Earthquake damage to the Kashiwazaki-Kariwa nuclear power station on 16 July appears to be limited and less than expected, according to an IAEA expert report released today and submitted to the Japanese authorities.
Although it appears that the earthquake of 16 July 2007 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.
In particular, the automatic shutdown of the reactors of Units 3, 4 and 7, which were at full power, and of the reactor of Unit 2, which was in the start up state, were performed successfully. According to the report's findings, this is probably due to the conservatisms introduced at different stages of the design process, the so-called 'design safety margins'. "The combined effects of these conservatisms were apparently sufficient to compensate for uncertainties in the data and methods available at the time of the design of the plant, which led to the underestimation of the original seismic input", it is said.