Integral Behavior of the Atlas Facility for a 3-inch Small Break Loss of Coolant Accident

Over the past 30 years, several large integral facilities have been utilized in order to understand transient behavior in nuclear power plants. The main objective of these integral facilities was to enhance our physical understanding of transient phenomena during LOCA or non-LOCA transients. An integral database was also used as a basis for the development and verification of thermal hydraulic safety analysis codes. A comprehensive literature survey on the world’s integral facilities can be found in the literature [1]. The important scaling features of these integral facilities and their major test items are summarized in Table 1. All these facilities were scaled down with respect to the reference plant by optimizing the costs and benefits. Most of them have a full height scale and a reduced volume scale. Each integral facility was established to reflect the design features specific to its reference plant and to focus on a certain thermal hydraulic phenomena. The integral test items were limited by the system configuration and the simulation capability of a facility. In Korea, the SNUF (Seoul National University integral test Facility) was the first attempt to carry out integral tests of nuclear plants for design basis accidents (DBAs). It aimed at a simulation of a large-break LOCA (LB-LOCA) and a direct vessel injection (DVI) line break under reduced pressure conditions [2, 3]. However, the SNUF is a small facility which is operated at reduced pressure conditions as compared with other facilities. It has a limited simulation capability for major event scenarios due to low core power and the fact that the secondary system is simplified as a lumped boundary condition. Recently, KAERI launched a large-scale integral effect test program sponsored by the Korean government and finished constructing a test loop, the ATLAS, at the end of 2005 [4]. The reference plant for the ATLAS is the APR1400, which is an advanced power reactor developed by Korean industry. The ATLAS was designed to have the capability of simulating manifold scenarios, including the reflood phase of the LB-LOCA, SB-LOCA scenarios including a DVI line break, a steam generator tube rupture, a steam or a feed line break, and a mid-loop operation, etc. The ATLAS has a particular focus on the reproduction of the multi-dimensional phenomena related to a DVI as well as on preservation of the surface tension effect and the flow regime in the design stage of the reactor vessel and downcomer. Accordingly, the ATLAS adopted an integrated annulus downcomer which is the A small-break loss of coolant accident (SB-LOCA) test with a break size equivalent to a 3-inch cold leg break of the APR1400 was carried out as the first transient integral effect test using the ATLAS (Advanced Thermal-hydraulic Test Loop for Accident Simulation). This was the first integral effect test to investigate the integral performance of the test facility and to verify its simulation capability for one of the design-basis accidents. Reasonably good thermal hydraulic data was obtained so that an integral performance of the fluid sub-systems was identified and control performance of the ATLAS was confirmed under real thermal hydraulic conditions. Based on the measured data, a post-test calculation was carried out using the best-estimate thermal hydraulic safety analysis code, MARS 3.1, and the similarity between the expected and actual data was investigated. On the whole, the post-test calculation reasonably predicts the major thermal hydraulic parameters measured during the SB-LOCA test. The obtained data will be used to enhance the simulation capability of the ATLAS and to improve an input model of the ATLAS for simulation of other target scenarios.