The moderator must not only have the good ‘nuclear physics’ characteristics of a high moderating ratio but also of course other desirable properties. For example, the moderator material must be:

• Readily available.

• Cheap.

• Machineable if a solid.

• Chemically compatible with reactor materials.

• Unchanged by neutron bombardment.

• Non-flammable.

• Non-corrosive.

• Non-toxic.

5,5 Possible moderator materials

From the previous sections the ideal moderator should have the following features:

• Solid or liquid.

• High scattering cross-section.

• Low absorption cross-section.

• Low mass number.

• Low cost.

• Where appropriate — ‘good’ mechanical, chemical and other physical properties.

In identifying materials that may be suitable for use as a moderator only the lighter elements need be con­sidered — up to oxygen say:

• Hydrogen, helium, nitrogen and oxygen are gases.

• Lithium is chemically very active and in nature contains 7 7 Щ Li-6 for which 6C = 70 b.

• The highly toxic beryllium is difficult to fabricate; BeO has been considered but large scale production is also difficult.

• Boron in nature contains 20% boron 10 for which ffc = 3836 barns.

• Carbon in the form of graphite is cheap, an easily machined solid, is relatively inert chemically and has the ‘nuclear’ properties of a reasonably low mass number (12.1), low ac (0.004 b) and accept­able cts (4.8 b).

Of the elements then, only carbon in the form of graphite is suitable for use as a moderator.

However, the potential of the low mass of the hydrogen nucleus may still be utilised in the form of ‘light’ water, H2O, or ‘heavy’ water, D2O:

• Light water is cheap, a liquid, has acceptable chem­ical behaviour and a high scattering cross-section (- 100 barns). Unfortunately it also has a highish

absorption cross-section (0,66 barns) with the re­sult that reactors using light water as moderator need slightly enriched (~ 3%) fuel to counter the neutron losses.

• Heavy water has nearly identical physical proper­ties to light water but, although the scatter cross — section is reduced to about 13 barns, the absorption cross-section is extremely low (0.001 b) giving heavy water its pre-eminence in terms of moderating ratio, see Table 1.7. But heavy water is expensive. It is obtained by isotopic separation from natural water which contains about 1 part of D2O in 6500 of H2O. Thus a decision whether or not to use heavy water as a moderator is a matter of balance between its excellent moderating property and the very high production costs.

To sum up this section on moderators, only three materials can be regarded as being viable for use as a moderator in commercial nuclear power reactors: light water, heavy water and graphite.