Safety cases for use of smart devices in existing nuclear power stations — “Getting the balance right”

Modern smart devices such as sensors, controllers and valve actuators provide many key advantages but there are wellknown difficulties in providing evidence to support the associated safety cases. This paper reminds the reader of these difficulties but mainly concentrates on the need for an “As Low as Reasonably Practicable” approach and hence the requirement to use expert engineering judgement to weigh up the advantages/disadvantages of using a smart device against the advantages/disadvantages of other possible options. The paper discusses the use of engineering judgement in the safety cases for installing smart devices and highlights there is more than just software faults to consider. Issues covered include:allocation of `best estimate' reliability data for use in Probabilistic Safety Assessments, `proven in use' arguments and the importance of understanding the wider safety case picture.