Abstract:
A method and apparatus for inspecting the upper portion of a core shroud of a nuclear power plant is provided. The upper shroud scanner mounts on an arcuic section of a steam dam of the core shroud and moves back and forth there along. A vertical arm with transducers thereon extend down from a Y-car portion of the upper shroud scanner. Transducers adjacent the core shroud emit and receive an ultrasonic sound to inspect for flaws and defects in the core shroud.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a method and apparatus of inspecting welds and, more particularly, inspecting welds in an upper core shroud of a reactor vessel of nuclear power plant. 
         [0003]    2. Description of the Prior Art 
         [0004]    In a nuclear power plant, the nuclear reaction occurs inside of a reactor containment vessel which further has a reactor vessel therein. Inside of the reactor vessel is located a core shroud in which a nuclear reaction occurs. The inside of the core shroud is subjected to wide variations in temperature and pressure. As a result of the wide variations in temperature and pressure, metal fatigue could occur in the core shroud. To ensure that does not occur, or monitor potential problems if they do occur, there are requirements by the Nuclear Regulatory Commission that the core shroud be periodically inspected, especially any welds in the core shroud. 
         [0005]    In the past, various types of inspection devices for inspecting the core shroud have been developed such as is shown in U.S. Pat. No. 5,586,155 to Erbes et al. However, the invention has shown in the Erbes patent has some practical problems. The assembly mounts on the steam dam and is propelled around the steam dam by conical tractor drive wheels. Because of the slippage of the tractor drive wheels, the operator at the top of the reactor containment vessel cannot tell exactly where the sensors are located within the core shroud. 
         [0006]    The inspection of the core shroud occurs when the particular reactor of a nuclear power plant is shut down. While that reactor is shut down, the top of the reactor containment vessel is opened, the top of the reactor vessel is opened and the top of the core shroud is opened. Due to the nuclear radiation, the person performing the inspection has to remain at the top of the opened reactor containment vessel. The inspection device must be operated entirely from the top of the reactor containment vessel. Typically at the time the inspection is being made of the core shroud and the welds therein, numerous other activities are occurring in the shut-down unit of the nuclear power plant. Therefore, numerous people performing other functions will be at the open top of the reactor containment vessel. Hence, space at the top of the reactor container vessel is limited. 
         [0007]    One of the problems that existed for prior inspection methods of a core shroud is that they required a ring to be mounted all the way around the top of the core shroud, typically on the steam dam. This meant a lot of room had to be taken at the top of the reactor containment vessel during the period of shut down, which is when other people are needing space to perform their functions. Also, the shroud at the steam dam was not perfectly circular and many times the rings would not fit on the steam dam. 
         [0008]    In addition to the Erbes patent described herein above, other patents have been published and/or issued on various tools that can be used to inspect core shrouds from the top of the containment vessel. Such patents or patent applications include Johnson (U.S. Pat. No. 6,322,011), Ortega (U.S. Patent Publication No. US 2008/0165911), Morris (U.S. Patent Publication No. US 2007/0125190) and Morris (U.S. Patent Publication No. US 2008/0205575). Each of these patents show various types of ways of inspecting core shrouds located within a reactor vessel of a reactor containment vessel. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the present invention to provide an apparatus to inspect the upper welds on a core shroud of a nuclear power plant. 
         [0010]    It is another object of the present invention to provide an apparatus for inspecting the upper core shroud of a unit in a nuclear power plant, when the unit is shut down, the reactor containment vessel opened along with the reactor vessel to allow access to the top of the core shroud. 
         [0011]    It is another object of the present invention to provide an arcuic section of a rail that connects to the steam dam at the top of a core shroud, which rail has mounted thereto transducers for inspecting weld joints in an upper core shroud. 
         [0012]    When a unit of a nuclear power plant is shut down, the reactor containment vessel and the reactor vessel opened, access can be obtained to the core shroud. At that time, an arcuic rail making approximately a 30° arc is lowered into and clamped on the steam dam at the top of the core shroud. A Y-car is attached to the arcuic rail and is driven along the arcuic rail by gears with a gear rack on the arcuic rail. As the Y-car moves back and forth, transducers attached thereto inspect a series of welds on the core shroud. A vertical arm extends downward to inspect lower welds within the core shroud. Air cylinders are used to position the transducers adjacent to the welds being inspected and to move the vertical arm in and out of contact with the core shroud. The bottom transducer arm may be pivoted in and out of contact with a lower weld on the core shroud. 
