There are more similarities than differences between the two predominate methods of steam ship propulsion, so I will first list these similarities:
1. Both nuclear and conventional propulsion plants use (with a few experimental exceptions) geared steam turbines. These turbine/reduction gear sets are connected directly to the forward end of the propeller shaft, thus one turbine set drives one propeller.
2. Both nuclear and conventional plants use seawater as the primary
coolant in the steam cycle (that is, sea water passing through the tubes
of a condenser is used to return the used steam to a liquid state so that
it can be returned to the steam generator).* Thus the temperature of the
near surface sea water can be a limitation on the maximum sustained performance
of either type of plant. This typically becomes a serious issue for conventional
plants at water temperatures at or greater than
*This is part of the "secondary cooling cycle" in nuclear plants, but it is still the terminal thermal transfer point.
3. Both nuclear and conventional plants use steam turbine powered turbine generators for ship's service electrical power. In warships, and in all nuclear plants, there are diesel (or, rarely, gas turbine) back up generators to power critical circuits.
4. Neither nuclear nor conventional plants develop more power based on the nature of the energy source. Conventional plants are more thermally efficient and tend to last longer than nuclear plants. Nuclear plants operate at lower temperatures and pressures and obviously require fewer refuelings.
5. The limitations on power produced for both types of plants lie in the design and construction of the gears, shafting, thrust bearings, and supporting structure, rather than in the turbines or steam generators.
6. High speed endurance is similar for both plants, until the conventional must slow for a few hours for refueling. The limitations on ship's speed are a product of hull design, propeller efficiency, horsepower, vibration, and thermal build up in the propulsion plant. Nuclear plants do not have a de facto advantage in any of these areas, although the smaller hull made possible by the reduction in liquid load is a factor in some designs.
Some of the significant differences between nuclear and conventional steam propulsion plants are:
1. Cost: Nuclear plants are far more expensive to build, maintain, operate, and dispose of. Fuel costs are actually lower for a conventional plant, considering the costs of a refueling overhaul for a nuclear powered ship. Manning levels are higher for a nuclear plant, as are training costs.
2. Flexibility: Nuclear plants require very heavy and complex foundations and shielding. This has a very significant influence on hull and machinery design. Conventional plants can be configured with many more options.
3. Efficiency: Conventional plants use superheated steam (that is, steam whose temperature is well above the vaporization point at a given pressure). This results in smaller turbine size and greater BTU content per pound of steam. This allows the use of smaller turbines to attain the same power levels. This also eliminates much of the "carry over" of contaminants that can erode and/or foul turbine blades. Nuclear plants use steam that is closer to its saturation point, and this results in greater turbine maintenance requirements.
4. Redundancy: Due to the high cost of reactors, modern nuclear plants are limited to two reactors, while conventional plants for capital ships may have up to eight boilers. The discrepancy in redundancy is obvious.
5. Diplomacy: (Sigh, I really hate this one!) Some "friendly" countries will not allow port visits from nuclear powered vessels. This could be significant in the event of battle damage.
6. Range: Nuclear, no contest.
7. Survivability: The actual survivability of the two designs is similar, with the nuclear plant being somewhat better protected, while the conventional plant is much easier to repair (including "jury rigging" in extremis).
8. The nuclear plant has a smaller above surface IR signature, and possibly
a lower radar image, as a result of the elimination of stacks and stack