Impact of Photo neutrons on Reactivity Worth of 3He In a Reactor with Beryllium Reflector

A study about the contribution of photo neutrons, produced in (gamma,n) – reactions of Be-9 into the reactivity worth of He-3 in a reactor with beryllium reflector is presented. Neutron interactions with beryllium lead to formation of 3T and strong neutron absorbers He-3 and Li-6 in the reflector (so called beryllium poisoning). After the reactor shut down, the concentration of helium – 3 increases in time due to tritium decay. The negative reactivity effect of neutron absorption by accumulated 3He has been considered in a few papers, concerning reactivity losses in reactors with beryllium reflector. The contributions of the photo neutrons for additional production of tritium during the reactor operation and enhanced build up of helium – 3 after the shut down have not been discussed previously. Calculations of the photonuclear reactions in beryllium are performed for the Belgian Material Testing Reactor BR2 and for a generic reactor model with beryllium reflector. The calculation analysis has shown an increase of about ~ 20% in the reactivity worth of helium – 3 by photo neutrons. The effective fraction of photo neutrons is estimated using MCNP5 with included (gamma,n) – reactions.