One of the main points of contention between the pro and anti nuclear lobbies is how to dispose of nuclear waste safely. So, why do we need to dispose of it at all? Nuclear waste is still radioactive, so why not just use the waste to generate more heat to make electricity out of?
Is there a reason why this is not done? I’m not entirely knowledgeable about nuclear power, so this is probably a stupid question.
I think a lot of countries reuse the fuel rods, but apparently not in the United States. We basically throw out rods that are 10% used up for a couple of reasons. First, the fuel is pretty cheap anyway. Second, there are fears that the recycling of fuel could lead to weapons grade Plutonium waste products, so the govt made rules about reuse and disposal.
How many times could a rod be recycled?
Reprocessing does have its hazards:
Huge radioactive leak closes Thorp nuclear plant (May 9, 2005)
As the fuel is used, the fragments of split uranium atoms (fission products) build up in the fuel. Some of these tend to absorb the neutrons that maintain the chain-reaction, so they “poison” the reaction. Over time they build up to the point where fission can’t work properly, and the rods need to be replaced.
The used fuel rods still have some uranium available, and also plutonium that was created while in the reactor. They also have a lot of highly radioactive fission products, which makes them dangerous to people and difficult to reprocess to reuse the uranium and plutonium.
The fission products also generate heat, but is is such a small amount compared to that generated by fission that it is not worth the trouble to use it.
Nuclear waste also includes non-radioactive objects that have been exposed. Things such as clothing, replaced parts of reactors, contaminated water and other such things.
In the US we decided to keep power and wepon uses of nuclear materials entirely seperate. So we burn Uranium in the reactors. Low Enriched (Uranium (LEU). You can’t make a bomb from LEU. Turning it into weapons-grade HEU is hard to do.
Most other countrys burn Plutonium, as it is way cheaper. Plutonium is what you get when you reprocess spent fuel that started out as either Uranium or Plutonium.
Problem is, there is no such thing as non-weapons-grade Plutonium. The international community relies on IAEA survielence and auditing to insure that Plutonium is not diverted to weapons production.
I long ago wondered about this – why not put thermocouples near the hot fuel? Surely you could get power out that way.
You could. suspect you could try getting it out in other ways. The problem is that I suspect it’s not worth it. The efficiency of power generators depends upon the difference in temperatures between the source and your sink, so the hotter the plant runs the better. This is why we’ve got one big nuke plant with very high temperatures turning water to steam to run turbines, rather tha a lot of little low-heat energy generators. You get a lot more usable power out of the big hot power plant, even if you do have to spend a lot of it in transmission to the eventual users.
Fine. But the spent fuel rods are still sitting there in your “swimming pool” storage chambers. Why not put that heat to use? So what if it’s not perfectly efficient? at least it would be doing something!
I suspect the answer is that the amount of energy you’d get out would be trivial for all your effort, and easily offset by building a few auto batteries.
This is a common method of powering spacecraft, especially interplanetary probes. They are called “radiothermal generators” or RTGs. The Apollo spacecraft used these too.
However, I suspect CalMeacham is right about it not being worth the trouble for terrestrial use.
You’re right about the efficiency being greater when the temperature difference is greater, but that’s not really the problem here. Spent fuel can get hot enough to melt if not actively cooled, and that is certainly hotter than the fuel in the reactor. The reactor generates much more power though.
Picture a block of metal cooled by flowing water on one side and heated on the other. For the reactor, the heat source is like a blow torch. For the spent fuel the heat source is like a match. The water flowing past the torched metal is going to carry away a lot more energy than the other. You can make both blocks the same temperature by adjusting the water flow, but you won’t get usable power from the match block unless your load is very small, like on a spacecraft.
Actually I’ve been reading a lot lately about the Radkowsky core design which is supposed to be “proliferation-proof” (proliferation-resistant is more accurate). This is a sort of breeder reactor using thorium-232 as the fertile material and a small amount of enriched uranium as a neutron source to produce the main fuel source, fissile uranium-233. U-238 is also used to complicate the separation of U-233 which could be used to make a bomb. The design should produce perhaps a quarter as much plutonium as a conventional reactor, but more importantly it would produce a small but significant amount of plutonium-238 along with the bomb material Pu-239. Pu-238 is difficult to separate from Pu-239 and even a small amount can cause a bomb to “fizzle.” You could still make a bomb with this mixture of Pu-239 and Pu-238, but the yield would be reduced by about 90%. Of course 10% of 15,000 kilotons is still pretty scary.
There are some problems with this design. For one thing, the enriched uranium used as a neutron source has to be more highly enriched than is normally the case. Not so highly enriched that you could make a bomb out of it directly, but enough to give the bomb makers a bit of a head start.
