INL/CON-14-31531 PREPRINT Irradiation Performance of AGR-1 High Temperature Reactor Fuel Proceedings of the HTR 2014 Paul A. Demkowicz, John D. Hunn, Scott A. Ploger, Robert N. Morris, Charles A. Baldwin, Jason M. Harp, Philip L. Winston, Tyler J. Gerczak, Isabella J. van Rooyen, Fred C. Montgomery, Chinthaka M. Silva October 2014 This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint should not be cited or reproduced without permission of the author. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, or any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party’s use, or the results of such use, of any information, apparatus, product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. The views expressed in this paper are not necessarily those of the United States Government or the sponsoring agency. 1 Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 Irradiation Performance of AGR-1 High Temperature Reactor Fuel Paul A. Demkowicz, John D. Hunn1, Scott A. Ploger, Robert N. Morris1, Charles A. Baldwin1, Jason M. Harp, Philip L. Winston, Tyler J. Gerczak1, Isabella J. van Rooyen, Fred C. Montgomery1, Chinthaka M. Silva1 Idaho National Laboratory P.O. Box 1625, Idaho Falls ID, 83415-6188, USA phone: +001-208-5263846, [email protected] 1Oak Ridge National Laboratory P.O. Box 2008, Oak Ridge TN, 37831-6093, USA Abstract – The AGR-1 experiment contained 72 low-enriched uranium oxide/uranium carbide TRISO-coated particle fuel compacts in six capsules irradiated to burnups of 11.2 to 19.6% FIMA, with zero TRISO coating failures detected during the irradiation. The irradiation performance of the fuel–including the extent of fission product release and the evolution of kernel and coating microstructures–was evaluated based on detailed examination of the irradiation capsules, the fuel compacts, and individual particles. Fractional release of 110mAg from the fuel compacts was often significant, with capsule-average values ranging from 0.01 to 0.38. Analysis of silver release from individual compacts indicated that it was primarily dependent on fuel temperature history. Europium and strontium were released in small amounts through intact coatings, but were found to be significantly retained in the outer pyrocarbon and compact matrix. The capsule- average fractional release from the compacts was 1×10-4 to 5×10-4 for 154Eu and 8×10-7 to 3×10-5 for 90Sr. The average 134Cs release from compacts was <3×10-6 when all particles maintained intact SiC. An estimated four particles out of 2.98×105 experienced partial cesium release due to SiC failure during the irradiation, driving 134Cs release in two capsules to approximately 10-5. Identification and characterization of these particles has provided unprecedented insight into the nature and causes of SiC coating failure in high-quality TRISO fuel. In general, changes in coating morphology were found to be dominated by the behavior of the buffer and inner pyrolytic carbon (IPyC), and infrequently observed SiC layer damage was usually related to cracks in the IPyC. Palladium attack of the SiC layer was relatively minor, except for the particles that released cesium during irradiation, where SiC corrosion was found adjacent to IPyC cracks. Palladium, silver, and uranium were found in the SiC layer of irradiated particles, and characterization of these elements within the SiC microstructure is the subject of ongoing focused study. I. INTRODUCTION experiment contained 72 fuel compacts, each containing approximately 4,100 TRISO particles The first in a series of tristructural isotropic with kernels comprised of a heterogeneous mixture (TRISO) coated particle fuel irradiation of uranium oxide and uranium carbide (termed experiments, initiated as part of the US Advanced UCO) [2]. The irradiation experiment was a major Gas Reactor (AGR) fuel development and success, achieving a peak compact-average burnup qualification effort [1], completed 620 effective full of 19.6% fissions per initial heavy metal atom power days in the Advanced Test Reactor at the (FIMA) with zero TRISO coating failures (i.e., Idaho National Laboratory (INL). The AGR-1 failure of all three dense coating layers) detected Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 during the irradiation based on monitoring of fission of each variant involved modification of the gas release from the fuel [3]. deposition conditions and properties of either the