Time of Flight Diffraction Technique (TOFD-T) as an Alternative to X-Ray Examination for Thick Welds: A Literature Review

Abstract

In the nuclear industry, and in particular, regarding large steel components including the nuclear steam supply system, weld integrity must be assessed and confirmed during the fabrication process, for the initial field-welds and periodically during in-service follow-up of some critical assembly welds. In France, this quality control is prescriptively carried out via nondestructive inspections in accordance with the RCCM code primarily via X-Ray Radiography or Gammagraphy (RT) coupled to conventional ultrasound (C-UT). These two techniques are integrated into code inspection requirements since the industry has good evidence that RT and C-UT are able to detect and characterize defects, and are well suited to the large weld thickness of the reactor pressure vessel, pressurizer or stream generators. C-UT is frequently used where it may be shown equivalent to RT. Use of RT however becomes more and more problematic because of the trend of regulatory restrictions to limit radiological source transport, extension of radiological exclusion zones to limit the dose to which workers are exposed, and pressure to increase production for new component fabrication and on-site assembly to support aggressive new-build schedules. Replacement of RT by another dose free technique such as the Time of Flight Diffraction Technique (TOFD-T) would be desirable. In this context, AREVA conducted a study of other industries and other countries management of RT particularly focusing on replacement of radiography by TOFD-T. Interviews were conducted surveying industries manufacturing pressure vessels and making similar welds to those within the nuclear industries, i.e. the oil and gas and the submarine industry. In addition, a literature review on the TOFD-T performances, existing codes and standards, and past approaches to justify the replacement of radiography by the TOFD-T was performed. For all this study, European, American and Japanese industries were surveyed or considered. This study showed that TOFD-T is widely used in the US oil and gas industry thanks to ASME code case, but the global nuclear industry has been reluctant to accept TOFD-T due to the lack of specific acceptance criteria. Follow-on work must be performed for TOFD-T to be proposed and accepted as an alternative to RT in France.