CRAFTY – A new separate-effect-test facility at LUT University for two-phase critical flow studies in transient nature

In 2024, a novel separate-effect-test facility for two-phase critical flow studies was commissioned at LUT University's Nuclear Engineering Laboratory. This CRiticAl Flow Test facilitY (CRAFTY) utilizes a straight, long tube for discharging subcooled water from an upstream pressure vessel. Prior, a plethora of two-phase critical flow experiments have been carried out in the world. A preliminary literary survey found that there is a lack of two-phase critical flow experiments utilizing very long length-to-diameter ratio tubes (>200). In a postulated primary-to-secondary leak in a pressurized water reactor, the length-to-diameter ratio of the tube can be upwards of 1000, depending on the steam generator design. In CRAFTY, the length-to-diameter ratio and tube diameter can be conveniently changed with interchangeable discharge tubes. In the tests conducted in 2024, a discharge tube with an inner diameter of 13 mm and closely resembling the VVER-440 steam generator tube (inner diameter of 13.2 mm) was utilized. The length-to-diameter ratio of the tube was 350, which is close to half of the average length of the VVER-440 steam generator tube. This paper presents experiment results from the initial proof-of-concept test, POC-04, and simulation results from in-house thermal-hydraulic system code developed in VTT. The experiment results show that the subcooling level of the discharged liquid plays a crucial role for the resulting critical mass flux. Higher subcooling shifts the choke plane closer to the discharge tube outlet, resulting in higher critical mass flux. This shift of the choke plane can be seen from the axial pressure profiles measured from the discharge tube. The simulation was able to capture the behavior of the axial temperature and pressure profiles. Yet, the simulated critical mass flux was approximately 10% greater than the experimental measurements. In addition, the relation between the inlet subcooling and the critical mass flux differed depending on the inlet subcooling level. The simulation overpredicts this relation.