Reactor core: The part of the nuclear reactor where the nuclear fuel assembly is located.
Moderator: The material in the core which slows down the neutrons released during fission, so that they cause more fission. It is usually ordinary water (used in Light Water Reactors) or heavy water (used in Heavy Water Reactors).
Control rods: These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or halt it.
Coolant: A liquid or gas circulating through the core so as to transfer the heat from it. This primary coolant passes through a steam generator (except in Boiling Water Reactors or BWRs), where the heat is transferred to another loop of water (in the so-called secondary circuit) to convert it into steam. This steam drives the turbine. The advantage of this design is that the primary coolant, which has become radioactive, does not come into contact with the turbine.
Pressure vessel: Usually a robust steel vessel containing the reactor core and moderator/coolant.
Steam generator (not in BWRs): Here, the primary coolant bringing heat from the reactor transfers its heat to water in the secondary circuit to convert it into steam.
Containment: This is typically a metre-thick concrete and steel structure around the reactor core. After the zirconium fuel cladding and the reactor pressure vessel, this is the last barrier against a catastrophic release of radioactivity into the atmosphere. Apart from a primary containment, many reactors have a secondary containment too, which is normally a concrete dome enveloping the primary containment as well as the steam systems. This is very common in BWRs, as here most of the steam systems, including the turbine, contain radioactive materials.
At a basic level, reactors may be classified into two classes: Light Water Reactors (LWRs) and Heavy Water Reactors (HWRs). LWRs are largely of two types, Pressurised Water Reactors (PWRs) and Boiling Water Reactors (BWRs). LWRs, and of them, the PWRs, are the most widespread reactors in operation today. Heavy Water Reactors can also be of different types, one of the most well known being the CANDU reactors developed by Canada, which are a type of Pressurised Heavy Water Reactors (PHWRs). Most of India's indigenous reactors are CANDU reactors.
Below, we discuss the most well-known type of nuclear power reactor—the PWR, and also the reactor design of most of India's reactors—the PHWR or CANDU reactor.
A PWR uses ordinary water as both coolant and moderator. It has three water circuits. Water in the primary circuit which flows through the core of the reactor reaches about 325°C; hence it must be kept under about 150 times atmospheric pressure to prevent it from boiling. Water in the primary circuit is also the moderator, and if it starts turning into steam, the fission reaction would slow down. This negative feedback effect is one of the safety features of this type of reactors.
The hot water from the primary cooling circuit heats the water in the secondary circuit, which is under less pressure and therefore gets converted into steam. The steam drives the turbine to produce electricity. The steam is then condensed by water flowing in the tertiary circuit and returned to the steam generator.
A PHWR uses heavy water as the coolant and moderator, instead of ordinary water. Heavy water is a more efficient moderator than ordinary water as it absorbs 600 times fewer neutrons than the latter, implying that the PHWR is more efficient in fissioning U-235 nuclei. Hence, it can sustain a chain reaction with lesser number of U-235 nuclei in uranium as compared to PWRs. Therefore, PHWR uses unenriched uranium, that is, natural uranium (0.7% U-235) oxide, as nuclear fuel, thus saving on enrichment costs. On the other hand, the disadvantage with using heavy water is that it is very costly, costing hundreds of dollars per kilogram.
Conceptually, this reactor is similar to PWRs discussed above. Fission reactions in the reactor core heat the heavy water. This coolant is kept under high pressure to raise its boiling point and avoid significant steam formation in the primary circuit. The hot heavy water generated in this primary circuit is passed through a heat exchanger to heat the ordinary water flowing in the less-pressurised secondary circuit. This water turns to steam and powers the turbine to generate electricity.
The difference in design with PWRs is that the heavy water being used as moderator is kept in a large tank called Calandria and is under low pressure. The heavy water under high pressure that serves as the coolant is kept in small tubes, each 10 cms in diameter, which also contain the fuel bundles. These tubes are then immersed in the moderator tank, the Calandria.