Activated sodium adjuster rod design for enhanced delayed-photoneutron yield in heavy water reactors

Abstract

A new adjuster rod design is proposed using natural sodium as the primary neutron absorber. In a generic heavy water lattice resembling an idealized, time-averaged pressure-tube-type heavy water reactor core, this adjuster rod design can achieve the performance objectives of conventional mechanical absorbing rods-flux flattening and excess reactivity for xenon override-but with the added benefit of producing the long-lived neutron activation product 24Na which emits a hard gamma ray. The conversion of the gamma rays in the heavy water moderator produces photoneutrons. Monte Carlo simulations quantify the saturated 24Na source yield at full power to be 2.3 x 1015n/s or an equivalent delayed neutron fraction of 1.03 x 10-5 which is a factor often to thirty larger than delayed-photoneutron yields from long lived-fission product precursors. Application of the 24Na source for robust subcriticality monitoring during a xenon-poisoning transient is demonstrated where all detector signals, even for the ex-core locations far from the source, are dominated by subcritical multiplication fission neutrons originating from the 24Na photoneutrons. The reactor kinetics parameters of large power reactors operating at full power can be beneficially modified through engineering and physical processes decoupled from the microscopic fission process.