Just wanted to share my thoughts with you, because i think this topic is very interesting.
IMHO RTGs have another disadvantage: They produce power continuously, meaning even when they are not needed, they produce power. This power needs to be dealt with. Either it is dissipated as heat, or it needs to be stored/used. And compared to space probes, subs are rather "dynamic" energy consumers.
Second, the power output of a RTG decreases exponetially over time, as the radioactive material decays. So the actual power produced over the lifetime of the system decreases. This is an important issue in space exploration, since the power consumption of the probe has to be adjusted to this. In a sub, you would have to replace the system after a couple of years.
A rather positive aspect is, that RTGs used for space exploration are very heavily shielded. The shielding could be reduced, since they don't need to withstand a possible uncontrolled atmospheric reentry. In addition with, lets say, stirling engines instead of thermoelectric elements the power density could be increased.
But the question remains, would you need a low power, ultra high endurance AIP system for something the size of a Kilo? I don't think so, because nations with the technology to built RTGs and advanced submarines opted either for full-nuclear propulsion or non-nuclear (for political reasons, cost savings, no strategic/tactical need).
By the way: RTGs are really expensive.
Phil
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