[en] At Tohoku University, 1/15 scale and 1/7 scale flow tests are being performed in order to clarify an unsteady flow structure in the cold leg piping and to investigate the scale effect. In FY2007, the 1/15 scale test was carried out using a double elbow geometry. In the test apparatus, the inner diameter is 56 mm and the curvature radius ratio is 1.0. For visualization without any image distortion, matched refractive-index PIV measurement was carried out using NaI solution as the working fluid. The Reynolds number was 43,000, and the inlet flow condition to the test section was a fully developed turbulent flow. It was confirmed that there existed a separation along the inner wall of the 1st elbow and a large swirling flow in the 2nd elbow. Furthermore, the unsteady flow formed in and/or behind the separation region was transported downstream and flows into the center area of the 2nd elbow. On the other hand, the working fluid of the 1/7 scale experimental apparatus is water. In this fiscal year, the apparatus for achieving high Reynolds number conditions was designed and each component was manufactured. At JAEA, a numerical analytical evaluation method is being developed to evaluate the feasibility of the reactor design. In FY2007, numerical analyses for the 1/15 scale test were performed using the commercial thermal-hydraulics code FLUENT. The analyses showed that the trend of calculated velocity distributions was in good agreement with the visualization test results. (author)