Abstract
In a hypothetical severe accident that happens in a light water reactor (LWR), the melt pool could be formed in the lower plenum of reactor pressure vessel (RPV). Remaining these molten materials inside RPV and preventing further leakage of these melt materials from RPV are treated as one of the severe accident management methods. In this aspect, one of the key issues is that the heat transfer from melt pool to RPV wall should be less than the corresponding critical heat flux (CHF) outside the vessel. The natural convection behavior in melt pools plays an important role in determining the heat flux from the melt pool to the RPV wall. Up to now, various experiments have been conducted to study the internally heated natural convection behavior in melt pools. In 1970s, several experiments were conducted, and mainly investigated the mechanism of internally heated natural convection phenomena. After TMI-2 and Chernobyl-4 severe accidents, such kinds of experiments were conducted worldwide, mainly aimed to further investigate the melt pool natural convection behavior in the lower plenum and conduct the experiment closer to the prototypical condition that may happen in a hypothetical severe accident. Some of these experiments were used to provide the data for severe accident management method assessment of their corresponding reactor types, and to solve the problems that were found or not solved by previous experiments. Especially the COPRA (COrium Pool Research Apparatus) experiment, of which the test section was the full scale and nitrate salt of which the physical properties are close to the prototypical material was used as the simulant material, improved the theoretical knowledge of the transient and steady-state phases of melt pool natural convection behavior. However, there are still uncertain and unclear phenomena related to the natural convection behavior in internally heated melt pools. This paper mainly summaries the melt pool natural convection experiments chronologically, and then points out the main remaining issues regarding the natural convection behavior in the melt pools in the RPV lower plenum.
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