The ultimate aim is to make nuclear power more sustainable through the use of ‘fourth-generation’ reactor designs in combination with a coherent re-use and recycling strategy.
The €9.4m (£7.9m) FP7 ASGARD project is led by Chalmers University of Technology in Sweden, with input from UK researchers at the National Nuclear Laboratory (NNL) and several universities.
Today most of the world’s nuclear fleet is based on thermal reactors that use oxide fuels and the so-called ‘once-through’ option — meaning that the fuel is run in the reactor for a period and then put in a final repository for around 100,000 years.
Only about one per cent of the energy in the fuel is actually used, although some countries such as the UK and France do recycle plutonium once before final repository.
Alternatives to oxides
‘It is more sexy to talk about the reactors, waste is waste, it’s not that fun,’ said Prof Christian Ekberg of Chalmers. ‘The expression “next-generation reactor” is inherently wrong, it’s not just the reactor — it’s a combination of a thermal reactor, fast reactor, recycling plant, and a fuel fabrication plant.’
Ekberg believes there may be better alternatives to oxides when thinking about future reactors.
‘The idea of a fuel is that it should be hot enough on the outside to boil water,’ said Ekberg.
‘But if you have a low thermal conductivity, in order to let’s say have 500ºC on the outside of the fuel pellet, you have to have a very high temperature in the centre of the pellet. Alternatively, if you have a high thermal conductivity you can have a much lower temperature on the inside, which is safer, and if you combine that with a higher melting point then you have a much larger margin towards core melt.’
As an example, metallic fuel has extremely good thermal conductivity but very low melting point, while oxide fuel has poor thermal conductivity and low melting point.
Nuclear fuels based on nitrides and carbides have both a good thermal conductivity and high melting point, and so could herald great potential. However, there has been very little research so far.
‘The idea of this project is to find a way of manufacturing these novel fuels that can be done industrially, and hopefully by the end of the project we’ll have been able to make fuels and irradiate them. We do have some plutonium zirconium nitrites that we will do experiments on … just a few pellets, not enough to run a reactor but enough to demonstrate the principles,’ said Ekberg.
As well as safety, one of the key considerations of the project is to have a more sustainable fuel cycle. While precise strategies will vary, the goal is to get to a situation where fuels can be at least 80 per cent recycled (relative to the one per cent average currently).
‘It’s all a matter of what your aim is — is the objective to burn away enough of the long-lived fission products so the final repository can be shortened, or is the main aim to increase the energy utilisation of the fuel?’ said Ekberg.
Meanwhile the UK side of the project will focus on carbide-based nuclear fuels.
‘It’s useful from the UK’s perspective to have a good skills base in understanding carbide fuels and how they behave, with regards to future reactors, but also how to treat our existing fuels — some of our earlier test reactor fuels and legacy materials are carbides,’ said Bruce Hanson, technical authority at NNL.
’If the UK is moving towards sustainable nuclear fuel cycles we will have to know about different types of fuel and how to treat them in a closed fuel cycle,’ he said.