FAILURE OF THE MAIN FEEDWATER PUMP DUE TO ABSENCE OF REQUALIFICATION TEST AFTER MAINTENANCE

During annual plant inspection which had been started in April of this year, the reactor was automatically shutdown due to signal "SG water level abnormally low". This event occurred during preparation of power generation following the reactor restart.
As a result of investigation, it was found that the wiring, which had been temporarily disconnected from the relay terminals during the functional test of the main feed water pump (secondary system), was inadvertently touched to adjacent terminals during restoring work and caused short circuiting. Due to the short circuiting, the no fuse breaker (NFB) for relay circuit was opened. Tripping the motor driven main feedwater pump. This caused the lowering of the steam generator water level, which led to automatic reactor shutdown. There were no releases of radioactive materials outside the reactor facility, and also there were no unplanned exposures of employees under radiation.
BASIS FOR RATING: The initiating event is a loss of feedwater flow in PWR seconday system or a reactor automatic trip, each of which is an "expected" event. All safety systems are available ("full"). A level 0 or a level 1 can be selected due to Table 1. A-1R. Operational limits and conditions were not exceeded, and reactor trip sequence proceeded normally, which selects a level 0 due to "5" below scale.
There was a single human error which caused the trip of a motor driven main feedwater pump, but it did not reveal deficiency in safety culture. This also supports to select a level 0 instead of a level 1 due to "5 (below scale). Thus, level 0 is selected.
DIFFICULTIES IN RATING:

Location: 
TSURUGA-2
Event date: 
Mon, 1989-06-26
Javascript is required to view this map.
»