A significant proportion of the radioactive waste that results from operating nuclear power stations is highly hazardous to health and some will remain so for a long time. Whatever our views on the wisdom of nuclear power, the question of what to do with the waste needs to be addressed. This is because after over half a century of nuclear power generation, involving more than 400 nuclear power stations, there is already a volume of over 20 000 cubic metres of high-level waste that needs looking after.
Nuclear waste is classified into three groups, according to their activity and hence their storage needs. Low-level waste includes things like contaminated overalls, lab gloves, paper towels, as well as some of the chemicals, which may be either solid, liquid or gas, that have come from different industrial processes along the way. These don’t need cooling and are considered suitable for storage in surface buildings for up to a few hundred years.
Intermediate-level waste is more active, and includes things like fuel cladding material that has become activated. These also require no or little cooling but do need to be more thoroughly contained, and are destined for storage below ground.
It is the high-level waste that concerns most people the most. This is the spent fuel from a reactor core. It has first to spend up to five years at the bottom of a pool of water, where the heat generated from the furious radioactivity can be taken away by the circulating water. The water also acts as a very effective radiation shield, even from the gamma rays the spent fuel is emitting. These pools make great photographs, as the blue glow from the Cherenkov radiation (light emitted by beta particles from the fuel travelling faster than the speed of light in water) is both eerie and (to some) strangely beautiful.
The time spent in the pool is short, relative to the tens of thousands of years it will languish in long-term storage, but the range of timescales involved tells us something about the nature of the spent fuel: it is a complex mixture of many radioactive materials, each having a different half-life, and hence activity. The short dip in the pool gives enough time for the short half-life, very active, reaction products to decay into other, longer lived, less active materials.
Many sites across the world have been investigated as possible long-term repositories for high-level nuclear waste, but actual completion of a site is still some way off. This is partly because it is hard to persuade any local community to host such a site. There are also some very challenging and interesting questions to answer that come from the 100 000 year timescale in relation to human history. The oldest known cave art in the world is dated to only about 40 000 years ago. Considering the changes in human culture and technology since then, and especially the rapid changes over the last few hundred years, is it possible to imagine what kind of human and human society will be around in the far future - over twice as far away as the time span back to the first faint traces of human culture?
This question is one of many that designers of long-term nuclear storage facilities, such as the one planned at Olkiluoto in Finland, are thinking about now. Should the site be marked and, if so, how? It is almost certain that all the languages we speak now will have transformed so that future people’s efforts to understand whatever warning sign we put up may well be fruitless. Perhaps it is better to eliminate all traces on the surface that anything of interest was ever built there, but what then of the possibility that a future human might unwittingly disturb, or even unearth, the still dangerous waste? But what if, for some reason not foreseen, the waste begins to contaminate our habitat in a dangerous way, for example by leaching into the groundwater? Our descendants may need to recover the waste to make it safe again.
If the site were marked, would our descendants’ natural curiosity not compel them to investigate, as we have done with every archaeological find we have ever made? Is there any way that we could guarantee conveying exactly what was contained below, perhaps by inventing some symbolic system that any civilisation capable of reaching the waste stored in sealed chambers hundreds of metres below would be bound to understand?
Another idea is to defer any decisions for as long as possible, storing everything in such a way as to relatively easily adapt to any new approach or technology that may emerge. This is the default approach that is currently being taken, but for how long is it viable, and how safe from natural or human disruption is the waste in the meantime?