Table 3-1 Characteristics of Pb-Bi cooled subcritical core for MA transmutation

Mixed nitride of plutonium and MA is used as fuel material at the initial loading to design the core with much improved MA transmutation performance. The annual MA transmutation rate is 10%, which corresponds to eliminating MA of approximately 250 kg annually (equivalent to that arising from 10 units of 1,000 MWe nuclear power stations).

Fig. 3-7 Changes of k-eff and beam power of the accelerator due to burnup of the transmutation system All the fuel is exchanged at the same time every two years. The additional fresh fuel contains MA only and plutonium is not loaded any more except initially. Changes of k-eff and accelerator power of the system are minimized during burnup, by adjusting the core parameters.

Fig. 3-8 Concept of Transmutation Plant Piping in the plant should be eliminated as much as possible, because of using heavy lead-bismuth. In this context, the core, the steam generator, the pump and the core support structure are installed in a reactor vessel, and then the plant has excellent aseismic and anti-thermal stress characteristics.

High-level radioactive waste produced from nuclear power generation contains extremely long-lived radioactive nuclides, such as minor actinides (MAs). Transmutation aims primarily at converting MAs to non-radioactive nuclides or very short-lived ones. In a transmutation system driven by a proton accelerator, a large number of spallation neutrons are generated in a target bombarded with a high energy proton beam from the accelerator. These neutrons are introduced to an MA fueled subcritical core and maintain the chain reaction in the core. MAs are eliminated mostly through fission reactions. The present system proposed here uses lead-bismuth (Pb-Bi) both as the spallation target and the coolant for the subcritical core. Chemically inert lead-bismuth coolant is more favorable for the safety of the system than sodium which was adopted in the previously designed system. The system achieves high MA transmutation capability (Table 3-1). Namely, the MA transmutation rate is approximately 250 kg/y, which corresponds to the amount of MA annually arising from 10 units of 1,000 MWe nuclear power stations. The core itself is maintained subcritical throughout the period of full power operation of the system (10 years). A variation of subcriticality during this period is minimized, resulting in a small variation of the proton beam power of the accelerator and resulting in efficient operation of the accelerator (Fig. 3-7). The plant concept based on the system is simple, in which the main components, such as steam generators, are integrated in a reactor vessel, together with the core (Fig. 3-8). This integrated plant can be expected to have substantially improved safety characteristics, such as the aseismicity.

Reference K. Tsujimoto et al., Conceptual Study of the Lead-Bismuth Cooled Accelerator-Driven Transmutation System, Proc. of AccApp 1998, 137 (1998).

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