ISO 21391:2019

Overview

ISO 21391:2019 provides guidance, requirements and recommendations for controlling geometrical dimensions used to ensure subcriticality in nuclear facilities and during transport of fissile materials (excluding nuclear power reactor cores). The standard defines how to determine subcriticality dimensions (item and layout dimensions), set subcriticality limits, and verify that actual dimensions comply with design requirements. Quality assurance of fabrication and layout is a prerequisite.

Key topics and requirements

• Identification of subcriticality dimensions: Nuclear criticality safety staff must identify item dimensions (e.g., tank diameter, thickness) and layout dimensions (e.g., spacing between items) that affect subcriticality.
• Subcriticality limits: Limits are established for identified dimensions using handbooks, standards, sensitivity calculations or specific subcriticality calculations. Limits must cover normal and abnormal conditions, ageing, and maintenance configurations.
• Calculation model dimensions: When performing specific calculations, choose conservative model dimensions that bound reality, account for manufacturing/layout tolerances, and limit the number of dimensions needing control.
• Actual-dimension control and verification: Verify as-built (actual) dimensions by measurement or qualified manufacturing processes and compare with subcriticality limits; manage non-compliance per the standard’s guidance.
• Conditions influencing limits: Account for mechanical stresses, thermal/pressure deformation, ageing, and other effects that can change geometry over a unit’s lifetime. Include additional dimensional margins where uncertainties exist.
• Documentation and early integration: Subcriticality dimensions should be defined early in design and communicated to designers, suppliers and operators so procurement and fabrication conform to safety limits.

Practical applications and users

Who uses ISO 21391:

• Nuclear criticality safety analysts and engineers
• Facility designers and mechanical engineers
• Procurement and manufacturing quality assurance teams
• Operations, commissioning and maintenance staff
• Regulators and transport planners for packaging/layout of fissile materials

Typical applications:

• Defining spacing and containment dimensions for storage, processing and handling units containing fissile material
• Setting design and inspection criteria for fabricated equipment and layouts
• Establishing commissioning and periodic verification checks for as-built geometry
• Applying dimensional controls for transport packaging and temporary storage outside nuclear establishment boundaries

Related standards

• ISO 11311 - referenced for critical values (homogeneous Pu-U oxide mixtures)
• ISO 12749-3 - vocabulary for nuclear fuel cycle terms used in this standard
• ISO 1709 - methods for nuclear criticality safety (context for control methods)

ISO 21391:2019 is a practical, design-to-operations standard that integrates nuclear criticality safety, geometrical dimension control, and quality assurance to ensure safe handling, storage and transport of fissile materials.