5 ms·

What I don't get is the following: If your nuclear waste is that "hot" (highly radioactive), then you're not draining as much energy out of the material. Idea

by pierat 3y ago

What I don't get is the following: If your nuclear waste is that "hot" (highly radioactive), then you're not draining as much energy out of the material. Ideally, the closer to lead you are, the more energy is drained out. And that's what I'd hope to see from the mining required for uranium. Instead, we seem to run it once and then go "whoop have to stop".

acidburnNSA 3y ago
Not sure I'm following. Fission reactors split uranium into smaller atoms like barium and krypton. The remaining energy that comes out as afterglow heat in the repository is a small fraction of what already came out during fission. less than 1% remains an hour after fissioning.
- nabla9 3y ago
  You have the numbers reversed. In spent nuclear fuel 96% of the mass is the remaining uranium is still usable. Spend nuclear fuel (=LEU) used in these reactors have almost all potential energy left. You can use them if you close the nuclear fuel cycle with fast neutron reactors. The downside of it is that they FNR's are also good for making fuel for nuclear weapons.
  - acidburnNSA 3y ago
    I don't have the numbers reversed. I'm talking about when a single atom actually undergoes fission, it releases 93% of its energy promptly as fission product kinetic energy, gamma rays, and neutron kinetic energy. An hour later, another 6% of the energy goes out as delayed betas and gammas. After that, only 1% remains to emerge as heat in the repository. What you're referring to is the fact that some reactors (aka breeder reactors, fast or thermal) can fission a much higher percentage of the total atoms loaded into the reactor, including most of the majority isotope (U-238) in addition to the minority (U-235). I wrote a bit about breeding and recycling here: https://whatisnuclear.com/recycling.html https://whatisnuclear.com/recycling.html
    - phkahler 3y ago
      >> don't have the numbers reversed. I'm talking about when a single atom actually undergoes fission... Well then maybe you should talk about the problem at hand, which is the stuff people are considering "nuclear waste" and wanting to bury.
  - pfdietz 3y ago
    That 96% of the mass is 238U. It's usable in breeder reactors. It's not useful for today's burner reactors. We have far too much 238U for those already; enrichment is the process of stripping out 238U from natural uranium. It would be utterly pointless to recover more 238U for them. We already have great stockpiles of depleted U sitting around. The stuff is not worth very much right now. Reprocessing can recover some plutonium, which can be reused (once more) in MOX fuel. Spent MOX fuel cannot be reprocessed further for use in today's thermal reactors, though.
    - flangola7 3y ago
      Since it if just sitting around can I buy some? I would be happy to have a kilogram cube of DU
      - ben_w 3y ago
        Yes. Legality might depend which country you live in, and the stores I've seen sell only tiny quantities for high prices because they can (only collectors really want it and they will pay), but you can totally just buy some.
Out_of_Characte 3y ago
Yeah, you could reprocess spent fuel but one of the byproducts would be plutonium. So we'd rather bury everything than using the plutonium for other projects because of fear that we can't handle nuclear byproducts.
- phkahler 3y ago
  I see what you did there.
- rektide 3y ago
  This was one of the advantages of Integral Fast Reactor. It had on site pyroprocessing, which doesn't have the same nuclear proliferation risks. https://en.wikipedia.org/wiki/Nuclear_reprocessing#Pyroprocessing https://en.wikipedia.org/wiki/Nuclear_reprocessing#Pyroproce...
photochemsyn 3y ago
There are three issues with trying to get more energy out of hot 'spent' fuel rods (1) the radiation levels increase, making it more dangerous for workers to do maintenance and safety operations (so overall reactor design would need to take this into account), (2) some fission products and transuranics (plutonium etc.) act as neutron poisons (absorbing neutrons rather than generating energy-productive fission events), resulting in reduced reactor output and instability issues as the accumulate in the fuel rods. (3) since you are actually transmuting elements, physical and chemical changes take place resulting in fuel rod corrosion and degradation, swelling and structural changes, etc. which could lead to catastrophic failure by clogging the reactor core and preventing coolant circulation. [edit] note that reprocessing the spent fuel (plutonium recovery) for use in MOX reactors is possible, but since this is also how you run a nuclear weapons program, there are additional concerns and the costs are really high: https://world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/processing-of-used-nuclear-fuel.aspx https://world-nuclear.org/information-library/nuclear-fuel-c...

