I share a visceral sense of unease whenever the phrase "nuclear waste" rears its head, since the term rings-in the implicit question of "where is it to go?" There have been various proposed scenarios, some of which are now considered impractical, others plain daft. For example, loading up a rocket with nuclear waste and firing it into the Sun (or into deep space - a gift from Earthlings to any extraterrestrials who might get in the way of it - or into a "stable orbit"...somewhere...from where it might come home again, some day). Firing it into the Sun is not such a bad idea, actually, until you think of what might happen if the vessel blew-up in mid-air - like the Space Shuttle did - and scattered its cargo among the four winds of the Earth. It would be a leviathan "dirty bomb", probably contaminating the entire planet.
One of the other more sentient suggestions, as I recall, was disposal via subduction zones - almost a supplication that Nature draws within herself what she has created, by placing our nuclear "offerings" on a geologic plate that is drawn under another on the ocean bed, and hence restored into the fire of the planet's interior, deep in the mantle which is in any case, at least partly fueled by the decay of radioactive elements. Given that geology does not operate exactly in this way, the idea is doomed. However, some of the more diehard protagonists insist that holes might be drilled into the descending plate which would be covered by the supra-bearing rock once sufficient millennia had elapsed. It is more probable that the molten magma that drives the plates of the Earth would be extruded upwardly through the weakened boreholes once they were drilled, casting back the radioactive detritus of our actions.
Other suggestions include disposing of waste under ice-sheets. Since high level waste (HLW; fuel rods and the like) produces heat, it could be simply allowed to melt its way a couple of miles through the Antarctic until it come to rest on the bedrock. A "recoverable" method holds the waste tanks with chains so it could be brought back to the surface once the more radioactive elements have decayed. After about 300 years, the more radioactive elements such as caesium and strontium will have decayed such that the level of radiation from it will be closer to that of natural uranium, and hence easier for final disposal. In a similar strategy, a "phased" disposal is now being considered, where the waste is kept initially closer to the surface for 300 years, and what remains is then disposed of in geological formations. This would also leave scope that the uranium fuel could be made available for reprocessing, if the availability of nuclear fuel were ever to become compromised.
Otherwise, the high level waste could simply be dropped into a deep (3 - 5 km) borehole. In Russia, a "direct injection" method is used, where liquid waste is pumped into geological reservoirs deep underground, where it is assumed that it will remain trapped. Another way to get rid of nuclear waste could be to buy it on to another country willing to take it for hard cash, in e.g. the former U.S.S.R. and Africa, but fears of it not being properly handled causing detriment to local populations, or that it might wind up in the hands of terrorists make this option unpopular. It appears that a, probably phased, geological disposal will be the method of choice in the U.K., since these islands have the correct geological structures to contain the waste within specially bored repositories. There is often a scare-story told that "Britain has 2.3 million cubic metres of nuclear waste stored around the country" (e.g. on the front page of "The Independent", Thursday January 24th) - "more than enough to fill the Albert Hall five times" - and while this is true, the vast bulk of this is low level waste, which carries only weak radioactivity, sometimes no more than the background level; the volume of high level waste occupies just under 2,000 cubic metres, most of it from Sellafield.
It is the latter that is the main problem, however, since although HLW constitutes only around 0.1% of the total volume of nuclear waste, it contains 94% of the total radioactivity. Research is ongoing as to what physical form it might be stored in. Nirex, as agreed by Defra/DTI has been given the mission to develop and advise on safe, environmentally sound and publicly acceptable options for the long term management of radioactive materials in the U.K. I attended a meeting at the Geological Society on January 9th, entitled: "Geosciences and the Long Term Management of Radioactive Wastes", from which a likely scenario emerged of a "Phased Geological Repository Concept" which allows a balance between long term storage and allowing future generations to do something else with it! Perhaps they would reprocess it for nuclear fuel or weapons, or put it down a deep borehole or somewhere else; who knows what they might decide? I often wonder what language any signs might be written in to alert those living as to the presence of a nuclear repository - English in the U.K.? - but will our by then equivalent of Shakespearean English be clear to those living hundreds of years from now, or even thousands?
The essential idea of form, is that HLW from nuclear fuel will be encased in a copper container, held within a structure of bentonite clay, and then stored in a deep rock depository. Copper is the metal of choice since stainless steel is prone to corrosion over long timescales. It is thought that 90% of the radioactivity will remain within the package, providing both retention and retardation such that only <1% of the total could ever escape into the geosphere. The figures given for the volume of nuclear waste refer only to the current levels, which will inevitably be increased by the government's plan to proliferate the nuclear industry and electricity production from nuclear power. One might argue that it would be more appropriate to get underway with disposing of the existing waste (an at estimated cost of £85 billion) before making more of it.