LAUSR

Abstract Natural convection based passive decay heat removal systems are increasingly being deployed in the newer designs of nuclear reactors with the expectation that they enhance reliability and robustness owing to the elimination of reliance on human operators and external electrical power supply. In the absence of demonstrated experience with passive systems, difficulty in testing them on the field and possible range of phenomena which can degrade their performance, in some designs part of the redundant passive circuits are fitted with pumps to alleviate concerns regarding passive system performance. In this study the two types of designs, one with a set of 4 passive thermo-siphon loops and another with electro-magnetic pumps in part or all of the thermo-siphon loops are studied to bring out the reliability aspects. The decay heat removal function failure frequency is analyzed considering, electromagnetic pump integrity, changes in human interaction aspects for the two different configurations. It is observed that the introduction of electromagnetic pumps in two out of four loops increases the decay heat removal function failure frequency by approximately 64%. If electromagnetic pumps are introduced in all the four loops, the decay heat removal function failure frequency increases by an order. The results are highly sensitive to electromagnetic pump reliability. This study indicates that, there is no change in the uncertainty propagated from input parameters to response parameters, there is no significant advantage in terms of flexibility gained for certain operator actions and higher levels of reliability need to be achieved for the electromagnetic pump if it is to be introduced in all the four loops as compared to introduction in only two loops.