Patent Number: 048266507
Section: description

Referring to FIG. 1, a reactor vessel V is shown shut down and open having its head H, dryer D and steam separator S all removed. Working personnel P1 and P2 are shown standing on a work platform overlying the top guide G some 50 feet deep within the reactor. Worker P1 through rod R1 manipulates frame F1 for the interrogation of the reactor top guide for vertical cracking; similarly worker P2 through rod R2 manipulates frame F2 for the interrogation of the top guide G for horizontal cracking. Typically, such an inspection will occur when portions of the top guide have been exposed to radiation in the order of 2.times.10.sup.21 neutrons/cm.sup.2. It is at this dosage level that the stainless steel of the top guide assembly G can begin to have that phenomenon known as irradiation assisted stress crack corrosion (IASCC). Referring to FIG. 2, a plan view of the top guide G is illustrated within the reactor vessel V. Top guide G is shown to be a lattice-like structure of intersecting bars. These bars typically overlie a core plate C (see FIG. 1) and define discrete cells 20. As is well known in the prior art, the cells 20 each brace the tops of four fuel assemblies. The cells 20 of the top guide G hold the fuel assemblies at their upward end remote from the core plate C which supports the weight of the fuel assemblies. The function of the top guide G is at least two fold in nature. First, the top guide G maintains the fuel assemblies in their upright position. Secondly, the top guide G maintains the fuel assemblies with their sides parallel to one another and spaced apart from one another. This enables among other things, the control rods to penetrate the cruciformed shaped interstices between four fuel assemblies. Such a configuration of four fuel assemblies A maintained by the top guide assembly is shown at cell 20' in FIG. 2. In order for a test of this invention to be conducted, it is preferred that each cell 20 have the fuel assemblies A removed. This prevents protrusion of the fuel channels of the fuel assembly from interfering with the test procedures herein set forth. The area of fuel assembly removal is shown in heavy solid lines. It will be understood that fuel assemblies on both sides of the portion of the top guide G designated by the heavy lines are removed. Having set forth the ambient within which testing occurs, the tests will now be described. First, a test frame F1 will be illustrated with respect to FIG. 3A and 3B. Its placement in testing a portion of the top guide G will be set forth with respect to FIG. 5. Thereafter and with reference to FIG. 4, a test frame F2 will be set forth. Its placement in testing top guide G will also be set forth with respect to FIG. 5. Referring to FIG. 3A, frame F1 includes a nose piece 40 having the shape of a finder and a main body 50 having the approximate section of a fuel channel of a typical fuel assembly. A carriage 60 is shown mounted for vertical excursion along two respective open and therefore exposed sides 51, 52 of frame F1. The carriage rides on three bars 53, 54, and 55. The carriage is propelled by a ball screw (imbedded in the carriage 60 and therefore not shown) following a rotating threaded shaft 57. Rotation of shaft 57 is monitored at shaft encoder 58 and caused by motor 59 at the top of the assembly. Conventional rotation of motor 59 and tracking of rotation at shaft encoder 58 enables precise positioning of the carriage 60 to be known. Carriage 60 includes a carriage face 61 parallel to open side 51 and a second carriage face 62 parallel to open side 52. Faces 51, 52 each confront a bar at the corner of a cell in a top guide. Referring to FIG. 3B, carriage 60 is illustrated in plan with its two faces 61, 62. Each of the faces 61, 62 have paired transducers. These transducers are 63 and 65 on face 61 and 64 and 66 on face 62. Transducer 63 sends a signal at 70.degree. way from face 61 towards the corner defined by the intersection of the faces 61, 62. Transducer 65 adjacent the corner of faces 61, 62 sends an acoustical signal horizontally at 70.degree. away from the corner defined by faces 61, 62. The acoustical signals of transducers 64, 66 on face 62 are correspondingly angularly incident towards and away from the swept cell corner. The purpose of these opposed angularly incident signals may readily be understood. Specifically, the transducers 63, 65 will pass immediately over the bar that they are interrogating. In such passage, the acoustical signals must be given an angle of incidence wherein penetration of the bar with the acoustical signal and detection of the returned acoustical signal is assured. By the specific orientation herein disclosed, thorough checking of a bar at the corner of a discrete cell in guide G is assured; one transducer interrogates to the corner, the remaining transducer interrogates away from the corner; as can be seen, vertical sweep across the entire width of the bar by the transducers thoroughly interrogates the full width of the bars forming the corner with horizontal ultrasound to detect vertical cracking. Turning to FIG. 5, positioning of the bar to the top guide G can be understood. Specifically, paired plates 71, 72 are positioned on the exterior of test frame F1. These plates define an inwardly extending angle, which angle braces frame F1 to a corner of the discrete cell illustrated in FIG. 3. Once the frame F1 is so positioned, excursion and acoustical interrogation in a horizontal plane of the illustrated transducers 63, 65 and 64, 66 occurs. It can be understood that all of the bars defining a discrete cell can be tested. This can occur by positioning frame F1 in each of the respective corners of a defined cell. With repeat of this procedure, acoustical sweeping of the bars of the top guide with horizontally interrogating acoustical signals for the detection of vertical cracks can occur. Referring to FIG. 4, frame F2 is illustrated. It includes longitudinal sides 101, 102 and ends 104, 105. These sides and ends form a rigid frame structure connected at a yoke 106 to rod R2. Paired rods 114, 116 form points of support for the excursion of a carriage 110. Carriage 110 has mounted there below an ultrasound transducer 112. Carriage 110 is driven by a threaded shaft 118 at a ball screw imbedded within the carriage 110 (not shown). Motor 119 causes shaft 118 to rotate. A shaft encoder 120 determines the precise position of the carriage 110. Feet 131, 132 rest upon a bar parallel to section 104 of the rotor. A forward foot from bar 105 (obscured in the view here shown) preferably rests on the bar tested at a portion within the next cell on top guide G. Turning to FIG. 5, placement of the frame F2 is illustrated. Referring back to FIG. 4 it will be understood that transducer 112 undergoes excursion the length of the frame. The single transducer 112 interrogates with vertical ultrasound waves a bar for horizontal cracking. Thus the fixtures set forth in Figs. 3A and 3B and FIG. 4 are capable of remotely interrogating the lattice of top guide G for horizontal and vertical cracking.