|Prof. Paul Wilcox|
In the UK alone, over 20,000 Intermediate Level Waste (ILW) drums are currently stored in above ground facilities. Confirming the integrity of the drums while in storage over extended time periods is challenging due to the restricted access, and the desire to avoid moving drums unnecessarily.
Guided waves in the 100-500 kHz frequency range can propagate in the plane of a plate-like structure over several metres, and are scattered by discontinuities such as corrosion or cracks. This enables guided waves to be used to monitor a large area of structure from a single transducer location, as illustrated schematically in Figure 1(a).
Guided wave transducers need to be ultrasonically coupled to the structure under test, typically using liquid or gel couplant. This makes deployment of transducers somewhat fiddly, and the couplant can potentially introduce contamination. To solve the coupling problem, transducers can be permanently bonded to the structure, but then there is the question of connectivity. Wireless technology can be employed for data transmission, but a power source is required at the sensor. Our proposed solution is illustrated schematically in Figure 1(b). A piezoelectric sensor is permanently bonded to the structure (the bond provides the ultrasonic coupling) and is inductively coupled to the outside world. To perform a test, a measurement probe (termed the “wand”) is brought within a few centimetres of the sensor.
The advantages of this approach are:
• The sensor is completely passive and contains neither batteries nor any electronic components.
• It takes <0.1 seconds to make a measurement and the wand uses special signal processing with low voltage electronics, allowing use in hazardous environments.
• The inductive coupling between the sensor and the wand is “dry” and does not require liquid couplant or precise alignment of the wand.
• The sensor’s position on the structure is precisely fixed and measurements are extremely repeatable, thus enabling changes in response to be tracked over time.
A sensor and prototype wand manufactured by our spin-out company, Inductosense Ltd, is shown in figure 2(a). This prototype is designed for handheld use with an integrated display. However, the actual measurement part of the wand is simply a pair of planar coils on a printed circuit board. This part can be made extremely long and thin as shown in figure 2(b), enabling it to be inserted between ILW drums in storage. Because neither ultrasonic couplant nor precise positioning of the wand are required, it is well-suited for deployment via robotic manipulators.
Wireless, inductively-coupled sensors could provide a means to periodically assess the integrity of ILW storage containers without disturbing them. Once the sensors are installed, the measurements could be made rapidly and non-invasively by deploying an extended measurement wand using, for example, an existing gantry crane or other manipulator. It is expected that 4-6 sensors would be sufficient to provide 100% coverage of each drum. Not only do multiple sensors provide some redundancy, they also enable triangulation to be performed to locate damage, as shown in the experimental example in Figure 3.