Patent Number: 047524332
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to pressurized water reactor systems and, more particularly, to a vent system and method of operation for controlling and actuating hydraulically operated displacer rod drive mechanisms for selective positioning of water displacer rods in the reactor vessel. 2. State of the Relevant Art As is well known in the art, conventional pressurized water reactors employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials known as poisons, which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relative to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor. Typically, the control rods, or rodlets, are arranged in clusters, and the rods of each cluster are mounted to a common, respectively associated spider. Each spider, in turn, is connected to a respectively associated adjustment mechanism for raising or lowering the associated rod cluster. In certain advanced designs of such pressurized water reactors, there are employed both reactor control rod clusters (RCC) and water displacer rod clusters (WDRC). In one such reactor design, a total of over 2800 reactor control rods and water displacer rods are arranged in 185 clusters, each of the rod clusters being mounted to a respectively corresponding spider. In the exemplary such advanced design pressurized water reactor, there are provided, at successsively higher, axially aligned elevations within the reactor pressure vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of generally cylindrical configuration, and an upper closure dome, or head. The lower barrel assembly may be conventional, having mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates. Within the inner barrel assembly there is provided a large number of rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively housing therewithin reactor control rod clusters (RCC) and water displacer rod clusters (WDRC); these clusters, as received in telescoping relationship within their respectively associated guides, generally are aligned with respectively associated fuel rod assemblies. One of the main objectives of the advanced design, pressurized water reactors to which the present invention is directed, is to achieve a significant improvement in the fuel utilization efficiency, resulting in lower, overall fuel costs. Consistent with this objective, the water displacement rodlet clusters (WDRC's) function as a mechanical moderator control, all of the WDRC's being fully inserted into association with the fuel rod assemblies, and thus into the reactor core, when initiating a new fuel cycle. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. As the excess reactivity level diminishes over the cycle, the WDRC's are progressively, in groups, withdrawn from the core so as to enable the reactor to maintain the same reactivity level, even though the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the control rod clusters are moved, again in axial translation and thus telescoping relationship relatively to the respectively associated fuel rod assemblies, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, for example in response to load demands, in a manner analogous to conventional reactor control operations. The calandria includes a lower calandria plate and an upper calandria plate. The rod guides are secured in position at the lower and upper ends thereof, respectively, to the upper core plate and the lower calandria plate. Within the calandria and extending between the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship, respectively aligned with the rod guides. Flow holes are provided in remaining portions of the calandria plates, intermediate the calandria tubes, through which passes the reactor core outlet flow as it exits from its upward passage through the inner barrel assembly. The core outlet flow, or a major portion thereof, turns from the axial flow direction to a radial direction for passage through radially outwardly oriented outlet nozzles which are in fluid communication with the calandria. In similar, parallel axial and aligned relationship, the calandria tubes are joined to corresponding flow shrouds which extend to a predetermined elevation within the head, and which in turn are connected to corresponding head extensions which pass through the structural wall of the head and carry, on their free ends at the exterior of and vertically above the head, corresponding adjustment mechanisms, as above noted. The adjustment mechanisms have corresponding control shafts, or drive rods, which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated spiders mounting the clusters of RCC rods and WDRC rods, and serve to adjust their elevational positions within the inner barrel assembly and, correspondingly, the level to which the rods are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the activity within the core. In the exemplary, advanced design pressurized water reactor, over 2,800 rods are mounted in 185 clusters, the latter being received within corresponding 185 rod guides. Of these clusters, 88 are of the WDRC type, divided into 22 groups of four clusters each, the clusters of each group being chosen such that withdrawal of an individual group, or multiple such groups, maintains a symmetrical power distribution within the reactor core. Since each WDRC is approximately 700 lbs. to 800 lbs. in weight, each group of four (4) such clusters