Australia's Aukus nuclear submarine fleet will leave behind hundreds of kilograms of weapons-usable uranium at the end of its working life, with experts warning the spent reactor fuel could be reprocessed into nuclear warheads, the Guardian has revealed.

Nuclear specialists told the Aukus public inquiry that the spent fuel from the submarines' reactors will still be highly enriched uranium — technically sufficient to build nuclear weapons. Each submarine will have a service life of about three decades and will leave behind hundreds of kilograms of high-level radioactive waste as spent reactor fuel, 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 enrichment figures are where the concern sharpens. US nuclear-powered submarines run on reactors containing uranium enriched to 93.5% U-235 at the start of their working lives. (U-235 is the fissile isotope — the part of uranium that can sustain a chain reaction.) After two to four decades of operation, the uranium in the spent fuel still averages 83% enrichment. The uranium left behind by Australia's submarines will likely still be enriched to about 80% — the same level as the atomic bomb dropped on Hiroshima. Modern nuclear weapons typically use uranium enriched to about 93% U-235.

For context, naturally occurring uranium contains about 0.7% U-235. Anything enriched above 20% is classified as highly enriched uranium and is technically sufficient to build a weapon. Some countries, including France and China, run their nuclear-powered submarines on low enriched uranium, between 3% and 5% U-235 — the same range used in most civilian nuclear power reactors. The Aukus boats will be in a completely different category.

A former White House national security adviser told the Guardian that Australia's Aukus deal, which involves the transfer of highly enriched uranium to a non-nuclear weapons nation, sets a dangerous precedent that other countries such as Iran could try to exploit. The US Virginia-class and UK Astute-class submarines that form the backbone of the Aukus arrangement are both fuelled with US weapons-grade uranium enriched to more than 90% U-235 that was declared excess to military needs Arms Control Association. The Bulletin of the Atomic Scientists reported in November 2022 that the reactors in submarines to be supplied under Aukus would likely operate on fuel enriched to around 93% U-235 Bulletin of the Atomic Scientists.

Ruff told the inquiry that no nation has resolved the problem of how to manage this category of high-level nuclear material and that it requires isolation for hundreds of thousands or millions of years. He warned that in 10,000 years, someone could still dig up the buried material and build nuclear weapons with it. Submissions to earlier Australian parliamentary inquiries have made the same point from a different angle: the uranium enrichment process is the technically demanding component in manufacturing nuclear weapons, meaning the spent fuel from these reactors hands any future state or non-state actor a shortcut past the hardest part of building a bomb APH submission.

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

The inquiry itself is a crowd-funded exercise launched at Parliament House in Canberra and headed by former Labor minister Peter Garrett, who was appointed to lead it 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 question does not arrive in a vacuum. A senior Australian defence official warned in March that Australia will be left with no submarines at all if it abandons the Aukus pact Guardian. In May, the government announced taxpayers would pay $11 billion to extend the lifespan of the Collins-class fleet amid Aukus delivery delays Guardian. The government's position is that walking away means having no submarines. The proliferation and waste disposal consequences of staying in the deal are now being spelled out in forensic detail.

The broader context here is that the Aukus arrangement sits at a structural tension in the non-proliferation architecture. The deal transfers weapons-grade highly enriched uranium to a non-nuclear weapons state under the Nuclear Non-Proliferation Treaty (NPT) — which is precisely what the treaty is designed to prevent. The government's argument that naval reactor fuel is exempt from safeguards because it remains in a military cycle does not address what happens to that fuel once the reactor is decommissioned. At that point the material becomes a permanent disposal problem, and it remains weapons-usable for the entire isolation period Ruff describes. The government has classified the quantities. The enrichment levels, the physics, and the precedents are all on the public record.