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I watched ‘Into Eternity‘ last night on More4 (It was titled ‘Nuclear Eternity’ on the channel here), and I must say the film was a wonderful work of art. The imagery was fantastic, the music was entertaining, the narrative compelling. However, being an engineer I was somewhat frustrated by the lack of technical discussion, and instead a heavy reliance on philosophical questions. If you’ve seen the film, and I suggest you do before reading this entry, you’ll notice that the interviewees (mostly technical staff working on the Onkalo repository) were somewhat taken-a-back by this kind of question.
So, in this entry, I’d like to discuss some of the technical issues which I believe were somewhat missed in the film. The first is the issue of how long the waste has to be stored for. The 100,000 years needed to store spent fuel is based on the radiological toxicity (how dangerous it is to humans).
As can be seen from the diagram, the danger from the fission products (what you get when you split the uranium atom) is more or less gone after between 500-1000 years. These are the worst bit of the waste, and they are what you want to guard against. After this, you have the actinides remaining. In a nutshell, these are what you get when uranium captures a neutron, but doesn’t split. These are still dangerous if you ate or came into contact with a large amount of them, but they will be largely diluted by the rest of the material in the repository. They also happen to be some of the heaviest elements known to man, and so are unlikely to ever migrate to the surface, even if they come into contact with water. As can be seen from the diagram, they are the reason why the spent fuel is still more radiotoxic than the original ore for a period greater than 1000 years.