Loss of off-site power transient analysis for a sodium-cooled fast reactor equipped with a gas power conversion system and preliminary optimisation of its operation

Abstract The French Commission for Atomic Energy and Alternative Energy (CEA) in collaboration with its industrial partners develops Sodiumcooled Fast Reactors (SFR) as industrial-scale demonstrators mainly guided by safety and operability objectives. In this paper, a SFR reactor associated to a nitrogen closed Brayton cycle for the Power Conversion System (PCS) is considered. This paper is dedicated to an alternative procedure to control a Loss Of Off-site Power (LOOP). Usually, in case of LOOP, the SFR standard procedure relies on passive Decay Heat Removal (DHR) systems to cool down the primary circuit. In this paper, an alternative solution substitutes the use of the latter by the gas Power Conversion System (PCS). This aims at reducing the delay to reach the cold shutdown state while fulfiling safety criteria dealing with thermal stress issues. The aim of this paper is to study the potential of the alternative sequence to cool down the reactor for a LOOP. It is interesting to integrate the alternative procedure to the reactor operation, in order to improve the reliability of the DHR function, by adding a medium strength safety defence line. Nevertheless, the high level of complexity associated to the gas PCS design makes its qualification as a safety grade system impossible. For this reason, the DHR systems cannot be replaced by the gas PCS and remain the main safety grade systems. The actuator of the sodium temperature regulation of the secondary circuit is chosen thanks to a global sensitivity analysis performed with a metamodel-based methodology. The setting of the controller associated to the TM rotation speed regulation is justified by the study of different proportional-integral-derivative (PID) controllers. A comparison of this alternative sequence with the reference one, based on simulations with the system thermalhydraulic code CATHARE2, is presented in this paper. The study indicates that the passive DHR systems allow the reactor to reach the cold shutdown state only after 24 h, whereas the procedure with the gas PCS required few hours to lead the reactor in this safety state. A Multiobjective Optimisation Problem (MOP) is solved in order to minimize simultaneously the delay to reach the cold shutdown state and the thermal stresses on the main vessel during the alternative procedure. These two objectives are conflicting, thus optimal compromises between them are required to solve the MOP and define a Pareto front. In this way, the alternative procedure allows the reactor to reach the cold shutdown state in a time ranging from around 36 min to 3 h and 52 min. The shortest observed time to reach the safety state induces a maximum of the thermal gradient through the main vessel around 40 percent higher than the procedure with passive DHR systems whereas a long delay to reach the safety state can divide the higher thermal gradient observed by 6.7. Thanks to the regulation of the TM rotation speed, the gas PCS is hence an adaptable system to optimize the thermalhydraulic behaviour of a SFR during a LOOP.