The process known as radioactive decay of the nuclear waste can take hundreds of thousands of years, depending on the material. During that time it is essential that the contents of the repository are isolated from the surface. We need to ensure that gases and liquids can’t migrate upwards. While it is accepted that the repository itself is likely to break down in a relatively short time, a good geological seal can trap the gases and liquids that escape, in much the same way that reservoirs of oil are formed. This process of isolating nuclear waste from the surface in the long-term depends almost entirely on the geology (rock formations) in which the repository is constructed.
There are several types of geology which are potentially suitable, but increasingly, types of clay are being favoured by the international community. Leaders in this field, France, Switzerland and Belgium have all chosen to build in clay. One of the key advantages of clays is that they tend to be self-sealing. Over hundreds of thousands of years there may well be some movement causing fractures or faults, and hard rocks which may have started out as an effective geological seal, may no longer be so after this movement.
The most important aspect to consider when looking for a site to bury nuclear waste, is to find simple geology. That means relatively flat-lying layers with very little faulting or fracturing and very similar over a wide area, so it is predictable. It is also extremely important to have as little underground water flow as possible as this will accelerate the breakdown of the repository and carry parts of the waste away. For this reason you should avoid hilly or mountainous areas as they drive water flow deep underground, in much the same way as a water tank in the loft of a house provides pressure. Geologists call this a high hydraulic gradient. High levels of rainfall are also undesirable.
If you were to restrict the search area to England, as seems likely for political reasons, the idea of further confining it to Cumbria seems a strange choice. Cumbria is not known for flat lying simply geology, quite the opposite in fact. It is also the most mountainous area with the highest rainfall in England. Precisely the opposite of what we should be looking for. Cumbria has highly complex geology with fast underground water flow.
This shouldn’t come as a surprise to the government who spent over £400m investigating Cumbria’s geology for this purpose in the 1990s. The conclusion reached by this Nirex inquiry was that the search should move to an area with simple geology, mostly found in the east and south of the country.