As the preceding section on fission emphasized, each fission reaction of an U235 atom leads to the release of an average of 2.5 neutrons. Hence, to sustain a continu­ous fission reaction (i. e., a chain reaction), these neutrons should be able to initiate the fission of at least another fissile atom. Note the schematic of a chain reaction involving U235 atoms in Figure 1.1. A majority of the neutrons (~99.25%) produced due to the fission reaction of U235, known as prompt neutrons, are released instanta­neously (within 10-14 s). But there are about 0.75% neutrons that are released over

a longer period (over ~20 s) and these neutrons are called delayed neutrons. These delayed neutrons play a very important role in controlling the fission chain reaction.

There is always a competition for neutrons between various processes, namely, (i) fission reaction of fissile atom nuclei, (ii) nonfission capture of neutrons by uranium and other reactions, (iii) nonfission capture of neutrons by other components in the reactor core, and (iv) leakage of neutrons from the core. The reaction can be termed as a chain reaction when the number of neu­trons consumed in the processes (ii)-(iv) is at least equal to or less than that consumed in the process (i). Thus, neutron economy plays a very important role in the design of a nuclear reactor. The need for a favorable neutron economy necessitates certain conditions to be met by a chain reacting system. For a given geometry, there is a certain minimum size of a chain reacting system, called the critical size (in terms of volume), for which the production of neu­trons by fission just balances the loss of neutrons by leakage and so on, and the chain reaction can be sustained independently. The mass corresponding to the critical size is called critical mass. Dependent on the relative generation of fission neutrons and their loss, the reactor is said to be in different stages: subcritical (neutron loss more than the production), critical (balance between the neutron production and loss, к = 1), or supercritical (the neutron produc­tion is more than the loss, к > 1). The multiplication factor к is often used to express the criticality condition of a reactor. This factor is basically the net number of neutrons per initial neutron.

1.6