A survey of defects in pressure vessels in the UK for the period 1962–1978 and its relevance to nuclear primary circuits

Abstract

Several surveys have been carried out in recent years to enable an assessment to be made of the reliability of high quality pressurised components. This survey extends available data and reviews experience from conventional plant in the UK. The paper also comments on the relevance of these data to nuclear circuit component assessments. The work has been compiled jointly between the Safety and Reliability Directorate of the United Kingdom Atomic Energy Authority and the Associated Offices Technical Committee (AOTC), a consortium of four major engineering insurance companies. In order to identify and compare events, a design equivalence criterion has been adopted. Reasons for using this, rather than other available approaches, are given in the text. Data are obtained from surveyors' reports in which detected faults are appraised and analysed and fitness for continued service considered. Two categories of failure are defined in the text: (i) potential and (ii) catastrophic (disruptive) failures. Results of the survey are given in detail and these indicate that, from more than 3 × 10 5 vessel years service accumulated by some 20 000 vessels, the potential failure rate is 6·9 × 10 −4 per vessel year averaged over the survey period and the catastrophic failure rate for the same period is 4·2 × 10 −5 per vessel year. In addition to failure rates it is important to obtain information on defect causes, mechanisms, locations and the material involved. These, together with other relevant data, are listed in the paper. Published American and German data are also referred to and commented on. Operating experience with nuclear reactor vessels and their associated heat exchangers in the UK and the USA amounts to approximately 4 × 10 3 vessel years. This is insufficient to provide a significant body of data from which to establish meaningful failure probabilities for reactor vessels. The problem of transferring data obtained from conventional plant to nuclear plant is discussed.