Nuclear marine propulsion

Nuclear marine propulsion is the method of moving ships and submarines using heat from a nuclear reactor.

This technology is vital for naval operations and specialized civilian vessels. It allows a ship to operate for very long periods without refueling. Since the fuel is contained entirely within the reactor, no space is lost to fuel tanks. There is also no need for exhaust stacks or large air intakes for combustion. While it offers great advantages, the high cost of infrastructure means most nuclear vessels are military.

The mechanism of a pressurized water reactor involves several distinct steps. First, nuclear fission generates intense heat within the reactor core. This heat is transferred to a primary water circuit. This water is kept under high pressure so it does not boil.

Pumps circulate this hot water to a steam generator. In the steam generator, the heat from the primary circuit warms a separate water circuit. This second circuit turns into steam and travels toward a steam turbine. As the steam expands, it hits the rotating blades of the turbine to impart energy. This spinning turbine can turn a propeller through a gearbox. Alternatively, the turbine can turn an electrical generator to power drive motors.

Marine reactors differ significantly from the large power plants found on land. Land-based plants can produce up to 1600 megawatts of electrical power. In contrast, a typical marine propulsion reactor produces only a few hundred megawatts. Space is a major constraint at sea, so marine reactors must be physically small. This requires them to generate much higher power per unit of space. Consequently, the components face much greater physical stresses. They must also withstand vibration and the pitching of a ship in rough seas. Furthermore, engineers cannot rely on gravity to drop control rods during a shutdown. This is because a ship does not always remain upright. Salt water corrosion also presents a constant maintenance challenge.

To achieve long operation times, marine reactors use specialized fuel and design features. The fuel is often a metal-zirconium alloy rather than the ceramic uranium dioxide used on land. This fuel is typically more highly enriched to ensure a sustained reaction in a small core. Some U.S. submarines use fuel enriched to over 96% U-235. This high enrichment increases power density and extends the life of the fuel. To manage the aging fuel, engineers include a "burnable poison" in the fuel elements. This substance is slowly depleted as the fuel ages. As the poison disappears, the reactivity of the core increases to compensate for the aging fuel.

The history of this technology is closely tied to the mid-20th century. In the United States, Admiral Hyman G. Rickover led the development of these plants. The first prototype naval reactor was tested in Idaho in 1953. The USS Nautilus, the first nuclear submarine, went to sea in 1955. This invention revolutionized the submarine by making it a true underwater vessel. It could stay submerged for long periods, limited only by the crew's endurance. In 1960, the Nautilus completed the first submerged circumnavigation of the Earth. The Soviet Union also developed nuclear submarines, such as the Project 627 type.

Today, various nations utilize different types of nuclear marine vessels. The Russian navy operates very large submarines, like the 26,500 tonne Typhoon class. The United States and France have both built nuclear-powered aircraft carriers. The French carrier Charles De Gaulle is a 42,000-tonne flagship. Different navies also use different propulsion methods. The Russian, American, and British navies often use direct steam turbine propulsion. However, French and Chinese ships use turbo-electric transmission to generate electricity for their motors.

Managing nuclear vessels also involves complex legal and environmental responsibilities. Decommissioning nuclear submarines is a major task for the U.S. and Russian navies. In the United States, the reactor section is often cut from the vessel for disposal. In Russia, some vessels or sealed reactor sections are stored afloat. Because an accident could cross national boundaries, insurance is handled differently than for regular ships. The 1962 Brussels Convention attempted to address this, but it was never ratified. In the United States, nuclear reactors under federal jurisdiction are insured via the Price-Anderson Act.