Optimization of OPDT protection for overcooling accidents

Abstract

Overcooling accidents are typically resulting in power increase due to negative moderator feedback. There are more protection set points responsible for terminating power increase. OPDT protection set point is typically protection from exceeding fuel centre line temperature due to reactivity and power increase. It is important to actuate reactor trip signal early enough, but to be able to filter out events where actuation is not necessary. Different concepts of coolant temperature compensation as part of OPDT set point protection were studied for decrease of feedwater temperature accident and for small main steam line breaks from full power for NPP Krško. Computer code RELAP5/mod 3.3 was used in calculation. The influence of different assumptions in accident description as well as nuclear core characteristics were described.