Post-irradiation examination (PIE) of the inner pyrolytic carbon (IPyC) or SiC layer relative AGR-1 experiment commenced in 2010 at INL and to the Baseline in order to explore the effect of Oak Ridge National Laboratory (ORNL). The various coating properties on irradiation primary objectives of the AGR-1 PIE are to analyze performance [7]. For Variant 1, the IPyC coating the fuel and irradiation capsule components to verify layer conditions were varied to provide a slightly fuel performance in the reactor and perform post- lower anisotropy and density. For Variant 3, the SiC irradiation high temperature safety tests to assess layer deposition conditions were varied to provide a fuel performance during postulated reactor accident finer grain microstructure relative to the Baseline scenarios. Fuel performance assessment focuses in fuel. Fabrication of the different fuel types also particular on the behavior of the kernel and coating resulted in minor differences in other average layers and the retention of fission products by the properties (including coating thickness, number of particles and fuel compacts, a key metric since particles per compact, packing fraction, and defect fission product retention by TRISO fuel is an populations) [8]. Due to time limitations, PIE did important component of the reactor safety basis. not include extensive investigation of AGR-1 Ultimately the goal is a better understanding of the Variant 2 fuel. causes of coating degradation and failure, the role The irradiation experiment consisted of six that fission products may play in these, and the separate capsules, each with independent sweep gas transport behavior of fission products in fuel flow and temperature monitoring and control. Each particles and compacts, both when coatings perform capsule contained 12 fuel compacts of a single fuel as designed and when they are damaged by type retained in a graphite fuel holder in three stacks irradiation or elevated temperature. oriented in a triangular array (Fig. 1), with each stack containing four compacts [3]. Each compact II. EXPERIMENTAL has a unique identifier in the format X-Y-Z that denotes the original position in the experiment: X The AGR-1 PIE consists of a diverse set of indicates the capsule, Y indicates the axial level analyses performed on the fuel compacts, particles, within the capsule, and Z indicates the stack. and the irradiation capsules. Due to the unique Compact 6-1-1 was irradiated in Capsule 6, Level 1 nature of coated particle fuel, specialized equipment (first level at the bottom of the capsule), Stack 1. has been developed at both INL and ORNL for performing some of these tests and analyses. The experimental results presented and discussed in this (a) paper are focused on assessing the performance of the fuel during the irradiation. In addition, high temperature safety tests have been performed on the AGR-1 fuel at INL and ORNL in order to explore the fuel performance under the conditions that may exist in the reactor core during a depressurized loss of forced cooling accident [4]. II.A. AGR-1 fuel and irradiation experiment The AGR-1 UCO fuel kernels were fabricated by Babcock and Wilcox Nuclear Operations Group [5]. Kernels were nominally 350 µm in diameter and (b) 235U enrichment was 19.74%. The TRISO coatings were applied at ORNL. Nominal coating thicknesses were 100 µm for the porous carbon buffer, 40 µm for the inner pyrolytic carbon (IPyC) layer, 35 µm for the SiC layer, and 40 µm for the outer pyrolytic carbon (OPyC) layer. The coated particles were formed into right cylindrical compacts at ORNL that were nominally 12.4 mm in diameter and 25.1 mm in length. Compacts contained an average of 4126– Fig. 1: Cutaway diagram of an AGR-1 capsule 4150 particles with a packing fraction of showing the key components (a) and cross-sectional approximately 37% [6]. A baseline fuel and three view of a capsule (b). different fuel variants were fabricated. Fabrication Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 The irradiation was performed from December the irradiation. As there were 12 compacts in each 2006 to November 2009 for a total of 620 effective irradiation capsule, the data provide information on full power days. The calculated compact average the total inventory of fission products released from burnup ranged from 11.3 to 19.6% FIMA, and all 12 fuel compacts; release from each compact compact