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. 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.
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. 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. Nuclear plants use steam that is closer to its saturation point, and this results in greater turbine maintenance requirements.