Ukraine: Passive shutdown of NBW fast reactor using the Autonomous Reactivity Control (ARC) system

We present the results of computer simulation of the ARC system performance in the NBW reactor at the loss of flow accident in the second cooling circuit and during the restart of reactor operation after the accident. The problem is studied by means of numerical simulation of the NBW propagation in such a reactor with the depleted uranium metallic fuel and sodium as a coolant. For the neutronics simulation we use the deterministic approach based on solving the non-stationary neutron diffusion equation using the effective multi-group approximation together with a set of burn-up equations for fuel components and equations of nuclear kinetics for precursor nuclei of delayed neutrons. These calculations are complemented with the thermal-hydraulics simulation of the first sodium coolant circuit in the interaction with ARC system. Our simulation shows that ARC system shutdowns the reactor in a few minutes without fuel and coolant overheating. We analysed also the interference between two negative reactivity feedback mechanisms inherent to the NBW reactor and ARC system, which appears in the form of certain oscillations of the flux in a few days after the reactor shutdown. As a result of this interference, the system stabilizes automatically at the acceptable temperature (below 550 ºC) when almost all intermediate isotope neptunium decays into plutonium