Reactivity Measurements on Fast Systems with Natural Uranium Metal Blanket by Pulsed Neutron Method

A study has been made on how simple analysis of pulsed neutron experiments could be applied to the measurement of reactivity of fast systems with a blanket of metallic natural uranium. Pulsed neutron experiments have been performed on three fast assemblies of the Fast Critical Assembly at the Japan Atomic Energy Research Institute. The decay constants have been successfully observed even at delayed critical. The energy and spatial higher modes have been also observed. Simple models of Gozani and Simmons-King have been applied to derive the reactivity in dollar units.It has been made clear that the reactivity could be measured by the pulsed neutron method down to -3, and in a certain case, down to -5, for fast systems with a core of medium size up to at least about 200l enclosed in a metallic natural uranium blanket. The establishment of the asymptotic decay has been experimentally investigated. Linearity has been ascertained to hold between the prompt neutron decay constant and the reactivity by the pulsed neutron method and between the reactivities by the pulsed method and by the positive period method down to a few dollars in negative reactivity.