Robotic ultrasonic testing of AGR fuel cladding

The purpose of the presented work was to undertake experimental trials to demonstrate the potential capabilities of a novel in-situ robotic ultrasonic scanning technique for measuring and monitoring loss of the cladding wall thickness in fuel pins of Advanced Gas-cooled Reactors using non-radioactive samples. AGR fuel pins are stainless steel cylindrical ribbed pipes of inner diameter of the rod being about 15 mm and wall thickness of about 300 μm. Spent AGR fuel pins are stored in a water pond and thus may be prone to corrosion and stress-corrosion cracking under adverse conditions. An ultrasonic immersion transducer with central frequency of 25 MHz was used to measure wall thickness of the AGR fuel cladding. The novelty of the approach consists in the usage of a frequency domain technique to measure the wall thickness combined with cylindrical ultrasonic scanning of the samples performed using an industrial robotic manipulator. The frequency domain approach could detect wall thicknesses in the range 96 μm to 700 μm with a resolution of about 10 μm. In addition to the frequency domain measurements, using conventional time domain techniques, it was possible to detect very short (2.5 mm long) and shallow (100 μm in depth) crack-like defects in the fuel cladding.