An extension of the two-zone method for evaluating a fission gas release under an irradiation-induced resolution flux

This paper describes an extension of the adaptive two-zone method whose accuracy is substantially enhanced when compared to the original formulation by Matthews and Wood. A diffusive problem under the presence of an irradiation-induced resolution flux is evaluated by applying a variational principle to the diffusion equation. Prior to a gas saturation in the grain boundaries, a constraint associated with a gas balance is added to the variational equation. The spherical grain is divided into two regions whose interface is relocated as the ratio of the number of gas atoms within a grain to that generated. The distribution of the gas concentration is calculated over the grain. During the calculations, the number of degrees of freedoms is reduced to provide a profile which decreases monotonically along the radius. Numerical verifications show that the present approach is viable in computing a gas release accurately and efficiently in fuel performance codes.