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That's a good link. But a fairly shallow dismissal. Both yours (you are quite an active participant on HN, so you are very likely aware of the site's guideline

That's a good link. But a fairly shallow dismissal. Both yours (you are quite an active participant on HN, so you are very likely aware of the site's guidelines) and theirs (the authors of the presentation). The wikipedia article on fusion-fission presents the downsides of the idea much better I think. The link you provided appears to be some type of high-school debate level of argumentation. The final report issued by MIT [1] at that 2009 fusion-fission forum is much more balanced (and more informative). But overall, the negative tone of the report rests on the assumption that fusion is "just around the corner". It is not. Their argument is roughly: we need research both for a pure fusion reactor and for a fusion-fission hybrid. Let's not get distracted with the fusion-fission, and invest fully in fusion. Which they call "the grand challenge" and "transformative". The thing is, we don't need "grand challenges" and "transformative" things for their own sake. We need stuff that works. Fusion-fission reactor may be complex, but is clearly achievable. Fusion by itself, not in the foreseeable future. [1] http://web.mit.edu/fusion-fission/Hybrid_Report_Final.pdf http://web.mit.edu/fusion-fission/Hybrid_Report_Final.pdf

pfdietz 4y ago
That link was on slides presented at a professional workshop. If you judge slides by whether they are flashy ppt files, well bless your heart. I agree that the slides maybe slanted too much in the direction of fusion being just around the corner. But that hardly helps hybrid reactors! If fusion is more problematic that promised, that tilts the table even more toward just using fission, with no fusion component whatsoever. The major problem of fission is not safety, or waste disposal, or (near term) fuel availability. The major problem is cost. It's difficult to see how hybrid reactors would improve cost. As the slides say, the focus on hybrids is more of matter of "how can we make our fusion work appear more relevant", not "what would customers actually want?"
- credit_guy 4y ago
  Maybe a little analogy would help: well before thermonuclear bombs were developed, people started using fusion in the so called "boosted bombs" [1]. In that design (which dominates to this day), the implosion of the Plutonium pit triggers a fission chain reaction, but it also triggers the fusion of a small amount of D+T mix. That fusion adds a tiny bit of energy to the whole explosion, but it also produces a huge amount of fast neutrons, which increases the number of Pu nuclei that split by about 30%. Of course, we eventually had above-breakeven fusion in nuclear weapons too. But the below-breakeven was and continues to be useful (although some people debate the entire idea of usefulness in the context of nuclear weapons). > The major problem of fission is not safety, or waste disposal, or (near term) fuel availability. The major problem is cost. You sound like one of those opponents of nuclear reactors who doesn't understand that cost is driven by regulations, which in turn are driven by safety. Increasing safety will decrease costs. But that's not all. A fusion-fission reactor can burn much more of the Uranium/Plutonium/Thorium core. While the fuel cost is not that important in the economics of a nuclear reactor, the costs associated with refueling are. If you don't need to refuel every 18 months, but rather every 15 years, then this changes drastically the economics. Of course, producing less waste is great too. Now, you can say (and you said) that fast fission reactors can achieve the same result. The Department of Energy recently let the startup Oklo operate an experimental liquid metal fast reactor [2]. That's great, and I really hope Oklo will succeed getting their NRC approval the second time around. But pure fission reactors have this problem with criticality. The presentation you linked to dismissed this Criticality control, while important, does not dominate fission reactor safety. This is a very strange thing to say. The safety of fission reactors is dominated by what is in people's mind. And two things are in people's mind: Chernobyl and Fukushima. The first got critical, and the second one may have. The world just does not have the appetite for another one of these. Consequently, the regulators are very concerned with criticality. Take a look at NRC's assessment of NuScale's design [3], and check how many times they talk about the control rods and their role in criticality. [1] https://en.wikipedia.org/wiki/Boosted_fission_weapon https://en.wikipedia.org/wiki/Boosted_fission_weapon [2] https://www.energy.gov/ne/articles/argonne-adds-new-testing-capability-liquid-metal-fast-reactors https://www.energy.gov/ne/articles/argonne-adds-new-testing-... [3] https://www.nrc.gov/docs/ML2022/ML20224A508.pdf https://www.nrc.gov/docs/ML2022/ML20224A508.pdf

pfdietz 4y ago

That link was on slides presented at a professional workshop. If you judge slides by whether they are flashy ppt files, well bless your heart. I agree that the slides maybe slanted too much in the direction of fusion being just around the corner. But that hardly helps hybrid reactors! If fusion is more problematic that promised, that tilts the table even more toward just using fission, with no fusion component whatsoever. The major problem of fission is not safety, or waste disposal, or (near term) fuel availability. The major problem is cost. It's difficult to see how hybrid reactors would improve cost. As the slides say, the focus on hybrids is more of matter of "how can we make our fusion work appear more relevant", not "what would customers actually want?"

credit_guy 4y ago
Maybe a little analogy would help: well before thermonuclear bombs were developed, people started using fusion in the so called "boosted bombs" [1]. In that design (which dominates to this day), the implosion of the Plutonium pit triggers a fission chain reaction, but it also triggers the fusion of a small amount of D+T mix. That fusion adds a tiny bit of energy to the whole explosion, but it also produces a huge amount of fast neutrons, which increases the number of Pu nuclei that split by about 30%. Of course, we eventually had above-breakeven fusion in nuclear weapons too. But the below-breakeven was and continues to be useful (although some people debate the entire idea of usefulness in the context of nuclear weapons). > The major problem of fission is not safety, or waste disposal, or (near term) fuel availability. The major problem is cost. You sound like one of those opponents of nuclear reactors who doesn't understand that cost is driven by regulations, which in turn are driven by safety. Increasing safety will decrease costs. But that's not all. A fusion-fission reactor can burn much more of the Uranium/Plutonium/Thorium core. While the fuel cost is not that important in the economics of a nuclear reactor, the costs associated with refueling are. If you don't need to refuel every 18 months, but rather every 15 years, then this changes drastically the economics. Of course, producing less waste is great too. Now, you can say (and you said) that fast fission reactors can achieve the same result. The Department of Energy recently let the startup Oklo operate an experimental liquid metal fast reactor [2]. That's great, and I really hope Oklo will succeed getting their NRC approval the second time around. But pure fission reactors have this problem with criticality. The presentation you linked to dismissed this Criticality control, while important, does not dominate fission reactor safety. This is a very strange thing to say. The safety of fission reactors is dominated by what is in people's mind. And two things are in people's mind: Chernobyl and Fukushima. The first got critical, and the second one may have. The world just does not have the appetite for another one of these. Consequently, the regulators are very concerned with criticality. Take a look at NRC's assessment of NuScale's design [3], and check how many times they talk about the control rods and their role in criticality. [1] https://en.wikipedia.org/wiki/Boosted_fission_weapon https://en.wikipedia.org/wiki/Boosted_fission_weapon [2] https://www.energy.gov/ne/articles/argonne-adds-new-testing-capability-liquid-metal-fast-reactors https://www.energy.gov/ne/articles/argonne-adds-new-testing-... [3] https://www.nrc.gov/docs/ML2022/ML20224A508.pdf https://www.nrc.gov/docs/ML2022/ML20224A508.pdf