Fast shutdown-margin calculation using perturbation theory with regionwise flux expansion

Shutdown margin is an important safety parameter in reload core design. In order to search for the most reactive control rod, it is necessary to calculate the worth of each rod, which will require much computing time if three-dimensional full-core calculation is performed. Recently, Smith developed a one-group model for fast shutdown margin calculations in SIMULATE-3. In this paper, a perturbation method with regionwise flux expansion is proposed for fast shutdown-margin calculations. Because it was observed that in cold conditions, with no voids present, the axial distribution of neutron flux will not change drastically when a single control rod is withdrawn from its full-in to full-out position, a two-dimensional model is adopted in this study.