Void effect on Static Neutron Multiplication Characteristics in Accelerator-Driven System

An accelerator-driven system (ADS) operates with a strong neutron source, and the neutron characteristics are subject to neutron source characteristics. The present study investigates the effect of different core void conditions on the static neutron multiplication characteristics by numerical simulations using a continuous energy Monte Carlo code. The neutron multiplication characteristics are evaluated for the single- and two-region cores with the parameters based on the effective multiplication factor keff derived from an eigenvalue calculation and the subcritical multiplication factor ksub derived from a fixed-source calculation. Numerical results show that the subcritical void reactivity calculated with ksub has a strong burnup dependence, while the conventional void reactivity calculated with keff has less dependence. Also, the power increase based on the keff is underestimated in comparison with that based on the ksub in the case of the inner fuel region void of the two-region core, although they are evaluated on the safe side in most cases. The parameter differences between eigenvalue and fixed-source calculations result from the discrepancies in the fission rate distributions, which are influenced by flux distributions, fuel compositions, and the positive streaming effect, as well as the spectrum hardening effect observed in a fast reactor.