Patent Number: 053316746
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before explaining the invention in detail, it is to be understood that the invention is not limited in its application to the detail of construction and arrangement of parts illustrated in the drawings since the invention is capable of other embodiments and of being practiced or carried out in various ways. It is also to be understood that the phraseology or terminology employed is for the purpose of description only and not of limitation. In order to enable the reader to better understand the following description of the invention, the following list provides typical water and system component elevations for a water cooled reactor and steam generator shown in FIG. 1. Values not pertinent to the explanation of the present invention are denoted not applicable, "N/A". ______________________________________ drawing feet Component designator elevation ______________________________________ refuel deck 16 2047.5 N/A mid loop level 70 2014.5 residual heat removal system RHR 90 2013.7 suction line top of fuel 94 2010.3 RHR discharge 18 1971.8 N/A to reactor (mid loop level) 70 2014.5 nozzle dam 20 2017 RHR loop 74 -- refueling level 80 2044.6 pressurizer 10 N/A steam generator 46 -- reactor 96 -- reactor coolant pump, RCP 38 N/A RHR pump 92 -- ______________________________________ Referring to FIGS. 1 and 2, nozzle dam 20 is installed over nozzle 26 of hot leg 30 of the hot side off partition 34 of bowl 40 of steam generator 46. Bowl drain 50 includes passage 54 which connects nozzle 26 with bowl 40, bypassing the hermetic seal 56 that is provided by the nozzle dam when in intimate contact with the nozzle wall 60. Drain 50 is sealed by screw-in plug 66. After the reactor coolant system "RCS" inventory water general level is lowered to mid loop level 70 of the residual heat removal system "RHR" loop 74 and of the hot and cold primary coolant inlet 30 and outlet 32 legs respectively of the steam generator bowl, nozzle dams are installed in the hot leg and cold leg nozzles, and plugs are installed in the individual bowl drains. After all nozzle dams are installed, the RCS inventory level is raised to refueling level 80 which causes a pressure head of about 27 feet of water (11.7 psig) directed upward against nozzle dam 20, and a pressure head of 43.5 feet (18.9 psig) at the lowest elevation of the cold leg, assuming that lowest elevation to be about 14 feet below mid loop in most plants. Referring additionally to FIG. 3, the nozzle dam inflatable seals 82 are each maintained at approximately 65 psig. Annulus 84 between the inflatable seals is maintained at 5 psig and monitored in order to learn of leakage in the seal system. Slow increase has been widely observed in annulus pressure. This may indicate that a very small amount of air can escape from the inflatable seals either by osmosis or from minute leaks. It is now believed by the present inventors that this air can also leak to the nozzle side of the seal. Pressurized seals presently seem to be the most efficient and safest way to seal a nuclear reactor steam generator nozzle. It is also now believed by the inventors that prolonged installation of a nozzle dam having an inflatable seal could allow enough osmotic or small leakage of air from the 65 psig source to displace the 11.7 to 18.9 psig water occupying the volume of the cold leg below the cold leg nozzle elevation. After having displaced the water, continuing air bubbles out toward RCS inventory storage. The nozzle dams remain in place while maintenance is completed on the reactor system, sometimes for as long as 30 days, then the water level is dropped from refueling level 80 to mid loop 70 in order to remove the nozzle dams. This reduces the pressure on the trapped column of air in the cold leg to a head of 14 ft. The trapped air exerts a pressure of approximately 6.1 psig totaling 7,662 pounds against the underside of the 40" diameter nozzle dam. Under this pressure, upward bounding can occur, driven by the trapped air as soon as a movable portion of a nozzle dam is unbolted. In a nozzle dam system relying solely upon inflatable seals, rather a combination passive and active inflatable system as in the BUSI Nozzle Dam, the trapped air is released around the dam when the seals are deflated, and bounding does not occur. Nevertheless loss of RCS inventory level can still occur. For steam generators with individual drain lines, bounding can be reduced by removing the drain plug after the reactor coolant system water general level is moved to mid loop level, and waiting for some period of time before removing the nozzle dam. Nevertheless loss of RCS inventory level can still occur. In steam generators with a common drain instead of individual drain