Now the entire safety I&C system is based on one kind of software and hardware platform, the Common Cause Failure (CCF) may impact the whole safety I&C system becomes to a potential risk. How to mitigate the effect of CCF in safety system and improve the safety of the nuclear power plant is considered by the system designer. Especially after the Fukushima nuclear accident, the Defense-in-Depth and Diversity (D3) should be more concerned by all designers. The diverse actuation system (DAS) plays a very important role in the D3 system. In this paper, the related codes and standards of DAS are analyzed firstly. Then, this paper expounds the approach to demonstrate the D3 analysis for the digital I&C systems applied to the nuclear power plant in detail. In the D3 analysis, all the safety functions of the digital safety system are assumed to be disabled by a CCF. DAS provides diverse automatic reactor trip and diverse safety injection actuation which are not impaired by the postulated CCF. DAS also provides manual actuation functions and plant parameter monitoring functions which can be used to cope with CCF. Finally, the paper takes the DAS of Fujian Fuqing Nuclear Power Plant as an example, introduce how to design the structure of the DAS and calculate the suitable setpoints.

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