Risk based methods
The nuclear industry has always had safety and reliability as its top priority. Methods to assess the risk of failure are a key requirement. TWI supports this objective with a range of technologies for evaluating the likelihood of failure of welded structures.
The frequency and size distribution of welding defects is a major factor influencing the likelihood of failure of reactor pressure vessels. For some years TWI has been identifying the metallurgical and process mechanisms that can lead to the formation of defects and the conditions under which they occur. Recent work has included an investigation into the causes of underclad cracking and an assessment of the maximum size of defect that could be reasonably foreseen in a weld from a review of the fabrication history.
The reliability of inspection was first highlighted during the safety acceptance for the construction of the UK's first civil pressurised water reactor at Sizewell B. It became necessary to qualify inspection and assess the capability of the NDT methods, procedures and operators by technical justification and blind trials. TWI has supported this development through theoretical work on the probability of detection and sizing by ultrasonic and radiographic NDT and the qualification of practical inspections.
TWI is active in European development of risk based inspection for the nuclear industry. As a partner in the EURIS Project, TWI contributed to a European status report of the technology. Further work is planned to develop a best practice handbook with a benchmark of quantitative methods through the NURBIM project and the European Network for Inspection and Qualification (ENIQ).
TWI's software for risk based inspection and maintenance, RISKWISETM , is capable of application in qualitative and semi-qualitative (levels 1 and 2) assessments of nuclear equipment. Based on a probability and consequence analysis, the software prompts users to think about the factors that affect the risk and evaluate these in relative terms.
A level 3 fully quantitative evaluation of risk for specific equipment items as a function of operating times is also used - in the form of TWI's software LIFEWISETM . This enables optimum refurbishment or replacement planning (for example of piping circuits) and also allows the life extension gains to be quantified in relation to factors such as improved inspection accuracy or lower welding residual stresses. The LIFEWISETM software is customised to specific applications.
Contact us at: power@twi.co.uk
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