[JumpCrisscross]

> Fusion power plants can't "melt down"

Eh, a core-containment failure (in any magnetically-contained system) would involve superheated hydrogen getting friendly with oxygen. That, in turn, would give neutron-impregnated barrier materials a free ride on propellant. It's not strictly a melt down. But it's in the same practical category of failure.

[gmueckl]

Ths is a massive misunderstanding of the technology. First of all, the amount of hydrogen in the reactor is tiny. The magnetic confinement severely limits the density of the plasma. The inner containment vessel is a ultra high vacuum chamber. The chemical energy that would be released by a reaction between the hydrogen in the reactor amd oxygen from the air would be less than what is released by popping a hydrogen filled balloon with a lighter.

The truly concerning failure modes would be related to release of radiation or activated materials. But that would require damaging the reactor in ways that the reactor is incapable of imparting on itself.

Overall, the technology is remarkably safe.

[JumpCrisscross]

> chemical energy that would be released by a reaction between the hydrogen in the reactor amd oxygen from the air would be less than what is released by popping a hydrogen filled balloon with a lighter

Thanks for the correction. If you're breeding lithium in the walls, might that be an incendiary concern?

[rnhmjoj]

The breeding blanket is entirely contained inside a vacuum vessel, so there isn't any oxygen to react with. Also, the are many blanket designs, but the lithium is never present in its elemental form (precisely because it would be very reactive), but in a stable chemical bond with some neutron multiplier (like lithium-lead alloys or beryllium ceramics). In some design the lithium is even immersed in the coolant itself, which is high pressure helium, so it's not going to ignite in any reasonable way.

[JumpCrisscross]

> breeding blanket is entirely contained inside a vacuum vessel, so there isn't any oxygen to react with

When the vessel works. If the vessel breaches, that lithium could ignite. Note a showstopper. But I suppose a risk to be thought about by the engineers (probably not by policymakers).

[leonidasrup]

Commonwealth Fusion Systems plan to use lithium in salt form FLiBe, a molten salt made from a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2). It does not violently react with air or water.

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

[pixl97]

There seems to be a number of different prototypes of blankets, but the average operating temperature seems to be 300-700C. Adding oxygen to some of these designs while that hot may cause metal burning. This said, many of them are ceramic designs and would likely resist combustion.

With all that said, it seems to be way less 'dangerous' material than would be in your average nuclear reactor, making it more of an industrial accident versus a planet contaminating mess.

[hunterpayne]

You are ignoring that the plasma would ignite the O2 in the air. You are also ignoring what happens when several hundred MW of energy (at about 1,000,000C) under pressure is released instantly. Anytime you have a powerplant with enough energy to be economically viable, releasing that energy at once will be a problem. Even FF PPs can explode quite violently.

[Dylan16807]

> You are ignoring that the plasma would ignite the O2 in the air.

What does this even mean?

> You are also ignoring what happens when several hundred MW of energy (at about 1,000,000C) under pressure is released instantly.

If you have a gram of hydrogen at a million degrees, it can continue putting out several hundred MW for about a fiftieth of a second.

Even if it somehow gets outside the machine with no heat loss to the structure, by the time it mixes with a few cubic meters of air it'll be down to 1000C or less.

[Symmetry]

There's only a few grams of hydrogen in the reactor's plasma, it's reaction with oxygen wouldn't be much more exciting than just losing containment. There are engineering challenges that have to be addressed but no worse than the 6 MW research reactor I used to walk by every day to my college classes in the middle of a dense city.

The proliferation risk of someone using the neutron flux to produce an atomic or dirty bomb are real but that exists no matter where it is.

[leonidasrup]

I think the proliferation risks will be in future the reason, independent of technological obstacles or costs, why US will not allow to build fusion power plants in all countries around the world.

Hybrid nuclear fusion–fission power plants have been already proposed and studied in theory.

"In general terms, the hybrid is very similar in concept to the fast breeder reactor, which uses a compact high-energy fission core in place of the hybrid's fusion core. Another similar concept is the accelerator-driven subcritical reactor, which uses a particle accelerator to provide the neutrons instead of nuclear reactions."

https://en.wikipedia.org/wiki/Nuclear_fusion–fission_hybrid

[bleepblap]

> Hybrid nuclear fusion–fission power plants have been already proposed and studied in theory.

I have a hand-wavy hard sci-fi universe I've been rolling around my head for years and I eventually came to the conclusion that fission-fusion drives would be really handy for spacecraft, since it would be much easier to start a fission reaction in a cold/dark ship than fusion because of the power requirements. Otherwise you need some other way to generate 10s or 100s of MW to start the fusion reaction.

[leonidasrup]

Using the energy of fission for spacecraft propulsion has been studied and some prototypes have been already constructed. Most of these nuclear engines should be used only outside of Earths atmosphere.

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

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

Most interesting and promising are direct nuclear propulsions, like fission-fragment rockets.

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

[DennisP]

Probably not all countries, but any NPT signatory has the right to build nuclear power plants, they just have to submit to inspections.

