Residual stresses in the stainless steel cladding of spent Advanced Gas-cooled Reactor fuel elements affect their behaviour during storage. For example, in a fuel storage pond the cladding can experience a type of material degradation known as Inter-Granular Stress Corrosion Cracking (IGSCC) which is caused by a combination of stress and electrochemical action. To ensure that spent fuel elements can be stored safely, it is important to be able to measure stresses inside the cladding material and understand how they develop during manufacture, irradiation and storage. This includes stresses at the scale of a complete fuel element but also at much smaller scales: micro-scale residual stresses between adjacent crystals of steel can also affect the material’s degradation behaviour.
In this project, we will develop specialised techniques for measuring stresses in AGR fuel cladding. These will include destructive methods which involve cutting specimens with Electrical Discharge Machining or a Focussed Ion Beam and measuring how they deform, as well as non-destructive techniques based on the scattering of X-rays and neutron radiation. During the development of these stress measurement methods, we will consider the challenges posed by the unique shape, thickness and microstructure of the cladding - as well as the activity of spent fuel cladding. We will then measure residual stresses in unirradiated cladding materials and small specimens of irradiated cladding. This will allow us to investigate the mechanisms of internal stress development within AGR fuel claddings. In the future, this could be used to provide better predictions about the integrity of fuel under storage and to determine better ways of storing spent fuel.