Thermal Mixing In-plant Test in Shinkori Unit 3 IRWST

The objectives of the in-plant test are to give the insights on the short term mixing phenomena and to generate validation data for the thermal mixing analysis tool. And eventually, this inplant test is expected to resolve the safety issues related with the adoption of IRWST. In-plant test in SKN3 on the thermal mixing was successfully conducted and it shows the more activated mixing compared with CFD analysis. IRWST (Incontainment Refueling Water Storage Tank), which was firstly incorporated in SKN3 and 4 (Shinkori Nuclear Power Plant Units 3 and 4) among PWRs (Pressurized Water Reactors), is an evolutionary engineered safety feature to enhance the safety of reactor in such way that it eliminates the switch-over process from a RWST (Refueling Water Storage Tank) to a containment sump in the long term cooling operation of ECCS (Emergency Core Cooling System). In addition to the above function it increases the quenching efficiency of steam and alleviates probable pressure surge induced by the sudden discharge of the high pressure steam during plant transient such as IOPOSRV (Inadvertent Opening of Pilot Operated Safety Relief Valve) accident or TLOFW (Total Loss of Feedwater) accident. The design of the IRWST seems originated from a suppression pool in BWRs (Boiling Water Reactors). However, the geometry and operational conditions of IRWST are far different from those of the suppression pool. Thus, in spite that there have been regulatory requirements on the design and operation of the suppression pool such as the local pool temperature limit concerning the pool mixing, the hydrodynamic load against the integrity of its structures, and so on, the direct application of the researches on the resolution of such regulatory requirements has not been fully accepted. Actually, domestic regulatory authority has required an in-plant test to check the appropriate thermal mixing and the validation of a mixing analysis tool since the design step of APR1400 (Advanced Power Reactor 1400MWe) which is a standard design of SKN3 and 4. This paper provides a procedure and the results of the in-plant test on thermal mixing in IRWST of SKN3 which was carried out on November 2012