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TWI helps to verify the reliability of radiography

 

During construction of the Magnox nuclear reactor pressure vessels (RPVs) in the late 1950s and 1960s, radiography was the main method of non-destructive testing (NDT) used for weld inspection. As part of a programme to extend the life of the Magnox Electric plc nuclear power stations, Magnox, in collaboration with Nuclear Electric Ltd, has been investigating the reliability of the original Magnox RPV radiography. As part of these investigations, TWI's NDT group was asked to carry out practical assessments of radiographic capability funded by the Industry Management Committee (IMC) comprising BNFL and British Energy plc.

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It is well known that radiography can detect volumetric flaws, but it was unclear how reliably this method could detect planar flaws. TWI has a large database of ultrasonic and radiographic inspection data for flaws examined over many years of past projects. This database was used to provide experimental data on the effectiveness of radiography in detecting planar flaws in steel welds 50-114mm thick; covering the range of thicknesses of Magnox RPVs.

To learn more about the flaws considered, sectioning was used to reveal their true type, size and morphology. Following this, new specimens were produced containing much larger flaws (>15mm through-wall size) to establish if these could be detected radiographically for a variety of experimental conditions.

Results showed that radiography is able to detect a wide range of planar defects of a size which may be of structural concern to Magnox RPVs, even when they are substantially misoriented to the radiation beam.

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