Development of an ISO standard related to geometrical dimensions for subcriticality control

Nuclear criticality safety is often based on requirements limiting geometrical dimensions. This is usually expressed as maximal dimensions for process equipment containing fissile material (e.g. maximal diameter of columns, maximal height of a stack...) or minimal dimensions for interactions (e.g. storage pitch) or neutron absorber dimensions (e.g. thickness of a cadmium sheet). These limits are derived from nuclear criticality safety calculations and assessment. The nuclear criticality safety specialist needs to deal with different topics: how to avoid an intractable amount of limits while maintaining an adequate safety level, how to comply with the requirements, what should be compared. In this context, an ISO (International Organization for Standardization) standard has been developed (ISO 21391) by Subcommittee SC 5 (Nuclear installations, processes and technologies) of Technical Committee ISO/TC 85 (Nuclear energy, nuclear technologies, and radiological protection), in order to provide guidance, requirements and recommendations related to the determination of the relevant dimensions to be ensured on the one hand, and to the comparison between actual dimensions and subcriticality dimensions on the other hand. This paper presents the main items, with examples illustrating how to avoid unnecessary complexities, why some dimensions are better suited for controls, how to deal with aging effects and deformations due to aggressions to be taken into account. These examples are analyzed either from the point of view of the design engineer or of the operation team. It also briefly touches which topics are commonly linked to the geometrical dimensions (e.g. neutron absorber content). (authors)