2.3.7 Solution: deep geological repository
The favoured option for nuclear waste management around the world at the time of writing, 2015, is to bury the waste in purpose-built underground repositories.
The siting of such a repository requires much thought as many of the fission products will remain active for thousands of years. Thus, a repository needs to be secure, both now and into the far flung future!
There are two main issues that need to be considered:
- The possibility of seismic activity. Earthquakes could bring the material to the surface.
- The problem of water. It is vital that water is not able to come in contact with the radioactive material now or in the future.
In order to reduce the possibility of either of these problems arising, the design of repositories includes containers with multiple layers enclosing the waste and other engineered barriers or seals around the containers. Great attention is also paid to the suitability of the surrounding environment, particularly the geology in terms of stability and rock composition, and the way water can move through it. Several processes combine to cycle water globally, and these in Figure 20.
At any stage in the water cycle where evaporation occurs, anything dissolved in the water is left behind. In particular, radioisotopes transported into the oceans would accumulate there; they would not evaporate and re-enter the water cycle.
How water flows through the ground is largely determined by the geology. Many of the rocks that make up the Earth’s crust contain voids, which can hold water. These voids can take various forms. In sandstones, for example, they consist of small interconnected pores between the grains of sand. In granites, which are made up of interlocking crystals, there may be fissures or fractures, which can be interconnected so allowing water to travel through the rock. Below a certain level, the rock voids are all filled with water. This level is called the water table, and the rocks below it are said to be saturated. By using the voids as a pathway, water can flow through the saturated rocks.
The ease with which water flows through rocks varies with the rock types. For example, if the rock contains large well-connected pores or voids, like the sandstone in (a), or extensive linked fractures, like the granite in (b), the water will flow easily through the rock. In (c) there are large pores in the rock, but the pores are not interconnected, so the water cannot flow easily through the rock. Any repository would ideally need to sit above the water table and within a rock that resisted the flow of water.
In the next section you’ll see a video of the planning for a depositary in Yucca Mountain, a mountain in Nevada, US. This site was deemed to be near perfect as regards its geology.