IMPROVING THE SAFETY OF BOLTING SYSTEM - USE OF THE ULTRASONIC TEST STRESS TIGHTENING METHOD

For the transport and interim storage of nuclear material TN International proposes to its customers a wide range of licensed cask design. The new transport & interim storage flask (TN®24E) for the German market with an advanced design to high efficiency has required to tighten the trunnion screws to a very high level of stress. New solutions for tightening these screws in compliance with all the German requirements in terms of standards and State of the Art had to be developed for this reason. Until now, the usual method to control the screw tightening implemented everywhere on a cask (for the bolts of lids, trunnions and protective covers) is to measure the torque using a torque-meter / torque-wrench (or equivalent). The qualification of this method leads to the implementation of typical accuracy around 5% on the tightening torque which corresponds consequently to 20% on the actual tightening force due to the uncertainty of the grease friction coefficient. That defines also the type of acceptable tightening equipment. In order to avoid damaging the screws due to an over-tightening, the accuracy of the force measurement had to be limited to 6.5% (instead of the usual 20%). The new method of control - The Ultrasonic Test stress tightening method - complies with this requirement. This technology is based on the measurement of the travel time of the ultrasonic wave along the elongating screw during the tightening. An ultrasound sensor is thus positioned inside a socket in order to measure the screw travel time in live. The information of the travel time combined with the calibration coefficient of the screw allows measuring the actual tightening force applied. The Ultrasonic Test stress tightening method is applicable for assembling the trunnion, control the tightening force after the loading test and the recurrent inspection of the cask. It was qualified by TN international and approved by TÜV Rheinland as expert of the German authority BAM to guaranty compliance with the guide BAM-GGR 011. This paper describes this new method, the first results from qualifications and its application within the TN®24E flask and the safety and design advantages.