Is the P-238/P-239 mix usable as reactor fuel, or does the '238 poison it for that use as well as making it unsutable for bombs?
First of all - WTF do thermocouples have to do with this topic? Thermocouples are devices used to measure temperature by the potential differences created by dissimilar metals. They do not generate power.
Secondly the last poster addresses newer reactor technology that is not currently employed in conventional domestic nuclear power plants and is therefore not germane to the OP’s question.
Thirdly, the nuclear reactors used in the US are the type where the amount of fissionable material produced is less than consumed. Breeder reactors (which produce more fissionable matter than what is consumed) have never really become a commercial success in this country. Fermi-1 was a liquid sodium breeder reactor that was built in the 60’s (IIRC) that almost resulted in the evacuation of Detroit (but this is another topic and another story)
The spent fuel rods from commercial power nuclear reactors have a limited amount of fissionable material, that for the most part, have significant half-lives that do not lend themselves to nuclear fuel recovery and also do not lend themselves to easy disposal.
Are you sure? The Apollo missions carried RTGs in experiments left on the lunar surface, and the Lunar Rovers (the dune buggies) were powered by them, too. But I can find no sign (in an admittedly brief Google search) that the Apollo spacecraft itself used them.
Sure they do:
Oh, and I forgot to mention: RTG stands for Radioisotope Thermoelectric Generator.
The only RTGs used in the Apollo missions were those powering the long-duration experiments left behind on the moon’s surface. The Apollo ship itself was powered by hydrogen/oxygen fuel cells, with battery backup. The lunar lander and the lunar rover were both battery-powered.
http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_lrv.html
The efficiency of BWR’s (Boiling water reactors) and PWR’s (Pressurized Water Reactors) used for commercial power production in the US (and most of the world) is based on the modified Rankine-Carnot) cycle for converting water to steam to drive a turbine and then converting that steam back to water by condensation and has nothing to do with the temperature that the reactor itself is operated at. It is not important whether you are using fossil or nuclear fuel to heat the water. The thermodynamic process, at best, gives rise to a thermal efficiency of about 40% since most of the heat generated by condensation (roughly 1,000 BTU/# of steam) cannot be recovered (the second law of thermo applies in this case).
In a simple analysis, nuclear reactors consist of a fuel source (the fuel rods), the coolant to remove the heat produced (water), a moderating material to slow down neutrons so the reactor can be controlled, and backup control and cooling systems. Again for BWR’s and PWR’s, which constitute virtually all commercial reactors built and operated in this country, the production of power is through the water-steam cycle. After obtaining the useful energy the spent fuel rods must be disposed of and this country has lacked a suitable means for accomplishing this for a long time. (See my previous post) A facility is being built in Nevada for longt-term disposal of spent fuel but in the meantime most spent fuel is being held in fuel pools at some of the nuclear power sites or other storage facilities.
Wrongful analysis here - fossil fired boilers operate at flame temperatures in the range of 2200-2400 degrees F. Nuclear reactors are not as “hot”. Again the issue has to do with the ecomomics of size and thus individual commercial nuclear reactors peaked out at around 1000-1200 MW capacities.
The amount of fissionable material, and therefore, the energy that could be obtained is not worth commercial production and see my previous post.
Whoops, I misread one of my cites for that bit about the Rovers being RTG powered. Sorry.
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[QB]The spent fuel rods from commercial power nuclear reactors have a limited amount of fissionable material, that for the most part, have significant half-lives that do not lend themselves to nuclear fuel recovery and also do not lend themselves to easy disposal.
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This is true ONLY of US power plants. Japanese, and European operators reprocess fuel routinely. Of course in those places, the reactors are state operated, so your statement is true in the strictest sense. It is the Plutonium based fuel cylce that makes this practical.
[Quote:Waterman]
[QB]
…a moderating material to slow down neutrons so the reactor can be controlled…
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The moderator is used to slow neutrons so that they WILL interact with fuel atoms and cause/sustain a chain reaction in the relativly ( compared to a warhead pit ) dispersed reactor core. Use of a moderator allows criticality with either less fissile material, or less compacted material. The moderator material is there to accelerate the reaction, not to slow it downt. Presence of moderator material can be compared to a catalyst in a chemical reaction.
Now the chain reaction does build MUCH more slowly in a dispersed core (which requires a moderator to work) and that does ease the problem of controlling it, so perhaps that is what was intended.
The reaction is controlled by the aptly named control rods which are not moderators and do not merrily slow neutrons, they absorb neutrons.