NRC SFST ISG-19: Moderator Exclusion under Hypothetical Accident Conditions and Demonstrating Subcriticality of Spent Fuel under the Requirements of 10 CFR 71.55(e)

This Interim Staff Guidance (ISG) provides review guidance for meeting the fissile material
package standards in 10 CFR 71.55(e). The provisions of 71.55(e) require that a fissile
material package be subcritical under hypothetical accident conditions assuming that the fissile
material is in the most reactive credible configuration consistent with the damaged condition of
the package and the chemical and physical form of the contents, and water moderation occurs
to the most reactive credible extent consistent with the damaged condition of the package and
the chemical and physical form of the contents.
Current regulatory guidance for addressing the requirements of 10 CFR 71.55(e) for spent fuel
are included in

NUREG-1617, "Standard Review Plan for Transportation Packages for Spent Nuclear
Fuel," and

NUREG/CR-5661, "Recommendations for Preparing the Criticality Safety Evaluation of
Transportation Packages."
The guidance in these two documents does not specifically and separately address the
requirements of 71.55(e). This ISG is intended to clarify the review of the criticality safety
evaluation for a single package under hypothetical accident conditions.