Problem 10 - A Complete Resource Book in Chemistry for JEE Main

Question

The moderator used in nuclear reactors is (a) TEL (b) \(\mathrm{D}_{2} \mathrm{O}\) (c) \(\mathrm{H}_{2} \mathrm{O}_{2}\) (d) \(\mathrm{R}-\mathrm{O}-\mathrm{R}\)

Step-by-Step Solution

Answer

The moderator used in nuclear reactors is \(\mathrm{D}_{2} \mathrm{O}\).

Step 1 - Understand the Question

The question is asking about a substance that is commonly used as a moderator in nuclear reactors. Moderators in reactors are used to slow down neutrons, making them more likely to cause additional fission reactions.

Step 2 - Examine Option (a) TEL

TEL, or tetraethyl lead, is not used as a moderator in nuclear reactors. It is a compound that was historically used as an anti-knock agent in gasoline.

Step 3 - Examine Option (b) \(\mathrm{D}_{2} \mathrm{O}\)

Heavy water, \(\mathrm{D}_{2} \mathrm{O}\), is a form of water where both hydrogen atoms are replaced by deuterium, an isotope of hydrogen. It is commonly used as a moderator in certain types of nuclear reactors, such as the CANDU reactor.

Step 4 - Examine Option (c) \(\mathrm{H}_{2} \mathrm{O}_{2}\)

Hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}\), is a chemical compound with uses that include bleaching and disinfection, but it is not used as a moderator in nuclear reactors.

Step 5 - Examine Option (d) \(\mathrm{R}-\mathrm{O}-\mathrm{R}\)

The formula \(\mathrm{R}-\mathrm{O}-\mathrm{R}\) represents an ether, which is a class of organic compounds, not related to moderating in nuclear reactors.

Step 6 - Identify the Correct Answer

Given the analysis, \(\mathrm{D}_{2} \mathrm{O}\) is the substance among the options provided that is widely recognized and used as a neutron moderator in nuclear reactors.

Key Concepts

Heavy Water (D2O)

Heavy water, symbolized as \( \mathrm{D}_{2} \mathrm{O} \), is a special kind of water where the normal hydrogen atoms are replaced by deuterium. Deuterium is simply a hydrogen isotope with an additional neutron, making it twice as heavy as normal hydrogen. Therefore, heavy water is slightly heavier than regular water. This small change gives it unique properties that make it useful in nuclear reactors.

In nuclear reactors, heavy water acts as an effective neutron moderator. The role of a moderator is crucial because it helps slow down fast-moving neutrons produced during fission reactions. Why slow them down? Slower neutrons increase the likelihood of further fission reactions, which are necessary for sustaining a controlled nuclear chain reaction. Heavy water is especially good at this because it absorbs fewer neutrons than normal water, allowing the reaction to be sustained efficiently.

One of the most well-known types of reactors using heavy water is the Canadian Deuterium Uranium (CANDU) reactor. This type of reactor can use natural, un-enriched uranium as fuel, a benefit directly tied to the neutron moderation ability of heavy water.

Nuclear Fission Reactions

Nuclear fission is a fascinating process that occurs when the nucleus of an atom splits into two or more smaller nuclei. This splitting releases a huge amount of energy, which can be harnessed in nuclear reactors to generate electricity.

Here's how it works:

• When a large, unstable nucleus, like uranium-235 or plutonium-239, absorbs a neutron, it becomes even more unstable.
• This instability causes the nucleus to split into two smaller nuclei, releasing energy and, importantly, additional neutrons.
• The emitted neutrons can strike other nearby nuclei, causing further fission reactions.

This chain reaction is what allows a nuclear reactor to produce a steady supply of energy. The key is to maintain a balance where each fission event causes exactly one more. Thus, keeping the reaction continuously but steadily going without going out of control.

Manually managing this reaction involves the use of moderators and control rods, ensuring the chain reaction runs smoothly, efficiently and most importantly, safely.

Neutron Moderation

Neutron moderation is an essential process in nuclear reactors, particularly in maintaining a steady chain reaction. When uranium or another fissile material undergoes fission, it releases high-energy, fast neutrons. These fast-moving neutrons can escape without furthering the fission reaction if not slowed down.

Moderation refers to the process of slowing these high-speed neutrons to thermal speeds. Thermal neutrons, or slow neutrons, have a much higher probability of inducing fission when they collide with fissile nuclei. This is why neutron moderation is crucial; it increases the efficiency and effectiveness of a nuclear reactor.

The substance used as a moderator can vary. It must have specific properties:

• A good moderator should scatter neutrons effectively while absorbing as few as possible.
• It should be stable under reactor conditions.
• Water and heavy water are commonly used, with heavy water being particularly efficient due to its low neutron absorption.

Using the right moderator ensures that the reactor can maintain a controlled and sustained nuclear reaction, providing a continuous supply of energy for power generation.