The seismic analysis for the OPR1000 (previously entitled Korea Standard Nuclear Power Plant, now Optimized Power Reactor 1000 MW-Class) was carried out based on the SSE to assure the integrity of the reactor coolant pressure boundary and to secure a safe shutdown of reactor as well as the OBE to ensure the functionality of all the necessary systems for the continued operation of the reactor without undue risk to the health and safety of the public. In fact, the OBE with some supplementary functions, however, tends to control the design of nuclear power plants instead of the SSE due to its conservative analysis methodology. As a result of analysis work by means of more sophisticated techniques and continuously raised questions by the nuclear industry about the adequacy of using the OBE with at least one-half the maximum vibratory ground acceleration of the SSE as a design input, the OBE elimination design is exceptionally allowed through the alternative design rule by KINS (Korea Institute of Nuclear Safety: Independent nuclear regulatory expert organization), as long as the OBE is established as less than or equal to one-third of the SSE. Evaluated in this paper was the effect of the application of the OBE elimination-related alternative design rule on the KEPIC (Korea Electric Power Industry Code)/ASME Code Class 1 piping systems of an OPR1000-based nuclear power plant. The SSE (e.g. 0.3g) at least three-folds larger than the OBE (e.g. 0.1g) ground acceleration could decrease the flexibility of piping systems by adding more restraints to mitigate the increased SSE-induced seismic loads. Therefore, the flexibility of four piping subsystems, whose acquisition is one of the main purposes of the OBE elimination, was evaluated for an APR1400-based nuclear power plant (Korea’s Advanced Power Reactor 1400 MW-Class), which has practically adopted the application of the OBE elimination for the first time in Korea.

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