Reactivity Associated with the Decay of Np²³⁹ in Irradiated Natural Uranium Dioxide

In the calculation of irradiation-induced changes in reactivity in natural uranium, the direct formation of Pu²³⁹ upon neutron capture in U²³⁸ is frequently assumed. The attendant error in reactivity during irradiation is shown to be significant for an illustrative case even after high exposure. This suggests that the consequences should be examined when the direct formation of Pu²³⁹ is assumed.