Effective outage planning and implementation is critical to the efficient and safe operation of commercial nuclear power plants in the UK. Statutory outages are necessary for refuelling, for preventive and corrective maintenance when shutdown conditions are required, and for major modification and improvement projects. Outages involve the support of many companies and individuals working together and, as such, require high levels of coordination. Planning of activities before the outage is critical to the overall success of the outage. Establishing the integrity of power plant piping and pressure vessels is a key objective as part of any outage and the methodology and processes involved are the subject of this paper.
Establishing the integrity of piping and pressure vessels requires an understanding of the specific threats, their relationship to the overall condition of the system, and the mitigating measures required to assure safe operation. Understanding the specific threats allows the engineering function of an organisation to advise on pipework and pressure vessel ‘Minimum Acceptable Thicknesses’ which can be used to assure integrity via comparison with thicknesses measured during outage inspections. Minimum Acceptable Thicknesses should be recorded in the outage management documentation so they are accessible during the outage implementation phase.
Historically a variety of different methodologies have been used to advise on Minimum Acceptable Thickness requirements including design drawing specified minimum thicknesses, design code based required thicknesses and thicknesses calculated based on Fitness for Purpose methods. It is important that a robust procedure be applied to promote consistency of approach as regards the calculation of pipework and pressure vessel Minimum Acceptable Thickness requirements across all power station assets. An additional consideration is that of ensuring that the approach adopted is consistent with high level safety case guidance, i.e., the assessment is appropriate for the failure tolerability of the plant item.
This paper provides an overview of the strategy, methodologies and processes employed to determine Minimum Acceptable Thicknesses for pipework components. These ensure that, over a specified inspection interval, were the weld/component to be defect free, it would not fail due to any of the relevant failure mechanisms, which typically are plastic collapse, creep rupture, fatigue, incremental collapse (ratcheting) or buckling.
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.