In recent years, nuclear industry has used many digital systems for instrumentation and control such as nuclear protection system which is safety critical. If these safety critical systems had serious failures, the consequences would have significant effects on the public health and wealth. Thus, safety critical systems must be of sufficiently high quality to assure safety and reliability. For that reason, the safety and reliability of the system must be considered important when a system is developed.
Safety analysis has been performed using several analysis methods. However, existing formal methods in view of safety analysis have some limitations. For example, safety checklists which are developed before are not complete, thus we cannot detect errors in requirements specification. To overcome these limitations, we suggest a new method in this work. A new method is proposed to perform safety analysis of the system in requirements phase. This method is composed of two steps. The first step is to convert the formal requirements specification based on statechart to SCR tables and ISO (Input-State-Output) tables. The second step is to detect errors formally in requirements specification using the SCR tables developed before and the ISO tables which have been newly developed in this work.
This method was applied to Water Level Monitoring System (WLMS) and Digital Online Active Test - Plant Protection System (DOAT-PPS). Using the suggested method, we could model the system requirements visually, and check the system formally with safety checklists. As the result of the safety analysis, two errors were detected in WLMS, and three kinds of errors were detected in DOAT-PPS. However, these errors could be resolved easily.
In conclusion, we come to know that formal safety analysis is necessary in order to improve the safety of safety critical systems, and that we can reduce the cost and efforts through the method which is newly developed in this work. Additionally, we can overcome the limitations of existing formal methods in view of safety analysis using our suggested method and also human errors that can be made in manual process by using supporting tools.
