Abstract:
An apparatus and method for maintaining contact between a pod of transducers and an inner surface of a reactor pressure vessel filled with water of a nuclear power plant is described. An underwater carriage carries the pod of transducers each of which is independently movable and are constantly urged against the surface of the vessel during inspection. Each transducer is independently pivotable about two axes. Each transducer emits and receives signals to detect any flaws of potential problems in the reactor pressure vessel.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an underwater carriage for the inspection of the inner surface of a pressure vessel of a nuclear reactor and, more particularly, to the mounting of the transducers on the underwater carriage. 
     2. Description of the Prior Art 
     Under the Atomic Energy Act of 1954, the United States Nuclear Regulatory Commission (“NRC”) has the authority to inspect nuclear power plants to protect public health and safety. A part of the NRC inspection program assesses whether the equipment is properly maintained to ensure safe operation. 
     Reactor inspections are conducted by independent inspectors to provide an assessment of the plant&#39;s condition and performance. One of the inspections that is routinely performed on a nuclear power plant is an inspection of the walls of the reactor vessel, reactor containment vessel, and/or core shroud. 
     There is a family of nuclear reactors known as light water reactors (LWR) which are cooled and modulated with ordinary water and tend to be simpler and cheaper to build than other types of nuclear reactors. LWRs can be sub-divided into three categories of (1) boiling pressure reactors (BPRs), (2) boiling water reactors (BWRs) and (3) super-critical water reactors (SCWRs). An LWR will have a containment vessel and a reactor pressure vessel. Generally, LWRs are divided into a BWR or BPR type of system. 
     Regardless of the type of nuclear power plant, the containment vessel, reactor vessel and other vessels used in generating nuclear power have to be inspected to make they are still structurally sound. Such inspections have to take place while the vessels are still filled with a fluid such as water, but while that particular unit of a nuclear power plant is not in operation. 
     In the past, carriages have been developed to move through water while simultaneously attaching itself to the walls of a vessel. A good example is U.S. Pat. No. 5,730,553, which is incorporated by reference, that has a skirt around the underwater carriage with thrust fans for attaching the underwater carriage to the wall of a vessel being inspected. The underwater carriage would then move around on the wall of the vessel being inspected. The underwater carriage may carry transducers for the inspection of walls of the vessel on which the underwater carriage is located. 
     Such an underwater carriage with the transducers being carried thereon is illustrated in U.S. Pat. No. 5,852,984, which is incorporated by reference. To ensure the transducers which are ultrasonic probes are in good contact with the surface being inspected, the &#39;984 Patent has sweeping units to make sure the surface of the walls of the vessel are clean and smooth. The ultrasonic probe can then be used to detect flaws and cracks in the walls of the vessel including nuclear reactor pressure vessels. However, in these prior designs, it was always a problem of maintaining the transducer or ultrasonic probe a uniform distance from the inside surface of the wall of the vessel. If a uniform distance is not maintained, false readings could be indicated to the operator at the surface, which could be misinterpreted as a flaw in the vessel. 
     Applicant has discovered a way to overcome these false readings. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus and method for attachment of a pod of transducers/probes to a carriage moveable in water. 
     It is a further object of the present invention to provide a method for attaching a pod of transducers/probes to an underwater carriage for inspection of the walls of a vessel filled with water. 
     It is a further object of the present invention to provide a method and apparatus for inspecting containment vessels, pressure vessels and/or nuclear reactor vessels while filled with a fluid such as water to determine the structural integrity of the vessels. 
     It is yet another object of the present invention to provide pods of transducers/probes attached to an underwater carriage that moves along the internal surface of a vessel, which pods are used to determine the structural integrity of the vessel while the underwater carriage moves along its internal surface. 
     In a nuclear power plant, numerous vessels such as a containment vessel, pressure vessel, nuclear reactor vessel, just to name few, have to be inspected on a regular basis to ensure their structural integrity. These vessels are normally filled with a fluid such as water. When the nuclear reactor unit is not in use, a reactor containment vessel and/or reactor pressure vessel can be opened at the top and an underwater vehicle lowered therein. The underwater carriage may have transducer/probes thereon which emit ultrasonic signals into the walls of the vessel. The ultrasonic signals will reflect off of any defect in the vessel, which reflected signals are received and recorded indicating a flaw or structural weakness in the vessel. 
