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VI Reactor Physics Studies for Post Fukushima Accident Nuclear Energy: Studies from the Reactor Physics Aspect for Back-End Issues Such as Treatment of Debris from the Fukushima Accident

Transmutation Scenarios after Closing Nuclear Power Plants

Abstract With consideration of the phase-out option from nuclear power (NP) utilization in Japan, an accelerator-driven system (ADS) for Pu transmutation has been designed and scenario analysis performed. The ADS is designed based on the existing ADS design for MA transmutation, and the six-batch ADS was selected as a reference design for scenario analysis. In the scenario analysis, the oncethrough scenario of light water reactor (LWR) spent fuel is referred to as a conventional scenario with a LWR-MOX utilization scenario. As the transmutation scenario, three cases of transmuters that are only-FR, only-ADS, and both-FR

+ADS are analyzed. The numbers of necessary transmuters are obtained as 15 to 32 units, and the necessary period for transmutation as 180–240 years. The benefit on repository by reduction of Pu and MA is reduction of repository area by a factor of five and of decay time of toxicity by one order of magnitude. The FR+ADS scenario would be a modest solution, although the ADS scenario is preferable if rapid transmutation is required.

Keywords ADS • Phase-out scenario • Scenario study • Transmutation

Introduction

After the Fukushima-Daiichi accident, Japan started a discussion of nuclear power (NP) utilization including a “phase-out” option in addition to the usual scenario of utilizing plutonium by deploying fast breeder reactors (FBRs). In the phase-out option, construction of new plants is limited and dependency on NP will be gradually reduced. One of the reasons supporting the phase-out scenario is an ambiguous prospect of conducting underground disposal of radioactive wastes. Increase of wastes can be limited or even stopped in the phase-out scenario, but spent fuels (SFs) containing plutonium (Pu) and minor actinides (MAs) will remain as a legacy of NP. “Direct disposal” to the underground of SFs confined in canisters is considered as a strong option to treat this legacy, but Pu and MAs that exist in the

underground can be utilized for nuclear weapons and can cause public dose in the very far future over several tens of thousands of years. Instead of direct disposal, transmutation of Pu and MAs (TRU, trans-uranic) has been studied in many countries for the purpose of eliminating them from the waste.

Transmutation can be performed by a “transmuter” that is dedicated for transmutation with the lesser role of electricity generation. It contains a fast reactor (FR) and an accelerator-driven system (ADS), which are fast neutron systems with metal coolant. FRs have been mainly developed as breeder reactors but they act as a burner reactor in the phase-out scenario. The burner reactor has no blanket region for breeding, larger Pu content, and shorter operation-cycle length [1]. The ADS has been designed as an MA transmuter with a smaller amount of Pu but changes to a Pu transmuter in this scenario.

In the present study, an ADS for Pu transmutation (Pu-ADS) is designed by neutronics calculation based on the ADS for MA transmutation (MA-ADS). In the original design for MA transmutation, drop of criticality during depletion is very small, and a long operation cycle is achieved because MAs behave as fertile material. In the Pu-ADS, criticality decreases much more rapidly and design modification is necessary.

After the design of the Pu-ADS, a scenario study is performed by a nuclear material balance (NMB) code that was developed by the authors. The following items are revealed by the study: accumulation of TRU in the LWR SFs, necessary number of transmuters, reduction of TRU, reduction of repository footprint, and radiotoxicity by transmuters.

In Sect. 19.2, calculation methods for neutronics design and scenario code are introduced. Section 19.3 provides the neutronics design and resulting ADS. Section 19.4 discusses assumptions and results of the scenario study. The results are concluded in Sect. 19.5.

 
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