Patent Number: 044420650
Section: description

DETAILED DESCRIPTION FIG. 1 shows a nuclear reactor including a containment structure 12 and a primary reactor vessel 14 containing the central core structure, arranged in a generally conventional configuration, with the surface of the earth being generally indicated by the line 16. The new structure which has been retrofitted to the nuclear reactor includes an isolation tube 18, and a core catcher heat exchanger structure 20. The floor of the containment structure 12 has been thinned down at 22, so that, in the unlikely event of a melt-down of the core 14, the floor 22 will be penetrated by the melted down fragments, and they will descend into the isolation tube 18 and eventually down into the core catcher heat exchanger structure 20. The isolation tube or conduit 18 includes transverse sheets, such as thin sheets of steel 24, and suitable layers of shock absorbing material which is relatively light in weight, such as sand, as shown by reference numerals 26, supported by the sheet material 24. Similar arrangements may be provided in the central area of the heat exchanger 20 to delay the descent of the molten core and related material, so that a slow controlled descent is achieved which will not destroy the heat exchanger walls by undue shock. The isolation tube 18 is preferably provided with an inner steel liner 28 (see FIG. 2), a layer of refractory material 30, such as carbon, and then an additional outer steel jacket 32. The entire assembly including the isolation tube 18 and the core-catcher heat exchanger 20 is enclosed in a reinforced concrete shell 34. As best shown in FIG. 2, the core-catcher heat exchanger includes the inner walls 36 and the outer walls 38 which may be extensions of the walls 28 and 32 respectively, which enclose the isolation conduit. Incidentally, it should be noted that the break 40 as schematically indicated between the isolation conduit 80 and the heat exchanger portion 20 of the structure is included to indicate that the action indicated by the circulation arrows 42, 44, and 46 would not occur with the support plates 24 and the sand 26 in the indicated position in FIG. 2, but these structures are merely shown for purposes of greater detail than is possible in FIG. 1. Of course, once the molten material has descended into the heat exchanger area 20, the sand and support plates 26 and 24 would have served their delaying function and would be reduced to molten form. In FIG. 2, the lower circulating arrow 46 represent the cooling action of the heaviest material, uranium oxide, which would descend to the bottom of the heat exchanger structure. Lighter weight material, such as molten steel and the like would be located in the next higher zone as indicated by the circulating arrows 44, while still lighter weight material, such as the sand, or other flux which may be provided within the isolation conduit and the heat exchanger structure, would be floating on top where the arrows 42 are present. The cooling jacket 48 between the inner wall 36 and the outer wall 38 is filled with water, and this automatically circulates through the lower input conduit 50 and the upper output conduit 52, which are connected as shown in FIG. 1 to the cooling structure 54. Molten uranium oxide normally has a melting point in the order of 2100 or 2200 degrees centigrade, which is well above the melting point of approximately 1400 or 1500 degrees centigrade for the inner steel lining 36 which faces the molten uranium oxide. However, the high thermal conductivity of the steel wall relative to that of uranium oxide insures that the steel will not differ significantly in temperature from the adjacent cooling water. In practice, therefore, the uranium oxide immediately adjacent the steel walls of the heat exchanger will solidify and form a "frozen" or solid layer against the steel wall, and the uranium oxide may thus be thought to form its own container within which the cooling action progresses. Of course, the water within the jacket 48 is heated rapidly and circulates through the large conduits 50 and 52 to the water tower 54, where much of the water may boil off, in the course of a month or two, as the molten core material and other miscellaneous molten debris cools down. Instead of a cooling tower 54, water may be drawn from, or returned to, any large body of water, such as a nearby lake, river, or ocean. Of course, with the thick steel walls and the construction as described above, the water does not become radioactive, and accordingly, there is no concern with the boiling off of the water from the water tower 54. Incidentally, the inner wall 36 of the heat exchanger 20 must be firmly supported by structural members, some of which are indicated schematically at 56, to support the very substantial weight of the molten core materials. In FIG. 1, the elevator or hoist structure 62, the vertical shaft 64, and the two horizontal shafts 66 and 68 have been shown to indicate one construction technique whereby the core-catcher arrangements could be retrofitted to an existing nuclear facility. Initially, the vertical shaft would be constructed in accordance with conventional mining techniques, with the two horizontal access tunnels 66 and 68 being dug to the indicated points directly under the core 14. With the relatively small diameter of the isolation tube and core-catching heat exchanger structure, most of the construction work can be accomplished while the reactor is still operating normally. This is particularly important, because the cost of shutdown may be in the order of several hundred thousand dollars per day. However, for a brief period of several days, while the reduced thickness floor 22, and the upper section of the isolation tube 18 are being constructed and positioned, the reactor