A stretch-distribution method based on perturbation theory for expansion reactivity evaluation in fast reactors

The Stretch-Distribution (SD) method is proposed for accurate efficient evaluation of expansion reactivity in fast reactors at a computational cost comparable to that of first-order perturbation theory using transport theory. In this method, the post-expansion neutron flux distribution is estimated by applying a coordinate mapping that accounts for expansion and a correction to the first-order angular moment based on the neutron current to the pre-expansion angular neutron flux. The reactivity is then evaluated based on perturbation theory using the estimated neutron flux distribution. The method is evaluated in cases involving uniform radial and axial expansion including control rods, non-uniform expansion, and changes in system size. For radial expansion, the SD method reduces the difference from the reference solution to below 0.5% and shows little dependence on the expansion ratio. For systems with inserted control rod, the difference of approximately 10% occurs based on first-order perturbation theory, whereas the SD method substantially reduces this difference. For axial expansion, the improvement is not consistent, although local reactivity differences are reduced from first-order perturbation theory. For different system sizes, the difference from the reference solution remains below 0.1%, with little dependence on neutron leakage. For non-uniform expansion, although the accuracy is lower than that for uniform expansion, it remains higher than that of conventional first-order perturbation theory. These results demonstrate the applicability of the SD method to expansion reactivity evaluation based on transport theory.