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The driving force behind the Royal Navy's submarines is the pressurised water reactor (PWR). 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 heavily shielded reactor compartment that completely protects the crew from radiation.

How the PWR Works

The PWR system is based on primary and secondary circuits. Water coolant travels round the primary circuit, through the reactor pressure vessel (where it is heated by the nuclear fuel elements) and on through tubes in a steam generator. A high pressure is maintained in the primary circuit to prevent the coolant from boiling. In the 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 to movement of the submarine. Steam is also used to drive the turbo-generators that supply the submarine with electricity.

The reactor and steam-generating facilities are operated through an array of automated and manual controls, while a network of electronic and mechanical failsafe devices monitor the reactor at all times. Because of the large amounts of energy stored in the uranium fuel elements, nuclear submarines can travel great distances for many years without refuelling.

Life at Sea

Cruising for weeks on end through the depths of the North Atlantic may seem a daunting prospect to most people but in truth Britain's modern ballistic missile submarines are well suited to these long-range patrols, both in terms of size and facilities. The messes in Vanguard are comfortable and well appointed, fulfilling the role of dining room, lecture hall, lounge, games area and venue for church services. The food on board is good and there is plenty of it.

One of the highlights of the week is the 'familygram', when members of the crew receive short messages from family and friends back home. Unfortunately these greetings cannot be returned, as this would reveal the submarine.s position. A deterrent patrol owes its success to total secrecy and the submarine has to remain completely hidden from the outside world.