No, I don't mean an ESBWR, as I said, I mean a compact liquid metal (typically liquid sodium) reactor. These are small enough for mounting in a sub, and are not too heavy.
The French nuclear subs were small because of the limitations of the French nuclear technology of the time, with larger reactors going for their SSBNs.
As I have already said, the diesel electric subs I am talking about are of the Collins size class, i.e. over 3,000 tons submerged before modification. I am not talking about trying to fit reactors to small coastal subs, e.g. the U212/214 or similar.
The actual concept has been applied before, and is in use on the Russian B-90 Sarov hybrid nuclear teakettle technology demonstrator submarine. This one doesn't use a proper compact reactor, but the basic concept is there.
The Toshiba 4S reactor, for instance, is supposed to be around 20ft x 6ft (by 6ft I believe); this is the perfect size, and though a civil reactor, I strongly suspect that it could be adapted for military use. The reactor produces around 10MWe, which would be enough for a Collins class size sub, and the reactor would be scaleable anyway. The Collins has a 5.25MW rated main motor, so 10MW should be plenty for the sub's propulsion, sensor and general electrical needs. Such a modification to the Collins would yield a sub with a modern combat system (it uses the AN/BYG-1 as fitted to USN SSNs), speeds in excess of 20 knots (officially, so probably a bit more) etc...
As for Americium batteries, these are very large, and only generate a couple of hundred KWs (150KW for your example, which only lasts 80 days!). In order to generate the equivalent sort of power to the compact Toshiba 4S, at 10MWe, you would be needing around 70 such batteries. All of these would need to be totally replaced after every 80 days, and rely on a man-made element that is difficult to generate. The 70 batteries, at 10 tons each would weigh in at around 700 tons, and that is just for the actual batteries!
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It all sounds like you're reinventing the Soviet Union's Alfa class SSN. At 2300mt, if this figure is true, the Alfa class is also the smallest nuclear submarine the world ever built. (1. Alfa class-2300, 2. Rubis class-2500, 3. Skipjack class-3000).
Liquid sodium reactor has been experimented in the Seawolf class (SSN-575, not your SSN-21) back in the late fifties. It proved to be troublesome and later replaced. Liquid sodium tends to be corrosive.
The Alfa class uses liquid lead, which offers even more density. It is inherently safe, since it won't blow, catch fire, release radioactive steam and gases. When leaked it would only solidify. For the most part, the Alfa class had a much better safety record than almost all Soviet subs.
The problem is, if the lead coolant solidifies, the whole circuit is dead and needs to be replaced entirely. That alone cost the scrapping of one of the Alfas. The remaining Alfas had to operate with their reactors always on even on port, to keep it hot and lead in liquid form. The Soviets tried other means to keep the lead hot but didn't succeed. Eventually all the boats were retired. Still the sub demonstrated its power potential by running 40 knots underwater under a NATO convoy, though not without some damage to the hull.
The Alfa has a high degree of automation. I believe it only has 30 crew members.
Given the state of 70s to 80s technologies, revisiting the concept in the 21st Century should be interesting and see what three decades of new technologies can change. The crucial part is to keep the lead molten in port while the reactor is shut down.