Novel Approach for Modeling Effective Delayed Neutron Fraction and Transient Reactivity Feedback Effects for Circulating Fuel Reactors

Abstract

The point reactor kinetics (PRK) model in GOTHIC∗ has been modified to consider the motion of delayed neutron precursors (DNPs) and Decay Heat Precursors (DHPs) when calculating reactor power. In a solid fuel reactor, delayed neutron precursors (DNPs) born during fission are typically treated as a time-lagged source of neutrons. Meanwhile, in fluid fueled circulating fuel reactors (CFRs), DNPs and DHPs are transported and decay outside the traditional “active core” region. This results in a reduction of the delayed neutron fraction, β, relative to the stationary fuel case. However, the reduction in β does not act equally on all DNP groups. It depends on the half-life relative to the loop transit time for the CFR as this determines where a DNP decays relative to the core. Due to the dependence on loop transit time, the reactivity feedback effect is not constant, but rather a transient phenomenon that must be considered when modeling startup, shutdown and flow anomaly scenarios. GOTHIC includes a detailed tracer transport equation that includes the effects of advection, molecular and turbulent diffusion, and radioactive decay, including both progeny formation and decay heat release within the fluid. This allows for time-dependent concentration for each DNP group to be calculated throughout the system. Integrating the tracer transport and PRK models in GOTHIC provides a novel approach to modeling the transient reactivity feedback effect due to the motion of DNPs in CFRs. This paper summarizes the new capability with validation against the Molten Salt Reactor Experiment (MSRE) data.