Introduction
The application of nuclear technology and nuclear power has become more and more popular around the world. Nuclear power plants use uranium as the fuel and moderators to enhance the rate of the chemical reaction according to the nature of nuclear fission. The material used for moderator has a great impact on the development of nuclear reactors. It has been widely used in the moderating process of a nuclear reactor, known as nuclear reactor slow materials.
Definition
In the terminology of nuclear engineering, a neutron moderator is any material that decreases the speed of a newly released neutron through a collision process. Moderators are essential components of the nuclear reactor, which are materials that interact with neutrons and their nuclei, slowing them down through a process called capture. Nuclear reactors generate heat from the interaction of neutrons with the fuel. A moderator allows the slowing down of thermal neutrons to increase the probability of fission, resulting in a chain reaction.
The moderating material must absorb the released neutrons without energy-amplifying, in other words, the moderating material is not allowed to cause neutron multiplication. Nuclear reactor slow materials are the materials used to reduce the energy and speed of neutrons, which are important to the operation and control of the reactor. That is to say, the moderating material helps to protect the reactor from unexpected release of high-energy neutrons, leading to instability or loss of chain reaction control.
Construction Principles
Nuclear reactor slow materials are typically composed of lightweight and low-cross-section elements, as the required kinetic energy of neutrons is proportional to their respective cross sections. Hydrogen-rich materials, such as water and deuterium-tritium, can absorb the neutrons more easily, due to the small mass of a single hydrogen atom compared to other light elements. The molecular structure of the hydrogen-rich material is important for the detail of the absorption and moderating process.
The main moderating material used in light water reactors is heavy water, a mixture of 20.3 percent deuterium and 79.7 percent ordinary water. As a result, heavy water has a high moderation capability due to its effective resonance scattering. Coolant, such as graphite, metal-clad sodium, and synthetic liquid-metal coolant, can act as moderators by absorbing neutrons and slowing them down. In addition, other materials such as boron, lithium, and beryllium can act as moderators.
Applications
Water is commonly used as a moderator in a wide variety of nuclear reactors, including boilers, pressurized water, and liquid metal reactors. Boron and beryllium are also commonly used as moderators in nuclear reactors.
Boron is used in pressurized and fast reactors due to its good thermal neutron absorption capability. The boron helps in controlling the reactor reaction speed and therefore inducing a power outage. The 10B isotope of boron has a very high resonance scattering effect, which increases the net reaction speed of the reactor.
Beryllium is used as a moderator in fast reactors because it can absorb neutrons faster than most other materials. It also has low neutron cross-section and good thermal conductivity. Beryllium is also used as a solid slow moderator in nuclear reactors because of its high neutron moderation and cross-section.
Conclusion
Nuclear reactor slow materials play a crucial role in the efficient and stable operation of a nuclear reactor. Although this material has been widely used in the nuclear industry, its development and application are still in the early stages. With the further development of nuclear technology and the decreasing costs of materials, the application of nuclear reactor slow materials will be expected to increase significantly in the future.
