Nuclear Fuels and Waste

[fenderzilla]

I want to know more about nuclear fission and different kinds of fuels, but all the literature I can find is denser than the fuels themselves. All I really want to know is; what machinery do you need to produce controlled nuclear fission and what wastes are produced by uranium, plutonium, and thorium fission?

[GeneralVeers]

CliffsNotes version:

You need a mechanism to control the fission rate (i.e. a piston that pushes a mass of uranium closer to another mass, which increases the fission rate); then you need a mechanism to convey the heat to a generator (water, liquid metal, or whatever else that can carry heat); then you need a generator (steam turbine, thermocouples, or whatever else that converts heat to electricity); you need a dense outer shell to keep waste radiation in; and finally you need a building staffed with lawyers to deal with environmentalist wingnuts.

Errrr.....wait......I just realized, that last one isn't a joke!

As to the wastes, that depends on the specific fuel. With uranium-235 it's krypton-92 and barium-141, both of which are radioactive. Other isotopes and elements (including thorium in some cases) actually produce uranium isotopes as "waste", which can be used in other reactors (hence the term "breeder reactor"). Usually the waste products are chemically useful for something or other--the only thing that really makes them "waste" is the fact that they'll give you a nasty sunburn if you get too close.

[lajoswinkler]

Reactor pile, which is the center of every fission reactor whether it's a graphite block or pressure vessel, is a system of closely spaced fissionable material immersed in moderator and permeated by stuff that eats neutrons called control rods. Depending on the configuration of the control rods, the pile can be more or less radioactive.

It's important to keep the area of criticality symmetrical. It's really a job for a computer.

Waste is not made out of krypton-92 and barium-141 because those are first daughter products of the fission and they decay rapidly.

In general, high level waste, which is the one we're discussing, is almost all of the starting uranium dioxide fuel (which is why it's not waste itself) intimately permeated by compounds made of a lot of other product elements. There's iodine-131, iodine-129, plutonium isotopes, caesium-137, strontium-90, europium-155, americium-241, samarium-151, tin-121m, krypton-85, cadmium-133m, .......

And each of these has its own daughter nuclides, some of which have very short half lives, which is what makes them very radioactive. You can see it's very complex. It's basically insane.

Edited December 31, 2015 by lajoswinkler

[GeneralVeers]

Well, I tried to simplify it, by sticking to the initial fission products, but noooooo, somebody had to come along and complicate things.

A favorite signature I saw somebody use once (I only ever saw this once, but it stuck in my brain forever): "nothing is so smiple that it can't get screwed up".