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Felchi
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If nuclear waste is more radioactive than the nuclear fuel it is derived from, why can't it be used in a reactor?
Evanish said:I tend to think that spent nuclear fuel is a vastly underutilized resource. Right now it spend years sitting in spent fuel ponds where energy is used to keep it cool. Instead of that it seems like it would make more sense to use the heat it produces to generate more electricity. Sort of a reactor within a reactor that could be used to provide back up power to cool the main reactor if needed or to supply more electricity to the grid during normal operations.
The idea is so obvious that it has been studied in detail - and as you can see from the non-existence of its usage, it is not economically viable.Evanish said:I tend to think that spent nuclear fuel is a vastly underutilized resource. Right now it spend years sitting in spent fuel ponds where energy is used to keep it cool. Instead of that it seems like it would make more sense to use the heat it produces to generate more electricity. Sort of a reactor within a reactor that could be used to provide back up power to cool the main reactor if needed or to supply more electricity to the grid during normal operations.
anorlunda said:The economics of nuclear power assumed that spent fuel would be reprocessed.
Perhaps your right about it not being economical for back up, but some people still seem interested in using it for some things.mfb said:The idea is so obvious that it has been studied in detail - and as you can see from the non-existence of its usage, it is not economically viable.
@anorlunda: Carter might have been relevant for the US, but nuclear power is used in many countries.
Felchi said:If nuclear waste is more radioactive than the nuclear fuel it is derived from, why can't it be used in a reactor?
anorlunda said:The flippant answer to the OP question is Jimmy Carter.
Before Carter became president, the economics of nuclear power assumed thst spent fuel would be reprocessed. When he came into office he forbade it. That, plus the incident at Three Mile Island doomed the industry.
To be fair, Carter had some good reasons. The first reprocessing plant at West Valley NY was a horrible fiasco. It gave a black eye to the whole idea of reprocessing.
http://en.wikipedia.org/wiki/Radioactive_waste#Nuclear_fuel_cycleUsed fuel contains the highly radioactive products of fission (see high level waste below). Many of these are neutron absorbers, called neutron poisons in this context. These eventually build up to a level where they absorb so many neutrons that the chain reaction stops, even with the control rods completely removed. At that point the fuel has to be replaced in the reactor with fresh fuel, even though there is still a substantial quantity of uranium-235 and plutoniumpresent.
My understanding is that reprocessing is done in many countries, though, yes, it is more expensive than once-through...though there can be political considerations that trump economics on either side of the issue.mfb said:The idea is so obvious that it has been studied in detail - and as you can see from the non-existence of its usage, it is not economically viable.
@anorlunda: Carter might have been relevant for the US, but nuclear power is used in many countries.
I think this thread was never about reprocessing.russ_watters said:My understanding is that reprocessing is done in many countries, though, yes, it is more expensive than once-through...though there can be political considerations that trump economics on either side of the issue.
http://en.wikipedia.org/wiki/Nuclear_reprocessing#List_of_sites
Thanks for your informative comment. It's nice to have a number giving me some idea of what kind of energy the energy spent fuel can give off. I'm curious, for how many years does it produce 1% of the energy that the fission process produces?Astronuc said:Spent fuel is considered high level waste, more or less, under the current strategy of once-through fuel cycle.
In the early decades (60s and 70s) of the commercial nuclear industry, there was some thought about recycling Pu and unused U in commercial fuel. This would require reprocessing plants to separate the 3 to 4% of fuel that became fission products as a result of the process. Back then, fuel cycles were designed on the basis of three or four annual cycles. Now most plants in the US operate on 18 of 24 month cycles, and discharge burnups are in the range of 4 to 5% of initial metal atoms. That is not very conducive for recycling, since with the higher burnups and residence times, the production of transuranics Pu, Am, Cm increases, and that requires more remote handling and shielding for reprocessing. In addition, one still has to separate all the fission products, which have to be immobilized in some vitrified (glass) form that is chemically stable (very little or no leaching) in storage for thousands of years.
The spent fuel generates very low levels of thermal energy, on the order of fractions of 1% of the power that the fission process produces. One could not develop much power from the heat generated for used/spent fuel in the pools. On the other hand, one could produce hot water and heat for several houses from the heat given off by one spent fuel assembly. However, spent fuel is considered 'special nuclear material', since it contains fissile and fertile isotopes in addition to fission products, in addition to being HLW, so one cannot simply purchase spent fuel for use in one's home. SNM/HLW requires approval and permits from the NRC and various state agencies, and most people are not qualified to take on the responsibility of possessing SNM or HLW.
