[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

[DennisP]

For the most part I think we're in vehement agreement. I wrote above that nuclear plants are permitted with inspection under the NPT, and I would expect D-T fusion to be the same. Your links also say that reactors are allowed in practice.

When I said "highly enriched fissile" I meant that to include plutonium. Weapons generally use Pu239, while nuclear waste plutonium is 58% Pu239 mixed with other isotopes, plus of course a lot of U238 and various transuranics.

There is one area where fusion reactors are clearly being treated differently, and that's by regulators such as the NRC, which has already decided to treat fusion reactors like particle accelerators and medical devices, rather than with the much more difficult process they use for fission reactors.

[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.