Australia has announced the most significant change in its defence and strategic direction in decades – a plan to make the Navy's next submarine fleet nuclear-powered.

It means a $90 billion program to build 12 French-designed diesel-powered submarines will now be scrapped, prompting many to ask – why are we doing this? 

There are many factors behind this decision, and many questions about what this means for Australia.  

Let's start by looking at the strategic advantages.

Silence is golden

Nuclear-powered submarines can stay quieter for longer. 

To put it simply, a nuclear-powered submarine has fewer moving parts than a diesel-powered alternative that could alert the enemy.

They have become so quiet that in 2009, British and French nuclear ballistic missile submarines reportedly collided in the Atlantic Ocean, unaware of each other's presence.

Diesel generators make a lot of noise when travelling on the water's surface and require a snorkel for air intake. That makes them more easily detected.

The subs can go quiet once they are submerged and switched to electric power, but this requires batteries to be charged and limits the total time spent underwater. 

Horses for courses

Another advantage is nuclear-powered submarines can go faster, and stay underwater for longer.

But they are often bigger in size, which makes them less nimble in shallow coastal waters.

So, there are strategic advantages and disadvantages. 

Nuclear-powered submarines would allow the Australian Navy to patrol more of the Indo-Pacific region for longer, which could be particularly handy at a time of competing territorial claims for strategic waters.

This could be done to deliberately make Australia's presence known near, or in, regions like the South China Sea, or in stealth mode. 

But diesel or electric-powered submarines excel in coastal waters like those to the north and north-west of Australia.

According to the former director of the Australian Submarine Corporation, Hans J Ohff, they are better suited to deployments in estuaries.

According to many analysts, they are better suited to defending coastlines or ports if invaded. 

Who wins the race?

This one is easy. It's the nuclear-powered alternative.

There are numerous reports they can reach speeds of 55 kilometres per hour or more when submerged. 

This is significantly faster than diesel/electric capacity. This is important when patrolling open oceans, but less important when operating close to a coast. 

Who's got what?

China already has nuclear-powered submarines, and this switch of strategy would see Australia match – or come close to – its capabilities. 

China has six Shang-class nuclear-powered attack submarines. Each one is 110 metres long and capable of carrying cruise missiles and torpedoes.

But it also has 50 diesel/electric attack submarines, making its submarine fleet significantly larger than Australia's.

Some naval experts predict China's fleet will soon be larger than that of the United States. 

Nuclear-powered submarines are also common among other major global powers.

As well as the US and China, Russia, France, the UK and India all have them.

What makes Australia different to all these nations is the absence of a domestic nuclear industry capable of supporting the submarines.

And unlike these other nations, Australia also does not have nuclear weapons capacity.

US President Joe Biden was eager to note this deal does not extend to weapons, only the propulsion system.

Do we need a nuclear industry?

The short answer is no.

Would it make it easier? Sure, but it's not essential. 

For example, the US had its first nuclear-powered submarine in service before its first nuclear power station. Australia could use fuel supplies from other nations, including the US and UK. 

The question of whether having a nuclear-powered fleet could lead to the expansion of the domestic nuclear industry is different, and that's open to debate.

It's something conservation groups are worried about and carefully monitoring. 

What happens to the nuclear fuel?

The details on this are unclear, despite it being one of the key questions. 

Given Australia would likely obtain the fuel from another nation, any waste could be returned to them, or it could be stored in Australia.

The waste could be sent to a proposed facility in South Australia. 

Another issue is what happens if there is a breach on a sub, or if there are nuclear material leaks. Again, answers to this are still unclear.

So… what do we do about the French?

We've already committed to spending a whopping $90 billion on French-designed diesel/electric submarines that would be built in Australia. 

At this stage, that contract will be torn up.

According to the Prime Minister, we've already spent about $2 billion on this project, and there'll likely be a cost to ending the contract early.

In 2019 the ABC revealed Australia would have to pay millions of dollars in in "break payments" to end the French deal.

From his perspective, ending the French deal now will save Australia in the long term. 

But the French are not impressed.

In a statement, French ministers Jean-Yves Le Drian and Florence Parly said France could only "observe and regret" as the deal was scrapped.

Can they call Australia home?

Good question. Yes, but perhaps not everywhere.

There is no overarching ban on nuclear-powered submarines in Australian ports.

But some ports, like Fremantle in Western Australia, have instituted their own bans, and negotiation would be required. Melbourne has also expressed concern in the past. 

The Australian government closely monitors which nuclear-powered ships visit our ports each year.

In 2017 for example, three US vessels spent 15 days at ports in Brisbane and HMAS Stirling in Western Australia.

Radiation checks were conducted to make sure they were safe. 

Can they visit other nations?

These submarines will not be able to dock in New Zealand, which has a long-standing ban on nuclear-powered vessels.

They may also not be able to visit some Pacific Islands that have taken similar measures.

This could reduce naval cooperation with these nations.