ANALYSIS: Only once in its history has America handed over a nuclear submarine propulsion plant, the crown jewels of military technology, to another country.

That was 63 years ago when America helped the Royal Navy to go nuclear. Now it will take that dramatic step again.

A new trilateral defence pact, Aukus, announced on September 15, will involve far-reaching defence co-operation between America, Australia and Britain.

The group’s first initiative, and its most important, will be American and British assistance to Australia in building a fleet of at least eight nuclear-powered submarines.

The precise form of assistance will be worked out over the next 18 months; it may involve Britain actually supplying the technology, with America’s blessing and support.

“This technology is extremely sensitive,” acknowledged an American official, speaking anonymously on September 15.

“This is, frankly, an exception to our policy in many respects...We view this as a one-off.”

Nuclear-powered subs are sensitive not just because of their range, speed and stealthiness. It is also because they are powered with the same stuff–usually, uranium enriched so that it has a higher proportion of the most fissile isotope, U-235–that is used in bombs.

The Nuclear Non-proliferation Treaty (NPT) forbids signatories who don’t already have a bomb from making one. It also says they must put sensitive nuclear material, like enriched uranium, under international safeguards, monitored by the International Atomic Energy Agency (IAEA), a watchdog. But the rules have a submarine-shaped loophole.

States are allowed to remove nuclear material from safeguards if they are for “a non-proscribed military activity”, such as submarine propulsion. No non-nuclear-armed state has ever tested that loophole–until now.

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Australia is unlikely to produce enriched uranium itself; unlike every other state which has operated a nuclear-powered sub, it has neither nuclear weapons nor any nuclear power stations. It is more likely to acquire reactor fuel from another country.

Once that fuel is in a working reactor, it becomes too radioactive to use for a bomb. But depending on how Aukus is implemented, it might still have fissile material hanging around before then.

Worse still, both America and Britain use highly enriched uranium (HEU), essentially weapons-grade, in their subs. It is possible to operate a sub with low-enriched uranium (LEU) – both France and China do so – but it has drawbacks, such as larger reactors and more frequent refuelling.

Most non-proliferation advocates are not terribly worried about Australia building a nuke (it once sought one, but ended that pursuit in the 1970s).

They are more concerned that the spread of nuclear submarine technology and fuel for propulsion reactors sets a dangerous precedent that will be exploited by others.

Countries that do want nuclear weapons, or simply want to keep the option open, might see submarines as a convenient excuse for making or acquiring bomb-usable HEU, out of sight of pesky inspectors.

Iran, whose nuclear programme is the subject of an increasingly tense dispute with the West, has toyed with the idea in the past.

South Korea, which faces a North Korean nuclear threat, and where opinion polls show plurality support for building nuclear weapons, has explored nuclear subs off and on since the early 1990s.

Brazil is actually building one, the Álvaro Alberto, as part of a partnership with France. “With the new Aukus decision, we can now expect the proliferation of very sensitive military nuclear technology in the coming years, with literally tons of new nuclear materials under loose or no international safeguards,” warns Sébastien Philippe of Princeton University, writing for the Bulletin of the Atomic Scientists, a research group.

Others believe that these concerns are overwrought. “It would be a matter of real non-proliferation concern if we are planning to produce our own fissile material but we are not,” writes Gareth Evans, a former Australian foreign minister, “and I have no doubt that complete safeguards discipline will be maintained.”

Evans dismisses the possibility that the move will encourage “problematic behaviour” by others.

Ian Stewart, based in Washington, DC, as director of the James Martin Centre for Nonproliferation Studies, says that Australian subs could be fuelled in Britain, with that fuel placed under permanent IAEA seal and subject to periodic inspection in a way that would meet both military requirements and the demands of safeguarding.

Even so, nuclear norms are being tested and stretched.

Notably, the past 16 years have seen two revolutionary agreements that prioritised geopolitics, namely, balancing against China’s rise, over non-proliferation sensitivities.

The first was America’s civil nuclear deal with India in 2005, which came only eight years after India, an NPT holdout, had tested nuclear weapons. Aukustreads a similar path.

After the cold war, much attention was paid to non-proliferation, observes David Santoro of the Pacific Forum, a think-tank. “Now power politics is back in force. Non-proliferation still matters but isn’t the sole consideration anymore”.