Measurement of residual stress in spent AGR fuel cladding using incremental centre hole drilling

Since reprocessing ceased at the Thermal Oxide Reprocessing Plant (THORP) at Sellafield in 2018, spent advanced gas-cooled reactor (AGR) fuel will be interim stored in cooling ponds until a disposal solution becomes available. Under these aqueous conditions, and with timescales of up to 80 years envisaged, susceptibility of the fuel cladding to SCC is possible, which would give rise to a risk of radioactive discharges into the storage pond water. As a result, research projects have been launched to investigate the causes of and identify solutions to the SCC of the AGR fuel cladding.

As the presence of a (tensile) stress state is necessary for SCC to occur, one of the pursued avenues is the quantification of residual stresses, present in spent AGR fuel cladding. Even though there exist many proven measurement techniques for residual stresses, none of these can be readily applied to AGR fuel cladding due to its high level of radioactivity and complex geometry. As a result, the objectives of the current project is to investigate the feasibility of modifying and applying existing residual stress measurement techniques.

One technique being investigated is incremental center hole drilling (ICHD). In ICHD, strains or displacements are measured during the drilling of a hole with increasing depth. This causes a relaxation of the stresses, and with it, a deformation of the surface around the hole. The stress state that existed in the material, prior to the introduction of the hole can then be reconstructed from the displacement field resulting from this relaxation.

For application on spent AGR fuel pins, a first requirement, is calibration data. For this, a finite element (FE) model of a fuel pin has been developed, this FE model was used to simulate ICHD experiments with a known stress state from which the displacement fields serve as calibration data for physical experiments. A second requirement is the application of an optical strain measurement, for which digital image correlation (DIC) has been chosen. And a third requirement is the development of an experimental stage capable of operating in a hot cell and which allows the combined application of ICHD and DIC.