In a nuclear fission reactor, each fission of a uranium nucleus is accompanied by the emission of one or more highspeed
neutrons, which travel through the surrounding material. If one of these neutrons is captured in another uranium nucleus, it
can trigger fission, which produces more fast neutrons, which could make possible a chain reaction (Figure 10.41).
However, fast neutrons have low probability of capture and usually scatter off uranium nuclei without triggering fission. In
order to sustain a chain reaction, the fast neutrons must be slowed down in some material, called a “moderator.” For reasons
having to do with the details of nuclear physics, slow neutrons have a high probability of being captured by uranium nuclei.
A slow neutron induces fission of U-235, with the emission of additional (fast) neutrons. The moderator is some material
that slows down the fast neutrons, enabling a chain reaction.
In the following analyses, remember that neutrons have almost no interaction with electrons. Neutrons do, however, interact
strongly with nuclei, either by scattering or by being captured and made part of the nucleus. Therefore you should think
about neutrons interacting with nuclei (through the strong force), not with entire atoms.
(a) Based on what you now know about collisions, explain why fast neutrons moving through a block of uranium
experience little change in speed.
(b) Explain why carbon should be a much better moderator of fast neutrons than uranium.
(c) Should water be a better or worse moderator of fast neutrons than carbon? Explain briefly.
Background: The first fission reactor was constructed in 1941 in a squash court under the stands of Stagg Field at the
University of Chicago by a team led by the physicist Enrico Fermi. The moderator consisted of blocks of graphite, a form
of carbon. The graphite had to be exceptionally pure because certain kinds of impurities have nuclei that capture neutrons
with high probability, removing them from contributing to the chain reaction. Many reactors use ordinary “light” water as a
moderator, though sometimes a proton captures a neutron and forms a stable deuterium nucleus, in which case the neutron
is lost to contributing to the chain reaction. Heavy water, D2O, in which the hydrogen atoms are replaced by deuterium
atoms, actually works better as a moderator than light water, because the probability of a deuterium nucleus capturing a
neutron to form tritium is quite small.