Reactor core performance calculation device

In order to investigate a critical state of a reactor, it is necessary to perform three-dimensional reactor core calculation with void repetition for many time, and it takes extremely much time for the estimation calculation. Then, a means is disposed for storing a mid-way convergence value of an inherent value that can be calculated on every void repetition, to calculate and estimate the final result of the convergence of the inherent value using a function of the stored mid-way conversion value. The inherent value of the reactor core has a characteristic tending to converge extremely monotonously compared with an amount that greatly changes locally on every void repetition such as an output distribution or a void distribution of the reactor core. Accordingly, by providing a means for estimating the final conversion value of the inherent value using a function of the mid-way conversion value of the inherent value determined by up to first three void repetition, one test calculation can be completed without requiring conversion of the three dimensional reactor core calculation to the last. This can greatly reduce the cycles of the void repetition thereby enabling to shorten the entire calculation time. (N.H.)