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A few points: * Uranium-Plutonium fuel in a closed cycle can burn all the transuranics and have very low radiotoxicity waste as well. * Molten salt fuels are

A few points: * Uranium-Plutonium fuel in a closed cycle can burn all the transuranics and have very low radiotoxicity waste as well. * Molten salt fuels are 10x less dense than typical solid fuels, implying an increase in waste volume. They're also water soluble and frequently liquid. This complicates waste disposal and transport. Not terribly, but still. A good summary reference on radiotoxicity vs. reprocessing is: https://www.iaea.org/publications/8359/assessment-of-partitioning-processes-for-transmutation-of-actinides https://www.iaea.org/publications/8359/assessment-of-partiti...

Gibbon1 7y ago
Something people dislike being pointed out. As far as reprocessing is concerned the best first step in reprocessing spent fuel rods is not fuck with them for a hundred years.
- acidburnNSA 7y ago
  I'd go so far to say we should just dispose of it all. I used to be all in on reprocessing the waste. Nowadays I only want to do it if it's actually cheaper (which I don't think it will be for many decades). Nuclear has economics issues, not uranium sustainability issues. Uranium from seawater with runoff can power the entire planed 10x over for a few hundred million years. It's borderline renewable. We need to be doing things that make nuclear reactors simpler and cheaper to operate, not more complex and expensive. Disposing of the waste from a once-through fuel cycle is totally doable. We just have to let the world know that the Finns are doing it at the Onkalo Repository, we can do it in WIPP, and Deep Isolation LLC is working on a pretty slick deep borehole option.
DennisP 7y ago
I didn't realize molten salt fuel is 10X less dense. But assuming either thorium or fast uranium reactors, they'll get 100X as much energy from the fuel, making 10X less fuel volume than conventional reactors. That doesn't necessarily imply anything about the waste volume. With high burnup, the waste is just fission products, which are entirely different elements than the fuel. One reason high burnup is easier in MSRs is that xenon, a neutron poison, is a gas that just bubbles out of the liquid fuel, instead of being trapped in a solid. With fast uranium or thermal thorium, you also don't have a large amount fuel that can't be fissioned. (Some early MSRs will be thermal uranium, and produce more waste.) For disposal of fission products, the general idea is to mix them into glass and bury them as solid blocks.
pfdietz 7y ago
A uranium-plutonium fuel in a closed cycle can do that if you have fast reactors. Thermal reactors, no. And the higher transuranics become increasingly difficult to burn, since the delayed neutron fraction goes way down. A reactor burning higher transuranics will be right on the knife edge of prompt supercriticality.
- acidburnNSA 7y ago
  > A reactor burning higher transuranics will be right on the knife edge of prompt supercriticality. Meh, that might be a bit of an overstatement. Yes, plutonium-driven fast reactors have a delayed neutron fraction around 0.0035 down from 0.0065 in a U-235-fueled fast reactor. The relatively large delayed neutron fraction from fast fissions of U-238 helps out along the way. You can reduce the radiotoxicity of TRU even if you continue to load some U-238. The inert matrix fuels with no U-238 at all get more interesting from this perspective (also from the Doppler effect), but most people don't envision using these. In fluid systems, more than half of the fuel inventory is off in a heat exchanger out of the core, which means the delayed neutron fraction is cut roughly in half in those systems too (the delayed neutron precursor decays happen where neutron importance is near zero). Plutonium-fueled fast reactors can be operated with good margins of stability.
  - pfdietz 7y ago
    I was talking about elements beyond plutonium.

Gibbon1 7y ago

Something people dislike being pointed out. As far as reprocessing is concerned the best first step in reprocessing spent fuel rods is not fuck with them for a hundred years.

acidburnNSA 7y ago
I'd go so far to say we should just dispose of it all. I used to be all in on reprocessing the waste. Nowadays I only want to do it if it's actually cheaper (which I don't think it will be for many decades). Nuclear has economics issues, not uranium sustainability issues. Uranium from seawater with runoff can power the entire planed 10x over for a few hundred million years. It's borderline renewable. We need to be doing things that make nuclear reactors simpler and cheaper to operate, not more complex and expensive. Disposing of the waste from a once-through fuel cycle is totally doable. We just have to let the world know that the Finns are doing it at the Onkalo Repository, we can do it in WIPP, and Deep Isolation LLC is working on a pretty slick deep borehole option.

DennisP 7y ago

I didn't realize molten salt fuel is 10X less dense. But assuming either thorium or fast uranium reactors, they'll get 100X as much energy from the fuel, making 10X less fuel volume than conventional reactors. That doesn't necessarily imply anything about the waste volume. With high burnup, the waste is just fission products, which are entirely different elements than the fuel. One reason high burnup is easier in MSRs is that xenon, a neutron poison, is a gas that just bubbles out of the liquid fuel, instead of being trapped in a solid. With fast uranium or thermal thorium, you also don't have a large amount fuel that can't be fissioned. (Some early MSRs will be thermal uranium, and produce more waste.) For disposal of fission products, the general idea is to mix them into glass and bury them as solid blocks.

pfdietz 7y ago

A uranium-plutonium fuel in a closed cycle can do that if you have fast reactors. Thermal reactors, no. And the higher transuranics become increasingly difficult to burn, since the delayed neutron fraction goes way down. A reactor burning higher transuranics will be right on the knife edge of prompt supercriticality.

acidburnNSA 7y ago
> A reactor burning higher transuranics will be right on the knife edge of prompt supercriticality. Meh, that might be a bit of an overstatement. Yes, plutonium-driven fast reactors have a delayed neutron fraction around 0.0035 down from 0.0065 in a U-235-fueled fast reactor. The relatively large delayed neutron fraction from fast fissions of U-238 helps out along the way. You can reduce the radiotoxicity of TRU even if you continue to load some U-238. The inert matrix fuels with no U-238 at all get more interesting from this perspective (also from the Doppler effect), but most people don't envision using these. In fluid systems, more than half of the fuel inventory is off in a heat exchanger out of the core, which means the delayed neutron fraction is cut roughly in half in those systems too (the delayed neutron precursor decays happen where neutron importance is near zero). Plutonium-fueled fast reactors can be operated with good margins of stability.
- pfdietz 7y ago
  I was talking about elements beyond plutonium.