The digitalized Instrumentation and Control (I&C) system of nuclear power plants (NPP) could provide operator easily Human-Machine Interface (HMI) and more powerful overall operation capability. However, some software errors may cause a kind of Common Cause Failure (CCF). As a consequence, the event of Anticipated Transients Without Scram (ATWS) will occur. In order to assure that the plant can be shutdown safely and to follow the requirements of 10CFR50.62, the utility builds up various ATWS mitigation features in NPP. The features include Fine Motion Control Rod Drive Run In, Alternate Rod Insertion, Standby Liquid Control System, Reactor Internal Pump Trip or Runback, Feedwater Flow Runback and Inhibition of Automatic Depressurization System. This research developed an evaluation method of diverse back-up means for computerized I&C system. A diverse backup of digital I&C system is the most important means to defend against CCF and un-detectable software faults. Institute of Nuclear Energy Research (INER) is developing a computerized I&C test facility, which is incorporated a commercial grade I&C systems with Personal Computer Transient Analyzer (PCTran)/Advanced Boiling Water Reactor (ABWR), a NPP simulation computer code. By taking the technology of Field Programmable Gate Array (FPGA) to implement the methods of ATWS mitigation, the research built up a diverse back-up of digital I&C system to expect to defend against CCF and undetectable software faults. According to the testing and evaluation, the work can be achieved the analysis of Diversity and Defense-in-Depth (D3).

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