must be briefly shut down. The input and output water conduits 50 and 52 may follow the access tunnels 66 and 68 during their substantially horizontal sections, and holes for the vertical sections of these conduits may be bored with conventional drilling equipment. In the foregoing discussion, attention has been concentrated on the below-ground structure; however, in some cases, when a melt-down would occur, the core structure might hang up within the containment structure 12, with high levels of heat being generated within this structure 12. In order to accommodate this eventuality, the heat exchanger structure 72 is provided to absorb heat in its section 74 extending within the containment structure 12, and dissipates the absorbed heat in the portion 76 which is within the cooling water 78 inside the cooling tower 54. A substantial protective shield 80 may be provided to protect the heat exchanger structure 74, from the possibly violent events associated with the possible melt-down of the reactor 14. With both the heat exchanger 72 and the core-catcher heat exchanger 20 being coupled to the single water tower 54, it has the capacity to absorb the core-decay heat, whether most of the heat is generated within the structure 12, or if the core, as expected, decends down into the core catching heat exchanger 20. When conventional reactors utilizing slow neutrons and water to slow down the speed of the neutrons, are employed, no special precautions need be taken with regard to the core catcher heat exchanger geometry to prevent it from going "critical" and generating additional heat. However, in the case of fast breeder reactors, there could be some possibility if the mass of the material was sufficiently great, that such criticality could occur. Accordingly, for reactors of this type, a longer and thinner vertically extending heat exchanger could be used, or alternatively a diverging geometry of the type shown in FIG. 3 could be employed. In FIG. 3, the structure, including the inner steel walls 36 and the outer steel walls 38, as well as the concrete enclosing structure 34 would be substantially the same, with a cooling jacket 48, all substantially as shown in FIG. 2. However, at the lowermost end of the heat exchanging structure, with sufficient volume to hold the heavy fast-breeder material, a series of branching arms 92 are provided. The input conduit 50 may be connected by suitable manifold piping 94 to the lower end of the water jacket enclosing each of the branching conduits 92. In addition, the concrete structure 34 is enlarged at its lower end 96 to fully enclose the lower end of the structure. It has been determined that, if the branching conduits 92 are oval, and if the distances across the conduits in the shorter cross-sectional direction are maintained less than about one foot, or about 30 centimeters, there will be no danger of the fast-breeder reactor material going critical. Incidentally, as noted above, the diameter of a nuclear reactor containment shell would normally be in excess of 100 feet, probably in the order of 125 feet. Further, the diameter of the isolation tube and the core catcher, is preferably in the order of two or three meters, or about 10 feet. Translated into the terms of cross-sectional area, the base of a nuclear containment structure would normally be greater than 10,000 square feet or 1,000 square meters, while the cross-sectional area of the isolation tube and the core catcher heat exchanger would normally be in the order of 100 square feet, or ten square meters. Translating these figures into percentages, the transverse dimension of the isolation tube and core catcher structure will normally be less than 10 percent or less than 20 percent of the transverse dimension of the base of the containment structure; and the cross-sectional area of the isolation tube and the core catcher structure will normally be less than 5 percent of the cross-sectional area of the base of the containment structure. For completeness, the following additional patents are cited as being of interest, although they have the shortcomings as noted hereinabove: U.S. Pat. No. 3,640,451, granted Mar. 14, 1972; No. 3,702,802, granted Nov. 14, 1972; No. 3,719,556, granted Mar. 6, 1973; No. 3,964,966, granted June 22, 1976; No. 4,003,785, granted Jan. 18, 1977; No. 4,028,179, granted June 7, 1977; No. 4,072,561, granted Feb. 7, 1978; No. 4,073,682, granted Feb. 14, 1978, and No. 4,113,560, granted Sept. 12, 1978. In closing, it is to be understood that the foregoing description and the drawings relate to specific embodiments of the invention. Other arrangements may be employed in the implementation of the invention without departing from the spirit and scope thereof. For example, instead of using steel for the lining of the isolation tube and the heat exchanger, other high temperature resistant, high strength materials could be employed. Similarly, instead of using sand and steel supporting sheets in the isolation tube and the heat exchanger, other inert material, such as plastic sheets and dirt, for example, or nearly any other material for slowing down the descent of the core, absorbing shock and avoiding steam explosions, could be employed. Further, other arrangements for conducting heat away from the core catching reactor in a passive manner, could be employed instead of the water cooling arrangement. In addition, the cross sectional dimensions of the water jackets and the strength of the supporting elements between the walls of the water jackets would be proportioned to accomodate the maximum heat flow and maximum stresses required by these portions for the particular nuclear reactor under consideration. It is to be understood, therefore, that such alternatives and other similar ones are within the scope of the present invention.