Neutron moderator

Neutrons are normally bound into an atomic nucleus and do not exist free for long in nature.

Some nuclei have larger absorption cross sections than others, which removes free neutrons from the flux.

This is only slightly modified in a real moderator due to the speed (energy) dependence of the absorption cross-section of most materials, so that low-speed neutrons are preferentially absorbed,[5][6] so that the true neutron velocity distribution in the core would be slightly hotter than predicted.

In a thermal-neutron reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits into two smaller atoms (fission products).

Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become a self-sustaining nuclear chain reaction under controlled conditions, thus liberating a tremendous amount of energy.

For thermal reactors, high-energy neutrons in the MeV-range are much less likely (though not unable) to cause further fission.

The newly released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down or "moderated", typically to speeds of a few kilometres per second, if they are to be likely to cause further fission in neighbouring 235U nuclei and hence continue the chain reaction.

Classically, moderators were precision-machined blocks of high-purity graphite[7][8] with embedded ducting to carry away heat.

In pebble-bed reactors, the nuclear fuel is embedded in spheres of reactor-grade pyrolytic carbon, roughly of the size of pebbles.

This design gives CANDU reactors a positive void coefficient, although the slower neutron kinetics of heavy-water moderated systems compensates for this, leading to comparable safety with PWRs.

[9] In the light-water-cooled, graphite-moderated RBMK, a reactor type originally envisioned to allow both production of weapons grade plutonium and large amounts of usable heat while using natural uranium and foregoing the use of heavy water, the light water coolant acts primarily as a neutron absorber and thus its removal in a loss-of-coolant accident or by conversion of water into steam will increase the amount of thermal neutrons available for fission.

Following the Chernobyl nuclear accident the issue was remedied so that all still operating RBMK type reactors have a slightly negative void coefficient, but they require a higher degree of uranium enrichment in their fuel.

In commercial nuclear power plants the moderator typically contains dissolved boron.

The Nazi Nuclear Program suffered a substantial setback when its inexpensive graphite moderators failed to function.

Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators.

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.

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.

[11] Only the Manhattan Project embraced the idea of a chain reaction of fast neutrons in pure metallic uranium or plutonium.

Other moderated designs were also considered by the Americans; proposals included using uranium deuteride as the fissile material.

[14] The motivation was that with a graphite moderator it would be possible to achieve the chain reaction without the use of any isotope separation.

In August 1945, when information of the atomic bombing of Hiroshima was relayed to the scientists of the German nuclear program who were interred at Farm Hall in England, chief scientist Werner Heisenberg hypothesized that the device must have been "something like a nuclear reactor, with the neutrons slowed by many collisions with a moderator".

[15] The German program, which had been much less advanced, had never even considered the plutonium option and did not discover a feasible method of large scale isotope separation in uranium.

The aim of the University of California Radiation Laboratory (UCRL) designs was the exploration of deuterated polyethylene charge containing uranium[16]: chapter 15  as a candidate thermonuclear fuel,[17]: 203  hoping that deuterium would fuse (becoming an active medium) if compressed appropriately.

If successful, the devices could also lead to a compact primary containing minimal amount of fissile material, and powerful enough to ignite RAMROD[17]: 149  a thermonuclear weapon designed by UCRL at the time.

For a "hydride" primary, the degree of compression would not make deuterium to fuse, but the design could be subjected to boosting, raising the yield considerably.

The core tested in Ray used uranium low enriched in U235, and in both shots deuterium acted as the neutron moderator.

Another effect of moderation is that the time between subsequent neutron generations is increased, slowing down the reaction.

According to Heisenberg: "One can never make an explosive with slow neutrons, not even with the heavy water machine, as then the neutrons only go with thermal speed, with the result that the reaction is so slow that the thing explodes sooner, before the reaction is complete.

"[20] While a nuclear bomb working on thermal neutrons may be impractical, modern weapons designs may still benefit from some level of moderation.

Helium is a gas and it requires special design to achieve sufficient density; lithium-6 and boron-10 absorb neutrons.