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.
"Safety related structures, systems and components of the plant seem to be in a general condition, much better than might be expected for such a strong earthquake, and there is no visible significant damage," the report states.
The report summarizes findings of an IAEA expert team during a four-day physical examination of the plant´s seven units and other facilities, as well as analysis of instrument logs and other records from the time of the event. The expert mission took place from 6-10 August at the invitation of the Japanese government.
In the report, the IAEA experts noted the Japanese authorities´ open cooperation. "This search for openness was shared by all the individuals with whom the team interacted", it is stated in the report. The Director General of the IAEA, Dr. Mohamed ElBaradei, had previously welcomed the cooperation and transparency the team had received from the Japanese authorities.
While the plant´s nuclear components appear to be in a satisfactory state, the IAEA team has also reported that "non-safety related structures, systems and components were affected by significant damage such as soil and anchorage failures and oil leakages."
In the IAEA report it is suggested that a re-evaluation of the seismic safety the Kashiwazaki-Kariwa NPP needs to be done taking into account the lessons learned from the Niigataken Chuetsu-Oki earthquake, using updated criteria and methods. 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, it is stated in the report, should address the issue of the potential existence of active faults underneath the site.
Another consideration is the possibility that the long-term operation of components could be affected by hidden damage from the earthquake. Thus, the potential interaction between large seismic events and accelerated ageing may be an important topic to consider in future inspection programmes.
In addition to the more detailed and comprehensive inspections and analyses to be performed by the Japanese authorities and their commitment to present a report at the Regulator´s Meeting during the 51st IAEA General Conference in September 2007, the IAEA will follow this mission with extensive international cooperation, including technical assessments and studies, as well as communicating the findings and lessons learned to nuclear operators, regulators and technical support organisations.
The IAEA Expert Mission was conducted at the Kashiwazaki-Kariwa nuclear plant following the 16 July earthquake upon request from the Government of Japan. The objective was to conduct a fact-finding mission and to identify the preliminary lessons learned that might have implications for the international nuclear safety regime.
Analyses of safety events at nuclear facilities are routinely communicated to other nuclear operators and nuclear regulators, so that lessons learned can be incorporated where relevant at other plants. An opportunity for such feedback on the earthquake that affected the Kashiwazaki-Kariwa plant will occur in September, when Japan will present a report on the event to a Senior Regulators Meeting at the IAEA General Conference.
With a total of 7,965MW net installed capacity and occupying a surface of 4.2 square kilometres, the Kashiwazaki-Kariwa facility is the largest nuclear power plant in the world. The plant is run by the Tokyo Electric Power Company (TEPCO), one of the world´s largest utilities.