Abstract:
A boiling water reactor is chemically decontaminated by circulating a decontamination solution through reactor recirculation loops and the annulus region of a reactor pressure vessel that surrounds the central core region while bypassing the central core region. The decontamination solution may also be circulated between the annulus region and a lower internals region while bypassing the central core region. The solution dissolves or breaks down metal oxide layers on the surfaces of the boiling water reactor. The metal oxide layers in the central core region and the activated metal ions contained in these layers, which do not substantially contribute to personnel exposure, are not released and, therefore, do not need to be removed from the solution.

Description:
CROSS-REFERENCE 
     This application claims the benefit of Provisional Patent Application No. 60/134,422, filed May 17, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for decontaminating nuclear reactors employed to generate electric power and more particularly to a method for performing full system decontaminations on boiling water reactors. 
     During on-line power generating operations of commercial boiling water nuclear reactors, thin layers of metal oxides tend to build up on the internal surfaces of vessels and other components and piping in contact with circulating primary coolant (essentially high temperature water). Activated metal ions in the central core regions in reactor pressure vessels are entrained in the primary coolant and then are absorbed in the metal oxides, which results in relatively high radiation levels on these surfaces. It is desirable to reduce the radiation levels to “As Low As Reasonably Achievable” levels in order to reduce the exposure of personnel working near the reactors during periodic plant outages and/or plant decommissioning operations. Thus, the industry may employ one or a combination of various known chemical decontamination treatments, e.g., acid permanganate, alkaline permanganate, Citrox, CAN-DEREM, LOMI and/or other processes, in order to dissolve or break up the oxide films. Conventionally, these decontamination processes involve the addition of permanganate, oxalate, citrate, EDTA and/or other ions to the primary coolant to form decontamination solutions and then the circulation of the solutions through the components to be decontaminated. In addition to removing the oxide layers, it may be desirable to remove several microns of base metal in order to better protect personnel during decommissioning processes. Dilute chemical decontamination solutions generally contain less than about 3-5% by weight of such decontamination agents. Chemical decontaminations may be performed upon full primary coolant systems or upon selected subsystems. Full system decontaminations are the preferred approach when the goal is to reduce dose rates on multiple subsystems throughout the plants. In addition, full system decontamination processes are generally performed with nuclear fuel assemblies out of the central core regions of the reactor pressure vessels, but the fuel assemblies may be retained in the central core regions in some cases. 
     The activated metal ions that are removed from the internal surfaces of the primary coolant systems in the course of the decontamination operations are collected on cation exchange resins. The activated resins must then be removed to remote disposal sites. 
     The majority of the activated oxide deposits in boiling water reactor primary coolant systems are located in the central core regions of reactor pressure vessels. These deposits do not substantially contribute to personnel exposure. Thus, it would be very desirable to decontaminate only those systems that substantially contribute to personnel exposure and bypass the central core regions. This would substantially reduce the total exposure of personnel while reducing resin and disposal costs. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to decontaminate a portion of a reactor pressure vessel in a boiling water reactor and its appurtentant recirculation system while bypassing its central core region. It is a further object to substantially decontaminate a boiling water reactor with lower overall personnel exposures to radiation and lower resin costs. 
     With these objects in view, the present invention resides in a method of decontaminating a boiling water reactor having a plurality of reactor recirculation loops hydraulically connected in parallel with a reactor pressure vessel. Such a reactor pressure vessel has: a central core region; an annulus region surrounding the central core region and in hydraulic communication with the recirculation loops; and a lower internals region in hydraulic communication with the central core region. In the practice of the present invention, a decontamination solution is circulated through at least one of the reactor recirculation loops and the annulus region of the pressure vessel without circulating through the central core region. In a preferred practice of the present invention, the decontamination solution also circulates between the annulus region and the lower internals region without circulating through the central core region. Thus, a boiling water reactor can be substantially decontaminated while reducing overall personnel exposure and generating less resin wastes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention as set forth in the claims will become more apparent from the following detailed description of certain preferred practices thereof, which may be performed in boiling water reactors shown, by way of example only, in the accompanying drawings, wherein: 
     FIG. 1 is a schematic representation of a boiling water reactor, including a reactor recirculation system comprising a plurality of reactor recirculation loops connected in parallel with a reactor pressure vessel. 