     Because a single transducer/probe may give inaccurate readings depending upon movement of the transducer/probe along the surface of the vessel, a collection or pod of transducer/probes is found to be much more effective in determining the structural integrity of the vessel. However, even using a pod of transducers/probes, it is important to maintain as many of the transducers/probes as close to or against the inside surface of the vessel as possible. Structural items in the vessel may limit movement of the underwater vehicle inside of the vessel. By extending the pod outward on either the left or right side, a much more thorough and complete inspection of the vessel can be obtained to more accurately determine if there is a structural flaw in the vessel. A complete and thorough inspection is extremely important in nuclear reactor vessels because the vessels are subject to extremes of temperature and pressure. 
     The present invention provides for an apparatus and method for attaching a pod of transducers/probes to an underwater carriage. The transducers/probes are contained in the pod and are independently articulated and urged against the walls of the vessel being inspected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative pictorial view of a nuclear power plant. 
         FIG. 2  is an enlarged perspective view of the inside of the reactor vessel of  FIG. 1  with the present invention located therein. 
         FIG. 3  is a top view of an underwater carriage having a pod of transducers pivotally mounted on bearing rails on a left front side thereof. 
         FIG. 4  is an enlarged view of a portion of  FIG. 1 . 
         FIG. 5  is an exploded perspective view of a pod of transducers shown in  FIG. 4 . 
         FIG. 6  is a top view of an underwater carriage having a pod of transducers pivotally mounted on bearing rails on a right front side thereof. 
         FIG. 7  is an alternative view for connecting a pod of transducers to a bearing rail mounted on an underwater carriage. 
         FIG. 8  is an exploded perspective view of the pod of transducers shown in  FIG. 7 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 . 
     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 release of other neutrons. The nuclear fission heats the water contained within reactor pressure vessel  19  to convert the water to steam. 
     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. 
     While in 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 . 
     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. 
     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. 
     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 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 . 
     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 . 
     The temperature and the pressure inside some of the vessels, especially the reactor pressure vessel  19 , can vary greatly. Pressure can be in the thousands of pounds per square inch. Temperatures inside of the reactor pressure vessel can be as much as 130° F. Because of the large variations in temperature and pressure, it is important to inspect all of the vessels, especially the reactor pressure vessel  19  to make sure the vessel is still structurally sound. The present invention is directed towards an underwater carriage that has transducers mounted thereon for inspecting the walls of a vessel (such as the reactor pressure vessel  19 ) while the vessel is filled with a fluid such as water. 
     Referring to  FIG. 2 , which is an enlarged partial view of the inside of the reactor pressure vessel  19 , an underwater carriage  83  is attached to the inner surface  85  of the reactor pressure vessel  19 . Nozzle  84  allows a fluid such as water to flow into the reactor pressure vessel  19 . Mounted on rail support  87  is a right Y-arm offset  89 . On the front of the right Y-arm offset  89  is a pod  91  of transducers, all of which will be explained in more detail subsequently. Bundled cable  93  carries the signals from underwater carriage  83  and the pod of transducers  91  back to the surface. 
     During an inspection as pictorially illustrated in  FIGS. 1 and 2 , the nuclear power plant  11  is shut down. The top of the reactor containment vessel  13  is removed. The top of the reactor pressure vessel  19  is also removed. With the reactor pressure vessel  19  still filled with water or some other similar fluid, the inspection of the reactor pressure vessel  19  occurs. The underwater carriage  83  is lowered into the water (not shown because the water is clear) and attached to the inner surface  85  of the reactor pressure vessel  19 . 
     As will be explained in detail subsequently, the underwater carriage  83  in combination with the pod  91  of transducers located thereon can inspect the reactor pressure vessel  19  with a maximum amount of contact by the transducers with the inner surface  85 . This allows the maximum of the nooks and crannies on the reactor pressure vessel  19  to be inspected with the maximum overall coverage. 
     Referring to  FIG. 3 , a top view of an underwater carriage  83  is shown. On the front of the underwater carriage  83  is located a rail support  87 . On the top of the rail support  87  are located bearing rails  95 . The left end  97  of the bearing rail  95  extends beyond the left side  99  of underwater carriage  83 . Left and/or right is determined by the direction of movement of the underwater carriage  83 , the same as left and right is determined on an automobile. 
     Mounted on the bearing rails  95  are bearing slide rails  101  and  103 . Pivotally attached to bearing slide rails  101  and  103  is left Y-arm offset  105 . Left Y-arm offset  105  is continually urged downward by spring  107 . Mounted on the bearing rail  95  and bearing slide rail  101  is Y-arm bracket  109 . 
