For the power produced, they are very compact. Our FB motor was a 6-cylinder, 12 piston (2-crankshaft) one that was the size of an F-150 pickup truck and produced 12.5 Megawatts of power, submerged, and 14 MW surfaced. When submerged, it has to push the exhaust through about 3 feet of water column, which is tremendous back pressure.
12.5 MW of power is over 16,000 horsepower, and that is AFTER generator losses.
They are also quite robust and hard to break if you run them right. We posted a watchmen on the intake sump to make sure it never ingested too much water. When submerged (at periscope depth), it sucks through a snorkel mast. Waves push water into the snorkel. The sump filters out most of that water.
Other than keeping a close eye on the intake for excessive water, all you have to do is keep the oil well filtered and fresh. Otherwise, they're pretty much bulletproof, literally. Mechanically, other than having 2 cranshafts synchronized in common, they're dead simple--compared to today's current 4-stroke turbodiesel passenger car engines, vastly simpler.
And yes, the fuel tank serves as some of the radiation shielding just forward of the reactor compartment. We carried enough to run across any of the world's oceans.
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