Abstract:
The present application discloses a magnetostrictive sensor (MsS) probe for guided-wave inspection of the entire length of a fuel rod. The probe includes a waveguide adapted to be clamped to a fuel rod, and an MsS adapted to generate guided waves into the waveguide such that the guided waves propagate down the waveguide into the fuel rod and back to the waveguide for detection by the MsS.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application claims the benefit of Provisional Application No. 61/230,147 filed on Jul. 31, 2009. 
     
    
       [0002]    Fuel rods in nuclear reactors are typically made of a zirconium alloy (Zircaloy) tube approximately 0.4 inch (10 mm) in diameter, 0.03 inch (0.76 mm) in wall, and 14 feet (4 m) in length. They are assembled in a 14×14 to 18×18 grid for structural support and control of fuel. During an outage, these fuel rod assemblies are removed from the reactor to the fuel pools and inspected to ensure integrity of fuel rods before they are placed back in service to preclude a potential reactor coolant contamination issue. To inspect fuel rods in a timely manner during the critical path of outage, efficient inspection methods are needed for detecting flaws in rods such as fretting wear, corrosion wall thinning areas, and cracks. 
         [0003]    Long-range guided-wave technique is a recently introduced inspection method for rapidly surveying a long length of pipe or tube for flaws from a given test location without mechanical scanning. Now widely used for examining pipelines in processing plants, this technique can provide a rapid and efficient inspection needed for fuel rods. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    These and other shortcomings of the prior art are addressed by the present invention, which provides a magnetostrictive sensor (MsS) probe for guided-wave inspection of the entire length of a fuel rod from its top end. 
         [0005]    According to one aspect of the present invention, an MsS guided wave probe for inspecting fuel rods includes a waveguide adapted to be clamped to a fuel rod, and an MsS adapted to generate guided waves into the waveguide such that the guided waves propagate down the waveguide into the fuel rod and back to the waveguide for detection by the MsS. 
         [0006]    According to another aspect of the present invention, an MsS guided wave probe for inspecting fuel rods includes a waveguide having a first end with at least one slit therein. The first end is adapted to slide over an end of a fuel rod. The probe further includes a clamp adapted to be pushed downward along the axis of the waveguide and over the first end; an actuator adapted to actuate the clamp such that the clamp squeezes the first end against an outside surface of the fuel rod; and an MsS adapted to generate guided waves into the waveguide. The guided waves propagate down the waveguide into the fuel rod and back to the waveguide for detection by the MsS. 
         [0007]    According to another aspect of the present invention, a method of inspecting a fuel rod includes the steps of providing an MsS guided wave probe; securing the probe to an end of a fuel rod; and generating guided waves and sending the guided waves into the waveguide such that the guided waves propagate down the waveguide into the fuel rod and back to the waveguide for detection by the MsS. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0009]      FIG. 1  shows a fuel rod MsS probe according to an embodiment of the invention; 
           [0010]      FIG. 2  is a detailed view of an end of the probe of  FIG. 1 ; 
           [0011]      FIGS. 3 and 4  are graphs showing examples of 250 kHz data obtained using the probe of  FIG. 1 ; and 
           [0012]      FIG. 5  shows the probe of  FIG. 1  inspecting fuel rods. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring to the drawings, an exemplary MsS probe for fuel rod inspection according to an embodiment of the invention is illustrated in  FIG. 1  and shown generally at reference numeral  10 . The probe  10  includes a waveguide  11 , a magnetostrictive sensor (MsS)  12  connected to an MsS instrument (not shown) by an MsS cable  14 , a clamp  16  for clamping a slit-end  17  (having slits  13 ,  FIG. 2 , therein) of the waveguide  11 , clamp rods  18  and  19 , and a clamp actuator  20  for actuating the clamp  16 . 
         [0014]    Referring to  FIG. 2 , for guided-wave inspection, the slit-end  17  of the waveguide  11  is sleeved over a top end  22  of a fuel rod  21  and mechanically clamped to the fuel rod  21  by pushing the clamp  16  downwards over the slit-end  17  and actuating the clamp  16  with the actuator  20 , thereby squeezing the slit-end of the waveguide against an outside surface of the fuel rod  21 . 
         [0015]    The MsS  12  generates guided waves in the waveguide  11 . The generated waves are then propagated down the waveguide  11 , coupled to the fuel rod  21 , and propagated along the fuel rod  21 . The reflected signals are coupled back to the waveguide  11  and subsequently detected by the same MsS  12  used for wave generation. 
         [0016]    The slit-end  17  of the waveguide  11  is tapered with a ridged area  23  near the tip  24 . When the clamp  16  is moved downward over the ridged area  23 , the clamp  16  presses the slit-end  17  of the waveguide  11  against the fuel rod  21 . The resulting intimate contact between them permits the coupling of the guided waves from the waveguide  11  to the fuel rod  21  and vice versa. 
         [0017]    To minimize the reverberation of guided waves in the waveguide  11  of the probe  10 , damping material (not shown) is placed at a sensor end  26  of the waveguide  11 . Also, the sensor end  26  of the waveguide  11  is mechanically fastened to the clamp actuator  20 . 
         [0018]    Both longitudinal (L) and torsional (T) guided waves can be used for fuel rod examination. However, T-waves are dispersion free and do not interact with water surrounding the fuel rods and the waveguide  11  of the MsS probe  10 . Therefore, T-waves are the preferred wave mode. To minimize the effects of grids placed at several locations along the length of fuel rods in a fuel rod assembly, guided waves over 200 kHz are typically used. 
         [0019]    The MsS utilizes the thin magnetostrictive strip approach disclosed in U.S. Pat. No. 6,396,262. 
         [0020]      FIGS. 3 and 4  show examples of 250-kHz T-wave data obtained using the probe  10  from 154-inch-long fuel rod samples with simulated defects. C 1  through C 4  are signals from simulated corrosion pits. A 1  through A 3  are signals from 1-inch-long and 0.01-inch-wide axial EDM (electrical discharge machined) notches. C 1  was approximately 0.18 inch-long, 0.12 inch-wide, and 28% wall deep; C 2  was 0.21 inch long, 0.14 inch wide and 50% wall deep; C 3  was 0.23 inch-long, 0.15 inch wide and 75% wall deep; C 4  was 0.25 inch diameter through wall hole. A 1  was 50% wall deep; A 2  was 75% wall deep; A 3  was 100% wall deep. 
         [0021]    Tests conducted on fuel rod samples in the laboratory showed that the invention can inspect the entire length of a fuel rod from the top end of the fuel rod with good performance (as shown in the examples given in  FIGS. 3 and 4 ) and that it can detect corrosion defects, wear and fretting defects, and cracks in any orientation (axial, circumferential, and 45-degree). 
         [0022]    Referring to  FIG. 5 , in use, probe  10  may be used on a fuel assembly  30 . Because the tip area of the probe  10  is only slightly larger than the fuel rod size, the probe  10  can couple to any fuel rod in the assembly  30  despite the close packed configuration of the assembly  30 . The overall inspection time of the entire assembly  30  may be shortened by using an array of the probe  10  and a multiplexer. For example, for a 14×14 grid assembly, an array of 14 probes  10  are coupled simultaneously to all fuel rods in a grid row or column and each fuel rod is examined at a time by multiplexing each probe  10  using a single MsS instrument. The data acquisition time per fuel rod takes only several seconds. Thus, the entire fuel rod assembly, including the time for mechanical coupling and decoupling, could be completed in less than 30 minutes. 
         [0023]    The foregoing has described an MsS probe for guided-wave inspection of fuel rods. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.