Optimization of a sodium-cooled fast reactor operation with a gas power conversion system during a loss of off-site power

International audienceThe French Commission for Atomic Energy and Alternative Energy (CEA) in collaboration with its indus-trial 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 con-ditions, the operation of 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). The operation 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.In the aim to optimize the alternative procedure for a LOOP a MultiObjective Problem (MOP) is solved. Two conflicting objectives are simultaneously minimized the time to reach the cold shutdown state and the maximum of the thermal gradient through the main vessel. The decision variables are the two de-scriptive parameters of the TM rotation speed target to solve the MOP. The multiobjective optimization step is supported by the study of different TM rotation speed targets; a Latin hypercube design of ex-periments 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 in a time ranging from 30 minutes to 4 hours, whereas passive DHR systems do not allow the reactor to reach this state 24 hours after the initiating events. A short time to reach the safety state induces a maximum of the thermal gradient through the main vessel about twice higher than the procedure with passive DHR systems; whereas a long delay to reach the safety state can divide the maximum of the thermal gradient through the main vessel by four. Furthermore, it is demonstrated that each decision variable is more influent on one of the two objectives. This property is exploited to favor a specific objective and large ranges for the decision variables define a great diversity of optimum compromises.Thanks to the regulation of the TM rotation speed, the gas PCS is hence an adaptable system to opti-mize the thermalhydraulic behavior of a SFR during a LOOP. Moreover, this alternative procedure strengthens the diversification of the systems to fulfil the DHR function