Published online 17 July 2007 | Nature | doi:10.1038/news070716-3


Japanese nuclear reactor under-designed for earthquake?

Rapid acceleration shakes up more than the ground in Japan.

The quake sparked a fire, which was soon extinguished.The quake sparked a fire, which was soon extinguished.AP/PA Photos

An earthquake off the western coast of Japan yesterday hit a nuclear plant with more than twice the jolt that the plant was expected to have to handle. The shock seems to have done little immediate damage, but has raised concerns about whether Japan's nuclear plants are designed to withstand the kind of shaking they are likely to experience.

The magnitude 6.6 earthquake killed at least 9, injured 1,000, and caused the evacuation of a further 10,000 people. At the Kashiwazaki-Kariwa reactor, 10 kilometres from the epicentre, the earthquake sparked a fire, which was soon extinguished, and caused a little more than a litre of radiation-contaminated water from an open pool to spill into the sea. The Tokyo Electric Power Company (TEPCO), which owns the facility, says that the concentration of radiation in the water was lower than national regulations and so poses no threat to people or the environment.

But the episode has sparked concern as to whether Kashiwazaki-Kariwa's seven reactors, as well as the rest of Japan's 55 operating nuclear facilities, are safe. All the Kashiwazaki-Kariwa reactors have been shut down until a safety evaluation has been done.

All shook up

“It could get much worse,”

Hideyuki Ban, of the Citizens' Nuclear Information Network.

An earthquake's effect on the surface depends on its magnitude, the depth of the epicentre, and the geological nature (hardness/density, formation) of the surrounding area - all of which affect the peak rate at which the ground accelerates in any given direction. This ground acceleration, which is particularly interesting to engineers calculating an earthquake's effect on a building, is measured in 'gal's (centimetres per second per second).

Based mainly on historical precedent, TEPCO designed the reactors at Kashiwazaki-Kariwa assuming that the area would have a maximum ground acceleration of 274 gal. Yesterday, the number 1 reactor at Kashiwazaki-Kariwa experienced an acceleration of 680 gal as the ground slid from east to west; the number 5 reactor accelerated at 442 gal east/west, and the number 6 reactor was hit with 488 gals up/down, as measured on site.

TEPCO is in the middle of re-evaluating the safety of its reactors in line with new guidelines implemented in Japan in September 2006, which call for more stringent safety measures. Surveys of active faults now have to take into account earthquakes going back 130,000 years, for example, compared with the previous standard of 50,000 years. TEPCO plans to finish this re-evaluation by December 2008. Whether this re-evaluation would have affected the maximum 'gal' limit for Kashiwazaki-Kariwa remains unclear.

Safety check

TEPCO now is patrolling the grounds and inspecting the damage. It has not yet given a date on when it will file a safety report. A preliminary check by members of the industry's agency for nuclear and industrial safety found some 50 problems, including radiation leaking from an exhaust filter.

With aftershocks expected for another two weeks, there is much concern. "It could get much worse," says Hideyuki Ban, of the Citizens' Nuclear Information Network in Tokyo.


In 2003, TEPCO was forced to shut down all 17 of its reactors over concerns about the accuracy of their safety data. This time, TEPCO has come under fire for being slow in reporting safety information to the government — it took them six hours to get word about the spilled water to the ministry of industry.

Depending on the outcome of the safety report, which will need to be evaluated by the ministry, it is possible that the seven reactors will be closed down. "We just want to carry out a thorough inspection and take any necessary measures needed to operate the facilities safely," says Yoshinobu Kamijima of TEPCO's press office.

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