Subgroup method with resonance interference factor table

• Fitted subgroup parameters based on IR approximation.
• Used resonance interference factor table to consider overlap effect.
• Used SPH factor to reduce the error of subgroup collapsing.
• The effect of “infinite mass” scattering and resonance scattering is taken into account.
• The results were consistent with predictions of the MCNP code.

Subgroup parameters based on the intermediate resonance (IR) approximation are generated by the constrained best fitting method to assure the positivity of the subgroup parameters, and the infinite mass scattering term introduced by the IR approximation is explicitly retained in the subgroup equation. The impact on the heterogeneous problem is investigated. To preserve the reference reaction rates obtained by the subgroup calculation, the Hébert SPH method is adopted. A resonance interference correction factor table is introduced to consider the resonance interference effect. The proposed resonance calculation method is tested on various problems, which cover homogeneous cases, 1D pin cell cases, as well as 2D assembly cases. It is demonstrated that (1) the proposed resonance interference factor table method can significantly reduce the error caused by resonance interference; (2) the SPH method can reduce the error of subgroup collapsing; (3) the infinite mass scattering term can significantly impact the result of a heterogeneous problem. Numerical results also revealed that all the three effects are in the same direction and add up to about 600 pcm reactivity increase, bringing a good consistency between the multi-group calculation and continuous energy Monte Carlo calculation.