Mechanical behaviour of fuel particles for AGRs and Generation IV reactors
Fuel for AGRs and some Generation IV reactors consists of individual particles bonded into larger pellets. Each particle comprises a fuel core, of around 0.5mm diameter, surrounded by a buffer layer of soft graphite and a composite shell of hard graphite, pyrolytic silicon carbide and hard graphite. The resultant particle is around 1mm diameter. As the silicon carbide provides the local containment, it is necessary that its mechanical properties be well characterised.
This exercise was to demonstrate a suitable mechanical testing route. Throughout, it was necessary to avoid any procedure that might scratch the brittle silicon carbide and thus promote early failure through unrealistic notch-sensitivity.
Mounting jigs were fabricated, in which batches of particles could be reduced to equatorial discs by mechanical polishing. These discs were then separated into their constituent rings by thermal expansion. The silicon carbide rings were then tested at constant strain rate in diametral compression.
Rings showed up to four failure points, involving initiation at first the inner, then the outer surfaces. Statistical distributions of inner and outer surface strengths, suitable for input to 3-D models of particle behaviour, were determined and comparative data on the effects of production variables on particle performance were obtained.