Patent Number: 
Section: description

Referring now to the drawings in detail and in particular to FIG. 1 there is generally illustrated a boiling water reactor 10 of a commercial nuclear power plant for generating electricity. The nuclear reactor 10 is generally characterized by a reactor pressure vessel 12 and by a RRS 14. The RRS has two substantially identical recirculation loops A and B hydraulically connected in parallel with each other and in series with the reactor pressure vessel 12. During on-line electrical power generation, coolant water (high purity water containing parts per million or lower levels of various ions and, in some cases, dissolved hydrogen gas) is pumped by feedwater pumps (not shown) from a turbine-generator unit (not shown) in a closed loop into the reactor pressure vessel 12 through an inlet nozzle 16 and steam is generated within the reactor pressure vessel 12. The steam flows out of the pressure vessel 12 through an outlet nozzle 18 back to the turbine-generator unit. The reactor recirculation loops A and B provide high velocity coolant water to a plurality of jet pumps 20 located within the reactor pressure vessel 12 for facilitating the flow of coolant water within the pressure vessel 12. As FIG. 1 shows, the reactor pressure vessel 12 includes a bottom head 22 with a sidewall 24 extending vertically to a flange 26. A removable head 28 has a flange 30 that may be bolted to the reactor pressure vessel flange 26. The reactor pressure vessel 12 has a core shroud 32 and a core plate 34, which define a central core region for containing removable fuel assemblies 38. The core shroud 32 has a removable upper end 40 that may be removed in order to remove the fuel assemblies 38. The pressure vessel wall 24, core shroud 32 and ring member 42 define an annulus region surrounding the central core region. The reactor pressure vessel bottom head 22 and the core plate 34 define a lower internals region which is in fluid flow communication with the central core region via flow holes 44 in the core plate 34. As FIG. 1 also shows, each reactor circulation loop A,B of the reactor circulation system 14 generally includes a centrifugal pump 50 having a pump suction nozzle and a pump discharge nozzle. The pumps 50 of commercial boiling water reactors may have nominal capacities of up to about 100,000 gallons per minute or more and the pump nozzles may have diameters of up to about 28 inches or more. Each pump suction nozzle is connected by suction piping 54 with a nozzle 52 in the pressure vessel wall 24 for fluid flow connection with the annulus region of the pressure vessel 12. In another commercial boiling water reactor design (not shown), the vessel nozzle 52 may connect the suction piping 54 with the lower internals region of a reactor pressure vessel. The discharge nozzle of each centrifugal pump 50 is connected by discharge piping 56 to a plurality of reactor pressure vessel nozzles, illustrated in FIG. 1 by nozzle 58. As is illustrated in FIG. 3, the discharge piping 56 comprises at least one manifold 64 with pipe reducers 66 which divide each loop A,B into parallel branches 68. In addition, the branches 68 may further subdivide each loop A,B into parallel pipes 70. In any event, each pipe 70 extends to one of the vessel nozzles 58. As is best shown in FIG. 2, each reactor pressure vessel nozzle 58 extends to a riser pipe, illustrated by riser pipe 72, in the annulus region of the reactor pressure vessel 12. Each riser pipe 72 extends upwardly to a ram""s head manifold 74 having two 180xc2x0 piping bends 76 disposed in parallel adjacent a pair of downstream jet pumps 20. It should be noted that FIG. 2 shows a jet pump assembly that has been modified to facilitate a full loop decontamination during a scheduled outage in accordance with the practice of the present invention. As shown, the piping bends 76 have been sealed with plugs 86, the jet pump inlet sections 78 (shown in FIG. 1) of the jet pumps 20 have been removed and the remaining portions 80 of the jet pumps 20 have been capped. After generating electric power during on-line operations for a year or more, commercial nuclear reactors exemplified by nuclear reactor 10 are taken off-line for refueling and/or performing scheduled maintenance or repairs. It is often desirable to first decontaminate the recirculation loops A and B but not the internal regions of the reactor pressure vessels 12 in order to reduce the radiation levels in the containment building before performing the scheduled maintenance or repairs. In a preferred practice in accordance with the present invention, a boiling water reactor 10 having a reactor pressure vessel 12 with two recirculation loops A,B piped thereto is decontaminated by: installing plugs 86 on both outlets of the jet pump ram""s head manifolds 74 in both of the recirculation loops A,B; pumping a decontamination solution in the loops A,B to wash the irradiated oxide layers on the piping surfaces; introducing compressed air, nitrogen or other monitoring gas into selected ram""s head manifolds 74 in the loops A,B through one of the plugs 86 for monitoring the pressure of the process in the manifolds; and determining the level of the decontamination solution in the recirculation loops A,B from the pressure of the monitoring gas in the manifolds 74. The preferred practice of the present invention may also include the additional steps of installing additional plugs 84 in the reactor vessel nozzles 52 connected with the suction pipes 54 extending to the suction connections of the recirculation pumps 50 in the recirculation loops A,B before pumping the decontamination solution into the loops A,B; introducing a monitoring gas through the plugs 84 into the suction pipes 54 for monitoring the monitoring