Patent Number: 053032748
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is concerned with provision of a satellite or stand-by passive containment cooling system for a nuclear reactor system. It can be embodied as part of an original nuclear reactor system construction, but it is primarily intended for retrofitting an existing nuclear reactor system with passive cooling capacity. More specifically, the invention is applicable to retrofitting nuclear reactor systems of the type wherein a suppression pool is confined in a separate space directly underlying the drywell space in which the nuclear reactor pressure vessel is located, this containment configuration being one in which a reactor core meltdown could result in rending of the floor separating the drywell and suppression pool space thereby creating but a single space in the containment from whence no venting of pressurized, heated non-condensables would be possible. Referring to the drawing, the nuclear reactor system 10 includes a heavy reinforced concrete containment structure 12 which has a base as at 13, a widened, generally cylindrical lower part 14 and a, e.g., tapered, conical or cylindrical upper part 16, there being a heavy cover unit 17 at the top of the structure. Containment structure 12 includes a horizontal floor 18 located proximal the juncture of parts 14 and 16, and this floor separates the containment interior into an upper or drywell space 20 and a lower or suppression pool space 22. The containment structure 12 can be located within a larger building enclosure shown generally at 24 and which serves to house other equipment and devices used in the system, this other equipment, devices, flooring etc not being depicted but being readily understood by one of ordinary skill in the art as being present in the building and employed in conjunction with operation, maintenance, fuel replenishment and like tasks. One of ordinary skill in the art will readily recognize the types of such stystems in which the invention can be employed, and also note the herein depicted system is representative of the Assignee's BWR Mark II system. A nuclear reactor pressure vessel 26 is located in the drywell space 20 of structure 12, being supported on a hollow cylindrical pedestal 28 extending up some distance from and generally formed as an integral part of the floor 18. The pressure vessel sits on a saddle part 30 at the top of the pedestal 28 and a concrete bio-shield 32 encircles the vessel. Nuclear reactor core 34 is located within the pressure vessel as shown in dashed lines. Other components of the system such as steam and condensate feed lines, and reactor core control rod assemblies are not depicted as same is not necessary for proper understanding of the invention. A plurality of vertically disposed vent pipes 36 are arrayed around the floor 18 and have entry ends at the floor which are in communication with the drywell space 20. The pipes 36 extend down through the floor and into a pool of water 38 in the space 22, lower outlet ends of these pipes locating submerged a distance below the level 40 of pool 38, there being an airspace 42 above the pool of water, gas flow communication between airspace 42 and drywell 28 being only possible via passage through the pool of water 38. In the event of a LOCA which may involve one or more of a break in a steam pipe or the pressure vessel, or a loss of coolant in the pressure vessel from other cause, there will be an immediate initial heat buildup in the drywell 28 represented by presence of highly heated steam and non-condensable gasses, chiefly nitrogen in the drywell. Due to the high steam/gas pressure, it will vent through pipes 36 into pool 38, the steam condensing, and the non-condensable gasses being cooled in the pool and venting therefrom to airspace 42. As noted earlier, recirculation of the pool water to a cooling operation outside the containment will be carried out since the buildup of heat in the pool will be rapid and of high magnitude. Recirculation of the feed water in the pressure vessel to a cooling operation as a containment heat removal agency also may be employed depending on whether or not the LOCA cause involves rupture in that vessel. Should the accident be an event that involves core meltdown with an ensuing breach in the structure of floor structure 18, the airspace 42 is opened to and becomes merged with the drywell atmosphere so there no longer exists a separate space to which non-condensable fraction of heated fluid in the drywell can be passed to effect cooling and venting of same. To offset this loss of wetwell cooling and venting in the containment structure as such, there is provided satellite heat removal means which can supplement drywell heat removal during an accident wherein the containment wetwell remains intact, but which also can assume all cooling function if core meltdown has rendered the containment wetwell cooling inoperable. This satellite heat removal means is described in detail next. Referring to the drawing Figure, a satellite building 50 is erected adjacent to large building enclosure 24, the satellite structure being either constructed as part of the original system erection, but more usually and in line with the type of system with which it is most efficaciously used, being a retrofit installation added on subsequent to system installation. Building 50 defines a structure having space enclosing an upper chamber 52 therein which is separated by a floor or slab 54 from a lower space constituting a lower chamber 56. A vent stack 58 communicates upper chamber 52 with outside or ambient environment. A pool of cooling water 60 is present in upper chamber 52, and at least one isolation condenser 62 will be submerged in pool 60. Lower chamber 56 also contains a water pool 63 which in volume should be at least about as large as the water volume capacity of suppression pool 38. Gas space 64 above pool 63 should be at least as large but more preferably about two to three times the volume of the containment wetwell space 42. An inlet to the isolation condenser 62 is connected to the containment drywell 20 by means of an inlet conduit 66, and an outlet from the isolation condenser is communicated to the containment drywell by an outlet conduit 68. One or more normally open valves 69 can be disposed in inlet conduit 66, and a condensate/non-condensable gas collector 70 can be fitted to the outlet end of the isolation condenser. The inlet and outlet conduits 66, 68 it will be noted have communication with the drywell elevated some location above floor 18 and other parts of the containment so that same are above any anticipated flood level of water as might be expected to invest the containment during a LOCA or core melt down where the suppression pool and water from the pressure vessel become one pool. It also is seen that these conduits are inclined so that the ends communicating with the containment are located at an elevation below the conduit ends connected to the isolation condenser. A vent pipe 72 connects the gas collector 70 with a submerged location in water pool 63, and a vacuum breaker 74 is fitted on the vent pipe at a location in the lower chamber gas space so it can operate to dmit gas from the gas space to the vent pipe whenever a reduced pressure condition in drywell 20 is such that it would induce a siphon effect draw of water from pool 63 to the drywell via isolation condenser 62. During an accident not involving core meltdown, the satellite heat removal means will provide supplemental cooling for the drywell. Highly heated fluid present in the drywell comprising steam and non-condensable gasses can access the isolation condenser 62 by way of inlet conduit 66, be cooled so that steam converted to water condensate will return by way of the outlet conduit 68 to the drywell. Non-condensables will be separated and collected in gas collector 70 for outlet via vent pipe 72 to the pool 63, from which the cooled gas vents to gas space 64. Cooling is by way of transfer of heat to the water in pool 60, water evaporating from the pool and this in turn venting to atmosphere by way of stack 58. Make up or replenishment for pool 60 can be provided in known manner. Where core meltdown occurs and the floor structure 18 is breached, all drywell heat removal will be accomplished with the satellite heat removal means. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.