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Fission has a few unavoidable issues: long radioactive isotope half-life, weapons proliferation, and the runaway scenario. Fusion has none of these issues plus

Fission has a few unavoidable issues: long radioactive isotope half-life, weapons proliferation, and the runaway scenario. Fusion has none of these issues plus the fuel being abundant. It's the answer to the question "how do we meet the growing energy demands of humans over the next thousand years?". The only real danger is that of a tritium leak, but the short half-life makes the prospect of a leak less concerning.

hairytrog 6y ago
Since the reactors would ostensibly be continuously operating, the danger is the same. 10+ year half life is about as dangerous as 10000 years because you are always generating the waste.
- reissbaker 6y ago
  Er, no. If you generate the same volume of waste per year, but for a fusion reactor the waste stops being a problem within a century and for a fission reactor it takes 10,000 years, at any point in time after the first 100 years, waste from a given fusion reactor will be less prevalent than waste from a given fission reactor. It also doesn't matter that no specific nuclear reactor will have a lifetime of 10,000 years. The problem is that per megawatt of energy generated, fission theoretically creates (much) longer-lived waste than fusion. Over a longer-than-one-hundred-year timeframe, equivalent amounts of energy generation result in vastly different waste carrying costs. Fusion's waste carrying costs are much lower. And obviously that number is even more in favor of fusion if it only takes 10 years. (ITER claims 100 years though: https://www.iter.org/sci/Fusion https://www.iter.org/sci/Fusion)
- willis936 6y ago
  The danger is not the same between fusion and fission. Firstly, 10 years worth of energy is inside a fission reactor and is capable of releasing most of that energy in an instant if not properly controlled. This cannot happen in a fusion reactor. A year's worth of fuel is in a gas tank on the wall and needs absurd conditions to ignite. It cannot happen spontaneously. Secondly, the exhaust is helium-4: a stable isotape of a valuable element. Thirdly, the neutron bombardment in a fusion reactor activate the materials they hit. If they hit lithium then they make tritium: a much needed isotape for fuel in first generation fusion reactors. The other materials they hit are chosen to have half-lives of less than 100 years. So you have a nuclear site that no one's allowed to touch for a while then you can recycle the materials. It's nothing like the transuranium nuclear waste from fission plants.
- roywiggins 6y ago
  If you had a half life of 5 minutes you'd only need to store the waste for a couple hours before burying it somewhere and there's a certain (smallish) amount of high-grade waste at any given moment, no matter how long you run the reactor. 10 years isn't 5 minutes, but it means you just need to keep it secure for a few decades before burying and forgetting it rather than many human lifetimes. Any leaks will be (to some extent) self-cleaning, insofar as they'll decay substantially within a human lifetime, so if you stop the leak you can wait a couple decades and it will have cleaned itself up. That's much better than the long-life stuff fission produces.
morning_gelato 6y ago
Why is the runaway scenario unavoidable for nuclear fission? Pressurized water reactors for example have negative void and temperature coefficients, and from what I've read about potential future reactors designs (e.g. high temperature gas reactors and some of the molten salt reactors) passive/inherent safety is a major selling point.
- willis936 6y ago
  All of those solutions involve better ways at keeping neutron multiplication factor below 1. Fission reactors need fuel that naturally have an eta above 1. Fission reactors are also designed to have large quantities of fuel inside of them. So if the control systems fail, even if those control systems are built-in chemically, then you have a disaster that lasts for timescales that humans would prefer not to be on the table.
  - morning_gelato 6y ago
    What's a scenario where passive safety systems, say those of a HTGR with TRISO fuel, fail and cause it to go prompt critical? I'm genuinely curious about this, as everything I've read suggests this is essentially impossible due to the design of the reactor, fuel, and coolant.
    - willis936 6y ago
      I don’t have an answer. Not knowing failure modes doesn’t mean they don’t exist. HBO’s Chernobyl series highlighted how dangerous surpressing information about fission reactors is. No one knew how the design could fail until it did and then it was painfully obvious. I’m not saying that HTGRs can meltdown nearly as readily as RBMKs, but the risk of the unknown needs to be given respect when the stakes are high. It’s difficult to be sure of safety in complicated systems when the only people with enough technical expertise to fully vet the systems have an interest in their success. I’m not saying it can’t be done, but I think it slows policy down significantly.
      - 6y ago

hairytrog 6y ago

Since the reactors would ostensibly be continuously operating, the danger is the same. 10+ year half life is about as dangerous as 10000 years because you are always generating the waste.

