Environmental qualification and functional issues for microprocessor-based reactor protection systems

Issues of obsolescence and lack of intrastructural support in (analog) spare parts, coupled with the potential benefits of digital systems, are driving the nuclear industry to retrofit analog instrumentation and control (I&C) systems with digital and microprocessor-based systems. This movement away from analog can be expected to increase in advanced light-water reactors (ALWRs), which will make extensive use of fiber optic transmission, multiplexing techniques, and microprocessor-based technology. Although these technologies have several advantages and, in fact, have been in widespread use in the non-nuclear industry for several years, their application to safety-related systems in nuclear power plants raises key issues relating to the systems` environmental and functional reliability. For example, does the new hardware introduce additional system aging degradation mechanisms that could adversely impact the safety of the plant? Do the systems introduce the possibility of new and different malfunction scenarios or increase the probability of common-mode failures that could reduce the reliability of the safety system?. Are current environmental qualification standards adequate for microprocessor-based I&C systems? Accordingly in 1991 the Nuclear Regulatory Commission (NRC) initiated the qualification of advanced Instrumentation and Control Systems program at ORNL to investigate issues that may arise with the use of advanced digital I&Cmore » in ALWRs. The results of our studies to date are summarized in this paper.« less