presents a combined weight of in the range of from 2,800 lbs. to 3,200 lbs., requiring that a drive mechanism and associated connecting structure for each group of four clusters have substantial strength and durability, and afford a substantial driving force. Due to the packing density, or close spacing, of the rod clusters and their associated guides, severe spacing requirements are imposed, both within the vessel and with respect to the rod drive mechanisms, including both the water displacer rod drive mechanisms (DRDM's) and the control rod drive mechanism (CRDM's). The critical spacing requirements were not experienced in reactors of prior, conventional types, which did not employ WDRC's and correspondingly did not employ DRDM's. In reactors of such conventional designs, ample spacing was available above the dome, or head, of the vessel for accommodating the required number of mechanisms for driving the RCC's. Particularly, the CRDM's of well known, electromechanical type associated with corresponding clusters of RCC's, were mounted in generally parallel axial relationship, vertically above the dome or head of the vessel and extended in sealed relationship through the head for connection by suitable drive rods to the associated RCC's, and provided for selectively controlled gradual raising and lowering of the RCC's for moderating the reactor energy level, or for rapidly lowering same in the case of shutdown requirements. In reactor systems of the advanced design herein contemplated, whereas the same mechanisms conventionally employed for the CRDM's functionally are acceptable for adjusting the WDRC's, due to the increased number of rod clusters (i.e., the total of RCC's and WDRC's), the conventional CRDM's are unacceptable mechanically, since they are too large. Various alternative mechanisms have been studied in view of this problem. For example, roller nut-drives were considered, but were determined to produce insufficient lifting force. Accordingly, a substitute DRDM has been developed which utilizes a hydraulically operated piston which is attached through a corresponding drive rod to each group of associated WDRC's, and which mechanism satisfies the spacing limitations, permitting mounting thereof above the head or dome of the vessel in conjunction with the conventional CRDM's. An example of such a hydraulically operated drive mechanism for a WDRC is shown in U.S. Pat. No. 4,439,054--Veronesi, issued Mar. 27, 1984 and assigned to the common assignee hereof. The provision of the hydraulically operated mechanism suitable for use with WDRC's as hereinabove set forth, however, has imposed a design requirement of a system to control and manipulate the hydraulic mechanism. No known systems are available for this purpose, in view of the fact that the requirement therefor has arisen out of the evolving design of the advanced design, pressurized water reactors of the type herein contemplated. SUMMARY OF THE INVENTION As before noted, a pressurized water nuclear reactor, of the advanced design type with which the vent system of the present invention is intended for use, employs a large number of reactor control rods, or rodlets, typically arranged in what are termed reactor control rod clusters (RCC) and, additionally, a large number of water displacer rods, or rodlets, similarly arranged in water displacer rod clusters (WDRC), an array of 185 such clusters containing a total of 2800 rodlets (i.e., the total of reactor control rods and water displacer rods) being mounted in parallel axial relationship within the inner barrel assembly of the reactor pressure vessel. The rods of each cluster are mounted at their upper ends to a corresponding spider, and the spider-mounted cluster is received in telescoping relationship within a corresponding rod guide. Each spider is connected through a drive rod to a corresponding adjustment mechanism, which provides for selectively raising or lowering the rod cluster relatively to an associated group of fuel rod assemblies. The adjustment mechanisms more specifically are mounted in generally parallel axial relationship on the head, or dome, of the pressure vessel. The control rod cluster drive mechanism (CRDM's) may be of conventional type as employed in the prior art, comprising electromechanically actuated mechanisms which provide for selectively raising and lowering the RCC's to provide the desired level of reactivity within the core and, alternatively, to lower the control rods rapidly in the event of a requirement for rapid shutdown. The drive mechanisms (DRDM's) for the water displacer rod clusters (WDRC's) may be of the type shown in the above referenced U.S. Pat. No. 4,439,054, which are driven hydraulically, and include a latch mechanism which mechanically latches at a fixed position adjacent the upper end of the stroke. The hydraulic mechanisms of the patented type are compatible in physical size with the CRDM's, and thus may be accommodated within the available spacings on the head, or dome, of the vessel. Each spider, and thus its associated vane assemblies, must be of considerable structural strength and weight. A typical water displacer rod (WDRC) cluster may comprise up to 24 water displacer rods mounted in alternating groups of two and four rods on corresponding ones of a total of eight vane assemblies, each of the four-rod assemblies including both a radially extending vane element and a pair of