Patent Number: 043009832
Section: summary

DESCRIPTION OF THE PRIOR ART Various proposals have been made with a view to avoiding the penetration of a nuclear reactor core melt flowing from the melting core of a nuclear reactor during a core melt-down accident through a containment surrounding the nuclear reactor. These proposals are mainly based on the attempt to cool the core melt by suitable means in a suitable core catcher. Heat is mainly removed from the core melt surface and it must be removed at such a rate that the core melt is solidified at latest upon reaching the core catcher walls so that its penetration through the reactor containment is impossible. Core catchers of metallic materials are, for example, known which have an inner liner of graphite and which include a cooling system. They are disposed within the reactor containment, and may include means for dividing the core melt into separate amounts in order to prevent the core melt from becoming critical. The German DOS No. 22 34 744 describes a core catcher arrangement which has a water spray system arranged above the core catcher so as to permit cooling of the core melt during core melt-down with a water spray. Cooling of the core melt with molten metal such as copper, lead or aluminum has also been taken into consideration. All the coolants proposed so far, however, have serious disadvantages. In particular, they do not dissolve the core melt. Therefore, at one hand, the fission products which are generating a substantial amount of residual heat remain in the core melt that is within only a relatively small volume and, on the other hand, the core melt forms a solid body after solidification which body is hard and difficult to remove mechanically so that it is difficult if not impossible to clean up the site of the accident. Also, the high concentration of the fission products in the core melt generates high core melt temperatures thereby increasing the risk of penetration of the core melt through the reactor containment and into the environment. Further, core catchers have been proposed which included concrete and basalt for melting and dissolution by the core melt. These arrangements, however, would after solidification of the core melt, again produce a solid radioactive product, which is difficult if not impossible to remove. In addition, concrete and basalt are relatively complex compounds which may generate unforeseeable reactions during the dissolution process (for example the generation of highly explosive hydrogen). A further proposal suggested to suspend the core melt in molten lead; but this would also result in the formation of a product which is difficult to remove from the reactor containment. U.S. Patent application Ser. No. 679,221 discloses a core catcher which includes a protective layer of oxides and salts covered by a metal foil. It is the purpose of this layer to reduce the risks of H.sub.2 generation so as to avoid possible explosions during a core melt-down accident of a nuclear reactor which is cooled by water, steam or gas. As oxides and salts used in the protective layer the application lists MgO, CaO, B.sub.2 O.sub.2, SiO.sub.2, borates, phosphates, chromates and carbonates. During an accident however, the core melt will melt through the single protective layer probably at one or a few points and underflow the protective layer and collect below the protective layer as a whole that is the oxidic as well as the metallic part of the core melt so that--as intended--the metallic part of the core melt is finally completely covered by the protective layer, which, meanwhile, has also melted. It is also possible that a part of the protective layer mixes with a portion of the oxidic part of the core melt thereby forming a compound which has a lower density than the metal melt and therefore forms a mixed layer above the metal melt. In both cases, however, a contact between the metal melt and water or steam is prevented. But in any case, it is difficult to remove the solidified and highly radioactive core melt at the bottom of the core catcher from the reactor containment after a core melt-down accident. SUMMARY OF THE INVENTION A method and arrangement for greatly facilitating removal of a core melt from the containment of a nuclear reactor after a core melt-down accident and without the risk of radiation exposure to humans wherein the core melt is permitted to gradually penetrate layers of inorganic, water soluble oxides or salts disposed in a core catcher arranged below a nuclear reactor, whereby the inorganic oxides and salts are dissolved by the oxidic part of the core melt thereby forming a solution melt. The solution melt, after a cool-down period and solidification, at a temperature at which hydrogen generating reactions do not take place, is leached with water and rinsed out of the containment.