lines, there is no access to the nozzle region just below the nozzle dam. The cause for loss of RCS inventory level is considered to be as follows. The volume of the trapped air is conservatively calculated to be 15.75 inches radius squared (or 248.06 square inches), times 3.14 (Pi), divided by 144, times 16.5 feet vertical height=89.25 cu. ft. or 669 gallons. When the cold leg nozzle dam is removed, the trapped air escapes and water rushes upward to equalize the reactor coolant system water general level, the RCS inventory could suddenly decrease by 669 gallons. Assuming 1191 gallons per vertical foot of RCS inventory, this event could result in a sudden decrease in RCS inventory level by about 7.2 inches. The residual heat removal system RHR suction line 90 is only 9.6 inches below mid loop 70. Taking in the above conservative calculation plus bends in cold legs, increase in volume near nozzles and water sloshing effects, it is conceivable that a single occurrence of this event can cause the localized inventory level close to the RHR suction line to fall, causing a risk of vortexing/cavitation failure of RHR pump, and subsequent loss of effectiveness of the RHR. Furthermore, unless RCS inventory is recovered after removal of the first cold leg dam of a system, further substantial risk exists when a second cold leg dam is removed, assuming that the second cold leg nozzle dam experiences a similar occurrence. If four cold legs in a system experience the same occurrence without water level recovery, the water level would drop to about 21 inches from the top 94 of the fuel in reactor 96. In order to avoid the above problem associated with use of nozzle dams with inflatable seals, it is advisable to bleed the trapped air from the cold leg before the RCS inventory drain down process is completed, so that the water level in the cold leg seeks the RCS inventory water general level. This process step can be accomplished preferably by passing the air from the nozzle leg into the bowl by way of a valve and passage through the nozzle dam. Another way is by passing the air from the nozzle leg into the bowl by way of a valve connected to passage 54 in place of the presently used plug. Referring to FIG. 4, valve assembly 100 passes through passage 108 of nozzle dam 20 body wall 102. If the nozzle dam is a BUSI Nozzle Dam, the valve is preferably mounted in the center section of the dam's three sections. Flared end 106 of fitting 104 sealingly engages rubber diaphragm 110 which spans the three sections. At the other end of fitting 104 is pipe means such as 1/4 inch reinforced hose 112 securely attached to the fitting. Adaptor 114 permits connection of hose 112 before the water level is to be lowered. Connector section 116 seals fitting 104 until hose 112 is attached for use of the fitting. Control valve 118, connected to passage 108 by way of hose 112 and fitting 104 is preferably located outside the generator bowl so that it can be easily controlled by an operator. Water-stop gas-conducting valve 130 releases the trapped air which flows via passage 108 from the nozzle in the region immediately below the nozzle dam, remaining open until it encounters water which it blocks. This valve design may be taken from ones presently used to automatically bleed air from water circulator systems. A valve assembly 118 similar to valve assembly 100 may be tapped into passage 54 by using a suitably modified fitting as shown in FIG. 5. In operation, valve 18 is turned on or opened shortly before or during lowering of the water level toward the mid loop level, and air is permitted to flow from the pipe until water reaches the valve. It is then known that the compressed air is removed, and valve 118 is turned off or closed. Automatic valve 130 can perform the same function. The hose is preferably positioned for directing the air out of the bowl. It may be for example passed out of the bowl through a manway, or connected to a bowl common drain. The hose may also be extended to a height that is higher than the level of the RCS water general level at which the air bleed is taking place. Visual or sensor indication may then be had from the raised hose to determine when the hydrostatic level of the water under the dam is equal with that of the RCS water general level. When the trapped air is removed, and the water general level is brought to mid-loop below the height of the nozzle dam, the dam may then be removed. Although the invention has been described in terms of specific preferred embodiments, it will be obvious to one skilled in the art that various modifications and substitutions are contemplated by the invention disclosed herein and that all such modifications and substitutions are included within the scope of the invention as defined in the appended claims.