Patent Number: 048266507
Section: summary

BACKGROUND OF THE INVENTION This invention relates to ultrasound testing. More particularly, the invention relates to remote ultrasound testing of a lattice-like nuclear reactor vessel top guide and sets forth a protocol wherein testing can occur without lattice disassembly. STATEMENT OF THE PROBLEM Reactors constitute extremely hostile environments for inspection of any kind. First, reactors are notorious for their radioactivity. Secondly, the internals of the reactors are frequently mechanically inaccessible. A classic example of such inaccessibility is the top guide used in a boiling water reactor. The top guide comprises a series of bars in the order of 1/4 inch thick and 9 to 13 inches in width. The bars each span the full diameter of the reactor vessel which can be in the order of 22 feet. The bars are grooved. The grooves extend through half the width of the bars. The grooves in parallel bars extend in the same direction. For example, one set of bars has its grooves upwardly disposed; bars at right angles have their grooves downwardly disposed. The bars are assembled in a lattice by confronting their respective grooves. They come together in a lattice-like structure that is not unlike the cardboard separators found in wine cases. This continuous lattice, once assembled, is welded at the side edges to a ring on the reactor vessel. The bars of the lattice are not otherwise welded or attached to themselves. The lattice defines a number of discrete square cells bounded by the intersecting bars forming parallel sides. The function of the top guide is for preserving the vertical and rotational orientation of square sectioned elongate fuel assemblies supported on a core plate some 14 feet below the top guide. The top guide braces and maintains the top of the fuel assemblies. The fuel assemblies are maintained vertical by the top guide. Moreover, the top guide forms the support surface from which the rotational orientation of the fuel assemblies is maintained. By bracing the fuel assemblies to the top guide, the vital cruciform shaped interstitial area between the fuel assemblies for control rod penetration and moderation of the reaction is maintained. Unfortunately, the top guide is in an ideal place for cracking to occur. First, the bars making up the top guide have numerous discontinuities. These discontinuities include the very grooves which enable the top guide bars to be assembled in their latticelike configuration. Additionally, various other discontinuities are present in the bars. For example, notches for the hanging of poison curtains used in the start-up of older reactors constitute such discontinuities. Further, the bars in spanning the reactor vessel and aligning the fuel assemblies are subject to stress. This stress is aggravated by the fact that the bars at their respective points of intersection are not fastened one to another. Additionally, the bars making up the top guide are subject to high radiation dosage. Consequently, the bars are ideal sites for irradiation assisted stress corrosion cracking (IASCC). IASCC occurs in many stainless steels when an irradiation dosage exceeds 2.times.10.sup.21 neutrons per cm.sup.2. Thus, when older reactors approach this dosage level, there is high motive to examine the top guide for IASCC. It goes without saying that removal and disassembly of the bulky radioactive lattice comprising the top guide is possible--but prohibitively inconvenient and expensive. SUMMARY OF THE PRIOR ART Ultrasound testing is a nondestructive technique well known. In the usual application, an ultrasound transducer is manually fastened to an article --typically piping--to be nondestructively tested. A transducer imparts an ultrasound pulse to the article to be tested. The pulse fully penetrates the article to be tested and is reflected. The pulse upon being reflected is detected, usually at the very transducer which initiated the pulse. The detection of the pulse at the transducer is recorded and analyzed. It is conventional to time the receipt of reflected pulses. By noting the reflected pulses, one can determine whether an intact part at an extremity causes the return pulse or an interfering crack at a location other than the extremity causes reflection. Since transducers, power equipment, recorders, monitors and computers for analyzing ultrasound are all known and conventional, further description will not be set forth herein. For more complete detail on the testing referred to herein, the readers attention is invited to the publication UT Operator Training for Intergranular Stress Corrosion Cracking published by the Electric Power Research Institute Nondestructive Evaluation Center of Charlotte, N.C., 1983. SUMMARY OF THE INVENTION In a boiling water reactor, an apparatus and process for ultrasound inspection of the top guide is disclosed. The top guide constitutes a lattice of stainless steel bars overlying the core plate and being assembled at confronting grooves with the lattice mounted at the side edges to the reactor pressure vessel. This lattice braces the upper ends of the vertically supported fuel assemblies in their requisite orientation and spaced apart relation to enable among other things the required spatial interval to be maintained for control rod moderation of the reaction. Because of the proximity of the top guide to the fuel assemblies, the individual bars making up the lattice need to be checked for cracking, especially that cracking produced by irradiation assisted stress crack corrosion. With a defined cell in the lattice emptied of its contained and adjoining fuel assemblies, there is disclosed an ultrasound test for cracking. A sound transducer on a first special frame sweeps horizontally across the top of a bar interrogating the bar with vertical longitudinal ultrasound waves for detecting horizontal cracks. Similarly, a sound transducer on a second special frame sweeps vertically across the side of a bar interrogating the bar with angularly incident horizontal shear ultrasound waves for detecting vertical cracks. Nondestructive testing of the lattice assembly occurs without required disassembly. Two test frames are disclosed, a first test frame for checking the lattice assembly for horizontal cracking and a second test frame for checking the lattice for vertical cracking. Before the test, it is required that all fuel assemblies adjacent the bars to be tested are removed. A first test frame has the cross section of of a fuel channel and is configured for detecting vertical cracking. This frame engages orthogonally disposed bars at their defined corners and comes to rest. A ball screw driven carriage is given vertical excursion along each bar of the defined corner. Each bar at the defined corner is interrogated by ultrasound from paired transducers. The paired transducers are oriented to have opposed acoustical angles of incidence to the bar in the order of 70.degree. in a horizontal plane towards the bars. One transducer sweeping each bar interrogates the bar with horizontal shear wave ultrasound towards the corner defined by the bar; the remaining transducer sweeping each bar interrogates the bar with horizontal shear wave ultrasound away from the corner defined by the bar. The vertically sweeping transducers therefore interrogate each bar with acoustical signals horizontally and in opposite directions to detect vertical cracking. The second test frame is configured for detecting horizontal cracking. The second test frame is configured for precise placement on the lattice overlying the portion of a bar to be tested. In the preferred embodiment, the carriage rest one leg on the bar to be tested and its rear two legs on an intersecting bar on the top of the top guide assembly. The second test frame includes a carriage mounted to a ball screw drive. This carriage contains an ultrasound transducer and sweeps horizontally the transducer immediately over the bar to be tested. The horizontally moving transducer interrogates the bar with vertically interrogating sound to locate horizontal cracking. In both types of test frames, each transducer (one transducer for vertical acoustical interrogation and four transducers for horizontal acoustical interrogation) couple through the demineralized water of the reactor to enable complete nondestructive testing of the top guide without costly disassembly.