Nuclear Technology / Volume 203 / Number 1 / July 2018 / Pages 17-33
Technical Paper / dx.doi.org/10.1080/00295450.2018.1426961
The nuclear accident at Fukushima has brought an increased research focus on nuclear safety and severe accidents. Radioactivity leakage into the environment has environmental, societal, and political impact, and further robustness of nuclear reactor design is essential. Thus, coolability and stabilization of corium within reactor containment in a severe accident scenario are important issues that need to be resolved. In this context, many new reactors have been envisaged with dedicated core catchers. In ex-vessel core catchers, corium coolability is one of the biggest concerns. Despite several efforts, melt pool coolability still needs to be understood in sufficient detail. Among the various cooling strategies, melt coolability using bottom flooding has been demonstrated to be one of the most efficient techniques so far. This paper presents the numerical and experimental study of melt pool coolability under bottom flooding with decay heat simulation. An experiment has been performed using a test section consisting of two parts: a lower part for melt retention and quenching, and an upper part for steam expansion and its outlet. To simulate the decay heat, ten radiative heaters equivalent to 10 kW were used to heat the lower part of the test section housing the melt. The experimental measurements showed that quenching of about 25 L of melt at 1200°C initial temperature took only a few minutes. The same phenomenon has been modeled using a mechanistic model. The model considers heat transfer in the melt pool, eruption in the melt pool by steam formed when water is injected at the bottom of the melt pool, and heat transfer from porous debris to a steam mixture. The model postulates the formation of crust below the melt pool when water is being inserted from the bottom. The model predicts the failure of this crust due to various stresses, resulting in an inverted cone–shaped melt eruption. The model captures the physics of this eruption cone along with the spatial variation in porosity. The model predictions have been compared with the measurements for the melt pool temperature during the cooling process. The results show that the model is able to capture quite accurately the multidimensional temperature fields in the melt pool during the cooling process.