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Nuclear power in submarines

Propulsion System Layout
Reactor Compartment
Reactor Compartment position

The driving force behind the Royal Navy's submarines is the pressurised water reactor (PWR).

Pressurised Water Reactor

In simple terms, this is a collection of fissile uranium fuel elements which provide huge amounts of energy to power the vessel (a ton of fissionable material releases the energy equivalent of 2.5 million tons of coal.

The entire process takes place inside a heavy shielded reactor compartment that completely protects the crew from radiation.

The PWR system is based on primary and secondary circuits. Water coolant travels around the primarily circuit, through the reactor pressure vessel (where it is heated by the nuclear fuel elements) and on through the tubes in a stream generator. A high pressure is maintained in the primary circuit to prevent the coolant from boiling.

In a steam generator, the heat from the primary coolant is used to convert the water outside the tubes into steam, which is used to drive the main turbine engines.

From there a system of clutches, gearing and propulsion transmits the power into movement of the submarine. Steam is also used to drive the turbo-generators that supply the submarine with electricity.

In a number of the elements, (such as the uranium used in the fuel plates of a nuclear reactor), the nucleus of the atom is just stable. If a neutron is fired at a uranium atom, the uranium nucleus absorbs the neutron and becomes unstable. The atom splits and releases a great deal of energy, which is used to generate steam to power the turbines.

Fission reaction creating energy to produce steam

However when this 'fission' happens not only is the energy produced, radiation and additional neutrons are also produced and the uranium atom changes into a variety of new elements or 'fission products'.

The extra neutrons produced help continue the reaction and in time sufficient neutrons are produced which enable the reactor to become 'critical'. The chain reaction is then in a self-sustaining state.

The nuclear fission process needs to be controlled to prevent overheating. Control rods are inserted into the reactor core, more neutrons are absorbed. The fission reaction can be controlled as fewer neutrons hit the uranium atoms.

When the control rods are withdrawn, a greater number of neutrons build up within the core increasing the chain reaction and the heat produced.

Reactor shutdown

The submarines reactor is shut down when the vessel is in harbour. Before the reactor is shut down, electricity from the shore is required to power the submarine's mechanical and electrical systems such as pumps and back up systems.

Although the reactor is shut down, fission products will still continue to decay and these produce heat. A shore cooling system is therefore required until the reactor has cooled down. There are also a number of backup systems to cool the reactor down.