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Ultrasonic detection and sizing of complex defects and implications for structural assessment procedures

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Abstract:

Ultrasonic inspections of superheater header closure welds at Wylfa Power station revealed numerous, closely-spaced defect indications which required careful interpretation and analysis. To accommodate the multiplicity of indications the inspection procedure included the use of focussed 0γ probes, which enabled detailed maps to be prepared for fracture assessment. The ultrasonic procedure provided a conservative estimate of the defective areas without requiring excessive time for the data analysis. The degree of conservatism has been investigated both using a mock-up with representative defects and by local grinding of a near-surface defect on the plant.

The fracture assessment employed a two-stage approach, firstly using a standard analysis to identify those headers for which good reserve factors could readily be demonstrated. Subsequently, a more refined analysis of the more defective headers was performed which included the results from material sampling and fracture toughness testing.

This paper demonstrates that there are several significant conservatisms implicit in the ultrasonic data analysis and the subsequent defect characterisation needed to make the fracture analysis tractable. Notwithstanding these conservatisms, all the welds were demonstrated to be fit for service without repair.

Document Type: Research Article

DOI: https://doi.org/10.1784/insi.45.9.594.52937

Affiliations: 1: BNFL Magnox Generation 2: Serco Assurance

Publication date: 2003-09-01

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