Abstract:
A temporary cover is plugged into an open handhole or inspection port in a steam generator or another pressure vessel of a nuclear power plant after bolted covers are removed for maintenance inside the generator or vessel. The temporary cover may be locked to prevent foreign objects from entering the generator or vessel. The cover is vented at inclined angles so that air or gas may pass therethrough but is shielded so that gamma rays, which are the most penetrating type of radiation, are greatly reduced and dispersed away from personnel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to and claims domestic priority from U.S. Provisional Patent Application Ser. No. 60/315,919 filed on Aug. 29, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a steam generator of a nuclear power plant and, in particular, to a temporary plug cover for a hole or a port in the steam generator. 
     2. Description of the Related Art 
     The prior art references developed as a result of a preliminary patentability search are listed below. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 U.S. Pat. No. 
                 Inventor(s) 
                 Issue Date 
               
               
                   
                   
               
               
                   
                 5,850,423 
                 Rusnica, Jr. et al. 
                 Dec. 15, 1998 
               
               
                   
                 4,948,981 
                 Wallace et al. 
                 Aug. 14, 1990 
               
               
                   
                 4,932,553 
                 Reich, Jr. et al. 
                 Jun. 12, 1990 
               
               
                   
                 4,860,919 
                 Weisel et al. 
                 Aug. 29, 1989 
               
               
                   
                 4,624,824 
                 Dooley 
                 Nov. 25, 1986 
               
               
                   
                 4,524,729 
                 Hill, Jr. et al. 
                 Jun. 25, 1985 
               
               
                   
                 4,192,053 
                 Blanco et al. 
                 Mar. 11, 1980 
               
               
                   
                   
               
               
                   
                 Foreign Patent 
                 Inventor 
                 Publ. Date 
               
               
                   
                   
               
               
                   
                 JP 5-172982 
                 Maekawa 
                 Jul. 13, 1993 
               
               
                   
                   
               
             
          
         
       
