INES-event
INES 0

REACTOR AUTOMATIC SHUTDOWN DUE TO INCORRECT FUNCTIONING OF A MAIN STEAM ISOLATION VALVE

During rated power operation, an automatic reactor shutdown occurred due to signal of "Neutron Flux High". As a result of investigation, it was estimated that since fastening of the rotation preventing pin had been insufficient, which had been attached at the connecting part of between a valve poppet and a valve stem in one of the 8 main steam isolation valves (MSIVs). This pin was damaged by vibration caused from steam flow and detached pilot- and main poppets plugged the main steam line causing increased reactor pressure which resulted in "Neutron Flux High". There was no release of radioactive materials outside the reactor facility. Also there was no unplanned occupational exposure of employees working under radiation.
BASIS FOR RATING: The occurrence probability of detachment of poppet from a main steam isolation valve is considered being the same as inadvertent closure of an MSIV due to malfunction. Then a real initiator of expected frequency can be assumed for this event. Thus rating of this event by INES resulted in a level-0 using the defence-in-depth criterion.
DIFFICULTIES IN RATING: Definition of occurrence frequency of an initiator like such a new event was rather difficult.

Location: FUKUSHIMA-DAIICHI-3
Event date: Sun, 09-09-1990
Nuclear event report
Legenda & explanation