Model of reactivity accident of the RBMK-1000 reactor

The reactor model was used to study the accident that destroyed the RBMK-1000 reactor at Unit 4 of the Chornobyl nuclear power plant on 26 April 1996. The model of reactivity accident of the RBMK-1000 reactor is based on equations of nuclear reactor kinetics, taking into account feedback in reactor reactivity. Reactivity changes as a result of both external influences – the movement of regulatory organs, changes in the reactor inlet coolant temperature, – and as a result of feedback by core parameters – changes in fuel temperature, coolant density, and 135Хе concentration. The model takes into account steam generation in the reactor core, which corresponds to the real physics of processes at the RBMK reactor, and allows obtaining simulation results that best match the recorded data and the consequences of the accident process. The study of reactivity accident on RBMK-1000 reactor is carried out for different combinations of values of control rods efficiency; reactivity coefficients by fuel temperature and coolant density; changes in the reactor inlet coolant temperature; the emergency protection time, as well as the reactor power level before closing the turbine generator stop valve. Different reactivity accident scenarios at RBMK-1000 reactor allow us to determine the most unfavorable combinations of external influences on the course of reactivity accident, namely: start time of main coolant pump rundown, time of activation of emergency protection, power level before the closing of turbine generator stop valves. In most reactivity accident scenarios, first of all, the critical values of fuel enthalpy are reached, at which the process of fuel destruction in the fuel element, destruction of the fuel assembly, and assembly channel start. Important results of studies are 1 – determination of the fact that time of activation of emergency protection after the closing of stop valves of turbine generator significantly affects the value of the maximum neutron power that is achieved during a reactivity accident; 2 – determination of the effect of reactor power before the closing of turbine generator stop valves on the course of the accident; 3 – it is not necessary to achieve supercritical on instantaneous neutrons, supercritical on delayed neutrons is enough to start fuel destruction.