Nuclear propelled attack submarine HMS Trenchant, of the Trafalgar-class (which preceded the Astutes) is a floating hulk in the UK. Trenchant awaits the huge cost and a UK nuclear waste dump option, involved in removing her radioactive parts. She can then be scrapped.
In July 1997, HMS Trenchant ran aground off the Western Australian (WA) coast. While approaching Fleet Base West (HMAS Stirling) just south of Perth, WA, Trenchant, at a depth of 200m, collided with Australia's continental shelf. She came to rest on a sloping patch of seafloor. Trenchant was able to free herself, and an inspection by divers (at 200m? shallower? or at Fleet Base West?) reported no significant damage.
If Trenchant had collided at high speed, like Los Angeles-class USS San Francisco, there could have been fatalities and $100s millions in damage. In 2005 USS San Francisco collided with an undersea mountain ("seamount") with 98 crew injured and tragically one fatality. Photo below:
When, or if, Australia buys Virginia-class submarines, operating out of Fleet Base West, RAN crews will need to get used to high submerged speeds, to avoid collisions.
Fortunately submarine reactors are towards the stern/back, so, in theory, not effected by head-on collisions.