Patent Number: 054066026
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic concepts of the invention, including a silo liner tank, a closable RVACS and backup air cooling of the leaked sodium, are shown in FIG. 3. The invention is utilized in conjunction with a conventional RVACS as described with reference to FIG. 1. In addition, however, RVACS inlet isolation valves 31 are provided in the four main RVACS air inlets 6 (only two of which are shown) and RVACS outlet isolation valves 32 are provided in the four RVACS air outlets 9 (only two of which are shown). The RVACS isolation valves 31 and 32 can be closed if leaks should develop in the reactor vessel 1 and containment vessels 2 that result in sodium entering the reactor cavity 14. Closing of the isolation valves 31 and 32 shuts off the oxygen supply to the reactor cavity 14, thereby suffocating any sodium fires that might have started initially when sodium entered the cavity. A bottom-supported, insulated silo liner tank 13 is provided to collect any sodium leaked and to prevent direct contact between sodium and the concrete of the reactor silo 8, thus preventing exothermic chemical reactions and maintaining the structural integrity of the reactor silo during the postulated double vessel leak event. As illustrated in FIGS. 4 and 5, the lower portion of the silo liner tank 13 consists of an inner steel liner 20 and an outer steel liner 21 containing a nonreactive granular insulating material 22 such as BeO. A multiplicity of split heat transfer tubes 23, of square cross section and containing tube divider plates 24, are attached to an annular tube plate 25 located at an elevation above the sodium leak level 11. Tube plate 25 is attached to the inner steel liner 20 and also to the outer steel liner 21. Above the tube plate 25, the inner steel liner 20 extends upwards and has a reduced diameter, as shown in FIG. 4. The multiple tube divider plates 24 are joined together to form a continuous cylinder referred to as the tube divider plate extension 26 above the elevation of the tube plate. A sliding seal 19 is provided at the top edge of the silo liner tank inner liner 20 where it meets the reactor facility base 16 to completely seal off the normal RVACS flow path from the outside atmosphere when the RVACS air supply isolation valves 31 and 32 are closed. Radiation shielding 27 is attached to the outer surface of the upper narrow-diameter portion of the silo liner tank inner liner 20 to limit radiation exposure in the seismic isolator gallery 28 between the base mat 17 and facility base 16. In accordance with the concept of the invention, decay heat removal is provided during a postulated double vessel leak event by a second backup RVACS consisting of multiple split heat transfer tubes 23 submerged in the hot, leaked sodium, as shown in FIGS. 3 and 4. Air for the backup RVACS enters the seismic isolation gap 35 via several backup air openings 34 and enters the cold air downflow path 36 via the seismic isolator gallery 28. The annular cold air downflow path 36 is formed by the space between the tube divider plate extension 26 and the reactor silo 8 above the elevation of the tube plate. At the tube plate elevation the cold air enters the outer halves of the split tubes 23 and flows downward as it is heated by the hot, leaked sodium flowing freely around the outside perimeter of the split tubes. The air reverses at the bottom of the split tubes and enters the hot air upflow path consisting of the inner halves of the split heat transfer tubes 23 below the tube plate elevation. Above the tube plate elevation, the air stream--now heated by the hot steel walls of the heat transfer tubes 23 which are submerged in hot leaked sodium--enters the hot air upflow path 37 formed by the upper narrow-diameter portion of inner steel liner 20 and the tube divider plate extension 26. The hot air upflow path 37 discharges into the seismic isolator gallery 28 and flows upwards in the seismic isolation gap 35. The hot air is discharged to the atmosphere at grade level through backup air openings 34. Simultaneous inflow of cold air and upflow of hot air in the seismic isolation gap 35 is possible because the annular gap extends completely around the reactor facility base 16. The huge flow area allows downflow and upflow zones to be established. Thus, the backup RVACS air flow path is separate from the normal RVACS air supply and discharge paths and is not closed off when the RVACS air supply isolation valves 31 and 32 are closed. In addition, there can be no direct contact between air and sodium and between sodium and concrete. The insulated double wall silo liner tank 13 prevents heat-up of the concrete silo 8 and is a durable containment for the leaked sodium via the use of a nonreactive granular insulation 22 (see FIG. 4) which protects the outer wall 21 of the silo liner tank 13. Reactor heat is removed by the backup RVACS at all times, including normal reactor operating conditions and RVACS decay heat removal operating conditions. However, heat removal by the backup RVACS increases significantly when the silo liner tank is partially filled with sodium following a postulated double vessel leak event and the hot sodium surrounds the split heat transfer tubes 23 to the double vessel leak level 11 (see FIGS. 3 and 4). Analyses have demonstrated that heat removal by the backup RVACS will maintain maximum bulk sodium temperatures below the design limit. However, limited fuel cladding failures are expected during this postulated event. Thus, the basic concepts of the invention are that an independent, backup passive shutdown heat removal cooling system is used in conjunction with the conventional RVACS and the capability is provided to isolate the conventional RVACS. These concepts have been illustrated by disclosure of the foregoing preferred embodiment. However, it is understood that these novel containment concepts are subject to change following tradeoff and detailed thermal performance evaluations without departing from the spirit and scope of the invention. Also, routine variations and modifications of the disclosed apparatus will be readily apparent to practitioners skilled in the art of passive air cooling systems for liquid metal-cooled nuclear reactors. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.