Patent Number: 061954055
Section: description

BEST MODES FOR CARRYING OUT THE INVENTION As shown in FIGS. 1, 2, 3, and 4, the central core (1) is a nuclear fuel region where the coolant flow removes the nuclear reaction heat. If an accident takes place and cooling capacity decreases, the core and neighboring structures may melt down and accumulate on the lower head. In this case the gap structure (3) will prevent the molten core from direct contact with the lower head (2) inner surface. In addition, instrumentation/control nozzles penetrate the reactor vessel lower head (2) and are welded to the vessel inner surface. The gap structure (3) of this invention will be installed at the bottom of the lower core support structures at an appropriate gap (5) or distance from the reactor vessel bottom and have sufficient strength and heat resistance to maintain coolable geometry and also support the dead weight of the molten core during core material meltdown accidents. The minimum gap size taking into account the boiling heat transfer and structural behaviors is determined to be about 2 centimeters. The gap structure (3) is of a parabolic or concave shape and has vertical flow holes (6). It (3) preferably covers the entire reactor vessel lower head (2) to prevent the molten core from flowing into the gap. The change of coolant flow distribution in the lower plenum due to the gap structure (3) of this invention must be minimized during normal operation or design transient of light water reactors (LWRs). FIG. 6 shows that the gap structure (3) of this invention is welded (11) or fastened to the lower core structures or instrumentation/control penetration (4) structure in the vessel lower head. Additional support structure may be attached to it (3) to resist deformation induced by heavy load. The means are used for spacing apart and maintaining the concave vessel in a spaced apart condition relative to the reactor vessel lower head such that a gap is formed and maintained between the concave vessel and the reactor vessel lower head in the event of a reactor core meltdown. These means enable the coolant circulating in the reactor vessel to circulate within the gap thereby preventing direct contact of the molten core debris with the reactor vessel lower head and removing heat from the molten core debris. Such coolant circulation ensures that the structural integrity of the reactor vessel lower head is maintained during the reception and retention of the molten core debris by the concave vessel. Examples of the means for spacing apart and maintaining the concave vessel in a spaced apart condition relative to the reactor vessel lower head include support beams (10A), deformation-limiting foot (9) secured to the concave vessel and structural stiffeners (10B). If the high temperature reactor core material melts down and accumulates in the reactor vessel lower plenum (FIG. 9), the gap structure (3) of this invention would catch the molten core material so as to avoid direct contact with the vessel and to enable removal of the heat from the relocated core material by the water coolant circulating in the gap (5) between the vessel and the debris. As shown in FIGS. 1, 2, 3, and 4, each gap structure (3) is illustrated in a single layer, however, multilayer gap structures may also be used where desired, as shown in FIG. 5. In this case the concave vessel of the gap forming and retention structure defines a first concave vessel (3A) and a second concave vessel (3B), wherein the first concave vessel (3A) is spaced apart from the reactor vessel lower head (2) to form a first gap (3AA) and the second concave vessel (3B) is spaced apart from the first concave vessel (3A) to form a second gap (3BB). Preferably, the diameter d1 of the second concave vessel 3B is less than the diameter d2 of the first concave vessel (3A) and the gap between vessels (3BB) and between the first concave vessel and the reactor vessel lower head (3AA) are independently uniformly or evenly spaced apart. A plurality of concave vessels may be positioned in the reactor vessel with the first being proximate the reactor vessel lower head and with the upper vessels having consecutively smaller diameters. In FIG. 5 the first and second concave vessels (3A, 3B) are secured to the structural guide sleeves (7) which house the instrumentation/control penetration structures (4) (not illustrated). Flow holes (6) distributed in each gap structure (3) may be introduced to reduce the temperature difference between the gap water and the bulk water in the lower plenum during normal operation and design transients. The flow hole length-to-diameter ratio exceeds so that the molten core debris is not expected to penetrate through. The relocated molten core may thermally attack the instrumentation/control penetration (4) to render further diagnosis of the core state difficult. FIG. 6 shows a structural guide sleeve (7) protruding up from the gap structure to form a vertical gap (8) that will have a similar cooling effect to protect instrumentation/control penetrations. The gap structure (3) of this invention should be made of materials that are both durable and tolerant to thermal and mechanical shock loads. Corrosion resistant metals possibly incorporating ceramic and/or composite materials would be expected to qualify for the required resistance. Cooling fins may be attached to the gap structure (3) to enhance the cooling capacity. While FIGS. 1, 2, 3, and 4 show the in-vessel gap structures, FIG. 7 shows an ex-vessel gap structure (3) installed outside the vessel lower head (2). In this case, required coolant would be supplied from a coolant reservoir (15) during the accident via a control valve (16) and coolant feed tube 17. Both in-vessel and ex-vessel gap structures may be installed such that they will not interfere with vessel inspection and maintenance. In the same manner as in the reactor vessel itself, a plurality of ex-vessel gap structures could be utilized (not shown for sake of brevity) if desired. The external gap forming and retention structure for use with a nuclear reactor vessel having a reactor core assembly positioned above a reactor vessel lower head and with a coolant circulating in the reactor vessel, comprises a concave vessel externally positioned below and spaced apart from the exterior surface of the reactor vessel lower head such that a gap is formed between the exterior surface of the reactor vessel lower head and the concave vessel. A coolant supply means supplies coolant to the gap in the event of a reactor core meltdown to enable removal of heat from the reactor vessel lower head heated by molten core debris from the reactor core assembly in the event of a reactor core meltdown thereby maintaining structural integrity of the reactor vessel lower head during the reception and retention of the molten core debris by the reactor vessel. Preferably, the coolant supply means further includes a control valve for controlling the rate of coolant flow into the gap. Cooling fins may also be secured to the lower head of the reactor vessel to aid in heat removal by the coolant as it flows in the gap. FIG. 8 illustrates severe accident development without the gap structures, while FIG. 9 illustrates the accident arrest with the help of the gap structure according to the present invention. In FIGS. 8 and 9, numbers 12, 13, and 14 represent water coolant (12), molten core debris (13) and failed vessel lower head (14), respectively. FIG. 10(A) shows a top view, FIG. 10(B) shows a cross-sectional view, and FIG. 10(C) shows a plan view of the gap structure (3) which is applicable to PWR, flow-channel type PWR (VVER-type), and BWR. The gap structure plan view of FIG. 10(C), is illustrated with grid lines to enhance three-dimensional display effect. This figure also shows structural guide sleeves (7) for the instrumentation/control penetration. FIG. 11(A) shows a top view, FIG. 11(B) shows a cross-sectional view, and FIG. 11(C) shows a plan view of the gap structure of the present invention (3) which is applicable to a pressurized heavy water reactor (CANDU-type). The plan view, FIG. 11(C), is illustrated with grid lines to enhance three-dimensional display effect. This figure also shows structural guide sleeves (7) for the instrumentation/control penetration. Design of the next generation nuclear reactors is required to have provisions for the protection against severe accidents. Proposed designs for this purpose, in the case of light water reactors, include two main features: namely, reactor cavity flooding method and advanced containment cooling method. These methods of protection against severe accidents would require large and costly facilities. In contrast, the gap structures of this invention provide vessel protection with relatively simple structural installations and can function largely in a passive manner. We have described the gap structure in detail. It is obvious that whoever has ordinary knowledge in this field will be able to easily adapt this invention to various applications. Therefore we want the use of this invention not only in the claimed range but also in the various applications.