Its too bad there isn’t a device that can generate electricity directly from radioactive material without any moving parts.
The Germans are moving ahead with their Hydrogen fuel-cell subs, which is as close as you can get to a “no moving parts” electricity generation. You still have the electrical motor and the screws though.
Nuclear subs are actually by far noisier and leave a stronger signature (noise, heat, radiation) than conventional electrics. Especially the german fuel-cell driven ones or the Swedish AIP sterling models.