SIL verification for SRS with diverse redundancy based on system degradation using reliability block diagram

Safety integrity level (SIL) verification is a critical step in safety lifecycle of safety-related systems (SRS). Introducing redundancy into SRS raises two issues: voting group configuration and common cause failures (CCF). In order to minimize CCF, diverse redundancy is widely adopted by SRS. However, in the past, almost all attention of SIL verification has been paid to identical redundancy, this is reflected in IEC 61508, ISATR84.00.02 and scientific literatures. Therefore, a novel method for SIL verification of SRS with diverse redundancy based on system degradation is proposed. Key idea of the method is to calculate average probability of dangerous failure on demand (PFDG) at each stage of system degradation, which is caused by failures of redundant channels. To validate proposed method, it has been applied on safety shutdown system of Nuclear Power Control Test Facility, and numerical result is compared with FTA and FRANTIC model. Sensitivity studies and comparison of numerical results indicate that the method has very good consistency with FTA and FRANTIC model. Moreover, two sets of general formulae for PFDG of any MooN(D) group with diverse redundancy are provided. From engineering practice point of view, it makes SIL verification process simpler.