Originally posted by frozenspark
What I don't understand is why can't they turn the damn things off? Or are they designed to give so much hell when the power is cut off? Isn't there
some shutdown mechanism in place to prevent meltdowns?
These reactors are Boiling Water Reactors (BWR) type. Any nuclear reactor requires moderation. It basically slows down/absorbs neutrons emitted by the
fuel (in this case, Uranium-235 and Plutonium-239).
Without moderation, an atom in the fuel rod emits a neutron, it flies into another atom, causing it to release more neutrons. More bombardments occur
and more neutrons are emitted. This is known as a chain reaction. This causes the fuel rods to get hot, and this heats the water.
There are two parts to the moderation' neither are sufficient in isolation to stop the chain reaction. The first are boron rods. These absorb
neutrons. The second is heavy water. This slows down the reaction as well as absorbing neutrons.
To shut down the plant, you fully insert all the control rods, and keep circulating the cooling water until the reactor is at ambient temperatures.
In the case of these reactors, the water is not being circulated. You still have the rods inserted, but this is insufficient. Initially, heat is
generated from the by-product of fission of the fuel. If left unchecked, fission can start again. During this period the heat generated is increasing.
The chain reaction can start again, and this heats the rods yet further. As they get hotter, they release more neutrons. This process accelerates like
crazy. The water gets hotter and hotter (beyond limits), and turns to steam. The pressure keeps rising, as does the temperature. Eventually the water
splits into hydrogen and oxygen. All that is left is for the hydrogen to gather in sufficient amounts for it to want to create helium or deuterium.
When it does so, the thing explodes.
It doesn't stop there. Assuming sufficient material is left, it can melt the very materials that comprise the core, and the thing melts. This is a
meltdown. It can occur before or after an explosion, but the conditions for an explosion exist prior to the temperatures required to melt the reactor
(1400 degrees C at very high pressure is required to split water - the reactor components start to melt about 2000 degrees C).
That is the situation.
They are reporting that the hydrogen exploded outside of the reactor, but in the building, during venting of pressure (to lower the pressure in the
reactor). This can't be so due to chemical nature of the hydrogen generation, and its reactivity. It is very likely to explode whilst still in the
reactor due to the heat of the fuel rods (remember at this points the rods are exposed, and extremely hot - maybe 2000 degrees C). The hydrogen
contacts these in sufficient quantity and BOOM.
Whilst the hydrogen is extremely volatile outside the reactor, it is unlikely to wait for the venting operation before deciding to fuse together. This
is what makes all the reports of the all reactors being intact and all suffering the exact same problem at the same time (exploding only once the
hydrogen was vented) so implausible.
There is some great footage of the aftermath of the explosion of reactor 3 on YT. There is clearly a void where the reactor should be. This proves
beyond doubt that it was the reactor that exploded. It is much clearer on the TV.
I hope this clarifies sufficiently for you. It is late here, and there is a lot to type on the subject, but this should be a good start.
on 14-3-2011 by mirageofdeceit because: (no reason given)