The reactor is an integral PWR in the 200-300 MWe range, e.g. the Westinghouse SMR.
All primary system components (core, control rods, pressurizer, steam generators, reactor coolant pumps) are housed within an integral Reactor Pressure Vessel (RPV) which is located within a compact pressure-suppression containment. The reactor + containment constitute the reactor module, which is housed inside the reactor hull, within a permanently flooded compartment at the bottom of the platform.
The gap between the containment and the reactor hull is normally dry, so the containment is not exposed to seawater. However, that gap can be flooded with seawater to provide passive containment cooling during low-probability accidents, as explained in the Safety Section.
The reactor hull can also be flooded with freshwater from the condensate storage tank during refueling operations. Refueling is performed every 4-5 years; spent fuel assemblies are transferred to the onboard spent fuel pool, which has storage capacity up to the plant lifetime, with a passive decay heat removal system that uses the ocean as its ultimate heat sink.
The steam lines run from the reactor module, to the turbine deck; the feedwater lines follow the inverse path. The power conversion cycle is a traditional Rankine steam cycle operating at typical nuclear BOP conditions and thus capable of reaching a thermal efficiency in the 31-35% range, depending on the local temperature of the ocean. Cooling water is drawn from the bottom of the ocean, and discharged at ambient temperature at the surface, thus eliminating thermal pollution.