French requirements for the design, manufacture and use of bolts and screws equipping packages for the transport and storage of radioactive material

In order to guarantee the good behavior of components important for safety in all conditions of transport, a specific attention should be paid on the bolted flange and screwed assemblies.In this regard, The French nuclear safety authority for defense activities (ASND) and its technical support, the Institute for radiation protection and nuclear safety (IRSN), initiated a working group to identify good practices related to:the design of screwed assemblies by specifying allowable criteria in all conditions of transport. Approaches which could be considered to limit the risk of self-loosening of the screws was also discussed;the representativeness of bolted and screwed components equipping mockup or specimen used during drop tests in terms of mechanical properties, dimensions and bolts preload;the hypothesis considered to model, by numerical calculations, the mechanical behavior of the screws;the manufacturing process to prevent notably the risk of hydrogen embrittlement and the supplying controls to check the as-built properties of the screws.Thereupon, it was recommended to avoid the use of 14.9 grade bolts and to introduce systematic hydrogen degassing for the bolts grades 10.9 and 12.9 coated with galvanic processes. A relevant final control procedure also has to be implemented to guarantee the compliance of the bolts; the use instructions by listing good practices to apply the torque values (cross pattern, time to reach the thermal steady state of the components, markings ?).The conclusions of this working group, taking into account transport events feedback and involving the main French applicants (CEA and ORANO TN), will be introduced in a dedicated guidance supported by ASND. The use of this guidance will allow a better understanding of the safety issues and contribute to ensure a high safety level for the radioactive material transportation and storage.