I understand that the lead boat of the Astute class is to be retired in 2035 and that the reactors cannot be refueled and have just a 25 year life , Im not aware if it's possible to replace the reactor but the present nuclear reactor is no longer in production
Actually the reactor does not have to be replaced. The fuel core needs replacement. This is a long, complex operation that requires cutting open the hull, extracting the core from the reactor and installing a new one. This work must all be done very precisely to a nuclear engineering standard. It is costly and may not be worthwhile on a 25 year old sub. But in fact UK has refueled PWR2 reactors on the Vanguard class SSBNs,, to extend their lives till the Dreadnoughts entered service.
If Australia goes for UK designed SSNs but with the US S9G reactor one advantage is that the US reactor core has a 32 yearlife, not 25.
You can see why SSN maintenance becomes a big deal and if Australian shipyards develop the skill to maintain SSNs, including the reactor, that will be invaluable to both the RN and USN. Both have a backlog of SSN maintenance. Some USN SSNs have been waiting over a year tied up in dock waiting for scheduled maintenance.
Whilst I support the RAN getting SSNs and AUKUS generally, IMO there were a few fibs told by politicians when it was announced. One was about us “not needing to touch” the reactors. That was false. They won't need refueling, but will need regular and thorough maintenance.
Some of the statements made about the French nuclear sub option were also false. It is powered by LEU uranium cores which need replacing every ten years (not seven). The Suffren design includes a special hatch to allow this in a few weeks. This would have a small impact on long term maintenance which would occur every ten years. In fact, since an LEU core does not breach the NNPT and spares could be stored in Australia, we could have refueled them here if we built the appropriate infrastructure. The French originally developed HEU reactors like USA and UK and did a lot of work to shift to LEU because it eliminated many risks. The Suffren is more automated and needs a smaller crew as a result. I’m not suggesting the Suffren is superior to an Astute, but it was a viable option that should not have been dismissed until we knew for sure that we could get US or UK designs built.
There is a lot of information about naval reactors at this site.
Over 160 ships are powered by more than 200 small nuclear reactors. Most are submarines, but they range from icebreakers to aircraft carriers. In future, constraints on fossil fuel use in transport may bring marine nuclear propulsion into more widespread use.
world-nuclear.org