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Uncertainty over the future integrity of graphite moderators in Advanced Gas-cooled nuclear Reactors (AGRs) could result in their premature shutdown before sufficient new nuclear or gas power stations come on-stream. This could leave a hefty 10% gap in the UK’s electricity supply. However new research carried out at ISIS to study the AGR graphite structure and its changes during irradiation, could help to extend the lifetime of AGRs.
Inside a nuclear reactor, heat energy is harnessed from the splitting, or fission, of atoms in the fuel to produce heat and hence steam. This steam drives the turbines to generate electricity. AGRs have a graphite core which acts as a moderator to slow down the neutrons released from the fission, in order to encourage more fission. The fuel, usually pellets of uranium oxide, is arranged in rods and assembled into the core, and a coolant gas such as CO2 circulates.
Dr Stephen King, ISIS, explains “The core of a graphite reactor is basically made up of a lot of graphite bricks and some of those bricks have holes in them for the fuel elements and control rods. What you don't want is for these bricks to start cracking to the extent that they break or move because this could affect the control of the reactor. So it’s vital to understand graphite cracking in order to predict the long term effects.”
Currently there is a divergence between the predicted and observed behaviour of the graphite core after long term irradiation. During its lifetime, open pores in the graphite bricks are subject to radiolytic corrosion due to the gamma irradiation. Moreover, fast neutrons displace carbon atoms which tend to fill the pores (which may be open to the surface or enclosed). The effects of this are damaging and limit the safe functioning of the reactors. A group of Scientists from the University of Salford, Manchester and ISIS have used small angle neutron scattering on the LOQ instrument, to develop a new way of characterising the porosity of graphite on a length scale going from a few atoms to 10nm.
Dr Zhanna Mileeva, who recently moved from Salford to the University of Manchester explains, “The structural deformation of graphite bricks exposed to fast neutron and gamma irradiation is not yet well understood, and currently presents a significant problem in extending the safe operational lifetime of nuclear power stations. Our research offers a new way of investigating the graphite structure and its changes under irradiation conditions”
“We have focused on the porosity of nuclear grade graphite and its accessibility to the external surface and hence the active species - those CO2 molecules that are excited by the gamma irradiation, and hence erode the surfaces of open pores in the graphite bricks. This process is called radiolytic oxidation and is one of the major concerns in reactor core integrity and thus operational lifetime.”
Partitally Complete AGR Graphite Core. Image courtesy of EDF Energy
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The team used two approaches, firstly looking at how porosity is linked to the surface and secondly how the SANS signal is reduced by increasing the temperature so as to uncover the link between pores and thermal expansion. In order to study porosity at the surface, the team used a technique called contrast matching. With this technique it’s possible to make the pores linked to the surface disappear by filling them with a liquid having the same neutron scattering length density as graphite. This method helps to distinguish between open and closed pores within the graphite.
This technique uncovered some remarkable results. Professor Keith Ross, University of Salford, explains, “One of the main advantages of using neutrons rather than X-rays in this study is that with neutrons we can saturate a sample with a liquid adjusted to match the graphite scattering. Using this technique we have demonstrated that about 70% of the small pores are linked to the surface of the graphite sample and that the probability of being linked to the surface is largely independent of pore size. This is important because these open pores are subject to radiolytic corrosion due to gamma rays in the presence of CO2, the coolant used in AGR reactors.”
The team also looked at how the pore width changes with temperature. “The second important result is that the small pores close up in a linear fashion as graphite is heated to 2000 oC such that they would be completely filled around 3000 oC.” explains Keith, “This is vivid proof of the concept of what are called Mrozowski Cracks.”
The formation of graphite takes place at around 3000oC and the Mrozowski cracks are formed during the cooling of the filler particles (coke) as the bunches of graphite layers split up due to thermal contraction. The team have found that the number of these cracks is proportional to the size of the graphite crystallites.
The results of the experiment throw a unique light on the behaviour of porosity on a length scale only visible to this technique. It should prove possible for the results to form a link between atomistic models and the macroscopic models being used to predict cracking. The team want to extend the work to investigate how the properties vary between different reactor graphites and as a function of position in irradiated graphite blocks.
Such research into the major components of AGR reactors is vital in order to satisfy safety regulators. Previous ISIS research into welded joints, led by the Open University and EDF Energy, supported year life extensions of four AGR reactors. Further understanding of the graphite structure in the moderator components of AGRs continues to ensure their safety.
The research has been published in Carbon and was funded by the EPSRC as part of the Fundamentals of nuclear graphite consortium.
Dr. Z. Mileeva, Prof. D.K. Ross, and Dr. S.M. King.
Research date: September 2013
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