Abstract
The digitalized Instrumentation and Control (I&C) system of Nuclear power plant can provide more powerful overall operation capability, and user friendly man-machine interface. The operator can obtain more information. However, while I&C system being digitalized, three issues are encountered: 1) software common failure, 2) the interaction failure between operator and digital instrumentation and control system interface, and 3) the non-detestability of software failure. These failures might defeat defense echelons, and make it more difficult to analyze the perfiormance of Diversity and Defense-in-Depth (D3). This research developed an integrated methodology to evaluate nuclear power plant safety effect by interactions between operator and digital I&C system, and then propose improvement recommendations. This integrated methodology includes component level software fault tree, and system level sequence tree method and nuclear power plant computer simulation analysis. Software fault tree can clarify the software failure structure in digital I&C systems. Sequence tree method can identify the interaction process and relationship among operator and I&C systems in each D3 echelon in a design basis event. Nuclear power plant computer simulation analysis method can further analyze the available backup facilities and allowable manual action duration for the operator when the digital I&C fail to function. Applying this methodology to evaluate the performance of digital nuclear power plant D3 design, could promote the nuclear power plant operation safety. The operator would trust the nuclear power plant than before, when operating the highly automatized digital I&C facilities.