transversely extending vane elements, the latter carrying the cylindrical support mounts at their outer extremeties. As before noted, the total weight of a water displacer rod cluster, thus configured, is approximately 700 lbs. to 800 lbs. The spiders must support not only the dead weight of the respective rod clusters, but additionally must accommodate the forces imposed thereon both by the environment of the relatively fast-moving core outlet flow which passes thereover and the rod height adjustment functions. The total of eighty-eight (88) WDRC's, in the exemplary vessel, are divided into 22 groups of four clusters each, the WDRC's of each group being selected such that withdrawal of a given one or more of the WDRC groups maintains a symmetrical power distribution within the reactor core. It follows that the total weight of a WDRC group is substantial, ranging from 2,800 lbs. to 3,200 lbs., and that a correspondingly high level force must be developed for raising the WDRC groups, as required, at successive stages of the fuel cycle. The vent system for the displacer rod drive mechanisms (DRDM's) of a pressurized water reactor in accordance with the present invention comprises an arrangement of valves, flow restricting devices, and a common orifice, for hydraulically actuating the DRDM's to drive the WDRC's between either the fully inserted or fully withdrawn positions, relative to the fuel assemblies. The pressure differential between the reactor vessel and the reactor coolant drain tank, which acts on the DRDM's, is used to achieve this function. The common orifice regulates the flow level, as may be required when two or more WDRC groups are selected for simultaneous withdrawal operations. The common orifice, in conjunction with the individual flow restricting devices, thus limit the rate of travel of the WDRC's to a safe value. The vent system is selectively operable in three modes, or flow conditions, including (I) normal withdrawal, (II) normal insertion, and (III) withdrawal employing a bypass line. By selective operation of the valve arrangement and through use of the flow restricting devices in condition (I), the pressure within the vessel head produces a pressure differential within the selected hydraulic mechanisms for withdrawing the corresponding WDRC's. When the WDRC's are fully withdrawn, the corresponding valves are closed and the pressure differential across the hydraulic mechanisms dissipates, due to a designed rate of leakage past the piston rings therein. Once pressure equilibrium is established, the WDRC's drop, or descend, by force of gravity, automatically engaging the mechanical latches within the DRDM's which then lock the WDRC's in their withdrawn, parked positions. Insertion of one or more WDRC groups into the core from a fully withdrawn and locked position is achieved by initially establishing the valve actuation of condition (I) for the withdrawal operation for a limited interval, sufficent to permit the hydraulic mechanisms to advance upwardly and release the mechanical latches, following which, condition (II) is initiated, in which a further valve is opened to communicate pressure from a head vent of the vessel to the respective hydraulic mechanisms (DRDM's) of the selected group or groups, thereby equalizing the pressure across the corresponding pistons and allowing the WDRC's of each group to descend, or fall, into the core under the force of gravity. The withdrawal using bypass, of condition (III) of the vent system, accommodates certain abnormal operating conditions, such as when a drive rod is stuck or a set of high leakage piston rings fails to seat properly, which in turn may prevent one or more WDRC clusters from being withdrawn under the normal withdrawal procedure of condition (I). The bypass mode serves to bypass the previously noted, common orifice, thereby increasing the pressure drop across the DRDM piston rings by several hundred pounds per square inch (psi), the increased pressure differential thus imposed being sufficient to seat any malfunctioning piston ring and raise the WDRC group to the desired, fully withdrawn position. This operation concludes, as in the normal withdrawal step, with closing of the associated valves, permitting the WDRC's to settle into the parked position, engaged by the mechanical latch of the DRDM's. Additionally, the vent system of the invention provides a recovery procedure for correcting any missequencing of the drive operations which may occur either due to unexpected impediments, as above described, or as may arise from inadvertent, premature actuation of the valves causing one or more of the individual DRDM's of a given group to be latched in the parked, fully withdrawn position, while others remain in an intermediate position, neither fully withdrawn nor fully inserted. Accordingly, the vent system of the present invention provides efficient and effective operation, utilizing the available hydraulic driving force of the high pressure within the reactor vessel, while employing a minimum number of selectively actuated conventional valves and flow restricting devices in combination with the noted, hydraulically actuated DRDM's, to afford a system which is safe and versatile in operation, yet low in cost of components and operation. These and other advantages of the present invention will become more apparent from the following detailed drawings and associated discussion.