A large-scale thermal-hydraulic integral effect test facility, ATLAS (Advanced Thermal-hydraulic Test Loop for Accident Simulation), has been operated by KAERI. The reference plant of ATLAS is the APR1400 (Advanced Power Reactor, 1400？MWe). Since 2007, an extensive series of experimental works were successfully carried out, including large break loss of coolant accident tests, small break loss of coolant accident tests at various break locations, steam generator tube rupture tests, feed line break tests, and steam line break tests. These tests contributed toward an understanding of the unique thermal-hydraulic behavior, resolving the safety-related concerns and providing validation data for evaluation of the safety analysis codes and methodology for the advanced pressurized water reactor, APR1400. Major discoveries and lessons found in the past integral effect tests are summarized in this paper. As the demand for integral effect tests is on the rise due to the active national nuclear R&D program in Korea, the future prospects of the application of the ATLAS facility are also discussed. 1. Introduction ATLAS (Advanced Thermal-Hydraulic Test Loop for Accident Simulation) is a large-scale integral effect test facility with a reference plant of APR1400 (Advanced Power Reactor, 1400？MWe), which is under construction in Korea [1]. It was designed to have the capability of simulating various transients and accident conditions at full pressure and temperature conditions, including loss of coolant accident (LOCA) series as well as non-LOCA series. The ATLAS program started in 1997 under a nuclear R&D program funded by the Korean government. The complete installation of ATLAS was finished in 2005. In 2006, extensive commissioning operations were carried out, including startup tests and preliminary tests [2]. In 2007, ATLAS was used for a wide range of integral effect tests on the reflood phase of a large break LOCA to resolve the safety issues of the APR1400 raised by a regulatory body [3]. Afterwards, at the beginning of 2008, ATLAS was modified to have a configuration for simulating the direct vessel injection (DVI) line break accidents of the APR1400. One of the unique design features of ATLAS is its ability to simulate the DVI of the emergency core cooling water. Sensitivity tests for different DVI line break sizes were performed and an integral effect database was established for various break sizes of 5%, 25%, 50%, and 100% [4]. After a series of DVI line break tests were completed, small break LOCA (SBLOCA) tests commenced at the end of 2008. In order to
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