A reactor at sea cannot count on gravity to shut itself down
On land, a reactor's control rods can simply drop into place under gravity. A ship pitches and rolls, so marine reactors need shutdown systems that work at any angle. That is just one reason naval reactors are built so differently, and by 1990 more reactors were driving ships than generating commercial electricity worldwide.
The basic idea resembles a steam ship with an unusual boiler. In most naval designs, water under high pressure carries heat from the fuel to a steam generator at around 250 to 300 degrees Celsius without boiling, keeping any radioactive contamination inside that loop. A second circuit turns to steam, spins a turbine, then condenses against cold seawater and returns. Russian, American and British vessels gear the turbine directly to the propellers, while French and Chinese ships use it to make electricity for drive motors.
Space is the big constraint. Land plants can sprawl over hundreds of hectares and produce up to roughly 1,600 megawatts, whereas a marine reactor delivers a few hundred at most and must pack more power into a small volume, putting its parts under greater stress amid vibration, salt corrosion and rough seas. A small core lets more neutrons escape before splitting an atom, so the fuel is usually more enriched. American submarines use uranium above 96 percent uranium-235, which allows a smaller, quieter core and very long fuel life, at higher cost and greater proliferation risk.
Designers stretch that life further with a burnable poison mixed into the fuel. It soaks up neutrons early on and fades as the fuel ages, balancing out the core's declining reactivity. Fuel is a metal and zirconium alloy rather than the ceramic uranium dioxide common ashore, and everything must run with little maintenance thousands of miles from port.
The work began in the 1940s in the United States under Hyman Rickover. Nearly all nuclear vessels remain military, because cheap fuel is outweighed by heavy operating and infrastructure costs, though icebreakers and a few cargo ships exist. Retirement is its own challenge: the US cuts out reactor sections for shallow land burial, while Russia has usually kept them afloat.
Source: Nuclear marine propulsion
