Time out of mind

What should we do with nuclear waste? It’s a question which has dogged the nuclear industry for decades, and it isn’t going away any time soon. The only practical answer is to bury it, somewhere where it’ll be secure and protected for as long as is necessary for its radioactivity to decline to safe levels. This week, the UK’s Nuclear Decommissioning Authority published its generic safety case for a depository to hold the UK’s wastes.

There are several slippery terms in that paragraph, and it’s worth going through them. The first is ‘bury’. We’re not just talking about digging a hole in the ground here. We’re talking about excavating a series of caverns, up to a kilometre below the ground, in carefully-surveyed rock that meets a number of stringent criteria; lining the caverns; filling them with waste which is either encased in cement (for intermediate-level materials) or set in molten and solidified glass and canned in steel and copper cases (for high-level waste and spent fuel), then sealing it up and leaving it while the radioactivity declines. The UK will have a total of some 50,000 tonnes of these categories of waste by the time the current fleet of nuclear reactors is decommissioned. Some of the legacy waste dates back to the 1940s.

The next tricky concept is ‘as long as necessary’. How long? A hundred thousand years. Just to put that into context, Stonehenge is about 5000 years old, and let’s face it, it’s not in good shape. Coincidentally, that’s also estimated to be the earliest date that humans started to write. The last ice age was at its peak 20,000 years ago. So a thousand centuries takes in the entire history of civilisation and reaches even further back. Designing something that’s going to remain intact — and, even more importantly, safe — for that distance in the future must surely be one of the most daunting engineering challenges possible.

Bruce McKirdy, the managing director of the NDA’s Radioactive Waste Management  Directorate, is the man in charge of the project, and he doesn’t seem phased by it. However, launching the generic safety study, he was scrupulous in pointing out that word ‘generic’. The reason it’s there is that the UK doesn’t have a site for its depository. The only site selection process that’s ever worked is a voluntary one, he explained, and the process of communities volunteering a site is still ongoing.

It has to be a big ask for any community to volunteer to house such a facility. It’s happened in Sweden, however, where a site in Forsmark was selected in 2009 and is now undergoing tests of its geology.

McKirdy is adamant that volunteerism is the only approach that can be considered, and if a suitable site doesn’t come forward, the process will continue until one does. ‘This isn’t the kind of thing that you could force on a community, and we can make the current waste storage facilities secure for as long as it takes,’ he told The Engineer. The reason for publishing the safety study now is that waste is currently being packaged and it needs to be placed in a form that would be suitable for a repository in a range of geological conditions, depending on the site that’s finally selected.

Clearly, a despository full of toxic, radioactive material is not the sort of legacy that any society would want to leave. But McKirdy believes that in the absence of any technology that can render waste less hazardous, it’s the only responsible course. For engineers, it’s a task which will have to be tackled.