Patent Number: 052232107
Section: description

DETAILED DESCRIPTION OF THE INVENTION Pool-type liquid-metal cooled reactors have sufficient surface area to accommodate dissipation of residual heat during reactor shutdown events. Overall, the reactor system has a relatively small heat capacity. The problem remaining is to dissipate the residual heat without significantly damaging the containment structures. A completely passive cooling system eliminates reliance on energy driven pumps and fans and the need for operator intervention. At the same time, the containment vessel itself must not be structurally modified due to the size constraints on modular reactors, and the necessity of a smooth, unperforated tank structure to prevent any areas where stresses might accumulate. Strict inspection requirements also require that the containment vessel be simple to inspect both during manufacture and erection of the structure. Referring to FIG. 1 of the drawings, an embodiment of a pool-type, liquid metal cooled nuclear reactor plant 10, comprises a reactor vessel 12, typically consisting of an cylindrical tank positioned with its longitudinal axis extending vertically upright, and having an open upper end provided with a removable cover. Reactor vessel 12 contains a pool of liquid metal coolant 14, such as sodium metal, with a heat producing core of fissionable fuel 16 substantially immersed within the liquid metal coolant pool 14 for heat transfer. Fission action of the fuel and the rate thereof is governed by neutron absorbing control rods 18 moving out from or into the fuel core 16. The reactor vessel 12 is enclosed within a concentrically surrounding containment vessel 20 in space apart relation. A baffle cylinder 22 encircles substantially the length of the containment vessel 20 in spaced apart relation. A guard vessel 24 concentrically surrounds the baffle cylinder 22 with the containment vessel 20 and reactor vessel 12, in spaced apart relation. A concrete silo 26 houses the concentrically combined and spaced apart arrangement of the guard vessel 24, the baffle cylinder 22, the containment vessel 20 and the reactor vessel 12. Preferably the concrete silo 26 is substantially buried into the ground to the extent that its contained reactor vessel 12 and adjoining vessels and cylinder are located at least below the ground surface, shown in the drawings as 28. Locating the liquid metal containing reactor vessel below ground surface precludes the escape of any liquid metal regardless of any loss of integrity of the plant. A containment dome covers the open top of the concrete silo to prevent any escape of radioactive contamination from the reactor plant out into the atmosphere. This arrangement of these combined components in surrounding or encircling and spaced apart positions, provides for their respective side walls forming a series of partitions with intermediate spaces. Specifically, a space 30 between the partitions comprising the side walls of the reactor vessel 12 and containment vessel 20; a space 32 between the partitions comprising side walls of the containment vessel 20 and the baffle cylinder 22; a space 34 between the partitions comprising the side walls of the baffle cylinder 22 and the guard vessel 24; and a space 36 between the partitions compressing the inside wall of the guard vessel 24 and the concrete silo 26, which in turn is divided by a second baffle cylinder. In a preferred embodiment of the invention wherein the above combined components are circular in cross-section and concentrically surround or encircle one another, the intermediate space 30, 32, 34, 36 and 37 are each substantially annular in cross-section. The containment vessel 20, the baffle cylinder 22, guard vessel 24 and concrete silo 26 are each provided with at least one upward projection or continuing wall extending above the uppermost portions of the reactor vessel 12, and up beyond the ground level 28. Thus the annular spaces 32, 34, and 36 formed intermediate the partitions are continued or in fluid communication with a corresponding space between projections or continuing walls extending above the heat producing reactor and its enclosing vessel 12. Specifically projection or continuing wall 38 extends from or adjoins the containment vessel 20, the projection or continuing wall 40 extends from or adjoins the baffle cylinder 22, the projection or continuing wall 42 extends from or adjoins the guard vessel 24, and projection or continuing wall 44 extends from or adjoins the concrete silo 26. These projections or continuing walls extending from or adjoining these vessels and cylinder which define the spaces 32, 34, and 36, with