Failure investigation

Atmospheric column after fire

From time-to-time, defects are detected in nuclear systems from inspection by non-destructive examination or the occurrence of leakage from pressurised systems. Whenever a defect is detected, there is a detailed investigation to characterise the defect type, its possible causes, and whether other similar defects are likely elsewhere. Defective parts may then be repaired and other mitigating actions put in place.

TWI's materials laboratory is recognised for its expertise in the examination of fracture surfaces and the characterisation of different defect types. Welding quality and fabrication problems are often issues when dealing with older plant. Clues to a sequence of events leading to failure can be evident from the fracture surface such as where fabrication defects lead to in-service crack growth followed by fracture.

Other problems within TWI's experience have resulted from:

• poor weld design
• incorrect heat treatment during fabrication
• the use of an inappropriate consumable
• arrested brittle fracture due to residual stress before PWHT

Typical service problems are:

• thermal fatigue
• aggressive environmental conditions
• stress corrosion cracking

In these instances, TWI's broad engineering experience has helped to identify the wider factors contributing to failure.

The investigation of structural failure is a multi-disciplinary activity and every case is different. TWI works closely with the nuclear industry to provide the appropriate expertise and techniques. These may include:

• testing of small material samples
• assessment of residual stress
• evaluation of welding procedures and records

For further information, please contact us at: power@twi.co.uk

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