As we saw in Chapter 7, the spent fuel from the reactor may be stored under­water in cooling ponds for typically 10 years or more. The current stage of de­velopment of the nuclear program in Europe, the United States, and Japan is such that final decisions about the next phase of spent fuel management, namely engineered surface storage, need to he taken in the next few years.

It would be possible, for instance, to continue with away-from-reactor un­derwater storage, perhaps with the fuel contained in an additional “bottle" to prevent the spread of contamination within such a large water basin store.

Alternatively, a dry storage system could he adopted. Essentially, two differ­ent dry storage systems have been developed: the cask or container system and the modular vault diy store. In Canada development of cliy store containers for CANDU fuel has been in process for 20 years. The latest design of dry storage container is shown in Figure 8.4. It consists of a box 25 m x 2 m x 3 5 m high, constructed from inner and outer steel shells filled with heavy concrete. It weighs 53 tons and contains some 384 CANDU spent fuel bundles: total mass,

7.3 tons. These are stored horizontally in four racks. The Canadian nuclear sta­tion at Pickering near Toronto will ultimately have 700 such containers storing nearly 5000 tons of fuel, making it, when complete the world’s largest dry store. For more highly rated PWR spent fuel Germany has developed a container using ductile cast iron (CASTOR) 2.4 m diameter. x 4.8 m high, weighing 100 tons when loaded and containing either 33 PVR spent fuel elements or 74 spent fuel elements—15 tons of spent fuel.

An alternative dry storage system is the modular vault dty store illustrated in Figure 8.5. In this concept the spent fuel is contained in individual vertical

sealed fuel storage tubes retained within a concrete vault that can be con­structed in modules. Air is drawn in by natural circulation between the array of storage tubes and is discharged via an outlet duct. The coolant air does not come in contact with the spent fuel and therefore neither it nor the concrete structure becomes contaminated. A facility of this type has been constructed to store the spent fuel from the gas-cooled reactor at Fort St. ViJ. in. ‘This particular

lesign has 45 fuel storage tubes, each holding six fuel elements. The fuel is noved in and out of the storage tubes by a fuel handling machine moved by he building crane. A modular vault dry store has been considered by Scottish..Juclear for dry storage of AGR fuel. Designs for both an 800-ton and a 1200-ton :apacity have been prepared.

Storage in these dry stores would continue for 50-100 years, during which he level of radioactivity gradually decays (Figure 8.2), as does the rate of heat )roduction (Figure 7.5). Surface storage in this form for extended periods is ad­vantageous since natural convection cooling can be arranged and the packages monitored systematically. Ultimately, the rate of heat generation will become low enough to permit storage without special arrangements for natural convec­tion cooling. At this stage long-term disposal may be considered.

The concepts being considered for ultimate disposal of spent or unre­processed fuel include disposal to underground salt formations or within hard rock geological formations.