Modelling long timescale effects of irradiation damage of nuclear graphite

There are currently 14 Advanced Gas-cooled Reactors (AGRs) in the UK which are operated by EDF. The reactor cores in these AGRs are composed of several tonnes of a synthetic form of graphite, commonly called nuclear graphite. This nuclear graphite serves two purposes: to moderate (slow down) neutrons to enhance nuclear fission, and to form the main structure of the reactor core.

Nuclear graphite is not a single crystal but is instead composed of a highly complex microstructure consisting of many mis-orientated graphite grains. This microstructure can be tailored by the manufacturing conditions.

A key problem is that the high temperature and high irradiation conditions of a nuclear reactor core causes the graphite bricks to swell considerably (dimensional change). This induces stress in the bricks which eventually results in cracks. Cracked and crumbling bricks would eventually lead to blocked channels in the reactor core which would prevent the insertion or removal of fuel rods and control rods. Therefore, materials (and particular microstructures) that are resistant to radiation damage and do not readily swell or crack under irradiation are desirable since they would lead to longer run times of future generation 4 nuclear reactors.

The key aim of this project is to develop understanding of how the microstructure of a graphite brick influences its properties (such as dimensional change) in the high temperature and high irradiation conditions of a reactor core.
This understanding will be gained by the following project objectives:

(1) Model the interaction of small and large atomic defect structures with a view to understanding the main causes of dimensional change in graphite. Uncover the key mechanisms for dimensional change by studying the motion of vacancy and interstitial atoms within graphite as well as the buckling of the graphene layers. We will use advanced computer simulation techniques that can model these graphite structures with atomic level detail. Long timescale techniques will be used to extend the simulation time of our atomistic simulations.

(2) Model the mechanical properties of micrometre sized thin graphite wafer structures containing realistic densities of defects in graphite. Computer simulations of indentation and scratching processes will be performed and compared with experimental results.

(3) Investigation of the thermal properties of graphite structures containing realistic distributions of defects and grain boundaries. Non-equilibrium molecular dynamics simulations will be performed on a range of these structures to determine the effect of the microstructure upon the thermal conductivity of the graphite. The coefficient of thermal expansion will also be simulated for a range of representative graphite structures. These results will also be compared to experimental results.

(4) Computer simulations are only ever as good as the underlying approximations made in fitting a usually limited data set in the model. The data generated in this project will be used to assess the suitability of existing models for graphite available in the literature. These existing models will be improved upon by identifying the best components of each model and combining them into a new hybrid model capable of accurately predicting a wide range of graphite material properties.