Abstract:
A seal arrangement for providing a seal between a nuclear reactor in-core instrument housing and an instrument contained within the housing includes a lower seal assembly surrounding an outer portion of the in-core instrument housing, an upper seal assembly surrounding an outer portion of the in-core instrument, a seal housing enclosing the lower and upper seal assemblies, and lower and upper compression assemblies positioned on respective ends of the seal housing. The compression assemblies each include a drive nut and a compression collar. The compression collars engage and apply an axial load on the seal assemblies to maintain a reliable seal between the seal housing and the outer portion of the in-core instrument housing, and between the seal housing and the outer portion of the in-core instrument.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/194,028 filed on Apr. 21, 2000 and is a division of U.S. patent application Ser. No. 09/818,534 filed on Mar. 28, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to nuclear power reactors, and more particularly, to a seal arrangement and method for sealing an in-core instrument housing (“ICI housing”) after reinsertion of an in-core instrument (“ICI”) during a refueling outage. 
     2. Description of the Related Art 
     On typical pressurized water nuclear reactors, a plurality of nozzles are located in an organized pattern across the head of a reactor vessel. A majority of the nozzles are used to provide a pressure boundary between neutron absorbing control rods and respective drive mechanisms which provide controlling motion to the control rods. A smaller number of the nozzles are used to provide access into the vessel for ICIs, such as thermocouples and the like. 
     At every refueling outage for typical nuclear reactors, the ICIs must be partially withdrawn to allow for movement of the fuel. In some cases, the process of withdrawing the ICIs has resulted in damage to the O-ring sealing surfaces of the ICIs and/or the ICI seal housing. In some cases, the sealing surfaces are damaged beyond repair and, because the ICIs have to be reinserted with a reliable seal prior to restarting, the damage must be repaired or an alternate sealing method used. 
     One currently available option to repair the damaged seal is to reinsert the ICIs without the O-rings, and perform a field seal weld of the ICI to the housing. However, this requires costly and time-consuming field welding. Also, at the next refueling outage, the seal weld must be machined out so that the ICI can be withdrawn, reinserted, and again seal welded. These operations are very time consuming and costly and can only be performed a few times before an alternate method must be used. 
     Another currently available option to repair the damaged seal is to withdraw the ICI completely and replace the damaged parts. However, because of high radiation contamination, there currently are no methods available to remove the ICI without cutting it up and disposing of the pieces underwater during the withdrawal process. Thus, even a working, undamaged ICI must be discarded and replaced, which is very costly. Moreover, replacement of the seal housing requires field cutting and ASME Code welding, which are very time consuming and costly. 
     Thus, there is a need for an improved seal arrangement and method for sealing ICI housings after reinsertion of the ICIs during a refueling outage. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a seal arrangement and method for resealing an ICI housing during a refueling outage that allows use of the existing components and minimizes the need for costly field cutting and/or welding, and which can be assembled and disassembled quickly and easily and reused over the life of the plant. 
     It is a further object of the present invention to provide a seal arrangement for sealing an ICI housing which is economical, efficient and reliable in use, capable of a long operating life, and particularly well adapted for the proposed use. 
     The present invention provides a seal arrangement for establishing a seal between a nuclear reactor ICI housing and an ICI contained within the ICI housing. The seal arrangement includes a lower seal assembly surrounding an outer portion of the ICI housing, an upper seal assembly surrounding an outer portion of the ICI, a seal housing enclosing the lower and upper seal assemblies, and lower and upper compression assemblies positioned on respective ends of the seal housing. The compression assemblies each include a drive nut and a compression collar. The compression collars engage and apply an axial load on the seal assemblies to maintain a reliable seal between the seal housing and the outer portion of the ICI housing, and between the seal housing and the outer portion of the ICI. Variations in the construction and methods for installing the seal arrangements to the ICI housing and ICI are disclosed. 
     According to a broad aspect of the present invention, a seal arrangement for a nuclear reactor ICI housing is provided, comprising: a first seal assembly surrounding an outer portion of an ICI housing; a second seal assembly surrounding an outer portion of an ICI inserted within the ICI housing; a seal housing having first and second ends, the seal housing enclosing the first and second seal assemblies; and first and second compression assemblies positioned on and engaging the first and second ends of the seal housing, respectively. 
