Patent Number: 056446080
Section: description

DETAILED DESCRIPTION OF THE INVENTION Although the principles of the invention are applicable to other cooling systems, the invention will be described in connection with a known type of spent fuel and reactor component cooling system. Such a system is illustrated in the accompanying drawings along with the added apparatus of the invention. As shown in FIG. 1, the known system comprises a spent fuel pool 1 filled with water 2 which can contain additives of a known type. Spent radioactive fuel assemblies 3 which have been removed from a reactor, not shown, are immersed in the pool of water 2. The water 2 must be kept at a temperature well below its boiling temperature, and the water 2 is cooled by pumping it out of the pool 1 by means of a pump 4 and sending it through pipes and valves 5 and 6 to a water conduit assembly 7, which can be a plurality of tubes, from which it returns to the pool 1 by way of a pipe 8. In a conventional cooling system, the assembly 7 through which the water 2 from the spent fuel pool is circulated is contained in a water-tight housing 9 of a heat exchanger 10 which receives and returns cooling water from and to another heat exchanger 11 of known construction which forms part of a cooling system for reactor components, e.g. reactor coolant pumps. Cooling water from a suitable source, e.g. a river, is supplied by a line 12 to the heat exchanger 11 and then dumped back to the source via line 11a. In the arrangement of the invention, the water from the spent fuel pool can be passed through the conventional heat exchanger 9, or the conventional heat exchanger can be bypassed, or the conventional heat exchanger and the heat exchanger of the invention can both be used. As examples of the spent fuel pool water cooling which can be required, it can be necessary to remove 13 million BTU/hour, 131 hours after a third of the reactor fuel assemblies are immersed in the water 2 and 12 million BTU/hour, 174 hours after such immersion of the assemblies. In a full core discharge case, i.e. when all of the reactor fuel assemblies are immersed in the water 2, the heat removal rate can be as much as 26 million BTU/hour, 360 hours after immersion of the assemblies in the water 2. When the reactor is shut down for maintenance and refueling, all of the assemblies are transferred from the reactor to the spent fuel pool. The procedure can take about twelve hours, which provides only a relatively short time for the maintenance or repair of the service water/component cooling system. Also, if there is a failure of the component cooling system 11 or the supply of water by way of the line 12, the reactor operation must be discontinued, and there would be a loss of spent fuel pool water cooling. In accordance with the invention, these problems can be overcome by the addition of the apparatus described hereinafter without a substantial modification of the known system, and with equipment of relatively small size and cost as compared to the size and cost of a conventional water-water heat exchanger such as the heat exchanger 10. The heat exchanger which is added in accordance with the present invention also is more reliable than a water-water heat exchanger. In the preferred embodiment of the invention, the added apparatus comprises a pump 13, valves 14, 15, 16 and 17, a heat exchanger 18 which uses air and water spray for the coolant and the interconnecting pipes shown in the drawing. The heat exchanger unit 18 comprises a spray water conduit W, a duct 22, one bank 20 of water spray heads or nozzles 20a and a fan 25. Air is supplied to the duct 22 from any convenient source, which can be the known spent fuel ventilation system normally used in the known cooling system, and water is supplied through the conduit W to the nozzles or spray heads 20a of the bank 20 from any convenient source thereof, e.g. public water mains, but preferably, the water is supplied thereto from a storage tank so that the spray water is always available and is independent of other sources which can more readily fail. When valves 14 and 17 are open and valves 5 and 6 are closed, water 2 of the spent fuel pool is circulated by the pump 13 and is returned to the pool 1 by way of the interconnecting pipe lines 30, 31, 32, as shown by the arrows in FIG. 1. As the water 2 passes through the heat exchange unit 18, a stream of air driven by the fan 25 impinges on the heat exchange surfaces, and water is sprayed into the flow of air and onto the unit 18 from the spray head nozzles 20a of bank 20 to thereby remove heat from the water 2. With the temperature of the water 2 at 150.degree. F., with the temperature of the ambient air entering the duct 22 at 75.degree. F., the heat exchanger 18 can remove heat from the water 2 in an amount equal to about 22 million BTU/hour. For this result, the operating conditions of the heat exchanger are as follows: ______________________________________ Air flow rate 72,000 cfm through duct 22 Total water flow 2,250 g./min. through unit 18 Air flow area 180 sq.ft. Air flow velocity 900 ft./min. Spray water flow 120 g./min. ______________________________________ About one-third of the water sprayed into the air stream is being evaporated to achieve the above result. If a larger fraction of the sprayed water is evaporated, the cooling effect will be enhanced. If the temperature of the air entering the duct 22 is lower, the amount of heat removed under the same conditions is greater. Let it be assumed that the heat exchanger 10 for the spent fuel water 2 is not available for cooling the water 2 or that the component cooling heat exchanger 11 or cooling water supplied from a river or other source by the line 12 is not available (first case), and that the heat removal requirements are 22 million BTU/hour. With the heat exchanger 18 and the operating conditions thereof described hereinbefore, the added equipment of the invention can assume the entire cooling load under the following conditions: ______________________________________ Component Condition ______________________________________ Pump 13 Operating Valve 14 Open Valve 5 Closed Valve 16 Closed Valve 17 Open Valve 6 Closed Valve 15 Open or closed ______________________________________ Let it be assumed that the heat exchanger 10 is operative but that supplemental cooling is required (second case), such as in the full discharge case previously described. With the heat exchanger 18 and the described operating conditions thereof, the added equipment can provide supplemental cooling with the components in the following conditions: ______________________________________ Component Condition ______________________________________ Pumps 4 and 13 Operating Valve 14 Open Valve 5 Open Valve 16 Closed Valve 17 Open Valve 6 Open Valve 15 Closed ______________________________________ Let it be assumed that the supply of water by the line 12 is lost and that it is desired to continue cooling of the water 2 and the reactor components, e.g. pump seals, etc. (third case). With the heat exchanger 18 and the described operating conditions thereof, the added equipment of the invention can provide such cooling with the components set as follows: ______________________________________ Component Condition ______________________________________ Pump 13 Operating Valve 14 Open Valve 5 Closed Valve 16 Open Valve 17 Closed Valve 6 Open Valve 15 Open or closed ______________________________________ Although not preferred, the added apparatus can be simplified by the elimination of the pump 13 and the valve 15, the valve 14 being connected directly to the pump 4 and the valve 5. For that modified apparatus, in the first case assumed hereinbefore, the heat exchanger 18 can assume the entire cooling load with the components set as follows: ______________________________________ Component Condition ______________________________________ Pump 4 Operating Valve 14 Open Valve 5 Closed Valve 16 Open or closed Valve 17 Open Valve 6 Closed ______________________________________ In the second case assumed hereinbefore, the modified apparatus can supply supplemental cooling with the components set as follows: ______________________________________ Component Condition ______________________________________ Pump 4 Operating Valve 14 Open Valve 5 Open Valve 16 Open or closed Valve 17 Open Valve 6 Open ______________________________________ In the third case assumed hereinbefore, the modified apparatus can continue cooling of the water 2 and the reactor components with the components set as follows: ______________________________________ Component Condition ______________________________________ Pump 4 Operating Valve 14 Open Valve 5 Closed Valve 16 Open Valve 17 Closed Valve 6 Open ______________________________________ The heat exchanger unit 18, which uses air entraining a sprayed mist of water as the coolant medium, is shown to be of known cross-flow plate-fin construction as illustrated in FIGS. 2 and 3, wherein the liquid to be cooled flows in channels between pairs of parallel sheets (the water side) while the coolant medium comprising air carrying a mist of fine droplets flows in channels arranged alternately with the water channels between the parallel sheets (the air side). That is, flows of mist-carrying air and of water being cooled flow past opposite sides of the parallel sheets for indirect heat exchange through the sheets. By cross-flow, it is meant that the flows of water and air are essentially directed at right angles to each other in a well known mode of operation, illustrated for example in FIG. 9-3 of Kays and London, Compact Heat Exchangers, second edition, 1954 and in the accompanying FIG. 3. Strip-fins can be employed only on the side of the heat exchange surfaces over which air carrying the water spray passes, or on both sides of the heat exchange surfaces. The strip-fins are preferably formed of copper. When strip-fins are employed on both sides of the heat exchange surfaces, a lower water spray rate can be used to produce a given cooling rate. Although the heat exchange unit of the type described is illustrated in FIG. 1, in many applications more than one heat exchange unit can provide the higher cooling rates as shown in FIG. 4. The number of heat exchange units to be arranged in tandem can be determined in accordance with the cooling capacity requirements of any given application. In certain cases, the first of a plurality of heat exchange units can be operated without any water spray, in which case, a downstream unit (or units) that is sprayed with water droplets, is more effective. Such an arrangement is shown in FIG. 4. FIG. 4 shows an alternate form of sprayed water heat exchanger according to the invention. In the embodiment of FIG. 4 three heat exchangers in tandem are employed, rather than the single heat exchanger unit 18 shown in FIG. 1. Each of the heat exchanger units 19, 20 and 21 can have the same structure as the single unit 18 that constitutes the heat exchanger of FIG. 1, and therefore the units 19, 20 and 21 are not described in detail. The heat exchanger units 20 and 21 are shown in FIG. 4 to have their own, individually operable banks 23 and 24 of water headers and respective spray nozzles 23a, 24a. The arrangement of FIG. 4 can provide greater cooling and more flexibility than that of FIG. 1. Experimental tests have been performed to compare the performance of similarly dimensioned tube-fin and plate-fin heat exchangers, both provided with means for spraying finely atomized droplets of water into the flow of coolant air through the heat exchangers. The results for the tube-fin heat exchanger demonstrated that the cooling capacity could be increased by a factor of four times that of the same heat exchanger with no water spray. Test results for the plate-fin heat exchanger demonstrated that the cooling capacity of the exchanger with sprayed water droplets exceeded by a factor of eight times that of the same heat exchanger with no water spray. The test heat exchangers represented approximately one-thirtieth of full scale air side frontal area of the heat exchanger which would actually be employed in an alternate spent fuel cooling system for a nuclear power generating plant, but the results are believed to demonstrate that either tube-fin or plate-fin air cooling heat exchangers augmented by spraying atomized water droplets having a mean diameter of 250 microns or less into the cooling air can be used for the purpose of the invention. The water spray can be produced by use of suitable high inlet pressure, hydraulic or air atomizing spray nozzles such as those available from suppliers such as Spraying Systems Co. of Wheaton, Ill. The nozzles can be arranged on headers that are opened and closed by solenoid valves for control of the amount of water sprayed. That is, one or more of a plurality of spray headers can be activated to provide the desired spray flow. Preferably the nozzles have built-in strainers. When the air flow through the heat exchanger is directed horizontally, the nozzles are arranged to spray water droplets concurrently with the air flow while the flow of water to be cooled on the water side of the plates is in a direction perpendicular to the direction of the flow of cooling air. Other arrangements will suggest themselves to those acquainted with the art of heat transfer.