4.1 Requirements of a good moderator

The function of the moderator is to transform fast neutrons of energy 2 MeV or so into low energy 0.025 eV thermal neutrons. The neutrons make elastic colli­sions with the nuclei of the moderator, each collision causing a reduction in the neutron’s kinetic energy, Fig 1.12.

Fig. 1.12 Fission, capture, moderation and leakage

During the slowing down period the neutron is at risk of meeting a U-238 nucleus and hence being captured in a resonance capture peak, Fig 1.6 (b). (As illustrated in Fig 1.12 there will be other neutron losses: absorption in U-235, only some of which lead to fission; capture in the moderator, not shown in Fig 1.12; capture in ‘reactor materials’ other than fuel or moderator and the occasional fission of U-238 by a fast neutron; finally, for a finite system, leakage out of the reactor.) Thus a good moderator should slow neutrons down quickly; an additional require­ment of course is that the moderator itself must not capture neutrons to any great extent.

THe moderator

The previous sentence is equivalent to saying:

• Each collision must reduce the neutron energy substantially.

• The collision rate between moderator and neutron must be high.

• The moderator must have low — neutron absorption.