Study the effect of beryllium reflector poisoning on the Syrian MNSR

Neutron interactions with beryllium lead to formation of (3)H and strong neutron absorbers (3)He and (6)Li in the reflector (so called beryllium poisoning). After the reactor shutdown, the concentration of (3)He increases in time due to tritium decay. This paper illustrates the impact of poisoning accumulation in the beryllium reflectors on reactivity for the Syrian MNSR research reactor. The prediction of (6)Li and (3)He poison concentrations, initiated by the 9Be(n,α) reaction, in the beryllium reflectors of the MNSR was also presented. The results were based on MCNP Monte Carlo calculations and solutions to the differential equations which describe the time dependent poison concentrations as a function of reactor operation time and shutdown periods. The whole reactor history was taken into account to predict reliable values of parasitic isotope concentrations. It was found that the (3)He and (6)Li accumulations in the beryllium reflectors during the actual working history decreased the excess reactivity by about 28%. While, the effect became more significant at the reactor life's end and the reactor became subcritical after 25,000 h operation. The results contained in this paper could be used in assess the safety analysis of the MNSR reactor.