For a 3-loop plant, a new reactivity model which is easily applicable in evaluating the safety of hot zero power steamline break accident was established. The applicability of this reactivity model for a 2-loop plant has been examined. As per the result, the weighting for the core inlet average density is optimized to be 0.8 and 0.2 respectively for each of a cold and a hot loop. Also, it is verified that the safety analysis reactivity coefficients suggested in this model are enoughly conservative as compared with the best-estimated reactivity coefficients. These results demonstrate that the established reactivity model for a 3-loop plant is applicable to a 2-loop plant. (Author).