   Due to the Great East Japan Earthquake and the accompanying tsunami, the Fukushima Daiichi Nuclear Power Plant lost all power, which led to the core meltdown. Nine years have passed since the earthquake, and Tokyo Electric Power Company (TEPCO) has aimed to establish procedures for removing molten debris. To reduce the exposure of workers and to work efficiently, understanding the spreading and deposition behaviors of the molten core are important.

   Severe accident codes can treat the behaviors of melting and dropping processes, deposition location, amount, and radioactivity of the molten core. However, the models include large uncertainty. For improved reliability, it is necessary to elucidate the dynamic characteristics of spreading behaviors of molten core and improve the model based on experimental results. There are few open works of literature of molten metal experiments with floor impingement.

   The objective of the study is to obtain data for verifying the analytical models for spreading and deposition behaviors of molten metals with free-falling and evaluate the influence of scaling parameters such as dimensionless numbers. Sn and Cu were used as a molten substance to evaluate the effects of physical properties such as thermal diffusivity, surface tension, and viscosity coefficient. The metals melted with a high-frequency induction heating device and fell to on a steel plate. We observed it with a high-speed camera. The spreading area and the deposition thickness of the solidified metal were also measured. The experimental parameters were nozzle diameter and fallen height.

   To consider scaling effects on the spreading and deposition behaviors, investigated the relationship between dimensionless numbers. Such as falling height to nozzle diameter ratio *L */ *d*, Reynolds number, Peclet number, Weber number, dimensionless spreading area *A~sp~*\*, and the ratio *t* / *δ* of deposition thickness *t* to thickness *δ* that minimizes the surface area of the material. The dimensionless spreading area is defined as *A~sp~*\*=*A~sp~*/(*M*/*ρ*)^2/3^ and the thickness *δ* is defined by *δ*=2(*σ*/(*ρg*))^1/2^, where *A~sp~* is the measured spreading area, *σ* is the surface tension, *M* is the mass of the molten metal, and *ρ* is the density of the molten metal.

   Experimental data of spreading area and deposition thickness were obtained for nozzle diameter of 4 mm to 10 mm and fallen height of 100 mm to 300 mm. Amount of falling molten metal was also changed from 10 cm^3^ to 100 cm^3^. The standard deviation for deposition thickness was about 3.9 % and for spreading area was about 4.2 %.

   There was not much influence of *L* / *d* on both the dimensionless spreading area* A~sp~*\* and the ratio *t* / *δ.* Re numbers had strong negative correlations with the ratio *t* / *δ*. Its correlation coefficient was about -0.87. It might have a large effect of viscous.

   For future work, obtaining additional experimental data under various conditions and creating correlation equations using these dimensionless numbers as fitting parameters from the experimental result. We believe that these studies help to improve the accuracy of correlation equations and calculation codes.


Experimental Study for Evaluation of Spreading Behavior of Free-falling Molten Core With Floor Impingement

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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