Audit Calculations of ATWS for Ulchin Unit 1 and 2 Power Uprate

In this study, the regulatory audit calculation for ATWS of Ulchin Unit 1 and 2 with 4.5% power uprate was performed to support the licensing review and to confirm the validity of licensee's calculation. In order to simulate the transient behavior of ATWS initiated by a loss of feed water, the systems of Ulchin Unit 1 and 2 was modeled with MARS-KS 1.3. In this study, the regulatory audit calculation of ATWS for Ulchin 1 and 2 with 4.5% power uprating and 99% MTC in the specific cycle designs was performed. It is conformed that the analysis results of ATWS for Ulchin 1 and 2 power uprate meets the RCS pressure acceptance criteria. An anticipated transient accompanied by a failure in the Reactor Trip System (RTS) to shut down the reactor is defined as an Anticipated Transient Without Scram (ATWS). Under certain postulated conditions, the ATWS could lead to Reactor Coolant system (RCS) pressure boundary fracture and/or core damage. For a conventional pressurized water reactor (PWR), the temperature corresponding to the NSSC notice No.2013.09(Performance Criteria for ECCS of the Pressurized Water Reactor Nuclear Power Plants), 1204 .deg. C and the pressure corresponding to the ASME Boiler and Pressure Vessel Code service level C stress, 221.5 bar is assumed to be an unacceptable plant condition against ATWS, above which the RCS pressure boundary could deform to the point of inoperability and the safe shutdown by injection of borated water could be challenged. Such potentially excessive RCS overpressure may occur in the ATWS initiated from a loss of heat sink. Currently, the modification of Ulchin 1 and 2 operating license for 4.5% power uprate is under review