Fissile material packages stored in transit must be limited so that the total sum of the CSI in any group of packages does not exceed:

• A. 50
• B. 100
• C. 25
• D. 75

Answer: (A)

Subcritical safety is controlled by limiting how much fissile content is allowed to be grouped together. Each fissile package has a Criticality Safety Index (CSI) value that represents its potential to contribute to a self-sustaining neutron chain reaction. When several packages are stored or transported together, you add their CSIs. The total must stay under a fixed regulatory limit to ensure that, even in the most unfavorable arrangement, the assembly remains safely subcritical.

That limit is chosen to provide a practical balance: it is high enough to allow shipping multiple packages together without excessive restriction, but low enough to maintain a very low chance of approaching criticality under normal handling and plausible accident conditions. If the limit were higher, the risk of a critical configuration could increase; if it were lower, operations would be unnecessarily constrained.

Other numeric options would either loosen safety margins too much or be overly restrictive, without providing additional safety benefit. This fixed limit is what keeps the grouping of fissile material packages within safe, subcritical bounds.