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Tritium is only necessary because the magnetic confinement is poor. If we assume that it gets much better then it becomes economical to use deuterium fuel.

Also, I question the explosion claim. Without confinement, the energy drops below the threshold necessary for fusion and the reaction stops. An explosion would require a self sustaining reaction of some kind and I'm just not sure where that would come from. It's not like there's much inertia in it.

But you're right about the neutrons. Maybe you can get away with a rice maker sized reaction chamber, but you're going to want it shielded in something the size of a couch. Probably best to just put it in a deep hole and pump water down there. Then you can provide the neighborhood with steam for heating and electricity for everything else.

> Also, I question the explosion claim. Without confinement, the energy drops below the threshold necessary for fusion and the reaction stops

I am not a physicist

I expect the sudden failure of magnetic confinement would lead to a chemical explosion because of super heated plasma

The amount of plasma in such a reactor would likely be fractions of a milligram.

What would scare me more is an asymmetric loss of the magnetic field, which would produce a huge and not-designed-for mechanical stress, potentially leading to pieces of the superstructure breaking off and flying at high speeds, pushed by the very strong magnetic field.

Also, a fraction of a milligram of plasma, suddenly free to roam about the entire to toroid rather than confined to a small part of it would lose temperature rapidly.

But yeah, magnets throwing around chunks of each other, and chunks of radioactive shielding, would be an unpleasant thing.

Not to necessarily feed into fear mongering, but: an explosion only needs pressure. A can of compressed air can explode. And residual heat from the reaction helps, too. So the question is whether there's enough hot gas under pressure to push past the volume of the reaction chamber...

- "Tritium is only necessary because the magnetic confinement is poor."

Startup hype notwithstanding, the other fusion reactions are only incremental improvements over D-T, in terms of emitting less neutron radiation. From the standpoint of radiation protection it's all utterly moot—you'll never have a fusion reactor in a "rice-maker" to "power homes and vehicles". The amounts of radiation are absurdly extreme, and of an exotic kind that's really, really, unshieldable. (Infamously, the whole point [0] of the neutron bomb was a Cold-War tactical antitank weapon. Something that heavy steel tanks, envisioned to be in a nuclear land war in the Fulda Gap or whereever, would have no chance of surviving—not even with all the efforts the superpowers threw at the military challenge).

A sobering fact for those unfamiliar with nuclear physics. The point of less-neutronic nuclear fusion reactions isn't to render them mild enough not to disintegrate humans. It's to make them mild enough to not disintegrate steel [1].

[0] https://en.wikipedia.org/wiki/Neutron_bomb#Use

[1] https://www.nature.com/articles/nphys3735

As much as a fallout nuclear energy world would be cool (with cars running on fusion), I think in our lifetime it’ll make more sense to just have the power plants run off fusion and delivery energy via the grid

We’re already seeing very promising improvements in batteries (some without even needing lithium), so probably sooner we’ll get wildly high battery energy densities

I agree, it's just not as practical as wind and solar and grid-connected energy storage for 95% of the things you'd want it for.

But the reasons are that its complex and expensive, not that it's accidentally a bomb.

> Plus, if the magnets fail while the fusion reaction is happening, then the superheated plasma will violently explode in all directions, killing anyone nearby, and spreading the radioactive remnants of the vessel all around.

No. Just...no. This is entirely wrong. The density of any proposed fusion plasma is 250,000 times less then the earth's atmosphere[1]. Fusion plasma would be crushed the surrounding air rushing in, not "explode".

Fusion plasma's are incredibly light, and incredibly thin. In the event of a full magnet quench, the only significant damage would be from magnetic quench boil off of coolant...which is a designed for failure mode, and would vent either liquid nitrogen (in HTS designs) or liquid helium in LTS designs like ITER.

[1] https://www.ipp.mpg.de/15144/zuendbedingungen

People always think that the heart of a big machine will be some massive block of dense metal. The heart of these machines is, for failure mode purposes, an empty void. Be more afraid of the associated refrigeration plant. That is the bit more likely to explode.

True, very thin but very hot

Yes It would initially collapse, but still a lot of heat to dissipate, and I would expect an explosion

The fact that it's very hot doesn't mean an explosion, especially if it's also very thin. The total amount of heat energy actually stored in a theoretical miniature fusion reaction needn't be much.

Not that much heat, because the heat comes from an ongoing process, rather then being extracted from a dense thermal mass of something under pressure. Intuition about mass, flow and inertia can be misleading. If there's even a tiny leak or magnetic wobble, the plasma just goes out instantly. The cooling blanket is at a steady state with the reaction so once the plasma goes out, excess heat from the walls is very quickly absorbed.

The thermal flux in the wall is high when the reaction is happening (multiple megawatts per square metre), but the temperature where the beryllium face is bonded to the copper-steel backing structure is below 300 Celsis, so there's not a huge heat storage of immense temperature in the structure.

The plasma itself is 1 gram of mass at 150 million degrees. The tokamak full of sea-level air would be 1 million grams of air, more or less. So it won't superheat 800 cubic metres of air into an explosion, even if there was a huge leak and pressure equalised very fast.

The bigger explosion hazard is the 800 cubic metre vacuum chamber imploding (unlikely, it's only 1 atmosphere difference, it's not a submersible), a magnet quench (which does store a lot of energy that can be released at once, just ask CERN, though the result isn't exactly catastrophic because it's part of the design, but it might not be good for the magnet) or some mundane but dramatic electrical, hydraulic, pneumatic or steam system failure that could happen in a coal power station. Future reactors may have a cooling blanket that could leak liquid lithium.

This is why you fuse He3-He3, no neutron emissions/everything is charged, much easier to contain.

The problem is getting it hot enough.

This is what you get when you learn fusion physics from sci fi comic books

I learned fusion from Doc Brown -- you get me some banana peels and stale beer and we're in business!