As soon as it was realized that pool capacity will not allow the long-term storage of all of the fuel from a reactor and as the final strategy for and destination of spent fuel were not clear, extended storage capacity became necessary. Dry storage systems started to be developed in the late 1970s and early 1980s. At the end of 2010 there has been more than 25 years of favourable experience on dry spent fuel storage technology. Since it began, the dry storage of spent fuel has evolved into a variety of systems.5,6 Examples are concrete canisters, metal casks (i. e. CASTOR casks in Germany), steel-lined concrete containers, concrete CANSTOR modules (in Canada), and vaults in France, Hungary, the UK and Canada. Many countries developed storage casks (under a variety of names for storage canisters or containers) as self-standing units for containment. In parallel to the development of designs for dry storage, research and investigations were carried out to determine the longevity and stability of materials and spent fuel during dry storage. This was motivated licensing of storage systems. Initially licensing periods were short (from 1 year in the Russian Federation, 2 years in Canada, and 20 years in the USA and Spain to 40 years in Germany). As the need for longer — term storage increased and the confidence in materials and designs grew, the licensing periods were extended. The CASTOR storage containers from Germany can now be licensed for 60 years and this is the tendency in other countries; in the USA they can grant a licensing extension for 40 additional years. The technology for manufacturing dry storage system is mature and we will show just a few examples since detailed information can be obtained elsewhere.6,7 The main requirements for dry storage systems are: containment of radioactivity with watertight or airtight barriers, biological shielding and decay heat removal (cooling).

In addition, the longevity and stability of all materials are becoming even more important when new strategies require very long-term storage (100-300 years) of spent fuel.

Several research projects have investigated the long-term integrity of spent fuel in dry storage.6 Potential mechanisms that may affect cladding integrity during dry storage are: [28]

to promote this mechanism and investigations so far have not shown that these conditions can exist.

Nevertheless, as long-term storage of 100 years or even more is becoming a possibility, and the kinetics of some chemical degradation processes may not be linear, investigations continue to confirm the long-term integrity of claddings. In addition, investigations these mechanisms are affected by transport and extreme accident conditions continue.