Monday, March 14, 2011

Nuclear disaster goes from bad to worse

The Fukushima disaster is now spinning out of control.


The explosion yesterday from Reactor 3 destroyed most of the pumps that were cooling Reactor 2. Reactor 2 started down the now familiar path of falling water levels, exposed fuel rods, fuel melting, and hydrogen generation.

However, this time the result has been a breach of the containment. At the base of the reactor there is a big circular steel tube called the "Wet well". This has the shape of a donut or torus, and it is normally partly filled with water. It is connected to the "Dry Well", which contains the reactor, by big steel tubes.

The Japanese now believe that this is leaking, maybe as a result of too much internal pressure, or maybe as the result of a hydrogen explosion.

The Wet well and the Dry Well together make up the containment. Both must stay intact to contain radiation if a meltdown occurs.


When I was annotating this diagram for the last post, I was wondering about something that looked like a spent fuel storage pond sitting at the top of the reactor. I was hoping that it was just temporary storage, which would be empty.

Today other people have started commenting on it, so I have marked it in tonight's post. It now seems as if there may be a lot of spent fuel stored in that pond.

It is in an extremely vulnerable position, right at the top of the reactor. Fortunately, the hydrogen explosions have so far blown away surrounding structure while leaving the spent fuel storage ponds intact. If the fuel was to be ejected from the pond it would subject the whole site to intense radiation which would be immediately detectable. If exposed to air, the fuel would probably also catch fire. This would release large amounts of radiation.

The fuel generates heat, and it will tend to boil water out of the pool. The pool needs to kept topped up.


When fuel is removed from a reactor it is extremely radioactive and still producing significant amounts of heat. A large crane lifts the fuel out of the reactor vessel and lowers it into a deep tank of water. This is the spent fuel pool. The water blocks the radiation, and keeps the fuel cool.
In time, the radiation and heat production drops. Eventually, the fuel is put into a large metal container, lifted out of the pool by a crane, and lowered onto a railway car for transport.


This is odd, because Reactor Four was supposed to be shut down for maintenance. It is possible that some supplies left behind by the maintenance workers caught fire. Things like electrical cables can burn, and there was a serious fire at Brown's Ferry many years ago which was started by a maintenance worker.

A far more disturbing possibility is that the spent fuel is burning. That might happen if all the water in the spent fuel pool boiled away. If fuel is burning, large amount of radiation will be detected.

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