Why submarine and surface-ship reactors differ

Beyond scale, how do submarine, cruiser, destroyer, and carrier reactors differ?

Submarine reactors face unique safety and buoyancy constraints, while surface ships vary mainly by power demand, reactor count, and hull arrangement.

Nuclear-powered submarines usually have a single reactor, while surface ships may use one, two, four, or more according to their scale and mission. A submarine plant must be exceptionally compact, quiet, radiation-tight, and reliable because faults occur inside a pressure hull far below the surface. Cooling, compartment layout, and emergency procedures consequently differ from those of a cruiser or carrier. Bringing emergency water into a submerged reactor also adds weight, so the crew may need to shed ballast at the same time to avoid an uncontrolled descent. Reactor count is therefore only the most visible difference between naval plant types.