Development of practical method using a Monte Carlo code for evaluation of optimum fuel pitch in a typical VVER-1000 core

Abstract

In this work, we developed a practical method to evaluate the optimum moderator-to-fuel ratio of a typical VVER reactor using a Monte Carlo code. To reach this aim, by analyzing the curves of the infinite multiplication factor (k∞) versus moderator-to-fuel ratio, the optimum fuel pitch is determined for different fuel diameters of the proposed fuel elements using the MCNPX code. Optimum moderator-to-fuel ratio and fuel pitch were determined from the curves where the value of k∞ is maximum for cold conditions and is located in an under-moderated region for hot conditions. Also, the void reactivity coefficient must be negative and smooth for the selected fuel pitch. Then, the gained optimum values of the proposed fuel elements are considered to evaluate the effective multiplication factor of the core, which should be located in the under-moderated region for all Cold Zero Power (CZP), Hot Zero Power (HZP), and Hot Full Power (HFP) conditions. Finally, we compared the calculated optimum values with the values given in VVER-1000 Final Safety Analysis Report (FSAR). The results show that the fuel diameter and pitch of 7.57 and 12.75mm, respectively, provide the best neutronic economy among all tested values.