Damage to the Kashiwazaki Kariwa nuclear power plant in Japan caused by an earthquake on 16 July 2007 appears to be “limited and less than expected”, according to an International Atomic Energy Agency (IAEA) report released yesterday and submitted to the Japanese  authorities.

Although it appears the earthquake “significantly exceeded” the level of the seismic input taken into account in the design of the plant, the installation behaved in a safe manner, during and after the earthquake, the report says.

In particular, the automatic shutdown of reactor units 3, 4 and 7, which were at full power, and of unit 2, which was in the startup state, were performed successfully.

The report suggests that a re-evaluation of the plant’s seismic safety needs to be carried out taking into account lessons learned from the earthquake, using updated criteria and methods. The report says: “In particular, detailed geophysical investigations are foreseen both on land and offshore in order to define the new seismic input to the plants.”

These investigations should address the possibility that there could be active fault lines underneath the site. Another consideration is the possibility that the long-term operation of components could be affected by hidden damage from the earthquake. The potential interaction between large seismic events and accelerated ageing could therefore be an important topic to consider in future inspection programmes.

Radioactive releases to the environment as a result of the earthquake were estimated to result in an individual dose well below the authorised limits for exposure of the public for normal operating conditions, the report says.

The report adds: “Further and thorough inspections and evaluations of all critical structures, systems and components of the seven units have not been completed and important components such as reactor vessels, the core internals and the fuel elements have not yet been examined.”

The earthquake led to the automatic shut-down of three units at the seven-unit plant, on Japan’s west coast. Units 3, 4 and 7 shut down safely. Units 1, 5 and 6 were already shut down at the time of the quake for periodic inspections. Unit 2 was technically undergoing a periodic inspection and start-up operations had just begun, but the unit was also shut down safely.