Effective outage planning and implementation is critical to the efficient and safe operation of commercial nuclear power plants in the UK. Outages are necessary for refuelling, for preventive and corrective maintenance and for major modification and improvement projects. Outages involve the support of many organisations working together and hence require high levels of coordination. Planning of activities before an outage is critical to their overall success. Establishing the integrity of power plant piping and pressure vessel components is a key objective of any outage and is the subject of this paper. It requires an understanding of the specific threats, their relationship to the overall plant conditions and the mitigating measures required to assure safe operation. Understanding the specific threats allows the engineering function of an organisation to advise on the component Minimum Acceptable Thicknesses (MATs) which can be used to assure integrity via comparison with measurements taken during outage inspections. Historically a variety of methodologies have been used to provide MATs including design drawing specified thicknesses and design code based or fitness-for-purpose methods. It is important that a robust procedure be applied to promote consistency of approach across all power station assets. This paper provides an overview of the strategy and methodologies employed to determine MATs which ensure that, over a specified inspection interval, a component would not fail by plastic collapse, creep rupture, fatigue, ratcheting or buckling and if the weld were to contain a crack-like defect at the time of inspection, it would not grow to a limiting size within the inspection interval. Readers of this paper will gain a valuable insight into the statutory outage process applicable to nuclear power plants in the UK. A particular focus of this paper is on the structural integrity assessments applied in a non-traditional sense prior to, during and after the statutory outage. As well as sharing a valuable insight into the assessment methodologies this paper highlights best industrial practice.
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