The Criticality Safety Index (CSI) is a dimensionless number assigned to the label of a fissile material package, to designate the degree of control of the accumulation of packages, overpacks or cargo containers containing fissile material during transport. It is used to prevent nuclear and radiation accidents resulting from an involuntary and self-sustained nuclear chain reaction. The CSI is calculated based on the value of N determined in matrix analysis and is rounded to the next tenth. It is important for criticality safety professionals to understand where this does not apply, such as high-temperature reactors or low-temperature cryogenic experiments. The sum of the safety criticality indices (CSI) for all Class 7 (radioactive) fissile material packages and overpacks on board a ship may not exceed the limits specified in Table IIIB.
To ensure this, inherently safe or fault-tolerant plant designs are implemented, or if such designs are not feasible, administrative controls such as operating procedures, work instructions, and other means are used to minimize the possibility of significant process changes that could lead to a nuclear criticality accident. A double contingency analysis is often applied to operations involving fissile material, where two or more independent, concurrent and unlikely changes in process conditions must occur before a nuclear critical accident can occur. Nuclear criticality safety professionals attempt to prevent nuclear criticality accidents by analyzing normal and credible abnormal conditions in fissile material operations and designing safe arrangements for fissile material processing.