Unit 1 was in mode 5 after core reload during a refueling outage. When the 1H EDG failed to start, the operator manually started the B RHR pump. Shutdown cooling was interrupted for approximately 2 minutes. Power was restored to its pre-event configuration in less than two hours and the reactor coolant pump was restarted. Inspection of the 1H EDG discovered lube oil on top of all 12 lower pistons.
Preliminary analysis indicates that the lube oil hydraulically locked the engine.
The licensee is still investigating the cause of the lube oil being on the pistons.