Nuclear disaster: radioactive radiation

Trinitiet

Hi,

With the possible meltdown in Japan, I've been thinking why a meltdown is that dangerous. I'm not sure whatever the kind of radiation is that exists behind a radiation but if it is:

-Alfa-decay: I think it's quite easy if the nuclear trash is put in a box with a thick border, I expect the intensity of the radiation to be low enough after being absorbed by a few inches thick border?

-Beta-decay: as these are just electrons, I'd catch them using magnetic fields, and then use them as electricity (so the trash would be useful)

-Gamma-decay: I'd use the photo-electric effect to charge a current.

So well, where's the problem I don't see to actually use the radiation as a source of energy, rather than a harmful piece of trash?

Trinitiet

JaredJames

Alpha and Beta are easily stopped by relatively thin materials. There's not much threat from them.
To stop gamma you need a lot - we're talking some major lead shielding / concrete. But even that isn't enough to entirely stop it.

We already use the radiation for energy but it's under extremely controlled conditions. A meltdown isn't under such controls and there are a lot more factors to consider.

Trinitiet

Why lead/concrete? I'd rather use some sort of metal (I'm not an engineer, maybe Cu?) shields so we can make a current of the radiation?

JaredJames

You need a heck of a lot of mass. Density is the key to stopping the radiation as quickly as possible.

You can certainly use metal, but there is a big difference in cost between using, for example, steel and using concrete.

Not only that, consider the temperatures around the reactor in meltdown.

EDIT: Lead is metal.

Trinitiet

Sorry, I meant to add good conductive metals (I don't think Lead is one?)

Anyway, I can imagine the cost to be bigger when using metals, but when you use concrete you don't get anything out of it. You have the concrete box and there it is, doing nothing; contrary to the energy source it could've been used like.

I also thought of this as a solution when the meltdown is over (temperature is back to normal)

Andrew Mason

The radioactive decay of fission products in nuclear fuel contributes about 7% of the fission energy of the reactor. A nuclear reactor is about 33% efficient. So when the fission reaction in a 1000 MWe reactor stops (ie the reactor is shut down), the reactor core continues to generate about 200 MW of heat. You can't turn this heat source off. It has to gradually reduce through radioactive decay.

So even after shut-down, the reactor core has to be cooled or the core will melt. If the core melts, it is not possible to remove the spent fuel. You cannot get near the core or you will die. All you can do is contain it, usually by pouring concrete over it. Three-Mile Island was a core melt-down after shut down due to decay heat.

AM

Trinitiet

Ah I think I get it, thanks

rbelli1

You also have to consider that if they get inside your body you can't put anything around them. You just wait to die.

If the material gets into the air you can breathe it. Also plants and animals (ie. our food sources) will also absorb the radioactive elements and you get them through ingestion.

Large scale radioactive release is really something you want to avoid at all costs. See Chernobyl.

BoB

Trinitiet

I wasn't really considering humans putting the frame around the nuclear trash :p I would've done that job with a robot :P

Drakkith

The problem in a meltdown is that to get to a meltdown you have to lose all control of the reaction. in a controlled reactor the fuel and control rods and everything else is kept cool by running a coolant through the core. The heat absorbed by this coolant is used to run a turbine to generate electricity.

Now if the coolant suddenly stops you have a large mass of radioactive fuel that continues to produce heat. If allowed to heat uncontrollably the fuel will melt eventually, usually preceded or followed by various components of the reactor core. Since the key to controlling a fission reactor is to moderate the chain reaction, melting fuel also becomes capable of causing a nuclear explosion, that while not near as bad as a nuke, can easily destroy the containment vessel and spread tons of radioactive material all over the place.

Borek

Does it? I was under impression that for explosion you need very particular geometries and high purity - as everything (fuel rods, moderating rods, zircalloy cladding, steel and whatever else is inside) melts together, you have no purity and geometry is rather random.

I would also guess that the outer containment is designed in such a way that in the case of meltdown liquid material is spread thin as fast as possible, but that's just my speculation.

Andrew Mason

I don't think you meant a nuclear explosion - rather an explosion in the reactor core.

AM