS (locations 1–3) with respect to the porosity outside of the high burnup region (locations 4–6) was apparent when comparing the reconstructions. An assessment of the porosities of each location, given in relative percentages in Table 1, showed that the expected trend of porosity increasing with burnup was followed, except for locations 3 and 6. The trend interruption at location 3 was due to the error in the reconstruction process for location 2. The micrographs gathered from the serial sectioning of location 2 were the hardest to reconstruct because of the instrument instability during data acquisition process. After the threshold was applied to the micrograph stacks, the images were then analyzed to determine the accuracy of the segmentation. At location 2, it was determined that the reconstruction process did not account for some smaller (~200 nm) bubbles. The missed bubbles were manually counted by progressing through the image stack and an average volume was calculated for them by approximating them as spheres. Using this method, it was estimated that approximately 8% of that region’s porosity was dropped, which when restored, would give location 2 approximately 4.2% porosity. Doing so corrected the expected trend of porosity increasing with burnup, except for location 6. From location 5 to 6, the porosity increased toward the pellet center, opposite the direction of burnup. This can be elucidated by understanding the role that burnup and temperature play in fission gas generation and mobility. The explanation that porosity depends on burnup held true for locations 1–5, but it does not explain the relative increase in porosity from location 5 to location 6, which can be explained by considering the high temperatures seen by the central region of the fuel pellet. The high centerline temperatures, as high as 900°C near the pellet’s EOL, resulted in greater mobility of the fission gases via the increased diffusion [26]. The increase in mobility allowed more of the fission gas atoms retained in the lattice to migrate to the bubbles via thermal diffusion at the centermost location, and thus increased the region’s porosity. At location 5, nanoscale intragranular bubbles were likely present, similar to those reported by Gerczak et al. in
9
unrestructured grains in the transition region [19]. It is postulated that these nanoscale bubbles, which would be undetectable by the technique used in this study, held most of the local inventory of fission gas at location 5. At the pellet rim the increased grain boundary density caused by the formation of the HBS promoted gas mobility and precipitation. In the central region of the pellet, increased mobility is favored by the higher temperatures. The mid-radial location lacked these two mechanisms; therefore, the bubbles in the mid-radial region were not able to coarsen, which caused it to have a lower porosity than the fuel center despite its higher burnup. Authors would like to acknowledge the limited statistical significance of the provided results, seeing that FIB tomography is limited by the size of analyzed volumes and random sampling, which could lead to inconsistencies in the reported size distribution. Our observations of the uniform bubble structures in the sampled regions allowed us to reach the conclusions shown here. We would like to note that to improve statistical significance of the results, further reconstructions are required to obtain a comprehensive understanding of the relationship between bubble volumes and porosity, which is beyond the scope of this work. 3.3 Bubble Shape Comparing the percentages of different bubble shapes given in Figure 7 shows that the regions outside of the HBS contained predominantly ellipsoidal bubbles, similar to the findings of Gerczak et al. in their 2D survey of the surface of this fuel segment [19]. The addition of a third dimension allowed for the characterization of the disk, football, and other volumes, which combined made up at least half of the bubble shapes found in each location. Due to the anisotropic nature of these shapes, they could not be properly assessed unless a volumetric approach was used.
Figure 7: Bubble shape percentages at each location.
3.4 Bubble Interconnection As can be seen from Table 1 and Figure 7, within HBS regions (corresponding to locations 1–3), bubble size and sphericity was highest as compared to other regions. This was likely due to large microscale bubbles growing at the expense of the smaller nanoscale bubbles or bubbles growing into each other, similar to previous observation by Spino et al. [27]. The enhanced mobility of atomic gas can favor enhanced precipitation and growth in the HBS bubbles, which could
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incorporate adjacent bubbles through static impingement. The reduced total number of bubbles in the same volume of material would provide additional space, as observed in this study. A similar trend was observed at location 6, which was attributed to the interconnection of the bubbles. The fission gas atoms at the center of the fuel pellet would be more mobile due to the higher local temperature, which would allow transport processes to facilitate bubble interconnection and reduce the number of bubbles while still increasing porosity. Another potential explanation for the observed interconnection is the influence of grain morphology, which was not considered in this work. Bubble accumulation can be impacted by the grain morphology. Our study did not include EBSD and we were not able to identify the nature of the grain boundaries and ascertain the presence of specific grain boundaries based on the collected secondary electron micrographs. The cube lift-out locations were selected randomly and were likely located inside an original grain. Regarding location 6, some bubbles can be assumed to be on grain boundaries based on their shape. However, given their proximity to other large bubbles in the same volume, combined with overall size of grain boundaries in this region, the likelihood of these bubbles being intergranular is low. 4. Discussion 4.1 HBS Bubble Interconnection In the HBS, where almost all of the fission gases are retained in the bubbles, there was no evidence of the interconnected bubble networks. It was postulated that as burnup increased, the large bubbles grew at the expense of the small bubbles or the bubbles coalesced, and thus increased the mean spacing between bubbles. If the bubbles were beginning to interconnect or on the verge of interconnecting, the mean spacing between bubbles should have decreased as they came closer together, but that was not observed in this investigation. Additionally, they did not create the oddly shaped bubbles (shown in Figure 8) that would indicate the formation of small isolated interconnected networks, as seen at the center of the pellet. Because of the limitations of the approach that were discussed previously, the authors cannot exclude the possibility of local bubble interconnection. If HBS experiences creep, matrix mobility is allowed, and bubble overpressurization accounted for, oddly shaped bubbles can relax into spheres to minimize the stress early on during irradiation. However, authors are unable to ascertain if this was the case based on the current analysis. For interconnection to occur, the porosity of the HBS would have to increase enough that the bubbles would grow into one another rather than migrating and interconnecting. This threshold porosity for bubble interconnection was originally analyzed by Barnes [28] and more recently refined by others using 3D simulations and experimental observations [13, 27, 29]. The more recent studies first stated that above 24% porosity, the fraction of open bubbles from the formation of interconnected networks would increase rapidly. Spino et al. challenged this statement after reconstructing the 3D bubble structure in a high burnup LWR fuel sample with maximum rim porosity of 25.3% [13]. This 25% threshold was believed to be conservative and it was later refined to state that the HBS porosity would remain closed up to 29% porosity [27]. Below that threshold limit, the bubbles of the HBS could not form the interconnected networks that would cause substantial FGR.
