Patent Number: 056446080
Section: summary

This invention relates to the cooling of spent fuel assemblies used in nuclear power plants. BACKGROUND OF THE INVENTION In nuclear power plants, it is customary to provide a pool of water for the purpose of cooling spent fuel assemblies, which are immersed in the pool of water. It is necessary to circulate and cool the water of the pool, and in most installations, a water-water heat exchanger is used to cool both the water from the spent fuel pool and the water that is used to cool components of the reactor. The component cooling water is circulated through the water-water heat exchanger through which the water from the pool is also circulated. Such water-water heal exchangers are relatively large and expensive, and the ultimate cooling therefor is obtained from a service water system, such as a river, lake or other source. If there is a loss of supply of water from the service water system, or if the water-water heal exchanger becomes inoperative, the spent fuel cooling system is inoperative. Also, with the present systems, the time allotted for repair of the component cooling system and the service water system is usually limited to the reactor refueling time during which all of the fuel assemblies from the reactor core are also in the spent fuel pool. Such time is on the order of twelve hours and can be inadequate. There exists a need to provide a redundant cooling system in order to permit repair of the main cooling system or to act as a standby system. To provide a redundant water-water heat exchanger to handle the cooling load in the event of such a failure or problem or during refueling is considered to be prohibitive not only because of cost and space limitations, but also because a redundant water-water system would not provide protection against loss of service water or component cooling water systems. The use of a conventional air cooled heat exchanger as a redundant cooler is also impractical because such an exchanger is relatively inefficient, and to provide the same cooling capacity as the water-water heat exchanger would require a large installation. It is known in the art that heat exchangers which employ pipe coils through which the liquid to be cooled is to circulate and which are subjected to a flow of air, such as ambient air, into which a spray of water is directed, are more efficient in cooling than heat exchangers which use only ambient air without a water spray for cooling. Compact plate-fin heat exchangers have been used, for example in aircraft. Such heat exchangers provide greater heat transfer surface per unit volume by the use of fins of very small cross section brazed or otherwise attached in good thermal contact with the primary heat transfer surface. The two principal arrangements employing extended surfaces are the plate-fin exchanger, which has no pipe coils, and the tube-fin exchanger. The primary heat transfer surface of the plate-fin design consists of multiple parallel plates connected by fins; the space between each pair of plates comprises a fluid passage. Alternate fluid passages are connected in parallel by suitable headers to form the two "sides" of the heat exchanger. In the strip-fin type of plate-fin exchanger, metal strips arranged either staggered or in-line serve as the fins. Tube-fin heat exchangers have fins on only one side of the primary surface, and tubes (either round or flattened) placed through holes in thin metal plates and to which the tubes are brazed. The preferred heat exchanger of the present invention has fins on both sides of the heat transfer surface and employs no tubes. U.S. Pat. No. 4,969,507 describes a falling film air-cooled surface condenser in which droplets of coolant liquid are detached from the falling film of water and entrained by a flow of coolant air, enhancing heat exchange. It has now been found that a heat exchanger of reasonable size and cost and which uses, as the coolant, ambient air into which a spray of water is directed concurrently with the flow of cooling air, can be used not only to substitute for the conventional water-water heat exchanger in the event the latter becomes inoperative, but also to provide component and spent fuel cooling in the event of failure of the water supply to the component cooling system, and hence, to the spent fuel water-water heat exchanger. BRIEF SUMMARY OF THE INVENTION In accordance with the invention, a plate-fin or tube-fin heat exchanger through which the water to be cooled is circulated and which employs a flow of air such as ambient air into which water from an auxiliary source, such as a storage tank, public water mains, river or lake, is sprayed, can, by means of appropriate valves and pipes, be employed, at least temporarily, as a substitute for the conventional water-water heat exchanger, as a supplemental cooler or as a heat exchanger for component cooling water and the spent fuel pool water in the event that the normal supply of water for such latter purposes fails. Accordingly, a heat exchanger or heat exchanger assembly with a set of individual plate-fin or tube-fin cooling surfaces, employing as the coolant an air-water spray and having a cooling capacity at least equal to the cooling capacity of the conventional water-water heat exchanger, is connected by valves and pipes to the conventional system so that: (1) The water-water heat exchanger is isolated from the spent fuel pool cooling loop and cooling is done by the air-water spray coolant heat exchanger; or (2) The air-water spray coolant heat exchanger is connected and operated in parallel with the water-water heat exchanger to provide supplemental cooling, in such situations as when there is a discharge of the reactor core or when the service water temperature is abnormally high; or (3) The air-water spray coolant heat exchanger provides cooling of the spent fuel pool water and the component cooling water system which cools the reactor components in such situations as when there is a failure of service water supply. The preferred air-water spray coolant heat exchanger of the invention achieves a significantly higher effectiveness than heat exchangers previously known. The term effectiveness as used in this description means the ratio of the actual temperature decrease of the fluid being cooled to the maximum theoretical temperature decrease which can be achieved with a coolant medium of a given temperature. To accomplish this effectiveness it is necessary to employ spray nozzles which produce very finely "atomized" water droplets which form a mist uniformly distributed within the heat exchanger. The effectiveness of the cooling can be greatly increased by using a mist of fine water droplets entrained in cooling air passed through the heat exchanger. Effectiveness can be increased as much as eight times as compared to cooling with air as the sole cooling medium. The mean diameter of the individual water droplets should be no greater than 240 microns, preferably less than 100 microns. The particularly preferred average water droplet diameter is about 50 microns. The spray of water is not directed through any slots or openings. The water spray in the form of very fine mist is injected into the air stream entering the heat exchanger as a result of the suction produced by a fan. The spray droplets are driven by small scale turbulence within the heat exchanger to collect on the fin and plate surfaces, providing a thin film of liquid which evaporates, transferring heat. When the temperature of the air stream as it enters the heat exchanger goes down slightly as a result of the presence of the water mist, the evaporative heat transfer from the air to the mist droplets significantly enhances the overall heat transfer from the water to be cooled by the heat exchanger. The heat exchanger can be of either the tube-fin or the plate-fin type. Preferably, the plate-fin type of heat exchange surface with either strip fins or louvered fins is used. Such surfaces are described in the reference textbook Compact Heat Exchangers by Kays and London, McGraw Hill, second edition, 1954 in Chapter 9 and illustrated in FIGS. 9-3, 9-5 and 9-6 of that text, which sections and figures are herein incorporated by reference.