2017 Water Reactor Fuel Performance Meeting September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju Jeju Island, Korea

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1 DEVELOPMENT STATUS OF MICRO-CELL UO2 PELLET FOR ACCIDENT TOLERANT FUEL Dong-Joo Kim, Keon Sik Kim, Dong-Seok Kim, Jang Soo Oh, Jong Hun Kim, Jae Ho Yang, Yang-Hyun Koo Korea Atomic Energy Research Institute, 111, Daedeok-daero 989Beon-gil, Yuseong-gu, Daejeon 34057, Korea ABSTRACT: As an Accident Tolerant Fuel (ATF) pellet, a micro-cell UO 2 pellet is being developed to enhance the accident tolerance of nuclear fuels under accident conditions as well as the fuel performance under normal operation conditions. The improved capture-ability for highly radioactive and corrosive fission product (Cs, I) is the distinct feature of a ceramic micro-cell UO 2 pellet, and the enhanced pellet thermal conductivity is that of a metallic micro-cell UO 2 pellet. The fuel temperature can be effectively decreased by the enhanced thermal conductivity. The material concepts of ceramic and metallic micro-cell UO 2 pellets were designed, and the fabrication process of microcell UO 2 pellets embodying the designed concept was developed. Micro-cell UO 2 pellets, in which the micro-cell concept is successfully implemented, can be obtained. In addition, an assessment of the out-of-pile properties of a micro-cell UO 2 pellet is being performed, and observation of the in-reactor performance and behavior of the developed pellets is in progress through a Halden irradiation test. According to the expectations, the excellent performances of the micro-cell UO 2 pellets are being shown by the online measurement data of the Halden irradiation test. KEYWORDS: Micro-cell UO 2 pellet, Accident tolerant fuel, Fission product retention, Thermal conductivity, Halden irradiation test. I. INTRODUCTION Several ATF (Accident Tolerant Fuel) concepts are currently being suggested and evaluated to mitigate the consequences of an accident. 1-7 At KAERI (Korea Atomic Energy Research Institute), a micro-cell UO 2 pellet as an ATF pellet is being developed to enhance the accident tolerance of nuclear fuels under accident conditions as well as the fuel performance under normal operation conditions The micro-cell consists of UO 2 grains or granules enveloped by thin cell walls, which are continuously connected. Fig. 1 shows a conceptual schematic of a micro-cell UO 2 pellet. The main purpose of the micro-cell UO 2 pellets is to minimize the release of highly radioactive and corrosive fission products (Cs and I) from the fuel, and to reduce the pellet temperature for enhanced fuel safety margin. There are two kinds of micro-cell UO 2 pellets under development at KAERI, classified according to the material type composing the cell wall. These are metallic and ceramic micro-cell UO 2 pellets with distinct features. The metallic micro-cell UO 2 pellet is a highly thermal conductive pellet having a continuously connected metallic wall. A cold pellet would reduce the fission product release by slowing down the diffusion. That is, the mobility of the fission gases is reduced by the lower temperature gradient in the UO 2 fuel pellet. The reduced temperature gradient mitigates the relocation and PCMI (Pellet-Cladding Mechanical Interaction) during a transient operation. In addition, a micro-cell UO 2 pellet with high thermal conductivity will significantly increase the safety margin under design basis accidents such as a LOCA (Loss-Of-Coolant Accident) as well as the thermal and operational margin under normal operation conditions

2 Fig. 1. Conceptual schematic of micro-cell UO 2 pellet. The ceramic wall in ceramic micro-cell UO 2 pellets is composed of an oxide phase with chemical affinity to volatile fission products (Cs and I), and acts as multiple traps to immobilize the volatile fission products. That is, the ceramic microcell walls can block the migration of fission products to the pellet outside. The increased retention capability of the fission products will reduce the stress corrosion cracking at the inner surface of the cladding as well as the rod internal pressure. In addition, a soft ceramic-wall facilitates the fast creep deformation of the pellets, thereby reducing the mechanical stress of the cladding under operational transients. These benefits are expected to enhance the robustness of a fuel rod during a severe accident as well as during normal operation. The large-sized grain and mesh-like rigid wall structures are also expected to prevent the massive fragmentation of pellets during a severe accident thereby reducing the release of retained radio-toxic fission products into the environment. At KAERI, to implement the designed fuel concept, the fabrication process of micro-cell UO 2 pellets embodying the designed concept was developed. In addition, assessments of the out-of-pile properties of micro-cell UO 2 pellet are being performed by various out-of-pile tests, and observations of the in-reactor performance and behavior of the developed pellets are in progress through Halden irradiation tests. In this paper, we describe the development status of the micro-cell UO 2 pellet for ATF. II. FABRICATION OF MICRO-CELL UO2 PELLETS Many candidate metals for a metallic micro-cell wall material have been assessed and screened, based on their physical properties such as the thermal conductivity, neutron absorption cross section, melting temperature and so on. Mo and Cr were primarily selected as the metallic cell wall materials because they have relatively high melting temperature, high thermal conductivity, and manageable neutron absorption cross section. Above all things, it is significantly required that the Mo and Cr are compatible with a UO 2 matrix under the sintering conditions of the pellet fabrication process. Fig. 2 shows the microstructure of a 5 vol% Mo and 5 vol% Cr metal phase containing metallic micro-cell UO 2 pellets, in which the micro-cell concept is successfully implemented. These pellets were fabricated by the co-sintering of metal powder over-coated UO 2 granules through a conventional sintering process. Metal powder over-coated UO 2 granules, which Fig. 2. The microstructure of a 5 vol% Cr metal phase containing metallic micro-cell UO 2 pellet (a panoramic image of pellet radial direction from end to end). 2

