Kashiwazaki-Kariwa Nuclear Plant (Japan)–Damage due to earthquake on 16th July, 2007

Earthquake damage to the Kashiwazaki-Kariwa nuclear power station (largest in the world, run by the Tokyo Electric Power Company , Japan) on 16th July 2007 appears to be limited and less than expected, according to an IAEA expert report on the incident. The design of the plant was not qualified to withstand earthquake of 6.8 magnitude that occurred.

In particular, the reactors, Units 3, 4 and 7, which were at full power, were shut down automatically probably due to the conservatisms introduced at different stages of the design process (design safety margins). Safety related structures, systems and components of the plant seem to be in a general apparently good condition, much better than might be expected for such a strong earthquake, and there is no visible significant damage, the report stated.

While the plant's nuclear components appear to be in a satisfactory state, non-safety related structures, systems and components were affected by significant damage such as soil and anchorage failures and oil leakages. There were two minor releases of radioactive material occurred after the quake. The first one was a release of radioactive material from the main turbine condenser through the main stack and the other was some leakage of contaminated water from the spent fuel pool to a non-controlled area, which was later pumped to sea. It was noted that there was some delay in reporting the releases to the authorities.

Some of the suggestions made in the report are: i) a re-evaluation of the seismic safety the Kashiwazaki-Kariwa NPP using updated criteria and methods and addressing the issue of the potential existence of active faults underneath the site, ii) the potential interaction between large seismic events and accelerated ageing to be considered in future inspection programmes and iii) communication of the investigation findings and lessons learned to other nuclear operators, regulators and technical support organizations.