Reliability analyses of digital instrumentation and control (I&C) systems of nuclear power plants are conducted within the licensing process to demonstrate that reliability targets imposed on the system are fulfilled by the design. The results of these analyses often show a strong contribution of common cause failures (CCF) of hardware modules, especially if generic parameters are used to calculate CCF probabilities. For this reason, the benefit of using component-specific experience to assess the CCF probabilities of the hardware modules involved in the I&C system is of high interest.
CCF parameter data collections are available for many populations of mechanical components, such as the Alpha Parameters in NUREG/CR-5497. However, such data collections are not available for digital hardware modules. The use of generic CCF parameters for digital modules (e.g., those reported in NUREG/CR-5497 for components for which no operating experience) leads to very conservative reliability results, which do not reflect the specific characteristics of the digital I&C systems operation. Another possibility is to use the less conservative Beta Factors suggested by the IEC 61508, however with difficulties to justify the proposed values during the I&C licensing process.
This paper presents a methodology for estimating CCF parameters of digital hardware modules using the TELEPERM XS safety system platform developed at Framatome for illustration purposes.
The methodology includes as the first step an analysis of the TELEPERM XS operating experience and identifies relevant events with the potential to lead to a CCF event. The TELEPERM XS operating experience is based on feedback collected during the last 20 years from several sources of information, such as feedback from the development, engineering, integration test fields and commissioning as well as feedback from customers worldwide, and includes quantitative (operating time, number of failures) as well as qualitative data (nature of failures, root-cause investigations).
The second step of the methodology involves a quantitative analysis, which follows the approach suggested in NUREG/CR-6268 to estimate CCF Alpha parameters using impact vectors. The paper illustrates the CCF parameters estimation for different groups of digital modules relevant for reliability analyses (e.g. input/output modules, priority control modules), including a discussion of the results and comparison to the Multiple Beta Factor model proposed in the
IEC 61508.