Abstract: A tank type nuclear reactor comprises an outer shielding wall structure, a reactor vessel installed in the shielding wall structure and provided with an inner sealed space in which a primary liquid coolant circulates, a reactor core disposed in the reactor vessel and charged with a number of fuel assemblies, a core support structure disposed to a bottom of the reactor vessel, a partition wall structure disposed on the core support structure defining the inner space of the reactor vessel into a hot pool region and a cold pool region, intermediate heat exchangers disposed in the reactor vessel for carrying out heat exchanging operation between the liquid coolant flow from the hot pool region and a secondary side heat medium, and electromagnetic pumps disposed in the reactor vessel for feeding the primary coolant into the core. The partition wall structure has a vertically cylindrical wall structure including a lower portion constituting a core tank portion surrounding an outer periphery of the core.