Understanding the Process of a Slow Neutron Fission Reaction in Simple Terms
Understanding the Process of a Slow Neutron Fission Reaction in Simple Terms
Introduction to Fission and Neutron Moderators
In layman's terms, a nuclear reactor is a controlled environment where atoms split into smaller atoms in a process called fission. This reaction is initiated by slow neutrons, which are slowed down to a specific energy level by materials called moderators. The right energy level—typically between 0.025 MeV to 0.999 MeV—makes it possible for the neutron to be absorbed by the fissile isotope uranium-235 (U-235), triggering the fission process.How Neutrons Are Slowed Down
Neutrons are slowed down in a nuclear reactor by surrounding materials like water, heavy water, or graphite. These materials effectively act as moderators, allowing the neutrons to lose energy through a series of collisions and interactions with the nuclei of the moderator material.When a neutron collides with a hydrogen nucleus (often hydrogen isotopes like deuterium in heavy water), it can slow down significantly because the hydrogen nucleus is very light compared to the neutron, much like how a billiard ball could transfer all its energy to another billiard ball in a head-on collision.
Types of Collisions: Elastcity and Inelasticity
Fission reactions involve both elastic and inelastic collisions of neutrons with nuclei.Inelastic Collisions: In this type of collision, the neutron loses all of its energy and the nucleus it implants itself into is left in an excited state. The excited nucleus then emits gamma rays to return to its ground state.
Elastic Collisions: These are similar to billiard ball collisions, where the neutron can gain or lose energy depending on the properties of the nucleus it collides with. Even though no nucleus is exactly the same weight as a neutron, materials like hydrogen can approximate this condition very well.
The Role of Neutron Energy Levels
Neutrons can be of different energies, with thermal neutrons starting at 0.025 MeV to 0.999 MeV, and fast neutrons starting from 1 MeV. The choice of moderator and the type of fissile material determine whether thermal or fast neutrons are used.Thermal neutrons are used with U-235 in fission reactions, while fast neutrons are used with U-238 due to their higher energy levels which allow for effective absorption and subsequent fission.
Commonly Used Materials as Moderators
Some of the most commonly used materials as moderators include: Water Heavy water (deuterium oxide) Graphite Paraffin (a type of wax)These materials help in dissipating the kinetic energy of neutrons through numerous elastic and inelastic collisions, eventually bringing them to an energy level suitable for fission.