Issues about implementation of geometry control to maintain nuclear criticality safety - 14438

The control of the geometry to establish and maintain Nuclear Criticality Safety (NCS) of a fissile unit is usually preferred by licensees and regulators since such a control is based on passive engineering and requires less operating constraints than other methods of control (mass, etc.). Nevertheless, several safety issues are associated with this method of control to ensure the geometry during all the unit lifetime, in normal and abnormal situations, from its manufacturing to its decommissioning. The first issue is to identify the dimensions defining the geometry, taking into account reflectors and mobile equipment, and the interaction conditions of the controlled unit to ensure the subcriticality. Thus, the dimensions important and to be controlled are identified in the NCS assessment. These dimensions are called Nuclear Criticality Safety Dimensions (NCSD) in this paper. Then, the second and the third issues are to define NCSD limits to be respected and to verify the compliance of the actual unit geometry with these limits at each stage of the unit life. Regarding the third issue, the challenges are the practicality of the control, its uncertainty and the management of non-compliances. Finally, the fourth issue is to properly document and record the NCSD and the compliance verification. This paper focuses on the main issues related to the implementation of geometry control on the NCS assessment and suggests advices to fulfill these challenges. (authors)