Study the effect of beryllium reflector poisoning on the Syrian MNSR

Neutron interactions with beryllium lead to formation of 3H and strong neutron absorbers 3He and 6Li in the reflector (so called beryllium poisoning). After the reactor shutdown, the concentration of 3He 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 6Li and 3He poison concentrations, initiated by the B9e(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 3He and 6Li 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.

► The impact of poisoning accumulation in the beryllium on reactivity for the MNSR was discussed.
► The 3He and 6Li accumulations in the beryllium reflectors were calculated.
► The parasitic accumulations of 6Li and 3He in the MNSR reactor lead to decrease in excess reactivity.