Boiling Water Reactor
In a time of accident like we are seeing at the Daiichi Nuclear Power Stations facts are a very good thing. So let me provide you with some facts, then we can talk about what is known at this point about the accident there.
Daiichi Nuclear Power Station station has 6 nuclear reactors on site that are all of the type known as Boiling Water Reactors. They are an older design that does not have a containment building but rather a containment vessel which holds the reactor core of rods and water that is used to generate steam. They were all built in the 1970′s. Reactor #1 is the one that is in danger at this station.
There is a second Daiichi NPS which has 4 reactors built in the 1980. These are of the same type and units 1,2 and 3 are in the same kind of danger that Unit 1 at Daiichi One is in, though as of now no explosions have been reported. This issue is that any of these plants could have a failure due to the inability to circulate cooled water around the core. If that happens it is called a meltdown.
A nuclear reactor works by fission, splitting the atom, which causes a lot of heat. The water in a reactor is used two ways. First to keep the nuclei which are generated in fission and cause other atoms to fission from escaping. This makes it a more efficient reaction.
The second purpose of the water is to carry away the heat of the ration. Water is great a conducting heat. It keeps the rods cool and the allows for generation of electricity from the steam.
The problem at Daiichi Unit 1 is that the pumps that circulate the water have shut down initially due to a lack of power. Since the reactor is a “single loop” there the water around the core has continued to heat up. Even with the control rods in, the reactor stays very hot for at least a couple of days.
A reactor works by having sets of fuel rods and control rods interspersed. When you remove the control rods (partially or fully) more and more of the nuclei escape from the uranium, they slam into other atoms of uranium and cause an escalating reaction. The water heats up and then turns to steam at the top of the containment vessel and moved to the generator.
Because the water in the core is turning to steam, there is a need to keep a constant watch on the level of water. If the core is exposed, the heat it generates will jump up very fast and become so hot that the rods, control and fuel, will melt. At this point you have the beginning of a meltdown.
The balance between control rods (which are often made of cadmium because it can catch and capture neutrons without fissioning) is a carefully planned thing. If the fuel rods melt and fall to the floor of the vessel they will no longer be inter-spaced correctly to prevent a run away nuclear reaction.
The fuel will all be in one place and there will be water all around it but nothing to prevent the neutrons from shattering other atoms of uranium. The puddle of super hot and reacting fuel will continue to heat up and in short order will burn through the containment vessel and fall to the floor of the reactor building.
Then things can get really bad. All the water in the containment vessel is going to pour out onto this very hot slag. It will flash into super-heated steam and could cause an explosion which could spread the melted fuel even further.
So far that does not seem to be what happened at Daiichi Unit 1. It appears that there was an explosion in the pumping mechanism from the super-heated and super-pressurized steam that the dormant reactor has been building up since the accident.
It is not good news, as you can see from the diagram the water in this type of reactor is highly radioactive from its contact with the core and the fact that there was a limited amount of water in the reactor to start with. Without being able to replenish this as it escapes or is released, you come closer and closer to uncovering the core.
Right now the Japanese officials are talking about flooding the core with seawater. This is a last ditch effort. The reason they have not done it up untill now is that seawater, especially hot seawater, is very corrosive. It will eat away at all the metal fittings in the system. But if they can pump enough in and keep pumping it in, it will prevent a core exposure and meltdown. It means completely scrapping any reactor they do this to, but it is far far better than a full meltdown.
More on this story as it develops
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