     
     We note at the outset that the search had not developed any references which taken alone or in combination might be considered to anticipate or render obvious the combination of features in the invention. So, the above references are considered as being of secondary interest. 
     Rusnica, Jr. et al., Weisel et al. and Dooley show various types of plugs for sealing holes or openings in reactor-related apparatuses. However, none have any significant teaching of the features of the present invention. 
     Wallace et al., Reich, Jr., et al. and Blanco et al. show the use of lead shielding in reactor-related covers or doors. In the patent to Wallace et al., see the language of col. 2 at line 53. In the patent to Reich, Jr. et al., see the language of col. 4 at line 8. The patent to Blanco et al. teaches both lead shielding as well as the use of openings in the shield for the insertion of a camera. See the embodiment of FIGS. 12 and 13 along with the description of col. 4 beginning at line 60. 
     Japanese Kokai No. 5-172982 to Maekawa provides a teaching in FIG. 4 of a scheme for securing a cover 20 to a steam generator opening using a nut screwed onto what could be a captive bolt in a blind hole anchored in a vessel wall. 
     SUMMARY OF THE INVENTION 
     The present invention is characterized by three distinct structural features of a cylindrical cover used temporarily to plug an opening in a steam generator. The features are: an integrally formed lead shielding layer; radiation channeling and diffusing ventilation holes; and a mechanical retainer. 
     The cover is particularly adapted for temporarily closing openings in a steam generator associated with a nuclear power plant. The cover is cylindrically shaped to fit snugly into round openings in the vessel walls, such as man holes, hand holes, inspection ports, and the like, during maintenance. 
     As disclosed in a first embodiment which is one of three illustrative embodiments, a cylindrical sleeve formed of stainless steel may have a diameter in the range of several inches and include a layer of lead plate shielding on its inner face. The layer of lead plate may have a thickness of two inches or more. An elongated retaining bar is affixed to its outer face. An array of nominally longitudinal vent holes are drilled through the lead plate. Each hole is typically oriented at a 7 to 25 degree angle to the longitudinal axis. This angle depends upon the diameter of the temporary cover used. The angled holes allow ventilation through the lead plate but channel and diffuse radiation that is axial to the plate away from an operator. During maintenance of the steam generator, any handhole or inspection port that is opened is temporarily fitted with a cover of suitable diameter and retained in place by the elongated bar which is anchored to a flange by a lock which engages a slot permanently cut in the elongated bar. 
     In a second embodiment, the elongated bar is secured directly to a flange of the handhole or the inspection port. In a third embodiment, two of the ventilation holes are enlarged in diameter to allow the insertion of a camera and/or a retrieval tool into the generator. 
     Other objects and features of the present invention will become apparent from the following detailed description when considered in connection with the accompanying drawings which illustrate preferred embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cutaway perspective view of a steam generator in a nuclear power plant. 
     FIG. 2 is a partially cutaway front elevation view of a lower portion of the steam generator. 
     FIG. 3 is a perspective view of a first embodiment of the present invention. 
     FIG. 4 is a front elevation view of the first embodiment. 
     FIG. 5 is a cross-sectional side elevation view taken along line  5 — 5  in FIG.  4 . 
     FIG. 6 is an enlarged, detailed, side elevation view of one end of the first embodiment shown in FIG.  3 . 
     FIG. 6A is a side elevation view of a pin which can be used instead of the bolt shown in FIG.  6 . 
     FIG. 7 is a cross-sectional top plan view of a second embodiment. 
     FIG. 8 is a front elevation view of a third embodiment. 
     FIG. 9 is a rear perspective view of the third embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a pressurized-water, nuclear-powered, electric generating plant, heat generated by a nuclear reactor is absorbed by a primary coolant that circulates through a reactor core and transfers heat to produce steam inside a steam generator  10  shown in FIG.  1 . The steam generator  10  is typically an upright cylindrical pressure vessel with an upper hemispherical end section  12  and a lower hemispherical end section  14 . The steam generator  10  has an upper external steel shell  16  and a lower external steel shell  18 . Note that the upper shell  16  has a wider diameter than the lower shell  18  and is separated therefrom by a transition cone  20 . 
     Inside the lower shell  18  and the transition cone  20 , there is a wrapper barrel  22  of thin carbon steel surrounding a tube bundle  24  which is an array of individual tubes extending from an upper portion of the transition cone  20  downwardly to a predetermined space at a bottom of the lower shell  18 . 
