Failure of the reactor component cooling water system and others by tsunami

Print View Posted on: 07 March 2012

Event Date: 11 March 2011 Event Type: Power Reactor
Event Location: Japan, ONAGAWA-2 INES Rating: 2 (Provisional)

Unit 2 of the Onagawa Nuclear Power Station started operation since 14:00 on March 11, and the Tohoku District – Off the Pacific Ocean Earthquake occurred at 14:46 on the same day. The reactor was automatically stopped and emergency diesel generator A (EDG (A)), emergency diesel generator B (EDG (B)), and high-pressure core-spray diesel generator (EDG (H)) automatically started. The off-site power was available and 3 EDGs were set up the standby position.
Cold shutdown of the reactor was completed at 14:49 on the same day because the reactor was in the subcritical state and the reactor coolant temperature was below 100 degrees Celsius immediately before the occurrence of the earthquake.
Following the earthquake, the tsunami hit Pumps of the power station. The reactor component cooling water (RCW) B system, and high-pressure core-spray component cooling water (HPCW) system stopped, which caused EDG (B) and EDG (H) to stop.
As a result of subsequent inspections at the site, it was estimated that tsunami sea water flooded from the water intake channel into the sea water pump room through the local box of the water level gauge which was installed for automatically stop the circulation water pump. Pump area of the reactor sea water (RSW) B system thus became submerged, and the water flowed further into heat exchanger rooms of the RCW B system and HPCW through the underground trench. The RSW B system, RCW B system, and HPCW system lost their functions as a result. The residual heat removal system B (RHR(B)) could not be used because pumps of the RCW B system and RSW B system were inoperable. And the high-pressure core-spray (HPCS) system could not be used because HPCW pump was inoperable. However, HPCS was not required, because reactor was cold shut down condition.
The functions of RSW A system and RCW A system were maintained, thus there was no impact on the cooling function of the reactor by RHR(A).

INES Rating: 2 - Incident (Provisional) as per 05 March 2012
Release beyond authorized limits? No
Overexposure of a member of the public? No
Overexposure of a worker? No

Contamination spread within the facility? No
Damage to radiological barriers (incl. fuel damage) within the facility? No

Degradation of Defence In-Depth Yes

Person injured physically or casualty? No
Is there a continuing problem? No

Toshiyuki Koganeya
Nuclear and Industrial Safety Agency (NISA/METI)

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