The Neutron Life Cycle.

The use of fissionable material as an energy source is currently the main type of nuclear energy used today.

A chain reaction caused by the production of neutrons is controlled through the use of design and implementation of the reactors to utilize the neutron life cycle (the process by which fission product neutrons produce further fission material) power plants are able to control the release of energy and operate safely.

This article will focus on the six factor approach to the neutron life cycle, focusing on the moderation of neutrons and the process by which thermal neutrons produce fission in uranium-235.

Chain Reaction

The animation on the left shows the propagation of fission by neutron induced fissions creating fission product neutrons (each fission produces ~2 neutrons and is caused by only one). This characteristic of uranium-235 fission is represented by the reproduction factor.

If this were the only one of the six factors contributing to fission rate nuclear fission would be wildly unsafe and uncontrollable. Each of the other factors reduces the amount of neutrons available for fission propagation so that in a critical nuclear reactor each neutron absorbed produces only one neutron that is absorbed in subsequent fission generations.

The Neutron life cycle reduces the propagation of neutrons to a safe and controllable level.

The main difference between a nuclear reactor and a nuclear bomb is the product of these values known as the multiplication factor.

If the multiplication factor increases above 1.0065 the reactor (or bomb) becomes prompt critical (which is what happened in the Chernobyl nuclear accident due to grossly negligent operation of the reactor during a test) this leads to a sudden and rapid runaway of fission. While operating on a slightly different concept this is how fission bomb produce the energy they do, the prompt neutrons propagate fission faster than the energy released can rip the core apart.

Thermal vs. Fast Neutrons

Fast neutrons are the product of fission and usually have ~1 MeV (mega electron volt) KE. thermal neutrons are neutrons that have the same average KE as particles at room temperature ~1 eV. Thermal neutrons absorbed by uranium-235 almost always produce a fission event in that nucleus, while most of the time a fast neutron being absorbed will be resonantly captured (the KE of the neutron will cause the nucleus to release the energy as a light particle which stabilizes the nucleus) and does not produce a fission event.

Reactor design must account for the need to remove the excess energy from the neutrons produced in the core so that they may be better utilized to produce and controllable, and sustainable, chain reaction. This is accomplished by the use of a moderator.