5 ms·

> Fast reactors can eliminate almost all the long-term waste, which consists of transuranic isotopes that fast reactors use for fuel. Not every transuran is vi

> Fast reactors can eliminate almost all the long-term waste, which consists of transuranic isotopes that fast reactors use for fuel. Not every transuran is viable fuel. Some of the stuff is actually actively getting in the way of an efficient fission chain reaction. The key benefit of fast breeder reactors is, that you can fuel them with stuff that's more abundant on Earth, so you're not getting into a fuel shortage (our supplies of U235 are quite limited, some 70 years or so, using conventional fission reactors). So you need to either go through a fast U238→Pu239 or a Th232→U233 process if you want to make use of the abundant stuff. Oh, and most of the fission products are not very pleasant either and you can't efficiently burn them in any way whatsoever. Burner Accelerators? I've got a few things for you to look up: Penetration epth, Bragg peak, small volume active zone. > The rest of the waste is fission products. Encase them in glass and bury them, they're back to the radioactivity of the original ore in a couple centuries. Unfortunately not. The generation zero of nuclides will decay in that timeframe. But all the other stuff to follow, less radioactive, but still dangerous, takes much more time to decay; and those daughter generation nuclides is where the headaches start. And it's not just the physics that's a problem, it's also chemistry. Some of the elements that are born are quite aggressive, chemically.

DennisP 11y ago
A large majority of the waste though. Most of the long-term radioactivity is from plutonium isotopes, all of which work in fast reactors just fine. I didn't claim all radioactivity would be gone, just that overall it will be comparable to uranium ore in a reasonable timeframe. If we really want to break up all the transuranics, neutrons from D-T fusion would crack them apart handily. We already have 70% energy return with D-T fusion; a hybrid fission-fusion reactor would probably achieve net power just with that.
- datenwolf 11y ago
  > I didn't claim all radioactivity would be gone, just that overall it will be comparable to uranium ore in a reasonable timeframe. As you correctly pointed out, radioactivity is inversely proportional to half-life. Or in other words, given the same chemical concentration the net half-life of the remaining nuclear waste would be on the same timescale as the of uranium ore. The most abundant uranium isotope us U238, with a half life of about 703Ma. Here's a list of half-lifes of the most common fission and spallation products: 99Tc 211ka (side note: used as radioactive tracer nuclear medicine) 126Sn 230ka 79Se 327ka 93Zr 1.53Ma 135Cs 2.3Ma 107Pd 6.5Ma 129I 15.7Ma Three things to note here: - Those are all isotopes, which you can't actively "break down" further without making things worse. A couple of those above are very efficient at neutron capture, thereby getting slightly more radioactive, still not enough to significantly shorten the half-life. - Those are still about 1000 to 20000 times more radioactive than U238, so it's a far cry to call that "comparable to uranium in a reasonable timeframe" - We're definitely in the Ma timescale here. Say you want that nuclear waste decay down to 0.1% of the original material. How long does it take? Easy enough to figure out: 0.001 > 2^-n → n = log2(1000) =~= 10. Or in other words, it takes 10 half life periods for the stuff to break down to a concentration, that's still far from being homeopathic. When it comes to technology IMHO a reasonable timeframe to deal with technological aftermath is 50 years (and not more). Anything beyond that and it will become a clusterfuck simply due to "human nature". Heck, today we've even got problem erecting structures that will last longer than 50 years without requiring major maintenance. And if you're honest about it, except for the few outliers of historical significance (the pyramids, port walls built by the Romans with their "super concrete" we still haven't reverse engineered, and such) we humans are quite terrible in building stuff that can go by without maintenance for more than a few decades. And that's the core problem in nuclear waste management. A problem I'd like to point out, we have to solve, because there's already plenty of nuclear waste around. Oh and then there's of course the problem how to treat and prepare the highly radioactive waste for long term storage in the first place. One of the proposed methods (and already employed by some nuclear energy companies) is to melt it into glass. But here's the problem: Glass is a quite complex material and its mechanical properties largely depend on its chemical composition. Radioactivity however means, that the chemistry is changing over time. Not to mention that some stages of the decay chain have a very high vapour pressure or are gaseous at ambient temperature/pressure. Which in turn means: Your once solid and smooth slab of glass will form lots of cracks, may even break down at some spots into fine sand (which could become air or waterborne). I did my graduation thesis at an accelerator facility where also some aspects of nuclear waste treatment were researched. If you see some of the principal investigators in that area doing research on the renewable energy stock market or even having invested in family members green energy startups and advertising for them, that's a very strong signal.
  - datenwolf 11y ago
    > given the same chemical concentration the net half-life of the remaining nuclear waste would be on the same timescale as the of uranium ore. I dropped a few words there. It should read: given the same chemical concentration and demanding the radioactivite density (decays per time per mass) of the remaining nuclear waste to be on a similar level as of that of uranium ore it has to have either a similar net half-life or be appropriately diluted (and uranium ore already is quite diluted by itself)

