5.87.
Particular consideration should be given to the treatment of dependencies in the logic model developed for the Level 1 PSA. In PSAs carried out in the past, dependent failures have often been found to be one of the dominant contributors to core damage frequency.
5.88.
Four different types of dependency can occur:
Functional dependencies include dependencies resulting from plant conditions (e.g. a failure to depressurize leads to the unavailability of low pressure injection) and dependencies owing to shared components, common actuation systems, common isolation requirements or common support systems (e.g. power, instrumentation and control, cooling, ventilation).
Physical dependencies (also referred to as spatial interaction dependencies) owing to an initiating event that can cause failure of credited SSCs. Failure can occur as a result of pipe whip, missile impact, jet impingement or environmental effects.
Human interaction dependencies owing to errors made by plant personnel that either contribute to, or cause, an initiating event, or lead to the unavailability or failure of one or more items of credited SSCs to the extent that they do not operate when needed following an initiating event.
Component failure dependencies owing to design similarities, manufacturing or installation errors, or errors made by plant personnel during plant operation. These are addressed by a common cause failure analysis (see paras 5.93–5.96).
5.89.
A systematic review should be performed of the design and operation of the plant to identify all the potential dependencies that could arise and that could lead to the unavailability of components of the credited system or a reduction in their reliability to protect against initiating events.
5.90.
All functional and physical dependencies should be modelled explicitly. Human interaction dependencies and component failure dependencies should also be modelled; these are discussed further in paras 5.93–5.96 on common cause failure analysis and paras 5.97–5.122 on human reliability analysis.
5.91.
All functional dependencies that could arise within systems should be taken into account in the fault tree model. These should be identified and modelled explicitly in the PSA model. It is good practice for the analysts to tabulate all these dependencies in a matrix of system dependencies, which can be used as a basis for constructing the fault trees and which is helpful to the reviewers when checking them. Functional dependencies should not be included among the component failure dependencies in the common cause failure probabilities of the system.
5.92.
The intersystem functional dependencies that could arise owing to shared components or support systems should be identified and modelled explicitly in the fault tree analysis. In the linked event tree approach (see para. 5.5), intersystem functional dependencies can be addressed using the boundary condition method. Such dependencies could arise in separate credited systems that perform the same safety function or in associated support systems. These need to be included explicitly in the fault trees.