Isotopic Transmutations in Irradiated Beryllium and Their Implications on Maria Reactor Operation

Abstract

Beryllium irradiated by neutrons with energies above 0.7 MeV undergoes (n,α) and (n,2n) reactions. The Be(n,α) reaction results in subsequent buildup of 6Li and 3He isotopes with large thermal neutron absorption cross sections causing poisoning of irradiated beryllium. The amount of the poison isotopes depends on the neutron flux level and spectrum. The high-flux MARIA reactor operated in Poland since 1975 consists of a beryllium matrix with fuel channels in cutouts of beryllium blocks. As the experimental determination of 6Li, 3H, and 3He content in the operational reactor is impossible, a systematic computational study of the effect of 3He and 6Li presence in beryllium blocks on MARIA reactor reactivity and power density distribution has been undertaken. The analysis of equations governing the transmutation has been done for neutron flux parameters typical for MARIA beryllium blocks. Study of the mutual influence of reactor operational parameters and the buildup of 6Li, 3H, and 3He in beryllium blocks has shown the necessity of a detailed spatial solution of transmutation equations in the reactor, taking into account the whole history of its operation. Therefore, fuel management calculations using the REBUS code with included chains for Be(n,α)-initiated reactions have been done for the whole reactor lifetime. The calculated poisoning of beryllium blocks has been verified against the critical experiment of 1993. Finally, the current 6Li, 3H, and 3He contents, averaged for each beryllium block, have been calculated. The reactivity drop caused by this poisoning is ~7%.