Patent Number: 043009832
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a core catcher 1 provided with cooling pipes 2 and having disposed therein blocks 3 consisting of a wrapper enclosing a water soluble borate. The reactor core catcher is arranged within a reactor containment 7 of, for example, a 1000 MWe gas cooled breeder reactor. The core catcher 1 has at its inner surface 4 a protective layer 5 consisting preferably of graphite which layer 5, however, is not part of the present invention. The water-soluble borate of the blocks 3 comprises preferably the easily soluble alkali borates i.e. lithium-, potassium-, sodium-, rubidium, and cesium-borates. In connection with gas cooled breeder reactors, however, Na.sub.2 B.sub.4 O.sub.7, borax is preferred because of its availability and because of its advantageous physical and chemical properties. As a wrapper material, steel is suitable. But also other metals such as iron, nickel and also metal alloys are usable. Through selection of the material of the wrapper or rather their melting points, as well as the size of the blocks 3, the penetration pattern of a core melt can be determined. The steel included in the core melt of a 1000 MWe gas cooled breeder reactor--if evenly distributed over a core catcher floor of 50 m.sup.2 --would form a layer of about 1011 cm thickness. In order to facilitate removal of such a metallic layer from the core catcher, there are provided at the bottom of the core catcher 1, that is below the borate blocks, bins 6 of graphite which have side walls extending upwardly above the expected height of the metallic layer. Borate blocks 3 may also be disposed within the bins 6. A heat shield 10 is provided above the borate blocks 3 for the protection of the cooled walls of the core catcher 1 against excessive heat radiation from the molten core materials flowing down into the core catcher 1. FIG. 2 illustrates the penetration of the core melt through the borate layer in the core catcher. The figure is based on time values calculated for the borate layer to reach the melting point of steel (about 1427.degree. C.) under the assumption of certain conditions. The weight ratio of sodium borate to the oxidic part of the core melt is preferably selected to be at least about 1:1. For the calculations of penetration times as given in FIG. 2, the ratio of Na.sub.2 B.sub.4 O.sub.7 :core melt was assumed to be about 4:3. Because of the relatively low atomic weights of sodium and boron as compared to uranium, the molar or atomic ratio is even greater. Such greater borate to core melt weight ratio is necessary if it is to be insured that the solid solution remains water leachable, even after a complete reaction with the core catcher material. It is, for example, possible that chemical reactions occur in the core melt which form Na.sub.3 UO.sub.4 (sodium uranate) and UO.sub.2 (BO.sub.2).sub.2 (uranyl-borate). Na.sub.3 UO.sub.4 immediately absorbs water and CO.sub.2 when only exposed to air; when exposed to water total hydrolysis will be the result. UO.sub.2 (BO.sub.2).sub.2 is also hydrolized with water by forming UO.sub.3.2H.sub.2 O. From this it can be learned that, if sufficient water soluble materials are present in the core catcher, the mixture of core catcher materials and core melt can be dissolved or hydrolized. The remainder of the oxidic part of the core melt assumes the state of a fine powder which can be rinsed from the containment as a slurry simply by suction. The dissolution and hydrolization can be achieved in a circuit in which the water is evaporated from the solution, any particles suspended in the solution are removed by filtration, and the water is continuously returned to the containment. With the method according to the present invention, it becomes unnecessary to provide remotely controlled machinery in the reactor containment for the removal of a core melt after solidification. Instead, there are provided merely water inlets 8, 8a and suction lines 9, 9a which are not necessarily permanently installed but may be introduced into the containment after a core melt-down accident. Also, the hydrolized core melt when removed from the water solution is obtained in a form well suited for reprocessing. Although the present invention was developed in connection with a gas-cooled breeder reactor, it should be understood that the invention can also find application, with certain modifications, in connection with water-cooled or sodium-cooled reactors. In the case of a light water reactor, provisions must be made that, during an accident, the water will not wash the water-soluble borate layer away from its location under the core. This can be achieved by fully enclosing the borate layer in the core catcher or rather encapsulating the layer. In connection with sodium-cooled reactors (such as breeder reactors) a borate is preferred which is essentially inactive chemically with regard to sodium, such as Na.sub.2 BO.sub.2, sodium metaborate. In connection with sodium-cooled reactors there must be provisions excluding the in-flows of water into the reactor containment in order to avoid its reaction with the sodium and the formation of hydrogen. With light water reactors no hydrogen or hardly any hydrogen will develop if the borate is in the form of blocks 3 enclosed in wrappers since, after melting of the wrappers of the uppermost blocks, the borate will prevent a direct contact between steel and water or steam. If, during an accident, only some of the blocks are melted, the undamaged blocks are removed in the normal manner and the remaining damaged blocks are removed by dissolution in a solvent circulated through the core catcher. If a large part of the blocks is damaged it is preferred to first expose the core catcher to the solution for dissolution of the damaged block material. The undamaged blocks can be removed afterwards. Dissolution of the core melt and core layer material and its removal by pumping the solution out of the containment provides for an easy and safe clean-up operation after a core melt-down accident and this operation can be initiated soon after an accident has occurred. With the arrangement according to the present invention it is furthermore insured that the decay heat still generated after a core melt-down can safely be absorbed by the core catcher without any additional active means. Also, the exposure of the containment and the reactor to heat and radiation is reduced. However, the additional safety measures have really no influence on the reactor design itself. They are simple and inexpensive. There is no disadvantage generated for the reactor and the containment by inclusion of an arrangement according to the present invention in a reactor installation. The arrangement according to the invention will also insure that the metallic part of the core melt will have only little contact with any water or steam present in the reactor containment as it readily melts into the top layer of blocks, which, upon melting cover the metallic part of the core melt so as to protect it from any water and steam in the containment. The blocks which as mentioned consist of water soluble alkali borates enclosed in a casing, may be arranged in layers of different thicknesses. Each layer includes preferably the same type or the same mixture of borates. Different layers may include different types of borates or different mixtures of borates. Different layers may also have blocks of different casing materials. The block casings generally consist of metal or metal alloys such as steel, iron, cast iron, nickel, iron alloys, or nickel alloys. But the casings may also consist of ceramic materials or glasses having a relatively high melting temperature however below the temperature of the core melt. The amount of core catcher material (alkali borates) arranged in layers in the core catcher is, by weight about equal the amount of the oxidic part of the core melt possibly to be received in the core catcher. Principally, during a core melt-down accident the molten core material flowing from the core penetrates the core catcher material layer by layer by melting the core catcher materials while being dissolved therein. This process consumes a large amount of heat thus taking up the residual decay heat and cooling the core melt. Once the solution of core material and core catcher materials has become solid and relatively cool, water may be introduced into the core catcher to leach the water soluble core catcher materials (borates) from the solid solution which leaves the oxidic part of the core melt in the form of a powder that may be washed out of the core catcher and sucked out of the reactor containment without the need for humans to be exposed to radiation during such clean-up operations.