3 Fig. 3. The microstructure and EDS mapping images of Si-Ti-O containing ceramic micro-cell UO 2 pellet. were prepared by mixing metal powders and UO 2 granules, were pressed into green pellets. After that, those green pellets were sintered under a dry hydrogen atmosphere. The sintered density and averaged cell (granule) size of fabricated 5 vol% Cr metallic micro-cell UO 2 pellet were 10.5 g/cm 3 and ~290 μm, respectively. The ceramic micro-cell wall materials require chemical affinity to Cs to form stable compounds with incorporated Cs. Because the fission yield of Cs is roughly ten-times larger than that of iodine, 14 the chemical affinity of the wall to cesium may deeply impact the retention capability of the fission products. Insolubility of the wall materials in a UO 2 matrix is also an important requirement to maintain the microcell structure. The recent test results for Al-Si-O doped UO 2 suggests the chemical trapping of volatile fission products in an SiO 2-based additive phase, and decreased possibility of the availability of aggressive species in the inside cladding We therefore selected several SiO 2-based mixed oxides as additive candidates for ceramic cell wall materials. To fabricate the ceramic micro-cell UO 2 pellets, a conventional liquid phase sintering technique has been applied. A powder mixture of UO 2 and additives was pressed into green pellets, which were then sintered at an elevated temperature at which the additives for the wall materials formed a liquid phase, penetrating through grain boundaries and enveloped UO 2 grains to make the designed ceramic micro-cell. Fig. 3 shows the microstructure of the ceramic microcell UO 2 pellet. This pellet was obtained by mixing a 0.6 wt% SiO 2- TiO 2 oxides mixture with UO 2 powder and then sintering the powder mixture at 1720 C for 4h in a dry hydrogen atmosphere. The sintered pellet density and averaged cell (grain) size were g/cm 3 and ~80 μm, respectively. EDS (Energy Dispersive Spectroscopy) mapping images revealed that the additive elements were isolated in the grain boundary. III. OUT-OF-PILE PROPERTIES OF MICRO-CELL UO2 PELLETS Several out-of-pile tests were conducted to evaluate the basic thermo-physical properties and fuel behaviors under the normal operating conditions as well as under accident conditions. The thermal conductivity and linear thermal expansion of the metallic micro-cell UO 2 pellets were measured. Fig. 4 shows the measurement results of 5 vol% Mo and 5 vol% Cr metallic micro-cell UO 2 pellets. The thermal conductivity of the pellet was remarkably enhanced, and the thermal conductivity of the 5 vol% Mo metallic micro-cell UO 2 pellet increased by 100% at 1000 C. The fuel temperature can be effectively decreased by the enhanced thermal conductivity of the pellet. The linear thermal expansion coefficient of the Mo metallic micro-cell UO 2 pellet decreased with increasing Mo contents because of the lower thermal expansion coefficient of Mo (~ /K at 25 C) than that of UO 2 (~ /K). Under the transient condition, the mechanical stress on the cladding due to pellet expansion can be reduced by this feature of the metallic microcell pellet. Maintaining pellet-structural stability under a high-temperature steam environment is an important property for fuel pellets. Thus, steam oxidation behaviors of Mo metallic micro-cell UO 2 pellets were investigated. Morphology changes of the polished surface of the sample pellets after steam oxidation tests were observed. In the oxidation test in 500~800 C steam, the oxidation even retarded in the Mo metallic micro-cell UO 2 pellets. The surface of the pure UO 2 pellets was damaged due to the oxidation along the grain boundary, whereas the Mo micro-cell UO 2 pellets showed sound surface morphology. A 3

