 INES 2 |
STEAM GENERATOR TUBE BREAK |
During rated power operation, at Unit no.2 of Mihama Power Station, Kansai Electric Power Company (PWR, 500 MWe), main condensor air ejector gas monitor alarm was sounded at 13:40, February 9, 1991. When decreasing of the reactor power was started, the reactor was automatically scrammed by "pressurizer pressure low" signal and the emergency core cooling system was activated by "coincidence of pressurizer pressure low and pressurizer water level low" signal.
As it was speculated that a tube of one of two steam generators was damaged, the operation procedures such as isolation of the steam generator were intiated and the reactor was safely shut down.
In the course of depressurization of the primary cooling system as mitigation of the event, as the pressurizer relief valves were not able to be opened, depressurization was done by using the pressurizer auxiliary spray. A main steam isolation valve of the steam generator was not completely closed and the valve was manually tightened.
The investigation of the tube break revealed that as the anti-vibration bars of the steam generator were not completely inserted, the fluid elastic vibration occurred in the U-bend portion of the unsupported tubes, and that one of the tubes was broken due to the fretting fatigue as the vibration amplitude was not limited. The cause of failure to open the pressurizer relief valves and incomplete closure of the main steam isolation valve were due to inappropriateness of the work management and the maintenance mangement.
There was no significant unplanned exposure of the personnel due to this event. The amounts of the radioactive release to the environment were evaluated as about 2.3 x 1E10 Bq of radioactive noble gases, about 3.4 x 1E8 Bq of radioactive iodines and about 7.0 x 1E6 of radioactive liquid. Those releases were well below the management target value of yearly release rates and the effective dose equivalent of the public at the surrounding of the site was evaluated as 0.01 microSv.
Basis for Rating of the Event:
Table A2, 2-C
Due to a circumfenrentially broken off tube of steam generator (possible), the reactor protection system and the emergency core cooling system were actuated as expected.
However, as the pressurizer relief valves failed to open, the pressurizer auxiliary spray had to be used for depressurization and cooling of the primary system. From this fact, the availability of the safety system is evaluated as "adequate".
As a result, the event rating of this event becomes level 2 or 3 according to the Table A2. As the safety margin in respect of redundancy was kept even if inoperability of the pressurizer auxiliary spray was assumed, level 2 is adopted to this event.
Event date: Sat, 09-02-1991