     FIG. 2 is a schematic representation of a boiling water reactor, including another reactor recirculation system comprising a plurality of reactor recirculation loops connected in parallel with a reactor pressure vessel and having jet pumps disposed in the reactor pressure vessel. 
     FIG. 3 is a partial perspective schematic representation of the boiling water reactor vessel of FIG. 2 which has been cut-a-way to show a conventional jet pump assembly arrangement. 
     FIG. 4 is a partial perspective schematic representation of the jet pump assembly arrangement of FIG. 3 including a first modification of the jet pump assembly for practicing the present invention. 
     FIG. 5 is a partial schematic representation of the jet pump assembly arrangement of FIG. 3 including a second modification of the jet pump assembly for practicing the present invention. 
     FIG. 6 is a partial schematic representation of the jet pump assembly arrangement of FIG. 3 including a third modification of the jet pump assembly for practicing the present invention. 
     FIG. 7 is a schematic representation of the reactor recirculation system illustrated in FIG. 2 including a fourth modification of the jet pump assembly for practicing the present invention. 
     FIG. 8 is a schematic representation of the boiling water reactor vessel of FIG. 1 including a modification for practicing the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings in detail and in particular to FIG. 1 there is generally shown a boiling water reactor  10  of a nuclear power plant for commercially generating electricity. The reactor  10  generally includes a reactor pressure vessel  12  and a reactor recirculation system  14 . The reactor recirculation system  14  generally comprises a plurality of reactor recirculation loops, illustrated by loops  16  and  18 , hydraulically connected in parallel with the reactor pressure vessel  12 . When generating power during normal online operations, primary coolant (high purity water containing ppm levels of various ions and, in some cases, dissolved hydrogen gas) is pumped by feedwater pumps (not shown) into the reactor pressure vessel  12  through an inlet nozzle  19  and steam is generated with the reactor pressure vessel  12 . The steam flows out of the pressure vessel  12  through an outlet nozzle  20  and then to a turbine (not shown) which generates the electrical power. The reactor recirculation system  14  facilitates the flow of primary coolant to fuel assemblies in the central core regions in the pressure vessel  12 . A commercial facility embodying this boiling water reactor design is the Oyster Creek Plant near Forked River, N.J. 
     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  36  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 core shroud  32  (or, equivalently, in a similar reactor design, a supporting skirt [not shown] supporting the core shroud  32 ) is spaced from the reactor pressure vessel wall  24  by a structural ring member  42 . The pressure vessel wall  24 , core shroud  32  and ring member  42  define an annulus region  44  surrounding the central core region  36 . The annulus region  44  frequently is referred to as a “downcomer” or a “downcomer annulus”. The reactor pressure vessel bottom head  22  and the core plate  34  define a lower internals region  46  which is in fluid flow communication with the central core region  36  via flow holes  48  in the core plate  34 . 
     Each reactor circulation loop  16  and  18  of the reactor circulation system  14  shown in FIG. 1 generally includes a centrifugal pump  56  with a pump suction nozzle and a pump discharge nozzle. The pump  56  may have a nominal capacity of up to about 50,000 gallons per minute or more. The pump suction nozzle is connected with piping  58  extending from one or more nozzles, illustrated by nozzle  60  in FIG. 1, in the pressure vessel wall for fluid flow connection with the annulus region  44 . The pump discharge nozzle is connected with piping  62  extending to one or more nozzles, illustrated by nozzle  64 , in the pressure vessel wall  24  for fluid flow connection with the lower internals region  46  of the reactor pressure vessel  12 . 
     When generating power during normal online operations, the primary coolant pumped through the inlet nozzle  19  into the annulus region  44  flows through the recirculation system  14 , into the lower internals region  46 , up through flow holes  48  in the core plate  34  and fuel assemblies  38  in the central core region  36  (where steam is generated) and up through steam/condensate separators (not shown) supported on the core shroud head  40 . The separated condensate drains back to the annulus region  44 . The steam flows up into the upper portion of the reactor pressure vessel  12 , through steam dryers (not shown) and then out of the pressure vessel  12  through outlet nozzle  20 . 
