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

International audienceThe French Commission for Atomic Energy and Alternative Energy (CEA) in collaboration with its industrial partners develops Sodium-cooled 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. In incidental and accidental conditions, the operation of the reactor must be defined to keep it under control and to fulfil safety requirements. 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 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 operating of the gas PCS required three regulations-The regulation of the Turbo-Machinery (TM) rotation speed to keep a gas flow in the PCS;-The sodium temperature regulation of the secondary circuit, once the cold shutdown state is reached, to adapt the heat removed by the gas PCS to the decay heat;-The regulation of the gas temperature at the inlet of the compressors, in the PCS, to keep an efficient heat sink.The actuator of the sodium temperature regulation of the secondary circuit is chosen among three different actuators 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 do not allow the reactor to reach the cold shutdown state after 24 hours, whereas the procedure with the gas PCS required few hours to lead the reactor in this safety state, without increasing thermal stresses on the main vessel. Based on this first result, 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. The multiobjective optimization step is supported by the study of different TM rotation speed targets. A Latin hypercube design of experiments is performed with the CATHARE2 code and is used to build the Pareto front. In this way, the alternative procedure allows the reactor to reach the cold shutdown state between 30 minutes and 4 hours. A short delay to reach the safety state induces thermal gradients through the main vessel about twice higher than the standard procedure, whereas a long delay to reach the safety state can divide the thermal gradients through the main vessel by four. To favour a specific objective of optimisation, this study highlights which TM rotation speed target must be chosen.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