gas pressure in the suction pipes 54; and determining the level of the decontamination solution in the suction pipes 54 from the pressure the monitoring gas introduced through the plugs 84. FIGS. 2 and 3 generally shows the recirculation loops A,B isolated from the reactor pressure vessel 12 by plugs 84 installed in the pressure vessel nozzles 52 connected with the suction piping 54 and by other plugs 86 installed in the outlets of the 180xc2x0 bends 76 of the ram""s head manifolds 74. As FIG. 3 illustrates, the pressure vessel nozzles 52 and their installed plugs 86 are located below the ram""s head manifolds 74 and their installed plugs 86. In commercial boiling water nuclear reactors, the pressure vessel nozzles 52 may be up to about ten feet or more below the ram""s head manifolds 74. The plugs 84 and 86 may be round aluminum or stainless steel plugs with inflatable rubber bladders. The plugs 84 and 86 may be installed by tooling operated by robots (not shown) working in the annulus region of the pressure vessel 12 after the reactor pressure vessel head 28 has been removed. It should be noted that the plugs 84,86 are installed while the reactor pressure vessel 12 is submerged in water. Accordingly, the loops A,B are solid with water after they have been isolated. Also, the loops A,B may be isolated from the nuclear reactor""s residual heat removal system by closing the valves in piping 92. In the preferred decontamination practice, a pumping unit such as a skid mounted decontamination unit 100 having a pump 102 as shown in FIG. 3 may be connected with the loops A,B at decontamination flanges 94 located near the suction connections of the recirculation pumps 50 for pumping the decontamination solution in the loops A,B. It is to be noted at this point that the decontamination solution is usually pumped through the recirculation pumps 50, which are not used because they are too big to be useful during a decontamination process. Flexible piping sections 104 may be used to facilitate the temporary connection of piping extending from the decontamination unit 100 with the decontamination flanges 94 of the loops A,B. In addition, the decontamination unit 100 may have valves 106 and associated piping that will permit the skid mounted pump 102 to pump the decontamination solution from one loop A,B to the other loop A,B. The arrangement of valves 106 shown in FIG. 3 advantageously permits the flow of decontamination solution to be reversed as often as is desired. In addition, in other practices of the present invention, jumper connections (not shown) may be employed to connect the manifolds 86 of one loop A,B with the manifolds 86 of the other loop A,B so that the skid mounted pump 102 may be employed to continuously recirculate the decontamination solution through the loops A,B. In the preferred practice of the present invention, both loops A,B may be isolated by plugging as described above and then drained down to an intermediate level. For example, the loops A,B may be drained down to a level where the RRS 14 is about half full. The volume in the loops A,B between the plugs 84, 86 and the fluctuating liquid levels of the decontamination solution in the loops A,B may be back filled by air, nitrogen or other suitable gas and vented through connections in the plugs 84,86 described below. Preferably, the pressure above the liquid levels of the decontamination solution throughout the RRS 14 are substantially equalized and nominally about atmospheric pressure throughout the decontamination process in this practice. As shown in FIG. 3, the decontamination unit 100 may also have a feed tank 110 and a feed pump 112 for feeding a suitable decontamination solvent or mixture of solvents (preferably diluted as an aqueous solution) to the suction side of the skid mounted pump 102. The skid mounted pump 102 may then mix the solvent with the coolant water within the pump body and pump the diluted solvent and at least some of the coolant water in one of the two loops A,B into the other of the two loops A,B, including the riser pipes 72. Thus, for example, the pump 102 first may pump some of the coolant water from loop A and the diluted solvent from feed tank 110 into loop B. Later, the valves 106 may be reversed and the skid mounted pump 102 may pump the decontamination solution from the loop B back to loop A. Advantageously, the energy input from the skid mounted pump 102 may also cause the decontamination solution to slosh and splash against the oxide layers on the surfaces of the piping of the recirculation loops A,B and the riser pipes 72. As is also shown in FIG. 3, the skid mounted unit 100 may have a heater 114 for heating the decontamination solution up to a desired operating temperature for effectively dissolving the oxide layers in reasonable time periods. For example, known dilute chemical decontamination processes may be performed at temperatures up to about 150xc2x0 F. or 180xc2x0 F. The skid mounted unit 100 may also have ion exchangers represented by ion exchanger 116 for cleaning up the decontamination solution at the end of the decontamination process to remove the decontamination solvents and dissolved ions from the coolant water. The levels of the decontamination solutions in the loops A,B must be known at all times during the course of decontamination processes to effectively decontaminate the loops A,B without running the risk of pumping the decontamination solvents into the reactor pressure vessels 12. In addition, it is often desirable that the solvents in the decontamination solutions not contact plugs 84 (and particularly aluminum plugs) in the pressure vessel nozzles 52. In the practice of the present invention, commercial gas bubbling level/pressure monitoring systems (not shown) are used