Asymmetric thermal-hydraulic conditions between loops in nuclear power plants (NPPs) may produce a nonuniform temperature distribution at the core inlet if the coolant is not mixed perfectly in the lower plenum. These uneven core inlet conditions, which may be formed remarkably during a postulated steam-line-break (SLB) accident, induce a distortion in the core power distribution, which can affect the thermal margin. Thus, to estimate the thermal margin under abnormal inlet conditions, it is necessary to predict correctly thermal mixing phenomena in the lower plenum. For this purpose, reactor internals scaled down with a flow-to-area ratio are added in the lower plenum of the loop test facility, manufactured with a scaling factor of 1/710 by volume and based on a Westinghouse-type two-loop NPP in Korea. The mixing tests in the lower plenum are performed under various loop temperature imbalances at low pressures. It is found that complete mixing hardly occurs in the lower plenum at any test condition. Also, the tests are simulated by the COMMIX-1B multidimensional thermal-hydraulic code. A comparison of the simulation results with the test results shows a good agreement, and thus it is concluded that COMMIX-1B can be applied to determine the mixing patterns under the asymmetric loop conditions of a real NPP. As for applications, the temperature distributions at the core inlet under asymmetric conditions induced by the postulated SLB accident in Kori Unit 1 are determined by COMMIX-1B, and thermal margins for the SLB accident are estimated. Analyses show that the thermal margins can be improved by using more realistic core inlet temperature patterns instead of NPP design patterns.
