A comparative study of reactivity feedbacks including in-pin motion and their impact on UTOPA in a 600 MW fast reactor

Abstract

The design experience of PFBR (Prototype Fast Breeder Reactor) gives confidence in designing advanced Sodium cooled Fast Reactors (SFR) with improved economy and passive safety features towards enhanced safety. In this regard, Fast Breeder Reactor (FBR) is optimised with 600 MWe leading to better economy. To prevent severe core damage & large radioactivity release to the public and also to enhance the safety, improved inherent safety characteristics are adopted with passive shutdown features. Important safety measures taken to improve inherent safety are the optimization of sodium void reactivity beneath 1 $, which is in line with the current international thinking. To incorporate the enveloping beyond design basis events (BDBE) within the Design Extension Condition (DEC), additional passive safety mechanisms are envisaged to strengthen the reactor safety and practically eliminate the probability of Core Disruptive Accidents (CDA). One such BDBE is Unprotected Transient Over Power Accident (UTOPA), in which uncontrolled withdrawal of one control rod leads to rise in power and fuel temperature. Under UTOPA, it is possible for fuel to melt and undergo in-pin fuel motion through the available central hole. Upon initiation of melting, fission gases trapped within the fuel microstructure are released into the pellet cavity along with molten fuel. Consequently, a multi-phase flow occurs inside the pellet cavity, with several hydrodynamic effects influencing the fluid motion. The relocation of molten fuel from high fuel void worth to low fuel void worth region results in a negative reactivity feedback. This in-pin fuel motion or squirting feedback reduces the total reactor power as well as the hot-spot clad and coolant temperatures. With change in reactor power during UTOPA, the balance of the plant gets affected. This results in a change in the inlet coolant temperature, which affects the overall core temperature profile and its respective feedbacks. Considering the above said arguments, UTOPA study has been carried out with different initial conditions such as Constant Inlet Coolant Temperature (CICT), Time Dependent Inlet Coolant Temperature (TDICT), with and without In-Pin Fuel Motion (IPFM) feedback. Comparisons of the results are made to get better understanding of various feedbacks and their impact on UTOPA, highlighting the importance of in-pin fuel motion reactivity feedback in a medium sized fast reactor core. From the study, it is concluded that the case without considering IPFM feedback gives the most conservative results and if the transients converge to a safe state without considering IPFM feedback then, the transients are expected to settle down to a safe state with IPFM feedback.