Dynamic Modelling for Shutdown-System-1 Depth Analysis

Abstract

Shutdown system 1 (SDS1) depth has traditionally been analysed as a static all effects calculation of the core condition 15 minutes after an in-core break, at which time operator action is credited. The postulated accident is intended to represent the most reactive core at that time and is arrived at by assuming that the discharging coolant is replacing and diluting heavily poisoned moderator as well as damaging nearby shutoff-rod guide tubes. The challenge is to demonstrate that SDS1 has sufficient reactivity worth to maintain this most reactive core subcritical. This paper describes a more accurate treatment of SDS1 depth through the time-dependent dynamic analysis of the event and demonstrates that the static calculation may underestimate the SDS1-depth margin or at least the time available before the core would become critical without operator action.