In order to improve the safety of Boiling Water Reactor (BWR), it is required to know the behavior of the plant when an accident occurred as can be seen at Fukushima Daiichi nuclear power plant accident. Especially, it is important to estimate the behavior of molten core jet in the lower part of the containment vessel at severe accident.

In the BWR lower plenum, the flow characteristics of molten core jet are affected by many complicated structures, such as control rod guide tubes (CRGT), instrument guide tubes and core support plate. However, it is difficult to evaluate these effects on molten core jet experimentally. Therefore, we considered that multi-phase computational fluid dynamics approach is the best way to estimate the effects on molten core jet by complicated structure.

The objective of this study is to develop the evaluation method for the flow characteristic of molten core jet including the effects of the complicated structures in the lower plenum. So we are developing a simulation method to estimate the behavior of molten core jet falling down through the core support plate to the lower plenum of the BWR. The method has been developed based on interface tracking method code TPFIT (Two Phase Flow simulation code with Interface Tracking). To verify and validate the applicability of the developed method in detail, it is necessary to obtain the experimental data that can be compared with detailed numerical results by the TPFIT. Thus, in this study, we are carrying out experimental works by use of multi-phase flow visualization technique.

For the experiment works, we constructed two experimental apparatuses, one is single-channel experimental apparatus and the other is multi-channel experimental apparatus. The single-channel experimental apparatus simply simulate single flow channel between four CRGTs. The multi-channel experimental apparatus is a 1/10 planar type model of a lower plenum of BWR. In the experiments, we will obtain the jet breakup behavior and surrounding velocity profiles of the jet by using LIF and PIV.

In this paper, the outline of two-experimental apparatuses are shown. And the results of the single-channel experimental apparatus with/without modelled complicated structures are also shown. In the results, it was confirmed that the complicated flow channel affects the jet injection and breakup behavior. And it was also confirmed that the complicated structures restrain diffusion of fragments of the jet.

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