Pressurized Water Reactors

Pressurized Water Reactors

The pressurized water nuclear reactor is the type of nuclear reactor more used worldwide in the nuclear power plants of generation of electricity. At present, there are more than 230 nuclear reactors in the world made with the pressurized water system. Also known by its abbreviations PW. Its main feature is the use of water under high pressure in the primary circuit to prevent it from boiling.

Within the naval engineering the pressurized water nuclear reactor (PWR) is widely used. In fact, this model was originally designed to be used on a nuclear submarine.

Pressurized water reactors use enriched uranium as a nuclear fuel.

Operation of the pressurized water nuclear reactor (PWR)

The operation of pressurized water nuclear reactors (PWR) can be divided into 4 main steps.

  • The core of the reactor inside the reactor vessel generates heat.
  • High pressure water from the primary circuit transports this thermal energy to a steam generator.
  • Inside the steam generator, the heat coming from the primary circuit turns the water in the secondary circuit into steam.
  • Generated steam drives a turbine that produces electricity.
  • During these four points, the pressurized water nuclear reactor has turned the nuclear energy of the nuclear fuel into electrical energy. From here, the cycle begins again: the residual vapor is converted back into liquid water through a condenser. This capacitor puts the secondary circuit in thermal contact with a tertiary circuit through which cold water flows from the outside (sea water, rivers, lakes, etc.). Once converted to liquid water it returns to the steam generator driven by a series of water pumps.

    The reactor core contains nuclear fuel rods that must be refrigerated. Cooling of the reactor core is done by water circulating through a set of water pumps. The water pumps in charge of cooling the core of the nuclear reactor run on mains electricity. In the event of a power failure, an alternative mechanism is activated to be fed by diesel heat engines, diesel generators.

    The heat exchange between the primary and secondary circuit must be done without the water mixing. This mixture should be avoided because the primary circuit water is radioactive.

    Nuclear power plant - Pressure water nuclear reactor

    Coolant in pressurized water nuclear reactors

    To cool the pressurized water reactors (PWR type reactors), ordinary water is used. In the field of nuclear engineering is called light water. Water can reach temperatures up to 315 ° C. The water remains essentially in the liquid phase (although there is nucleated boiling in the core area) due to the high pressure at which the primary circuit operates.

    In the primary reactor circuit the water works under a pressure of around 16MPa.

    Nuclear moderator in pressurized water nuclear reactors

    Nuclear fission reactions occurring within the reactor core generate fast neutrons. Like all thermal reactors, the pressurized water nuclear reactor (PWR) requires fast neutrons to lose speed in order to maintain the chain reaction.

    The charge that neutrons lose speed is the nuclear moderator. In PWR nuclear reactors water used as a refrigerant also acts as a neutron moderator material. Since the mass of hydrogen nuclei found in a water molecule is similar (actually somewhat larger) to the mass of a neutron, neutrons are slowing down as they collide with water molecules.

    The higher the density of water, the greater the power of moderation. As water density increases there are more water molecules with which fast neutrons can collide. This feature is very important in terms of safety: in case of an increase in the temperature of the moderator (of water), its density decreases, reducing the effect of moderation. If they lose moderation they will remain fast and will be more difficult to generate more fission reactions. This effect makes PWR reactors very stable.

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    Last review: June 16, 2017