     The tube bundle  24  inside the wrapper barrel  22  is the main source of heat transfer and also is a radiation source emitting most of the doses absorbed by nuclear power plant workers. 
     The wrapper barrel  22  is supported by wedges  26  and anti-rotation devices (not shown) inside an annular space  28  formed between the wrapper barrel  22  and the lower shell  18 . 
     The upper shell  16  has its interior space dedicated to separating moisture from wet steam which is generated during an operating cycle. Primary moisture separation is accomplished through angled vanes  30  inside swirl barrels  32  that throw off excess moisture through centrifugal steam action. 
     Entrance into the interior space of the upper shell  16  is made by a worker through one of two manways  34  of which each has a circular opening with a 16-inch diameter. Because of the distance from each manway  34  to the tube bundle  24 , there is generally a low field of only one to five millirads of gamma rays per hour at the opening such that the radiation dosage is not a cause for concern about the worker&#39;s health. 
     The main concern at this height of the generator  10  is venting for a safe atmosphere, i.e. oxygen. Also, there is a need to document equipment that is introduced and persons who enter through the manway  34 . No lead shielding is necessary at this upper level. 
     For the sake of completeness, other elements of the upper shell  16  shown in FIG. 1 are a steam nozzle  36 , positive entrainment steam dryers  38 , and a feed water nozzle  40 . At the lower shell  18 , there is a thick horizontal plate called a tubesheet  42  surrounded by a support ring  44 . 
     In FIG. 2, the lower hemispherical end section  14  and the bottom of the lower shell  18  are shown. Also, the wrapper barrel  22 , the tube bundle  24 , and the annular space  28  are illustrated in two cutaway views. 
     Along the curvature of the end section  14 , there is a primary water inlet nozzle  46  and a primary water outlet nozzle  48 . At a transition area between the end section  14  and the bottom of the lower shell  18 , there is the support ring  44 . 
     Access to the area of the tubesheet  42  of FIG. 1 inside the support ring  44  of FIG. 2 is made through either two or four six-inch openings in handholes  50  situated at either 180° or 90°, respectively, around the bottom of the lower shell  18 . However, the six-inch diameters of the openings in the handholes  50  may vary, depending upon the model of the steam generator  10 . 
     The handholes  50  in FIG. 2 have the same general purpose as the manway  34  shown in FIG. 1, i.e. to maintain, service and inspect vital components inside the generator  10 . However, the three embodiments deal primarily with the handholes and the inspection ports due to the higher level of radiation inside the lower shell  18  than inside the upper shell  16 . 
     Other circular openings, such as visual inspection ports  52  seen in FIG. 2, can have the same but generally smaller diameters than the diameters of the handholes  50 . These ports  52 , like the handholes  50 , may be arranged around an outer circumference of the lower shell  18  at either 90° or 180° from each other. In FIG. 2, three of the four handholes  50  and their corresponding inspection ports  52  are seen at 90° from each other. 
     Each handhole  50  have a flange  51  with bolt holes  51 A. Likewise, each inspection port  52  has a flange  53  with bolt holes  53 A. 
     In FIG. 3, a perspective view of a first preferred embodiment of the present invention is shown. A temporary cover  60  has a cylindrical metal sleeve  62  with an outer diameter which is slightly smaller than an inner diameter of the opening. In other words, the handhole  50  or the inspection port  52  in FIG. 2 is matched with the sleeve  62  in FIG.  3  and the sleeve  62  is inserted in the selected handhole  50  or port  52 . 
     A solid lead plate shield  64  has a thickness which reduces by absorption the radiation from a source inside the tube bundle  24  of FIGS. 1 and 2 to approximately one-tenth or less of its initial energy. Depending upon the necessary shielding required and the intensity of the radiation source, the shield  64  with a thickness of two inches of lead is usually sufficient to obtain the desired reduction in harmful gamma rays. 
     Vent holes  66  are machined by drilling into the shield  64  to provide for adequate circulation of air. The plurality of vent holes  66  typically comprises approximately five cubic inches of volume in the cylindrical shield  64  which has a six-inch diameter and a two-inch thickness. A mesh screen  68  is placed entirely over an external surface of the shield  64  to prevent the accidental or purposeful insertion of foreign objects into one or more of the vent holes  66 . 
     An elongated bar  70  is fillet-welded to an outer circumferential edge  72  of the sleeve  62  in order to prevent the sleeve  62  from slipping into the opening in either the handhole  50  or the inspection port  52  in FIG.  2  and falling down on top of the tubesheet  42  seen in FIG.  1 . 
