JAPAN | Japan mulls checking twin disaster risks in nuclear safety tests

Posted on July 14, 2011

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JAPAN | MAINICHI | 14 July 2011

In this March 11, 2011 file photo released by Tokyo Electric Power Co., waves of tsunami come toward tanks of heavy oil for the Unit 5 of the Fukushima Dai-ichi nuclear complex in Okuma, Fukushima Prefecture, northeastern Japan. (AP Photo/Tokyo Electric Power Co.)

TOKYO (Kyodo) — The government’s nuclear regulatory agency is considering focusing on five scenarios, including the possibility of both an earthquake and tsunami hitting a reactor, when assessing nuclear safety under the so-called “stress tests” to be introduced in the wake of the Fukushima Daiichi crisis, government sources said Wednesday.

The scenarios, worked out after referring to the European Union’s stress tests, include the possibility of earthquakes or tsunami hitting a reactor separately as well as in combination, as seen in March at the Fukushima Daiichi nuclear power plant, the sources said.

The Nuclear and Industrial Safety Agency is aiming to compile a draft plan, possibly later this week, on how it will conduct the safety assessments, which will be carried out in two stages. Another oversight body, the Nuclear Safety Commission, will check whether the draft plan is appropriate.

Nuclear reactors idled for regular checks would undergo the first stage in order to resume operation, with utilities studying the extent to which key installations would be able to withstand damage in the event of extreme natural disasters on a scale greater than expected.

In this March 11, 2011 file photo released by Tokyo Electric Power Co., waves of tsunami gush into a complex near the Unit 4 reactor at the Fukushima Dai-ichi nuclear complex in Okuma, Fukushima Prefecture, northeastern Japan. (AP Photo/Tokyo Electric Power Co.)

In this March 11, 2011 file photo released by Tokyo Electric Power Co., waves of tsunami gush into a complex near the Unit 4 reactor at the Fukushima Dai-ichi nuclear complex in Okuma, Fukushima Prefecture, northeastern Japan. (AP Photo/Tokyo Electric Power Co.)

The second stage of the tests will be more comprehensive and carried out at all nuclear plants. Depending on the results, Japanese nuclear reactors that are currently in operation could be suspended.

The sources said that the first stage of analysis may adopt a simpler method than the second stage, in a way that does not go so far as checking the actual limit but studying whether it can withstand a certain level of impact.

A method called “probabilistic safety assessment,” used in such countries as the United States, is expected to be introduced for the second stage with the aim of analyzing the risk of accidents, the sources said.

The so-called PSA method uses numeric data representing the frequency of all possible accidents and earthquakes that may occur at nuclear power plants and their impact, the sources said. But not all of the five situations may be assessed by the method.

Of the five scenarios, the remaining two are when a reactor loses power and when the function to take away heat from a reactor by using seawater fails.

In this March 11, 2011 photo released Monday, April 11, 2011 by Tokyo Electric Power Co.,(TEPCO), the access road at the compound of the Fukushima Dai-ichi nuclear power plant is flooded as tsunami hit the facility following a massive earthquake in Okuma town, Fukushima Prefecture, northeastern Japan. (AP Photo/Tokyo Electric Power Co.,)

In this March 11, 2011 photo released Monday, April 11, 2011 by Tokyo Electric Power Co.,(TEPCO), the access road at the compound of the Fukushima Dai-ichi nuclear power plant is flooded as tsunami hit the facility following a massive earthquake in Okuma town, Fukushima Prefecture, northeastern Japan. (AP Photo/Tokyo Electric Power Co.,)

The first and second stages of analysis conducted by the utilities operating nuclear power plants would be submitted to the nuclear safety agency, and the agency’s decision would be examined by the Nuclear Safety Commission.

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