     On the front of the left Y-arm offset  105  is located a pod  111  of transducers  113 . Each of the transducers  113  connect through electrical connection  115  via the cable bundle  93  (see  FIG. 2 ) to the top of the reactor containment vessel  19  where the measurements are physically being recorded. 
     Referring to  FIG. 4 , there is an enlarged view of the left end  97  of the rail support  87  showing the left Y-arm offset  105  with the transducers  113  mounted thereon. As can be seen in  FIG. 4 , the spring  107  continually urges the transducers  113  against the surface being inspected. The transducers  113  are connected to the left offset bracket  105  by shoulder bolt  117 . Rotational movement of the transducers  113  may be limited by limiting bar  119 . 
     Referring to  FIG. 5 , an exploded perspective view of the transducer pod  91  is shown. Each of the transducers  113  are further designated by the letters A, B, C or D. Likewise, the electrical connections  115  are designated with letters A, B, C and D corresponding with transducers  113  that bear the same letters A, B, C, or D. 
     Between transducers  113 A and  113 C is the shoulder bolt opening  121  to receive the shoulder bolt  117  therein (see  FIG. 4 ). The shoulder bolt opening  121  is located in pivot bracket  123 . Pivot bracket  123  holds the limiting bar  119  and is connected on the bottom thereof to the transducer center rod  125 . 
     The transducer center rod  125  extends through transducer center rod opening  127  located through the bottom of pivot bracket  123 . Each end of the transducer center rod  125  extends into a back side opening (not shown) in transducer end bracket  129 . The transducer end bracket  129  is held onto the end of transducer center rod  125  by end screws  131 . The transducer end bracket  129  is held to transducer  113 C by side screws  133 . Side screws  133  extend through hole  135  in transducer end bracket  129  to threadably connected in threaded hole  137  in each side of base  139  of transducer  113 C. 
     While not shown and explained in detail, transducer  113 A connects in the same manner as transducer  113 C explained herein above. Transducers  113 A and  113 C can pivot around the center line of the transducer center rod  125  or around the center line of the shoulder bolt  117  (see  FIG. 4 ). 
     Pivotally connected on the transducer center rod  125  is a rear transducer bracket  141  and a front transducer bracket  143 . Rear transducer bracket  141  is connected to transducer  113 D by rear transducer screws  145  threadably connecting to the base  147  of transducer  113 D. While not visible in  FIG. 5 , another rear transducer bracket connects to the other side of transducer  113 D in the same manner as rear transducer bracket  141 . 
     Front transducer bracket  143  also is pivotally connected on transducer center rod  125  and is bolted to the base  149  of transducer  113 B by front transducer screws  151 . It is important that each of the transducers  113 A,  113 B,  113 C and  113 D have some movement capability so that they can as a group continually be urged in contact with the surface being probed. It is also important that the transducers  113 A,  113 B,  113 C and  113 D move independently of each other to the degree possible. Transducers  113 B or  113 D can rotate independently of any other transducer around transducer center rod  125 . Springs  153  continually urge transducers  113 B and  113 D downward around the transducer center rod  125  and against whatever surface is being probed. To prevent excessive rotation by transducers  113 B and  113 D, limiting rods  119  may abut against shoulder  155  of front transducer bracket  143  or shoulder  159  of front transducer bracket  143 . 
     By having independent pivotal movement of transducers  113 A,  113 B,  113 C and  113 D, the respective transducers can maintain better contact with the surface being inspected. For example, the reactor pressure vessel  19 , as seen in  FIG. 2 , has a curved inner surface  85 . On the curved inner surface  85  there may be some roughness of the surface due to manufacturing or deposits forming thereon. Hence it is important for the transducers  113 A,  113 B,  113 C and  113 D to maintain as good a contact with the inner surface  85  as possible. Therefore, each of the transducers are independently moveable while the entire pod  91  is continually being urged against the inner surface  85  by spring  107  (see  FIG. 3 ). 
     In addition to the pivotal movement of the transducers  113 A,  113 B,  113 C and  113 D as described herein above, each of the transducers can rotate about the axis formed by the screws holding the transducers to respective brackets. For example, transducer  113 C can rotate about the axis formed by screws  133 . Transducer  113 A can rotate in a similar manner. 
     Likewise, transducer  113 B can rotate about the axis formed by front transducer screws  151 . Transducer  113 D can rotate about the axis formed by rear transducer screws  145 . 
