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News reports continue to shed light on the damage inflicted on Tokyo Electric Power Co.’s Kashiwazaki-Kariwa nuclear power plant by the magnitude-6.8 earthquake that struck Niigata and Nagano prefectures July 16. Most worrying is a report that the tremors were more than double the quake-design benchmark of the world’s largest nuclear-power-generation complex.

What happened at the Kashiwazaki-Kariwa plant has raised doubts about the safety of the nation’s 55 power-generation reactors. Power companies must review the capability of their nuclear power plants to withstand seismic tremors under the tougher design guidelines revised by the Nuclear Safety Commission in September. In addition, the government and the commission should not hesitate to further revise plant-design guidelines after taking into consideration the damage wrought by the July 16 earthquake.

The focus of the earthquake was about 9 km northeast of the nuclear power plant and about 17 km below the Sea of Japan. Worryingly, the Meteorological Agency analyzed the distribution of aftershocks and ascertained the possibility that the fault that caused the main quake runs directly beneath the nuclear plant at a depth of 20 km. The approximately 15-km-wide aftershock zone extends some 30 km southeast of the quake’s focus. The nuclear power plant lies within this zone.

The government’s earthquake study group is scheduled to carry out a large-scale study to pinpoint the fault that caused the earthquake. Hopefully the study will confirm whether the fault runs beneath the nuclear power plant.

A study by a group of experts also indicates that TEPCO may have underestimated the size of a fault whose existence was confirmed by the power company around 1980 — before it expanded the plant. Documents filed by TEPCO for plant expansion identified the fault as located beneath the Sea of Japan, about 20 km west of the plant. TEPCO estimated its length at 7 to 8 km and concluded that there was no need to consider the fault in plant design since evidence suggested that the fault had long been inactive. The group of experts, however, says the fault extends about 30 km in a north-south direction and that it may have caused the July 16 quake.

Construction of nuclear power plants is approved on the condition that active faults do not lie beneath the proposed sites. If an unknown fault does exist beneath a planned nuclear power plant, it is assumed that such a fault would cause a maximum magnitude-6.5 quake.

Confirmation that the fault responsible for the July 16 quake runs below the Kashiwazaki-Kariwa plant would undermine the basic assumptions on which the plant was designed.

Unlike older design guidelines that applied uniform assumptions about the effects of possible earthquakes to all nuclear power plants, the September 2006 guidelines call for examining geological features and the history of past quakes, and determining whether faults exist or not near planned nuclear power plants. They do not specify the maximum magnitude of an anticipated earthquake, but magnitude 6.8 is accepted as a rough yardstick. However, even a magnitude increase of just 0.2 means a roughly twofold increase in the energy released by a quake. In fact, the July 16 earthquake registered magnitude 6.8, and it was strong enough to cause the ground around a diesel-oil storage tank at the Kashiwazaki-Kariwa plant to fracture and sink 1.6 meters.

It is said that under the September 2006 design guidelines, the earthquake resistance of nuclear power plants will increase by 20 to 30 percent. Given the damage suffered by the Kashiwazaki-Kariwa plant, however, the government and the Nuclear Safety Commission should review the guidelines.

The quake highlighted the extent to which nuclear power plants are vulnerable to major earthquakes. A 310-ton ceiling crane above the pressurized chamber of the No. 6 reactor at the plant was damaged although the building housing the reactor and crane was supposed to be quake-resistant. Water overflowed from a pool storing spent nuclear fuel, some of which leaked into the sea.

A ventilator at the No. 7 reactor continued to operate for two days after the quake, spewing radioactive materials into the atmosphere. Oil leaked from the transformers for four reactors, one of which caught fire. And it was found that no power companies have in-house firefighters on duty around the clock.

Both the power industry and the government must make enormous efforts if they are to restore people’s trust in the safety of nuclear power plants.

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