     Returning to FIG. 3, the bar  70  has a slot  74  cut into one end through which a D-shaped ring  76  on a head of a bolt  78  is secured by a lock  80 . Although the lock  80  may be opened and closed with a key (not shown), a resettable combination is preferred because it can be changed from time to time. 
     The temporary cover  60  is now ready to be plugged and secured into the opening of either the handhole  50  or the inspection port  52  of FIG. 2 by the elongated bar  70  which serves as a handle. Thus, the cover  60  functions as a radiation block via the lead plate shield  64 , a vent through the plurality of holes  66 , a foreign object excluder due to the mesh screen  68 , and a nonremovable device because of the D-shaped ring  76  which is secured in the slot  74  of the bar  70  by the lock  80 . 
     In FIG. 4, there is shown a top plan view of the cover  60  without the mesh screen  68  of FIG.  3 . The elongated bar  70  is fillet-welded onto the outer circumferential edge  72  and extends over two sides of the sleeve  62 . The plurality of vent holes  66  is arranged around an outer periphery of the lead plate shield  64 . In this first embodiment, there are  12  vent holes  66  in an equally spaced array from the center of the lead plate shield  64 . Of course, more or less vent holes  66  of small or larger diameters, respectively, may be drilled as long as the total area of openings for the vent holes  66  and their angles do not impede shielding. The size of the vent holes  66  and their angles will vary with the diameter and the thickness of the shield  64 . The slot  74  is seen in the one end of the elongated bar  70  without the D-shaped ring  76  and the lock of FIG.  3 . 
     In FIG. 5, a cross-sectional, side elevation view taken along line  5 — 5  in FIG. 4 shows the elongated bar  70  of the cover  60  to be a hollow square tube. Inside the outer edges  72  of the sleeve  62 , there is the mesh screen  68  fillet-welded in front of the solid lead plate shield  64 . Two of the 12 vent holes  66  drilled through the lead plate shield  64  are shown to be inclined at an angle. This angle may vary from 70 to 25° from the longitudinal axis of the sleeve  62 . 
     Because gamma rays travel in a linear manner, i.e. in a straight line, from their radiation source and do not bend around corners, the inclined vent holes  66  prevent the majority of the gamma rays from escaping therethrough while simultaneously allowing air to circulate in and out of the openings. Gamma radiation that emerges through the vent holes  66  is channeled away from the operator. In other words, the majority of gamma rays from the radiation source on the right side of the lead shield  64  are absorbed and diffused by the lead in the plate shield  64  at the same time that air is adequately vented through the holes  66 , thus protecting a worker on the left side of the temporary cover  60 . 
     FIG. 6 shows a close-up detailed view of the one end of the elongated bar  70 . At this one end, the slot  74  receives the D-shaped ring  76  formed integrally on a head  78 A of the threaded bolt  78 . A body  78 B of the bolt  78  is threaded, with reference to FIG. 2, into either one of the bolt holes  51 A in the flange  51  or one of the bolt holes  53 A in the flange  53  so that the temporary cover  60  of FIG. 3 may be secured by the lock  80  into either the handhole  50  or the inspection port  52 , respectively, seen in FIG.  2 . 
     FIG. 6A shows an unthreaded pin  71  which may be substituted for the threaded bolt  78  in FIG.  6 . The pin  71  has a head  71 A and a body  71 B. The D-shaped ring  76  is welded to a distal end of the body  71 B instead of to a top of the head  71 A. The lock  80  in FIG. 3 is secured through the D-shaped ring  76  to prevent the pin  71  from slipping out of the slot  74  in FIG.  6 . 
     FIG. 7 shows a second embodiment of the temporary cover  60  plugged into the inspection port  52 . Inside the outer edges  72  of the cylindrical sleeve  62 , there is a mesh screen  68  placed over the lead plate shield  64  through which vent holes  66  are inclined at an angle in the range of 7° to 25° from the front outer periphery towards a rear center of the lead plate shield  64 . 
     However, the cover  60  of the second embodiment differs from the cover  60  of the first embodiment shown in FIGS. 3-6A in that the elongated bar  70  in FIG. 7 extends beyond only one side of the outer circumferential edge  72  of the sleeve  62 . 
     The slot  74  at the one end of the bar  70  receives a leg  82  of the lock  80  instead of receiving the D-shaped ring  76  of the threaded bolt  78  illustrated in FIGS. 3 and 6. In FIG. 7, the leg  82  of the lock  80  extends through one of the bolt holes  51 A in the flange  51  of the handhole  50 . Alternatively, the leg  82  may extend through one of the bolt holes  53 A in the flange  53  if the inspection port  52  of FIG. 2 is being temporarily plugged by the cover  60 . 