     According to another broad aspect of the present invention, a seal arrangement is provided in combination with a nuclear reactor ICI housing and an ICI inserted within the ICI housing. The seal arrangement comprises: a first seal assembly surrounding an outer portion of the ICI housing; a second seal assembly surrounding an outer portion of the ICI; a seal housing having first and second ends, the seal housing enclosing the first and second seal assemblies; and first and second compression assemblies positioned on the first and second ends of the seal housing, respectively. The first compression assembly engages the first seal assembly to maintain a seal between the first end of the seal housing and the outer portion of the ICI housing. The second compression assembly engages the second seal assembly to maintain a seal between the second end of the seal housing and the outer portion of the ICI. 
     According to another broad aspect of the present invention, a method of sealing a nuclear reactor ICI housing is provided, comprising the steps of: positioning a first compression assembly and first seal assembly around an outer portion of the ICI housing; positioning a seal housing over an end of the ICI housing; positioning a second seal assembly and second compression assembly around an outer portion of an ICI contained within the ICI housing; and threading respective drive nuts of the first and second compression assemblies onto the seal housing to maintain a compressive sealing force against the first and second seal assemblies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more clearly appreciated as the disclosure of the invention is made with reference to the accompanying drawings. In the drawings: 
     FIG. 1 is a schematic elevation view of the upper portion of a nuclear reactor vessel, showing a conventional ICI housing connected to a reactor vessel nozzle extending from the reactor vessel head. 
     FIG. 2 shows a cross-sectional elevation view of a seal arrangement for an ICI housing according to a first embodiment of the present invention. 
     FIG. 3 shows a cross-sectional elevation view of the seal arrangement of FIG. 2 with a compression tool for preloading the seal arrangement during installation. 
     FIG. 4 shows an enlarged section view of a sealing portion of the seal arrangement of FIGS. 2 and 3. 
     FIG. 5 shows a cross-sectional elevation view of a seal arrangement for an ICI housing according to a second embodiment of the present invention. 
     FIG. 6 shows a cross-sectional elevation view of a seal arrangement for an ICI housing according to a third embodiment of the present invention. 
     FIG. 7 shows an enlarged section view of a sealing portion of the seal arrangement of FIG.  6 . 
     FIG. 8 is a plan view of a portion of a compression collar having an anti-rotation key used in the seal arrangement of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A seal arrangement and method for sealing an ICI housing according to the present invention will now be explained in detail with reference to FIGS. 1 to  8  of the drawings. 
     FIG. 1 shows a portion of a nuclear reactor system  10  having a reactor  12  including a substantially upright cylindrical vessel  14  and a substantially hemispherical head  16 . The vessel has an upper flange  18 , and the head  16  has a lower flange  20 . The flanges  18  and  20  are bolted together in a known manner for normal operation of the reactor system  10 . As is well known in the field of nuclear engineering, the operating conditions within the reactor  12  can be monitored by ICIs, such as thermocouples and so forth. This is typically accomplished by a plurality of ICIs  23  that pass through respective nozzles  22  into the reactor  12 . For the purpose of simplifying the present description, only one of the reactor vessel nozzles  22  on the head  16  is shown, but it should be appreciated that normally there are several nozzles  22  positioned in an organized array across the head  16 . Each ICI  23  is housed in an ICI housing  24  connected to a respective one of the nozzles  22 . The ICI  23 , ICI housing  24 , and other components shown in FIG. 1 are conventional. 
     As explained above, the ICIs  23  must be partially withdrawn at every refueling outage to allow for movement of the fuel. The process of withdrawing the ICIs  23  often results in damage to the sealing surfaces of the O-rings installed in the grooves  25  (FIG. 2) of the ICIs  23  and/or to the ICI seal housing  24 . The present invention provides seal arrangements for resealing the interface between the ICI  23  and the ICI housing  24  after the ICIs  23  are reinserted following the refueling. 
