Patent Number: 052710513
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to pressurized water reactor nuclear power plants, and more particularly to systems for cooling and purifying coolant in the spent fuel pit, refueling cavity and refueling water storage tank, and for transferring coolant between these components. 2. Background Information In pressurized water reactor nuclear power plants, coolant in the form of light water is passed through fuel assemblies in a reactor core housed in a reactor vessel. The coolant is circulated in a primary loop between the reactor core and a steam generator which utilizes the heat removed from the reactor core by the coolant in the primary loop to generate steam in a secondary loop which drives a turbine generator to produce electricity. Periodically, the reactor core must be refueled. This is accomplished by flooding a refueling cavity over the reactor vessel with coolant from a refueling water storage tank. The fuel assemblies to be replaced are lifted from the reactor core into the refueling cavity and, while remaining immersed in coolant, are conveyed through a transfer tube and a fuel transfer canal to a spent fuel pit outside containment for on-site storage. A spent fuel pit cooling system removes decay heat released by the stored fuel assemblies. The cooling system pumps coolant from the spent fuel pit through heat exchangers which dump the decay heat to a component cooling water system. The current spent fuel pit cooling systems are required by regulation to be safety grade and that includes energization of the cooling system pumps by a safety grade bus. In order to meet these requirements, two completely separate spent fuel pit cooling systems are provided. During refueling operations, the operator manipulates a refueling machine by visual observation of the fuel assemblies through the coolant. The operators ability to do so is dependent on the clarity of the coolant. Presently, the refueling cavity is flooded with coolant from the refueling water storage tank through the reactor core. Debris and sediment dislodged from the core leads to poor clarity of the coolant in the refueling cavity. Often auxiliary purification equipment in the form of filters, dimineralizers and skimmers must be brought in to improve coolant clarity during refueling. Clarity can also be a problem in the spent fuel pit. Presently separate purification systems, independent of the spent fuel pit cooling system, are used to improve the clarity of the coolant in the spent fuel pit. Separate skimmers are also used to remove foreign matter from the surface of the spent fuel pit. All of these separate, and in some cases redundant, cooling and purification systems require a great deal of equipment including six or seven pumps some of which must be safety grade. SUMMARY OF THE INVENTION It is the primary object of the invention to provide improved apparatus for cooling and purifying coolant in the spent fuel pit, refueling cavity, and refueling water storage tank in a pressurized water reactor nuclear power plant. It is a further object of the invention to provide such apparatus which does not have to be qualified as safety grade. It is also an object of the invention to reduce the need for purification of coolant in the refueling cavity. It is a more particular object of the invention to reduce the need for purification of refueling cavity coolant by transferring coolant from the refueling water storage tank to the refueling cavity without it passing through the reactor core. It is an additional object of the invention to reduce the total equipment needed to perform all the required cooling and purification functions. These and other objects are realized by the invention which is directed to improvements in a nuclear reactor which include a spent fuel pit having a capacity to absorb decay heat from a specified number of fuel assemblies stored in the pit for the time interval required by regulation without the need for electric power so that the spent fuel pit cooling system does not have to be safety grade. As another aspect of the invention, non-safety rated spent fuel pit cooling means is connected in series with the purification means to form a combined cooling and purification system having a common non-safety rated pump. This combined cooling and purification system simultaneously cools and purifies spent fuel pit coolant in a single loop. Preferably, the single loop has two branches, each with cooling means such as a heat exchanger and purification means such as a demineralizer and filters and its own non-safety rated pump. Coolant is drawn from the spent fuel pit by each branch of the combined cooling and purification system through common suction piping and is discharged back into the spent fuel pit through common discharge piping. The skimmer means is connected to the common suction piping, thereby eliminating the need for a separate skimmer circuit. The invention further includes cooling and purifying coolant in the refueling cavity by circulating it through the combined cooling and purification system. This eliminates the need to bring in additional cooling and purification equipment during refueling. As another aspect of the invention, the combined cooling and purification system is used to transfer coolant between the refueling water storage tank and the refueling cavity without the coolant passing through the reactor core. This improves the clarity of the coolant in the refueling cavity as the debris and sediment in the core is not flushed into the refueling cavity as is currently the case. The clarity of the water in the refueling cavity can be further improved, by prior to refueling, circulating the refueling water storage tank coolant through the combined cooling and purification system. The invention is directed to both the apparatus and method related to the above improvements in a pressurized water reactor nuclear power plant.