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In the fuel segment examined in this study, the maximum porosity was found to be around 6.3%, which is significantly lower than the threshold limit. The H. B. Robinson rod in this study was run in the reactor to 72 MWd/kgU, which is significantly less (by ~ 26 MWd/kgU) than the high burnup fuel examined by Spino [27]. The higher burnup of the Spino fuel rod is not standard, but in attempts to maximize the time that these rods can be run in the reactor, it may be approached. Future attempts to increase fuel rod burnups should consider Spino’s studies, and the limit predicted (~29%) before the fraction of open bubbles increases rapidly. 4.2 Central Bubble Interconnection The greatest evidence of interconnection was found at the centermost location on the fuel segment where the temperatures were the highest (location 6). In this region, there were isolated interconnected bubbles (shown in Figure 8) that made up less than 1% of the total number of bubbles. The small interconnected fraction of bubbles is not sufficient to cause substantial FGR at the pellet’s EOL, which is supported by the low fractional release of fission gas (~2.1%) from this fuel. Even with the limited resolution of the tungsten filament SEM, and the large step size used for the serial sectioning, if the bubbles had been forming large interconnected networks such as those seen by White, it would have been evident since the features are clearly visible at the microscale [7]. The oddly shaped bubbles at the center of the pellet were also seen previously by Noirot et al. in an examination of a section of UO2 with an average burnup of 73 MWd/kgU, similar to the one examined in this study [15]. Their study also concluded that there were no significant interconnected bubble networks at the center of the pellet [15]. In an effort to compare both results with White’s investigation, the temperatures in these studies need to be considered. In White’s examination, the interconnected networks were seen only after the fuel was held at 1800°C for 30 minutes [7]. This temperature is over 800°C higher than the highest temperature that the fuels in this study (900–925oC) and in Noirot’s study (800–900oC) experienced during their last few cycles in the reactor. High centerline temperatures promote mobility of fission gas atoms, which can allow them to interconnect if the temperature is high enough.
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Figure 8: Isolated interconnected bubbles from location 6 (axes are scaled in nanometers).
The formation of interconnected bubble networks that could release the significant amounts of fission gases that accumulate in the HBS bubbles by the pellet’s EOL is unlikely to occur under normal reactor operating conditions due to the current low operational lifetimes for standard fuel rods. The region most susceptible to bubble interconnection is at the center of the pellet where the temperatures are the highest. However, the fuel pellet in this study did not experience the temperatures needed for full bubble mobility and interconnection. 5. Conclusions A unique LWR fuel pellet irradiated to high burnup at relatively low temperatures with limited FGR was examined in this work. Six separate locations on the UO2 fuel pellet were serially sectioned using a FIB/SEM so that the 3D bubble structure at each location could be reconstructed from the micrographs. The bubbles were analyzed based on their size, shape, number density, and percentage of the total volume they occupied. This analysis was also used to determine the possibility of bubble interconnection that would result in the release of fission gas. Based on this investigation, a relationship between burnup, temperature, and the resulting bubble characteristics was determined. As expected, the porosity increased with burnup, but in central regions where burnup was uniform, increased temperature also caused the porosity to increase. Consistent with prior literature, in the high burnup region, where bubble volume fraction was the largest (6.3%), no interconnection that could facilitate FGR was observed. The centermost bubble reconstruction, the location of highest temperature, showed the most evidence of 13
interconnection, but there were still no observed large interconnected networks like those needed for FGR pathways. Acknowledgments The authors of this work are grateful to the many people involved in the specimen preparation and handling at the hot-cell and radiological facilities at Oak Ridge National Laboratory. Authors acknowledge Joseph Burns for burnup calculations, and Andrew Nelson, Tyler Gerczak, and Jason Harp for their support and critical review of the manuscript prior to publication. This research was supported in part by an appointment to the Oak Ridge National Laboratory Higher Education Research Experience Program, sponsored by the US Department of Energy and administered by the Oak Ridge Institute for Science and Education.
