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I don't drop into this sub too often because my interest is mostly in fusion as a power source, and questions on the topic typically come up about every month or so. By the time I see them, questions on the topic have almost always been closed, often as "opinion based", and I can't answer them.

The problem is that the questions are perfectly fine. Consider the latest example, which asks if there are accident scenarios in fusion similar to Chernobyl and Fukushima. This was closed as "opinion based", apparently only moments after it was posted.

The question, as asked, does not ask for an opinion. It is asking a yes or no question. Are there accident scenarios similar to these examples? There either are or there aren't.

The issue of fusion power safety is the topic of hundreds of reports and has well-known fact-based answers. In particular, the possibility of tritium release is a concern that is going to require confinement systems similar to those used on large fission plants - particularly, those on the CANDU fleet. That is the answer, and it is not opinion based.

Is there something I'm missing here, a historical precedent?

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  • $\begingroup$ My issue is that most questions are way too broad. Which fusion reaction? Which fusion concept? What power output do they imagine? The hazards of a scaled-up ITER concept are different from a scaled-up NIF concept. $\endgroup$
    – Jon Custer
    Feb 28, 2023 at 21:25
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    $\begingroup$ "The hazards of a scaled-up ITER concept are different from a scaled-up NIF concept" - the primary hazard from either is tritium release through lithium fire, which can occur in both designs and most any other. $\endgroup$ Mar 1, 2023 at 1:35
  • $\begingroup$ That is your opinion. I would disagree. $\endgroup$
    – Jon Custer
    Mar 1, 2023 at 1:53
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    $\begingroup$ It is the opinion of the ANS, INS, DOE, LLNL, LANL, ORNL and that many international bodies that all have reports on this stretching back many decades, along with many peer-reviewed articles in major journals. $\endgroup$ Mar 1, 2023 at 2:07
  • $\begingroup$ So reference those, linking to the reports at osti.gov for the various national lab reports. The current private fusion ventures, with very different concepts, likely have yet other possible failure mechanisms. $\endgroup$
    – Jon Custer
    Mar 1, 2023 at 2:34
  • $\begingroup$ If the ITER magnets quench, an energy equivalent to a 747 crashing into the machine is released. If the lasers on NIF don’t fire, nothing happens. Those are two very different things to consider in an FMEA. $\endgroup$
    – Jon Custer
    Mar 1, 2023 at 2:50
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    $\begingroup$ I would love to reference these many (quite readable) reports, but as I note, the question is closed so I can't. If you care to open it, I would be happy to provide an in-depth answer. The private ventures you mention are either using the same physics (TE, CFS, FLF), are too private to really say one way or the other (Helion, ZAP) or likely are not "real" for reasonable definitions of the term (TAE, n-tau). This seems out of the scope of the question, however, which would be limited to the physics of the issue, namely the alternative fuels. $\endgroup$ Mar 1, 2023 at 2:50
  • $\begingroup$ Look, I agree that many experimental physics questions are hastily closed around here. But an analysis of failure modes of a fusion reactor isn’t really physics, and is specific to one fusion reaction in a particular conceptual design. $\endgroup$
    – Jon Custer
    Mar 1, 2023 at 2:54
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    $\begingroup$ That tritium is radioactive, flammable and highly mobile, and thus presents a safety risk in any design using large inventories (even non-fusion) is neither engineering related nor "specific to ... a particular conceptual design". $\endgroup$ Mar 1, 2023 at 3:15
  • $\begingroup$ And the tritium inventory exposed to major failures is quite different in different designs as well. I could throw a NIF capsule away in the garbage and nobody would care. $\endgroup$
    – Jon Custer
    Mar 1, 2023 at 3:30
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    $\begingroup$ "Opinion-based" feels like the wrong objection even for fusion questions not answerable here. Experimental designs/technology are on-topic, while speculation isn't. I suspect we're not only closing too many fusion questions, but misdiagnosing why some should be closed. (Also, often what feels "speculative" is really just divisive among experts, or near-experts.) $\endgroup$
    – J.G.
    Mar 2, 2023 at 11:23
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    $\begingroup$ Tritium inventory in a power-producing ICF design would be several kg in the blanket. And at something like a million dollars per target, I certainly hope someone would care! $\endgroup$ Mar 4, 2023 at 0:13
  • $\begingroup$ as an fyi: you can also edit such questions yourself to ensure they are better phrased and more specific. That will certainly help to avoid closure/make it way more likely that they'll get reopened $\endgroup$
    – glS
    Mar 7, 2023 at 15:16

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  1. "apparently only moments after it was posted" If you hover over the "x hours ago" time indicator almost everywhere on SE sites, you can see a precise timestamp. The question was posted at 11:06 UTC and closed at 11:25 UTC, i.e. 19 minutes after it was posted.

  2. This specific "question" doesn't even really ask a question, it just states that the asker thinks there could be dangers, but even if we interpret it as asking "Are there accident scenarios similar to nuclear fission reactors for fusing?" it is not clear to me this makes the question on-topic. I'd agree it is likely not opinion-based, but this seems like an engineering question as the possible accident scenarios would seem to me to heavily depend on the specifics of how the actual real-world reactor is constructed, and not merely on the general physics of fusion as such.

  3. The historical precedent around fusion is that the early site had a lot of trouble with "cold fusion" - the meta search for "fusion" has almost exclusively years-old discussions around questions focussing on cold fusion. I can't find any major discussion about the topic of fusion in general; it is possible that the "cold fusion" discussions have bled into heightened scrutiny against any post mentioning fusion.

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    $\begingroup$ "could there be" sounds like a question to me, and I try to be lenient in these matters given this is an international forum. I do not see how this is engineering related; fusion reactors using D-T do have inherent radiological risks as part of the fundamental physics. $\endgroup$ Mar 1, 2023 at 2:18

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