TEPCO has taken the first step towards installing new heat exchangers in wrecked reactors at its Fukushima 1 nuclear power plant that have suffered core damage after a cooling system failure. The objective is to achieve “cold shutdown”, which means that cooling water in the reactor remains below 100 degrees C and the pressure is at atmospheric level. Currently the water inside units 1, 2 and 3 is still boiling.

During normal operation, sea water cools steam coming out of the power turbine that generates electricity. It condenses it back to water, which goes back into the reactor core to be turned into setam again. After a shutdown, electric pumps and a separate heat exchanger, known as the residual heat removal system (RHR), take over the core cooling function. TEPCO had been aiming to restore the RHR since about March 20, when it reestablished a grid connection into the turbine hall, but found that the pumps are no longer usable.

The new system will not cool the core directly, which after the fuel rods partially melted is highly contaminated, but instead will circulate water to the containment vessel that surrounds the core, cooling the central part of the reactor from the outside rather than from inside. To that purpose, water will be pumped from a pipe connected to the containment to a heat exchanger, which passes the heat to an external water cycle that will be cooled using air. The original cooling system depended on sea water for condensing steam in the inner cycle, but the sea water intake system was flooded and destroyed by the tsunami, along with most of the diesel generators that provided power to all emergency pumps.

Before any work can be performed inside the reactor building, which no human had entered since the hydrogen explosions around March 12-15, the level of radiation there has to be brought down considerably. To that purpose a pair of hose pipes has been laid through the airlock and hooked up to a filter. Until at least May 16 air will be pumped through a filter set up outside the building. US-made robots sent into units 1 and 3 measured radiation levels of up to 49 mSv/h in unit 1 and up to 57 mSv/h in unit 3 on April 18. The dose inside the airlock of unit 1 was 270 mSv/h, in unit 3 it was 170 mSv/h. The typical background radiation dose for a civilian is 2-3 mSv per year. TEPCO hopes to be able to reduce radiation inside the building by a factor of 20.

Work will start with unit 1, because unlike unit 2 and 3 its containment vessel is assumed to be intact. Unit 2 and 3 are at atmospheric pressure. If they can’t maintain steam pressure they may also not hold water once they’re flooded. Also, because of a damaged torus (pressure suppression chamber), a lot more radioactivity has been leaking from unit 2 than from unit 1 and 3. That makes unit 1 by far the best candidate for trying out ideas on how to regain control and limiting further damage to the environment. However, there is no guarantee that, even if everything were to go perfectly on unit 1, any of the lessons learnt could be applied to the other units, since they may be too badly damaged to be repaired this way. The worst “patient” probably will be unit 2, which seemed to be leaking the most water. It isn’t clear how TEPCO is planning to deal with the leaking containments or the damaged torus. One approach might be to pump leaking water from the basement of the building back into the cooling system. TEPCO estimates 6 to 9 months for its “cold shutdown” plan.

Fallout prediction system went blind

Meanwhile it was reported that two systems meant to collect data from nuclear reactors and make predictions for nuclear fallout in Japan have failed to perform as intended. The Emergency Response Support System (ERSS) was supposed feed data from reactors into the System for Prediction of Envionmental Emergency Dose Information (SPEEDI). Both systems combined cost JPY 28 billion (about US$350 million), but ERSS failed along with everything else in Fukushima 1 when power was lost. It has yet to gather any data on the damaged units.

Radiation found on ocean floor

Caesium levels about 1000 times higher than normal have been found on the ocean floor 15-20 km from the Fukushima power plant, NHK reports:

The plant’s operator, Tokyo Electric Power Company, conducted its first contamination analysis of the seabed near the plant using samples from 2 points 20 to 30 meters deep on Friday.

Samples collected about 15 kilometers north of the plant contained 1,400 becquerels of cesium-137 per kilogram and 1,300 becquerels of cesium-134.

Samples taken around 20 kilometers south of the plant contained 1,200 becquerels each of cesium-137 and cesium-134 per kilogram.

The samples from the 2 points were also found to be contaminated with iodine-131.

TEPCO says it’s difficult to evaluate the readings as there are no official limits for these substances, but it will continue monitoring the radiation levels and their impact on seafood.

See also:

• Death toll climbs in Japan as work goes on at nuclear power plant (CNN, 2011-04-18)
• Another gov’t prediction system failed in Fukushima nuke accident (Kyodo, 2011-05-02)