Question

(a) What is the function of the moderator in a nuclear reactor? (b) What substance acts as the moderator in a pressurized water generator? (c) What other substances are used as a moderator in nuclear reactor designs?

Short answer

(a) The function of the moderator in a nuclear reactor is to slow down neutrons produced during nuclear fission, increasing the likelihood of these neutrons interacting with fissile materials and maintaining a consistent chain reaction. (b) In a pressurized water reactor (PWR), ordinary water (H₂O) acts as the moderator. (c) Other substances used as moderators in nuclear reactor designs include heavy water (D₂O) and graphite.

Step-by-step solution

(a) Moderator Function in a Nuclear Reactor

The function of a moderator in a nuclear reactor is to slow down the neutrons produced during nuclear fission. When neutrons move at slower speeds, known as thermal neutrons, there is a greater likelihood of these neutrons interacting with fissile materials, such as uranium-235 or plutonium-239, causing further fission reactions and maintaining a consistent chain reaction in the reactor.

(b) Moderator Substance in Pressurized Water Generator

In a pressurized water reactor (PWR), the substance that acts as the moderator is ordinary water, also known as light water or H₂O.

(c) Other Substances Used as Moderators

Other substances that are used as moderators in nuclear reactor designs include: 1. Heavy water (D₂O) – In heavy water-moderated reactors, such as the CANDU (CANadian Deuterium Uranium) reactors, deuterium oxide acts as the moderator. Heavy water has a higher effectiveness in slowing down neutrons compared to light water. 2. Graphite – Graphite is used as a moderator in some reactor designs, such as the RBMK (Reaktor Bolshoy Moshchnosti Kanalny) reactors. Graphite is a good moderator due to its ability to slow down neutrons without absorbing them, thereby maintaining the chain reaction.

Key concepts

Nuclear Fission

Nuclear fission is a process in which the nucleus of an atom splits into two or more smaller nuclei, along with a few neutrons and a large amount of energy. This splitting is often triggered by the absorption of a neutron by a fissile nucleus, like uranium-235. When fission occurs, it releases more neutrons, which can then cause further fission reactions in a chain reaction. These fissions release a significant amount of energy, which is the basis for both nuclear power and nuclear weapons.

For a sustainable and controlled nuclear reaction in power plants, a critical mass of the fissile material is required along with a proper arrangement to ensure that new fissions are induced. If too many neutrons escape or are absorbed by non-fissile material, the chain reaction will dwindle; Conversely, if too many fission reactions occur, it may lead to an uncontrolled release of energy—a situation that safety mechanisms in nuclear reactors are designed to prevent.

Pressurized Water Reactor

A pressurized water reactor (PWR) is one of the most common types of nuclear reactors found around the world. In a PWR, the nuclear reactor core generates heat through fission, and this heat is transferred to water under high pressure. This water acts both as a coolant and as a moderator. Because the water is pressurized, it does not boil even at high temperatures.

The heated, pressurized water is then passed through a heat exchanger, where it transfers its heat to a secondary loop of water. This secondary loop is not pressurized to the same extent and can boil into steam. The steam then drives turbines, which generate electricity. This two-loop system separates the radioactive water in the reactor from the water-steam system that drives the turbines, enhancing safety.

Neutron Moderation

Neutron moderation is a critical process in a nuclear reactor to sustain a chain reaction. As mentioned earlier, neutrons released from fission reactions are typically very fast. To continue the chain reaction, these fast neutrons must be slowed down to thermal energies, where they are more likely to cause further fission in the fissile material.

Moderation is accomplished by a material called a ‘moderator.’ As fast neutrons pass through this moderator, they collide with its nuclei and lose energy in the process. The neutrons are thereby slowed to thermal energies, making them effective at inducing fission in other fissile nuclei. The effectiveness of a moderator depends on its ability to slow down neutrons and its low neutron absorption properties. For this reason, materials such as ordinary water (light water), heavy water, and graphite are commonly used as moderators.

Fissile Materials

Fissile materials are a subset of nuclear materials that are capable of sustaining a nuclear fission reaction. The most common fissile materials used in nuclear reactors are uranium-235 and plutonium-239. These materials have large enough nuclei that are unstable enough to split when hit by a neutron, and crucially, the fission of these materials releases additional neutrons.

Alongside being fissile, these materials should be present in the reactor core in sufficient quantities (known as critical mass) to maintain the chain reaction. The careful handling and enrichment of fissile materials are key to the operation of nuclear reactors, with various safety measures in place to ensure that the nuclear reaction remains controlled and the risk of accidents is minimized. Alternative fissile materials can include uranium-233, which can be bred from thorium-232—a process being researched for future reactor designs.