         [0013]    Because the arcuic rail is a fairly short arc, i.e., of approximately 30°, not that much space at the top of the reactor containment vessel is needed during inspection. After the arcuic section of the core shroud of approximately 30° is inspected, clamps on either end of the rail are loosened from the steam dam. Simultaneously a lug clamp clamps to one of the lugs on the outside of the core shroud. By turning the gear that meshes with the gear rack, the entire arcuic rail and the Y-car mounted thereon is moved around the steam dam to inspect the next 30° section of the core shroud. 
         [0014]    During the inspection of a section of the core shroud, different transducers may be operated at different times, each of which would be inspecting a weld or an area around a weld. 
         [0015]    After the next section of the core shroud is inspected, the lug clamp is again loosened from its prior core shroud lug and moved around the arcuic rail so that it now clamps to a new core shroud lug that is at the opposite end of the arcuic rail. Then, rail clamps are loosened and the gear motor that turns the gear meshing with gear rack is again turned which slides the arcuic rail around the steam dam to inspect another section of the core shroud. Clamps are again clamped so that the arcuic rail securely attaches itself to the steam dam and the inspection process repeated. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a pictorial flow diagram of the operation of a nuclear power plant. 
           [0017]      FIG. 2  is a partial section pictorial view of a reactor vessel with a core shroud being shown therein with an upper shroud scanner located thereon. 
           [0018]      FIG. 3  is a partial top view of a reactor vessel and core shroud when the reactor containment vessel is opened with the upper shroud scanner being located therein and attached to the steam dam. 
           [0019]      FIG. 4  is a side view of the upper shroud scanner. 
           [0020]      FIG. 5  is a pictorial view of the upper shroud scanner being mounted on a section of the steam dam of the core shroud. 
           [0021]      FIG. 6  is a side pictorial view illustrating inspection of the lower welds, but a transducer is mounted on the bottom transducer arm. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    An illustrative flow diagram for a nuclear power plant for generating electricity is shown in  FIG. 1  and is represented generally by reference numeral  11 . The nuclear power plant  11  has a reactor containment vessel  13  that has a Taurus  15  with an auxiliary water feed  17 , which is a backup water supply for the nuclear power plant  11 . 
         [0023]    Inside of the reactor containment vessel  13  is located a reactor pressure vessel  19 . A bundle of fuel rods  21  absorb a neutron to cause nuclear fission and releases of other neutrons. The nuclear fission heats the water contained within reactor pressure vessel  19  to convert the water to steam. 
         [0024]    To ensure the bundle of fuel rods  21  remain immersed in water an internal reactor recirculation pump  23  continues to recirculate water over the bundle of fuel rods  21 . Also, an external reactor recirculation pump  25  circulates water within the reactor pressure vessel  19  to ensure the bundle of fuel rods  21  remain cool and immersed in the water. 
         [0025]    Inside the reactor pressure vessel  19  different fluids have been used, including gas, liquid metal or molten salts to ensure that the nuclear reaction does not run away. Control rods  27  are located in the bottom of the reactor pressure vessel  19 . The control rods  27  absorb some of the released neutrons to prevent too large of a nuclear reaction with the bundle of fuel rods  21 . 
         [0026]    Above the bundle of fuel rods  21  is located heat exchanger  29 , which is used to convert the water to steam. Steam generated in the reactor pressure vessel  19  enters steam line  33  through outlet nozzle  31 . The steam flows through the steam line  33  and the main steam isolation valve  35  to enter steam turbine  37 . As the steam turns the steam turbine  37 , steam turbine  37  turns generator  39 , which generates electricity. 
         [0027]    After the steam flows through the steam turbine  37 , a major portion of the steam flows through the main steam exit conduit  41  to condenser  43 . Circulating through the condenser coil  45  is cooling water received from the cooling tower  47  via condenser cooling water pump  49 , cooling water control valve  51  and cooling water inlet conduit  53 . The cooling water returns to the cooling tower  47  via cooling water return conduit  55  and cooling water return valve  57 . The cooling water can be of any convenient source such as lake water or river water. The cooling water does not have to be refined or processed. 
         [0028]    From condenser  43  through the feed water return conduit  59 , the water is being pumped by condenser pump  61  through water return valve  63  into a feed water heater/preheater  65 . The feed water flowing back to the reactor pressure vessel  19  is heated/preheated inside of feed water heater/preheater  65  which receives some of the steam flowing through steam turbine  37  through preheater steam conduit  67  and control valve  69  to feed through water heater/preheater  65 . The feed water heater/preheater  65  increases the temperature of the feed water significantly prior to returning to the reactor pressure vessel  19  via reactor feed pump  71 , main feed water isolation valve  73  and main feed water return conduit  75 . The main feed water is discharged into the reactor pressure vessel  19  through return nozzle  77 . 
