Neutronic study of a combined fast fission FFHR and thermal fission FFHR system using thorium bearing molten salt fuel

In this paper, an integrated fusion-fission hybrid reactor energy system aiming at utilizing the thorium fuel efficiently with Th-U closed cycle is presented. The ultimate purpose is transition to the all thorium fueled energy system with the bred 233U fuel burning in-site. The system consists of two fusion-fission hybrid reactors. The first reactor is a fast breeder which provides the starting fuel 233U for the second reactor (thermal burner). The bred 233U is burnt in the second reactor with the thorium bearing molten salt fuel blanket. In order to reach the multi-system performance, i.e. high energy multiplication M, tritium self-sufficiency and fissile fuel self-sufficiency, using limiting 233U start fuel, the seed-blanket concept is adopted in the thermal burner design. The thermal burner blanket is divided into two main regions. The front region (seed) plays the dominate role in the energy production, neutron multiplication and 233U breeding. Excess neutrons produced by the front region are then used to drive backward region (blanket) to breed tritium. The Couple 2.0 code developed by Tsinghua University is used to calculate the fuel depletion process of the whole system. The simulated results show, the thermal burner could operate with excellent performance, energy multiplication M (8-10), tritium breeding ratio TBR (>1.05) continuously and only natural thorium fuel is added into the system.