When a U235 nucleus is joined by a slow neutron, it can split into two nuclei further down in the periodic table, plus two or three neutrons, as shown in Fig. 3.59. In this case, the fragments are Ba144 and Kr89. Adding the mass numbers will show that three neutrons are released in this case. A chain reaction occurs when one, and only one, of these neutrons splits another U235 nucleus to make more neutrons to keep the chain of reactions going. If two neutrons cause further fissions, the reaction will run away. Figure 3.59 shows another way to continue the chain. As we shall see, there are many more U238 nuclei in the fuel than U235s, so a neutron can enter a U238 nucleus to form U239, which then beta-decays into Pu239, which is fissionable. A neutron hitting the Pu239 will cause fission and keep the chain going.

Moderation is the Key

Of course, things are not this simple. The neutrons from a fission have energies around 2 MeV (about 20 billion°K). (Definition of electron-volt units is given in Chap. 5). They have to be slowed down to normal temperatures before a nucleus will accept them. Room temperature is about 0.025 eV. A moderator is used to do this. Here, a moderator is not the chair of a panel discussion; it is an element that slows down neutrons efficiently without absorbing them. The most common mod­erators are light water [ordinary H2O, heavy water (D2O), and graphite (very pure carbon)]. Only light elements (those with low atomic masses) can be moderators. The reason is that neutrons are light, and they will bounce off a large nucleus with­out losing much energy. A marble striking a billiard ball will just bounce off. A cue ball striking an 8-ball can come to a complete stop, losing all its energy to the 8-ball, because it has the same mass. Light water is a better moderator than heavy water because the H is closer to the neutron in mass than the D, but it’s not twice as good because the H can capture the neutron to make a D. A deuteron is less likely to capture yet another neutron to make triply heavy tritium. Carbon has mass 12, so graphite is a weaker moderator than water, but it has other properties, like staying solid at high temperature. Moderators are so important that nuclear reactors are classified according to their moderators.