     FIG. 2 shows a different boiling water reactor design which employs internal jet pump assemblies  76  disposed in the annulus region  44  in the reactor circulation loops  16  and  18  for circulating coolant from the annulus region  44  to the lower internals region  46 . Each jet pump assembly  76  includes inlet piping  78  with a jet nozzle  80  in fluid flow communication with one of the reactor recirculation loops  16  or  18 . A mixing assembly  82  has a suction inlet end  84  spaced from the jet pump nozzle  80  in fluid flow communication with the annulus region  44 . The primary coolant around the suction inlet end  84  in the annulus region  44  is entrained by the primary coolant flowing out through the jet pump nozzle  80  and the two fluids are mixed together in the mixing assembly  82 . The mixing assembly  82  is connected with a diffuser assembly  86  having an outlet end  88  in fluid flow communication with the lower internals region  46 . U.S. Pat. No. 5,515,407 entitled “Jet Pump Assembly For Recirculating Coolant Through a Recirculation Loop Of A Boiling Water Reactor Vessel” is incorporated by this reference for its detailed description of the structure of such an assembly. 
     As is shown in FIG.  3  and as is described in U.S. Pat. No. 5,515,407, jet pump assemblies  76  are conventionally arranged in pairs with an inlet riser pipe  90  extending directly from an inlet nozzle  64  or via a header (not shown) to a header  92  and then to the inlet piping  78 , which may be a piping elbow with a lifting eye  94 . The assembly comprising the header  92  and the inlet piping  78  to each jet pump assembly  76  is frequently referred to as a “ramshead”. The riser pipe  90  and the ramshead are maintained in place by a holddown assembly  96 . The mixing assembly  82  and the diffuser assembly  86  are held against the riser pipe  90  by restrainers  98 . Also, the diffuser assembly outlet end  88  may be fit into a fitting  100  extending to or through the ring member  42  and to a connector (not shown) for providing primary coolant to the lower internals region  46 . 
     As discussed above, after generating electric power during online operations, it is desirable to decontaminate boiling water reactors and their recirculation systems  14  but not necessarily the central core regions  36 . In accepted commercial decontamination processes such as, e.g., the LOMI, CAN-DEREM, CAN-DECON, Citrox and various permanganate processes, low oxidation state metal ions, permanganates, oxalates, citrates, EDTA and other agents are added to the primary coolant to generate a decontamination solution. The decontamination solution is then circulated past the activated surfaces to dissolve and break up the radioactive oxide films that have formed and release the activated metal ions. 
     In accordance with the practice of the present invention, a decontamination solution is circulated through at least one of the reactor recirculation loops  16  or  18  and the annulus region  44  without circulating the decontamination solutions through the central core region  36 . Preferably, the decontamination solutions are circulated through all of the recirculation loops  16  and  18 . 
     FIGS. 4-7 show various modifications to jet pump assemblies  76  of boiling water reactors, such as the paired jet pump assembly arrangement illustrated in FIG. 3, for at least restricting the flow of the decontamination solution from the annulus region  44  into the lower internals region  46  so that the decontamination solution flowing through the annulus  44  and the recirculation system  14  will not circulate through the central core region  36 . Preferably, the turbulence in the lower internals region  46  (if any) is sufficiently low that substantial amounts of the decontamination solution will not splash through the holes  48  in the core plate  34  and into the central core region  36  because this could unnecessarily generate additional activated ions in the decontamination solution which would need to be removed and thereby reduce the efficiency of the process. 
     FIGS. 4 and 5 show modified jet pump assemblies  76  of FIG. 2 fed by a riser pipe  90  from which at least part of each jet pump assembly has been removed. The remaining part of each jet pump assembly  76  in the pressure vessel  12  which is in fluid flow communication with the lower internals region  46  is covered by a cap  110 . Each cap  110  may be placed over the remaining part of the jet pump assembly  76  to provide an umbrella type cover that substantially prevents the liquid which may fall or splash from entering into the diffuser assembly  86 . A liquid tight cap seal may be employed where the liquid level in the annulus  44  were to be maintained near or above the level of the cap  110  and it is desired to substantially prevent decontamination solution from flowing into the lower internals region  46 . 
