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Comment by wolverine876 | Hacker News Reader

Why go through the expense of engineering and manufacturing a bomb, and then use it for a different purpose? If you want an electricity generator, engineer and manufacture for that. Or are you thinking of using excess bombs?

I want to say that fusion bombs require more energy input than they output, because until recently nobody has been able to create fusion reactions otherwise (and now only in a very limited way). But is that true? Fusion bombs release an incredible amount of energy. Maybe it's that they release it all at once, but still, how does that equation work out?

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> Why go through the expense of...

Good question. I should have asked strictly about fusion bombs. These being a known-working example of energy production by fusion, why not use them when we can't get 'controlled' fusion working?

From my recollection, even in a fusion bomb, most of the energy comes from the fission. I don't fully remember/understand the physics involved. But apparently the fission reaction triggers the fusion reaction (that part I get) and then that fusion reaction in turns boosts the fission reaction.

So my takeaway is that fission bombs are not very efficient to begin with. You're after all trying to extract as much energy as possible as quickly as possible from a system that is literally self destructing. We do not have that constraint with current fission plants, the plant is designed to run in a steady state for years, the fissile materials stay in place and my intuition is that this significantly helps efficiency.

Yes, the gist is that in a fusion device, you've got a chain reaction where the splitting uranium or plutonium is making neutrons, and those neutrons strike other uranium or plutonium, which makes more neutrons, etc. But you have a very short period of time to take advantage of this chain reaction, because before long, your device has blown apart, so a large percentage of the fuel in a conventional fission device never undergoes fission (the device has blown up before it has a chance). The job of the fusion reaction in a fission-fusion-fission device (how "modern" thermonuclear weapons work) is, in addition to producing energy of its own, to rapidly produce lots of lots of neutrons, which can very quickly fission a second quantity of fissile material.

One of the biggest challenges is that the fusion reaction can't be fully contained. There are particles without electric charge that simply go through your magnetic containment field and smash into the reactor wall, continuously destroying it. You don't really care about preserving the shell of your bomb. You care about preserving the shell of your reactor.

How do fusion researchers handle that problem?

Replacement parts! That may sound flippant but yeah, replace the components/structural members subject to neutron flux before they get too compromised, basically.

This is yet another reason that fusion research is expensive and time consuming, the cost involved in manufacturing these components and time lost in painstaking teardowns and overhauls between test runs.

Let me elaborate on thmsths's answer. The United States developed and tested large "clean" fusion bombs that produced only a very small fraction of their energy from fission. Those bombs could produce raw thermal energy at a cost below any fossil fuel or conventional nuclear reactor. For example, the Housatonic test shot of Operation Dominic in 1962 yielded about 10 megatons and 99.9% of the energy released came from fusion [1].

The problem is that explosions of that magnitude are not usable for controlled energy generation. Confining them underground is impractical, due to the scale of energy release. The largest underground US test was 5 megatons in 1971, and it was so disruptive (induced surface motion equivalent to an earthquake) that it had to be conducted at a remote Alaskan island instead of the usual Nevada test site [2].

For those reasons, Project PACER considered bombs of only up to 50 kilotons. At those sizes (0.5% the energy yield of Housatonic), there is no known design to get 90%+ of the bomb's energy yield from fusion. The efficient fusion designs don't gracefully scale down to that range. And at that much smaller scale where underground confinement was practical, bombs were not actually any cheaper as a source of raw thermal energy. Quoting Wikipedia about the project here, "In a 1975 review of the various Plowshares efforts, the Gulf University Research Consortium (GURC) considered the economics of the PACER concept. They demonstrated that the cost of the nuclear explosives would be the equivalent of fuelling a conventional light-water reactor with uranium fuel at a price of $328 per pound. Prices for yellowcake at that point were $27 a pound."

[1] "Ripple: An Investigation of the World’s Most Advanced High-Yield Thermonuclear Weapon Design" http://web.mit.edu/zoz/Public/jcws_a_01011.pdf

[2] https://en.wikipedia.org/wiki/Cannikin

Great answer, and from others too, thank you all.

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