There are two main ways to store SNF, 'dry' and 'wet (Romanto, 2011). Wet storage involves placing the spent nuclear fuel in large pools of water. Water is a good 'shield' for the radiation and it also helps to cool the hot radioactive waste. Over summer I was actually lucky enough to visit the pools in Sweden where their SNF is stored, the level of shielding the water provided meant we could stand right next to the pools! However my worries did prompt a question aptly answered by one of my favourite comic series: xkcd. I remember being very surprised by two things, the fact that the pools literally looked like a large public Olympic sized swimming pool, and also the incredible heat in the room emanating from the pool, it was 5C at the surface but almost 28 in the storage area. Dry storage instead involves a process called 'vitrification'. The SNF is mixed with glass forming chemicals in a furnace to form molten glass that then solidifies in containers that can be stored (Jantzen, 1991). Most reactors will involve some form of wet storage as nuclear waste is too hot to handle when it's first taken out of a reactor, however whether or not the SNF will remain in wet storage or be moved to dry storage differs from country to country.
Countries that mostly leave their fuel in only wet storage (for now) include Sweden, Finland and Russia (Kadak, 2012) In other countries, most notably the UK, Canada, Japan, China, the US, the Czech Republic, the waste is vitrified and put in dry storage after spending some time (a decade) in wet storage. In the UK the dry SNF is stored on a site called Sellafield in Cumbria, England (BBC, 2014). Sellafield is currently the world's largest stockpile of plutonium with 140 tonnes of plutonium being stored on site. It's not a comparison that I like to make when discussing nuclear power, but just to put things in some form of perspective, a nuclear bomb will typically contain 5-10 kilograms of plutonium (Thompson, 1998). Obviously with so much dangerous radioactive material, safety concerns are a big issue, and a £39 million firearm training base has been opened next door to assist with raising the standards of policing at the site (News & Star, 2016).
A third option other than wet or dry storage exists. Reprocessing. Reprocessing (without getting too technical*) is based on the fact that most spent nuclear fuel in fact still contains 95% of its potential energy, this energy is just too inefficiently accessed to warrant further use of the fuel. However through several processes invented by people far smarter than you or I, SNF can be reprocessed to be efficient and usable in reactors (Nash & Lumetta 2011), In fact this is one of the most important aspects of the nuclear energy debate as without reprocessing Uranium will run out in ~45-60 years, making it a fairly useless replacement for fossil fuels. However with reprocessing experts say we could at the very least triple that. So why don't all countries employ reprocessing techniques? Because of failures of the market. Uranium is very cheap (see graph) for various economic reasons, that there's no incentive for some countries to invest in reprocessing (SNF workshop I attended in 2017). Alongside that, countries like Sweden and Germany are constantly debating whether or now to slow and/or close their nuclear programs, which I feel is making them even more reluctant to invest more money into reprocessing. Countries like Russia, Korea, Japan, France, Taiwan and the UK all reprocess to some extent.The big issue is that in general governments are not satisfied with leaving SNF in these surface-level storage facilities. In terms of cost the operational staff required to man the facilities,and the money which must be spent on containing the high amounts of radiation are big on energy comissions' minds. The other big worry is that of safety, a well placed missile could irradiate swathes of a country if not of a continent.
So what can we do with nuclear waste? I'll explore some ideas in my next post!
*If you want to get technical check out the BBC's nice resource on what reprocessing in the UK looks like: http://news.bbc.co.uk/1/hi/uk/647981.stm
Hey Luca! Nice neat summary of wet, dry and reprocessing techniques.....also loving the fidget spinners reference! I had two questions for you:
ReplyDelete1. Regarding the wet storage technique, surely radioactivity's danger does not just depend on its temperature? Do you know how else water might act as a shield?
2. If we introduced market incentives to reprocess all our nuclear fuel, do you think it would then be a good energy alternative to fossil fuels?
Yes! You're right, the temperature is far from the only danger. The lethal radiation it admits is also stopped by the water, in fact if you look at the cartoon in the post, it details how you could probably swim safely at the top of the pool, but would definitely die if you dived to the bottom.
DeleteAnd for number 2, you'll have to wait till I tackle that controversial topic in my later posts! ;)