Kashiwazaki-Kariwa, Japan

Japan's Kashiwazaki-Kariwa nuclear station has escaped major damage from the earthquake that hit it on 16 July 2007. A follow-up fact-finding mission by IAEA in February 2008 reported "no significant damage" to safety equipment.

Kashiwazaki-Kariwa is the world's largest rated nuclear power station. With seven reactors generating 8,212MW, the station, owned and operated by the Tokyo Electric Power Company (TEPCO), can provide electricity to 16 million households.

The 4.2km² site is located in the Niigata Prefecture city of Kashiwazaki and the town of Kariwa, approximately 135 miles north-west of Tokyo, on the coast of the Sea of Japan. Kashiwazaki-Kariwa is also the world's fourth largest electric-generating station behind three hydroelectric plants: Itaipu on the Brazil-Paraguay border, Three Gorges Dam in China and Guri Dam in Venezuela.

Like all power plants in Japan, Kashiwazaki-Kariwa is built to strict earthquake-resistance standards. However, the 2007 earthquake caused the plant to leak radioactive substances into the air and water. The plant, which has been operating since 1985, has been closed until safety checks following the earthquake are complete.

KASHIWAZAKI-KARIWA REACTORS

Kashiwazaki-Kariwa has seven conventional nuclear reactors, each with an average output of 1,067MW and a power rating of 1,100MW. Work on the first reactor began in 1980, which came on line in 1985. The last came into operation in 1994.

In 1996 Kashiwazaki-Kariwa became the first plant in the world to use an advanced boiling water reactor (ABWR) for commercial use. The ABWR, designed by General Electric, is a Generation III reactor and has an average output of 1,315MW and a power rating of 1,356MW.

Another ABWR was opened in 1997. All reactors use low-enriched uranium as nuclear fuel.

In 2002, the reactors were shut down after data from the plant was found to have been deliberately falsified. Units 1 to 3 were taken offline for the whole of the 2003 fiscal year.

THE EARTHQUAKES

Japan sits on top of four tectonic plates and is one of the world's most earthquake-prone countries. As a safety measure, four storeys of the plant's foundations are fixed underwater into a layer of sturdy bedrock and sand.

In October 2004 an earthquake measuring 6.9 on the Richter scale struck the Niigata Prefecture, killing 40 people and damaging 6,000 homes.

Kashiwazaki-Kariwa withstood the tremor well – all the reactors continued to work as normal during the quake, although one was forced to close during an aftershock when a trip signal was activated.

In July 2007 another earthquake struck Niigata. Measuring 6.8 on the Richter scale, the quake killed 11 people, left 1,000 injured and destroyed 300 buildings. It significantly exceeded the level of seismic activity for which the plant was designed and, this time, Kashiwazaki-Kariwa didn't hold up so well.

Although all the reactors that were running were shut down, a fire burned in an electrical transformer for two hours, pipes burst, drums of nuclear waste toppled over and 1,200l of contaminated water escaped into the sea.

EARTHQUAKE AFTERMATH

Despite TEPCO reportedly wanting to re-open Kashiwazaki-Kariwa the day after the earthquake, the Trade Ministry and the Mayor of Kashiwazaki ordered the plant to remain closed until the relevant safety checks could be completed.

In August 2007 investigators from the International Atomic Energy Agency (IAEA) carried out a four-day inspection of the site. They concluded that the plant's safety measures performed well during the quake and damage to the environment was limited.

Kashiwazaki-Kariwa remains closed until further safety checks have been completed and the plant's safety can be guaranteed.

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