Patent Number: 051456370
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, a boiling water reactor system 1 includes reactor pressure vessel 2 with bottom head 3, as shown in FIG. 1. Below reactor core 4 sits core support plate 5 with circular openings on which sit which fuel assemblies, dry-tubes, and other internal parts of the core. Dispersed below core 4 are incore guide tubes 6. These are welded to the top of incore housing tubes 7 that penetrate bottom head 3 of the pressure vessel wall. These penetrations are sealed with incore housing welds 8, which are crucial since the inside of pressure vessel 2 is under pressure and is radioactive. Access to this region, as well as core 4, is facilitated by top guide 9, which is a metal grid with a matrix of square openings. Equipment and parts in this region are handled from refueling bridge 10, which is a trolley that rides across the refueling platform on rails. A hoist on refueling bridge 10 uses hoist cable 11 to raise and lower equipment and parts into this region. Adjacent to pressure vessel 2 is a refueling pool 12, which contains spare reactor fuel. In order to check the region around weld 8 for flaws a combination of ultrasonic testing of the weld region, and eddy current testing of the inner surface of housing tube 7 is employed. A submersible device for performing such an examination is shown in FIG. 1; hoist cable 11 is used to lower a unit including scanning tool 21, probe 22, and probe tube 23 into position at the weld to be tested. Probe 22 is lowered through incore guide tube 6 and incore housing tube 7 until it is level with weld 8, and scanning tool 21 is clamped to top guide 9 with a mechanical clamping device. Probe tube 23 is clamped to scanning tool 21 at a previously determined location so that when scanning tool 21 and top guide 9 are at the same level, probe 22 and weld 8 are at the same level. Scanning tool 21 moves probe 22 automatically to perform the inspection, and data is sent to electronic processing and control equipment outside pressure vessel 2 via cable bundle 24 for analysis. Wiring to probe 22 runs through probe tube 23. The nature of any defects found during inspection is determined with reference to a previously manufactured set of defects found in calibration standard 26, which is a duplication of the housing weld configuration. Support bracket 25 is used for storage as well as calibration of the inspection equipment. Positioning of the incore housing inspection equipment is most easily seen from above the reactor, as shown in FIG. 2. The inspection equipment is moved using refueling bridge 10 until scanning tool 21 is located above an incore housing. These are below the intersections in the metal matrix of top guide 9. Scanning tool 21 is mounted at such an intersection for weld inspection. The hoist on refueling bridge 10 moves equipment in the vertical direction. Bridge 10 moves along the refueling platform on rails, which in turn moves between pressure vessel 2 and refueling pool 12, also along two parallel rails. Cable bundle 24 sends commands to scanning tool 21 and the probe (beneath the scanning tool), as well as carries data back to work station 27, which is outside the pressure vessel. FIG. 3 shows a side view of the inspection equipment, and also shows a box diagram of the electronic gear used to control and receive data from the inspection equipment. Relative electrical impedance data (both vertical and horizontal components) is digitally recorded for the four eddy current coils with the eddy current instrument, eddy current computer, and storage tape. The eddy current instrument records raw data, and the eddy current computer prepares it for storage. The eddy current instrument also drives the eddy current coils, and can drive each one independently at different frequencies. Reflection data for the six ultrasonic transducers is recorded with a pulser/receiver, data aquisition unit (which includes a central processing unit), and an optical disk recording module. Both systems are connected to the motion controller, which is connected to the motors and position encoders in scanning tool 21 to move probe 22 automatically. Both systems also have printers for hard copy read-outs. The ultrasonic system also has a monitor for an electronic read-out. The method of determining the direction of the high side of the housing welds, is shown in FIG. 4. Pressure vessel 2 is assigned a permanent coordinate system with a central origin. Scanning tool 21 is mounted on top guide 9 with clamp 28 secured to top guide 9. When the inspection equipment is first lowered into pressure vessel 2 and clamped into place the probe (the straight-on transducer on the probe) faces the 0.degree. reference direction. The angle .beta. which the probe must be rotated to face the high side of the weld is determined by the location of scanning tool 21 relative to the origin in the pressure vessel's coordinate system, since the bottom head of pressure vessel 2 is rotationally symmetric. Thus the probe can be oriented to face the high side of the weld by remote-control, given the scanning tool's coordinates, which are the same coordinates as those of the incore housing. If the initial rotational orientation of the probe is known, then its rotational orientation is known throughout the inspection. Knowing the rotational orientation of the probe gives clues as to the type of indications which might be found, what their size is, and whether