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2011年5月12日 (木)


--The Asahi Shimbun, May 10
EDITORIAL: Tsunami not the only safety concern for Hamaoka plant

Chubu Electric Power Co. decided on May 9 to halt, for the time being, operations of all reactors at its Hamaoka nuclear power plant, which is located right above what seismologists say will be the focus of an expected Tokai earthquake.

The company made the decision in an emergency board meeting in response to Prime Minister Naoto Kan's unusual request for the action.

The March 11 Great East Japan Earthquake has triggered a disastrous accident at the Fukushima No. 1 nuclear power plant, operated by Tokyo Electric Power Co., shattering the myth of the safety of nuclear power generation.

There is clearly a compelling case for shutting down the nuclear power plant in Shizuoka Prefecture, which is considered the most dangerous in Japan, and taking fresh measures to bolster the safety of the plant.

One troubling element in the move is that the suspension of reactor operations requested by Kan is limited to the period until medium- to long-term safety measures, like the construction of levees, have been implemented at the plant.

This raises the question of whether the plant is strong enough to withstand the violent shaking of the ground caused by powerful seismic waves in the first place.

The medium- to long-term safety measures have been called for by the Nuclear and Industrial Safety Agency in light of the crisis at the Fukushima No. 1 plant. They are aimed at enhancing defenses against massive tsunami and potential losses of power sources.

Besides building levees, Chubu Electric says that it will also install watertight doors, a backup reactor cooling system and additional power sources for emergencies.

But the blueprint doesn't contain measures for checking and enhancing the safety of the plant against quakes themselves.

The principal concern about the Hamaoka plant has been whether it can withstand the force of the expected Tokai earthquake, a mega-quake that occurs at the boundaries of tectonic plates.

In response to the concern, Chubu Electric announced in 2005 plans to strengthen the quake resistance of the plant. Other electric power companies have followed suit.

But NISA's assessment of the quake resistance of the Hamaoka plant based on the new quake safety standards adopted in 2006 is not yet over.

The assessment has been delayed by the safety implications of some recent seismic events, including the 2007 Niigata Chuetsu-oki Earthquake, which shook the Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture with a far stronger intensity than TEPCO had expected.

It is also necessary to glean lessons from the Great East Japan Earthquake.

Regarding the accident at the Fukushima No. 1 plant, attention has been focused on the height of the tsunami that struck the facilities.

But it is possible that the piping of the reactors might have been seriously damaged by the intense shaking before the tsunami hit.

The March 11 earthquake was marked not just by its strong peak ground acceleration but also by the unusual length of the shaking.

What is crucial in taking safety precautions is not to focus too much on a single aspect.

The Fukushima No. 1 plant suffered unexpectedly serious damage from a tsunami, a factor that had not been given much attention in preparations for a natural disaster.

But it would be foolish to go to the opposite extreme and concentrate only on the danger of tsunami in taking precautionary measures to make the Hamaoka plant safer.

Let us not forget the destructive force of the shaking of a mammoth quake.

New important discoveries have been made almost annually about the mechanisms and risks of earthquakes that occur in areas surrounding the Japanese archipelago.

Such new knowledge should be used for the sake of making more informed decisions on whether to keep or stop operating specific reactors.

A greater dose of flexibility is needed for the government's nuclear power policy.


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