     FIG. 4 illustrates a jet pump assembly modification wherein the jet pump nozzles  80  and the mixing assemblies  82  have been removed, e.g., by underwater cutting, and the diffuser assemblies  86  covered by caps  110 . Also, as shown, the jet pump nozzles  78  may be removed and adapters  112  such as orifice plates or other flow devices may be welded or attached to the ends of the inlet pipes  92 . The adapters  112  may be employed to direct decontamination solution downwardly to avoid creating a geyser in the annulus  44 . This can be particularly important to reduce splashing over the core shroud  32  in a decontamination practice where the core shroud head  40  and fuel assemblies  38  are removed. FIG. 5 illustrates another jet pump assembly modification wherein only the suction inlets  84  of the mixing assemblies  82  have been removed and the remaining portions of the mixing assemblies  82  capped. 
     The flow of decontamination solution in each recirculation loop  16  and  18  usually may be controlled by variable speed circulation pumps  56  or by flow control valves (not shown) to maintain the required net positive suction head (NPSH) of the pumps and to limit vibrations on the temporary adapters  112 . Also, the recirculation pumps  56  may be operated at a rate that will provide sufficient energy input into the decontamination solution in order to heat up and maintain the system at a temperature of about 180° F. to about 250° F. for the chemical decontamination agents to be effective in an acceptable period of time. An overpressure with a gas such as air or nitrogen may be provided in the reactor vessel  12  to prevent boiling and to provide the required NPSH. Alternatively, where it is not possible or undesirable to operate the recirculation pumps  56 , the Residual Heat Removal Pumps (not shown) or external pumps (not shown) may be employed to circulate the decontamination solution. 
     FIG. 6 illustrates a jet pump assembly modification wherein some of the jet pump assemblies  76  are modified and other assemblies  76  are not modified. This modification permits decontamination solution to be pumped into the lower internals region  46  and circulate upwardly through the remaining parts of the modified assemblies  76  without circulating the solution through the central core region  36 . It is noted, however, that substantial turbulence in the lower internals region  46  should be avoided because turbulence may cause some decontamination solution to splash through the core plate  34  depending upon the numbers of assemblies  76  which are modified. 
     FIG. 7 illustrates a piping arrangement where one or more jet pump nozzles  80  of modified jet pump assemblies  76  (as shown in FIG. 6) of one recirculation loop are connected by jumper pipes or hoses  120  with jet pump nozzles  80  of modified jet pump assemblies  76  (as shown in FIG. 6) of another recirculation loop. A recirculation pump  56  of one recirculation loop may then be operated to pump decontamination solution up through riser pipes  90  and headers  92  of the one recirculation loop, through jumpers  120 , into headers  92  and down riser pipes  90  of the other recirculation loop, through the other recirculation loop including its recirculation pump (which would not be operating), and into the annulus region  44  of the pressure vessel  12 . Advantageously, the flow could be reversed by operating the other recirculation pump  56 . 
     FIG. 8 illustrates a modification of the reactor pressure vessel  12  of FIG. 1 (which does not employ jet pumps) wherein flow holes in the ring member  42  or in the lower portion of the core shroud  32  below the core plate  34  would permit decontamination solution in the lower internals region  46  to circulate into the annulus region  44  without circulating through the central core region  36 . The flow holes could be uncovered annulus manways  130  and/or flow holes cut into the ring member  42  or in the lower part of the core shroud  32  (or, equivalently, its supporting skirt) below the core plate  34 . 
     With each of these modifications, additional plant pumps could be operated to circulate the decontamination solution through appurtent systems if needed. For example, Reactor Water Clean-Up, Residual Heat Removal, High Pressure System Injection, Low Pressure System Injection Systems and others could be decontaminated if desired. 
     Entrained particulates and activated ions in the circulating decontamination solutions may be removed in filters (not shown) and on cation resins (not shown), respectively, during the course of the decontamination operations. Then, at the conclusion of the decontamination operations, the primary coolants may be cleaned up on the resins. If plants are to be decommissioned, decommissioning operations may continue without restoring the reactor pressure vessels to their initial conditions. However, if plants are to be returned to online power generating operations, then pressure vessels may need to be repaired or further modified. 
     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.