they are classified as acceptable or non-acceptable. The basic mechanical components of scanning tool 21 are shown in FIG. 5. Scanning tool 21 comprises a fixture with two DC motors with positioning encoders attached. The DC motors drive the probe in the circumferential and verical directions with encoders providing the positioning data. The mechanical portion of the tool is housed in a cylindrical can which is locked in position on the pressure vessel top guide by a mechanical clamping device. Probe tube 23 fits through shaft 31, and is clamped thereto with clamp 32 after the length and angular orientation of probe tube 23 is set so probe 22 will initially face the high side of the weld after a rotation through an angle .beta., as discussed in connection with FIG. 4. The length of the lower extension of probe tube 23 is set by sliding it vertically through shaft 31 until probe 22 will be at weld level. The rotational orientation is set by aligning scribe marks on probe tube 23 and shaft 31, so the probe will point in the same direction as the mechanical clamping device that fastens scanning tool 21 to the pressure vessel top guide. Brackets 33 also hold scanning tool 21 in place on the top guide. Once in place the circumferential drive rotates shaft 31, probe tube 23, and probe 22 clockwise through the angle .beta. to put probe 22 in its initial position before weld inspection. Once probe 22 is in its initial position the weld and housing tube inspection is performed automatically. Probe 22 is driven with vertical and circumferential drive motors in such a way as to inspect the housing weld and housing tube from at least 40 mm above the weld to at least 40 mm below the weld. In practice, this distance is about 2 inches. The eddy current and ultrasonic inspections are done independently. Probe 22 moves vertically from above the weld to below the weld, then rotates 5.degree., and moves upwards to above the weld, then rotates 5.degree.. This repeats until it has rotated 360.degree.. The two drive motors each have encoders to control them, based on the position of the probe. Vertical movement of probe 22 coincides with the movement of travel plate 34, since they are connected via probe tube 23 and shaft 31. Stops 35 and 36 are the upper and lower limits of motion for the travel plate. The START SCAN position of travel plate 34 before the respective eddy current (EC) and ultrasonic (UT) inspections is different; on probe 22 the eddy current coils are located about 2.7 inches below the ultrasonic transducers. To inspect the same region above the weld, travel plate 34, and hence probe 22, must be raised 2.7 inches higher for the eddy current inspection than for the ultrasonic inspection. The vertical drive moves travel plate 34 down until the eddy current coils are about two inches below the weld for the eddy current inspection, and unitl the uppermost transducer (the straight-on transducer) is about two inches below the weld for the ultrasonic inspection. Before the automatic scan the initial position of the straight-on transducer is such that it faces the high side of the weld, as mentioned above. More specifically, it faces the top of the high side of the weld. In this way the probe is raised the same amount to inspect the required region above each weld. The amount the probe is lowered is then varied to inspect the required region below the weld. Welds higher up on the bottom head of the pressure vessel are at a greater angle, so the amount the probe is lowered is correspondingly greater. Probe 22 has six piezo-electric transducers (T1-T6), arranged as shown in FIG. 6. All are turned on during the ultrasonic scan. Transducers T2-T6 are all simultaneously focused so as to interrogate both of the weld fusion zones. They are focused at the same point at the interface between the incore housing and the weld. This permits examination of the entire weld region including the interface between the weld and the bottom head of the reactor. Transducer T1 is focused at the interface between the incore housing and the weld, but above the others. An arbitrary indication (flaw) shows up if it reflects a portion of the beam back to the transducer that sent it, with the greatest reflection coming back if the indication is perpendicular to the direction of the beam. The transducers are pulsed sequentially, with each pulse followed by a time interval for reception. The elapsed time until reception reveals the location of an indication since the speed of sound in the various materials the beam travels through is known. The magnitude of a reception reveals the size of an indication, due to prior instrument calibration. All examination data is stored by computer techniques, and can be presented graphically with a hard copy printer. Space considerations cause straight-on transducer T1 to be located about 2 inches above transducers T2-T6. Transducer T1 is aligned with the top of the high side of the weld in question after the scanning tool is clamped to the top guide, as discussed above. Thus, to position transducers T2-T6 about two inches above the top of the high side of a weld, the vertical drive raises the travel plate 4 inches to the UT START SCAN position. The vertical drive then lowers the travel plate until T1 is about 2 inches below the bottom of the weld. (At this point the other five transducers T2-T6 are 4 inches below the weld, which results in additional data in the interval from 2-4 inches below the weld. When