average fast neutron fluence ranged from to cannot be derived from these data. 2.17 to 4.30×1025 n/m2. The time-average, volume- The metal capsule shells were leached in acid to average compact temperatures were 955–1136°C remove deposited fission products and the solutions and the time-average maximum compact analyzed with gamma spectrometry, inductively temperatures were 1069–1197°C. Based on the low coupled mass spectrometry (ICP-MS), and gas-flow fission gas release-to-birth ratios in all of the proportional counting (for 90Sr). The graphite capsules, there were zero TRISO-coating failures holders were gamma counted to determine the during the irradiation out of a total of approximately inventory of gamma-emitting fission products, and 2.98×105 in the experiment [3]. then crushed, oxidized, and leached and the solutions analyzed with ICP-MS and gas-flow II. B. Analysis of metallic fission product release proportional counting. The graphite spacers at the axial ends of each capsule were gamma counted. The release of metallic fission products from the Details of the experimental procedures and the fuel during irradiation was assessed in several ways inventories of specific isotopes on the various during post-irradiation examination, as briefly components are available in References [10] and summarized here. In all cases, the measured [11]. inventories of fission products were decay-corrected to the end of the AGR-1 irradiation plus 1 day (7 II.D. Gamma scanning of individual fuel compacts November 2009, 12:00 GMT) and compared to the predicted inventory, based on physics simulations of Fifty-six of the 72 AGR-1 compacts were the AGR-1 irradiation [9], to convert the measured individually gamma scanned at INL with sufficiently value to a fractional inventory. Table 1 lists the types long counting times to quantify 110mAg inventory of analyses performed and summarizes the key [12]. The experimental apparatus consists of a high- information pertaining to fission product release purity germanium detector with a Compton from the fuel that each data set provides. suppression system, and a rectangular collimator with a 2.5-mm opening. Each compact was counted Table 1: Post-irradiation analysis of fuel particles, in 2.5-mm axial “slices”. The gamma scanning compacts, and capsule components and the key system was calibrated so that the total activity in a information provided on fission product release. compact could be measured by adding the measured activity from all scans corresponding to a single Key information pertaining to Analysis fission product release compact. Interference from 137Cs prevented the use of the primary 110mAg gamma-ray necessitating the Cumulative inventory of fission use of the second- and third-most intense gamma- Capsule products released from all fuel ray lines for activity determination. The components compacts in a capsule during experimental setup for gamma scanning the irradiation compacts for 110mAg was very similar to that Fuel compact Retained inventory of fission described in Reference [13]. By comparing products in each individual fuel gamma scanning measured inventories with the predicted values, the compact data provide information on the level of silver Inventory of fission products retention in each compact analyzed. Deconsolidation- retained in the compacts outside of leach-burn-leach the SiC layers II.E. Deconsolidation and leach-burn-leach analysis Particle gamma Retained inventory of fission of fuel compacts counting products in the particles A total of nine AGR-1 compacts were electrolytically deconsolidated in the as-irradiated II.C. Analysis of irradiation capsule components for state and leach-burn-leach analysis was performed at deposited fission products both INL and ORNL. The deconsolidation-leach- burn-leach (DLBL) analysis effectively measures The major components of the irradiation the total inventory of fission products located in the capsules—including the metal shells and structural compact outside of the SiC layer (i.e., it includes components, the graphite fuel holders, and graphite both the OPyC and matrix), provided that no spacers at the ends of the capsules—were analyzed particles with a failed SiC layer are present (in this at INL to determine the cumulative inventories of article, “SiC failure” refers to loss of integrity of the fission products released from the compacts during Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 SiC layer, with at least one pyrocarbon layer solution for each cycle. The leachate solution from remaining intact, and is distinct from complete each of the cycles was assayed for fission products TRISO-coating failure). It is therefore a measure of and actinides. The entire thimble was then fission products that (a) are retained within the transferred to an oven where a burn step is compact outside of the SiC layers, and (b) generated performed for 72 hours at 750°C in air. This from uranium contamination in the compact matrix oxidized all exposed carbon, including any or OPyC and retained within the compact during remaining matrix debris, the outer pyrolytic carbon irradiation. As the level of uranium contamination in layer, and the inner pyrolytic carbon and buffer the AGR-1 compacts was generally very low layers of any particles with fractures or holes in the (average uranium contamination fractions were less SiC layer. The thimble was then transferred back to than 4×10-7 for all AGR-1 fuel types), the inventory the Soxhlet extractor and two more 24-hour leach measured from DLBL is typically dominated by cycles were performed, with each solution again release from the particles. In some cases, if assayed for actinides and fission products. If any measured values are very low (as is often the case particles had failed SiC, the post-burn leach steps with cesium, where release through intact SiC is dissolved the kernel exposed by the prior removal of found to be extremely low), then hot cell the carbon layers during the burn step. Thus, the contamination can constitute a significant post-burn leach data can be used to determine the contribution to the total measured values. The nine number of particles with failed SiC that were present AGR-1 compacts analyzed are listed in Table 2 with based on the measured inventory of uranium in the the fuel type and selected irradiation properties. solutions (a single irradiated AGR-1 kernel contains approximately 0.18 mg of uranium). Table 2: Irradiated AGR-1 fuel compacts used for Analysis of the leach solutions included gamma DLBL analysis and particle gamma counting. spectrometry for gamma-emitting fission products, separation and gas flow proportional counting for Compact a Fuel type bTAVA Temp [14](°C) cTA Max Temp [14](°C) Burnup [9](%FIMA) Fast fluence [9]252(10 n/m) sgoR90pafSe emfrtce,hm trereoa namA ceneGemdst Rr i[yt-t1 i1i(5nn I]gdCD auPfLnci-sdBtMis vLi[oS1e nl6)ey ]xf p.op rreoc rdaoimucutcpietnnlsei.td dsAe sad rdapeni ltadpiso rmoenstaahel endrt eemndtaoa inislnss- 6-3-2 B 1070 1144 11.4 2.55 II.F. Gamma counting of individual coated particles 6-1-1 B 1111 1197 15.3 2.43 3-2-1 B 1051 1143 19.1 4.21 Individual particles were gamma counted at 5-2-1 V1 1057 1140 17.4 3.71 various stages of the DLBL process to determine the 5-2-3 V1 1059 1141 17.4 3.77 inventory of gamma-emitting fission products. 5-3-1 V1 1040 1122 16.7 3.60 4-4-2 V3 1024 1139 16.6 3.59 Analysis of the fission product inventory present in 4-1-1 V3 1072 1182 19.4 4.13 the particles allows the relative degree of retention 1-3-1 V3 1092 1166 15.3 3.22 to be evaluated. This analysis has a twofold a B=Baseline; V1=Variant 1; V3=Variant 3 objective. b Time average, volume average temperature Individual particles with abnormally low c Time average maximum temperature inventories of specific fission products can be identified, and this may be indicative of a defective Compacts were deconsolidated in a glass tube SiC layer, as in the case of high cesium release. with the bottom ~1 mm submerged in concentrated While cesium diffuses with relative ease through nitric acid. A platinum-rhodium wire was placed on intact pyrocarbon, it is effectively retained by intact the top of the compact (the anode) and a second SiC. However, a SiC layer defect or in-pile failure electrode (the cathode) was placed into the nitric may allow cesium to readily escape from the particle acid solution. An electric potential with a net power at normal irradiation temperatures. Therefore, level of less than 10 watts was applied, resulting in measurements of the relative 137Cs inventory can be electrolytic oxidation of the compact matrix, used to identify particles with failed SiC. Such liberating the particles as the reaction proceeded. particles can then be retained for subsequent The loose particles and deconsolidated matrix debris