Core Loading and First Approach to Critical: Darlington Unit 2 Return to Service after Refurbishment

Abstract

This paper describes how the Darlington Unit 2 reactor was reloaded with fresh fuel (November 2019) and brought to criticality (April 2020), after a refurbishment outage in which all fuel channels, calandria tubes and adjuster rods were replaced. Neutron monitoring during return to service was provided by three one-inch-diameter fission chambers, positioned in-core in a temporary guide tube. Gamma fields from irradiated structures interacted with the moderator heavy water to produce a significant photo-neutron background that influenced detector count rate behaviour during fuel loading. The new fission chambers also monitored neutron flux throughout the first approach to critical and power raise, which are also discussed. During the approach, after removing moderator poison to bring the reactor sufficiently close to criticality, changes in Average Zone Level were used to effect a series of small reactivity step increases until first criticality was achieved. Small AZL changes were then used to initiate and control a power raise until control was transferred to the Reactor Regulating System.