     Referring now to  FIG. 6 ,  FIG. 6  is identical to  FIG. 3 , except left offset bracket  105  has been replaced with right offset bracket  161  and the rail support  87  has been relocated on the underwater carriage  83  so that the pod  111  is now located on the right end  163  of the rail support  87 . Depending upon the obstruction within the vessel being inspected, the pod  111  of transducers  113  can be located on the left side of the underwater carriage  83  as shown in  FIG. 3 , or on the right side of the underwater carriage  83  as shown in  FIG. 6 . This allows for versatility in the position and location of the pod  111  of transducers. 
     The arrangement of transducers  113  in the pod  91  as shown in  FIG. 5  is commonly referred to as a “little t” arrangement. There are other arrangements for the transducer pods. A different arrangement is shown in  FIGS. 7 and 8 , which is commonly referred to as a “big T” arrangement of transducers. In  FIG. 7 , the underwater carriage  83  has a rail support  165  mounted on the front thereof. The bearing rails  95  are the same as previously described along with bearing slide rail  101  and bearing slide rail  103 . Also, spring  107  continually urges the pod  167  of transducers  169  against the surface being inspected. The transducers  169  are mounted on the forward end of triangular bracket  171 . Triangular bracket  171  is pivotally mounted at the rear thereof to bearing slide rail  101  and bearing slide rail  103 . 
     Referring now to  FIG. 8 , which is an exploded perspective view of the pod  167  of transducers  169 , the entire pod  167  is mounted on the front end of the triangular bracket  171  (see  FIG. 7 ) by connecting to center pivot bracket  173 . A bolt  179  extends through opening  177  in side transducer bracket  175  and opening  181  in center pivot bracket  173  prior to threadably connecting to the front end of triangular bracket  171  (see  FIG. 7 ). The bolt  179  has a smooth shank on the portions thereof extending through openings  177  and  181  to allow for free pivotal movement around the axis of bolt  179 . Each of the transducers  169 A,  169 B,  169 C and  169 D electronically connect through electrical connections  183 A,  183 B,  183 C and  183 D, which form a part of the cable  93  shown in  FIG. 2 , back to the area at the top of the reactor containment vessel  13  (see  FIGS. 1 and 2 ) where test personnel are located. 
     As shown in  FIG. 8 , the center pivot bracket  173  has a bolt  185  threadably connecting in each side thereof. The bolts  185  extends through opening  187  in rear transducer bracket  189  and opening  191  in center transducer bracket  193  to threadably connect with threaded hole  190  in center pivot bracket  173 . Rear transducer bracket  189  and center transducer bracket  193  can freely rotate around the smooth portion of the shank of bolt  185 . While not visible in  FIG. 8 , identical rear transducer bracket  189  and center transducer bracket  193  are located behind transducers  169 A and  169 D in a similar manner to the connections just described. The rear transducer bracket  189  is connected to transducer  169 A by rear transducer screws  195 . Transducer  169 A may rotate about the axis of rear transducer screws  195 . The center transducer  169 D is connected to the center transducer bracket  193  by center transducer screws  197 . Transducer  169 D is free to rotate about the axis of the center transducer screws  197 . However, the amount of rotation is limited due to the length of the slot  199  in center pivot bracket  173  through which electrical connection  183 D extends. While not shown in  FIG. 8 , springs similar to springs  153  as shown in  FIG. 5 , will be utilized in  FIGS. 7 and 8 . The springs will continually urge transducers  169 A and  169 D into contact with the surface being inspected. 
     Attached to the front lower portion of side transducer bracket  175  are side transducer brackets  201 . Side transducer brackets  201  are attached to the lower ends of side transducer brackets  175  by side transducer bracket screws  203 . Transducer  169 B is free to rotate around the axis of side transducer bracket screw  203 . Side transducer screws  205  pivotally attach transducer  169 B to side transducer bracket  201 . Side transducer screws  205  threadably connect to the base  207  of transducer  169 B. Transducer  169 B is free to rotate about the axis of side transducer screws  205 . While not explained in further detail, transducer  169 C is connected in a manner similar to  169 B. 
     By mounting of the transducer pods  167  in the manner as just described in conjunction with  FIGS. 7 and 8 , the independent movability of each of the transducers  69 A,  69 B,  69 C and  69 D is maintained to give the maximum contact with the surface being inspected. Also, the pivotal mounting of triangular bracket  171  and the continual urging of spring  107  urges the entire pod  167  in contact with the surface being inspected. The individual movability of each of the individual transducers  169 A,  169 B,  169 C and  169 D also helps ensure the maximum contact with the surface of the vessel being inspected. This gives a better signal, which in turn gives a more accurate reading and determination as to the condition of the vessel under inspection.