A Study on Improvement Through Analysis of Cold Hydrostatic Test Cases in Korea Nuclear Power Plants

The objective of this study is to analyze the effectiveness of the Cold Hydrostatic Test (CHT) process and determine the optimum method of completing a CHT through case studies in the Korea nuclear power plants. In this study, all the 9 CHT cases, performed for the past sixteen years (1993 to 2009) in Korea nuclear power plants, will be examined and evaluated. There are twenty (20) operating Units and eight (8) Units under construction at 3 nuclear facility sites in Korea. Among the 20 Units, only 4 Units at the Wolsong site are pressurized heavy water reactors (PHWR), the others are pressurized light water reactors (PWR). CHT is based on the requirements of ASME NB-6200 & NC-6220. CHT is a mandatory test to verify integrity of weld points and interfaces associated with the equipment and pipes of the Reactor Coolant System (RCS) pressure boundary. The design pressure of the RCS is 2,500psia (175.8 kg/cm² a). The major steps of test sequence of a CHT is RCS filling, venting, heat up, pressurization and inspection. Reactor Coolant Pump (RCP) operation is utilized as thermal input to raise RCS temperature over 120 °C. The Chemical and Volume Control System (CVCS) Charging Pump, or temporary hydro pump is used to pressurize the RCS. CHT requires pressure to be raised and maintained more than 10 minutes at 1.25 times of design pressure, and then be depressurized and inspected at the design pressure of 2,500psia (175.8 kg/cm² a). According to the analyzed results of the CHT cases, all CHTs were successfully conducted but there are several items which need to revised and modified for increased effectiveness of the CHT. These items include pressurizer manway gasket leakage, improper process of the procedure and others. In conclusion, the results of this study will be used to prevent similar errors and improve the effectiveness of the CHT for future nuclear power plants projects in Korea.