Interactive System Design Using the Complementarity of Axiomatic Design and Fault Tree Analysis

Design is divided primarily into two processes: ‘synthesis’ and ‘analysis’. For the purposes of this paper, synthesis is regarded as the process of decision-making regarding parameters, and analysis as the process of optimizing those parameters. It is known from experience that the mistakes made during the synthesis process are never completely corrected in the analysis process. For example, in the design of a safety-critical system where a critical concern is its reliability, if the upstream synthesis process results in an unreliable system, the system is rarely transformed into a highly reliable system. The downstream detailed analysis process only slightly improves its reliability. It is commonly known that it is possible to achieve higher reliability with less cost if reliability is built into the system from the early design phase. Many approaches have been proposed in order to integrate synthesis and analysis into a single framework that minimizes the overall design efforts toward maximizing the reliability. This paper shares that goal with those proposals. The claim here is that as the synthesis and analysis processes more closely interact, a high level of system reliability becomes more achievable. Because uncertainty and subjectivity are inherent to the synthesis process while the analysis process cannot tolerate uncertainty and subjectivity, their consummated integration poses significant challenges. If synthesis and analysis methodologies can be found with the following properties, their integration should be more promising: It should be applicable to all of the design phases, from the early design phase to a detailed design phase. It should provide a means to quantitatively as well as qualitatively evaluate design decisions. Qualitative decision-making is usually dominant in the early design phase, while quantitative evaluation is necessary in the detailed design phase. It should share a fundamental framework in performing the synthesis and analysis processes to reduce the effort and resources required for the integration. An integrated framework is valuable only if the effort to carry out it To efficiently design safety-critical systems such as nuclear power plants, with the requirement of high reliability, methodologies allowing for rigorous interactions between the synthesis and analysis processes have been proposed. This paper attempts to develop a reliability-centered design framework through an interactive process between Axiomatic Design (AD) and Fault Tree Analysis (FTA). Integrating AD and FTA into a single framework appears to be a viable solution, as they compliment each other with their unique advantages. AD provides a systematic synthesis tool while FTA is commonly used as a safety analysis tool. These methodologies build a design process that is less subjective, and they enable designers to develop insights that lead to solutions with improved reliability. Due to the nature of the two methodologies, the information involved in each process is complementary: a success tree versus a fault tree. Thus, at each step a system using AD is synthesized, and its reliability is then quantified using the FT derived from the AD synthesis process. The converted FT provides an opportunity to examine the completeness of the outcome from the synthesis process. This study presents an example of the design of a Containment Heat Removal System (CHRS). A case study illustrates the process of designing the CHRS with an interactive design framework focusing on the conversion of the AD process to FTA.