     With reference to FIGS. 2 and 7, the reader must realize that usually, during operation of the pressurized steam generator  10  shown in FIG. 1, the handholes  50  and the inspection ports  52  are tightly and permanently plugged by a heavy cover (not shown) that is secured over a gasket (not shown) by bolts or studs and nuts (not shown) threaded through the bolt holes  51 A and  53 A of the flanges  51  and  53 , respectively. 
     However, when the nuclear power plant is shut down periodically for routine maintenance, each steam generator  10  is taken out of service and cleaned. To prevent radiation from escaping through the handholes  50  and the inspection ports  52  seen in FIG. 2, a plurality of the temporary covers  60  are plugged therein. 
     Although the second embodiment of FIG. 7 is used where the bolt holes  51 A and  53 A of FIG. 2 are unthreaded bores extending completely through the flanges  51  and  53 , respectively, the first embodiment of FIGS. 3-6A can also be used where the bolt holes  51 A and  53 A of FIG. 2 are unthreaded holes in the flanges  51  and  53 , respectively, because the pin  71  of FIG. 6A can be inserted from a rear of the flanges  51  and  53  and still hold the cover  60  of FIGS. 3-6A in either the handhole  50  or the inspection port  52  of FIG.  2 . 
     In FIG. 8, there is illustrated a third embodiment of the temporary cover  60  with its elongated bar  70  fillet-welded to the outer circumferential edge  72  of the cylindrical sleeve  62 . Inside the sleeve  62 , there is the lead plate shield  64 . 
     However, instead of having the plurality of small vent holes  66  as in the first embodiment of FIGS. 3-6A and the second embodiment of FIG. 7, the third embodiment has only two large vent holes  66  which allow an operator&#39;s hand H to insert two guide tubes (not shown). Through these guide tubes, a small camera  84  or a retrieval tool (not shown) are inserted for finding and grasping a dropped tool (not shown) or a foreign object (not shown). 
     Note that the elongated bar  70  does not have a slot  74 . The reason for this omission is that this third embodiment is intended for immediate use and is not supposed to be left unattended after a foreign object is removed. Unlike the first embodiment of FIGS. 3-6A and the second embodiment of FIG. 7, the third embodiment of the temporary cover  60  cannot be locked in place and left overnight. Thus, when the operator is finished making a search and a retrieval, he or she must promptly replace the temporary nonlocking cover  60  of FIGS. 8 and 9 with one of the temporary locking covers  60  of FIGS. 3-6A or  7 . The permanent cover (not shown) that is usually bolted to flange  51  or  53  of the handhole  50  or the inspection port  52 , respectively, seen in FIG. 2, is then installed after all maintenance is finished. 
     In FIG. 9, there is illustrated a rear perspective view of the third embodiment of the temporary cover  60 . The cylindrical sleeve  62  is seen with the shield  64  having the two slightly inclined vent holes  66  bored therethrough and opened to the outer circumferential edge  72  of the sleeve  62 . The elongated bar  70 , which serves as a handle to put the cover  60  into place and to prevent the cover  60  from falling inside the steam generator  10  of FIG. 1, is attached to the outer edge  72  of the sleeve  62 . 
     The cover  60  is kept in place in the handhole  50  or the inspection port  52  during an inspection being made when the two guide tubes (not shown) are inserted through the large vent holes  66  illustrated with open sides extending to the outer circumferential edge  72  of the cylindrical sleeve  62  in FIGS. 8 and 9. The small camera  84  is slipped through one guide tube (not shown) to search for and find the foreign object. When it is located, the retrieval tool (not shown) is slipped through the other guide tube (not shown) to grasp and remove the foreign object (not shown). 
     The outer circumferential edge  72  of the cylindrical sleeve  62  is smooth so as to allow quick and easy removal of the cover  60  from the handhole  50  or the inspection port  52 . Thus, the foreign object will not be dropped back in and possibly lost again inside the steam generator  10 . Furthermore, when the cover  60  is in place in the handhole  50  or the inspection port  52 , it provides protection by reducing radiation to less than one-tenth of its value before the gamma rays strike the shield  64 . 
     Because the two vent holes  66  are so large and are inclined at only a slight angle, more radiation escapes through the holes  66  in this third embodiment than the first embodiment of FIGS. 3-6A and the second embodiment of FIG.  7 . However, the operator is aware of the increased exposure to the gamma rays. Therefore, he or she will know to stay out of the direct path of the radiation or stay behind lead shielding before undertaking any work using this third embodiment of the cover  60 . 
     The above-described embodiments are not intended to be the only manner in which the invention is made. Instead, the scope and the spirit of the invention are defined by the appended claims.