4 Fig. 4. The measurement results of metallic micro-cell UO 2 pellets. 9 Fig. 5. Comparison of the steam oxidation behavior of UO 2 pellet and Si-Ti-O ceramic micro-cell UO 2 pellet steam oxidation test at higher temperature is being performed to indicate whether the micro-cell pellet-structural stability is damaged by the volatilization of the Mo-oxide phase. The melting temperatures of both Cr (~1907 C) and Mo (~2623 C) are lower than that of UO 2 (~2865 C). Therefore, what will happen in a metallic micro-cell UO 2 pellet when the pellet temperature is increased to the temperature higher than the melting point of a metallic wall is one of the main concerns in a metallic micro-cell UO 2 pellet. A thermal transient annealing test was carried out to verify the structural integrity for both Cr and Mo containing metallic micro-cell pellets, and it was shown that the micro-cell pellet was preserved well even after being annealed at higher temperature than the melting point of the wall. The thermal diffusivity and thermal expansion test results showed that the thermal properties of a ceramic micro-cell UO 2 pellet are similar to those of an UO 2 pellet. In addition, the compressive-creep deformation of ceramic micro-cell pellets at high temperature was faster than that of an UO 2 pellet. The fast creep deformation indicates that the ceramic micro-cell pellets can reduce the mechanical stress on the cladding during a transient or accident, as well as during normal operation. In contrast with the metallic wall, the ceramic wall is an oxide phase having inherent stability under a steam environment. The experiment results showed an enhanced resistance to steam oxidation in a ceramic microcell UO 2 pellet. This enhancement may be related with the large grain size of ceramic microcell UO 2 pellets. A thermal transient annealing test at a higher temperature than the melting point of the ceramic wall revealed that the structural integrity of pellets was preserved well after the test. 4

5 Fig. 6. Online measurement data of fuel centerline temperature of UO 2 reference pellet and Cr metallic micro-cell UO 2 pellet as a function of the operation time. IV. IN-REACTOR PERFORMANCE OF MICRO-CELL UO2 PELLETS In the development of nuclear fuel, the irradiation behavior of the developed fuel material must be considered based on various aspects (pellet microstructure, pellet structural integrity, cell wall soundness, cell wall material behavior and redistribution, reaction between cell wall material and fission product, etc.). Above all, it is important that the designed inreactor fuel performances of the micro-cell UO 2 pellet are verified. Through cooperation with Thor Energy, Norway, a Halden irradiation test of the micro-cell UO 2 pellets is now in progress. KAERI is participating in the International Thorium Consortium organized by Thor Energy. Two kinds (ceramic and metallic) of micro-cell UO 2 pellets were fabricated in accordance with the requirements for the Halden irradiation test. The number of samples (~60 EA) was sufficiently provided for manufacturing the instrumented rigs. The prepared pellet specifications are shown in Table I. The irradiation test rigs with instrumentations were manufactured at the Institute for Energiteknikk (IFE), Norway. Fig. 6 shows the online measurement data of the fuel centerline temperature of a Cr metallic micro-cell UO 2 pellet that is being irradiated in the Halden reactor. The online data of the fuel temperature are in good agreement with the predicted data based on a computer simulation. 17 Until now, the temperature difference between the UO 2 reference pellet and Cr metallic micro-cell UO 2 pellet is being consistently maintained. The irradiation test of micro-cell UO 2 pellets is ongoing at above ~10,000 MWd/mtU according to the plan, and is proceeding to reach a higher burnup. TABLE I. Specifications of prepared pellet samples. Ceramic micro-cell pellet Metallic micro-cell pellet Cell wall materials 0.6 wt% Si-Ti-O (2 vol%) 3.4 wt% Cr (5 vol%) Averaged cell size * (μm) ~80 ~290 Pellet density (g/cc) 10.73± ±0.03 Pellet diameter ** (mm) 8.190± ±0.001 Pellet height (mm) 9.4± ±0.03 Pellet weight (g) 5.15± ±0.02 U enrichment 4.5%-U %-U 235 * A ceramic micro-cell consists of one grain. Therefore, the cell size of a ceramic micro-cell pellet is equal to the grain size. On the other hand, a metallic micro-cell consists of a lot of grains (6-7 μm). ** Pellet diameter after centerless grinding process. 5

