Licensing experience of the evaluation of surveillance test for digital I and C systems important to safety

The introduction of digital systems in nuclear power plants becomes inevitable nowadays. This trend applies not only to the newly constructed plants but also to the operating plants which are to be upgraded. But the existing code and standards for I and C systems important to safety do not appropriately incorporate the characteristics of digital systems yet. This paper presents licensing experience of the evaluation of surveillance test for the Digital Plant Protection System (DPPS) of Ulchin Nuclear Power Plant unit 5 and 6. As a standard for surveillance test, IEEE Std 338-2006 is almost same as 1987 edition except the risk based guidance for the determination of test intervals. It does not incorporate the design features of digital system regarding the surveillance test method. But IEC 60671 published in 2007 strengthens the role of computer self supervision as an alternative to periodic surveillance testing. We have evaluated the change of surveillance test methodology for DPPS according to IEC 60671. We have reviewed the characteristics of self diagnostic programs, self test programs, their test coverage and the integrity of checking algorithms. And we have reviewed the failure mode and effect analysis of DPPS to verify the acceptability of new test methodology. In this paper, we discuss the major issues related to the surveillance test for digital systems important to safety. And we describe our approach and evaluation results for the change of test methodology for DPPS. (authors)