The event involved the unavailability of both low head safety injection (LHSI) motors for an extended period for certain accident scenarios that went undetected by all the present processes currently in use for surveillance, maintenance and testing.
On 2005-12-05 both LHSI motors were declared inoperable because of reduced motor cooling water flow. An Operating Technical Specification Limiting Condition of Operation required the unit to be taken from a plant state of Intermediate Shutdown Residual Heat Removal system conditioned to Cold Shutdown within 24 hours.
An engineering assessment of this condition concluded that the motor/pump combinations would not have been able to meet their design base accident functions with only 66% of the designed cooling flow to the motor. The motors would either have been damaged or destroyed if used as required by design or would have had to be taken out of service, to prevent damage, before they could complete their intended/designed mitigating functions during design base accident conditions.
The investigation determined that the incorrect re-assembly of the motor coolers resulted in the cooling flow to the motors to be restricted. This condition rendered both the low head safety injection (LHSI) motors inoperable. An analysis of archived records confirmed that the last interventions had been performed in 1993.
Event date: Mon, 05-12-2005