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

The function of a moderator in a nuclear reactor is to slow down neutrons produced during the fission process, ensuring a controlled and sustained chain reaction. In a pressurized water generator (PWR), light water (H2O) is used as the moderator. Other substances used as moderators in nuclear reactor designs include heavy water (D2O), graphite, and beryllium.

Step-by-step solution

Function of a Moderator

The primary function of a moderator in a nuclear reactor is to slow down the neutrons produced during the fission process. By slowing down the neutrons, the moderator increases the likelihood of further reactions and ensures a controlled and sustained chain reaction. Slow neutrons are more likely to cause fission events in the fuel, which in turn generates more neutrons, continuing the nuclear chain reaction.

Moderator in Pressurized Water Generator

In a pressurized water generator (Pressurized Water Reactor or PWR), the substance that acts as a moderator is light water (ordinary water, H2O). Moreover, in PWRs, light water also serves as a coolant and transfers the heat generated during the fission process to the steam generators, which is later used to produce electricity.

Other Substances Used as Moderators

Apart from light water, there are several other substances that can be used as moderators in nuclear reactor designs. Some examples of these substances include: 1. Heavy water (D2O): Heavy water is a form of water containing a larger than normal amount of the hydrogen isotope deuterium and is used as a moderator in certain types of reactors, such as the CANDU reactor. 2. Graphite: Graphite is a commonly used moderator in gas-cooled reactors (such as the Advanced Gas-cooled Reactor, AGR) due to its excellent neutron moderation properties and high thermal conductivity. 3. Beryllium: Beryllium has some advantages as a moderator due to its low neutron absorption cross-section and high scattering cross-section. However, its high production cost and the issue of handling beryllium-associated health risks have limited its use in commercial reactors. These substances slow down neutrons by scattering collisions with their nuclei, thus increasing the probability of further fission events and maintaining a sustained chain reaction within the reactor.

Key concepts

Nuclear Fission Process

The nuclear fission process is a fundamental concept in nuclear reactors. It involves the splitting of a heavy atomic nucleus into two smaller ones, along with the release of a significant amount of energy and additional neutrons. These additional neutrons are critical because they perpetuate the process by colliding with other fissile nuclei, causing further fission events.
The chain reaction is maintained by these neutrons, which need to interact with the fuel effectively. For the fission process to be sustainable, it is crucial that enough neutrons from each fission event go on to trigger further fissions. However, the neutrons released are initially fast and need to be slowed down to increase the likelihood of collisions with other nuclei. This is where moderators become essential in the fission process. They help to slow down the neutrons to thermal energies, making them more likely to induce further fission reactions.

Pressurized Water Reactor

The pressurized water reactor (PWR) is one of the most common types of nuclear reactors in use today. One of its key features is using ordinary light water as both a coolant and a moderator. In a PWR, the reactor core is filled with water, which acts as a medium to decelerate neutrons and also absorb the heat produced during the fission process.
Unlike other reactor types, the water in a PWR is kept under high pressure to prevent it from boiling within the reactor core. This pressurized water passes its thermal energy on to a secondary water system via a heat exchanger called a steam generator. This secondary system produces steam, which then drives turbines to generate electricity. This method is efficient and contributes to the widespread adoption of PWRs worldwide.

Heavy Water

Heavy water, or deuterium oxide ( D_2O ), serves as another efficient moderator in certain types of reactors, such as the CANDU reactors. Heavy water contains a larger concentration of deuterium, an isotope of hydrogen, compared to normal water. This gives heavy water its unique properties as a moderator.
One notable advantage of heavy water is its lower neutron absorption rate compared to light water. This means more neutrons remain available to continue the fission process, enhancing the reactor's efficiency. Furthermore, heavy water reactors do not require enriched uranium as fuel, which is a distinct advantage in certain nuclear designs. However, heavy water is also more expensive to produce than ordinary water, which is an important consideration when designing reactors.

Graphite Moderators

Graphite is another commonly used material for moderating neutrons in nuclear reactors. Its primary advantage lies in its ability to slow down neutrons through scattering without absorbing them, making it effective in maintaining a chain reaction.
Graphite moderators are often used in gas-cooled reactors, such as the UK’s Advanced Gas-cooled Reactor (AGR). Graphite is preferred for its high thermal conductivity, which helps in managing the heat produced and maintaining the reactor's efficiency.
One of the challenges with graphite moderators is the potential for them to oxidize at high temperatures. Hence, reactors using graphite moderators must be designed to control this risk effectively.

Beryllium in Reactors

Beryllium, although not widely used, has certain properties that make it a potential moderator in reactors. It has a very low neutron absorption cross-section and a high scattering cross-section, which are desirable characteristics for a moderator.

• Beryllium has a low density, which can be advantageous when designing reactors that require lightweight components.
• Its ability to reflect neutrons back into the reactor core can help maintain the fission process.

Despite these benefits, beryllium is expensive to produce and handle. It also poses health risks due to its toxic nature, which limits its use in commercial reactors. However, in specific applications where these issues can be managed, beryllium may still serve as an effective moderator in nuclear designs.