microstructural analysis to investigate the nature of were collected in a Soxhlet thimble for subsequent the SiC failures. Identification and subsequent processing. analysis of specific particles with failed SiC layers is At the completion of the deconsolidation step, the topic of a separate paper in these proceedings the thimble was transferred into a Soxhlet extraction [17] and is not discussed here. apparatus and two 24-hour extraction cycles were In addition to finding particles with a defective performed using fresh concentrated nitric acid SiC layer, the distribution of fission product Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 activities among a population of particles can particle from that fuel compact to determine an provide information about the varying degree of estimated fraction of 110mAg retained. To minimize release through intact SiC. This is primarily of the effect of particle-to-particle variation in fissile interest for silver, which may exhibit significant content (due to variation in kernel size, density, and release from particles (several percent or more under stoichiometry) and burnup, the ratio of measured normal irradiation conditions) such that differences 110mAg activity to the calculated 110mAg activity was can be detected above the uncertainty of the gamma normalized using the 137Cs activity, as in the counting technique. By measuring the 110mAg following equation, retained in the particles, specific particles with high or low release can be selected for microstructural AAg−110m examination that may elucidate the causes of the i   behavior. For example, variations in SiC   microstructure might be correlated with the level of AAg−110m× AiCs−137  silver release. calc ∑n 1 ACs−137  Counting all of the approximately 4,100 particles  j=1n j  in each compact is made possible by the relatively short count times required for 137Cs and 144Ce. Other important fission products such as 110mAg and 154Eu where AiAg−110m is the decay-corrected measured tend to have much lower total activity in the 110mAg activity of particle i, AAg−110m is the average particles (generally <3×104 Bq for 110mAg and calculated 110mAg activity focra lca single particle, ~1×105 for 154Eu versus 4×107 Bq for 144Ce and ACs−137 is the decay-corrected measured 137Cs 4×106 Bq for 137Cs) and therefore require much i activity for particle i, and n is the total number of longer counting times (generally several hours) to particles counted. obtain statistically relevant quantification. As a consequence of the longer counting times, II.G. Microanalysis processing of all particles would be prohibitively time consuming. Therefore a subset of particles Microanalysis of the AGR-1 fuel consisted of (approximately 40–120) is counted to establish a characterization of both whole compacts and distribution of activities for these isotopes. individual particles. Six irradiated AGR-1 fuel Irradiated particle gamma counting was compacts were sectioned axially and longitudinally performed at both INL and ORNL. At INL, and the polished cross-sections were examined using individual particles (typically 60 per compact) were optical microscopy as described in Reference [19]. manually placed in glass vials and spectra acquired Approximately 1000 particles exposed in the with a lithium-drifted germanium gamma compact cross-sections were examined and spectrometer. The energy efficiency of the spectrometer system was calibrated using a 3.48×106 characterized based on the kernel and coating Bq 152Eu point source. Count times were typically behavior during irradiation. In addition, individual particles taken from varied from 1 to 12 hours in order to minimize total numerous deconsolidated fuel compacts were experimental duration while still obtaining sufficient counting data to quantify 110mAg activity. mounted in epoxy, ground, and polished to prepare cross-sections for examination. Several hundred Gamma counting of particles at ORNL was such particles were successively ground and accomplished using the Advanced Irradiated polished to expose particle cross-sections at Microsphere Gamma Analyzer (Advanced-IMGA) increasing displacements into the particle. This was [15][17][18], an automated system developed at undertaken to better understand the morphology of ORNL specifically for TRISO particle examinations. irradiated kernels and coating layers in three The system uses a vacuum needle with