This report presents the investigations performed to validate the coupled code RELAP5/PARCS using data of a pump restart-up test carried out at the Kozloduy nuclear power plant (KNPP). An integral plant model—including the relevant primary and secondary systems as well as a detailed model of the reactor pressure vessel and of the core—was developed for RELAP5 and PARCS to simulate the pump restart-up test. From the comparison of the thermal-hydraulic data obtained during the tests with the code’s predictions, it can be stated that the overall trends of most plant parameters are in a reasonable agreement with the experimental data. The investigations have shown some limitations of 1D thermal-hydraulic coarse models of the downcomer, lower, and upper plenum. Hence, multidimensional thermal-hydraulic models are needed for a more realistic description of the coolant mixing phenomena within the reactor pressure vessel. iii FOREWORD This validation report presents the investigations performed by the Forschungszentrum Karlsruhe GmbH to validate the coupled code system RELAP5/PARCS. The plant data used for this work was distributed in the frame of the OECD/NEA VVER-1000 Coolant Tansien
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