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Chapter 21: Problem 60

(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

Expert verified
(a) The function of the moderator in a nuclear reactor is to slow down fast neutrons produced during the fission process, increasing their probability of causing further fission in neighboring fuel nuclei, and thus maintaining a controlled chain reaction. (b) In a pressurized water reactor (PWR), ordinary water (H鈧侽) acts as the moderator. (c) Other substances used as moderators in nuclear reactor designs include heavy water (D鈧侽), graphite, and beryllium.

Step by step solution

01

(a) Function of the Moderator in a Nuclear Reactor

The moderator is an important component in a nuclear reactor. Its main function is to slow down the neutrons produced during the fission process. Fast neutrons, generated when the nucleus of a fuel atom undergoes fission, have too much energy for them to effectively sustain the chain reaction. Slowing down these neutrons increases the probability of them causing further fission in neighboring fuel nuclei, thus maintaining a controlled chain reaction.
02

(b) Moderator in a Pressurized Water Generator

In a pressurized water reactor (PWR), the most common substance used as a moderator is ordinary water, also known as light water or H鈧侽. The water in the reactor core not only acts as a coolant, removing heat generated during the fission process but also serves as a moderator, slowing down fast neutrons to sustain the chain reaction.
03

(c) Other Substances Used as Moderators in Nuclear Reactor Designs

Apart from ordinary water, there are other substances that are used as moderators in various nuclear reactor designs. Some of these substances include: 1. Heavy water (D鈧侽): Deuterium oxide, or heavy water, is used as a moderator in heavy water reactors such as the CANDU (Canadian Deuterium Uranium) reactor. It is more effective at slowing down neutrons than ordinary water without absorbing many of them, thus allowing the use of natural uranium as fuel. 2. Graphite: Graphite, a crystalline form of carbon, is used as a moderator in certain types of reactors, such as the RBMK (Reaktor Bolshoy Moshchnosti Kanalnyy) design, originally developed in the Soviet Union. Graphite is an excellent moderator because it can slow down neutrons effectively without capturing many of them. 3. Beryllium: Beryllium is another substance used as a moderator in some reactor designs due to its excellent neutron reflection and low neutron absorption properties. Beryllium is used primarily in research reactors and some advanced reactor designs.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Moderator Function
In a nuclear reactor, the moderator plays a crucial role. This essential component serves to slow down the neutrons resulting from nuclear fission. When a nucleus splits, it releases fast-moving neutrons with high energy. If these neutrons remain too speedy, they might escape the vicinity of the nuclear fuel without contributing further to the fission process. By lowering their velocity, the moderator increases the likelihood that these neutrons will collide with and split other nuclei in the reactor fuel. This action is critical, maintaining a continuous chain reaction. Without this moderation, a reactor wouldn't efficiently sustain the fission process needed for controlled energy production.
Pressurized Water Reactor
A Pressurized Water Reactor (PWR) is a popular type of nuclear reactor used worldwide. It uses ordinary water, or light water, as both a coolant and a moderator. The mechanism involves keeping the water at high pressure, ensuring it remains in liquid form even at high temperatures within the reactor core. This water absorbs heat from the nuclear reaction. Not only does it remove this heat away to be used for energy generation, but the water also moderates. It slows down the fast-moving neutrons produced during fission, ensuring they are effective in maintaining the chain reaction. Thanks to this dual function, PWRs are efficient and widely utilized.
Nuclear Fission
Nuclear fission is the process that fuels nuclear reactors. During fission, a heavy nucleus, like uranium or plutonium, splits into smaller nuclei, releasing vast amounts of energy, along with several fast-moving neutrons. These neutrons then can cause additional fissions, maintaining the chain reaction. It's a naturally occurring process, but in reactors, it is controlled meticulously. When neutrons from each fission event initiate further splits, the reaction eases the production of continuous energy. This process is pivotal for the generation of nuclear energy, converting the enormous energy released into electrical power for everyday use.
Heavy Water
Heavy Water, or D鈧侽, distinguishes itself by containing deuterium, an isotope of hydrogen with one extra neutron. This unique composition makes heavy water an effective nuclear reactor moderator. In reactors like the CANDU, heavy water reduces the speed of high-energy neutrons, allowing them to effectively initiate fission in nuclear fuel. The added advantage of heavy water is its low neutron absorption, making it possible to use natural uranium as fuel. This capability makes heavy water a valuable resource in certain reactor designs, enabling efficient and sustainable nuclear reactions.
Graphite Moderator
Graphite moderators are used in specific reactor types, known for their capacity to slow down neutrons effectively without absorbing them excessively. Graphite, made from carbon atoms arranged in a crystalline structure, stands out because of its ability to moderate neutrons and maintain a sustained chain reaction. Reactors like the RBMK, developed in the Soviet Union, rely on graphite for moderation. Its use demonstrates the adaptability of reactor designs, applying different materials to match the distinct moderation needs required for efficient fission processes.
Beryllium Moderator
Beryllium emerges as a noteworthy choice for moderation in some cutting-edge reactors and research facilities. Known for its low neutron absorption and excellent reflection properties, beryllium enhances the reactor's ability to maintain a steady chain reaction. Though often less commonly used than water or graphite, it serves specific roles primarily in experimental or specialized reactors. Beryllium's properties allow it to return "bounced" neutrons back into the reactor core, thus contributing effectively to an ongoing fission process without significant neutron loss. This quality makes it a valuable resource in advanced reactor applications.

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Most popular questions from this chapter

The Sun radiates energy into space at the rate of \(3.9 \times 10^{26} \mathrm{~J} / \mathrm{s} .\) (a) Calculate the rate of mass loss from the Sun in kg/s. (b) How does this mass loss arise? (c) It is estimated that the Sun contains \(9 \times 10^{56}\) free protons. How many protons per second are consumed in nuclear reactions in the Sun?

According to current regulations, the maximum permissible dose of strontium-90 in the body of an adult is \(1 \mu \mathrm{Ci}\left(1 \times 10^{-6} \mathrm{Ci}\right)\). Using the relationship rate \(=k N\), calculate the number of atoms of strontium-90 to which this dose corresponds. To what mass of strontium-90 does this correspond? The half-life for strontium-90 is \(28.8 \mathrm{yr}\)

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