by pierat 3y ago

acidburnNSA 3y ago

Not sure I'm following. Fission reactors split uranium into smaller atoms like barium and krypton. The remaining energy that comes out as afterglow heat in the repository is a small fraction of what already came out during fission. less than 1% remains an hour after fissioning.

nabla9 3y ago
You have the numbers reversed. In spent nuclear fuel 96% of the mass is the remaining uranium is still usable. Spend nuclear fuel (=LEU) used in these reactors have almost all potential energy left. You can use them if you close the nuclear fuel cycle with fast neutron reactors. The downside of it is that they FNR's are also good for making fuel for nuclear weapons.
- acidburnNSA 3y ago
  I don't have the numbers reversed. I'm talking about when a single atom actually undergoes fission, it releases 93% of its energy promptly as fission product kinetic energy, gamma rays, and neutron kinetic energy. An hour later, another 6% of the energy goes out as delayed betas and gammas. After that, only 1% remains to emerge as heat in the repository. What you're referring to is the fact that some reactors (aka breeder reactors, fast or thermal) can fission a much higher percentage of the total atoms loaded into the reactor, including most of the majority isotope (U-238) in addition to the minority (U-235). I wrote a bit about breeding and recycling here: https://whatisnuclear.com/recycling.html https://whatisnuclear.com/recycling.html
  - phkahler 3y ago
    >> don't have the numbers reversed. I'm talking about when a single atom actually undergoes fission... Well then maybe you should talk about the problem at hand, which is the stuff people are considering "nuclear waste" and wanting to bury.
- pfdietz 3y ago
  That 96% of the mass is 238U. It's usable in breeder reactors. It's not useful for today's burner reactors. We have far too much 238U for those already; enrichment is the process of stripping out 238U from natural uranium. It would be utterly pointless to recover more 238U for them. We already have great stockpiles of depleted U sitting around. The stuff is not worth very much right now. Reprocessing can recover some plutonium, which can be reused (once more) in MOX fuel. Spent MOX fuel cannot be reprocessed further for use in today's thermal reactors, though.
  - flangola7 3y ago
    Since it if just sitting around can I buy some? I would be happy to have a kilogram cube of DU
    - ben_w 3y ago
      Yes. Legality might depend which country you live in, and the stores I've seen sell only tiny quantities for high prices because they can (only collectors really want it and they will pay), but you can totally just buy some.

Out_of_Characte 3y ago

Yeah, you could reprocess spent fuel but one of the byproducts would be plutonium. So we'd rather bury everything than using the plutonium for other projects because of fear that we can't handle nuclear byproducts.

phkahler 3y ago
I see what you did there.
rektide 3y ago
This was one of the advantages of Integral Fast Reactor. It had on site pyroprocessing, which doesn't have the same nuclear proliferation risks. https://en.wikipedia.org/wiki/Nuclear_reprocessing#Pyroprocessing https://en.wikipedia.org/wiki/Nuclear_reprocessing#Pyroproce...

photochemsyn 3y ago

There are three issues with trying to get more energy out of hot 'spent' fuel rods (1) the radiation levels increase, making it more dangerous for workers to do maintenance and safety operations (so overall reactor design would need to take this into account), (2) some fission products and transuranics (plutonium etc.) act as neutron poisons (absorbing neutrons rather than generating energy-productive fission events), resulting in reduced reactor output and instability issues as the accumulate in the fuel rods. (3) since you are actually transmuting elements, physical and chemical changes take place resulting in fuel rod corrosion and degradation, swelling and structural changes, etc. which could lead to catastrophic failure by clogging the reactor core and preventing coolant circulation. [edit] note that reprocessing the spent fuel (plutonium recovery) for use in MOX reactors is possible, but since this is also how you run a nuclear weapons program, there are additional concerns and the costs are really high: https://world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/processing-of-used-nuclear-fuel.aspx https://world-nuclear.org/information-library/nuclear-fuel-c...