Development of Quantitative Safety Assessment Methodology for NPP I&C Systems Including Operators
In an effort to design a more reliable control system, many studies have been performed on instrumentation and control (I&C) systems, Man-Machine Interface(MMI) and the behavior of human operators. Reliability analysis methods for digital systems, development methods for high reliability software, verification and validation of high reliability software, complexity analysis of MMI, the design of computerized procedures, and qualitative and quantitative models for human behavior have been among the focuses of the research.
Even though those studies share the same goal, the design of a more reliable control system, there have been few discussions concerning their quantitative contribution to the reliability of the control system integrating I&C systems, MMI and human operators.
Generally, research thus far has assessed qualitative contributions rather than quantitative.
A. The Importance of Quantitative Analysis
A great many components are related in designing a more reliable control system, especially when the control system includes human operators. From the integrity of a small transmission wire to the teamwork of human operators, it would be almost impossible to mention every single component that is related to the control system.
In other words, there are innumerable tasks that have to be done to improve the reliability of the control system.
Perhaps it is easy to talk about why a specific component is important in a plant. But, at this point, we believe that it is relatively much more difficult to talk about how important a component is in a plant. In other words, we can discuss the importance of a component in a qualitative way, but not in a quantitative way.
We think that this is mainly because there have been few quantitative models for the control system, which includes the I&C systems, MMI and human operators altogether. Therefore, we believe that at this point it is necessary to establish a quantitative model which takes key factors related to the I&C systems, MMI and human operators into account.
B. The Objective of Our Research
The purpose of our research is to propose a model for the quantitative analysis of the control system, which includes the I&C systems, MMI and human operators. Even though fault tree analyses have successfully served as a general method for quantitative analysis for decades, the fact that they only represent two kinds of system states, success and failure, imposes limitations on the range of applications. The limitation will be overcome by introducing a method based on discrete functions.