Dependent failure assessment in the development of a defuelling facility for nuclear submarines

This paper describes how dependent (i.e. common cause and common mode) failures have been accounted for in the development of a safety case for the ´defuelling´ of nuclear submarines. A brief description of the submarines, the defuelling process and the safety case is given, followed by a detailed description of the assessment of dependent failures using the Unified Partial Model. The results of the assessments were fed back into the deterministic and probabilistic models of the defuelling process. Cut-off values were applied to multiplicative claims in the deterministic model, whilst beta factors were applied to equipment reliability claims in the probabilistic model. Beta factors have also been used in the derivation of the Probability of Failure on Demand (PFD) and associated Safety Integrity Levels (SΓLs) for the electrical protection system. Finally, an assessment of alternative methods of calculating beta factors was made and the results were found to be comparable, although the UPM proved to be the most flexible and transparent technique.