Since the accident of the Fukushima Dai-ichi Nuclear Power Station, analysis evaluations for stress tests have been conducted on the prototype fast breed reactor MONJU. In these evaluations, it has been necessary to evaluate the plant characteristics including the Ex-Vessel fuel Storage Tank (EVST) under the severe accident conditions such as station blackout. The EVST is a coaxial cylindrical tank to store spent fuels (SFs) stored in fuel transfer pots (FTPs) until the radioactivity enough decays. It is able to store 252 fuel transfer pots in the rotating rack and cool them by internal natural circulation of sodium coolant under both the severe accident and normal operation. The heat produced by SFs removed by the helical cooling coils installed in the annular space.

Evaluations of natural circulation in the EVST have been performed with a one-dimensional flow-network code. However, it would be difficult to predict its behavior exactly, because it would include multidimensional flow such as local natural convection. Then, in order to clarify the natural circulation behavior and multidimensional effects and evaluate appropriateness of this flow network model, we have performed a thermal-hydraulic analysis using a three-dimensional model which has high resolution meshes and the almost same geometry as the actual equipment. This model makes it possible to take into account the following multidimensional phenomena, the heat distribution of the FTPs, the mixing in plenums, the bypass flow through the flow holes and the other geometry effects.

In this study, we have used a commercial computational thermal-hydraulics code, “FrontFlow/red”. As a result of steady analyses, we have confirmed the following: The coolant temperature in the plenums is almost uniform and its difference is in a few degrees. The influence of the flow holes is also limited because its flow rate is relatively low to main flow rate. On the other hand, pressure loss at supporting plates of the rotating rack, which are main causes of flow resistance in the EVST, is larger than the case without multidimensional effects because of the natural convection concentrated in the high temperature region near heated FTPs. The result leads to our presumption that the flow network model of the EVST is almost appropriate. It should be noted that flow resistance coefficient of the supporting plates or the heat transfer center of the cooling coils should be set to conservative for the safety analysis on the EVST.

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