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Quote by: Domino  Personally, I have trouble understanding why nuclear waste can't be used for power generation. It sure seems to have a lot of energy to be worthless. |
Sounds good. Have you started working on a way to do that? You see, it is called waste because it can no longer be used to generate power. The level of fission by-products is so high in the fuel that it is no longer possible to achieve a critical mass. Normally, power reactors use enriched uranium as fuel. Natural uranium is about 99.3% U238 and 0.7% U235. For use as a power reactor fuel it is enriched until the U235 is about 3% (I think the average is around 3.3%), the rest still being U238 (about 97%). Spent fuel rods are about 94.3% U238 and about 3.5% fission products (really nasty, highly radioactive, nonfissile stuff). That leaves about 2.2% other stuff. It has 0.81% U235 (the fissile isotope of uranium), 0.51% U236 (also nonfissile), 0.52% Pu239 (fissile), 0.21% Pu240 (fertile), 0.10%Pu241 (fissile), 0.05% Pu242 (nonfissile). If you aren't aware of it, the Pu comes from neutron capture by U238, like this: U238 + n => U239 => (- Beta particle) Np239 => (- Beta particle) Pu239. The other isotopes of Pu are also the result of neutron capture from Pu239. However, Pu239 is also fissile, as is Pu241. In fact, about 1/3 of the total energy output from a nuclear reactor comes from the fission of the Plutonium derived from U238 by neutron capture.
Plutonium for weapons is made in a different type of reactor, known as a fast breeder reactor. Such a reactor may be liquid sodium cooled. In these reactors the fission pile is surrounded by a blanket of uranium oxide (U238). This blanket is outside of the fission reaction but close enough that fast neutrons from the reaction are captured by some of the U238, which then becomes Pu239.
Any way, the "fission product" part of the waste actually would stop the nuclear reaction if it were not taken out of the pile from time to time (about 1/3 of the fuel is replaced at each refueling). There are several products that are produced in pretty standard ratios from fission. They have not been found to be useful, as far as I know.
Depleted uranium comes from the process of enrichment of natural uranium. It has a lower percentage of U235 and therefore a higher percentage of U238. It is less radioactive than natural uranium. It is used for armor piercing tank shells.