T2-T6 are 2 inches above the weld T1 is 4 inches above the weld, which also results in additional data for the region 2-4 inches above the weld.) The probe rotates 5.degree., then the travel plate returns to the UT START SCAN position, the probe rotates another 5.degree., and repeats the vertical sweep. This continues until probe 22 has rotated 360.degree. to complete the ultrasonic inspection. Transducer T1 is a longitudinal tranducer with a frequency of 2.25 MHz. T1 looks straight-on, i.e. perpendicular to probe 22 in a horizontal direction, and is aligned with the top of the high side of the weld before the scan. Its purpose is to provide indication, thickness, and depth information, and to provide information as to the condition of the weld, e.g. cracking, lack of fusion, inclusions, porosity, etc. There are four 45.degree. shear wave transducers with a frequency of 5.0 MHz. Transducers T2 and T3 look right and left, while transducers T4 and T5 look up and down. Transducers T2 and T3 examine the incore housing and weld circumferentially to detect indications oriented in the axial direction. T4 and T5 examine the volume of material in the axial direction to detect circumferentially oriented indications in the housing and weld. The downward-looking transducer is also used to examine pressure vessel material below the normal plane of coverage. Transducer T6 is a 60 degree refracted longitudinal wave transducer with a frequency of 2.25 MHz. T6 looks down, and is used to ascertain the condition of the weld build-up area which is present in some incore housing weld designs. In these designs a build-up of weld material is applied to the pressure vessel in such a manner that all the incore housing weld attachments are horizontal. The eddy current assembly on probe 22 has four coils 40, as shown in FIG. 6. The coils are positioned 90.degree. apart around the lower end of the probe. Two of the four coils are of the absolute type with one coil, and the other two are of the differential type that use two coils for reference and stabilization purposes. The absolute coils are used to provide the required depth of penetration, which is near surface. The differential coils are used to minimize the effect of conductivity and magnetic permeability variations in the heat-affected zone surrounding the weld. The eddy current assembly is used to examine the inner surface and near surface of the housing for defects. Eddy current coils 40 induce a current in the surface of a conductor, i.e. metal. Variations in the surface of the conductor cause changes in the surface impedance. The changes in impedence have characteristic patterns corresponding to dents, corrosion, or any other flaw with an associated impedence pattern. All examination data is retained digitally on magnetic tape, and can be presented graphically on computer screen and/or be presented in hard copy form with a printer. The EC START SCAN position of the scanning tool's travel plate is higher for eddy current coils 40 than for the transducers because of their lower position on probe 22, but since coils 40 are at the same level the vertical drive moves them from 2 inches above to 2 inches below the weld without taking into account a coil that is not on the same level. (The vertical sweep is longer for the ultrasonic inspection because transducer T1 is above the others, as previously described.) The eddy current inspection is otherwise identical to the ultrasonic inspection, i.e. vertical sweeps are made in 5.degree. increments until probe 22 has rotated 360.degree.. Examination of the inner surface and near surface of a housing tube may be done with only one eddy current coil since both the absolute and differential coils provide adequate sensitivity. All four coils are placed into service, however, in case a mechanical problem diminishes the performance of a primary coil. The absolute coils are 0.25 inches in diameter, and the differential coils are 0.125 inches in diameter. Both types operate at nominal frequencies of 100 KHz, but they may be individually driven at other frequencies to provide additional information for analysis. Each coil is spring loaded to maintain contact with the inner surface of the housing tube in order to minimize the effect of lift-off. Spring-loaded balls 41 roll along the inner surface of the incore housing, and help protect eddy current coils 40 from physical damage. FIGS. 7-12 show the paths of the six transducer beams as they traverse an incore housing weld. In general, indications that present cross-section to the ultrasonic beams will send a reflection back to the probe and be detected. FIG. 7 shows the path the beam from transducer T1 follows as the probe passes weld 8 going either up or down. Above weld 8, the beam reflects at the interface between the wall of housing tube 7 and the water inside the pressure vessel. Below weld 8, the beam reflects at the interface between the wall of housing tube 7 and air gap 50, which is present in the region below weld 8 between the wall of housing tube 7 and bottom head 3 of the pressure vessel. When T1 is level with weld 8, the ultrasonic beam diverges at the interface of the wall of housing tube 7 and weld 8, and it passes into bottom head 3 of the pressure vessel, or is reflected at the interface between weld 8 and the water inside the pressure vessel. Arbitrary indications that cause a sufficient reflection back to the probe as it traverses weld 8 will be detected. FIG. 8 shows the