[en] Thermal stratification after a scram is one of main thermal loads of a reactor vessel in sodium cooled fast reactor. Water experiments using a 1/10th scaled model were carried out for an advanced loop type sodium cooled reactor, which was designed by the fast reactor cycle technology development project (FaCT) in Japan. The reactor vessel is highly compact and has an upper inner structure (UIS), which consists of perforated baffle plates and has a slit in radial direction for fuel handling. The jet from core outlet goes through the UIS slit and makes asymmetric and locally high velocity field in the reactor upper plenum. Steep temperature distribution across the stratification interface and temperature fluctuations were found near the UIS slit in the experiment. It was revealed that they were resulted from the impingement of the jet through the slit at the interface. Parameter experiments for velocity and temperature drop after the scram at the core outlet showed that the rising speed of the stratification interface was well correlated with the Ri number. Dominant period of temperature fluctuations at stratification interface was influenced by the jet velocity through the UIS slit and also temperature difference across the interface. Parameter experiment of the shape of structures in the upper plenum showed that the temperature gradient across stratification interface was decreased by the UIS with the perforated outer cover and by the FHM Plug penetrated deeply. (author)