reissbaker 6y ago
Er, no. If you generate the same volume of waste per year, but for a fusion reactor the waste stops being a problem within a century and for a fission reactor it takes 10,000 years, at any point in time after the first 100 years, waste from a given fusion reactor will be less prevalent than waste from a given fission reactor. It also doesn't matter that no specific nuclear reactor will have a lifetime of 10,000 years. The problem is that per megawatt of energy generated, fission theoretically creates (much) longer-lived waste than fusion. Over a longer-than-one-hundred-year timeframe, equivalent amounts of energy generation result in vastly different waste carrying costs. Fusion's waste carrying costs are much lower. And obviously that number is even more in favor of fusion if it only takes 10 years. (ITER claims 100 years though: https://www.iter.org/sci/Fusion https://www.iter.org/sci/Fusion)
willis936 6y ago
The danger is not the same between fusion and fission. Firstly, 10 years worth of energy is inside a fission reactor and is capable of releasing most of that energy in an instant if not properly controlled. This cannot happen in a fusion reactor. A year's worth of fuel is in a gas tank on the wall and needs absurd conditions to ignite. It cannot happen spontaneously. Secondly, the exhaust is helium-4: a stable isotape of a valuable element. Thirdly, the neutron bombardment in a fusion reactor activate the materials they hit. If they hit lithium then they make tritium: a much needed isotape for fuel in first generation fusion reactors. The other materials they hit are chosen to have half-lives of less than 100 years. So you have a nuclear site that no one's allowed to touch for a while then you can recycle the materials. It's nothing like the transuranium nuclear waste from fission plants.
roywiggins 6y ago
If you had a half life of 5 minutes you'd only need to store the waste for a couple hours before burying it somewhere and there's a certain (smallish) amount of high-grade waste at any given moment, no matter how long you run the reactor. 10 years isn't 5 minutes, but it means you just need to keep it secure for a few decades before burying and forgetting it rather than many human lifetimes. Any leaks will be (to some extent) self-cleaning, insofar as they'll decay substantially within a human lifetime, so if you stop the leak you can wait a couple decades and it will have cleaned itself up. That's much better than the long-life stuff fission produces.

morning_gelato 6y ago

Why is the runaway scenario unavoidable for nuclear fission? Pressurized water reactors for example have negative void and temperature coefficients, and from what I've read about potential future reactors designs (e.g. high temperature gas reactors and some of the molten salt reactors) passive/inherent safety is a major selling point.

willis936 6y ago
All of those solutions involve better ways at keeping neutron multiplication factor below 1. Fission reactors need fuel that naturally have an eta above 1. Fission reactors are also designed to have large quantities of fuel inside of them. So if the control systems fail, even if those control systems are built-in chemically, then you have a disaster that lasts for timescales that humans would prefer not to be on the table.
- morning_gelato 6y ago
  What's a scenario where passive safety systems, say those of a HTGR with TRISO fuel, fail and cause it to go prompt critical? I'm genuinely curious about this, as everything I've read suggests this is essentially impossible due to the design of the reactor, fuel, and coolant.
  - willis936 6y ago
    I don’t have an answer. Not knowing failure modes doesn’t mean they don’t exist. HBO’s Chernobyl series highlighted how dangerous surpressing information about fission reactors is. No one knew how the design could fail until it did and then it was painfully obvious. I’m not saying that HTGRs can meltdown nearly as readily as RBMKs, but the risk of the unknown needs to be given respect when the stakes are high. It’s difficult to be sure of safety in complicated systems when the only people with enough technical expertise to fully vet the systems have an interest in their success. I’m not saying it can’t be done, but I think it slows policy down significantly.
    - 6y ago