Yasushi Tsuboi

Development of the SESAME Metallic Fuel Performance Code

A simple fuel performance code, SESAME, has been developed to analyze the steady-state irradiation behavior of metallic fuels. Important characteristics of metallic fuels such as fission product gas release, swelling, material redistribution, bonding sodium infiltration, and axial elongation are studied. Simple models have been developed for the SESAME code, which are successful in predicting irradiation data and useful in investigating the basic mechanisms that contribute to these particular irradiation behaviors of metallic fuels.

Design of the 4S Reactor

Abstract The Super-Safe, Small and Simple (4S) sodium-cooled fast reactor plant incorporates innovative design features, such as a nonrefueling reactor, passive safety, low maintenance requirements, and inherent security. Major components such as the reflector drive mechanisms, the electromagnetic pumps, and the double-wall tube steam generator have been optimized for efficient and safe operation. The nonrefueling reactor concept is made possible by incorporating a 30-yr refueling interval for the reflector-controlled metallic fuel core. Sodium-cooled, metallic-fueled fast reactors have a good conversion ratio due to fast neutron usage, thus extending the core life. Passive safety is achieved with redundant residual heat removal systems that function using only natural circulation, and a metallic core with a negative reactivity coefficient. Low maintenance requirements are achieved by simplifying the design and minimizing the use of active components, and by using electromagnetic pumps, which have no moving parts. The inherent security of the nuclear materials is significantly enhanced by the nonrefueling reactor concept and the minimal maintenance requirements. In addition, the reactor building is located below ground level, providing substantial protection against an aircraft impact and thus further enhancing the security of the design. The demonstration of key components such as the electromagnetic pumps and the steam generator is part of an ongoing testing program that has already confirmed many of the 4S engineering solutions. This paper describes the current status of design and component tests for the 4S reactor.

Mechanistic model of fission gas behavior in metallic fuel

A mechanistic model of fission gas swelling and release has been developed for metallic fuel under operational conditions. This swelling model is based on gas bubble coalescence and growth by gas bubble random and/or biased migration occurring in grain boundaries. The gas release is modeled by open bubbles and tunnel formation. Experimental data are analyzed by this model. The result suggests that the model can predict fission gas bubble growth of metallic fuel.

Thermal conductivities of granular UO2 compacts with/without uranium particles

The thermal conductivities of granular UO2 compacts with and without uranium particles were measured to evaluate the thermal performance of vibro-packed granular MOX fuels containing metallic fine particle oxygen getters. The thermal conductivity of the compact with 10 wt% of uranium particles was higher than that of the compact without uranium particles. After heating beyond 1,408 K, the melting point of the uranium particles, the thermal conductivity increased further. The evaluation model for analyzing such phenomena was developed. The model predicted that once the UO2 compact with uranium particles was exposed to a temperature beyond 1,408 K, the uranium particles should melt and provide interconnecting areas between the UO2 granules and uranium particles, and between other uranium particles. The resulting increase of the thermal conductivity was reasonably expressed by the effect of necks in the compact on the heat conduction.

Neutronic feasibility of an LMFBR super long-life core (SLLC)

Abstract The LMFBR Super Long-Life Core (SLLC) concept has evolved over the last few years as one of the targets of innovative approaches for future FBR cost reduction. An idea for SLLC has been developed wherein the core lifetime is extended up to the plant life of about 30 years by applying the radially and axially multi-zoned core concept (the improved homogeneous core concept). The main purpose of the present study is placed on the evaluation of neutronic feasibility of the 1000 MWe class SLLC concept. The core size of the present SLLC, which is approximately 3 to 4 times as large as those of the current 1000 MWe core design, was determined by the limit of the maximum fast neutron fluence level, which was tentatively assumed to be 5–6 × 10 23 nvt as the target of the future development of advanced cladding materials. Emphasis is placed on the discussion of neutronic performances of cores with oxide fuels rather than metal or carbide fuels. The present study has shown that proper zoning of the different plutonium enrichment fuels at the initial core makes it possible to achieve small enough reactivity loss during 30-year burnup while satisfying mild variation of the subassembly power distributions using a higher fuel volume fraction of about 50%. Effects of important neutronic parameters on the core performances are also discussed.

Vibro-Packing Experiment of Non-Spherical Uranium Dioxide Particles with Spherical Metallic Uranium Particles

Japan Atomic Energy Agency has developed vibro-packed fuel as one of the candidates for commercial fast breeder reactor fuels. In this study, vibro-packing experiments were carried out to investigate particle movement during vibro-packing as well as particle distribution after the completion of vibro-packing. Non-spherical uranium dioxide particles and spherical metallic uranium particles were used to simulate mixed oxide particles and oxygen getter particles, respectively. These experiments revealed the following facts. It is important to prevent segregation by means of feeding each size of the fuel particles uniformly into a cladding tube in order to disperse oxygen getter particles uniformly. “Simultaneous feeding” with volumetric powder feeders is useful to realize the above.

Development of Uranium-Free TRU Metallic Fuel Fast Reactor Core

A TRU-burning fast reactor cycle associated with a uranium-free trans-uranium (TRU) metallic fuel core is one of the solutions for radioactive waste management issue. Use of TRU metallic fuel without uranium makes it possible to maximize the TRU transmutation rate in comparison with uranium and plutonium mixed-oxide fuel because it prevents the fuel itself from producing new plutonium and minor actinides, and furthermore because metallic fuel has much smaller capture-to-fission ratios of TRU than those of mixed-oxide fuel. Also, adoption of metallic fuel enables recycling system to be less challenging, even for uranium-free fuel, because a conventional scheme of fuel recycling by electrorefining and injection casting is applicable.