Criticality is an uncontrolled nuclear chain reaction (divergence) within fissile materials, which are the odd-numbered isotopes of uranium and plutonium (233U, 235U, 239Pu and 241Pu). The risk of criticality is due to the presence of a significant amount of fissile material in some part of the facility and this is therefore broken down into several units so that adequate control modes may be defined. These modes can be characterized by limits given by up to three parameters while also taking into account the presence of neutron poisons, if any:

• fissile material mass

• geometrical dimension of the piece of equipment

• moderation ratio

To measure the damage resulting from a nuclear incident or accident, there is an international nuclear and radiological event scale, the INES scale (Fig. 16.24).