Hardware Reliability prediction of Computer Based safety Systems of Indian Nuclear Plants

Computer Based Systems (CBS) are employed in safety critical and safety related systems of Indian Nuclear Plants; for protection, control and monitoring purpose. These systems should meet the target reliability figures. These CBS are configured using a family of microcomputer boards. For calculating the system reliability metrics, failure rates of microcomputer boards used in system are required. For forthcoming CBS, a new standardized Versa Module European (VME) bus based family of microcomputer boards is developed by Reactor Control Division, BARC. The family has CPU board, Ethernet communication board, analog input and output board, digital input board, digital output board, relay output board, protocol translator board, signal conditioning board, bus extender boards and motherboards. These boards are compact, low power, high speed, and intelligent modules designed as per latest state of the art technology. These boards and the systems configured using these boards need to be qualified to stringent requirements of nuclear industry. The paper briefly outlines the microcomputer boards' description and qualification tests carried out on these boards. Board failure rate estimation is done by summing the component failure rates. This board failure rate is then modified by various factors corresponding to process, environment, reliability growth and infant mortality characteristics. MIL-STD-217 Plus methodology is adopted for failure rate calculation of the components and boards. Complete design details of the boards being available, Part Stress Method of MIL-STD-217 Plus was used. Comparison of various prediction methodologies is presented in the paper along with detailed description of MIL-STD-217 Plus methodology. A fully integrated framework of reliability analysis tools was used. It supports Reliability prediction, Reliability block diagram, Fault Tree and Event Tree evaluation, Failure Mode and Effect Analysis as per MIL-STD-1629A and Weibull Analysis. Previous- experience about system reliability prediction and failure data of the installed systems is also mentioned. These systems were configured using 8086 based family of boards with similar functionalities as above mentioned boards. The paper includes details of failure rate analysis of microcomputer boards and results. The paper also presents system reliability analysis carried out for CBS system built using these boards.