Nuclear Power Generation

Power generation in all nuclear reactors are based on nuclear fission but the design of reactors may be different. The commonly used reactor types in the world are the Pressurised Water Reactor, Boiling Water Reactor and Pressurised Heavy Water Reactor.

In addition to these three widely used reactors, there are other types of reactors under commercial operation in nuclear power stations worldwide. They include Gas-cooled, Light Water Graphite and Fast Neutron Reactor.

Pressurised Water Reactors (PWRs) – 60%

PWR is the most common type of nuclear reactor, representing about 60% of all nuclear power reactors in the world. PWRs are adopted in countries such as the United States, France, Japan, Russia and China.

PWRs use light water (ordinary water) for neutron moderation* and reactor heat removal. The water inside the primary cooling circuit of PWR is under high pressure, and it will not turn into steam even under high temperature. The primary circuit and the secondary circuit are completely separated, and heat energy will be transferred from the primary circuit to the secondary circuit. With a lower pressure in the secondary circuit, steam is raised to drive a turbine-generator to produce electricity. These reactors use U235 of a typically 3%-4.5% enrichment.

PWR's two cooling systems separate the reactor cooling water and steam for power generation. In the event of necessary venting, steam released will be free from radioactive products.

*Moderation slows down the speed of the neutrons so that fission may take place with U235 at a low enrichment.

Boiling Water Reactors (BWRs) – 20%

BWRs use U235 of a typically 2%–4% enrichment and account for around 20% of all nuclear power reactors. BWRs are adopted in countries such as the United States, Japan and Sweden.

BWRs use light water for neutron moderation and reactor heat removal. The heat raises steam directly in their reactor pressure vessel to drive a turbine-generator to produce electricity.

BWR's basic design is similar to that of PWR, except that it uses only one single circuit in which the water is at lower pressure. As the water around the core of the reactor always contains some traces of radionuclides, should necessary venting occur, any steam released could contain radioactive products.

Pressurised Heavy Water Reactors (PHWRs) - 10%

Usually referred as CANDU, PHWR was developed in the 1950s and is widely used in countries such as Canada and India. PHWR represents about 10% of all nuclear power reactors in the world.

This type of reactor uses Uranium at its natural level of around 0.7% U235 concentration with no enrichment. It uses heavy water* for neutron moderation and reactor heat removal. Heavy water absorbs the fewest neutrons among common moderator material so that it will least suppress the chain reaction. This heavy water flows inside pressure tubes filled with Uranium, taking away reactor heat and delivering it to an adjoining circuit to raise steam and drive a turbine-generator for production of electricity.

PWHR's pressurised tube design enables refueling of the reactor during operation, by isolating individual pressure tubes from the cooling circuit.

*Heavy water refers to water in which the ordinary hydrogen atoms (containing only 1 proton in the nucleus) are replaced by heavier hydrogen atoms (containing 1 proton and 1 neutron in the nucleus), which can help achieve a more efficient fission process.