6 V. SUMMARY As an accident tolerant fuel (ATF) pellet, a micro-cell UO 2 pellet is being developed to enhance the accident tolerance of nuclear fuels under accident conditions, as well as the fuel performance under normal operation conditions. The material concepts of ceramic and metallic micro-cell UO 2 pellets are designed, and the fabrication process of micro-cell UO 2 pellets embodying the designed concept has been developed. In addition, assessment of the out-of-pile properties of a micro-cell UO 2 pellet are being performed, and observation of the in-reactor performance and behavior of the developed pellets is progressing through Halden irradiation testing. According to the expectations, the excellent performances of the micro-cell UO 2 pellets are being verified by the online measurement data of Halden irradiation tests. ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017M2A8A ). REFERENCES 1. I. Younker and M. Fratoni, Neutronic Evaluation of Coating and Cladding Materials for Accident Tolerant Fuels, Progress in Nuclear Energy, 88, 10 (2016). 2. M. Q. Awan, L. Cao, H. Wu, Neutronic Design and Evaluation of a PWR Fuel Assembly with Accident Tolerant-Fully Ceramic Micro-Encapsulated (AT-FCM) fuel, Nuclear Engineering and Design, 319, 126 (2017). 3. K. D. Johnson, A. M. Raftery, D. A. Lopes, J. Wallenius, Fabrication and Microstructural Analysis of UN-U 3Si 2 Composites for Accident Tolerant Fuel Applications, Journal of Nuclear Materials, 477, 18 (2016). 4. C.P. Deck, G.M. Jacobsen, J. Sheeder, O. Gutierrez, J. Zhang, J. Stone, H.E. Khalifa, C.A. Back, Characterization of SiC-SiC Composites for Accident Tolerant Fuel Cladding, Journal of Nuclear Materials, 466, 667 (2015). 5. X. Wu, T. Kozlowski, J. D. Hales, Neutronics and Fuel Performance Evaluation of Accident Tolerant FeCrAl Cladding under Normal Operation Conditions, Annals of Nuclear Energy, 85, 763 (2015). 6. J. H. Park, H. G. Kim, J. Y. Park, Y. I. Jung, D. J. Park, Y. H. Koo, High Temperature Steam-oxidation Behavior of Arc Ion Plated Cr Coatings for Accident Tolerant Fuel Claddings, Surface and Coatings Technology, 280, 256 (2015). 7. J. G. Stone, R. Schleicher, C. P. Deck, G. M. Jacobsen, H. E. Khalifa, C. A. Back, Stress Analysis and Probabilistic Assessment of Multi-layer SiC-based Accident Tolerant nuclear Fuel Cladding, Journal of Nuclear Materials, 466, 682 (2015). 8. Y. H. Koo, J. H. Yang, J. Y. Park, K. S. Kim, H. G. Kim, D. J. Kim, Y. I. Jung, K. W. Song, KAERI's Development of LWR Accident-tolerant Fuel, Journal of Nuclear Technology, 186, 295 (2014). 9. H. G. Kim, J. H. Yang, W. J. Kim, Y. H. Koo, Development Status of Accident-tolerant Fuel for Light Water Reactors in Korea, Nuclear Engineering and Technology, 48, 1, 1 (2016). 10. D. J. Kim, Y. W. Rhee, J. H. Kim, K. S. Kim, J. S. Oh, J. H. Yang, Y. H. Koo, K. W. Song, Fabrication of Micro-cell UO 2-Mo Pellet with Enhanced Thermal Conductivity, Journal of Nuclear Materials, 462, 289 (2015). 11. K. A. Terrani, D. Wang, L. J. Ott, R. O. Montgomery, The Effect of Fuel Thermal Conductivity on The Behavior of LWR Cores during Loss-of-coolant Accidents, Journal of Nuclear Materials, 448, 512 (2014). 12. N. R. Brown, A. J. Wysocki, K. A. Terrani, K. G. Xu, D. M. Wachs, The Potential Impact of Enhanced Accident Tolerant Cladding Materials on Reactivity Initiated Accidents in Light Water Reactors, Annals of Nuclear Energy, 99, 353 (2017). 13. X. Wu, W. Li, Y. Wang, Y. Zhang, W. Tian, G. Su, S. Qiu, T. Liu, Y. Deng, H. Huang, Preliminary Safety Analysis of The PWR with Accident-Tolerant Fuels during Severe Accident Conditions, Annals of Nuclear Energy, 80, 1 (2015). 14. G. Brillant, F. Gupta, A. Pasturel, Fission Products Stability in Uranium Dioxide, Journal of Nuclear Materials, 412, 170 (2011). 15. J. Matsunaga, Y. Takagawa, K. Kusagaya, K. Une, R. Yuda, M. Hirai, Fundamentals of GNF Al-Si-O Additive Fuel, TopFuel 2009, Paris, France, September 6-10, American Nuclear Society (2009). 6

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