Ceramographic Examinations of Irradiated AGR-1 Fuel Compacts [report]

Paul Demkowicz, Scott Ploger, John Hunn
2012 unpublished
The AGR-1 experiment involved irradiating 72 cylindrical fuel compacts containing tri-structural isotropic (TRISO)-coated particles to a peak burnup of 19.5% fissions per initial metal atom with no inpile failures observed out of almost 300,000 particles. Five irradiated AGR-1 fuel compacts were selected for microscopy that spanned the range of irradiation conditions (temperature, burnup, and fast fluence). These five compacts also included all four TRISO coating variations irradiated in the
more » ... rradiated in the AGR-1 experiment. The five compacts were cross-sectioned both transversely and longitudinally, mounted, ground, and polished after development of careful techniques for preserving particle structures against preparation damage. Approximately 40 to 80 particles within each cross section were exposed near enough to midplane for optical microscopy of kernel, buffer, and coating behavior. The microstructural analysis focused on kernel swelling and porosity, buffer densification and fracturing, debonding between the buffer layer and the inner pyrolytic carbon (IPyC) layer, and fractures in the IPyC and SiC (silicon carbide) layers. Three basic particle morphologies were established according to the extent of bonding between the buffer and IPyC layers: complete debonding along the interface (Type A), no debonding along the interface (Type B), and partial debonding (Type AB). These basic morphologies were subdivided according to whether the buffer stayed intact or fractured. The resulting six characteristic morphologies were used to classify particles within each cross section, but no spatial patterns were clearly observed in any of the cross-sectional morphology maps. Although positions of particle types appeared random within compacts, examining a total of 830 classified particles allowed several specific relationships among morphological types to be established: Buffer-IPyC debonding was prevalent in irradiated AGR-1 particles. Nearly 62% of the irradiated AGR-1 particles showed a radial gap between buffer and IPyC layers from complete debonding while less than 6% of the particles had fully intact buffer-IPyC bonds. Partial debonding was found in approximately 33% of the classified particles. These percentages varied only slightly among the cross-sectioned compacts, so differences in irradiation temperature and neutron exposure (burnup, fast fluence) apparently had little influence on buffer-IPyC bond integrity. Buffer layers remained intact in over three-fourths of the AGR-1 particles. Fractured buffers were nearly three times as likely in particles with partial buffer-IPyC debonding (Type AB) than in particles with complete debonding (Type A). This tendency was found in all five cross-sectioned compacts, so differences among them in irradiation temperature and neutron exposure apparently had little influence on buffer fracturing. Buffer fractures in Type A particles tended to have smooth radially oriented contours. Buffer fractures in Type AB particles tended to have jagged contours (mixture of tangential, circumferential, and radial directions). Most buffer fractures in Type AB particles appear to have initiated tangentially at the kernel interface, while most buffer fractures in Type A particles apparently initiated radially. In addition to buffer fractures, all classified particles were inspected for fractures in other TRISO coating layers. No irradiation-induced fractures were found in OPyC layers. OPyC layers also resisted fracturing during the occasional cases where the carbonaceous compact matrix fractured when compacts were sawed and mounted. The only instances found of OPyC-matrix debonding were caused by preparation damage, especially sawing of the compacts. Although partial IPyC fractures and more severe through-IPyC fractures were found in only 50 of the 830 particles (6%), fractures through the entire IPyC layer were much more likely than average in particles from the Variant 1 compact, which had the lowest density IPyC layer. Significant variations in IPyC fracturing behavior also were seen among morphological types: vi All buffer fractures in Type B particles propagated into IPyC fractures, presumably because of the intact buffer/IPyC interface. Through-IPyC fractures were more likely in particles with partial buffer-IPyC debonding (Type AB) than with complete debonding (Type A). In Type AB particles, IPyC fractures tended to occur where buffer-IPyC debonds ended (where buffer-IPyC bonding was locally strong). In Type AB particles, IPyC fractures were much more likely