Numerical Reliability Analysis Techniques for Optimisation of Decay Heat Rejection Systems

Up to the last decade Engineers have assimilated data on plant reliability as part of plant performance assessment procedures. However, the data collecting process was not conducted on a formal basis because there were no formalised methods for assessing plant reliability requiring such data. Rather was plant reliability assessed on a subjective basis. Nonetheless some of the solutions aimed at improving plant reliability were applied but the techniques for numerical evaluation to arrive at these solutions with any degree of precision were not available. The advent of advanced technologies with associated higher risks has now given considerable impetus to the adoption of numerical reliability analysis techniques. The space and atomic energy industries may be quoted as prime examples in providing this impetus. Furthermore once risk targets had been defined numerical analysis methods logically followed. In the UKAEA nuclear industry the Farmer criterion defined objectives which set the scene for considerable activity in the formulation and adoption of reliability analysis techniques. This means the addition of a further dimension in the design process. Hitherto designers had confined themselves to their traditional activities in assessing plant performance, in arriving at a design compromise when evaluating various choices available to them. Now the design criteria have been expanded so that designers ask themselves additional questions such as - what is the chance of failure of a particular engineered safeguard? Will it meet the nominated risk targets? Current techniques allow an improvement in the precision with which answers to : such questions may be made. At the same time however, it must be admitted that with the present state of the art one may only be able to predict answers within an order of magnitude