The key to Australia's nuclear ace in the hole

Scott Morrison, Boris Johnson and Joe Biden had just agreed to an alliance between Australia, the United Kingdom and the United States, dubbed AUKUS.

It would become the alliance through which Australia will become the seventh nation to acquire nuclear-powered submarines.

Australian Prime Minister Anthony Albanese will unveil the full details of the plan alongside UK Prime Minister Rishi Sunak and US President Joe Biden in San Diego on Tuesday (AEDT).

Australia is expected to buy US Virginia class submarines in the 2030s before a jointly built next-generation sub using a UK design integrated with an American weapons system towards the end of that decade follows.

It's only the second time in history that the US will share its coveted nuclear secrets.

The secret pact had been in the works since late 2019 when Mr Morrison had two main doubts over the French submarine program.

The shipbuilder was repeatedly missing key construction targets, and the diesel-powered subs would be obsolete upon delivery.

This was all against the backdrop of China's military build-up.

Mr Morrison said the AUKUS alliance and nuclear submarines would become one of the most significant checks on Chinese assertion in the region in 20 years.

"The best way to stop a war is to make sure the one who might want to start it thinks twice," he told AAP.

First, he informally tasked defence officials to determine whether a new plan to acquire nuclear submarines was feasible for Australia.

It then started to gain momentum as Australia began engaging with UK and US officials.

Mr Morrison's original plan was to acquire British submarines, but with the nuclear technology belonging to the Americans, Washington needed to be brought into the tent and assured of Australia's commitment.

"This wasn't a tender, the US own the tech. (If) they're not happy with it, it's not happening," he said.

It never reached the desk of then-president Donald Trump and was first elevated at a political level in the US to Mr Biden's national security advisors, Jake Sullivan and Kurt Campbell, after the change of administration.

Australia's ambassador to Washington Arthur Sinodinos said once it hit Mr Biden's desk, he had to satisfy himself that the agreement wouldn't jeopardise America's commitment to nuclear non-proliferation.

He also had to be assured Australia was capable of committing to a multi-decade agreement.

"Once he satisfied himself that the strategic stakes justified doing this, (Mr Biden) was all in," the ambassador said.

Australia's national security committee meetings - a cabinet subcommittee of fewer than 10 people - had been kept to the smallest possible attendance with as few officials and advisors as possible when the discussion arose.

Mr Morrison told the full committee of his plan in May 2021 in order to get their endorsement to approach the US and UK leaders with an official government policy.

The British prime minister was already on board before they took the plan to the US president.

"He loved it. He loved the idea of it. Boris was all in on the challenges we faced here," Mr Morrison said.

The plan was then formally put to Mr Biden and agreed to in principle on the sidelines of a G7 meeting in Cornwall in June 2021.

It all hinged on the president's nod.

"We shook hands and we walked out of there together and did the G7 leaders' photo," Mr Morrison said.

The main aim of the high level of secrecy preceding and following the handshake was to prove that Canberra could keep America's most coveted secrets.

The plan didn't reach Australia's cabinet until the last minute, just ahead of the public announcement in mid-September 2021.

The chief of the defence department's nuclear powered submarine task force says Australian access to US nuclear technology has never been greater.

Up until just over a year ago, no Australian had set foot in a naval nuclear reactor, attended a US naval reactor training school or even entered their headquarters.

Vice Admiral Jonathan Mead said hundreds of Australians had since been through that system and "significant inroads" had been made in granting access.

Then-finance minister Simon Birmingham was first alerted by Mr Morrison personally ahead of the NSC being brought in.

The cancellation of the multibillion-dollar French submarine contract would have a devastating impact on Adelaide's shipbuilding industry.

The South Australian MP was initially shocked by the scale of the proposal.

"Why would we do this? Should we do this? Are we capable of doing this?" Senator Birmingham told AAP.

"All had to be asked and answered."

But the advice from the defence department always proved compelling, not only on the strategic circumstances but also on how historical limitations on Australia's technical ability to handle such technology would be addressed.

The pair came quickly to the conclusion something had to be done to shore up jobs in Adelaide to make sure there wouldn't be a mass exodus from the industry that would cripple other defence projects in the future.

Defence Minister Richard Marles says while the submarines will be capable of war, their primary purpose is to ensure peace.

The nuclear model means they'll be able to run quieter and operate for longer, becoming a crucial piece of the puzzle to the defence force's new "impactful projection" mantra, which focuses on striking adversaries further away from Australia's shores.

"As we acquire this nuclear-powered submarine capability, we do so as part of making our contribution to the peace and stability of our region and of the world," Mr Marles said.

Almost 21 months to the day the initial handshake sealed the foundation of Australia's defence policy for decades to come, the decision that spanned three defence ministers and two prime ministers will finally rise to the surface.

Australian Associated Press