Subcritical fission situation

In addition to the product nuclides, neutrons are produced in the fission reactions of This makes it possible to produce a self-sustaining fission process—a chain reaction (see Fig. 21.12). For the fission process to be self-sustaining, at least one neutron from each fission event must go on to split another nucleus. If, on the average, less than one neutron causes another fission event, the process dies out the reaction is said to be subcritical. If exactly one neutron from each fission event causes another fission event, the process sustains itself at the same level and is said to be critical. If more than one neutron from each fission event causes another fission event, the process rapidly escalates and the heat buildup causes a violent explosion. This situation is described as supercritical. 

Figure 23.8 shows two types of fission reactions. For a chain reaction to occur, enough uranium-235 must be present in the sample to capture the neutrons. Otherwise, many of the neutrons will escape from the sample and the chain reaction will not occur, as depicted in Figure 23.8(a). hi this situation the mass of the sample is said to be subcritical. Figure 23.8(b) shows what happens when the amount of the fissionable material is equal to or greater than the critical mass, the minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction, hi this case most of the neutrons will be captured by uranium-235 nuclei, and a chain reaction will occur. 

---