Untersuchung zur Verbesserung des Wärmetransports durch flüssiges Blei im Stabgitter eines Druckwasserreaktors nach einem schweren Kühlmittelverluststörfall

In pressurized water reactors with high capacity, accidents with heavy loss of cooling water and the simultaneous loss of all emergency cooling systems will lead to a core melt down due to missing decay heat removal. If no cooling system can be activated, the residual water inside the reactor vessel will be evaporated soon. The fuel assemblies above the water level will heat up. The heat transport inside the pin bundle without surrounding water is not sufficient to remove decay heat by heat radiation although the temperature of the fuel cladding in very high. Fuel and cladding materials will reach their melting temperatures. To prevent the release of radioactive material and for environmental protection, the fuel pellets of an innovative pressurized water reactor are encapsulated inside small, very tight ceramic containers and the specific heat production during normal operation is limited to 50 MW per cubic meter. For temperature limitation on values lower than the melting temperatures of core materials, lead granulate is filled inside the reactor pin bundle. To release the filling of the reactor with lead, a melting safety device is installed which melts at a temperature higher than the temperature under normal operation conditions and below the melting temperature of the reactor core materials. The release is self-driven if temperatures are high enough. After melting of all injected lead the decay heat will be taken away from inside the fuel assembly to the edge of the fuel assembly by natural convection of liquid lead. From there the heat will be transferred to the reactor vessel and further to a surrounding water pool of the reactor. For determination of the heat transfer phenomena inside a pin bundle filled with liquid lead experiments have been done at a test facility that simulates the fuel bundle of a reactor. Specific heat rates in order of the decay heat production inside the reactor were realized. Heat transport values were evaluated by measuring temperature profiles inside the liquid lead filled pin bundle of the test facility. With the knowledge about the heat transport inside the test facility the heat transport inside the reactor filled with liquid lead can be estimated. Considerations of the whole accident show, that the temperature rise can be limited to lower values due to liquid lead inside the fuel assembly. The low heat conduction and thickness of the reactor vessel lead to a high temperature difference between the outer and inner side of the vessel. This is the reason why temperatures around 1500 degrees Celsius inside the reactor are reached over the time period of the accident. The longer time between appearance of the accident and the time the pin bundle reaches high temperatures will be longer than 30 hours due to the high melting heat capacity of the inserted lead granulate. If a core melt down occurs is a question of stability of the used core materials