space 36 being divided into subspaces 62 and 64 by baffle cylinder 60 provide corresponding continuations or ducts extending upward from each of said spaces. The projections or extension wall 38 continuing from or adjoining the containment vessel 20 and the projections or extension wall 40 continuing from or adjoining the baffle cylinder 22 form a channel or at least one duct 46 in fluid communication with space 32 and extends therefrom upward opening out into the ambient atmosphere. The projections or extension wall 40 continuing from or adjoining the baffle cylinder 22 and the projections or extension wall 42 continuing from or adjoining the guard vessel 24 form a channel or at least one duct 48 in fluid communication with space 34 and extends therefrom upward opening out into the ambient atmosphere. The projections or extension wall 42 continuing from or adjoining the guard vessel 24 and the projections or extension wall 44 continuing from or adjoining the concrete silo 26 form a channel or at least one duct 50 in fluid communication with space 36 and extends therefrom upward opening out into the ambient atmosphere. The space 30 intermediate the reactor vessel 12 and containment 20 is typically filled with an inert gas, such as argon or nitrogen, and sealed. The containment vessel 20 and intermediate inert gas serves as a protective measure against the occasion of a breach of the reactor vessel 12 and in turn leaking of liquid metal coolant such as sodium. The channel or duct(s) 46 and channel or duct(s) 48, are provided with isolation valves 52 and 54, respectively, for closing off the spaces 32 and 34 along with their adjoining channels or ducts from the atmosphere. In operation, heat produced by the fuel core 16 is conveyed to the reactor vessel 12 by natural convection of the surrounding liquid metal coolant 14, then transferred mainly by thermal radiation across the inert gas containing space 30 from the reactor vessel 12 to the containment vessel 20. The heat is absorbed by the air in space 32 which is in contact with the outer surface of the containment vessel 20 and is carried along in the air rising upward due to its decreased density from heating thereby inducing a natural draft in space 32. The heat induced air flow continues upward from space 32 through channel or duct(s) 46 and out into the atmosphere where the heat is vented. This heat induced air flow up and out through space 32 and duct 48 draws air in from the atmosphere down into channel or duct(s) 48 and through space 34. From space 34 the cool atmospheric air flow continues passing beneath the lower edge of the baffle cylinder 22 and up into space 32 where it is heated from the hot outer surface of the containment vessel 20 to perpetuate the circulating cooling flow through the auxiliary safety cooling system for venting the carried heat out into the atmosphere. This heat motivated cooling course of through channels or ducts and/or spaces 48, 34, 32 and 46 comprises auxiliary safety cooling circuit or loop 56. Continued emissions of heat from the fuel core and transfer to the air within space 32 perpetuates the cooling air flow through the auxiliary safety cooling circuit or loop 56, and dissipation of heat out into the ambient atmosphere. Thermal performance studies of the system indicate that the maximum average core sodium outlet temperature for a decay heat removal transient is about 1140 degrees F. which is well below the current ASME service level temperature limit of 1200 degrees F. based on nominal calculations. An extreme and improbable postulated event proposed for safety considerations is the unlikely rupturing of both the reactor and the containment vessels 12 and 20. Such an event would permit the leakage of the liquid metal coolant, typically sodium, contents from the reactor vessel 12 and through the containment vessel 20 out into spaces 32 and 34, possibly blocking the cooling air flow therethrough, as well as imperiously reducing the level of coolant remaining in the reactor vessel for conveying heat away from the fuel core 16. Hot liquid metal coolant such as the commonly used sodium escaping out from the confines of the reactor and containment vessels could result in exothermic chemical reactions, sodium fires and/or a severe release of radioactive material out into the ambient atmosphere. Such an event and coolant leakage which obstructs the cooling air circulation through spaces 32 and 34 prevents the operation of the passive cooling safety system for the removal of decay and sensible heat whereby resultant overheating can cause significant damage to structural components of the reactor plant which propagates further