     Three embodiments of seal arrangements according to the present invention are described herein and shown in FIGS. 2 to  8  of the drawings. All three embodiments utilize external seal rings, preferably made of a graphite material, on the external surfaces of the existing ICI  23  and ICI housing  24 . To effect a seal against full system design pressure of up to 2500 psi, the seal rings must be compressed under very high loads to prevent leakage. The sealing principle is the same in each of the three embodiments. The method used to compress the seal rings differs. 
     As shown in FIGS. 2 to  4 , a seal arrangement  30  according to the first embodiment of the present invention includes a first lower seal assembly  31  surrounding an outer portion  32  of the ICI housing  24 , and a second upper seal assembly  33  surrounding an outer portion of an ICI  23  contained within the ICI housing  24 . The lower seal assembly  31  includes a first pair of seal rings  34 ,  35  positioned in abutting relationship with each other. The upper seal assembly  33  includes a second pair of seal rings  36 ,  37  positioned in abutting relationship with each other. The seal rings  34 ,  35  of the lower seal assembly  31  have an inside diameter that fits closely to the external surface of the ICI housing  24 . The seal rings  36 ,  37  of the upper seal assembly  33  have an inside diameter that fits closely to the external surface of the ICI  23 . The seal rings  34 - 37  are preferably fabricated of a graphite material. 
     A seal housing  38  is provided which spans the interface between the ICI  23  and the ICI housing  24 . The seal housing  38  has a first recess  39  formed in a first lower end  40  and a second recess  41  formed in a second upper end  42 . The lower seat assembly  31  is positioned in the first recess  39  so as to be enclosed by the lower end  40  of the seal housing  38 . The upper seal assembly  33  is positioned in the second recess  41  so as to be enclosed by the upper end  42  of the seal housing  38 . 
     The seal housing  38  has a housing portion  43  and an integral retainer portion  44 . The retainer portion  44  has an inner diameter  45  that fits closely around the external surface of the ICI  23 , and an outer diameter having external male threads  46 . The retainer portion  44  is secured to the ICI housing  24  by threadably engaging the external threads  46  of the retainer portion  44  to corresponding internal threads  47  formed in the upper end of the ICI housing  24 . The retainer portion  44  has an abutment seat  48  at the top of the external threads  46  that engages and seats against an upper end surface  49  of the ICI housing  24 . A lower end of the retainer portion  44  has an abutment surface  50  against which the existing spacers  51  surrounding the ICI  23  can be engaged. A lower end  52  of the lowest spacer  53  is engaged by an annular shoulder  54  formed on the ICI  23 . Thus, the retainer portion  44  provides positive retention against slipping of the ICI  23  and other parts relative to the ICI housing  24  when system pressure is applied. 
     The housing portion  43  of the seal housing  38  is molded together with the retainer portion  44  in a single, integral piece. The housing portion  43  has external male threads  55  on each end, which threads may extend along the entire length of the housing portion  43 , as shown in FIG.  2 . The seal housing  38  is preferably fabricated from a stainless steel alloy that resists galling and seizing of threads, such as NITRONIC 60™ Stainless Steel. 
     A first lower compression assembly  56  and a second upper compression assembly  57  are positioned on the lower and upper ends  40 ,  42  of the seal housing  38 , respectively. The lower compression assembly  56  includes a first lower drive nut  58  and a first lower compression collar  59 . The lower drive nut  58  has internal threads  60  threadably engaged on the external threads  55  of the housing portion  43 , and a flange  61  that extends inwardly at a lower end. The lower compression collar  59  has a first annular portion  62  engageable by the flange  61  of the lower drive nut  58 , a second annular portion  63  which extends through the flange  61  and protrudes from a lower end of the lower drive nut  58 , and a third annular portion  64  facing the lower seal assembly  31 . A first lower spacer ring  65  is positioned between the third annular portion  64  and the seal rings  34 ,  35  of the lower seal assembly  31 . The lower compression collar  59  is axially movable against the lower spacer ring  65  using a compression tool, which will be described below, to compress the lower seal assembly  31  to form a seal between the ICI housing  24  and the seal housing  38 . The lower drive nut  58  is threaded onto the housing portion  43  of the seal housing  38  until the flange  61  is engaged snugly against the first annular portion  62  of the lower compression collar  59  to maintain the compressed seal between the ICI housing  24  and the seal housing  38 . 