DennisP 11y ago

A large majority of the waste though. Most of the long-term radioactivity is from plutonium isotopes, all of which work in fast reactors just fine. I didn't claim all radioactivity would be gone, just that overall it will be comparable to uranium ore in a reasonable timeframe. If we really want to break up all the transuranics, neutrons from D-T fusion would crack them apart handily. We already have 70% energy return with D-T fusion; a hybrid fission-fusion reactor would probably achieve net power just with that.

datenwolf 11y ago
> I didn't claim all radioactivity would be gone, just that overall it will be comparable to uranium ore in a reasonable timeframe. As you correctly pointed out, radioactivity is inversely proportional to half-life. Or in other words, given the same chemical concentration the net half-life of the remaining nuclear waste would be on the same timescale as the of uranium ore. The most abundant uranium isotope us U238, with a half life of about 703Ma. Here's a list of half-lifes of the most common fission and spallation products: 99Tc 211ka (side note: used as radioactive tracer nuclear medicine) 126Sn 230ka 79Se 327ka 93Zr 1.53Ma 135Cs 2.3Ma 107Pd 6.5Ma 129I 15.7Ma Three things to note here: - Those are all isotopes, which you can't actively "break down" further without making things worse. A couple of those above are very efficient at neutron capture, thereby getting slightly more radioactive, still not enough to significantly shorten the half-life. - Those are still about 1000 to 20000 times more radioactive than U238, so it's a far cry to call that "comparable to uranium in a reasonable timeframe" - We're definitely in the Ma timescale here. Say you want that nuclear waste decay down to 0.1% of the original material. How long does it take? Easy enough to figure out: 0.001 > 2^-n → n = log2(1000) =~= 10. Or in other words, it takes 10 half life periods for the stuff to break down to a concentration, that's still far from being homeopathic. When it comes to technology IMHO a reasonable timeframe to deal with technological aftermath is 50 years (and not more). Anything beyond that and it will become a clusterfuck simply due to "human nature". Heck, today we've even got problem erecting structures that will last longer than 50 years without requiring major maintenance. And if you're honest about it, except for the few outliers of historical significance (the pyramids, port walls built by the Romans with their "super concrete" we still haven't reverse engineered, and such) we humans are quite terrible in building stuff that can go by without maintenance for more than a few decades. And that's the core problem in nuclear waste management. A problem I'd like to point out, we have to solve, because there's already plenty of nuclear waste around. Oh and then there's of course the problem how to treat and prepare the highly radioactive waste for long term storage in the first place. One of the proposed methods (and already employed by some nuclear energy companies) is to melt it into glass. But here's the problem: Glass is a quite complex material and its mechanical properties largely depend on its chemical composition. Radioactivity however means, that the chemistry is changing over time. Not to mention that some stages of the decay chain have a very high vapour pressure or are gaseous at ambient temperature/pressure. Which in turn means: Your once solid and smooth slab of glass will form lots of cracks, may even break down at some spots into fine sand (which could become air or waterborne). I did my graduation thesis at an accelerator facility where also some aspects of nuclear waste treatment were researched. If you see some of the principal investigators in that area doing research on the renewable energy stock market or even having invested in family members green energy startups and advertising for them, that's a very strong signal.
- datenwolf 11y ago
  > given the same chemical concentration the net half-life of the remaining nuclear waste would be on the same timescale as the of uranium ore. I dropped a few words there. It should read: given the same chemical concentration and demanding the radioactivite density (decays per time per mass) of the remaining nuclear waste to be on a similar level as of that of uranium ore it has to have either a similar net half-life or be appropriately diluted (and uranium ore already is quite diluted by itself)