Developing Cleaner Graphite For Next Generation Nuclear Reactors.

We are offering a fully funded PhD scholarship to examine the behaviour and performance of graphite to be used in Gen IV nuclear systems.

Graphite has been used for neutron moderation and structural support since the beginning of the nuclear reactor era. While the research activities have abated over the years, there is a renewed interest in graphite motivated by its use in the next generation of Very High Temperature and Molten Salt Reactors. The retention of the activated fission products (FP) is paramount during normal operating and accident conditions, and a mechanistic understanding of the physio-chemical and transport processes is vital for predicting the release rates and designing appropriate barriers to ensure inherent safety and lifetime performance.

A number of experimental studies conducted with TRISO fuel show that pyrolytic carbon layers offer some resistance to noble gases while FPs such as caesium, silver and strontium escape under normal and accident conditions. Metallic FPs such as caesium and silver are thought to get trapped in the intercalated structure of graphite but are extremely volatile at temperatures greater than 900oC. Finally, FPs can interact with air/water during an accident situation and potentially enhance the release rates. At the microstructure level, transport and retention behaviour is thought to be dependent on the graphitic filler phase, binder insoluble particles, micro Mrozowski cracks and macrocracks however the fundamental mechanisms of FP interactions and the dependence of diffusion/transport on microstructure have not been quantified to date in adequate detail.

Key aims of the project at Manchester is to better understand mechanisms associated fission product transportation in nuclear graphite, we will study the microstructural and spectroscopic properties of graphite using state of the art 4D X-ray and TEM tomography in order to understand the migration, retention and release of particular fission products in graphite with the aim to correlate microstructure characteristics to long term graphite behaviour. Complimentary analysis such as Raman spectroscopy, X-ray photon spectroscopy (XPS), nitrogen porosimetry and diffractometry may also be used to understand FP interactions.