New approach to the determination of the lowest achievable effectiveness of emergency shielding of VVÉR reactors

Abstract

A conceptual approach is proposed to determining the minimum achievable number of control organs in the safety and control system and the minimum effectiveness of emergency shielding of VVÉR reactors. This approach includes a new probabilistic method of determining the effectiveness of emergency shielding with failures of different numbers of control organs and a decrease in the excess conservatism in performing thermohydraulic analysis. The modern safety ideology is used. For VVÉR-1000, it is shown that emergency shielding with 49 control organs is sufficient for the determining anticipated accident with steam-pipe rupture.One of the key principles used to validate the safety in deterministic approximation is a quantitative principle according to which the action of the emergency shielding is simulated neglecting one control rod with maximum efficiency which is stuck in the extreme top position.The emergency shielding of a reactor together with the inherent properties of the core (reactivity coefficients) is essentially intended for bringing the reactor into a safe state. A safe state is characterized by reactivity control, i.e., preventing uncontrollable growth of reactivity and limiting the power in a manner so as to ensure that the physical safety barriers are maintained (fuel, fuel-element cladding, boundaries of the coolant pressure loops, protective shell) at levels established for normal operation and its disruptions. In accordance with the norms [1], it is necessary to ensure and maintain (possibly together with actuation of an additional system) the reactor in a subcritical state. For comparison, we note that the modern safety ideology used abroad, specifically, the requirements of the European operating organizations (European Utility Requirements Rev. C 2002) requires that the emergency protection compensate the power effect of reactivity and ensure subcriticality in a hot state at or near zero power, admitting short-time and limited increase in power with a substantial decrease of coolant temperature, for example, in an accident with rupture of a steam pipe. In the process, the design limits for the fuel should not be exceeded.Analysis showed that the greatest demands on the emergency shielding, among all design demands, refer precisely to accidents with rupture of a steam line. Consequently, in the present paper the adequacy of emergency protection is analyzed for the example of such an accident. The question of the minimum number of control rods adequate for guaranteeing safety is posed. A probabilistic approach is proposed for solving this problem. A concrete solution is presented for VVÉR-1000 with 49 control rods.In currently operating VVÉR-1000 reactors, different numbers of control organs in the form of a clusters consisting of a different number of rods are used to shut down the reactor -109 control organs consisting of 12 rods (No. 5 unit of the Novovoronezh nuclear power plant), 49 control organs with 18 rods (No. 1 unit of the Southern Ukraine nuclear power plant), 61 control organs with 1...