Development of a kinetic model for safety studies of liquid-fuel reactors

Liquid-fuel reactors offer very fascinating problems in neutronics due to the effects of the fuel movements in the reactor core and loop. In the present study, a spatial kinetic model taking the fuel flow effects into account for liquid-fuel reactors is derived without approximation from the time-space-dependent equations of neutrons and delayed neutron precursors. The relation between such spatial kinetic model and the point kinetic model for the solid-fuel reactor is set up as well, which is useful for extending the safety codes developed for conventional solid-fuel reactors to liquid-fuel ones. The spatial kinetic model together with the other two approximated kinetic models are applied to the safety analysis of a typical liquid-fuel reactor MOSART (MOlten Salt Actinide Recycler and Transmuter). The steady state calculation and the ULOF (Unprotected Loss of Flow) calculation are performed, and the liquid-fuel flow effects are studied particularly. The steady state results show that the fuel flow influences the distributions of the delayed neutron precursors significantly. The ULOF results by all models show that the behavior of the relative power, fuel salt temperature, graphite temperature, and reactivity feedbacks are similar due to strong negative reactivity feedbacks. However, the developed spatial kinetic model obtains more safety margin to the fuel temperature limit.