At the request from the Government of Japan through the Nuclear and Industrial Safety Agency (NISA), an IAEA expert mission was conducted at the Kashiwazaki-Kariwa nuclear power plant (NPP) following the strong Niigataken Chuetsu-Oki earthquake that affected the plant on 16th July 2007.

This article is based on the preliminary findings of that mission, which was lead by IAEA director Philippe Jamet and included experts from Bulgaria, Sweden, Japan and Turkey.

Kashiwazaki-Kariwa is the biggest nuclear power plant site in the world (Fig.1) and is operated by Tokyo Electric Power Company (TEPCO). The site has seven units with a total of 7965MW net installed capacity. Five reactors are of the boiling water reactor (BWR) type with a net installed capacity of 1067MW each. Two reactors are of the advanced boiling water reactor (ABWR) type with 1315MW net installed capacity each. The five BWR units entered commercial operation between 1985 and 1994 and the two ABWRs in 1996 and 1997 respectively.

At the time of the earthquake, four reactors were in operation: units 2, 3 and 4 (BWRs) and unit 7 (ABWR). Unit 2 was in start-up condition but was not connected to the grid. The other three reactors were in shutdown conditions for planned outages: units 1 and 5 (BWRs) and unit 6 (ABWR).

A strong earthquake with a magnitude of 6.6 occurred at 10.13am on 16 July with its epicentre about 16km north of the Kashiwazaki-Kariwa NPP site.

The earthquake caused automatic shutdown of the operating reactors, a fire in the in-house electrical transformer of unit 3, release of a very limited amount of radioactive material to the sea and the air and damage to non-nuclear structures, systems and components of the plant as well as to outdoor facilities, as reported by TEPCO on its web page.

Preliminary data indicated that the design basis ground motion for the plant may have been exceeded, with possible significant effects on the behaviour of the plant systems, structures and components.

The IAEA was informed of this event on the same day and in view of subsequent information on its severity, the IAEA expressed on 18th July its readiness to send to Japan an international team of experts to join the Japanese government in assessing the event and its consequences and to share the findings and lessons learned with the international nuclear community.

On 23rd July NISA requested the IAEA that a team of international experts be sent to Japan observe the current conditions at the Kashiwazaki-Kariwa NPP.

Scope of the mission

The scope of the mission was limited to three subject areas:

• Area 1. Seismic design basis ­– design basis ground motions. Preliminary investigations of the actual earthquake and its ground motions and comparison with the design basis ground motions for the plant seismic design.
• Area 2. Plant behaviour – structures, systems and components. Observation of the damage that occurred as a consequence of the earthquake on the basis of the information gathered and made available by TEPCO and by performing limited but representative plant walkdowns.
• Area 3. Operational safety management. Preliminary investigations of the operational safety management response and releases of radioactive material during and after the earthquake, on the basis of the examination of documents and of discussions with TEPCO.

Main findings and lessons learned

Although it appears that the Niigataken Chuetsu-Oki earthquake significantly exceeded the design basis ground motion as indicated by the response spectra comparison at the level of the foundation base mat in all units, the operating plants were automatically shutdown and all plants behaved in a safe manner, during and after the earthquake.

The three fundamental safety functions of reactivity control, removal of heat from the core and confinement of radioactive materials were ensured with the exception of very minor radioactive releases that occurred shortly after the earthquake. The radioactive releases to the environment were estimated to result in an individual dose well below the authorised limits established by the regulatory authority for exposure of the public for normal operating conditions.

Based on the reports from TEPCO experts and the limited in-plant walkdowns and visual observations performed by IAEA experts, safety related structures, systems and components of the plant seem to be in a general condition much better than expected for such a strong earthquake, with no visible damage. This is probably due to the conservatisms introduced at different stages of the design process. The combined effects of these conservatisms were apparently sufficient to compensate for uncertainties in the data available and the methods applied at the time of the design of the plant, which led to the underestimation of the original design basis ground motions.

However, observations and conclusions relating to the behaviour of structures, systems and components require validation from the results of the ongoing investigations. Further and thorough inspections and evaluations of all critical structures, systems and components of the seven units have not been completed and important components like the reactor vessels, the core internals and the fuel elements have not yet been examined. TEPCO is accomplishing what is to be the first stage of a more comprehensive inspection plan, namely visual observations. Presently, detailed checks of the integrity and operability of all safety systems and components of the frontline and supporting safety related systems are ongoing even though no apparent damage has been sustained. All these activities should be thoroughly and fully documented.

Due to proper functioning of the automatic seismic scram system, the Kashiwazaki-Kariwa units that were in operation, as well as unit 2 which was in startup state, shut down safely when the earthquake occurred. In accordance with Japanese safety regulations, in order to restart plant operation TEPCO needs the permission of the regulatory authority. Furthermore, in accordance with the new seismic guidelines of the Nuclear Safety Commission (NSC) (issued in September 2006) a re-evaluation of the seismic safety needs to be done taking into account the effects of the Niigataken Chuetsu-Oki earthquake.

Any re-evaluation of the seismic safety of the plant, including possible upgrades, requires the input from a new seismic hazard evaluation, seismic analysis and comparison of results with the original seismic design, including also the need to address the issue of the potential existence of active faults underneath the site.

Another consideration is the possibility that a component remains functionally available under normal operating conditions but sustains hidden damage. This could affect the capability of the component to function as required during potential accidents and its safe long-term operation.

“Therefore, the potential interaction between large seismic events and full functionality also under accident conditions should be analysed and inspected prior to restart of the plant, and accelerated ageing may be an important topic to consider in future inspection programmes,” concludes the IEAE report.