[en] The overpressure protection for OPR1000 Reactor Coolant System (RCS) and the Steam Generators (SG) is provided by means of Pressurizer Safety Valves (PSV), Main Steam Safety Valves (MSSV) and Plant Protection System (PPS). MSSVs are periodically tested in accordance with the ASME OM Code inservice test requirements to verify that their opening uncertainties are maintained within the limits specified in technical specification. And MSSV inservice tests carried out in OPR1000 showed the valve opening uncertainty may have greater value than expected due to the uncertainties originating from the setpoint measurement accuracy and setpoint drift. To evaluate the effect of the increased MSSV opening uncertainty on the plant overpressure protection, an analysis has been performed in a manner that verifies the validity of the OPR1000 overpressure protection which was determined at the stage of plant licensing