Patent Number: 054917310
Section: summary

Field of the Invention The present invention relates generally to a method and system for maintaining pressure in a nuclear power plant primary loop having a pressurizer vessel operatively attached to it for maintaining pressure in the primary loop and, more particularly, to such a method and system which automatically inject nitrogen into and vent nitrogen from the pressurizer vessel for maintaining the pressure in the primary loop during startup and shutdown which, in turn, eliminates temperature gradients between the pressurizer vessel and the primary loop or laborious methods caused by presently known and utilized methods and apparatuses. Description of the Related Art A typical nuclear power facility includes a nuclear reactor wherein a controlled nuclear reaction, which generates heat, is occurring. Typically, borated water is contained in the reactor for controlling the nuclear reaction process and for passing the generated heat away from the reactor. A primary loop communicating with the reactor functions to pass the borated water (i.e., the heat) away from the reactor and to transfer the heat to a secondary loop. A reactor coolant pump is attached to the primary loop for pumping the borated water through the primary loop. The primary loop then returns the borated water back into the reactor where the above described process is repeated. The secondary loop is isolated from the primary loop and generates steam from the heat passed from the primary loop. The steam of the secondary loop is used to produce electricity as is well known in the art. A pressurizer vessel is connected to the primary loop for maintaining a constant pressure in the primary loop. The pressurizer vessel includes a protective shell forming an interior portion for containing any water and steam therein. The protective shell includes a cylindrical shaped side terminating at a hemispherical shaped head at both its top and bottom end, with the bottom end attached to a cylindrical support skirt. A flange extends radially from the skirt bottom for attaching it to its support structure, typically a floor. A nozzle at the bottom of the lower hemisphere connects to piping which attaches to the primary loop for allowing the primary loop and the pressurizer vessel to pass the borated water therebetween which, in turn, functions to maintain proper pressurization of the primary loop. A heater support plate is located in a lower portion of the shell interior for receiving a plurality of electrical heaters which, during plant operation, are turned on to further heat the water or to maintain the water at a constant temperature. A spray nozzle is positioned at an upper portion of the shell interior for spraying water in the shell interior which condenses the steam back to water. During operation of the power plant, a transient event that could decrease system pressure, for example, is counteracted by increasing the water temperature via the electrical heaters which, in turn, causes a portion of the water to flash to steam. An increasing pressure transient is limited by spraying cooler water from the primary loop via the spray nozzle into the shell interior which, in turn, causes a portion of the steam to condense to water. During startup and shutdown of the nuclear power generating process, there are presently two known and utilized methods and systems for maintaining the pressure in the primary loop, which is necessary to allow the reactor coolant pumps to properly operate. Using the first method and system during startup, the heaters are energized for heating the borated water in the pressurizer which, in turn, creates the necessary pressure in the primary loop for allowing the reactor coolant pumps to be operated. The reactor coolant pumps are started in conjunction with the heat production process of the heaters for thoroughly mixing the borated water contained in the reactor vessel and the primary loop. Obviously, mixing is necessary for maintaining all of the borated water at a constant temperature for eliminating temperature gradients. Once the primary loop is pressurized, the reactor vessel begins its heat producing process for initiating the power production process, as described hereinabove. The reactor vessel then brings the borated water up to its operating temperature, and the normal operational procedure is started. Using first method and system during shutdown, the reverse procedure for startup is used. Obviously, the normal operating procedure is in process, and to initiate shutdown of power generating process, the reactor vessel gradually ceases its heat production process. However, to dissipate the heat of the borated water, the primary loop continues to circulate through the primary loop and, more particularly, to the steam generator for passing and dissipating the heat of the primary coolant to the secondary loop. This circulation requires the reactor coolant pumps to be in operation which, in turn, requires pressurization of the primary loop. This pressurization is maintained by energizing the pressurizer heaters for heating the borated water therein. After the primary heat of the primary loop is substantially dissipated, the reactor coolant pumps are turned off which, in turn, obviates the need for pressurization of the primary loop. Although the presently known and utilized method and device for starting up and shutting down the power generating process are satisfactory, they are not without drawbacks. One drawback is that a temperature gradient exists between the borated water in the pressurizer (i.e., created by the pressurizer heaters for pressurization of the primary loop) and the borated water in the primary loop. This, in turn, causes the pressurizer and the primary loop to have a temperature gradient between them, which causes thermal loads and stresses on these components and their interconnections. To eliminate this temperature gradient, a second known and utilized device eliminates the need for pressurization via the pressurizer heaters and, instead, creates pressurization of the primary loop by inserting and withdrawing nitrogen from the pressurizer vessel. This method and system, however, is performed by manually manipulating valves and the like. Although this manual method and system is satisfactory, it is not without drawbacks. Manual operation of the valves and the like requires that maintenance personnel be within the containment building which exposes them to low levels of radiation. Consequently, a need exists for an improved method and system for maintaining pressurization of the primary loop during startup and shutdown. SUMMARY OF THE INVENTION The present invention provides an improvement designed to satisfy the aforementioned needs. Particularly, the present invention is directed to an automated method for maintaining pressure within a nuclear power plant primary loop. The method comprises the steps of: (a) partially filling a portion of a pressurizer vessel, in fluid communication with the primary loop, with a liquid for maintaining pressure in the primary loop; (b) circulating a primary coolant through the primary loop; (c) automatically inserting an inert gas by a first automated device, operatively connected to the pressurizer, into the pressurizer vessel when the pressure in the pressurizer vessel is less than a first predetermined pressure; and (d) automatically venting the gas by a second automated device, operatively connected to the pressurizer, from the pressurizer vessel when the pressure in the pressurizer vessel is greater than a second predetermined pressure. In another broad forms the invention resides in an automated system for maintaining pressure within a nuclear power plant primary loop. The apparatus comprises (a) a pressurizer vessel operatively connected to the primary loop for maintaining pressure in the primary loop; (b) a first valve in pneumatic communication with said pressurizer for sensing the pressure in said pressurizer and for supplying an inert gas into said pressurizer vessel when the pressure sensed by said first valve is less than a predetermined pressure; and (c) a second valve in pneumatic communication with said pressurizer for sensing the pressure in the pressurizer and for venting the inert gas from said pressurizer vessel when the pressure sensed by said second valve is greater than a predetermined pressure. It is an object of the present invention to provide an improved method and device for maintaining pressure in a nuclear power plant primary loop during startup or shutdown. This and other objects will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.