The Origin and Consequences of Radial Helium Profiles in Fast Reactor Cladding

The inner surfaces of both fuel pin cladding and helium-pressurized creep tubes develop near-surface enhancements of helium above that generated in the cladding by (n,α) events. The amount and distribution of the additional helium can be predicted from knowledge of the neutron spectrum, component geometry, and adjacent materials. The major sources of injected helium are ternary fission events in the fuel, recoil-injected helium from the cover gas, and (n,α) events occurring both in the tube and its surrounding materials. While the near-surface enhancement is shown not to be solely responsible for the loss of strength and ductility observed in fuel cladding, it appears that the additional helium acts synergistically with other phenomena associated with fuel adjacency to cause a reduction of cladding lifetime.