Unit 7 was being started up after a long planned shutdown. The startup involved removing “poison” from the unit’s moderator. The “poison” is a material added to the moderator to keep the reactor sub-critical. When the poison was being removed and the neutron activity began to increase, the start-up instrumentation failed to show any effect. Bruce Power operations staff recognized this instrumentation failure while the unit was still deeply sub-critical and returned the unit to the over-poisoned shutdown state.
Investigation found that the instrumentation had been giving incorrect readings from November 30, 2002 to December 23, 2002. The startup instrumentation is also connected to one of the reactor’s two shutdown systems, providing an additional backup trip for the system. This backup trip was impaired, representing a reduction in defense in depth. The credited trips for both shutdown systems use different instrumentation and were not affected.
The root causes of this event were inadequate understanding and training on the use of the start-up detectors which led to their incorrect installation, calibration and testing procedures.
Event date: Tue, 24-12-2002