Calculation Model of Radial Core Expansion Reactivity Feedback in Sodium-cooled Fast Reactors

Fast reactors is ensured by self-regulating reactor power characteristics. The self-regulation is achieved by both negative reactivity feedback effects upon a core power excursion and large thermal inertia of a primary sodium inventory. Therefore higher fidelity analyses for the reactivity feedback effects may eliminate an unnecessary conservatism and increase safety margins by beneficial design changes. A high-accuracy reactivity feedback calculation requires mechanistic reactivity feedback models in conjunction with multi-dimensional core thermal-hydraulic models. It has been found a radial core expansion reactivity, among various reactivity feedbacks, is dominant in view of its magnitude and response time for most postulated ATWS events in a metal-fueled core. However, the accurate calculation of its reactivity feedback effect is very difficult because of its associated complex phenomena of thermal, hydraulic, mechanistic, and neutronics. This paper discusses analytical methods to represent the radial core reactivity feedback to be employed into a system-wide safety analysis code