So I understand spent nuclear fuel is hot, both physically and radioactively. You wouldn’t want a ton of drums in your basement.
I also understand that fresh nuclear fuel has more energy compared to spent fuel, of course. But if the waste is so hot, why can’t it be used to generate something, like boiling water to make electricity? If it is so hot, why isn’t it useful?
I suspect the answer is that boilers, generators, etc. work most efficiently with high-potential energy sources, and waste would not be of that quality. Still, I would imagine SOME electricity could be generated by a warm pile even if it wasn’t as good as a hot one. Is that the case?
If so, what would be the chances of making generators more efficient so we could use the waste all the way down to room temperature and/or low radiation, where it wouldn’t be a problem to dispose of anymore?
Previous threads on the topic:
[thread=397384]What is it about reactor fuel that makes it spent?[/thread]
[thread=362711]What’s the big deal about reprocessing nuclear waste?[/thread]
[thread=327833]How much nuclear power do we have left?[/thread]
Short answer: because the high level waste products of nuclear fission are dangerous to handle and it isn’t economical to extract energy at the level they provide. Depending on what fission process you use and your willingness to perform elaborate and hazardous operations to seperate usable isotopes from the waste products, you can recycle a good portion of material almost indefinitely.
Thanks, Stranger. My question was posed in one of those threads, but I don’t feel satisfied with the answers given. Basically, if something is too hot to handle, it seems to me that it would be hot enough to be useful for something, even if less efficient than ideal.
And if it will remain hot (radioactive or temperature-wise) for 1000’s of years, creating a long-term disposal problem, that sounds like a pretty good source of fuel to me. Maybe a pound of waste wouldn’t light Chicago for a century, but would it light my house for a year? After all, it’s useless otherwise.
I guess we need to get down to some actual numbers.
Here’s the problem; with a heat engine (like the steam-cycle turbine used almost universally for electric power generation) the amount of useable work isn’t proportional to the temperature differential between ambient and what the pile generates; once you get below a certain temperature, the efficiency of the cycle drops precipitously, perhaps not even enough to overcome other mechanical inefficiences in the system. It’s like a child pushing a car; theoretically, because the child can generate some force, the car should start rolling at some small amount of speed, but the rolling resistance of the car (flexure of the tires, friction in bearings, et cetera) will keep it in place until you apply a force above this threshold.
This, combined with the fact that the waste is too dangerous from to handle directly (many of these elements and compounds are, in addition to their radioactivity, highly toxic) makes the heat they generate unsuitable to use to any practical purpose. Could we be more efficient in using and recycling used fuel elements to generate less resultant waste products; without a doubt, and this is an active area of research. Unfortunately, in the United States nuclear power is a highly politicized issue and nobody who values their political life wants to become the kind of unabashed champion for nuclear power that the industry needs in government to support the design and construction of modern nuclear fission plants.
We could talk about specific numbers but first you’d need to have an understanding of basic thermodynamics as it applies to steam-electric cycles and power generation. Try this site for some basic info.
Nope. First of all, the efficiency of a Carnot cycle isn’t necessarially proportional to the temperature differential, and second, Carnot is an idealization which assumes two isothermal and two adiabatic processes; it models the maximum possible thermal efficiency of a process; any real world heat cycle is going to be less than a Carnot cycle (and decidedly nonlinear with respect to temperature differential, even after dispensing with the effect of phase changes of the cycle medium and mechanical efficiencies of the process).
You would have to reprocess it for an atom bomb. Hard to do, as you said. But you don’t have to if you want to make a dirty bomb. All this stuff is really dangerous as it is, even if no longer useful.
Plus, “the nuclear club” takes some care in making sure the stuff is not falling in the “wrong hands”. Look out for waht comes off the former soviet republic and Russia herself as they fall into chaos and nobody is watching over the stuff anymore.
Would there be any more creative uses for it than just generating heat?. I first thought of using it to keep storage places hot and roach free. What could you possible store that would benefit from this but not be made worse by the radiation, I don’t know.
I guess there are not that many uses for non-selective top-grade self-warming poisons. Maybe to sterilize our mission to Mars so we don’t contaminate the planet with our microbes?
Don’t they use spent nuclear materials for bullets? (because of their high density)
Depleted uranium is a residue of both pitchblende ore processing and spent nuclear fission fuel (a combination of [sup]238[/sup]U and [sup]236[/sup]U, with trace amounts of other isotopes). Note that depleted uranium can’t be directly seperated from ore or waste; it first has to be put into solution to make the highly corrosive and toxic uranium hexaflouride (UF[sub]6[/sub]), then seperated via gas centerfuge or diffusion processes to remove the more radioactive [sup]235[/sup]U and [sup]233[/sup]U, then condensed back down to elemental state, a costly and hazardous process. Note that the result is still radioactive, just at a substantially more tolerable level.
DU (also sometimes called depletalloy) is very high density, as noted, and very hard. It is used in numerous applications where those qualities are paramount, such as high velocity kinetic energy penetrators, tank armor, ballistic re-entry vehicle structural shielding and support, trim ballast for aircraft, et cetera. Basically, any application in which you would use tungsten for structural purposes you can also DU. However, there are some significant contamination concerns and possible health risks with DU, and in powdered form it is highly pyrophoric (it burns readily) and so use of it is restricted.
