Transient behavior of ASTRID with a gas power conversion system

•CATHARE2 transient calculations have been performed for ASTRID with a gas PCS.•The behavior of the reactor is close for gas and for water PCS in case of LOOP.•The gas PCS enables to cool the core for at least 10h for pressurized transients.•The depressurization of the PCS induces an over-cooling for breaches on low pressure pipes.•The spurious opening of a by-pass line of the turbomachine can be controlled without scram. The present article is dedicated to preliminary transient studies carried out for the analysis of the system overall behavior of the ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) demonstrator developed in France by CEA and its industrial partners. ASTRID is foreseen to demonstrate the progress made in SFR technology at an industrial scale by qualifying innovative options, some of which still remain open in the areas requiring improvements, especially safety and operability. Among the innovative options, a gas power conversion systems (PCS) is envisaged. In this innovative PCS, the working gas is nitrogen whose flow rate delivers power to a turbine driving with the same shaft two compressors (low and high pressure) separated by an intercooler. The other part of the work delivered by the gas is used to drive the alternator that produces electricity. The main objective of such a PCS consists in avoiding physically the possibility of a sodium/water reaction with the secondary circuit but the impact of this PCS on the control of incidental and accidental transients has also been studied. The main purpose of the studies presented in the paper is to assess the dynamic behavior of ASTRID including a gas PCS with the CATHARE2 code. The first transient presented deals with a loss of off-site power and has been calculated for the gas PCS but also for a classical steam/water PCS for comparison purpose. Then typical transients of gas system have been investigated. Several families of initiating events affecting the PCS are studied: breaches in the ternary circuit, a loss of power supply, a loss of main heat sink and the spurious opening of the turbine by-pass valve. Regarding this last transient, sensitivity studies have shown that an adequate design of the turbomachine by-pass lines allows a control of the event without shutdown. Moreover, regulating actions following the scram are also investigated in order to take benefit of the PCS in order to remove the decay heat.