Hydrothermal oxidative decomposition is a method for detoxifying polychlorinated biphenyls. To effectively decompose PCBs in a pressure vessel, thermo-fluid analysis considering heat conduction in the vessel and heat transfer between the vessel wall and the internal fluid is required. In this study, a multi-region analysis of a pressure vessel incorporating chemical reactions was performed using the OpenFOAM solver chtMultiRegionFoam. The analysis considered not only conjugate heat transfer on the solid-fluid interface but also the mass production and consumption rates of chemical species and the heat release associated with chemical reactions. An unsteady thermo-fluid analysis was conducted by solving the compressible Navier-Stokes equations, transport equations for mass fractions including mass source terms, and an energy equation with a heat generation term. The analysis confirmed buoyancy-driven flow induced by density differences in the internal fluid and an increase in the internal temperature due to chemical reactions.