two-axis dimensions, as two-dimensional cross-sections often directional movements to automatically select a limit complete characterization of important single particle from a vial located on a rotating features. Major results of optical microscopy are carousel, reposition the particle in front of a high- discussed in this paper. purity germanium gamma detector, gamma count the Detailed microanalysis of individually mounted particle for a predetermined time interval, place the irradiated particles included scanning electron particle into a designated receptacle vial based on microscopy (SEM) with elemental analysis specified criteria, and then repeat the process until including energy dispersive x-ray spectroscopy all particles have been counted. (EDS) and wavelength dispersive x-ray Subsets of particles from nine AGR-1 compacts spectroscopy (WDS) to examine the coating (listed in Table 2) were gamma counted. The measured 110mAg inventory in each particle was microstructure and distribution of fission products and actinides within the coating layers. compared to the calculated activity in an average Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 Following this characterization, selected components from each capsule (detailed data for particles were examined with more advanced isotopic inventory found on each capsule component techniques to further explore the coating can be found in Reference [11]). Values are microsctructures (especially the SiC layer) and presented as a fraction of the predicted inventory in investigate the migration of fission products and the capsule (includes the contribution from all actinides within the layers in order to better twelve compacts in each capsule). understand transport behavior. A focused ion beam The inventory of key fission products found (FIB) was used to prepare small specimens from the outside the SiC layer in the nine compacts listed in kernels and coating layers for analysis with Table 2 is given in Table 4 (based on DLBL data), transmission electron microscopy (TEM) and where the totals include exposed fission products in scanning transmission electron microscopy (STEM). the fuel compact detected by both the pre-burn and Elemental analysis was performed on selected post-burn leaches. The values are presented as a specimens using TEM-EDS and electron energy loss fraction of the predicted inventory in the compact, spectroscopy (EELS) in order to help identify and represent the inventory of fission products specific elements in the coating layers, often at the released from the particles but retained within the nanometer scale. Finally, work has begun using OPyC or compact matrix during irradiation. Note atom probe tomography (APT) to further expand the that in Table 3 and Table 4 134Cs data have been existing database with information down to the presented instead of 137Cs, as the 134Cs atomic level. measurements were found to be less subject to bias from the presence of hot cell contamination due to III. FISSION PRODUCT RELEASE the much shorter half life (2.07 y for 134Cs compared to 30.07 y for 137Cs). In instances where hot-cell III.A Experimental results contamination had a minimal effect on the cesium measurements, the agreement between fractional The inventory of key fission products released releases calculated for 134Cs and 137Cs was generally from the compacts to the capsule components is very good. Similarly, although only 154Eu is summarized in Table 3. These values represent the reported, 155Eu was also measured and yielded sum of inventories measured on the various matching results. Table 3: Fractional inventory of fission products on the AGR-1 capsule components. Capsule # Fuel type 110mAg 144Ce 134Cs 154Eu 90Sr 6 Baseline 3.8E-1 9.7E-6 1.3E-5 4.6E-4 3.1E-6 5 Variant 1 2.3E-1 <2E-6 1.2E-5 1.4E-4 7.1E-6 4 Variant 3 1.2E-1 <4E-6 <3E-6 1.3E-4 9.7E-6 3 Baseline 1.2E-2 <4E-6 <3E-6 4.3E-4 2.2E-6 2 Variant 2 5.5E-2 <2E-6 <2E-6 1.6E-4 8.4E-7 1 Variant 3 3.6E-1 <3E-6 <3E-6 1.3E-4 2.8E-5 Note that a fraction of 2.0E-5 corresponds to the equivalent inventory of a single particle (based on approximately 49,200 particles per capsule). Table 4: Fractional inventory of fission products in the DLBL solutions. Compact 110mAg 144Ce 134Cs 154Eu 105Pd 90Sr 238U 6-3-2 2.2E-4 1.7E-4 7.3E-5 5.9E-3 4.3E-4 2.8E-4 6-1-1 1.7E-1 6.9E-4 2.1E-5 1.3E-2 1.1E-2 5.9E-4 <1E-4 3-2-1 6.6E-3 <4E-6 <3E-6 7.4E-4 1.5E-6 5.1E-6 5-2-3 3.3E-3 1.5E-3 4.4E-5 6.0E-3 3.2E-2 3.1E-3 6.5E-5 5-2-1 3.9E-3 1.6E-3 4.8E-5 5.8E-3 3.0E-2 1.9E-3 8.2E-5 5-3-1 9.4E-3 3.5E-4 2.5E-6 1.2E-3 6.3E-3 1.9E-4 2.4E-5 4-4-2 2.3E-2 1.3E-5 1.2E-5 5.9E-4 1.1E-2 1.6E-5 6.3E-5 4-1-1 3.3E-2 6.6E-7 <4E-7 2.4E-4 1.7E-6 1.3E-5 1-3-1 3.6E-3 1.4E-4 5.4E-6 6.3E-3 8.6E-3 2.6E-3 2.0E-5 Shaded rows indicate compacts known to have one or more particles with failed SiC. Note that a fraction of 2.4E-4 corresponds to the equivalent inventory of a single particle. Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 Three of the compacts in Table 4 were found to simulation (in this case the implication is that have one or more particles with failed SiC, as predicted values could be biased low), or a discussed further below. These compacts are shaded combination of both. in the table. In the case of Compacts 5-2-3 and 5-2-1, the particles with failed SiC were first identified using particle gamma counting following the pre-burn leaching steps and removed from the population prior to burn-leach analysis, therefore the SiC failures kernels of these particles were not leached during that analysis. Compact 6-3-2 contained a single particle with failed SiC, and this particle remained in the population when burn-leach was performed. As a result, the contents of this kernel were dissolved in the burn-leach solutions and are included in the totals in Table 4 (note the inventory of 238U for Compact 6-3-2, which is close to the equivalent fraction of a single particle, 2.4×10-4, indicating that the kernel was dissolved in the leach solution). Therefore the values in this table for Compact 6-3-2 that are at or below a value of approximately 2.4×10-4 (including 110mAg, 144Ce, and 134Cs) are not Fig. 2: The range of inventory fractions found a reliable measure of the inventory retained in this retained in irradiated compacts outside of the SiC compact outside of the SiC layer because of the layer (red columns) and on the capsule components additional contribution from the kernel leached (blue columns). The portions of the data circled and during the post-burn leach. labeled as “SiC failures” are related to the presence The data in Table 3 and Table 4 are summarized of particles with failed SiC layers, as discussed in graphically in Fig. 2, which indicates the range of the text. The cross-hatched regions indicate values fractional fission product inventory retained in the that correspond to measured inventories below the compacts outside the SiC layer (red columns) and detection limit, and therefore represent a released from the 12 compacts in each capsule (blue conservative upper-bound on the range. columns). In Fig. 2, the separate upper data ranges for 134Cs (circled in the figure) are attributed to compacts or capsules containing particles with failed SiC layers, while the lower data ranges are due to 1.2 Capsule 6 Capsule 5Capsule 4 Capsule 3 Capsule 2 Capsule 1 fuel with only intact SiC (discussed further below). The cross-hatched regions of several of the data 1.0 columns indicate that the measured inventories on n o some capsule components were below the detection acti 0.8 limit of the technique, and the sum of contributions g fr from all components therefore represents a mA 0.6 conservative upper bound for the total inventory in 110d e several of the capsules. ain 0.4 The estimated fraction of 110mAg retained in 56 Ret Stack 1 of the 72 AGR-1 compacts at the end of irradiation 0.2 Stack 2 is shown in Fig. 3. This was determined from the Stack 3 0.0 fuel compact gamma scanning data, with the total Vertical position measured inventory for each compact compared to Fig. 3: Fraction of retained 110mAg inventory in 56 of the predicted inventory from physics simulations. the 72 AGR-1 fuel compacts after irradiation. Data The data are plotted as a function of the vertical are plotted as a function of vertical position in the position in the AGR-1 test train, with the top of the experiment (top of the experiment at the left) and by test train at the left, and compacts in each of the the stack number. three fuel stacks plotted separately. Note that the fraction slightly exceeds 1.0 (the maximum value is The fraction of 110mAg retained in individual 1.09) for several compacts in Capsules 2, 3, and 4. particles often varied considerably within a single This could be due to uncertainties in the gamma compact. The distributions of 110mAg fraction