Remote detection of clogging in thin pipes using laser-generated ultrasound

A remote detection method for clogging in pipes using the Scanning Laser Source technique (SLS) was discussed, aiming to address the need for inspecting inaccessible pipes during the decommissioning of the Fukushima Daiichi Nuclear Power Station. The images were obtained for a stainless-steel pipe simulating a Hydraulic Control Unit (HCU) pipe in different modulation frequency bands. The results confirmed that a uniform vibration distribution is generated along the longitudinal axis of a pipe when using a frequency band near the cut-off frequency, which allows for stable signal acquisition independent of the receiver position. Furthermore, detection experiments were conducted using a frequency band near the cut-off frequency of the F(2,1) mode on a simulated clogging approximately 40 mm long that filled the entire cross-section of the pipe under conditions where the interior of the pipe was filled with air and water. We demonstrated that the location of the clogging in the HCU piping at positions 4.0 m and 6.2 m from the measuring device could be identified, as areas of localised reduction in peak values of the frequency spectrum.