Internal corium catcher of a nuclear reactor

Full text of publication follows: A corium catcher is one of the main devices of a nuclear reactor that provides corium melt and fission products retention within a containment during severe accidents. Several studies and design developments have shown that corium retention within a reactor vessel can be attained with a moderate capacity of the latter (up to 600 - 650 MW el.). With a higher reactor capacity external corium catchers are applied both at Russian (VVER-1000) and European (EPR) reactors. In the external catcher of a VVER-1000 reactor, most technological problems are solved due to using sacrificial material. They are as follows: (a) endo-thermal interaction of corium and sacrificial material reduces a level of the temperatures in the final melt pool; (b) solution in the melt of a great amount of the sacrificial material reduces the specific heat release density and the heat flux density at the boundaries of a melt; (c) due to changing of the oxide-component density an inverse stratification of the metallic and oxide components of the corium takes place, thus excluding heat-flux focusing in the zone of the metallic layer and making it possible to supply water on the free surface of the corium without a danger of incipience of the vapor explosion; (d) final oxidation of zirconium occurs without hydrogen generation. The above principles have been realized in the external catcher of the VVER- 1000 reactor at Tyanvan NPS that is presently under construction in China. Successfully solving of the problems concerning to the external catcher makes it possible to return on the new conceptual and technological basis to the idea of retention of the corium melt inside the vessel of a nuclear reactor of large capacity, that is, to provide the reactor vessel to play a role of an internal catcher. For this purpose, a reactor vessel is elongated by approximately two meters. In the lower part of the vessel, on elliptical bottom, pieces of sacrificial material are arranged in a certain geometrical order and with proper porosity. They are covered by a steel sheet to prevent contact of the sacrificial material with first-circuit water during normal operation. During severe accident, from outside the vessel is cooled by boiling water. The arrangement of the external circuit of natural circulation is similar to that of the circulation circuit of the heat exchanger of the VVER-1000 external catcher. Estimates conducted show that the heat flux density at the external surface of the reactor vessel do not exceed the allowed one with the proper margin to the boiling crisis, while residual thickness of the vessel wall and additional supports installed provide integrity of the vessel. In the paper, we consider the specifics of the catcher design, the main parameters of the thermal a thermally stressed conditions of the corium-melt pool and the reactor vessel itself; and the requirements for the sacrificial material. (authors)