Prolonged use of spent fuel would require some level of assurance that over the course of the use, the cladding integrity would not be compromised such that fission products would be released to the immediate systems, e.g., one's habitat, or to the environment. Most people probably wouldn't want to bother with the necessity of a formal program to ensure that cladding integrity or control of fission products is maintained.
Evanish said:Thanks for your informative comment. It's nice to have a number giving me some idea of what kind of energy the energy spent fuel can give off. I'm curious, for how many years does it produce 1% of the energy that the fission process produces?
What really interests me is the possibility of portable power sources for things like ships and mining operations. Do you think you could power a cargo ship with something like Strontium 90, and if you could do you think it could be economical and allowed?
Look at the fourth column in the table on this page - http://mitnse.com/2011/03/16/what-is-decay-heat/ .Evanish said:Thanks for your informative comment. It's nice to have a number giving me some idea of what kind of energy the energy spent fuel can give off. I'm curious, for how many years does it produce 1% of the energy that the fission process produces?
What really interests me is the possibility of portable power sources for things like ships and mining operations. Do you think you could power a cargo ship with something like Strontium 90, and if you could do you think it could be economical and allowed?
Interesting idea. I was wondering what is the cost per mole of neutrons produced by tokamak?Cibo Matto said:http://spectrum.ieee.org/energy/nuclear/could-fusion-clean-up-nuclear-waste
This seems like a good idea.
Astronuc said:Look at the fourth column in the table on this page - http://mitnse.com/2011/03/16/what-is-decay-heat/ .
Decay heat drops below 1% in few hours after shutdown. In 30 hours, it's below 0.5% of full power.
Thanks for the link. It was very interesting. I used the numbers from the wiki article, and It seems like it might technically feasible to power cargo ships with Strontium 90. I think it would take around 50 to 350 tons (check my math here). A far better option than strontium 90 would be Am-241. It would take more (170 to 1,200 tons) but it would last for a very long time (its half life is 432 years!) and it’s less active. As for safety is this really any more dangerous than burning barely refined bunker fuel releasing all kinds of particles into the air and adding to climate change?mfb said:Large cargo ships are at power levels of small nuclear reactors.
radioisotope thermoelectric generators are used where there is no reasonable alternative - mainly where maintenance is hard to impossible but you need a reliable long-living power source, like in spacecraft s. To produce some hot water or electricity for a house or small ship, the safety concerns are just too problematic.
Evanish said:Thanks for the link. It was very interesting. I used the numbers from the wiki article, and It seems like it might technically feasible to power cargo ships with Strontium 90. I think it would take around 50 to 350 tons (check my math here). A far better option than strontium 90 would be Am-241. It would take more (170 to 1,200 tons) but it would last for a very long time (its half life is 432 years!) and it’s less active. As for safety is this really any more dangerous than burning barely refined bunker fuel releasing all kinds of particles into the air and adding to climate change?
Ten percent seems kind of low to me. Are you sure they couldn't get more? What about what I talked about before with insulation? Seem like if you have enough temperature difference you can get much higher efficiencies.QuantumPion said:Am-241 has a specific power of 0.1 W/g. To make a 1000 HP engine (enough for a small boat), with 10% thermal efficiency, you would need 75,000 kg of Am-241. I doubt you could produce this much Am-241 even if you reprocessed all of the spent fuel in the entire world.
Cargo ships have to carry a huge weight in fuel because they burn tons of it a day. If you could get any kind of decent efficiency from decay heat then the weight needed wouldn't be that different. The radioisotopes can be encased in something like glass.mfb said:The small boat would need a large fraction of those 1000 HP just to carry around the americium and the machinery around it.
And you really don't want a ship to sink with so much radioactive material.
The ocean is a giant heat sink. At least the heat from nuclear powered ships will dissipate relatively quickly. The heat being added to the oceans from burning fossil fuels will keep being added for a very long time.mheslep said:Can't "turn off" that 75 tons of radioisotope either. Not a problem for spacecraft on an eternal journey, but a ship is another matter. While sitting in port the 7.5 MW of heat has to be continually dissipated.