Eddy current inspection of AGR fuel channels: keyway cracks and density mapping

摘要

There have been efforts to develop eddy-current inspection for the graphite moderator bricks in Advanced Gas Reactor (AGR) cores for a number of years. The only access to the graphite bricks is down the fuel channel bore whilst the reactor is off load and the fuel has been removed from the channel. There is a need for a technique that will detect cracks that have the potential to propagate from the external keyways of the bricks, but do not reach the bore; i.e. the cracks remain subsurface. As graphite is an electrical conductor, albeit with a high resistivity, eddy current testing is a suitable technology to apply to this problem. In 2009 a proof-of-principle eddy current tool was deployed at Hartlepool and has since had three further deployments at Heysham 1 and Hinkley Point B reactors. Variations in electrical resistivity over the bore surface of the bricks are the greatest limitation on defect detection. However, the sensitivity of eddy current probes to resistivity - and therefore capability to map the graphite density - has provided an additional opportunity to gain further information on the condition of the graphite bricks. In March 2012, a prototype tool was deployed for the first time at Heysham 1. This paper provides a summary of the work undertaken from development through to deployment of the prototype tool in 2012.