         [0029]    Any remaining portion of the preheater steam received in the feed water heater/preheater  65  flows to condenser  43  through preheater steam conduit  79  and preheater steam control valve  81 . 
         [0030]    Inside of the reactor pressure vessel  19  is a core shroud  83  where a bundle of fuel rods  21  are located. The nuclear reaction occurs inside of the core shroud  83 . In  FIG. 2 , a perspective view of the reactor pressure vessel  19  and the core shroud  83  are shown. Connecting between the reactor pressure vessel  19  and the upper portion of the core shroud  83  are a series of downward extending pipes called down corners  85 . The down corners  85  have a tendency to interfere with devices that may be used to inspect core shroud  83  for defects or flaws. 
         [0031]    Referring to  FIGS. 2 and 3  in combination, a steam dam  87  is located on the top of the core shroud  83 . The steam dam  87  is a flange that extends upward about two or three inches above the top of the core shroud  83 . Mounted on the top of core shroud  83 , attached to the steam dam  87  and extending downward outside of the core shroud  83  is an upper shroud scanner  89 . The upper shroud scanner  89  has an outside flange  91  and an inside flange  93  clamped to the steam dam  87  by clamps  95  and  97 . Each of the clamps  95  and  97  are operated by air cylinders  99  and  101 , respectively. Mounted on the outside flange  91  and inside flange  93 , which are both clamped to the steam dam  87 , is a Y-car  103  that is driven by Y-car motor  105 . 
         [0032]    Referring to  FIGS. 4 and 5  in combination, the upper shroud scanner  89  will be explained in more detail. A gear rack  107  is located on the outside flange  91 , which is clamped to the steam dam  87  (see  FIG. 3 ). Y-car motor  105  drives gear  109  meshes with the gear teeth in gear rack  107 . For the turning of gear  109 , the entire upper shroud scanner  89  may be moved left or right on the outside flange  91  and inside flange  93 , which are clamped to the steam dam  87 . 
         [0033]    The Y-car  103  has a pivot arm base  111  extending outwardly therefrom. Extending downward from the pivot arm base  111  on pivot pin  113  is vertical arm  115 . Vertical arm  115  may be pivoted about pivot pin  113  by air cylinder  117 . The vertical arm  115  has a mounting plate  119  extending downward from pivot pin  113  to which everything is attached. The upper shroud scanner  89  is used to inspect an upper weld  121 , middle weld  123  and lower weld  125  in the upper portion of the core shroud  83 . Transducers will be used to inspect above and below each of the welds  121 ,  123  and  125 . 
         [0034]    Mounted on the Y-car  103  below the Y-car motor  105  is upper transducer  127 . Mounted on an upper lead screw  129  is upper moveable transducer  131 . Mounted on a lower lead screw  133  is a lower moveable transducer  135 . A transducer motor  137  turns pulley  139 , which operates belt  141 . The turning of belt  141  turns upper lead screw  129  and/or lower lead screw  131  to adjust upper moveable transducer  131  or lower moveable transducer  135  either up or down. Upper moveable transducer  131  should be adjusted until its positioned at or just below upper weld  121 . The lower moveable transducer  135  should be adjusted until it is adjacent or just above middle weld  123 . 
         [0035]    Mounted on lower bracket  143 , which is attached to mounting plate  119 , is roller  145 . The roller  145  sets the distance between the vertical mounting plate  119  and the core shroud  83  and allows for ease and movement of the entire upper shroud scanner  89  around the core shroud  83 . 
         [0036]    Mounted on a bottom transducer arm  147  are upper bottom transducer  149  and lower bottom transducer  151 . The upper bottom transducer  149  is used to check below middle weld  123  and above lower weld  125 . Lower bottom transducer  151  is used to check below weld  125 . The entire bottom transducer arm is pivotally connected around pivot pin  153 . The bottom transducer arm  147  may be pivoted out of the way by actuation of air cylinder  155  connected between the outside of roller bracket  143  and bottom transducer arm  147 . The entire bottom transducer arm  157  and everything mounted thereon can be pivoted out of the way when the upper shroud scanner  89  is being lowered into position or removed. 
         [0037]    During use of the upper shroud scanner  89 , the top of the reactor containment vessel  13  is removed and the top of the reactor pressure vessel  19  is also removed. From the top of the reactor containment vessel  13 , the upper shroud scanner  89  is lowered into position with the vertical arm  115  being between the reactor pressure vessel  19  and core shroud  83 . After the upper shroud scanner  89  is secured in position on the steam dam  87  by clamps  95  and  97 , the Y-car  103  may be positioned along the outside flange  91  by turning gear  109  which meshes with gear rack  107 . This permits the Y-car  103  along with vertical arm  115  to move around an approximately 30° arc formed by outside flange  91  and inside flange  93 . 
