Flow Particulars of Some Transient Regimes with Load Shedding on VVER-1000

The results of an investigation of the operating regime of a nuclear power plant with VVER-1000 with actuation of accelerated preventative protection at power 100% N nom with one of two turbo pumps switched off are presented. The modeling was performed using the improved accuracy RELAPSCDAPSIM/Mod3.4 code and a corresponding model for the No. 2 unit of the Khmelnitsky NPP. It is shown that the actuation of emergency protection based on the fact that the three main circulation pumps are switched off because of a drop in the feed water in the steam generators cannot be avoided. On the basis of the results obtained, it is proposed that for the second half of the fuel run an emergency protection signal instead of an accelerated preventative protection be immediate generated when one of the turbo pumps is switched off at power level >95% N nom ; this will make it possible to secure more forgiving temperature regimes for fuel and equipment, avoid additional thermal cycling loads, and prevent violation of the safe operating limits on the basis of the feed water level in the steam generators. The fuel component plays an important role in the competitive production of electricity by NPP; it can reach 20‐30% of the total costs. For this reason, the necessity of optimization and efficient use of fuel is obvious, and one of the main methods is to increase burnup [1]. In addition, nuclear fuel producers increase the initial enrichment and the amount of fuel in the core. A consequence of this approach is that the water-uranium ratio and the reactivity coefficients change. Such changes can be substantial and cause deviations from the anticipated flow of transient processes associated with, among other things, an increase in the rate of change of the neutron power as a result of feedback effects. In the present article, we present the results of modeling and analyze the particulars of the flow of the accelerated preventative protection regime in a situation where one turbo pump is switched off with a power-generating unit with VVER1000 operating at power N nom = 100%. The reactivity coefficients of the core of a reactor with TVS-A fuel assemblies adopted in Ukraine for use in VVER1000 vary over a wider range in the course of a fuel run than with the fuel assemblies used previously. This trend will remain in the future due to the use of fuel assemblies with greater fuel-kernel heights and fuel pellets with no central opening. A change in the reactivity coefficients strongly affects the flow of the anticipated regime of accelerated preventative protection. Specifically, the neutron power excursions recorded in the last few years have been significantly higher than those observed at the start of operation. Such regimes flow with the reactor in a subcritical state, but in the process the growth rate of and gain in neutron power reach the limits of safety and in consequence the feed water level in the steam generators drops below the levels at which the main circulation pumps are switched off and the emergency protection is actuated.