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Nuclear Requirements for Moderator Graphite

There are two very basic nuclear requirements for moderator graphite, or, for that matter, any modera­tor material. First, it must be effective in slowing fast neutrons down to thermal energies, and, second, it must have a small cross section for neutron absorption. The slowing down of neutrons results princi­pally from energy transfer during elastic collisions between the neutrons and the moderator atoms. For a material to be an efficient moderator, the collision rate per unit volume must be relatively high. The collision rate is proportional to the number of moderator nuclei per unit volume, and for this reason the density of moderator graphite should not be too low if the volume of the reactor core is to be mini­mized.

The graphite should absorb as few neutrons as possible during the moderating process because neu­trons absorbed in any reactor component other than the fuel (parasitic absorption) are lost from the chain reaction. The typical impurities occurring in graphite have a greater propensity for absorbing neutrons than do the carbon atoms. It is, therefore, desirable to have the graphite as free as possible from such impurities.

It is of interest to consider the relative importance of high-purity graphite for thermal natural-uranium reactors and for thermal enriched-uranium reactors. For natural-uranium reactors a low neutron-absorp­tion rate in the moderator is especially important. Such reactors must be very large to have any excess reactivity available at all, and a loss of excess reactivity as a result of a high rate of absorption in the moderator is especially undesirable. Excessive absorption in the graphite also causes non­pro­ductive consumption of U²³⁵ atoms, thereby decreasing plutonium formation.

Neutron absorption in the moderator is basically undesirable in enriched-uranium reactors for the same reasons that it is undesirable in natural-uranium reactors. The decrease in the excess reactivity resulting from absorption in the moderator is not as critical a problem, however, because it is possible to gain excess reactivity by increasing the enrichment of the fuel. If the enrichment is increased in lieu of further purification, .the formation of plutonium is decreased because there is competition for neu­trons by absorption in U²³⁵ as well as in the impurities. The loss in plutonium production amounts to about 2.5 per cent for each 0.05 per cent of added enrichment. [1] Thus whether to increase the purity of the graphite or to increase the degree of enrichment becomes a question of economics. 

Graphite-moderator Physics

The material in this section introduces the basic concepts of moderator physics and shows how they apply to graphite. For more detail the reader in referred to textbooks on reactor physics. [2], [3]

Reactor-Physics Concepts

Very simply, the neutron cycle in a reactor consists of:

1.     Absorption of slow neutrons by fuel.

2.     Production of a new generation of fast neutrons from fission in the fuel.

3.     Slowing down (moderation) of fission neutrons.

4.     Leakage of neutrons and parasitic absorption of neutrons at all stages of the cycle.

5.     Absorption by fuel of the new generation of slow neutrons.

1. The neutron loses energy in elastic collisions with the moderator nuclei.

2. The moderator nuclei are initially at rest in the laboratory system of coordinates.

3. The scattering of neutrons by the moderator nuclei is spherically symmetric in the center-of-mass system of coordinates.

Two of the quantities in Table 4.1 are sometimes used as figures of merit of a particular substance as a moderator; these are discussed below:

1. The slowing down power, x×S_s, is an index of the ability of a modera­tor to slow down neutrons. How­ever, since this parameter is independent, of the absorbing properties of a material, it is not a good representation of the usefulness of a particular material as a moderator. Boron, for example, has a high value of x×S_s but would he useless as a moderator material because of its high absorption cross section,

2. The moderating ratio, defined by x×S_s/S_a, is a better index of the merit of a particular material as a moderator. A shortcoming of the moderating ratio is that it is independent of density. For example, helium has a high value for the moderating ratio, but it would be a poor moderator because it has such a low density at normal pressures and temperatures. Perhaps the best specification for a moderator is that both moderating ratio and density be relatively high.