Why can't we use nuclear waste in nuclear reactors?

[Felchi]

If nuclear waste is more radioactive than the nuclear fuel it is derived from, why can't it be used in a reactor?

 

[mfb]

Nuclear reactors mainly use (induced) nuclear fission. Radioactive decays of fission products contribute to the power, but not enough to make a power plant based on them worth the effort.

 

[QuantumPion]

If by nuclear waste you mean spent nuclear fuel, you can, but the fuel must be reprocessed first to remove fission products which hinder the nuclear chain reaction. This is expensive, and historically it has been cheaper and easier to just make new fuel than reprocess spent fuel.

If by nuclear waste you mean just radioactive material in general, there are some radioactive materials which can be used to generate power in radioisotope thermal generators. However these RTG's use specialized materials made specifically for them, not just random radioactive waste. The energy output of low level radioactive waste is simply insignificant compared to conventional macroscopic energy sources.

 

[Evanish]

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]

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.

 

[QuantumPion]

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.

Spent fuel does not generate enough heat to be economically useful. There are already other more robust sources of backup power.

 

[mfb]

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.

@anorlunda: Carter might have been relevant for the US, but nuclear power is used in many countries.

 

[Nugatory]

The economics of nuclear power assumed that spent fuel would be reprocessed.

As I read the question, OP isn't asking about extracting fissile material from the waste for use as fuel (which is what is usually meant by "reprocessing") but rather about using the energy released by the ongoing decay of the fission products themselves.

 

[Evanish]

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.

Perhaps your right about it not being economical for back up, but some people still seem interested in using it for some things.
http://en.wikipedia.org/wiki/Spent_fuel_pool#Other_possible_configurations

 

[GiuseppeR7]

interesting...as someone as already said they are already used in some fields...some satellites use them to produce directly electrical energy.

 

[DrStupid]

If nuclear waste is more radioactive than the nuclear fuel it is derived from, why can't it be used in a reactor?

It can be used but it is tricky and less profitable if the costs for waste disposal are not considered. There are pilot projects e.g. MYRRHA in Belgium (http://www.siler.eu/public/DeBruyn.pdf).

 

[DrDu]

I don't remember where and when, but some physicist wrote a request to use a barrel of highly radioactive fuel to generate hot water for its own house. For some strange reason, his request was declined :-)

 

[Quantum Defect]

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.

The Wikipedia page has some interesting history on nuclear reprocessing. Worries of nuclear proliferation were responsible for Ford/Carter's actions in the US. Reagan lifted the ban, and there was some recent construction at Savannah River for a mixed oxide fuel fabrication facility, but nothing has been completed. Other countries have continued fuel reprocessing, as outlined in the table in the wiki article: http://en.wikipedia.org/wiki/Nuclear_reprocessing

See also: http://www.fas.org/sgp/crs/nuke/RS22542.pdf for a very nice timeline...

 

[russ_watters]

A more direct answer to the OP's question, perhaps:

Used 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.

http://en.wikipedia.org/wiki/Radioactive_waste#Nuclear_fuel_cycle

 

[russ_watters]

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.

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

Perhaps more to the point, for the US, right now the economic analysis of the issue is incomplete or even moot: The US has tried and failed (thus far) to get a permanent waste storage facility commissioned and as a result, the true cost of the principal alternative to reprocessing can't readily be calculated. Sure, storage could and probably should be relatively easy and cheap, but then again, so should nuclear power itself!

 

[Evanish]

Here is one fascinating possibility.
http://en.wikipedia.org/wiki/Atomic_battery

http://www.extremetech.com/extreme/...ont-value-your-life-or-sperm-count-too-highly

 

[Khashishi]

Fast neutron reactors can burn most of the nuclear wastes as fuel. The USA used to have some fast neutron reactors, but as far as I know, they have all been shut down for political reasons and safety concerns. Criticality accidents are more dangerous in fast neutron reactors than light water reactors since there are fewer negative feedback mechanisms in place if the coolant evaporates. These issues can probably be solved but there hasn't been a political will, since there is a proliferation risk. I think worrying about proliferation is stupid, since plentiful clean energy will do a lot more for peace than any political blustering.

 

[mfb]

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

I think this thread was never about reprocessing.
@Felchi asked why the radioactivity of nuclear waste is not used in a reactor.

 

[Astronuc]

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]

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.

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?

 

[mfb]

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.

 

[anorlunda]

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?

Not something as large as a ship. It would be unsafe and probably prohibited. But we do power space probes such as Pioneer with somewhat similar ideas. See wikipedia about snap reactors.

