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Comment by ccleve | Hacker News Reader

A number of these steps become easier if the reactor is physically small. A plant that fits in a shipping container has a way easier path to commercial viability.

Small plants require less capital. They're easier to manufacture. They can iterate faster. They have less environmental impact, and therefore fewer regulatory hurdles. They require less labor to build and a smaller supply chain.

I expect that the engineering is harder, because scale can bring efficiencies. But still, I think the winner is going to be a small device, not a behemoth, if only because a small device can come online years earlier.

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I expect that you lose efficiencies with respect to security. I.e., it's not easy to secure one large nuclear reactor (fission or fusion), but it's a lot easier than securing dozens of small reactors.

If that’s a problem, you can locate hundreds of small reactors together.

There might also be less security needed if you have dozens of smaller reactors. You have to secure the one because losing it affects so much. If losing a couple is not as big a problem, you an rely more on the redundancy of the system than on the redundancy of the component.

This depends on the threat model to some degree, but there's security in knowing that it's vastly harder for a threat to affect multiple targets at once, no matter the threat.

Isn't this just the same scenario as we currently have, just with different underlying technology? Do coal-powered plants have additional security outside of a few private guards?

There's not much security around existing power plants, even nuclear ones. There's vastly more security around a casino or even a large shopping mall. I don't think it's a limiting factor, especially for non-weaponizable fusion reagents.

For fission reactors that is not the case, a fission core can be as small as few liters in volume without losing power, but are artificially kept to less dense forms, such that runaway supercritical events(intended or not) can only result in Fukushima and not a Nagasaki.

That might be different for this fusion device, but for outdated looking fission reactors it’s artificial dilution for NPT purposes and by no way a technological limitation.

Nuclear fission reactors need more security than other power plants because of the implications of stolen fuel/waste, or fallout from sabotage. A fusion reactor like the one this group is working on probably doesn't have as high as security concerns. In fact the only concern might be that it is critical infrastructure(like our entire power grid), and a distribution of smaller reactors would be better for reliability anyway.

Traditional fusion reactors do produce fast neutron radiation, which, in addition to causing problems like weakening the structure (which can be mitigated with better materials, better engineering, and by replacing subcomponents over time) is able to irradiate U238 into PU239.

Drop a chunk of U238 in/near the fusion reactor somewhere, let it absorb stray neutrons, and you'll soon have some plutonium. That's bad for control of nuclear weapons proliferation, which relies on high-energy neutron sources like breeder reactors being difficult to build and expensive.

Not sure about the radiation profile from this system though.

I'm not very knowledgeable about fusion, but it looks like this system is aneutronic fusion, so those concerns would be lessened too. https://en.wikipedia.org/wiki/Aneutronic_fusion#Boron

A simple security camera or hank the security guard could easily guard against someone setting up shop and producing plutonium.

If you have the uranium, couldn’t you afford to bribe Hank?

The general anti-proliferation challenge is harder than that. It includes guarding against the production of weapons material with the collusion of the host country and plant operators. Consider e.g. the degree of assurance needed to provide North Korea with a new power plant that can never aid its nuclear weapons program.

If the security concerns decrease, perhaps they can get to the point of how security is done for ISP and power substations. If so, many smaller nodes across a city are much more feasible.

If these reactors are low heat, not radioactive, and use plentiful fuel, what's the threat model? Is it just worried about availability of power?

"Not Radioactive" is the trick. Even if the fuel isn't hazardous you have lots of radiation being kicked off by the reaction that irradiates the surrounding equipment and the waste products.

The HB11 reaction doesn't produce neutron radiation. I've seen claims that there would be side reactions, generating neutron radiation totaling under 1% of the energy output, but the only source of that I can think of is if deuterium isn't removed from the hydrogen, and we could do that if necessary. It might not be, since neutrons from deuterium fusion are a lot less energetic than D-T neutrons.

The energy output of HB11 consists of fast-moving alpha particles, which is good because that's the energy you capture, along with x-rays. None of this would activate reactor components. The waste product is non-radioactive helium.

It’s not that simple. 1% of even 1MW is a lot of radiation so it comes down to what the walls are made of more than the total neutron flux. ITER style lithium blankets would be recycled for tritium without producing much net waste. Light water nuclear reactors have several feet of water to absorb fast neutrons, which makes a huge difference. The walls of this are a more open question.

So I looked it up, and found there actually are two side reactions from the primary fuels, no deuterium required. In a thermal plasma the resulting neutrons would carry less than 0.2% of the energy output. I don't know what the percentage would be for this reactor, since the fuel isn't a thermal plasma while fusion is happening.

https://en.wikipedia.org/wiki/Aneutronic_fusion#Boron

Neutron energy would be a bit under 3 MeV, comparable to fast fission neutrons but in much lower quantity for the power output.

https://en.wikipedia.org/wiki/Neutron_temperature

Interestingly, that page says the thermal neutrons in a conventional nuclear reactor are better at activating materials than fast neutrons.

According to a presentation I saw by the leader of MIT's fusion program, the inner wall of a D-T tokamak would be activated by the neutrons, but would only need to be stored for a few decades. D-T releases 80% of its energy as 14 MeV neutrons.

According to LPP, which is working on a different boron fusion design, a 20MW reactor would activate the walls with short-lived radiation, but it could be safely opened for maintenance after about twelve hours (iirc).

According to the article, the energy is generated by alpha particles, which are in principle a very dangerous form of radiation. It's not clear to me, though, whether the radiation stops as soon as the fusion does or not.

Substances which continuously emit alpha particles are dangerous.

This is something different. With every laser shot there will be a burst of alpha particles. Then they'll slow down and just be plain old helium. If the direct energy conversion works, they'll get slowed down very quickly.

Substances which continuously emit alpha particles are dangerous.

Only if you eat or inhale them. A sheet of paper will stop alpha particles.

Up until a decade or two ago, tritium--which emits alpha radiation--was commonly on wristwatch dials, precisely because they were not dangerous. Alpha radiation is easily stopped; it's only dangerous if you breathe it (or drink it, if that's even possible).

Tritium emits beta radiation. It would be an interesting trick for an atom to emit radiation with a higher weight than itself.

It might also be worth pointing out that "It must be safe! They even put it on wristwatch dials!" is not, historically, a sound argument.

Fusion shouldn’t need any more security than a coal plant.

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