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.
The film repeated the fact that no human made structure has ever lasted a fraction of the 100,000 years needed. The pyramids in Eqypt and Medieval castles in the UK have held up pretty well for over 1000 years, and it should be noted that the Onkalo repository is placed 430 meters within a rock formation – not a man made structure. However, is there any physical evidence that the waste from nuclear reactions can be stored for the time-scales required? You may be surprised to find that actually, there is!
Oklo was a natural formation of uranium at Gabon, Africa. Miners digging in the 70s discovered something was a bit weird about this place. After investigation it was realised that this had once been the location of a series of natural fission reactions – a chain reaction just like in today’s nuclear reactors, with all the same waste products.
After further investigation, it was found that in the one million years or more since the nuclear reactions took place at the Oklo reactor, the waste products had not moved more than a few inches from their place of origin. The reason being is the multiple barriers in place – clay, rock etc. The designers of Onkalo in Finland take these ‘natural analogues’ into account when designing the repository. In Onkalo, each fuel assembly will be surrounded by a massive copper canister, and placed in conditions which make rusting highly unlikely for hundreds of thousands of years. Just in case though, the tunnels will be back-filled with a kind of clay which isn’t permeable to water.
This leads on to the other major theme in the film – human intrusion into the repository by some future civilisation. The film seems to completely ignore the fact that the tunnels will be filled when the repository is closed in the 2120s. Instead, it seems to be suggesting that the tunnels will be left open for future generations to stroll down and take a look at the spent fuel rods which will of course be left lying around on the floor. However, there exists one further barrier to the removal by some future dark-age civilisation. If the 2.5 miles of tunnel leading down could be dug out, or somehow the 430 meters of rock leading directly down could be blasted away, there exists a hazard down there that is equal or greater than the radiation emitted – a lack of breathable air.
As can be seen, as well as the access and and personnel tunnels for people and vehicles, there exists two vertical shafts to keep the area being excavated for the spent fuel habitable. Why was this not discussed in the film? Why did they not ask the designers, engineers and experts interviewed what physical barriers will be placed to prevent future generations accessing the repository? If necessary the tunnels could be collapsed – the ultimate backfill.
Finally, I was a bit disappointed by how at the end of the film, the issue of waste transmutation was dismissed almost out of hand as being a pipe dream. It was also claimed that uranium would run out ‘just like oil’ in around 100 years. It just so happens that transmutation of nuclear waste and extension of nuclear fuel supplies are both accomplished in the same way – by using an advanced type of nuclear power station called a fast reactor.This type of reactor uses the actinides discussed earlier as fuel, meaning that only the fission products remain (see first graph) and the supply of fuel can be extended by up to 100 times. I probably will not be discussing this type of reactor on this website, as it already has been done excellently elsewhere.
In conclusion, the Onkalo repository is a major step forward on the disposal of radioactive waste. The Finnish example should certainly be followed by other countries, including our own. As discussed in the film, it is irresponsible to leave potentially hazardous (nuclear and non-nuclear) lying around for future generations, and the Fins should be applauded for their efforts.