destruction and hazards. In accordance with the invention, a backup or secondary auxiliary safety cooling course or system 58 is provided to coupe with significant liquid metal coolant leaks due to a breach of both the reactor and containment vessel 12 and 20. Referring to the drawing, a baffle cylinder 60 is provided extending down between the concrete silo 26 and the guard cylinder 24 substantially surrounding the length of the guard cylinder. The baffle cylinder 60, which does not extend down to the floor of the concrete silo, divides space 36 into two annular subspaces, subspace 62 between the concrete silo 26 and baffle cylinder 60, and subspace 64 between the baffle cylinder 60 and the guard vessel 26. Subspaces 62 and 64 are in fluid communication below the lower end of baffle cylinder 60. The backup safety cooling course 58 comprises at least one upward projecting or continuing wall 44 continuing outward and upward above the ground level 28 extending from or adjacent to the concrete silo 26. Wall 44 and wall 42 form an annular area which can be divided into sections by radial-like partitions extending from one wall to the other to provide several ducts or flues leading from the atmosphere down into the space 36 between the concrete silo 28 and the guard vessel 24. At least one section forming duct(s) 66 makes fluid communication with subspace 62 between the concrete silo 28 and baffle cylinder 60, and at least one section forming duct(s) 68 makes fluid communication with subspace 64 between the baffle cylinder 60 and the guard vessel 24. Thus both subspaces 62 and 64 are in communication with the outer atmosphere and are in fluid communication with each other in the area beneath the lower end of the baffle cylinder 60. A seal 70 can be provided between the upper portion of the guard vessel 24 and wall 42. Accordingly, in the event of a double breach of the reactor and containment vessels 12 and 20 resulting in substantial leakage of liquid metal coolant into spaces 32 and 34, the valves 52 and 54 in ducts 46 and 48 are closed to prevent radioactive containments escaping out into the atmosphere. Moreover, the heat of the leaked liquid metal into spaces 32 and 34 carries to space 36 and thus induces air in subspace 64 to rise upward carrying heat entrained therein and out into the atmosphere through duct(s) 68 venting the heat. This heat induced air flow from subspace 64 draws air in from the atmosphere down through duct(s) 66 into subspace 62 and then under baffle cylinder 60 and reversing direction into subspace 60 whereby a circulating cooling air flow through the backup safety cooling course 58 is perpetuated and continues as long as heat is generated and transferred to space 36 or subspace 64. Thus, this backup safety cooling course or system 58 is separate from the primary auxiliary safety cooling course 56, and the cooling air subspaces 62 and 64 and ducts 68 and 66 are not closed off when the isolation valves 52 and 54 are closed. Moreover there is no direct contact between the cooling air and the liquid metal coolant typically comprising sodium. Reactor heat is removed by this backup course 58 at all times, including normal reactor operating and auxiliary safety cooling 56 decay heat removal operating conditions. However, heat removal by the backup course 58 increases significantly when the guard vessel 24 is partially filled with hot liquid metal coolant following a postulated double vessel 12 and 20 leak event and the normal operating liquid metal coolant level 80 of the reactor vessel 12 has dropped to the double vessel leak level 82 within the reactor vessel 12. Analysis demonstrate that heat removal by this backup system will maintain maximum bulk liquid metal coolant temperatures below a design limit for precluding destructive and/or hazardous results. An embodiment of this invention comprises a design to cope with the effects of earthquakes. This design comprises a composite of the reactor vessel 12 and its enclosing containment vessel 20 being suspended from an overlying superstructure 72 which includes other seismic sensitive plant components. The vessel carrying superstructure 72 is mounted on and supported with seismically isolating means resting on a fixed structure foundation 74 comprising an upper structural portion of the earth embedded concrete silo 26. Thus the superstructure 72 carrying the reactor and containment vessels 12 and 20 can be mounted and supported on shock absorbers 76, such as springs, rubber pads, hydraulic absorbers and the like which are fixed to an upper annular surface or flange 78 extending around the earth embedded silo 26 comprising the fixed structure 74.