         [0038]    As the Y-car  103  moves around by the turning of the gear  109  and gear rack  107 , upper transducer  127  monitors the top surface  157  of the core shroud  83 , which in turn monitors the area above upper weld  121 . At the same time, upper moveable transducer  131  monitors the area below upper weld  121  in the core shroud  83 . Simultaneously, lower moveable transducer  135  monitors the area just above middle weld  123  of the core shroud  83 . 
         [0039]    Assuming the bottom transducer arm  147  is in the position as shown in  FIGS. 4 and 5 , upper bottom transducer  149  will monitor the area between middle weld  123  and lower weld  125 . Lower bottom transducer  151  will monitor the area below lower weld  125 . 
         [0040]    By moving the upper shroud scanner  189  back and forth along the arc formed by outside flange  91  and gear rack  107 , if there are any flaws in that arcuic portion of the core shroud  83 , they can be discovered. 
         [0041]    To move the upper shroud scanner  89  to a different arcuic section of the core shroud  83 , the clamps  95  and  97  are released by air cylinders  99  and  101 , respectively. Immediately prior to the release of the clamps  95  and  97 , the lug clamp  159  is secured between one of the lug pairs  161  shown in  FIGS. 3 and 5 . By knowing which of the lug pairs  161  the lug clamp  159  is between, the operator will know exactly where the upper shroud scanner  89  is located. 
         [0042]    With the lug clamp  159  securely in place between one of the lug pairs  161 , and the clamps  95  and  97  loosened from the steam dam  87 , now if the gear  109  is turned while meshed with gear rack  107 , the outside flange  91  and inside flange  93  will move arcuicly around steam dam  87  until the Y-car  103  reaches one end of the gear rack  107 . At that point, the clamps  95  and  97  are re-secured to the steam dam  87 . Thereafter, the lug clamp  159  is disconnected from one of the lug pairs  161  so that now when motor  105  turns gear  109  meshed with gear rack  107 , the Y-car  103  along with its vertical arm  115  all move along the gear rack  107 . Now another arcuic section of the core shroud  83  may be inspected. 
         [0043]    By the above described process of clamping and unclamping clamps  95  and  97  and lug clamps  159 , different arcuic sections of the core shroud  83  may be inspected. 
         [0044]    Because the area at the top of the reactor containment vessel  13  is at a premium when the reactor is shut down, the operator of the core shroud scanner  89  will only need to use a small area at a time. In that manner, there is less likelihood that the operation of the upper shroud scanner  89  will interfere with any other activities occurring while the unit of the nuclear power plant  11  is shut down. 
         [0045]    Referring to  FIG. 6 , more detail concerning the lower portion of the vertical arm  115  is shown. The middle weld  123  and lower weld  125  of the core shroud  83  is shown in further detail. As can be seen, the lower, moveable transducer  135  is inspecting near or above the middle weld  123 . The upper bottom transducer  149  is checking below middle weld  123  and above lower weld  125 . Lower bottom transducer  151  is checking below bottom weld  125 . The entire bottom transducer arm  147  may be pivoted on pivot pin  153  (see  FIG. 4 ) to get around the bottom lip  163  of the core shroud  63 . This has to occur when the upper shroud scanner  89  is being inserted between the reactor pressure vessel  19  and the core shroud  83 , or removed therefrom. 
         [0046]    Using the process just described, the upper welds of the core shroud  83  can be inspected by using very little of the area at the top of the reactor containment vessel  13 . The entire vertical arm  115  can be removed when the Y-car  103  moves behind some of the down corners  85  as shown in  FIG. 2 . Also,  FIG. 2  illustrated therein various inlet nozzles  165  or outlet nozzles  167 , which have to be worked around. 
         [0047]    While the transducers  127 ,  131 ,  135 ,  149  and  151  may be of any particular type, ultrasonic transducers have been found to be particularly good for this type of inspection. 
         [0048]    The entire upper shroud, not just the welds  121 ,  123  and  125 , can be inspected by appropriate movement of the upper moveable transducer  131  or lower moveable transducer  135 . The upper lead nut  169  causes removable transducer  131  to move up and down on upper lead screw  129  as it is turned by transducer motor  137  via pulley  139  and belt  141  (see  FIG. 4 ). Likewise, lower moveable transducer  135  may be moved up and down by lower lead nut  171  on lower lead screw  133  as it is turned by transducer motor  137 . In that manner, by adjusting upper moveable transducer  131  or lower moveable transducer  135  up and/or down and by back and forth movement of the Y-car  103 , the complete surface of the upper portion of the core shroud  83  can be inspected and a picture painted of its physical condition. Any flaws or defects would be detected.