Hydrothermal oxidative decomposition is one of the methods for detoxification of polychlorinated biphenyls (PCB), which are decomposed into water, carbon dioxide, and sodium chloride under high temperature and pressure (370 °C, 26.5 MPa). This chemical reaction consists of the dechlorination of PCB with sodium carbonate and the oxidative decomposition of biphenyl with liquid oxygen. In these reactor vessels, wall thinning due to corrosion was observed on the bottom inner wall. At present, the reactors have been safely maintained and operated by adding a bottom partition to prevent chemical sinking and supplying hot water to the reactor vessel bottom to control the temperature. The purpose of this study is to clarify the corrosion mechanism by thermal fluid analysis of the hydrothermal oxidative destruction reactor. A finite volume analysis solver of OpenFOAM is used to perform a multi-regionally coupled analysis of the internal fluid flow and heat conduction in the reactor vessel considering the conjugate heat transfer on the solid-liquid interface. The internal fluid is a two-component fluid, PCB and water, with different mass densities and viscosity without chemical reaction. The advection equation for the volume fraction of two fluids based on the volume of fluid method, the equation of continuity, the compressible Navier-Stokes equation with gravity term, and the energy equation are staggeringly solved for unsteady thermal fluid analysis. In this study, the numerical results of supplying hot water to the bottom of the reactor to maintain the vessel temperature, and with and without the bottom partition to prevent the settling of chemicals are compared. The percentage of PCB on the partition and at the bottom of the reactor decreased when the viscosity was increased by 50 times with and without the partition. The temperature distribution results with a partition showed that the temperature at the center of the bottom of the reactor decreased when the viscosity of PCB was 50 times higher than that of water. The velocity distribution shows that at the bottom of the reactor, the upward flow from the bottom water inlet blocks the downward flow cooled by the oxygen nozzle. The results with a viscosity factor of 50 indicate that the downward flow at the bottom of the reactor is reduced. As described above, when the viscosity was increased, the flow state of PCB in the reactor changed.