Optimization of STI and AOT of RPS/ESFAS for Kori Unit 2

Reactor protection system(RPS) and engineered safety actuation feature system(ESAFS) of a nuclear power plant are critical safety systems to protect a reactor by inserting control rods into a reactor core and by actuating safety systems such as safety injection pumps under abnormal situations. To implement high reliability of these systems, they are designed to have redundant structures and periodically tested. Surveillance test interval(STI) and allowed outage time (AOT) are well defined and provided in the technical specifications, which are determined by engineering judgment without analytical analysis. Operating experiences and maintenance records reveal that currently adopted STI and AOT are too frequent and short, respectively, which may cause negative effects on safety. In other word, a trivial human error during the test and maintenance may directly cause reactor trip and these works require lots of cautiousness so that the site engineers get stressed a lot. It is necessary to optimize STI/AOT of RPS/ESFAS considering risk change with respect to relaxation of STI and AOT. This paper shows the methodology and the results of STI/AOT optimization of RPS/ESFAS in Kori Unit 2