Irrespective of which option is chosen, the first step in the spent fuel man­agement is interim storage. The spent nuclear fuel element is mechanically the same as fresh fuel and can be handled as an intact fuel element. It is, however, highly radioactive and needs shielding and cooling during han­dling. The shielding is mainly for gamma and neutron radiation from the fuel. The spent fuel element is thus, after removal from the reactor vessel, handled and stored under water that can provide adequate shielding and cooling.

All water-cooled reactors store the spent nuclear fuel in deep water-filled pools. The water depth is typically 10 metres or more to ensure that the water provides adequate shielding (3-4 metres coverage) during all han­dling of the fuel. As the original intention in many cases was to reprocess the fuel, the size of the pools was designed to store a few years’ production only in these pools. In later reactors larger storage capacity has been pro­vided, in some cases corresponding to 30 years’ production or more.

As reprocessing currently is used in only a few programmes, it has been necessary to expand the storage capacity for most reactors. Different methods have been used. In some cases it has been possible to pack the fuel closer in the existing pools by introducing neutron absorbers or by taking into account the fact that the reactivity of spent fuel is lower than that of fresh fuel for which the storage racks in the pools were designed (burn-up credit). In other cases new storage facilities have been built, either at the reactor site or at a central site away from the reactor, to which the fuel can be transferred. In some cases these are also built as deep storage pools, while in other cases the fuel is stored under dry conditions in metal or concrete casks similar to transport casks, or in vaults or silos. Dry storage can be used when the heat release has diminished sufficiently after 5-10 years of storage. Although there are several wet-type storage facilities in operation, the trend at present is to use dry storage facilities for long-term storage. These have the advantage of requiring less long-term maintenance and can also more easily be expanded as the needs arise. The most obvious example of the latter is the dry storage casks, which essentially can be pur­chased as they are needed (Fig. 14.6). A typical modern storage cask can accommodate 20-40 PWR fuel elements or 50 — 100 BWR fuel elements, which means that a large reactor will require only about two to three casks

per year. The casks can be stored in simple warehouses and do not require strong buildings. There are also proposals for multipurpose casks that can be used for storage and transport and possibly even disposal. An overview of existing storage types is given in IAEA (2007c).

There is ample experience of long-term storage, up to 50 years, of spent fuel in water. So far no degradation of the fuel has been seen if the water quality is kept under control. The experience of dry storage is also good, although for a shorter period (less than 30 years). It is expected that the storage times can be extended without problems to at least 100 years, but the proof of such extension will require some further studies, in particular to ensure that the fuel can be removed after such a long period.

The main difference between wet storage and dry storage is the need for continuous cooling and chemical clean-up of the pool water to ensure a low fuel temperature and avoid long-term corrosion of the fuel or the spent fuel pools. Wet storage thus normally requires more staff for operation. Dry storage represents a much higher fuel and fuel cladding temperature, but a more benign environment, normally helium or argon gas. It is, however, important to have a follow-up programme of the fuel to ensure that the fuel can still be removed from the dry storage when the fuel is transferred to the next step. Most storage facilities are built above ground, but like the Swedish CLAB facility they could be built in a rock cavern to provide a better physical protection over long time periods (Fig. 14.7).