Patent Number: 054105773
Section: summary

TECHNICAL FIELD This invention relates to the field of nuclear reactors and core meltdown damage prevention and more specifically to core catchers. BACKGROUND ART In the field of water cooled nuclear reactors it is well known that the most probable accident in the nuclear reactor is a reactor core meltdown which occurs when the capability to remove heat from the nuclear reactor core is lost. When a reactor core meltdown occurs, the core melts through the pressure vessel and the molten material drops onto the containment floor, which is typically fabricated from a concrete material. The reactor is surrounded by a containment building which is intended to prevent the radioactivity from escaping into the environment. The resultant mixture of molten and solid core material from a water cooled nuclear reactor core melt accident is "corium". Corium results from a zirconium clad fuel and is generally comprised of uranium oxide (UO.sub.2), steel (Fe), zirconium oxide (ZrO.sub.2), zirconium (Zr) and fission products. In a typical coremelt accident, the molten corium core is an uncoolable, high power density mass which exits the bottom of the reactor pressure vessel. The geometry of the molten corium is ill defined. The high temperature corium can penetrate the containment floor by high temperature decomposition of the concrete. When this occurs, the reaction between corium and the concrete generates flammable, noncondensable gases. More specifically, the decomposition of concrete generates carbon dioxide (CO.sub.2). The resultant carbon dioxide reacts with the zirconium in corium to generate carbon monoxide (CO). Further, zirconium reacts with water in the containment and in the concrete to generate hydrogen (H.sub.2). The noncondensable gases, hydrogen and carbon monoxide, may overpressurize the reactor containment causing its failure. Also, the hydrogen and carbon monoxide may burn or detonate with air in the containment building which could cause containment failure. Furthermore, with failure of the containment, the potential for leakage of radioactive material to the environment is extremely high. Many methods for preventing containment failure have been proposed. Generally, these methods require spreading the molten material out so it can cool and freeze by flooding the underreactor cavity with water. The basic drawback to all such approaches is ensuring the distribution or geometry of the molten core material during a core-melt accident. For example, lead core catchers have been developed in an effort to solve the problems involved with a core-melt accident. Lead has a sufficiently high density to float the core debris but there is no way to absorb the core debris such that the power density of the molten core is lowered. Further, a lead core catcher can not prevent reaction of zirconium in corium with the water which drains down onto the corium with time. A core-melt source reduction system has been developed for a gas cooled fast reactor which utilizes a stainless steel clad fuel. The major difference between the system developed for the stainless steel clad fuel and the present invention for a zirconium clad fuel is that stainless steel does not react with most materials in a core-melt accident. Unlike stainless steel, zirconium is highly reactive. Because of this highly reactive nature, there will be fundamental compositional differences between the system developed for the stainless steel clad fuel and that for zirconium clad fuel. Therefore, it is an object of this invention to provide a core-melt source reduction system which stops the progression of a high temperature core through the containment floor during a core-melt accident. It is another object of the present invention to provide a core-melt source reduction system which prevents the generation of noncondensable gases when the core materials react with the containment floor. It is a further object of the present invention to provide a core-melt source reduction system which does not require any assumptions about the geometry or timing of the molten core material in a core-melt accident. It is yet another object of the present invention to provide such a core-melt source reduction system which can be incorporated into or replace the existing containment floor. Further, it is object of the present invention to provide a core-melt source reduction system which minimizes heat rejection to the containment floor early in the core-melt accident. It is yet another object of the present invention to provide a core-melt source reduction system which ends the accident sequence with a long term, cold, stable state. It is a further object of the present invention to provide a core-melt source reduction system which traps radionuclides in a solidified matrix. DISCLOSURE OF THE INVENTION Other objects and advantages will be accomplished by the present invention which serves to provide a system which can contain a molten core after a nuclear reactor core meltdown accident such that containment failure is prevented. The core-melt source reduction system, hereinafter "the system", of the present invention includes alternate layers of two different materials as part of the floor under the reactor. The first material reacts with and absorbs the core material in a manner such that the core spreads out and the progression of the core is slowed and ultimately halted. The second material acts as a barrier to slow the descent of the molten core through a layer of the first material to provide time for dissolution of the core materials within the first material. During the reaction between the molten core and the layers of the first and second materials, no noncondensable gases are generated such that the containment will not fail as a result of overpressurization. Further, the resultant material is a solidified thermally hot stable waste material which can ultimately be cooled to a stable state matrix. The stable state matrix is such that radionuclides are trapped within the matrix to minimize the consequences of leakage.