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Steam generator tube rupture scenarios

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Steam generator reliability and performance are serious concerns in the operation of pressurized water reactors. The aim of the SGTR project was to provide a database of fission product retention in steam generator tube rupture sequences and models, which could be applied to estimate the effectiveness of different accident management strategies in these kind of accidents. The data have been urgently needed, since there has not been any reliable database available on fission product retention, and since these types of accidents are risk dominant in many PWRs operated in European countries. In WP1 the important accident scenarios for steam generator tube rupture were determined from existing PSA level 2 analysis, and by performing additional analyses with system level codes. From these results the boundary conditions for the experimental studies were defined. In WP2, scaled-down models of a western PWR type steam generator and a VVER-440 steam generator were used to analyze the retention of fission products in the steam generator tubes and in the secondary side. Small-scale facilities were used to study the local deposition phenomena. The parameters in the experiments enveloped a wide range of predicted conditions for the reference plants. The main boundary conditions considered were dry and wet conditions in the secondary side, number of broken tubes, the break location(s) and dimension(s), the break flow, the simulated fission product concentrations and flooding rate. It can be concluded that in the primary side highly turbulent flow in the broken tube is expected and existing deposition models predict high DF in these conditions. However, re-suspension becomes dominant in these conditions and the experimentally observed DF is only 1.1-1.2 for dry aerosols. Hygroscopic aerosols would increase the DF, but this effect was not measured in the SGTR project. In the secondary side DF is larger than in the primary side, but still rather low at dry conditions and dry aerosol. With the flooded bundle the DF was significant and it varied from 100 to 5000 depending on the water height, mass flow rate and the steam mass fraction. When steam was present, condensation inside the tube caused aerosol deposition on the tube surface and nearly blocked the break. In the separate effect studies it was observed that the aerosol retention at the break stage of a dry steam generator was low and non-uniform. Neither break type nor orientation affected the results significantly, when the gas flow rates exceeded 100 kg/h. In the primary side the effect of flow and aerosol material on deposition and re-suspension were measured with radiotracer techniques on-line. The results are of great value for deposition/re-suspension model development. The plant evaluations showed that the accident management measures and the newly developed models influence not only the deposition of fission products, but also the thermal hydraulics, the sequence of events and as a result the fission product behaviour. The SGTR project made an important step forward to resolve uncertainties of physical models, especially in the aerosol deposition and mechanical re-suspension in turbulent flows, even though there are still open questions. The way to reach more exact prediction for aerosol retention in the steam generator during a SGTR accident should be looked for in the future.

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