Influence of the processing and salt composition on the thorium molten salt reactor

Molten salt reactors (MSRs) are one of the six systems retained by Generation IV as a candidate for the next generation of nuclear reactors. The MSR is a very attractive concept especially for the thorium fuel cycle, which allows nuclear energy production with a very low production of radiotoxic minor actinides, so it has been selected by the Generation-IV International Forum. Its main characteristic is a strong coupling between neutronics and salt processing. Such nuclear reactors use a liquid fuel that is also the coolant. Elements produced during the reactor's operation, like fission products or transuranic elements, modify the neutronic balance of the reactor by capturing neutrons. As the fuel is liquid, partial processing of a limited amount taken from circulating salt is possible, in order to remove the poisoning elements, without stopping reactor operation. In this paper, we present a configuration that we consider to be a reference one for a thorium molten salt reactor (TMSR), and we study the influence of efficacy of different types of processing on the neutronic behavior of this reactor. BY considering both the possibilities in chemistry and the neutronic effects, our aim is to work out an efficient, reliable, and realistic processing scheme. The processing includes in fact two components: an in-line bubbling system within the reactor that extracts the gaseous and metallic fission products quickly and a slower external processing unit that extracts the other fission products. A salt volume equal to the core volume is thus cleaned in several months. We have studied the influence of different processing rates on the reactor's behavior. This mainly affects the breeding ratio. Properties of the salt are also crucial. We choose in our simulations of the TMSR a 78 mol% LiF-22 mol% [heavy nuclei (HN)] F-4 salt for the fuel, but lower HN proportions in the fuel salt are also examined in order to minimize the U-233 inventory in the reactor. The neutron spectrum is largely modified by the HN proportion and has a deep impact on the reactor behavior. Our simulations evaluate the degradation of the breeding ratio from >1 for the reference configuration down to 0.86 due to a decrease of the HN proportion in the fuel salt. We conclude that the simplification of the salt processing that is addressed in this work improves the feasibility of the TMSR system.