Development of active and passive reactivity control systems for a fast spectrum small modular reactor

Abstract

This paper presents the development of active and passive reactivity control systems implemented in a fast-spectrum small modular reactor (SMR). The design of the control assembly serving as an active system still relies on a conventional rod-typed geometry. To enhance its worth, a thermalizing ZrH_1.6 pin is introduced, which softens the neutrons around the control assembly, leading to increased neutron absorption in the absorber material. Additionally, a control assembly with a partial-length absorber is strategically placed at a certain location to mitigate high peak power levels. Two passive systems have been developed to enhance reactor safety: the gas expansion module (GEM) aimed to mitigate the unprotected loss of flow (ULOF), and the liquid lithium-6 absorber intended to address unprotected transient overpower (UTOP) and unprotected loss of heat sink (ULOSH) scenarios. With a total of 24 GEMs and 18 lithium-6 channels installed, they provide negative reactivity ranging from about 932 to 693 pcm (1.17$ to 1.03$) and 521 to 602 pcm (0.65$ to 0.89$), respectively.