Problem 85 - Master Resource Book in JEE Main Physics

Question

A moderator is used in nuclear reactors in order to (a) slow down the speed of the neutrons (b) accelerate the neutrons (c) increase the number of neutrons (d) decrease the number of neutrons

Step-by-Step Solution

Answer

Option (a): The moderator slows down the speed of neutrons.

Step 1: Understanding Moderation

In a nuclear reactor, moderation is a process where fast neutrons are slowed down. This is crucial because slower neutrons are more likely to induce fission. Thus, the moderator is an essential component that helps maintain the chain reaction within a reactor.

Step 2: Analyzing the Options

Let's examine each option: (a) A moderator is indeed used to slow down neutrons, making them effective in sustaining nuclear reactions. (b) Accelerating neutrons is not the function of a moderator. (c) Increasing the number of neutrons is not directly the role of a moderator; it instead makes the available neutrons more effective. (d) Decreasing the number of neutrons is typically not desired and is not the role of a moderator.

Step 3: Selecting the Correct Answer

Given the role of a moderator to slow down fast-moving neutrons so they can effectively sustain a fission chain reaction, the correct choice aligns with option (a), which states the moderator slows down the speed of neutrons.

Key Concepts

Neutron Moderation

Neutron moderation is an essential process in the functioning of nuclear reactors. Imagine a pinball machine where the balls must move at a precise speed to hit the targets — in this case, those targets are the atomic nuclei ready to fission. Without control, the balls would speed through without effectively striking the targets. Similarly, in a nuclear reactor, neutrons emitted during nuclear fission move extremely fast. Unfortunately, high-speed neutrons tend to pass by atomic nuclei without causing them to fission.

To enhance their chances of successfully inducing fission, these neutrons need to be slowed down. This is where a moderator comes into play. The moderator is a material placed within the reactor to reduce the speed of fast neutrons, converting them into thermal neutrons. Thermal neutrons are much more likely to sustain the chain reaction necessary for a stable and controlled release of energy in a nuclear reactor. Hence, neutron moderation is vital to converting an erratic and potentially dangerous reaction into a controlled and productive one.

Some common materials used as moderators include water, heavy water, and graphite. These substances are particularly effective because they decrease neutron speed without absorbing them significantly.

Chain Reaction

A chain reaction is a self-sustaining series of reactions where the products of a reaction initiate further reactions. In the context of nuclear reactors, a chain reaction typically refers to the repeated process of nuclear fission. Here's a simple analogy: think of a line of standing dominos, where tipping one leads to the rest falling in sequence.

In a nuclear chain reaction, a neutron collides with a fissile atomic nucleus, such as uranium-235 or plutonium-239, causing it to split and release energy alongside additional neutrons. Each of these newly released neutrons can then collide with and split other fissile nuclei, perpetuating the process.

For a chain reaction to be sustained, the number of neutrons produced must be sufficient to cause further fission reactions at a steady rate. This ongoing repeat of reactions is critical for generating energy consistently. If the chain reaction is allowed to progress uncontrolled, it can lead to disastrous consequences, including core meltdown. Therefore, precise control of the chain reaction is imperative in nuclear reactor safety.

• Gradual and continuous monitoring of the neutron flux is vital.
• Control rods and neutron moderators help manage the rate of reaction.
• Sustaining a controlled chain reaction ensures a stable energy output.

Nuclear Fission

Nuclear fission is the foundational process that powers nuclear reactors. It occurs when a large and unstable atomic nucleus splits into two smaller nuclei along with the release of a significant amount of energy. This splitting is usually prompted by the collision of the nucleus with a neutron.

This process of fission releases a tremendous amount of energy, primarily in the form of kinetic energy of the resulting fission products, which is then converted into heat. This heat is used to produce steam, which ultimately drives turbines to generate electricity.

Another crucial aspect of fission is its ability to produce more neutrons. These surplus neutrons can further induce fission reactions in nearby fissile material, thereby sustaining a chain reaction. However, not every fission event results in a net loss or gain of neutrons. The balance is contingent upon the reactor's design and the materials used, particularly the choice of neutron moderators and control mechanisms.

By understanding and utilizing nuclear fission, humanity has accessed a potent source of energy capable of powering cities and offering alternatives to traditional fossil fuels. Nonetheless, the use of nuclear fission also comes with responsibilities, as managing the byproducts and ensuring safety are paramount.