First Approach to Critical (ATC) for Bruce Units 1&2 Following Refurbishment

Abstract

Following refurbishment, Bruce Units 1 and 2 achieved first criticality in March (Unit 2) and July (Unit 1) 2012.During fuel loading, three in-core fission chambers confirmed that the neutron source was from spontaneous fission of 238U only. Therefore, first criticality would occur on manual control of power well before the permanent ion chambers became rational at 10-7.2 full power.For Unit 2, after the moderator gadolinium had been reduced to slightly above the critical concentration, incremental control absorber withdrawals achieved administrative criticality at - 0.2 mk. Decreases in fission chamber count rates were then observed and attributed to ingress of poisoned heavy water from the expansion tank into the calandria. The reactivity fell below administrative criticality.Modifications to the procedures were approved by the duty manager to continue the ATC using intermittent poison removal rather than incremental control absorber withdrawals. These modified procedures were also used for the first ATC for Unit 1 four months later.