The Fermi Chronicles - Part 14: Neutron Moderation

Not moderation in the political sense, as that word is equivalent to 'waffling,' or 'flip-flopping.' In the nuclear industry, moderation of neutrons necessary for fission to occur is extremely important in controlling the core reaction rate. If left to its own devices, nuclear fission would not only be self-sustaining, but would chain-react to consume all the fuel in a very short time period releasing such a large amount of energy that the core would melt down, or, depending on design, would halt the nuclear reaction as some reactors are dependent upon slower neutrons. Thus, some means of control must be available. As a backdrop, recall from earlier that there are two types of neutrons - fast and thermal. When fission occurs, all neutrons are fast (about 10% of the speed of light!), but slow down as they bump into other atoms (neutron scattering). Once a few bumps occur, the fast neutrons have lost pretty much all of their kinetic energy and have the same speed as the ambient atoms floating around. They have been thermalized, or slowed down to thermal neutrons. Also, recall that fast neutrons can cause fission in all fissionable material, while thermal neutrons can only cause fission in fissile material (U-235, P-239), of which there is very little in a typical nuclear reactor core. The material used to slow down or thermalize the neutrons is called a moderator.

A good moderator should slow down fast neutrons in very few collisions, and should not absorb them to any significant extent. If it took too many collisions to thermalize the fast neutrons, then there would be a good chance of what is termed 'neutron leakage.' The ability of a material to moderate, absorb, detect, etc. neutrons is quantified in terms of a cross-section, and has units of area. For instance, in the case of a moderator, ideally it should have a large scattering cross-section, meaning that their is a very good probability that a majority of fast neutron is thermalized. A small cross-section here would be undesirable since it would certainly lead to neutron leakage. Other aspects of the moderator, however, such as the absorption cross-section, should be absolutely minimal (thus maintaining a constant neutron flux). A moderator should ideally scatter the neutrons elastically, as I wrote about in a prior entry. The most ideal moderator would be one with equal mass to the neutron - the hydrogen atom (not practical).

As it turns out, H2O ('light' water) is an excellent moderator and is used in BWRs. Heavy water, D2O is used in some PWRs, including the CANDU reactor types. Russian reactors use graphite, which was the moderator of the infamous Chernobyl reactor. Helium has been used in GCRs (gas-cooled reactors don't seem to be ideal candidates in my book). Taking standard water as an example, it takes on average about 19 collisions to thermalize a neutron, and the H2O was given a great deal of energy in the process. The only downside is that water has a greater absorption cross-section than say heavy water, D2O, but D2O has a smaller scattering cross-section. So there is typically a trade-off with most moderators. This causes the water to increase in temperature. Water is also a good moderator from the standpoint of safety. In a PWR, if a loss of coolant accident were to occur, moderation would be lost and the reaction shut down simply from the fact that thermal neutrons would no longer be available. Loosing coolant, however, would negate the cooling of the hot fuel rods and lead to loss of geometry rendering the fuel rods useless.

Unlike the moderator, control rods have a high absorption cross-section as they are not meant to moderate, but rather to stop. I'll talk about control rods, typically made of mostly boron (I'll try to make the discussion more lively), with some halfnium, in a later installment.

Previously:
The Fermi Chronicles - Part 13: Nuclear Reactor Types
The Fermi Chronicles - Part 12: Generating Electricity
The Fermi Chronicles - Part 11: Worldwide Uranium Availability
The Fermi Chronicles - Part 10: Utilizing Nuclear Reactions To "Breed" More Fuel
The Fermi Chronicles - Part 9: Nuclear Fission
The Fermi Chronicles - Part 8: Neutron Interaction
The Fermi Chronicles - Part 7: Radioactive Decay and Half-Life
The Fermi Chronicles - Part 6: Atomic Structures
The Fermi Chronicles - Part 5: Nuclear Waste Storage
The Fermi Chronicles - Part 4: Radiation Types and Radiation "Dose"
The Fermi Chronicles - Part 3: Radiation Types
The Fermi Chronicles - Part 2: A week of training
The Fermi Chronicles - Part 1: The alpha post