Analysis of U-Pu breeding in molten salt fast reactor

[Background] Molten salt reactor has been selected as one of the candidates for the fourth generation reactors due to its superior performance, and molten salt fast reactor has become one of the hot research subjects in the world. [Purpose] This study aims to find the most suitable molten salt fuel scheme in the uranium-plutonium cycle to achieve the purpose of high proliferation of fissile nuclides. [Methods] Based on the two-fluid cooling cycle scheme, independent cooling cycles for fission molten salt fuel and breeding molten salt, a comprehensive simulation program SCALE (Standardized Computer Analyses for Licensing Evaluation) was employed to calculate the neutron spectrum and reactivity temperature coefficient of three feasible molten salt fuel schemes (LiF+PuF₄+UF₄, NaF+PuF₄+UF₄ and NaCl+PuCl₃+UCl₃). High breeding ratio (BR) in the molten salt fast reactor was realized by utilizing the high solubility characteristics of plutonium and uranium heavy metal salts in fluorinated or chlorinated molten salts at high temperatures. Influence factors of BR, such as the sizes of fission zone, breeding zone and ZrC reflection layer, the isotopic abundances of ⁶Li and ³⁵Cl in the molten salt, as well as the dynamic change of BR with running time, were computationally analyzed. [Results] The BR of two fluorinated molten salt schemes reaches about 1.06 whilst BR of the chlorinated molten salt scheme reaches 1.46 when the diameter and height of the fission zone are both 260 cm. [Conclusions] Combined with the phase diagram of molten salt, the variation curve of BR with the molar concentration of heavy metals and the variation curve of the maximum value of BR with the average operating temperature of molten salt, the operating temperature of molten salt, the molar concentration of heavy metals and the reactor BR can be be quickly determinated in the conceptual design of the molten salt fast reactor. (authors)