Investigation of thermal hydraulic behavior of SBLOCA tests in SMART-ITL facility

An experimental investigation of the thermal hydraulic behavior of small break loss of coolant accident (SBLOCA) tests in an integral test loop (ITL) of a system-integrated modular advanced reactor (SMART) was performed, especially on the pressurizer safety valve (PSV) line and safety injection (SI) line breaks. Compared to typical phases of SBLOCAs in conventional and advanced active pressurized water reactors (PWRs), four sequential phases were identified: a blowdown to the upper downcomer (UDC) under saturation conditions, a pressure plateau under forced circulation, boil-off after the reactor coolant pump (RCP) trip, and core level restoration after the safety injection tank (SIT) injection or long-term cooling. The pressure plateau was deeply dependent under UDC saturation conditions and there were mass transfers among the reactor vessel (RV) regions during the pressure plateau. In the boil-off phase, a short reverse flow occurred from the lower dowoncomer (LDC) to the primary side of steam generator (SGP) owing to the occurrence of vaporization in the LDC region. The core level restoration is mainly dependent on the SIT injections. In the secondary system, the fluid conditions on the secondary side of steam generator (SGS) outlets were changed during the tests from superheat to saturation, from saturation to subcooling, and from subcooling to superheat. In the passive safety injection system (PSIS), there was a short reverse flow in the pressure balance lines (PBLs) of the core make-up tanks (CMTs) just after the break, and sufficient injection flowrates of the CMTs were achieved after the partial clearing/blocking of the PBL. In the case of a SIT actuation signal, duration times for the hydraulic equilibrium in pressure among the CMTs, SITs, and RV were needed.