As mentioned in Chapter 7, the nitric acid stream containing the fission products after solvent extraction in the reprocessing plant is concentrated by evaporation and then held in storage tanks. A photograph of one of these tanks under construc­tion is shown in Figure 8.7. Nearly all the high-level waste from the nuclear work in the United kingdom, accumulated over the past 25 years, is stored in 15 such tanks at Sellafield in Cumbria, which contain a total of about 100m3 of liquid.

The stainless steel tanks are contained in concrete vaults, which are them­selves lined with stainless steel to provide further containment in the improba­ble event that the primary container should fail. The space between the tanks and the vaults is monitored, and provision is made for transferring the contents to spare tanks should the need arise. Heat is removed by several independent sets of cooling coils. Reinforced concrete, typically 2 m thick, in which the tanks are sited, protect the operators from direct radiation. Provided cooling is main­tained, there are essentially no radiological hazards. The possibility and conse­quences of an accidental loss of coolant were considered at the Public Enquiry on Windscale in 1978. In the extremely unlikely event of a total loss of coolant (estimated to have a probability of occurrence of 1 in 1 million for each year of operation), it would take hours for the contents to boil and days for them to evaporate, allowing ample time to take remedial action. During the period in which the fission products are generating significant quantities of heat, keeping them in a liquid form facilitates cooling. However, for long-term storage it is considered preferable to convert the waste into solid form. and a number of

processes have been considered for this.

Work on solidification of nuclear waste started in the 1950s, and by the mid — 1960s incorporation of wastes into glass (vitrification) was established on a lab­oratory scale. The method has been used on an industrial scale in France for a number of years, and the French AVM process (illustrated in Figure 8.8) has been adopted in other countries, including the United Kingdom. Among the al­ternative processes being investigated is the microwave vitrification process il­lustrated in Figure 8.9. A range of glass compositions have been developed that enable the constituents of the waste to he incorporated. The glasses have been shown to survive the effects of heating and radiation from the wastes without significant deterioration. They would dissolve veiy slowly over many thousands of years in freely flowing water. Dissolution in the sort of repositories likely to he used, where access to water is severely restricted, would he very much slower. Other solidification techniques include incorporation into various ce­ramics and forms of crvstalline rocks.

The vitrified waste is typically cast into stainless steel canisters and these can­isters dry-stored in a manner illustrated in Figure 8.5. The vitrified waste canis­ters will be stored in these natural convection air-cooled stores up to 50 years before final disposal. A typical glass block might be 30 cm in diameter and 1 m long, weighing about 0.2 tons. About 20% of the weight of such a block would be the fission products from the reprocessing plant, the rest being added mate­rials to help form the glass. As in the case of the spent fuel, the heat release is dominated by the caesium-strontium decay with a half-life of 30 years. It is gen­erally considered that surface temperatures for the block in the long-term store should fall below 100°C, and at this surface temperature a heat rejection rate of about 1 kW is achievable by conduction into the surrounding rocks. To avoid interactions between blocks within the rock matrix, a spacing of about 10 m in all directions is required. This could be achieved by tunneling to the required 1 km depth and then constructing a gallery from which holes, say, 200 m in depth, are drilled. The blocks could then be dropped in and the required 10 m

spacing achieved by infilling before dropping in the next block. The holes themselves would also be spaced out on an array of 10 m square.

For the British program, it is estimated that some 10,000 blocks will have been produced before the end of the century. This would imply the use of an array of around 50 x 50 blocks arranged, say, in a cube.

Again, the problem of leaching of fission products from the block and their transfer through the strata must be considered, and the thermal circulation and thermal buoyancy effects mentioned are very important in the medium term. Enough is now known about these systems to be sure that safe disposal of nu­clear waste is possible.