Why the USN has no non nuclear subs I should have typed, is partly a matter of politics and opinion, not 100% cut and dried difference in capabilities of subs and mission of the USN.
The German fuel cell system uses metal hydrides for hydrogen storage. The Stirling type AIP as in Swedish and Japanese subs burns diesel fuel. Both need the sub to store a large amount of pure oxygen, far beyond what previous subs had to in order for the crew to breathe. That’s more the potential safety issue. We think of hydrogen as ‘dangerous’ because or if it’s combusted with oxygen.
However at this point one would have to say that the AIP systems developed operated now for a pretty long time by several countries have proved basically safe. Just as nuclear propulsion is potentially dangerous, but has proved basically safe given the particular designs and institutional safe guards in place in Western navies at least. And likewise a different AIP design or one not maintained and operated properly by its crew could surely cause the loss of a submarine, same with nuclear.
The difference though is a nuclear sub’s potential to contaminate a large area. Which hasn’t provably been a big problem with the limited number of nuclear subs lost in fairly deep water including the couple of US losses believed to have been accidents (Thresher and Scorpion). But you sometimes see professional USN stuff about subs playing their part in the ‘littoral’, shallower areas, where it’s actually not impossible that mines could breach the reactor protection, or a near peer opponent manage to simply sink one with good ASW, and the leaking radiation would not be 1,000’s of feet down. But that possibility seems to factor zero into USN opinions about nuclear v non-nuclear. I’m not saying that consideration, nuclear releases that are not accidents but a result of combat, should rule out nuclear subs. It’s just interesting how it gets seemingly zero weight in semi-official debates about nuclear/non-nuclear (like occasional debates about it in Naval Institute Proceedings over the decades I’ve been reading it).
AIP and diesel boats are basically mobile minefields. They cannot actively intercept a target, they have to lie in wait. This is a low percentage deal unless you are at a choke point, in which case there will be more focused and thorough ASW measures taken.
Also, nuclear power provides plenty excess for things like oxygen generation and air cleaning.
The sunken USN submarines are not leaking in any measurable amount (and we do check). Solid fuel and fission products are encapsulated in the zircaloy and reactor vessel, and any in the coolant would be diluted by the seawater. Contamination would be the same unless the vessel was run up onto the beach.
I do not know what is in the public domain regarding Soviet/ Russian hulls and reactors in the open sea and off Novaya Zemlya (and the recent accident).
The Virginia-class fast attack submarines (17 active) are powered by the S9G reactor, which is supposedly able to cool itself at some fraction of full power via natural circulation, like its predecessor.
Or so the pundits are guessing… nobody who knows is talking right now.
Not necessarily as benign a situation if a nuclear sub was sunk by mine or torpedo in shallow water, where it is sometimes contemplated they’d operate in combat.
A mine or torpedo does not have the power of seawater 2,000 feet below. Scorpion is in several largish chunks, Thresher is in bits and pieces, but the reactor vessels are still intact. Minus any shut isolation valves.
The sea is still a big dilution volume.