Analysis of the Self-Loosening of Packages Impact Limiter Fastening Bolts under Vibrations during Transport

Bolted impact limiters generally equip extremities of packages used to transport irradiated fuel or high-level waste. These shock absorbers, whose mass can reach 2 tons for some package designs, commonly include steel casing filled with wooden blocks aiming at absorbing mechanical impacts and protecting the closure system of the package containment system in case of fire. Over the last 5 years, around twenty events were declared in France after the observation, during the package unloading operations, that some of these bolts could be loosened by hand.The causes identified by the package designers and the involved consignors, such asnon-compliances in the tightening operations, have led to additional controls of the bolt tightening before shipment. The decrease in the number of events declared subsequently show that these actions seem to be relevant but not sufficient to completely solve this issue.In this regard, IRSN decided to analyse the phenomenon which can initiate a self-loosening of the fixing bolts of the impact limiters. Several theoretical and experimental studies performed in the past decades show that a self-loosening of fastening bolts can be generated under external loads and vibrations. This phenomenon basically depends on the geometry of the bolt design (diameter, length and thread pitch), the mass of the bolted components and the vibrations transmitted by the transport frame. It especially appears that the bolts self-loosening is favoured by an unsufficient tightening torque, the use of lubricant on the bolt threads and the vibrations characterised by high frequencies and amplitudes in the perpendicular axis of the bolts. Considering the vibrations measurements performed during road and rail transports, these conclusions conducted IRSN to recommend, when possible, an increase in the tightening torque of the impact limiters fastening bolts for some package designs. The first results of this analysis also put in the light the necessity to perform a dedicated experimental program to determine precisely under which conditions the self-loosening of packages impact limiter fastening bolts is initiated.