New techniques for designing the initial and reload cores with constant long cycle lengths

Abstract

Several utilities have increased the output power of their nuclear power plant to increase their income and profit. Thus, the utility increases the power density of the reactor, which has other consequences. One consequence is to increase the depletion of the fuel assemblies (FAs) and reduce the end-of-cycle (EOC) sum of fissionable nuclides in each FA, ∑EOC. The power density and the ∑EOC remaining in the FAs at EOC must be sufficiently large in many FAs when designing the loading pattern, LP, for the first and reload cycles to maintain constant cycle lengths at minimum fuel cost. Also of importance is the cycle length as well as several other factors. In fact, the most important result of this study is to understand that the ∑EOCs in the FAs must be such that in the next cycle they can sustain the energy during depletion to prevent too much power shifting to the fresh FAs and, thus, sending the maximum peak pin power, PPPmax, above its constraint. This paper presents new methods for designing the LPs for the initial and follow on cycles to minimize the fuel costs. Studsvik’s CMS code system provides a 1000MWe LP design in their sample inputs, which is applied in this study. The first 3 cycles of this core are analyzed to minimize fuel costs, and all three cycles have the same cycle length of ∼650days. Cycle 1 is designed to allow many used FAs to be loaded into cycles 2 and 3 to reduce their fuel costs. This could not be achieved if cycle 1 was a low leakage LP (Shi et al., 2015). Significant fuel cost savings are achieved when the new designs are applied to the higher leakage LP designs. There are many factors, such as the core power density, cycle length, fuel cost, time between core shutdown and return to power, cost of replacement power during shutdown, loss of income during shutdown, cost of storing used FAs, and the income accrued over the same period of operation, which the utilities must consider when trying to increase their profit. The purpose of this paper is to provide the information to give guidance in making these decisions. Relative cost calculations are presented in establishing this guidance by comparing the utility’s profit accrued over a total cycle for the different core designs and cycle lengths.