 

[Astronuc]

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/ .

Decay heat drops below 1% in few hours after shutdown. In 30 hours, it's below 0.5% of full power.

 

[Cibo Matto]

http://spectrum.ieee.org/energy/nuclear/could-fusion-clean-up-nuclear-waste

This seems like a good idea.

 

[mheslep]

Extensive reprocessing of spent fuel / waste before reuse in a reactor is a prerequisite of existing, *solid* fueled reactors. Such is not a requirement for other designs, in particular liquid fueled reactors where fission products that interfere with the reaction are allowed to out gas relatively simply. With efficient use of neutron spectrum in thermal and fast regions, the transuranic wastes produced in traditional lightwater PWRs would be consumed.

http://transatomicpower.com/white_papers/TAP_White_Paper.pdf

 

[Evanish]

http://spectrum.ieee.org/energy/nuclear/could-fusion-clean-up-nuclear-waste

This seems like a good idea.

Interesting idea. I was wondering what is the cost per mole of neutrons produced by tokamak?

 

[Evanish]

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 helpful. Even though all the world's nuclear waste together might produce a useful amount of power it seems that we are talking about low grade heat here which I imagine is why generating electricity from it is not practical. I was wondering if you think this idea has any potential.

https://www.physicsforums.com/threa...ower-density-possible-with-decay-heat.798802/

 

[mheslep]

mfb addressed uses of decay heat above in this thread

 

[Evanish]

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.

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?

 

[QuantumPion]

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?

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.

 

[mfb]

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.

 

[Evanish]

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.

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.

 

[mheslep]

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.

 

[Evanish]

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.

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.

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.

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.

 

[mfb]

Direct heat is not an issue for global warming, as it does not change the equilibrium temperature (and the nuclear waste is there anyway).

Diesel has an energy density of ~50 MJ/kg, spread over one week this gives 80W/kg - a factor 800 above the value QuantumPion calculated for americium. Ships would not be happy if they had to carry 800 times more fuel, even if that fuel would last "forever".

 

[mheslep]

The ocean is a giant heat sink.

The problem's not the ocean but preventing the radioisotope stack from melting in the same way that radioisotopes melted the Fukushima cores. It would require constant liquid cooling with a fairly large pumped flow, always running at sea and in port.

 

[mfb]

Well, a ship always has cooling water around while operational. 7.5 MW is ~20kg/s heated to boiling temperature or ~4kg/s boiled away.

 

[Evanish]

Direct heat is not an issue for global warming, as it does not change the equilibrium temperature (and the nuclear waste is there anyway).

Diesel has an energy density of ~50 MJ/kg, spread over one week this gives 80W/kg - a factor 800 above the value QuantumPion calculated for americium. Ships would not be happy if they had to carry 800 times more fuel, even if that fuel would last "forever".

Cargo ships burn between https://people.hofstra.edu/geotrans/eng/ch8en/conc8en/fuel_consumption_containerships.html of bunker fuel a day. Bunker fuel has an energy density of around http://www.people.hofstra.edu/geotrans/eng/ch8en/conc8en/energycontent.html. 50 ton* 907.185 kg/ton * 40 MJ/kg = 1,814,370 MJ a day

350 ton* 907.185 kg/ton * 40 MJ/kg = 12,700,590 MJ a day1,814,370 MJ * 0.277777777778 kwh/MJ = 503,992 kwh a day

12,700,590 MJ * 0.277777777778 kwh/MJ = 3,527,942 kwh a day503,992 kwh / 24h = 21,000 kw

3,527,942 kwh / 24h = 146,998 kwLets say the ships achieve a staggering 50% efficiency. 21,000 kw * .5 = 10,500 kw

146,998 kw * .5 = 73,499 kwSo cargo ships need between roughly 11 and 73 MW. From Wikipedia we get some power densities. Sr-90 = 0.46 kilowatts per kilogram

Pu-238 = 0.54 kilowatts per kilogram

Am-241 = ¼ Pu-238 = 0.135 kilowatts per kilogramSo if you got 100% efficiency you would need…10,500 kw / .135 kw/kg = 77,778 kg or 86 short tons

