Helium, effect slow reaction

A further consequence of irradiation is the change in composition arising from neutron interactions with the nuclei of the material. This is predominantly a slow neutron effect, but the formation of helium gas as a result of the fast neutron-induced (n, a) reaction in boron has an important effect on the... 

It should be noted that irradiation of steels in thermal reactors also produces embrittlement, though in this case the main contributor of helium is the (n, a) reaction of slow neutrons with boron impurity in the steel. Because of the high thermal absorption cross section of the boron, however, this burns out at fairly moderate exposures, leading to saturation of the effect. 

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