One of the difficult challenges for decommissioning of Fukushima Daiichi Nuclear Power Plants (1F) is the retrieval of fuel debris which was formed during the nuclear severe accident. Based on the three-dimensional reconstructed images of the fuel debris in Unit 3 by Tokyo Electric Power Company (TEPCO), numerous structures, including Control Rod Guide Tubes (CRGTs) and gratings, have been found embedded within the sedimented fuel debris bed. A comprehensive understanding of the spatial distribution, material characteristics, and phase states of the fuel debris is essential for the safe and effective execution of decommissioning operations.
Since the interior of the reactor is highly radioactive and physical access is extremely challenging, numerical simulation is one of the essential approaches to directly estimate the fuel debris distribution. Specifically for 1F3, the numerical simulation of Fluid-Structure Interaction (FSI) which deals with the interaction between corium and structures is crucial. However, existing numerical simulation methods have limitations and challenges, particularly modeling in highly viscous corium-structure interactions, phase change between solid and liquid (i.e. melting and solidification) and scaling issue.
To address these limitations and challenges, the objective of this research is to develop a numerical simulation method namely MPH-PMS which can accurately estimate the FSI phenomena in 1F3. Moving Particle Hydrodynamics (MPH), one of the particle methods, is applied to calculate the behavior of corium and structures with a unified manner.
Furthermore, to stably compute the highly viscous corium flow, implicit time integration for both pressure and viscosity terms is employed. In addition to that, passively moving solid (PMS) is applied to structures to model rigid body dynamics. In PMS, the rigid body is first calculated as fluid and the coordinates are modified to maintain the initial shape. The advantage of PMS is that it applies the same governing equations as those used for fluids to rigid bodies. For calculation of the rigid bodies interaction, spring-dashpot model which is applied to DEM is employed. Finally, phase change model that mimics the solid-liquid phase transition by varying the viscosity depending on the solid fraction is applied.
To verify and validate MPH-PMS, several benchmark studies were conducted, including the transition and rotation of single rigid body, Cylinder Drop, ice melting underwater, and the melting of multiple rigid bodies in a highly viscous fluid.
First, we focused on simulating the motion of single rigid body. For transition case, we prepared a cube rigid, accelerated at 1.0 m/s2 for 2.0 seconds and turned off the acceleration. For the rotational case, we applied an initial angular velocity to the same cube and tracked the displacement of the specific particle on the object’s perimeter. As the results, the displacement and velocity obtained in the simulation matched the theoretical values, suggesting that this method can preserve both linear and angular momentum of the rigid body.
Next, we examined the floating behavior of a rigid cylinder in water. A cylinder with a diameter of 0.1 m was dropped from a height of 0.05 m into a pool having 0.5 m in width and 0.2 m in depth. The final orientation of the cylinder depended on its aspect ratio (H/R) in which H and R stand for the height of the cylinder and radius, respectively. The simulation demonstrated that for aspect ratios less than √2, the cylinder remained upright, while for larger aspect ratios, it tilted and reached a stable angle, consistent with a theoretical prediction.
The Phase change behavior under low-viscosity conditions was validated by simulating the melting of an ice block in hot water. A rectangular ice block was placed vertically in water heated to 60°C. As the ice melted, its shape and center of gravity changed, which caused it to slowly rotate. Around 80 seconds, it started to tilt, and by 160 seconds it had leaned more than 45 degrees. The result was similar to experimental observation, demonstrating that the simulation could capture both melting and the resulting motion.
In the case of high-viscosity, paraffin wax heated to 200°C was poured over metal spheres made of a low-melting-point alloy. The top spheres melted while the lower spheres stayed solid due to the insufficient heat transfer and the thermal stratification. A two-dimensional simulation showed similar results. However, the temperature did not match exactly, suggesting that a 3D simulation is needed for more accurate results.
Finally, a real-scale 1F3 calculation which employs highly viscous corium- structures was performed. The results indicated that the latent heat and superheat of the highly viscous corium are quite important for the structure degradation.
Overall, the method shows great potential for contributing to the understanding of FSI happened in.