with fractured buffers than with intact buffers, although IPyC fractures in these particles normally did not propagate from buffer fractures. Extra attention was paid to extremely rare SiC fractures because SiC constitutes the primary barrier to migration of metallic fission products in TRISO-coated particle fuel: Tangential SiC fractures were found in only five particles from the five cross-sectioned AGR-1 compacts. All of the observed fractures were apparently confined close to the IPyC interface, and none appeared to breach the entire layer thickness during the AGR-1 experiment. Two of these particles contained foreign material along a portion of the IPyC-SiC interface, which may be soot picked up from the coating chamber wall. The SiC fractures in these two particles initiated tangentially where the foreign material ended and IPyC was able to bond to the SiC. Two Type B particles showed tangential SiC fractures at the ends of extensive IPyC-SiC debonds, where the debonding apparently began at the tips of radial fractures through both buffer and IPyC. One unclassified particle exhibited a tangential SiC fracture that may have propagated directly from a tangential IPyC fracture. Tangential SiC fractures appeared more likely in conjunction with extensive IPyC-SiC debonds. Very short IPyC-SiC debonds were found at tips of IPyC fractures in eight Type AB particles with fractured buffers, but no SiC fractures were discovered in these eight particles. SiC fractures also seemed somewhat more likely in particles from the Variant 1 compact, which probably is related to its much higher frequency of through-IPyC fractures. Observations of kernels in the five AGR-1 compacts suggested that kernels often swelled into fractured buffer cavities, at least where buffer fractures occurred relatively early during irradiation. Kernel pores generally appeared somewhat larger inside fractured buffers than nearby intact buffers, perhaps reflecting less constraint on kernels after buffer fracturing. Similar side-by-side comparisons indicated that pores were even larger inside Type B kernels, where outward buffer densification imposed even less constraint on kernel growth. Insights on kernel and buffer behavior were obtained by extracting measurements from a total of 95 calibrated particle images taken from three transverse cross sections, where the three compacts were selected to cover extremes of irradiation temperature and neutron exposure. To reduce geometric errors, only those particles ground close to midplane were measured. Movement of loose kernels and buffers inside IPyC layers compromised accurate measurements, but dimensions in the plane of polish were obtained and summarized to look for trends. The following observations could be made despite the wide distributions in measurement results: Cross-sectional kernel areas increased during the AGR-1 irradiation. Distributions were similar for all three compacts, so differences in irradiation temperatures and neutron exposures apparently had no major influence on amounts of kernel swelling. Although kernels often swelled into buffer fracture cavities, area increases in kernels surrounded by fractured buffers were similar to those inside intact buffers. This probably reflects constraint vii on kernel growth before buffers fractured. Buffer fractures apparently had more influence on where kernels swelled than on how much they swelled. Kernel area increases were slightly larger on average in Type B particles, where uniformly strong buffer-IPyC bonding and the associated outward buffer densification imposed the least constraint on kernel swelling over the course of the AGR-1 experiment. Porosity (area percentage inside kernel exteriors) and pore enlargement (as indicated by the diameter of the largest pore in each kernel) were greatest in Type B kernels with the least constraint on kernel swelling and smallest in Type A and Type AB kernels with intact buffers (maximum constraint on kernel swelling from inward buffer densification). All measured buffers showed appreciable decreases in area and thickness from as-fabricated values. No differences in the measured buffer parameters could be discerned among the three compacts. Buffer area and thickness reductions appeared similar for radially inward densification (Types A and AB) and outward densification (Type B). Inward buffer densification created a thick buffer-IPyC gap in Type A and Type AB particles with intact buffers. Average gaps with fractured buffers were approximately one-half as thick, evidently because buffer pieces separated after fracturing and moved closer to IPyC interiors. viii
doi:10.2172/1044196 fatcat:f52s2jaed5eyvg5u77nzvdies4