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References [1] D. R. Olander, "Fundamental Aspects of Nuclear Reactor Fuel Elements," Technical Information Center, Springfield, VA, 1976. [2] J. Rest, M. Cooper, J. Spino, J. Turnbull, P. Van Uffelen and C. Walker, "Fission gas release from UO2 nuclear fuel: A review," Journal of Nuclear Materials, vol. 513, pp. 310-345, 2019. [3] A. H. Booth, "A suggested method for calculating the diffusion of radioactive rare gas fission products from UO*sub 2/ fuel elements and a discussion of roposed in-reactor experiments that may be used to test its validity," Atomic Energy of Canada Ltd. Chalk River Project, Chalk River, Ont., 1957. [4] M. V. Speight, "A calculation on the migration of fission gas in material exhibiting precipitation and re-solution of gas atoms under irradiation," Nuclear Science and Engineering, vol. 37, pp. 180-185, 1969. [5] D. R. Olander and D. Wongsawaeng, "Re-solution of fission gas-A review: Part I. Intragranular bubbles," Journal of Nuclear Materials, vol. 354, pp. 94-109, 2006. [6] J. A. Turnbull, "The distribution of intragranular fission gas bubbles in UO2 during irradiation," Journal of Nuclear Materials, vol. 38, pp. 203-212, 1971. [7] R. J. White, "The development of grain-face porosity in irradiated oxide fuel," Journal of Nuclear Materials, vol. 325, pp. 61-77, 2004. [8] J. -P. Hiernaut, T. Wiss, J. -Y. Colle, H. Thiele, C. T. Walker, W. Goll and R. M. Konings, "Fission product release and microstructure changes during laboratory annealing of a very high burn-up fuel specimen," Journal of Nuclear Materials, vol. 377, pp. 313-324, 2008. [9] K. Govers, C. L. Bishop, D. C. Parfitt, S. E. Lemehov, M. Verwerft and R. W. Grimes, "Molecular dynamics study of Xe bubble re-solution in UO2," Journal of Nuclear Materials, vol. 420, pp. 282290, 2012. [10] V. V. Rondinella and T. Wiss, "The high burn-up structure in nuclear fuel," Materials Today, vol. 13, no. 12, pp. 24-32, December 2010. [11] M. Kinoshita, T. Kameyama, S. Kitajima and H. Matzke, "Temperature and fission rate effects on the rim structure formation in a UO2 fuel with a burnup of 7.9% FIMA," Journal of Nuclear Materials, vol. 252, pp. 71-78, 1998. [12] C. T. Walker, S. Bremier, S. Portier, R. Hasnaoui and W. Goll, "SIMS analysis of an UO 2 fuel irradiated at low temperature to 65MWd/kgHM," Journal of Nuclear Materials, vol. 393, pp. 212223, 2009. [13] J. Spino, D. Papaioannou and J. -P. Glatz, "Comments on the threshold porosity for fission gas release in high burn-up fuels," Journal of Nuclear Materials, vol. 328, pp. 67-70, 2004. [14] W. Beere and G. L. Reynolds, "The morphology and growth rate of interlinked porosity in irradiated UO2," Journal of Nuclear Materials, vol. 47, pp. 51-57, 1973. [15] J. Noirot, I. Zacharie-Aubrun and T. Blay, "Focused ion beam-scanning electron microscope examination of high burn-up UO2 in the center of a pellet," Nuclear Engineering and Technology, vol. 50, pp. 259-267, 2018. [16] R. Restani, M. Horvath, W. Goll, J. Bertsch, D. Gavillet, A. Hermann and M. Martin, "On the condition of UO2 nuclear fuel irradiated in a PWR to a burn-up in excess of 110 MWd/kgHM," Journal of nuclear materials, vol. 481, pp. 88-100, 2016. [17] F. Cappia, D. Pizzocri, A. Schubert, P. Van Uffelen, G. Paperini, D. Pellottiero, R. Macian-Juan and V. V. Rondinella, "Critical assessment of the pore size distribution in the rim region of high burnup UO2 fuels," Journal of Nuclear Materials, vol. 480, pp. 138-149, 2016.
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Three-Dimensional Bubble Reconstruction in High Burnup UO2 • • • •
The evolution of fission gas bubbles was investigated in light water reactor fuel. The high burnup structure (HBS) showed no interconnected bubble networks. The central region of the fuel showed little evidence of bubble interconnection. Interconnection did not cause substantial fission gas release.
Casey McKinney: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing, Visualization Rachel Seibert: Writing – review & editing, Supervision Grant Helmreich: Software, Formal analysis, Data curation, Writing – original draft, Writing – review & editing, Visualization Assel Aitkaliyeva: Writing – original draft, Writing – review & editing Kurt Terrani: Conceptualization, Writing, - original draft, Writing – review & editing, Project administration, Funding acquisition
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: