Chapter 11. Neutron Chain Reactions
The possibility in a mass of nuclear fuel such as uranium depends on (a) nuclear properties such as cross sections and neutrons produced per absorption (Section 6.3) and (b) the size, shape, and arrangement of the materials.
11.1. Criticality and Multiplication
To achieve a self-sustaining chain reaction, one needing no external neutron supply, a “critical mass” of uranium must be collected. To appreciate this requirement we visualize the simplest nuclear reactor, consisting of a metal sphere of uranium-235. Suppose that it consists of only one atom of U-235. If it absorbs a neutron and fissions, the resultant neutrons do nothing further, there being no more fuel. If instead we have a small chunk of uranium, ...