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Disposition of radioactive waste is one of the key issues to make nuclear energy as a sustainable power resource for future power generation. To solve this issue, a long-term program for research and development on partitioning and transmutation called “OMEGA” (Options Making Extra Gains from Actinides and fission products) was adopted in 1988 by the Japan Atomic Energy Commission (Japan Atomic Energy Commission, 2009). The aims of the OMEGA Program are to widen options for future waste management and to explore the possibility to utilize high-level radioactive wastes as useful resources. To proceed the OMEGA program, partitioning and transmutation (P-T) is a key technology to reduce environmental impacts and source terms contained in the high level radioactive waste which was discharged from commercial nuclear power reactors. By using partitioning technology, the long-lived radioactive nuclides such as minor actinides (MAs) and longlived fission products (LLFPs) can be extracted from high level radioactive waste. From the mass balance study performed under the OMEGA program, the lifetime of final disposal site can be prolonged 3 to 10 times more than current designed lifetime (Oigawa, et al. 2005). Under the framework of the OMEGA program, Japan Atomic Energy Agency (JAEA) has proposed a double-strata fuel cycle concept (Takano, et al. 2000) which consists of two fuel cycles; one is a commercial fuel cycle including current LWR cycle and also future FBR fuel cycle and the other is a small fuel cycle dedicated to the transmutation of MA and LLFP. By the double-strata fuel cycle shown in Fig. 1, long-lived radioactive nuclides are confined into the second-stratum small fuel cycle that includes an innovative nuclear system which is optimized to the transmutation of MA and LLFP. JAEA carries out research and development of accelerator-driven transmutation systems (ADS) as an innovative dedicated transmutation system under the OMEGA Program. In the P-T, the most effective reaction to transmute MA separated from high level radioactive waste by partitioning process into short-lived/stable nuclides is fission reaction. By fission reaction, target MA nuclides can be directly transmuted into short or stable nuclides. However, to make fission reaction as dominant reaction in the nuclear reactor, the reactor core should be formed a fast neutron spectrum field. It requires rather large fuel inventory and diminishes some safety characteristics such as Doppler coefficient and delayed neutron fraction. Another point of view, MA discharged from current light water power reactors is roughly composed of 55% of neptunium, 40% of americium and 5% of

Shielding Analysis of the Secondary Coolant Circuit of Accelerator Driven Systems