73,499 kw / .135 kw/kg = 544,437 kg or 600 short tonsOf course you won't get 100% efficiency. If you had a 10% efficiency you would need ten times that or 860 to 6,000, but it’s hard for me to believe more than 10% isn’t possible. With insulation you could increase the temperature difference and with it the efficiency. If you could get up to twenty percent then you would only need 5 times or between 430 to 3,000 tons. If you could manage 30 percent then you would only need between 287 and 2,000 tons. Now let's consider bunker fuel masses.A cargo ship going from Europe to the Us takes http://www.quora.com/Approximately-how-long-does-it-take-for-a-cargo-ship-to-go-from-Europe-to-the-USA depending on speed. Note that fuel use also varies with speed. So we get a range of...50 tons/day * 9 days = 450 tons

350 tons/day * 7 days = 2,450 tonsSo the range for bunker fuel is 450 to 2,450 tons.Compared to Am-241…

10% between 860 to 6,000 tons

20% between 430 to 3,000 tons

30% between 287 and 2,000 tonsThese masses seem to be in the same ballpark to me. If I made some mistakes please point them out.

 

[mfb]

Hmm, I relied on the values @QuantumPion used, apparently they are a factor 1000 too low. Okay, total mass would be acceptable. You still have the problem that you cannot have a single hot place where you burn fuel, because your whole fuel produces the heat throughout its volume. The large scale of a ship would make more than 10% efficiency plausible, but probably not the 50% the burning of fossile fuels gives.

And where do we get it from?
"A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes" according to Wikipedia - let's assume we just get the isotope we want and not the longer-living one - so we have to multiply those ~3000 tons by 10000 to get the initial fuel we have to burn to power a single ship from americium in its nuclear waste: ~30 megatons.

At ~40GWd/t that gives 1,200,000 TWd or 200 times the energy consumption of the world 2010 that has to be processed in nuclear power plants. If we satisfy the whole energy demand of the world (not just electricity, also transportation and heating) with nuclear power, and do so for 200 years, then we have enough americium to power a single ship. Well, not exactly, because 200 years is not short compared to the lifetime.

 

[Evanish]

Hmm, I relied on the values @QuantumPion used, apparently they are a factor 1000 too low. Okay, total mass would be acceptable. You still have the problem that you cannot have a single hot place where you burn fuel, because your whole fuel produces the heat throughout its volume. The large scale of a ship would make more than 10% efficiency plausible, but probably not the 50% the burning of fossile fuels gives.

And where do we get it from?
"A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes" according to Wikipedia - let's assume we just get the isotope we want and not the longer-living one - so we have to multiply those ~3000 tons by 10000 to get the initial fuel we have to burn to power a single ship from americium in its nuclear waste: ~30 megatons.

At ~40GWd/t that gives 1,200,000 TWd or 200 times the energy consumption of the world 2010 that has to be processed in nuclear power plants. If we satisfy the whole energy demand of the world (not just electricity, also transportation and heating) with nuclear power, and do so for 200 years, then we have enough americium to power a single ship. Well, not exactly, because 200 years is not short compared to the lifetime.

The whole fuel produces heat through its volume which is why you need some kind of coolant circulating through the fuel helping to keep the heat averaged out over the whole volume. Then you only remove heat from one non insulated area, but that heat is quickly replaced by heat from the whole volume by the circulating coolant. I don't imagine you would get 50%. Maybe 30%. It seems Am-241 wouldn't work, but I'm guessing you could do it with Sr-90. It has higher power density and a lot more of it is produced by nuclear reactors (the fission product yield from U-235 is 5.7%, from U-233 6.6%, but from Pu-239 only 2.0%).

 

[mheslep]

Well, a ship always has cooling water around while operational. 7.5 MW is ~20kg/s heated to boiling temperature or ~4kg/s boiled away.

Not enormous cooling volumes, granted. If existing LWR nuclear plant pumps are any guide, scaling down to 7.5 MW heat generation requires only a ~30HP pump, though it (or its backup) needs to be running continuously for the life of the vessel.

 

[mheslep]

I'm guessing you could do it with Sr-90. It has higher power density and a lot more of it is produced by nuclear reactors (the fission product yield from U-235 is 5.7%, from U-233 6.6%, but from Pu-239 only 2.0%).

Finding sufficient Sr-90 is also going to be impractical. If it were coming out of reactors in sufficient volume, the spent fuel casks would molten slag heaps and not merely warm to the touch cylinders.

 

[nikkkom]

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

Sr-90 has a specific activity of 5.21 TBq/g. 100 tons of Sr-90 is about 500 exabequerels of activity.

It's unsafe to have so much activity on a vessel which can sink, be hijacked or sabotaged.

 

[Evanish]

Ok, I surrender already. I still think ships need to be nuclear powered. I hope fission reactors replace bunker fuel in the near future.