Ageing Management of DPCs : some Aspects of Bolted Systems at Packages

Dual Purpose Casks (DPCs) are currently loaded and stored for decades. Stored, but ready to be transported at any time: some facilities have to maintain a valid agreement for transport for all stored casks. The new edition of IAEA Transport Regulations (SSR-6 Rev 1) introduces the obligation of taking into account ageing mechanisms in the design phase of the package. In the same time, some preparations of DPCs for transport after storage have been performed. This article focusses on the issues raised by the assessments of the package design safety report (PDSR) of DPCs and by the preparations of DPCs for transport concerning bolted package component systems. Bolts are usually very important components for the safety of a package e.g. for lids or trunnions. Replacement of bolts before transport is not easy and can be impossible, e.g. bolts of thermal dissipators. Depending on the design, replacement of threaded holes may be impossible. Moreover, replacement of components may lead to high doses to the operators and should be limited in a radioprotection point of view. Even verification of the tightening torque may be difficult. The main expected ageing mechanisms of the bolted connection deal with the tightening torque. Many parameters can influence the tightening torque: of course, the torque applied after loading, with uncertainties from the tool; but also the grease applied on the bolts, the emplacement of the grease; temperature of the bolt; creep during storage etc. This article discusses some aspects of ageing management of the bolted systems related with data traceability such as initial torque, grease used and practical application on real preparation for transport.