Robust Subcriticality Monitoring of Xenon-Poisoned CANDU-6 Core using Intense Delayed-Photoneutron Source from Activated-Sodium Adjustor Rod Design

Abstract

This study evaluates the performance of an activated sodium adjuster rod design as an intense delayed-photoneutron (PN) source for subcriticality monitoring in large heavy water reactors. The proposed adjuster rod design utilizes NaF powder sandwiched between concentric Al tubes while maintaining the reactivity worth and flux shaping performance target of conventional stainless steel adjuster rods. The activated sodium (24Na) generates long-lived delayed-PN in the heavy water moderator from the high-intensity 2.754 MeV gamma ray through D(ℝ?,ℝ?)1H reaction. The saturation activity of the 24Na source is orders of magnitude larger than other long-lived fission-product PN groups, and the activation-product source dominates the intrinsic source in the reactor from tens of minutes to several days after the reactor shutdown and Xenon poisoning transient. Detector signals during a Xe-poison transient at all locations (in- and ex-core), are primarily (~99%) fission neutrons from subcritical multiplication and over 90% of these fission chains originate from the 24Na PNs. These detector signals from the Xe-well at 10 h after shutdown to approach to critical between 39 hours and 43 hours are factor of 10 to 100 larger than from the 140La PN source alone, providing robust detector response for reactivity monitoring of CANDU-6 core.