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    geomancer's Avatar

    Fukushima is not Chernobyl (no graphite in the core)

    [It appears that the Japanese nuclear disaster cannot approach the scale of Chernobyl because graphite is not used in their reactor design. It was the burning of 1200 tons of graphite that sent the Chernobyl plume into the upper atmosphere and then over much of Europe. The jet stream, and hence North America, should not be heavily impacted by Fukushima.]

    Institution [of mechanical Engineers] comments on the main engineering challenges at Fukushima

    Fukushima nuclear plant: the main issues

    Following the massive earthquake and tsunami in Japan, the Fukushima Daiichi nuclear plant is faced with two grave problems: a lack of electrical power to operate the site, and no means of absorbing the energy stored in the reactors, other than boiling water. Dr Colin Brown, the Institution’s Director of Engineering gives an overview of the main issues facing engineers working to contain the plant.

    Q: What are the two main engineering challenges?

    A: There are two main engineering problems. Firstly, the safe operation of the reactors relies on the supply of electrical power. The earthquake and subsequent tsunami wiped out the electrical power to the plant, so there is no power to operate safety systems and especially power the high pressure cooling pumps.

    This is a double negative as the failed grid means that there is no way to take energy from the site. Currently, there is nothing to absorb the decay heat in the reactors, and the only solution is to boil water, and releasing the steam it generates to take the heat away.

    Q: Looking at the world’s responses to the emergency, the public seems to have three questions about how the stored energy in the reactors can be absorbed; why the buildings are exploding; and lastly, whether Fukushima will turn into the next Chernobyl.

    A: Firstly, the engineers have got to dissipate the decay heat. The only effective technique is to boil water, and so as much water as possible has to be pumped into the reactors. This will be combined with venting and releasing steam and then more pumping. Overall, the engineers are faced with the challenge of trying to dissipate the equivalent of the power of 100 large wind turbines using boiling water.

    Secondly, the public rightly wants to know why explosions are happening when the engineers tell us that the reactors are safe. The answer to this is that during the period when there was no power, the reactor cores were uncovered, and they decomposed. Hydrogen was formed, and the result was that the venting system, which was designed to vent steam, instead had to vent large quantities of hydrogen and this is why the buildings exploded. There may well have been a failed catalytic converter system on the vent relying on electrical power, that would normally have safely have converted hydrogen to water. Prior to the third explosion, engineers had started to tak panels out of the buildings trying to release the hydrogen.

    Thirdly, a concern for the people not just of Japan but the Pan Pacific area is whether Fukushima will turn into the next Chernobyl with radiation spread over a big area. The answer is that this scenario is highly unlikely, because of the wildly different design of the two reactors. The reason why radiation was disseminated so widely from Chernobyl with such devastating effects was a carbon fire. Some 1,200 tonnes of carbon were in the reactor at Chernobyl and this caused the fire which projected radioactive material up into the upper atmosphere causing it to be carried across most of Europe. There is no carbon in the reactors at Fukushima, and this means that even if a large amount of radioactive material were to leak from the plant, it would only affect the local area. The Japanese authorities acted swiftly and decisively in evacuating people living within 20km of the plant, and ensuring people living within 30km of the plant remained in their homes, with windows and doors closed. The radiation measured so far at Fukushima is 100,000 times less than that at Chernobyl.

    Appraising the situation at Fukushima as it stands now, it seems as if the engineers have enough cooling water pumping into the reactors, and all three of the buildings have exploded, so in my opinion, the engineers on site now have to continue their efforts for 5 – 10 days to get the core heat down, and then hopefully the plant will be finally safe.
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