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In a nuclear fission reactor, each fission of a uranium nucleusis accompanied by the emission of one or more highspeed neutrons,which travel through the surrounding material. If one of theseneutrons is captured in another uranium nucleus, it can triggerfission, which produces more fast neutrons, which could makepossible a chain reaction. However, fast neutrons have lowprobability of capture and usually scatter off uranium nucleiwithout 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 nuclearphysics, slow neutrons have a high probability of being captured byuranium nuclei.

A slow neutron induces fission of U-235, with the emission ofadditional (fast) neutrons. The moderator is some material thatslows down the fast neutrons, enabling a chain reaction.

In the following analyses, remember that neutrons have almost nointeraction with electrons. Neutrons do, however, interact stronglywith nuclei, either by scattering or by being captured and madepart of the nucleus. Therefore you should think about neutronsinteracting with nuclei (through the strong force), not with entireatoms.

1. Based on what you now know about collisions, explainwhy fast neutrons moving through a block of uranium experiencelittle change in speed.

2. Explain why carbon should be a much better moderatorof fast neutrons than uranium.

3. Should water be a better or worse moderator of fastneutrons than carbon? Explain briefly.