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November 22, 2011

Fukushima reactors are nearing cold shutdown

Japan Times - Fukushima is nearing cold shutdown

The government will declare that the facility has achieved cold shutdown once it confirms the complex can maintain stability over the next several years even if it is hit by an earthquake or suffers malfunctions

The amount of radioactive substances currently leaking from the crippled reactors has further declined to a maximum of 60 million becquerels per hour, or around a one 13-millionth of the level seen in the early days of the crisis, which was triggered by the devastating March 11 earthquake and tsunami.

The estimate means someone standing in the crippled plant for one year, would be exposed to up to 0.1 millisieverts of radiation, far below the government-set limit of 1 millisievert per year.

The cold shutdown is the key goal of "Step 2" of the road map, with the government defining it as a situation in which the bottom part of a reactor pressure vessel at the plant is kept below around 100 degrees and exposure from the release of radioactive substances is significantly held down.
Most of the surrounding area was flattened by the tsunami

Temperatures recorded at the bottoms of the reactor vessels for units 2 and 3 are below 70ºC, while unit 1 is cooler still at 37ºC. Being below the landmark 100ºC, these basically fulfil the conditions for the declaration of cold shutdown although this has not been officially recognised by the government.

A complicating factor is the uncertainty over the melted core. Water leaking from holes in the bottoms of the reactor vessels, has lead to concerns that corium may have followed. But this theory is not supported by radiation readings from the drywell below, with the exception of unit 1 where a damaged sensor is fluctuating wildly. The drywells contain large pools of water at below 50ºC.




Other tasks included in the phase have almost been completed, with workers starting to build a sunken wall between the reactors and the shore to prevent water containing radioactive substances from leaking into the Pacific Ocean.

They also finished installing a cover over reactor 1 on Oct. 28 as the building housing the unit was badly damaged by a hydrogen explosion.

After Step 2, the government and Tepco aim to start removing nuclear fuel in the spent-fuel pools of reactors 1 to 4 within two years and the melted fuel from inside reactors 1 to 3 within 10 years.


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