for monitoring the levels of the decontamination solutions in the loops A,B. These monitoring systems actually determine the level of a process liquid at a particular location by sensing a differential pressure due to the vertical height (or static head) of the liquid above a reference level and then calculating the level based upon the differential pressure and the specific gravity of the liquid. Commercially available monitoring devices sense the pressures at various locations in a process by introducing (or, as it is sometimes known in the field, xe2x80x9cbubblingxe2x80x9d) compressed air or nitrogen at known, low flow rates into the process and sensing the pressure of the monitoring gas. Each sensed pressure is converted to an appropriate signal by a transducer and the signal is sent to a programmable logic controller or other suitable calculating device by a pressure transmitter. In the preferred practice of the present invention, the monitoring systems are employed to sense the pressure within the loops A,B at the plugs 84, 86 and at the decontamination flanges 88 on the suction sides of the recirculation pumps 50, which flanges 88 are physically located low in the drywells of nuclear reactors. Additional monitoring systems may be optionally employed to sense the pressures at other locations. Advantageously, the monitoring systems may be located remotely from the pressure vessel 12 in relatively low radiation level areas, e.g., on the refuel floor, and connected with one or both recirculation loops A,B via small bore monitoring gas sensing lines 120. The monitoring gas sensing lines 120 may extend through the one of the plugs 86 in the bends 76 of the manifolds 74 for introducing the monitoring gas into the manifolds 74 and associated riser pipes 72. Although it may be sufficient to introduce the monitoring gas into only one of the manifolds 74 of one of the loops A,B to determine the level of the decontamination solution, it is preferred to introduce the monitoring gas into at least two of the manifolds 74 in each loop A, B in order to assure the accuracy of the measurement. Similarly, monitoring gas sensing lines 122 may extend through the plug 84 in the reactor pressure vessel nozzle 52 connected with the suction piping 54 for sensing the pressure of the decontamination solution in the suction piping 54 extending to the suction connection of the recirculation pumps 50. As is shown by FIG. 3, the ends of the monitoring gas sensing lines 120, 122 preferably have flexible lengths 104 near the connections with the plugs 86 and 84, respectively. In the preferred practice of the present invention, the changing gas volumes above the surface levels of the decontamination solution in the loops A,B adjacent the plugs 84,86 and suction pipes 54 are vented by vent lines 130 to reduce the effects of process changes that might substantially affect the back pressure on the monitoring gas sensing lines 120,122 and thereby result in inaccurate sensed pressures. Preferably, for the manifolds 86 that are connected with the monitoring gas sensing lines 120, the monitoring gas is introduced through one plugged outlet of each such manifold 74 and the monitoring gas is vented through the other plugged outlet of the manifold. The vent lines 130 may be small bore hoses, piping or tubing of about xe2x85x9c inch diameter or larger. As is shown in FIG. 3, the vent lines 130 from the recirculation loops A and B are preferably directed to one of two vent tanks 132 and the vent tanks 132 are vented through vent lines 134 to a vacuum tank 136. Advantageously, this arrangement permits the overpressure on the decontamination solution in the several portions of the loops A,B to be substantially equalized as the decontamination solution is pumped from loop A,B to loop A,B in the course of a decontamination process. The vacuum tank 136 may be connected with a vacuum pump 138 for pulling a vacuum on the vacuum tank 136 and on the vent lines 130 and 134. Advantageously, the vacuum may be employed to clear any trapped water in the 180xc2x0 bends 76 of the ram""s head manifolds 74. It should be noted that the structure of the 180xc2x0 bends 76 will inherently trap coolant water on the plugged outlet side at the time the plugs 84,86 are installed. This water will remain trapped even though the level of the coolant water throughout the loops A,B is lowered. Later, during the decontamination process, the decontamination solution may flow over the central portion of the bends 76 and become trapped in the plugged outlets. The accumulation of water in the plugged outlets may be indicated by excessively high indicated pressure levels. In the preferred practice, the vacuum may be periodically employed when the indicated pressure indicates that the vent paths have become blocked. Also, the vent tanks 132 preferably have drain pipes 140 extending to the vacuum tank 136 for draining coolant water and decontamination solution which may have vented into the vent tanks 132. An eductor 142 or other pumping means may be connected by pipe 144 to the vacuum tank 136 for pumping decontamination solution out of the vacuum tank 136. The vent tanks 132, vacuum tank 136, vacuum pump 138 and eductor 142 may be located on the skid mounted unit 100 together with the monitoring system on the refuel floor or at another suitable location remote from the reactor pressure vessel 12. The ram""s head manifolds 74 and the vent tanks 110 may be drained on an xe2x80x9cas neededxe2x80x9d basis whenever the solution begins to block the vent path. While a present preferred embodiment of the present invention has been shown and described, it is to be understood that the invention may be otherwise variously embodied within the scope of the following claims of invention.