paths the beams from transducers T2 and T3 follow as the probe passes weld 8 going either up or down. Above weld 8, the beams reflect at the interface between the wall of housing tube 7 and the water inside the pressure vessel. Below weld 8, the beam reflects at the interface between the wall of housing tube 7 and air gap 50 between housing tube 7 and bottom head 3 of pressure vessel 2. When T2 and T3 are level with weld 8, the ultrasonic beams pass through the interface of the wall of housing tube 7 and weld 8, and go into bottom head 3 of pressure vessel 2, or are reflected at the interface between weld 8 and the water inside the pressure vessel. T2 and T3 are specifically intended to find indications that are axially oriented (lie in the direction of housing tube 7). FIG. 9 shows a plan view of the ultrasonic beam paths from transducers T2 and T3 at a single location when they are at weld 8 level. Each beam is oriented 45.degree. in the circumferential direction from T1's beam. Probe 22 essentially fills housing tube 7, and the beams pass through housing tube 7, into weld 8 and bottom head 3 of the pressure vessel. Axial indications are "double checked" from both the clockwise and counter-clockwise directions. FIG. 10 shows the path the beam from transducer T4 follows as the probe passes weld 8 going either up or down. Above weld 8, the beam reflects at the interface between the wall of housing tube 7 and the water inside the pressure vessel. Below weld 8, the beam reflects at the interface between the wall of housing tube 7 and air gap 50, between housing tube 7 and bottom head 3 of the pressure vessel. When T4 is level with weld 8, the ultrasonic beam passes into weld 8 until it is reflected at the interface between weld 8 and the water inside the pressure vessel. T4 is specifically intended to find indications that are circumferentially oriented (tend to lie in a horizontal plane in a direction perpendicular to housing tube 7). FIG. 11 shows the path the beam from transducer T5 follows as the probe passes weld 8 going either up or down. Above weld 8, the beam reflects at the interface between the wall of housing tube 7 and the water inside the pressure vessel. Below weld 8, the beam reflects at the interface between the wall of housing tube 7 and air gap 50, between housing tube 7 and bottom head 3 of the pressure vessel. When T5 is level with weld 8, the ultrasonic beam passes into weld 8 and down into bottom head 3 of the pressure vessel below the normal plane of coverage. T5 is also specifically intended to find indications that are circumferentially oriented. Also, circumferential indications that might tend to be slightly oriented upwards in the direction of the T4 beam will form a greater angle with the T5 beam, and circumferential indications that might be oriented downwards in the T5 direction will form more of an angle with the T4 beam, so circumferential indications are also "double checked" as the probe traverses the weld. FIG. 12 shows the path the beam from transducer T6 follows as the probe passes weld 8 going either up or down. Above weld 8, the beam reflects at the interface between the wall of housing tube 7 and the water inside the pressure vessel. Below weld 8, the beam reflects at the interface between the wall of housing tube 7 and air gap 50, between housing tube 7 and bottom head 3 of the pressure vessel. When T6 is level with weld 8, the ultrasonic beam passes into weld 8 and down into bottom head 3 of the pressure vessel below the normal plane of coverage, and closer to housing tube 7 than the T5 beam. Indications that are circumferentially oriented in weld build-up regions, which are employed in some reactor housing attachment welds to make them level, will cause reflections back to the probe as the T6 beam traverses the weld. Since all the transducers are on during the inspection, an indication will generally show up on more than one transducer read-out. The status of the attachment weld is then known. With the additional information available from the eddy current examination, the status of the entire weld region is determined. It is then possible to decide whether or not repairs are needed. Because the examination is normally done when the incore instrumentation is tested (and replaced), there is a considerable savings of time and money inspecting the welds from the inside of the incore housing tubes with access from above. This savings is in addition to the greatly increased margin of safety over the prior method of examining the welds from below, in which workers were exposed to high radiation levels. Also, since the entire scanning process is automated, there is a higher standard of precision than the prior method, which was manual. The invention provides for other embodiments than those described above. For instance, the invention provides for inspection of any circumferential weld about the outside of a tube with access from above, and an overlay suitable for supporting a scanning tool. The weld need not be inside a nuclear reactor pressure vessel. This invention provides for different numbers and types of transducers, depending on the individual circumstances. The transducers need not be focused at the interface of the tube and the weld, but may be focused at other regions of interest. These and other modifications to and variations upon the described embodiments are provided for by the present invention, the scope of which is limited only by the following claims.