EXTENSION OF CONTAINER LIFE THROUGH LOW-COST PROBABILISTIC ASSESSMENT

Three sets of ageing containers, approaching the end of their service life in new fuel transport, were discovered to have worn screw threads in some of the lid attachments. This wear manifested itself as an oversize female screw thread in hexagon nuts welded to the structure. Following a quarantine period of random sampling and characterisation, the problem was confirmed to be widespread. Options considered to recover the situation included the renewal of all nuts, repair by helical insert, use of over-size fasteners, and use of a backing nut on lengthened fasteners. The practical difficulties of all of these approaches, together with the costs and disruption to the transport programme associated with repair of over 4,000 screw threads, favoured the use of a non-intrusive solution. A programme was therefore initiated to provide detailed characterisation of samples of worn nut threads by putty replication, with test specimens made up by artificially creating a range of worn threads. By ensuring that the test specimens covered a wide spectrum of thread wear, and by testing these for bolt pull-out under controlled conditions, a probabilistic case was made that the incidence of a container thread failure occurring at a load below the design intent was very small. This design intent load is characterised by failure of the flanges, which occurs before that of the bolted connection, and so the concept of a 'weak link' was generated that sets a minimum value for fastener strength. As part of the justification, potential causes of the thread damage were investigated. Factors relevant were found to be weld distortion during container manufacture or re-build, and the use of thread-cutting taps to clear deposits during the course of maintenance activities. The probabilistic approach to justification of these container designs has been reviewed and accepted by the UK Competent Authority, with the award of a Fissile certificate for shipment of each set.