nrg said:
Spent reactor fuel continues to produce heat for a very long time and I am curious as to why this heat cannot be used for something, such as district heating. Obviously there are some barriers out there or it would be done. I'm wondering what these barriers are. Any discussion surrounding this topic would be appreciated. Thanks.
Well, as mathman said, spent fuel tends to be quite radioactive. New fuel rods contain just uranium, plus the structural materials fo the fuel pellets/rods/tampers/etc. Since most of commercial fuel is U-238 with a half-life of 4.6 Billion years, it's specific activity is very low. Low activity = low dose. You could walk into a room full of unused fuel rods and receive very little radiation.
Spent fuel rods, on the other hand, contain all the radioactive decay products produced from the fission. Many of these have a relatively short half-life, so you would get a much, much larger dose of radiation standing near a spent fuel rod than an unused one. It's also not all that uncommon for a fuel pellet to rupture during it's life cycle and if that happens the decay products can escape.
If you simply put the spent fuel rods into a pool of water and pumped the water through your house, the escaped decay products would begin to coat the pipes and before too long the pipes would become radioactive too. Also rememer that you'd need electricity to power the pump and you'd probably spend more in electricity for the pump than it would take to heat your home.
What you *don't* want to do is let the water boil off the spent fuel rods. If you have enough spent fuel rods close enough together, the decay heat may be great enough to melt the rods, releasing *all* of the radioactive decay products rather than just a small portion. Without the water, you also have no shielding to stop the radioactivity.
The goals of the cooling pools at nuclear power plants are (as I understand them) twofold. First, the water provides a shield that stops most of the radiation from the fission products. Secondly, you want the fuel rods placed far enough apart so that there is no danger of the water boiling. The idea is to keep them there until a significant amount of the short-lived fission products have decayed away and the rods are safer to handle and produce significantly less heat.
I guess my overall answer to your question would be safety. In order to *safely* extract useful energy from the spent rods you would probably need a set up that resembled the reactor itself: pressure vessel, containment building, primary and secondary coolant loops, radiation scrubbers for the primary loop, etc. Reactors aren't cheap and this setup would produce *significantly* less power than a reactor. You'd probably come out better financially if you just took all that money to the bank, converted it to $1 bills, and burned it.