spectra analysis (total uncertainty in the final values retained in particles from several compacts are is estimated to be approximately 5%), a small bias in shown in Fig. 4. In some cases (e.g., Compact 3-2- the predicted 110mAg inventories from the physics Proceedings of the HTR 2014 Weihai, China, October 27-31, 2014 Paper HTR2014-31182 1), there was a relatively high degree of silver Table 5: Average fraction of 110mAg retained in a retention in the sample of particles analyzed (mean sample of particles taken from the irradiated AGR-1 of the distribution near 1.0). In other cases (e.g., compacts. Compact 5-2-3), the distribution was extremely Average retained Compact broad, with a portion of the particles exhibiting fraction of 110mAg almost complete release of silver during the 6-3-2 0.36 – 0.51 irradiation (particles plotted in the zero bin had 6-1-1 0.39 – 0.45 3-2-1 0.92 activities below a quantification limit of ~0.08). The 5-2-3 0.43 – 0.45 presence of particles in these distributions with 5-2-1 0.30 – 0.45 estimated retained 110mAg fractions as high as ~1.2 is 5-3-1 0.48 – 0.57 believed to be due in part to an appreciable variation 4-4-2 0.93 in the 110mAg inventory generated in particles based 4-1-1 1.02 on their original location within the fuel compacts. 1-3-1 0.12 – 0.28 Based on the particle gamma counting data, the average fraction of 110mAg retained in the subset of III.B. Discussion particles gamma counted from each compact was determined, and is presented in Table 5. These Cesium values represent an estimate for the fraction of 110mAg retained within the entire population of Past experience with TRISO fuel has particles for each compact. For several of the demonstrated that the SiC layer provides the primary compacts, a number of gamma counted particles did containment of cesium in the particles, and that not have detectable 110mAg activity. In these cases, a cesium release is therefore an effective indicator of minimum detection limit was determined, and was SiC failure [20][21]. The total fractional release of used as an upper bound on the activity of 110mAg in 134Cs from the AGR-1 compacts was <3×10-6 in the particle, while a value of zero was used as the Capsules 1 – 4 (Table 3), but notably higher in lower bound. The range provided in Table 5 for Capsules 5 and 6 (~10-5). some of the compacts uses these two bounding Detailed spatial gamma spectrometry of the values to account for the unknown activity in these AGR-1 graphite fuel holders initially identified particles. specific compacts in Capsules 5 and 6 that released higher-than-average quantities of cesium during 8 irradiation [10]. The identified compacts were the Compact 3-2-1 focus of detailed examination to locate and cy 6 59 particles n characterize any particles containing a failed SiC e u 4 q layer that could be responsible for the cesium e Fr 2 release. This effort is detailed in a separate paper in these proceedings [17]. The results of this analysis 0 0 2 4 6 8 0 2 4 have shown that three AGR-1 compacts (two in 0. 0. 0. 0. 0. 1. 1. 1. Capsule 5 and one in Capsule 6) contained particles 20 Compact 5-2-3 with failed SiC and were responsible for a y 15 57 particles significant portion of the total cesium that was c en 10 released in those capsules. Compact 5-2-3 contained u eq 5 two such particles while Compact 5-2-1 contained Fr one. All three of these particles were located with 0 0 2 4 6 8 0 2 4 the IMGA and detailed analysis has been performed. 0. 0. 0. 0. 0. 1. 1. 1. The level of 134Cs fractional release from these 20 particles ranged from 0.38 to 0.65, which shows Compact 4-4-2 that, while SiC is the most effective cesium barrier 15 90 particles cy in the TRISO particle, it is not the only defense n ue10 against cesium release [17]. Other analyzed Freq 5 radioisotopes (95Zr, 106Ru, 125Sb, 144Ce, and 154Eu) were retained in these three particles within the 0 0 2 4 6 8 0 2 4 normal distribution measured by the IMGA on 0. 0. 0. 0. 0. 1. 1. 1. Estimated fraction of retained Ag-110m randomly-selected particles with intact SiC. A single particle with failed SiC was identified in Compact Fig. 4: Distributions for the fraction of 110mAg 6-3-2; this particle was identified during DLBL retained in particles from three AGR-1 compacts. analysis, as evidenced by the level of uranium in the burn-leach solutions (see Table 4), which was equivalent to the inventory in a single kernel.