     The upper compression assembly  57  includes a second upper drive nut  66  and a second upper compression collar  67 . The upper drive nut  66  has internal threads  68  threadably engaged on the external threads  55  at the upper end of the housing portion  43 , and a flange  69  that extends inwardly at an upper end of the upper drive nut  66 . The upper compression collar  67  has a first annular portion  70  engageable by the flange  69  of the upper drive nut  66 , a second annular portion  71  which extends through the flange  69  and protrudes from an upper end of the upper drive nut  66 , and a third annular portion  72  facing the upper seal assembly  33 . A second upper spacer ring  73  is positioned between the third annular portion  72  of the upper compression collar  67  and the seal rings  36 ,  37  of the upper seal assembly  33 . The upper compression collar  67  is axially movable against the upper spacer ring  73  using a compression tool, which will be described below, to compress the upper seal assembly  33  to form a seal between the ICI  23  and the seal housing  24 . The upper drive nut  66  is threaded onto the housing portion  43  of the seal housing  38  until the flange  69  is engaged snugly against the first annular portion  70  of the upper compression collar  67  to maintain the compressed seal between the ICI  23  and the seal housing  38 . 
     As shown in FIG. 2, the diameters of the threaded portions  60 ,  68  of the lower and upper drive nuts  58 ,  66  are the same. The inner diameter of the flange  69  of the upper drive nut  66  and the corresponding portions of the upper compression collar  67  are smaller than the inner diameter of the flange  61  of the lower drive nut  58  and the corresponding portions of the lower compression collar  59 . 
     An installation tool  74  for installing the seal arrangement  30  shown in FIG. 2 over the existing ICI  23  and ICI housing  24  during a refueling outage is shown in FIG.  3 . The installation tool  74  includes a pair of leg assemblies  75 ,  76  each having a lower end  77  with a gripping portion  78  protruding inwardly toward the ICI housing  24 , and an upper end  79  supporting a hydraulic cylinder  80 . The gripping portions  78  at the lower end  77  of the leg assemblies  75 ,  76  engage the protruding second annular portion  63  of the lower compression collar  59 . The pair of leg assemblies  75 ,  76  can be easily positioned over and removed from the seal arrangement  30  after the compression assemblies  56 ,  57  are installed with the drive nuts  58 ,  66  threaded hand tight. 
     The installation tool  74  also includes an upper compression plate  81  surrounding the ICI  23  above the upper compression collar  67 . A lower surface  82  of the compression plate  81  engages the protruding second annular portion  71  of the upper compression collar  67 . A piston  83  protrudes from each of the hydraulic cylinders  80  into engagement with the upper compression plate  81 . When the tool  74  is placed over the seal arrangement  30 , as shown in FIG. 3, a predetermined pressure can be introduced into the hydraulic cylinders  80  to force the respective pistons  83  against the upper compression plate  81 , which in turn pushes the upper compression collar  67  against the upper seal assembly  33 . At the same time, a corresponding force is transmitted through the leg assemblies  75 ,  76  to force the gripping portions  78  against the lower compression collar  59  to compress the lower seal assembly  31 . The tool  74  is thus operable to provide a compression load to the lower and upper seal assemblies  31 ,  33  simultaneously. 
     While the tool  74  is installed and a compression preload is applied to the seal assemblies  31 ,  33 , the lower and upper drive nuts  58 ,  66  can be threaded further along the seal housing  38  until the flanges  61 ,  69  of the drive nuts  58 ,  66  are engaged snugly against the respective lower and upper compression collars  59 ,  67 . For example, the drive nuts  58 ,  66  can be seated hand tight while the seal assemblies  31 ,  33  are under compression from the installation tool  74 . When the hydraulic pressure is released from the tool  74 , the drive nuts  58 ,  66  pick up the load and maintain the compression preload on the seal assemblies  31 ,  33 . 
     Having explained the construction of the seal arrangement  30  according to a first embodiment of the present invention, a method of installing the seal arrangement  30  during a nuclear reactor refueling outage will now be described. 
