TRANSIENT RESPONSE CHARACTERISTICS IN MULTI-LOOP NATURAL CIRCULATION SYSTEM USING FREQUENCY RESPONSE TESTING METHOD

Natural circulation is one of the significant passive cooling mechanisms in the designs of many advanced light water reactors (LWRs) as well as integral reactors such as small modular reactors (SMRs). Under accident conditions, it can be employed to cool the core without the involvement of any active system. The natural circulation occurs due to buoyancy forces caused by density gradient. However, presence of subcooled boiling will affect the density gradient in the closed loop which may lead to consequential effects. Passive residual heat removal system and reactor primary and secondary circuit systems are in a coupling state of multi-natural circulation loops under accident conditions. Because of the nonlinear characteristics of natural circulation system, the interaction of these loops has obvious feedback effects which make response characteristics of the multi-loop natural circulation in the passive residual heat removal system to be extremely complex. Therefore, this paper employs frequency response testing method to numerically study the transient response characteristics of the natural circulation system using RELAP5/SCDAPSIM/MOD3.4 in coupled loops. The RELAP5 code calculation results are validated with experimental data under under steady-state condition and consistency is observed within a reasonable range. Having validated the code, the study is carried out under two-phase flow heat transfer condition using RELAP5 code. It focuses on the study of void fraction response to sinusoidal power fluctuations under different frequencies. At period of oscillation, T=2700, void fraction is seen to respond out of phase with the sinusoidal power fluctuation. However, for T=270s and 27s, the phase difference reduces signicantly. This research can serve as a guide to develop method of 2 testing response of thermal hydraulic parameters to transients in passive residual heat removal system (PRHRS) of nuclear reactors during decay heat removal operation.