Neutron moderator

Explanation

In a thermal nuclear reactor, the nucleus of a heavy fuel element such as speed of light, must be slowed down or "moderated", typically to speeds of a few kilometers per second, if they are to be likely to cause further fission in neighbouring uranium nuclei and hence continue the chain reaction.

A good neutron moderator is a material full of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off. In this process, some energy is transferred between the nucleus and the neutron. More energy is transferred per collision if the nucleus is lighter; see elastic collision. After sufficiently many such impacts, the velocity of the neutron will be comparable to the thermal velocities of the nuclei; this neutron is then called a thermal neutron.

A fast reactor uses no moderator, but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction.

In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully thermalised than others; For example in a CANDU reactor nearly all fission reactions are produced by thermal neutrons, while in a PWR a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled SCWR, the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor.

Form and location

The form and location of the moderator can greatly influence the cost and safety of a reactor. Classically, moderators were precision-machined blocks with embedded ducting to carry away heat. Also, they were in the hottest part of the reactor, and therefore subject to Wigner energy. At Windscale, this problem led to the infamous Windscale fire.

Some pebble-bed reactor's moderators are not only simple, but also inexpensive: the nuclear fuel is embedded in spheres of reactor-grade Wigner energy.

Moderator impurities

Good moderators are also free of neutron-absorbing impurities such as heavy water moderators. In the U.S., Leo Szilard, a former chemical engineer, discovered the problem.

Non graphite moderators

Some moderators are quite expensive, for example chemical bonds in almost the same ways, at only slightly different speeds.

The much cheaper light water moderator ( essentially very pure regular water ) absorbs too many neutrons to be used with unenriched natural uranium, and therefore uranium enrichment or nuclear reprocessing becomes necessary to operate such reactors, increasing overall costs. Both enrichment and reprocessing are expensive and technologically challenging processes, and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material, causing proliferation concerns. Reprocessing schemes that are more resistant to proliferation are currently under development.

The CANDU reactor's moderator doubles as a safety feature. A large tank of low-temperature, low-pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss-of-coolant accident conditions. It is separated from the fuel rods that actually generate the heat. Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high "neutron economy".

Materials used

• enriched uranium to operate.
• natural uranium.
• Wigner energy in the material. Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium.
• Beryllium, in the form of metal. Beryllium is expensive and toxic, so its use is limited.

Other light-nuclei materials are unsuitable for various reasons. boron absorb neutrons.

See also

• Nuclear cross section