The Reactor Pressure Vessel, also know has the RPV, is where all the nuclear reactions take place. The fuel rods and controls rods, collectively known as the core, are arranged in a matrix to carefully control the reactivity across the core.

The RPV is constructed of steel with a stainless steel lining to prevent corrosion occurring due to the high levels of radiation and temperatures. The RPV in a PWR will be completely full of water that is kept as liquid at all times.

The water enters the bottom of the reactor core at about 275°C and is heated as it flows upwards through the reactor core to a temperature of about 315°C, the pressure varies between 150 and 160 bar. The normal pressure is around the 155 bar mark.

Flow rate is the critical reading for this in litres per second and the nominal level would be around 330 lps.

The coolant in a Pressurised Water Reactor (PWR) is H₂O, normal water. It is used to transfer heat out of the reactor.

In a PWR the water is at high pressure to prevent it boiling away as steam. Other coolants that are used include carbon dioxide, heavy water D₂O and even liquid metals such as sodium or lead.

The containment shield is usually made of reinforced concrete plus thick steel plate. The containment shields of modern reactors are designed to withstand the impact of an aeroplane and also prevent the release of radioisotopes from internal accidents such as a core meltdown.

The control rods are used to control the reactivity of the reactor and for shutdown, when refueling can take place.

They will contain boron or cadium as these elements are able to absorb neutrons which therefore stops the neutrons initiating fission reactions.

No fission reactions mean no heat output.

The control rod length will be the same as the fuel rod, about 3 metres.

The Fuel Rod in a modern PWR is typically about 3 metres in length and contains around 300 fuel pellets which are approximately 1cm in diameter and slightly longer than 1 cm in length.

The uranium fuel has a ²³⁵U enrichment level of around 3 - 5% and is in the form of a UO₂ ceramic.

Uranium fission reactions produce neutrons, which give the possibility of a chain reaction but these fission neutrons are moving too fast to be captured by other uranium nuclei and induce further fissions.

The moderator is needed to slow the neutrons to allow the capture to occur.

The optimum transfer of energy occurs between two objects of the same mass.

The PWR moderator is the water that is also used to cool the reactor. It has lots of single protons, which are almost the identical mass of the neutrons so is very efficient at slowing the neutrons.

Carbon, which is a relatively light element, in the form of graphite is also used for moderation.

The moderator will slow the neutrons down from about 2 MeV to about 40 eV which takes about 15 collisions with the protons.

In a PWR the pressuriser is required to maintain the water as liquid with the consequence that the water can be heated to well over 300°C without boiling.

To achieve a pressure of 155 bar, the pressuriser temperature is maintained at 345°C.

86% of the world's electricity is produced by an electrical generator from the rotation of wires in a magnetic field.

Turbines are connected to the electrical generator and the blades of the turbine are rotated by steam from steam generators.

The primary coolant loop contains only liquid water which passes through the steam generator and heats the water in the secondary cooling loop which turns to steam and is fed to the turbine.

The steam generated by the interaction of the primary and secondary cooling loops rotates the turbine.

It is connected to the electrical generator which consists of many metal wires being rotated in a magnetic field which the produces alternating current ( A.C. ) which is used to for our lighting and power.