Fuel design evaluation of the 4S

Typical fast reactor fuel residence time is less than 5 years. Extending the life of the fuel beyond this range is an important feature of the battery type (non-refueling) fast reactors. This type of reactors requires operation without refueling for long periods of time between 15 to 30 years, after which the full core is removed from the reactor and sent to disposal or reprocessing with the possibility of replacement with new core. Extending the fuel residence time to such long periods requires an advanced fuel design that mitigates the effects of extended fuel element exposure to different severe conditions which are present in fast reactor environment. Those conditions include high temperature, large irradiation doses, possible interactions between fuel and cladding, and sodium corrosion of the outer cladding surface. One such advanced fuel design is the metallic U-10Zr fuel design proposed for use in the sodium cooled Super-Safe Small and Simple (4S) reactor which is expected to operate without refueling for 30 years. This paper describes the 4S advanced fuel design and evaluation of its expected performance over a 30 years life time. The first implication of the required long fuel life, from the neutronics prospective, is that the fuel has to be longer and wider than a typical fast reactor fuel. Beginning of life fuel length is 2.5 m compared to typical heights of up to about 1.5 m or less (PRISM design is about 1.34 m while EBR-II and FFTF heights were about 1/3 m and 1 m, respectively). Pin diameter is 14 mm compared to a range between about 4-7 mm for other reactors. Mitigation of the fuel cladding thermal creep due to gas pressure over the long irradiation period is achieved through the use of a plenum region on top of the fuel slug (ratio of plenum to fuel volume is 1.3) and operating at peak cladding temperature that maintain low thermal creep rate (hot channel peak cladding temperature is 609 deg. C). Low plenum pressure combined with low average fuel burnup (less than 5 at%) reduce the stresses on the cladding and reduce corresponding creep strains. Lower burnup combined with the use of appropriate fuel smeared density (allowing for enough fuel-cladding gap) eliminate possible concerns regarding fuel cladding mechanical interactions (FCMI). Meanwhile fuel cladding chemical interaction (FCCI) is reduced significantly given the reduction in its driving forces for this particular design. Those driving forces include temperature gradient over the fuel cross section, fission products accumulation, contact period between the fuel and cladding and contact temperature. As shown in Fig.1, the fuel-cladding contact period is reduced at the top of the fuel where cladding temperature is the highest and fuel burnup (that is, available lanthanide fission products) is the lowest. In addition, thermal properties of U-10Zr alloy, low linear power and wider fuel cross section reduce the temperature gradient across the fuel to less than 100 deg. C. In addition, the cladding thickness is about double the typical cladding thickness used in EBR-II and FFTF, allowing for further mitigation of FCCI effects. Concern regarding cladding corrosion due to long exposure period between the sodium coolant and cladding outer surface are reduced by controlling the oxygen content in the sodium to levels that are know to limit this phenomenon (e.g., oxygen limits used in EBR-II, where some of the blanket fuel elements remained in the core for the full reactor life time of about 30 years without showing corrosion related problems). Finally, issues related to constituents' redistribution in metallic U-Zr fuel are eliminated based on analysis of the phenomenon for the 4S fuel where it is shown that the redistribution is not expected to take place under the 4S operating conditions. Design criteria are established for evaluation of the 4S fuel performance, which are based on past experiences from different fast reactor programs. The LIFE-METAL fuel performance code was used to evaluate the performance of the 4S fuel design given those design criteria. The performance evaluation shows the design meets those pre-set design criteria. Sensitivity analysis was performed to look at the fuel performance at conditions that are beyond expected operating conditions, that is higher cladding temperature and fuel swelling behaviour that increases FCCI. The sensitivity study shows that none of the design criteria are violated under those extreme conditions. The paper includes discussion of the experimental metallic fuel database and extrapolation of the database to the 4S fuel characteristics and operating conditions. Also included are discussions related to licensing of the 4S reactor in the U.S., and interactions with the U.S. Nuclear Regulatory Commission on issues related to this fuel design and previous fast reactor designs in the U.S.