Designs for deep-disposal facilities are dependent on the type of waste and the geology. For SNF and HLW, there is a strong emphasis on physical containment through the use of corrosion resistant canisters or, alternatively, through a corrosion allowance approach that, typically, deploys steel or cast iron canisters with thick walls, which are expected to remain leak-tight for the requisite length of time. Below, we provide three examples of these approaches.

The best-known example of the corrosion resistance approach is the Swedish KBS-3 system,35 which has also been adopted by Finland. The lowland geology of these countries is dominated by granite so that the geological environment corresponds to hard rock in low relief terrain. In the KBS-3 concept, PWR or BWR fuel assemblies are placed inside a nodular cast-iron insert that is surrounded by a 50-mm-thick copper shell: the first supplies mechanical strength and the second corrosion resistance (Fig. 18.5). The complete package, over a metre in diameter, more than 4 m long and weighing over 20 tonnes, is lowered into a vertical deposition hole drilled into the floor of a subterranean passage. Between the container and the rock is an annulus of dried, compacted bentonite, a form of clay. More bentonite is placed on top of the container to seal the hole and, when all the deposition holes have been filled, the passage is backfilled with compacted

rock spoil and sealed. On resaturation, the bentonite becomes plastic and increases in volume, creating a swelling pressure by expanding against the rock and the backfill. The intention is that the bentonite should protect the container from water ingress and rock movement so that it remains intact for a very long period. A variant of the system allows the containers to be put into horizontal deposition holes.

Andra, the national waste management organisation of France, has developed a deep-disposal system for a site that is being investigated in the east of the country.36 Here semi-indurated[32] clay is found at depth on the edge of the Paris basin. The site corresponds to the ‘deep sedimentary basin’ class and groundwater flow is extremely low. France has a policy of reprocessing its SNF so that, for the most part, heat-generating waste consists of vitrified fission products held in relatively thin-walled stainless steel containers; these are produced in various
sizes but, in broad terms, are generally a little over 1m long and 0.4 m diameter. For disposal, each of these is to be placed in a carbon steel overpack with a wall thickness of 55 mm, chosen to provide leak-tightness for several thousand years (Fig. 18.6). The external surface of the overpack has ceramic runners that enable it to be slid into a disposal module — a 40 m long, steel-lined horizontal borehole drilled into the side wall of an underground tunnel. Depending on thermal output, each module will contain up to 20 disposal packages with appropriately sized spacers between them so that, after allowing for a 10 m long closure zone, the whole of the module is used.

For the SNF that will not be reprocessed, carbon steel containers are deployed once again. These have a diameter of about 1 m, a length of more than 4 m and a wall thickness of 110 mm designed to provide a lifetime of at least 10 000 years. Depending on heat output, each container can hold up to three fuel assemblies so that the resulting waste packages are considerably larger and heavier than those for vitrified waste. Up to four waste packages can be placed, equally spaced as before, into a 40 m long, steel-lined, horizontal disposal module. In this SNF variant a bentonite buffer separates the steel liner from the surrounding rock.

A salt dome near the village of Gorleben in northern Germany was designated as a preliminary site for radioactive waste disposal as long ago as 1977. Subsequently, two shafts were sunk to 930 and 840 m and an extensive site

18.2 POLLUX® casks used in the German deep deposal system for spent nuclear fuel (Courtesy of Dr Bernhard Droste, Berlin, Germany).

characterisation programme was performed. Political difficulties have prevented the programme from progressing as quickly as was originally envisaged but, nevertheless, the engineering concepts are well defined. PWR and BWR fuel assemblies are to be dismantled and the fuel rods placed inside thick-walled, self — shielded steel containers known as POLLUX® casks (Fig. 18.7). These have a maximum length of 6.5 m, a diameter of 1.65 m and a weight of 65 tonnes. Transported on a railway system, they are to be laid horizontal in 300 m long, closed-end disposal tunnels and then backfilled with crushed salt.3137 A different system is envisaged for vitrified waste from reprocessing.38 Here, the packages themselves, with no additional containment or shielding, are to be lowered into 300 m deep unlined boreholes drilled into the floor of the repository access tunnels. These two concepts make use of three useful properties of rock salt: its

high thermal conductivity, its stability at high temperatures and its ability to creep to close up voids between the emplaced waste, the backfill and the surrounding host rock. These characteristics allow heat-producing waste to be packed more densely than would be possible in another geological context. The dry environment provided by the rock salt produces very low rates of corrosion so that the containers remain leak-tight for very long periods of time. In this respect, POLLUX® casks are not deployed because a corrosion-allowance approach is being used but, rather, because the shielding they provide removes the need for remote handling.