1. A non-transitory computer readable media, comprising a nuclear-characteristic calculating program executable by a processor for calculating a nuclear characteristic of a fuel rod that contains nuclear fuel and burnable poison that loses neutron absorption capability with a neutron absorption reaction of absorbing a neutron, wherein the nuclear-characteristic calculating program includes: a first scattering model representing an isotropic scattering of the neutron with lighter calculation load than the calculation load of a second scattering model as a selected scattering model for representing isotropic scattering of the neutron; the second scattering model representing an anisotropic scattering of the neutron with heavier calculation load than the calculation load of the first scattering model as the selected scattering model for representing anisotropic scattering of the neutron; a first nuclear-characteristic calculation module for calculating, when a burnup of the fuel rod is before a set burnup at which the burnable poison is considered to lose the neutron absorption capability, the nuclear characteristic of the fuel rod by using the second scattering model; a scattering-model switching module for switching, when the burnup of the fuel rod is the set burnup, the selected scattering model from the second scattering model to the first scattering model; and a second nuclear-characteristic calculation module for calculating, after switching the selected scattering model, the nuclear characteristic of the fuel rod by using the first scattering model, wherein each of the first nuclear-characteristic calculating module and the second nuclear-characteristic calculating module calculates the nuclear characteristic of the fuel rod based on a prediction nuclear characteristic that serves as a predictor and a correction nuclear characteristic that serves as a corrector, and at a time of performing the scattering-model switching, the nuclear characteristic of the fuel rod is calculated based on the prediction nuclear characteristic calculated by the first nuclear-characteristic calculating module using the second scattering model and the correction nuclear characteristic calculated by the second nuclear-characteristic calculating module using the first scattering model.