[leonidasrup]

This is the basic promise behind the "Treaty on the Non-Proliferation of Nuclear Weapons" (NPT).

https://en.wikipedia.org/wiki/Treaty_on_the_Non-Proliferatio...

In the text of the treaty there are promises to NPT signatories, such as:

Article IV "1. Nothing in this Treaty shall be interpreted as affecting the inalienable right of all the Parties to the Treaty to develop research, production and use of nuclear energy for peaceful purposes without discrimination and in conformity with articles I and II of this Treaty."

https://en.wikisource.org/wiki/Nuclear_Non-Proliferation_Tre...

But the hard reality of U.S. nuclear politics in regard to other countries can be read here:

"The Nuclear Fuel Cycle and The Bush Nonproliferation Initiative"

https://www.iaea.org/sites/default/files/neff.pdf

"The world’s leading nuclear exporters should ensure that states have reliable access at reasonable cost to fuel for civilian reactors, so long as those states renounce enrichment and reprocessing."

"The 40 nations of the Nuclear Suppliers Group should refuse to sell enrichment and reprocessing equipment and technologies to any state that does not already possess full-scale, functioning enrichment and reprocessing plants."

For example when United Arab Emirates wanted to build the Barakah nuclear power plant, (supplied by Korea Electric Power Corporation, not by an U.S. company), it had to sign an the Section 123 Agreement with United States of America. As part of the agreement, the UAE committed to forgo domestic uranium enrichment and reprocessing of spent fuel.

https://en.wikipedia.org/wiki/U.S.%E2%80%93UAE_123_Agreement...

To be fair, it's not only U.S. who want's the control access to nuclear technology and nuclear materials. For example India wanted to become a member of Nuclear Suppliers Group for a long time. As of 2019, China has thwarted every attempt of India's inclusion into NSG and has made it clear that status quo will remain citing "lack of consensus" among NSG members.

https://en.wikipedia.org/wiki/Nuclear_Suppliers_Group#India

Another example is South Korea. South Korea is constrained in its nuclear power policy by the 1974 Korea-US Atomic Energy Agreement. Only in November 2025 did the USA formally affirmed support for South Korea’s civil uranium enrichment and spent fuel reprocessing for peaceful uses.

https://www.armscontrol.org/act/2025-12/news/us-supports-sou...

https://world-nuclear.org/information-library/country-profil...

[DennisP]

Good points, but in all of that they are specifically targeting enrichment and reprocessing, while allowing nuclear power plants.

A fusion reactor is a power plant. It produces neutrons but so does fission; in fact, conventional fission plants get a third of their power from plutonium that they breed from U238, and plutonium accounts for most of the long-term radioactivity in the waste.

By comparison, a fusion plant would have no uranium present for any legitimate reason, and assuming it's D-T it would need those neutrons to breed tritium. Tritium has a use in thermonuclear weapons but not without highly enriched fissiles, and tritium is also a byproduct of fission plants.

[leonidasrup]

Precisely because fusion reactor could be used for plutonium breeding they will be subject to the same controls and political pressures as fission power plants.

Operation of all fission power plants, outside of countries with nuclear weapons, is subject to very strict controls by International Atomic Energy Agency, which reports to United Nations Security Council. Also because each civilian nuclear power reactor breeds enough plutonium to produce multiple nuclear weapons each year, spent nuclear fuel is subject to the same very strict controls. (I know the the isotopic composition of this plutonium is miserable for weapon production, but nuclear weapons using reactor-grade plutonium have been build and tested).

https://en.wikipedia.org/wiki/Reactor-grade_plutonium

I don't know any thermonuclear weapon using highly enriched uranium. Highly enriched uranium is used only in few designs of nuclear weapons: nuclear weapons which you build when you don't have access to weapon grade plutonium, or nuclear weapons which have to survive strong external shocks - nuclear bunker busters. Plutonium Pu-239 based nuclear weapons are physically smaller and lighter, easier to place into missile nose cone.

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

[cyberax]

Fission-fusion or accelerator-driven fission is pure BS. It combines the disadvantages of _both_ and none of the advantages.

Modern fission power plants are designed with a reactor vessel to last a century and to withstand high pressures and temperatures. It's built and emplaced permanently in a large concrete shielding structure.

In a hybrid design this just won't work. Fuel will need to be right next to a high-vacuum chamber that will need periodic maintenance.

[Aardwolf]

What's the effect of this in a populated area in a certain radius? Compared to nuclear power plants...

[cyberax]

Radiologically? Pretty much nothing. The regular industrial safety concerns will matter more.

The plant will have some tritium, and the material in reactor walls will get activated by the neutron flux. Some of the activated materials can disperse in case of a catastrophic explosion (e.g. a couple of large airplanes being flown the reactor building).

But the material of the walls is not volatile, so it'll stay on the site. And tritium is very volatile, so it'll quickly disperse to safe levels. You'll be able to detect them with sensitive equipment, but it won't be dangerous.

[JumpCrisscross]

> What's the effect of this in a populated area in a certain radius?

I'd imagine this is, like with fission plants, deeply dependent on the specific design.