ULTRASONIC DEFECT DETECTION | INSTRUMENTATION & CONTROL
Welds with cracks
Pipe elbow
Above: On 21 October 2021, following ultrasonic checks scheduled during the second 10-yearly outage of Civaux NPP reactor 1, EDF informed ASN that it had detected indications of cracks on welds on the elbows of the safety injection system piping of the reactor’s main primary system Source: ASN
● Reduction in thermal cycling associated with outages for inspection can be reduced
The project brought together specialists in ultrasonic transduction and monitoring technology from KANDE and mathematical modelling specialists from the University of Liverpool (UoL) Department of Mathematical Sciences and was funded by the UK Government’s Game Changer scheme.
Sensor and array design KANDE supplied the sensors used in this application, which are 0°, relatively small and low profile, suitable for permanent installation below thermal lagging. The transducer crystal size is a key design parameter as divergent beams reduce the need for a closely spaced array at the expense of reduced beam intensity and possible reduction in signal to noise ratio (SNR). Owing to logistic issues associated with the support
grant, it was necessary to obtain results from the study within a three-month period which meant procurement, fitting, design, instrumentation and soak-test setup needed to be completed within one month of the grant award. In practice, the soak-test ran on for six weeks after the conclusion of the project. The array has n elements and is used by pulsing on one element and receiving on all n elements. The transmitter element is progressed through the array and for each pulsed element, all elements receive. In this way a total of n2
signals are collected in one data
collection operation. In this trial, n=28. To be effective in detecting, sizing and characterising any defects within the inspection volume, ultrasonic beams
from the sensors in the array need to: ● Insonify the full inspection volume ● Provide sufficient beam overlap that a credible defect
could scatter signals to multiple member elements of the array
● Generate signals from the defect with sufficient signal to noise to enable defect detection and sizing
● Produce propagation paths from the array elements to points in the inspection volume that are sufficiently diverse to enable location with minimal ambiguity.
A first draft array design was produced manually on the basis of physical reasoning and simple beam spread and timing considerations. This preliminary design (see Figure 1, over leaf) was then tested and refined on the basis of finite element (FE) modelling using Pogo software. UoL members of the project used FE modelling to give insight into the monitoring arrangement considering 2-dimensional and 3-dimensional modelling exercises. Calculations were performed to tailor the draft design. As Pogo can output time series data (see Figure 2 over leaf: Right pane), it was possible to test the monitoring system analysis tools for the particular application, prior to sensors being installed.
The experimental study The study was intended to replicate conditions which might be encountered in pressurised water reactor (PWR) nuclear power plant (NPP) applications by using typical pressure vessel steel, plant operating temperatures, and seeking defects of a type and size similar to those sought in typical NPP inspections. The sample used in the study was a flat, parallel-sided, forged SA508 ferritic specimen which is both similar acoustically to other ferritic pressure vessel steels and commonly used as a pressure boundary material in NPP. This material would be fine grained, homogeneous and isotropic. The synthetic defect introduced into the component
was a slot ~1.0mm wide, 20mm long and 5mm deep. While critical defect sizes vary depending upon the plant and component, this is similar to defects commonly sought in pressure vessel inspections. The specimen was 52mm
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