Burnup and Analytical Analysis of Neutron Poison Accumulation in the Beryllium Reflector of the Jules Horowitz Reactor

The Jules Horowitz Reactor (JHR) is a Materials Testing Reactor (MTR) under construction at CEA Cadarache in southern France. Its primary function is to provide materials and fuel irradiation testing capabilities in its adjustable experimental positions, while also being able to produce medical radio-isotopes, for an international consortium of industry and research partners.[1]
Like many other MTRs, JHR has a beryllium reflector to reduce neutron leakage from the core which not only reduces core size and the amount of fuel necessary for criticality, but also provides a more uniform power distribution. This last point is particularly of interest for running experiments in order to assure homogeneity of conditions across tests. Beryllium has many excellent properties such as a high neutron multiplication, low absorption cross section, and high scattering cross section of neutrons. It is expensive and toxic, however, making replacing beryllium during a MTR’s lifetime unappealing.
The focus of this paper is how neutron poison accumulation affects the reflector’s performance over time. Two different approaches were used with Serpent 2 Monte Carlo neutronics code[2] , focusing on 6Li, 3He and 3H, which are all part of the same beryllium transmutation chain that could affect the primary function of the beryllium reflector. The results indicate that after ten years, the accumulation of 6Li and 3He would result in reductions in the thermal neutron flux of up to 2% and 5%, respectively, in the reflector experimental sites, while the effect of 3H was negligible. Furthermore, the decrease in reactivity induced by poisons during this period was also minimal. Therefore, this neutronic assessment suggests that it will be unnecessary to replace beryllium components during the first ten years of operation.