Tomographic array design for online, non-invasive, non-intrusive measurement of magnox slurry during nuclear decommissioning

Conducting ultrasound measurements in a nuclear environment necessitates a non-invasive and non-intrusive transducer design, capable of operating through thick walled stainless steel vessels. Decommissioning of legacy infrastructure requires transducers capable of retro fitting. Achieving spatial measurements across vessel or pipe cross-sections requires a tomographic approach, typically using multiple invasive or intrusive point source transducers with a fanbeam profile enabling point to multipoint measurements. Non-invasive point source transducers are unable to create fan beams within the vessel. Differing path lengths within the vessel wall combined with the ultrasonic velocity mismatch between the wall and vessel contents creates a plane wave ultrasound beam within the vessel which is unsuitable for tomography. The objective of this study is to create a non-invasive transducer array with the spatial diversity to allow point to multipoint measurement. A non-intrusive array design methodology, overcoming the inherent focusing effects of curved inner pipe surface, is proposed and illustrated through the design, manufacture and testing of an 18 virtual point tomographic array on a stainless steel 6″ pipe with 18mm thick walls. The pipe transfers a slurry of corroded Magnox nuclear fuel rods during decommissioning activities. A point to multi-point transducer arrangement is achieved through the creation of virtual sources, located on the inner surface of the pipe wall. Transducers are arranged at discrete angles around the virtual point such that refraction at the inner pipe surface creates multiple beams, each directed at individual, virtual points, to create point to multipoint measurements. The transducer array is constructed using an electrical discharge machined (EDM) stainless steel support with random 1″3 piezo-composite transducers. Finite element verification of the design PZFlex and the manufactured array is experimentally characterized.