     After the seal welds have been cut (where applicable) and the surfaces cleaned, the existing retainer nut (not shown) is removed and discarded. The ICI  23  is partially withdrawn in accordance with existing procedures. The existing ICI O-rings in the grooves  25  can be discarded because they are not required with the seal arrangement  30  of the present invention. The refueling is then completed in accordance with existing procedures. The ICI  23  is then reinserted to the proper depth. The reinsertion can be done by hand without using an insertion tool because the grooves  25  do not have O-rings causing a friction drag during reinsertion. Spacers  51  are added according to the existing procedure to a defined height so that the seal housing  38  will seat properly. 
     The lower drive nut  58 , lower compression collar  59 , lower spacer ring  65 , and lower seal assembly  31  are lowered over the outside of the ICI housing  24 . The retainer portion  44  of the seal housing  38  is threaded (e.g., hand tight) into the internal threads  47  of the ICI housing  24  until the seal housing  38  is seated on the ICI housing  24 . After the seal housing  38  is installed, the ICI  23  is pulled up slightly by hand to close any gaps with the spacers  51 . The lower seal assembly  31 , lower spacer ring  65 , lower compression collar  59  and lower drive nut  58  are then installed to the lower end  40  of the seal housing  38  (e.g., hand tight), as shown in FIG.  2 . The upper seal assembly  33  and upper spacer ring  73  are installed in the upper end  42  of the seal housing  38 . The upper compression collar  67  and upper drive nut  66  are then installed (e.g., hand tight), as shown in FIG.  2 . 
     The installation tool  74  shown in FIG. 3 is used to seat the lower and upper seal assemblies  31 ,  33 , preload the seal arrangement  30  during installation, and unload the seal arrangement  30  for removal. Because the tool  74  loads both seal assemblies  31 ,  33  simultaneously, the number of operations required to install or remove the seal arrangement  30  is reduced, thereby saving installation and removal time. 
     The upper and lower compression collars  59 ,  67  are axially loaded simultaneously by the hydraulic tool  74  to compress the lower and upper seal assemblies  31 ,  33  to the desired preload. While under compression with the hydraulic tool  74 , both drive nuts  58 ,  66  are seated (e.g., hand tight). When the hydraulic pressure is released from the tool  74 , the drive nuts  58 ,  66  pick up the load and maintain the preload on the seal assemblies  31 ,  33 . The tool  74  can then be removed. 
     A seal arrangement  85  according to a second embodiment of the invention will now be described with reference to FIG.  5 . 
     The seal arrangement  85  shown in FIG. 5 is similar in most respects to the seal arrangement  30  shown in FIGS. 2 to  4 . The main difference is that the seal arrangement  85  of FIG. 5 includes a retainer nut  86  as a separate component from the seal housing  87 . The retainer nut  86  has external threads  88  which are threaded into the internal threads  47  of the ICI housing  24  until the retainer nut  86  is seated against the upper end of the ICI housing  24 . The seal housing  87  is installed over the retainer nut  86  after the retainer nut  86  is seated in the ICI housing  24 . An inwardly directed flange  89  at the upper end of the seal housing  87  engages an upper surface of the retainer nut  86  to maintain the vertical positioning of the seal housing  87 . The seal housing  87  and retainer nut  86  are both preferably fabricated from a stainless steel alloy that resists galling and seizing of threads, such as NITRONIC™ 60 Stainless Steel. The installation tool  74  shown in FIG. 3 is used for installing and removing the seal arrangement  85  of FIG. 5 in the same manner described above. 
     A seal arrangement  90  according to a third embodiment of the invention will now be described with reference to FIGS. 6 to  8 . 
     The seal arrangement  90  shown in FIGS. 6 to  8  is similar in many respects to the seal arrangement  85  shown in FIG.  5 . The main difference is that the seal arrangement  90  of FIGS. 6 to  8  relies upon the torque applied to the lower and upper drive nuts  91 ,  92  to compress and load the lower and upper seal assemblies  93 ,  94  during installation, rather than a hydraulic installation tool. 
