Australia's Aukus nuclear submarines will leave behind hundreds of kilograms of waste uranium that could be used to build nuclear weapons, according to experts who gave evidence to a public inquiry reported by the Guardian.

Here's the problem in plain terms. The submarines run on a type of uranium that is packed full of the stuff that makes nuclear reactions — and nuclear bombs — possible. Even after the submarines have finished their 30-odd-year working lives, the leftover fuel will still be rich enough to be turned into weapons. That's according to Associate Professor Tilman Ruff, founding chair of the Nobel prize-winning International Campaign to Abolish Nuclear Weapons (ICAN), who gave evidence to the inquiry Guardian.

The numbers explain why experts are worried. US nuclear submarines start out running on uranium enriched to 93.5% — meaning 93.5% of it is the kind that can power a nuclear reaction. After decades of use, the leftover fuel still averages 83%. The waste from Australia's submarines will likely still be around 80% enriched. The atomic bomb dropped on Hiroshima used uranium at a similar level. Modern nuclear weapons typically use uranium enriched to about 93%.

For comparison, uranium as it comes out of the ground contains only about 0.7% of the reactive kind. Anything above 20% is called highly enriched uranium and is technically enough to build a weapon. Some countries, including France and China, run their submarines on uranium enriched to just 3% to 5% — the same level as civilian nuclear power plants. The Aukus submarines are in a completely different league.

A former White House national security adviser told the Guardian that transferring this kind of uranium to Australia — a country that doesn't have nuclear weapons — sets a precedent other countries like Iran could try to exploit. The US Virginia-class and UK Astute-class submarines at the heart of the Aukus deal are both fuelled with US weapons-grade uranium, enriched to more than 90%, that was declared surplus to military needs Arms Control Association. The Bulletin of the Atomic Scientists reported in November 2022 that the submarine reactors supplied under Aukus would likely run on fuel enriched to around 93% Bulletin of the Atomic Scientists.

Ruff told the inquiry that no country in the world has figured out how to safely store this kind of nuclear waste permanently. It needs to be kept isolated for hundreds of thousands — possibly millions — of years. He warned that someone could dig up the buried material 10,000 years from now and still use it to build nuclear weapons. Earlier submissions to Australian parliamentary inquiries made a related point: the hardest part of building a nuclear bomb is enriching the uranium. The spent fuel from these reactors hands any future government or group a shortcut past that hardest step APH submission.

The Australian government has repeatedly said the exact amount of nuclear waste from the submarine fleet is classified. Documents released under freedom of information laws confirm Australia will be responsible for each submarine's nuclear reactor compartment. Where and how that waste will be stored is still an open question — one of the core issues the public inquiry is examining.

The inquiry is a crowd-funded exercise launched at Parliament House in Canberra, headed by former Labor minister Peter Garrett, appointed in June Guardian. It has already heard warnings that Australia is not prepared for a potential nuclear accident at the naval base HMAS Stirling in Western Australia SMH.

This waste problem doesn't exist in isolation. A senior Australian defence official warned in March that Australia will be left with no submarines at all if it walks away from the Aukus deal Guardian. In May, the government announced taxpayers would pay $11 billion to extend the life of the existing Collins-class submarines because the Aukus replacements are running late Guardian. The government's position is simple: quit Aukus and you get no submarines. The proliferation and waste consequences of staying in the deal are now being laid out in detail.

The broader context is that the Aukus deal creates a tension at the heart of the global rules designed to stop the spread of nuclear weapons. The international treaty meant to prevent the spread of nuclear weapons — the Nuclear Non-Proliferation Treaty, or NPT — is supposed to stop weapons-grade uranium being given to countries that don't already have nuclear weapons. The government argues that naval reactor fuel doesn't count because it stays in a military program. But that argument doesn't cover what happens when the submarine is retired and the reactor is shut down for good. At that point, the waste becomes a disposal problem that lasts effectively forever — and it stays weapons-usable the entire time. The government has classified the quantities. The enrichment levels, the physics, and the precedents are all on the public record.