Risk-based technical specification optimisation methodology for complex systems

Technical specifications (TS) are safety rules for Nuclear Power Plant (NPP) Operations that are approved by the regulatory authority. The technical specifications define limits and conditions for operations, testing and maintenance activities as a way to assure that the plant is operated safely in a manner that is consistent with the assumptions made in the plant safety analyses. The TS define limiting conditions for operations (LCOs) and surveillance requirements (SRs). Traditionally, the method used to set the limiting conditions for operations and maintenances, is deterministic in nature, which in most cases is based on the deterministic criteria, expert judgement or as recommended by the manufacturer, and in most cases are conservative and prescriptive in nature. This paper proposes a risk-based maintenance methodology for research reactors, to determine Surveillance Test Interval (STI) and Allowable Outage Time (AOT), as a decision support tool. The quantified risk value obtained from the PSA model has been used in the development of this methodology. In this paper, the concept of analysing the effect of random failures, human failures during maintenances and ageing failures on the system unavailability has been applied. System unavailability due to test outage has also been included to quantify the risk. However, quantification of all the parameters requires exhaustive data, which is essentially required to use this methodology.