Experiments are carried out to investigate the effects of the natural convection of superheated gas as well as those of the standpipes on the temperature distributions of the components and the heat removal performance in the water-cooling panel system for the MHTGR for decay heat removal, and to verify reliability of the design and evaluation methods. The numerical results of the code THANPACST2 are compared with the experimental data to verify the numerical methods and axi-symmetric model proposed, which can simulate the three-dimensional configuration of the standpipes on the upper head of the pressure vessel by using porous body cells. The experiments revealed that temperatures increased with elevation on the upper head, because the standpipes restrict radiation heat transfer to the upper cooling panel and reduce the heat transfer area on the upper head, which was superheated by natural convection of helium gas in the pressure vessel. In the presumed accident condition in which thermal radiative heat transfer is responsible for the majority of the total heat transfer, the numerical methods were able to closely duplicate the pattern of the rising temperature profile with elevation around the top of the upper head as observed in the experiments.