Thermal Neutrons
Definition and meaning of Thermal Neutrons in chemistry.
Thermal neutrons are neutrons that have been slowed to low kinetic energies, typically around 0.025 electronvolts (eV) at room temperature, through collisions with moderator materials. They are in thermal equilibrium with their surroundings and are essential for sustaining controlled nuclear fission reactions in reactors.
In more detail
These low-energy neutrons are produced when fast neutrons released by nuclear fission collide with light atomic nuclei such as hydrogen and carbon, gradually losing kinetic energy. In nuclear reactors, thermal neutrons induce fission in uranium-235 and other fissile nuclei far more effectively than fast neutrons do, making moderators like water or graphite necessary to slow down fission neutrons. Thermal neutrons are also widely used in scientific research, particularly in neutron diffraction studies for determining crystal structures and in boron-based reactor control rods where they are readily absorbed.
Key facts
| Energy range | Approximately 0.025 eV at room temperature (293 K) |
|---|---|
| Common moderators | Water, graphite, beryllium, heavy water |
| Primary use | Sustaining controlled fission in nuclear reactors |
| Field | Physical Chemistry |
In a pressurized water reactor, fast fission neutrons lose energy through multiple collisions with water molecules, becoming thermalized to energies suitable for sustaining the chain reaction.
Frequently asked questions
Why are thermal neutrons preferred in nuclear reactors?
Thermal neutrons have much higher absorption cross sections for fissile nuclei like uranium-235, making them far more likely to induce fission than fast neutrons, allowing for controlled chain reactions.
How are thermal neutrons produced?
Fast neutrons from nuclear fission are slowed down through elastic scattering collisions with light nuclei in moderating materials such as water or graphite.