Wednesday, March 16, 2011

Fukushima: Japanese for Chernobyl

What's the big picture here? Deep within the radioactive ruins of the Fukushima
Daiichi nuclear complex, there lie four swimming pool like structures. These are
the spent fuel ponds. These are full of used fuel rods from the nuclear reactors.
Spent fuel must be kept covered with water to absorb the radiation from it and
keep it cool.

Heat from the spent fuel is slowly evaporating the water in the ponds. Under
normal circumstances topping up the water level would not be a problem, but the
situation at Fukushima is not normal. Over the past few days, the reactor
buildings have been ripped apart by hydrogen explosions. The ponds are located on
the upper levels in the buildings, and normal access routes to them are probably
destroyed or blocked by debris. The blasts may have cracked the concrete
structure of the ponds themselves, causing water leaks. Piping and
instrumentation cables leading to the ponds have probably been destroyed.

What makes this much worse is that the hydrogen explosions have also punctured at
least two of the reactor containments. The whole Fukushima site is now covered in
dangerous radiation, and levels near the reactors are reported to be lethal.
The bottom line here us that there little chance that the plant workers will be
able to maintain the water levels in the fuel rods. That fuel is going to be
exposed to the air.

What happens then? The fuel will heat up, and the metal cladding will react with
the water to liberate hydrogen. This will lead to further hydrogen explosions. At
some point the water will all boil away. Fuel temperatures will then rise much

Expert opinion differs on what happens next. Some experts say that the fuel will
not catch fire. Others say that the fuel will burn, and I strongly agree with
that. Put a large pile of flammable stuff in a metal lined basin which tends to
retain heat, then raise the temperature, and something is going to ignite.
Experts I have read seem to agree that a spent fuel pond fire is one of the worst
things that can happen in a nuclear plant. It is as bad as a core meltdown, if
not worse. It will release huge amounts of radiation into the air, which will
then go where ever the winds take it.

Major spent fuel pond fires are now all but inevitable. This will be every bit as
bad as Chernobyl. That is why the French are telling their citizens to leave the
country, and why the US is advising Americans to stay 50 miles away.


Some US experts believe that the water in the spent fuel pond at Reactor 4 has
already gone. The reactor building has been blown apart by a hydrogen explosion,
and they think that the exposed fuel rods are the source of the hydrogen. There
have been signs of a fire inside the building.

I think the US experts are wrong. The spent fuel pond is at the top of the
structure. Hydrogen tends to rise, and it would not sink down into the lower
levels of the building. Hydrogen from the fuel ponds would blow the roof off, but
would not produce the kind of damage visible in building 4. The hydrogen that
blew up building 4 came from somewhere else. However it is possible that the
explosion damaged the pond and that all the water leaked out.

The Japanese say that the water is still there and that the temperature is 86
Celsius, far above normal. I suspect that the Japanese are correct, but it
doesn't really make much difference. There is no feasible way to top the water

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