Physics Aspects of Reload and Approach-to-Critical of the NRU Reactor After Vessel Repair

Abstract

The National Research Universal (NRU) reactor at Chalk River shut down on 2009 May 14 and there was a subsequent outage of 15 months to repair leaks from the vessel. On 2010 August 17, NRU returned to full power operation and resumed isotope production. This paper describes the physics aspects of reload, and the approach-to-critical (ATC) tests conducted to restart the reactor safely. Five ATC’s, each at a different number of reloaded assemblies, plus a final one before reactor startup, were completed to confirm the calculated physics predictions of the subcritical state and critical point. Activities for preparation of the ATC tests, the responsibilities of the physicists during execution of the ATC’s, and plots of neutron signal data during the ATC’s are presented. The final measured critical point of CR#14 @190 cm agreed well with the calculated physics prediction of CR#14 @185 cm, or within ~0.5 mk.