The key difference between control rod and neutron moderator is that control rods are able to absorb neutrons whereas neutron moderators can slow down the neutrons.
Control rod and neutron moderator are two components of nuclear reactors. These two components have two different but important roles to play. The control rod controls the nuclear chain reaction by absorbing neutrons, but neutron moderator controls the nuclear chain reaction by reducing the speed of neutrons.
CONTENTS
1. Overview and Key Difference
2. What is Control Rod
3. What is Neutron Moderator
4. Side by Side Comparison – Control Rod vs Neutron Moderator in Tabular Form
5. Summary
What is Control Rod?
A control rod is a component in a nuclear reactor which can absorb neutrons. The name of this component is given as such because its major role is to control the fission rate of uranium or plutonium used in the nuclear reactor by absorbing neutrons. The composition of control rods usually includes chemical elements such as boron, cadmium, silver, etc. These chemical elements are capable of absorbing neutrons without undergoing any fission reaction. Moreover, these chemical elements have different neutron capture cross sections for neutrons having various energies.
Figure 01: Control Rod
The control rods are placed inside the core of the nuclear reactor. They are then adjusted to control the nuclear chain reaction that takes place in the core. It is important mainly in controlling the thermal output of the reactor, the rate of steam production and the output of the electricity.
The number of control rods that are inserted into the core and the distance to which the rods are inserted has a very strong influence on the reactivity of the nuclear reactor. Usually, a newly built nuclear reactor has its control rods fully inserted. They are partially removed when the nuclear chain reaction starts up.
What is Neutron Moderator?
A neutron moderator is a component in a nuclear reactor that can slow down the neutrons. It is considered as a medium that can slow down fast neutrons by reducing their speed. However, this component is very important because it can slow down neutrons without capturing any of them. The neutrons are left with minimal kinetic energy by this component. These neutrons are then called the thermal neutrons and they are more susceptible than fast neutrons for the propagation of the nuclear chain reaction.
The most commonly used neutron moderator in a typical nuclear reactor is “light water”. As alternatives, we can use solid graphite and heavy water. The reduced kinetic energy from the neutron is transferred to the moderator. Here, the energy is converted into the potential energy of the moderator material.
The fission of uranium-235 in thermal-neutron reactors form two fission products: fast-moving free neutrons and energy. This starts up a chain reaction because the neutron releasing process can be self-sustaining. Further, this can liberate very high energy. The fission cross-section is the component where the further fission events are determined. The fission cross-section depends upon the speed of the neutrons. Therefore, as a control measure, using a neutron moderator is very important. However, in fast reactors, there are no neutron moderators.
What is the Difference Between Control Rod and Neutron Moderator?
Control rods and neutron moderator are two components in a nuclear reactor. The key difference between control rod and neutron moderator is that control rods are able to absorb neutrons whereas neutron moderators can slow down the neutrons. The control rods control the nuclear chain reaction by capturing neutrons but the neutron moderator does not capture any neutron.
Below infographic summarizes the difference between control rod and neutron moderator.
Summary – Control Rod vs Neutron Moderator
Control rods and neutron moderator are two components in a nuclear reactor. The key difference between control rod and neutron moderator is that control rods are able to absorb neutrons whereas neutron moderators can slow down the neutrons.
Reference:
1. Wheeler, Mason WheelerMason. “How Do Control Rods Work?” Physics Stack Exchange, 1 Nov. 1964, Available here.
Image Courtesy:
1. “PWR control rod assemby” By Nic Ransby – (Public Domain) via Commons Wikimedia
2. “U235 Fission cross section” – Plot made with Origin software package on my PC. Data from public database http://t2.lanl.gov/data/ ENDF/B-VII library of evaluated incident-neutron data (Public Domain) via Commons Wikimedia