     The seal arrangement  90  of FIGS. 6 to  8  includes a first lower seal assembly  93  enclosed by a lower end of a seal housing  95 , and a second upper seal assembly  94  enclosed by an upper end of the seal housing  95 . A first lower compression collar  96  and a second upper compression collar  97  are installed against the respective lower and upper seal assemblies  93 ,  94 . The lower and upper compression collars  96 ,  97  each have anti-rotation keys  98 ,  99  protruding radially outwardly, as shown in FIG. 8, for example. The lower and upper ends of the seal housing  95  have mating keyways  100 ,  101  formed therein into which the anti-rotation keys  98 ,  99  of the compression collars  96 ,  97  are received. The compression collars  96 ,  97  act as a bearing surface for the applied thrust loads and prevent rotational loads on the seal rings of the respective seal assemblies  93 ,  94 . 
     Thrust bearing rings  102 ,  103  are installed between each of the drive nuts  104 ,  105  and the respective compression collars  96 ,  97 . The thrust bearing rings  102 ,  103  reduce the rotational frictional drag as the drive nuts  104 ,  105  are tightened. The thrust bearing rings  102 ,  103  can be eliminated if the compression collars  96 ,  97  are fabricated of a good bearing material. 
     The seal housing  95  has at least a pair of flat surfaces  106 ,  107  formed intermediate its ends on an external perimeter for engagement by a wrench (not shown) to prevent rotation during torquing of the upper and lower drive nuts  104 ,  105 . For example, the external perimeter of the seal housing  95  can be hexagonal shaped. As shown in FIG. 6, the diameter of the threaded portion of the lower drive nut  104  is larger than the diameter of the threaded portion of the upper drive nut  105 . The inner diameter of the flange  108  of the lower drive nut  104  and the corresponding portions of the lower compression collar  96  are also larger than the inner diameter of the flange  109  of the upper drive nut  105  and the corresponding portions of the upper compression collar  97 . 
     A method of installing the seal arrangement  90  of FIGS. 6 to  8  will now be described. 
     As in the embodiments described above, the existing O-rings in the grooves  25  on the ICI  23  can be removed and discarded because they are not required with the seal arrangement  90  of the present invention. After the refueling is completed, the ICI  23  is reinserted to a proper depth. Spacers  51  are added according to the existing procedure to a defined height. The retainer nut  86  is then seated in the upper end of the ICI housing  24 . 
     The lower drive nut  104 , lower thrust bearing ring  102 , lower compression collar  96 , and seal rings of the lower seal assembly  93  are lowered over the outside of the ICI housing  24 . The seal housing  95  is then placed over the retainer nut  86  with its upper flange  110  seated against the upper surface of the retainer nut  86 . The lower seal assembly  93 , lower compression collar  96 , lower thrust bearing ring  102 , and lower drive nut  104  are then installed to the lower end of the seal housing  95 . The lower drive nut  104  is torqued using a wrench (not shown) until a sufficient load is placed on the lower seal assembly  93  to prevent leakage under full system pressure. A second wrench (not shown) is used to engage the flat surfaces  106 ,  107  on the seal housing  95  to prevent rotation of the seal housing  95  when the lower drive nut  104  is being torqued. 
     The upper seal assembly  94 , upper compression collar  97 , and upper thrust bearing ring  103  are then installed over the ICI  23  and seated into the upper end of the seal housing  95 . While pulling the ICI  23  up slightly by hand to close any gaps with the spacers  51 , the upper drive nut  105  is threaded onto the upper end of the seal housing  95  and torqued using a wrench until a sufficient load is placed on the upper seal assembly  94  to prevent leakage under full system pressure. 
     The seal arrangements  30 ,  85 ,  90  described above provide the following advantages over the existing technology: (1) no field cutting or welding are required; (2) the existing ICIs and ICI housings can be reused, even with damaged O-ring sealing surfaces; (3) the seal assemblies will seal the expected pressure without machining or polishing the existing parts; (4) the seal arrangements can be assembled quickly and easily by hand; (5) the ICIs do not need to be withdrawn any further than normal to install the seal arrangements; (6) the existing O-rings can be eliminated making the ICI removal and insertion process easier; (7) the seal arrangements can be fit and installed into smaller areas of access; and (8) the seal arrangements can be reused throughout the life of the plant. 
     While the invention has been specifically described in connection with specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.