Method and apparatus for examining obstructed welds

A method and apparatus is provided for inspecting material arranged within an annular recess such as weld beads, particularly weld beads between the shroud, core plate and support structures within a boiling water nuclear reactor (BWR). The apparatus comprises a track support, a generally arcuate track, a carriage, a sensor support and a sensor. The track is configured to correspond and align generally with an inner surface of a shroud or other encompassing structure with the sensor support and sensor configured to be extended into and then moved horizontally along the annular recess. The sensor may be configured with a plurality of sensor elements, such as ultrasonic transducers or eddy current sensors that may be configured to analyze different properties or different regions relative to the sensor location.

BACKGROUND OF THE INVENTION

This invention relates generally to the in situ and non-destructive examination of large circumferential surfaces, particularly including welds, and more particularly, obstructed and recessed peripheral welds. Such surfaces and welds may be found throughout boiling water nuclear reactors and, in particular, welds between ring structures that support the core plate and the core shroud arranged above the ring structures, sometimes referred to as the H6A weld.

A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide, sometimes referred to as a grid, is spaced above a core plate within the RPV. A core shroud, or shroud, surrounds the core plate and is supported by a shroud support structure. The core shroud is a reactor coolant flow partition and structural support for the core components. In most instances, the core shroud will have a generally cylindrical shape and surround both the core plate and the top guide. A removable shroud head is coupled to a shroud head flange at the top of the shroud.

Above the H6A weld, the core plate will typically be spaced from the shroud using a series of irregularly spaced core plate wedges set into a thin annular opening formed between the core plate and the inner surface of the shroud. The core plate wedges obstruct access to the welds and surfaces within the annular opening and the irregular spacing between the core plate wedges further complicates access. During operation of the reactor, however, the circumferential weld joints may experience intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC) in weld heat affected zones which can diminish the structural integrity of the welds. In particular, lateral seismic/dynamic loading could cause relative displacements at cracked weld locations and may produce leakage and misalignment of reactor components that could compromise the safety or performance. Given the complex configuration of the attachment between the shroud and core plate, however, in situ examination of the welds has proven very difficult.

It is desirable, therefore, to provide an apparatus and a corresponding method for inspecting the welds used to attach the shroud and the core plate to support rings arranged below the core plate that is reliable and is capable of examining the majority of the circumference of such welds and the associated surfaces. When using ultrasonic sensors to examine a weld, the focus point, direction and frequency of the ultrasonic beam may be selected to align with a predetermined fusion line between a weld and the attached structures. The ultrasonic beam may then be repeatedly refocused to move the focal point along the weld fusion line in discrete increments of about 0.25 to about 12.7 mm (about 0.01 to about 0.5 inch). One method for such incremental scanning is disclosed in U.S. Pat. No. 6,332,011, the contents of which are hereby incorporated by reference.

A variety of mechanisms have been devised for the examination of welds, particularly for use in hostile environments such as the interior of RPVs. One such apparatus is disclosed in U.S. Pat. No. 5,568,527, the contents of which are hereby incorporated by reference, and provides a remotely operated apparatus with clamping, sliding, rotational and sensor mechanisms to scan an ultrasonic transducer over specific core spray “T-box” welds including the T-box to cover plate attachment the T-box to thermal sleeve attachment welds.

BRIEF SUMMARY OF THE INVENTION

In an exemplary embodiment, an apparatus is provided for inspecting a majority of the length of one or more obstructed welds used in attaching the shroud and the core plate to supporting structures. The exemplary apparatus may be embodied in a range of configurations, but will tend to include a support, a frame which may include integral holding means, a connector, a carrier and a sensor. In an exemplary embodiment, the disclosed apparatus is positioned adjacent a portion of the inner surface of the shroud near the core plate and temporarily held in place. The sensor is positioned sufficiently close to the weld or surface of interest and activated, either continuously or in a pulsed fashion, as carrier moves, either continuously or in a stepwise or incremental fashion, along the frame to move the sensor(s) along the scanned feature to acquire data corresponding to the character and quality of scanned feature.

It should be noted that these Figures are intended to illustrate the general characteristics of methods and materials of exemplary embodiments of this invention, for the purpose of the description of such embodiments herein. These drawings are not, however, to scale and may not precisely reflect the characteristics of any given embodiment, and should not be interpreted as defining or limiting the range of values or properties of embodiments within the scope of this invention. In particular, the relative sizing and positioning of the various elements may be reduced or exaggerated for clarity.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1Ais a partial sectional view of a portion of a boiling water nuclear reactor (BWR) pressure vessel (RPV). RPV typically have a generally cylindrical shape that is closed at one end by a bottom head and at its other end by a removable top head with a side wall extending between the bottom head and the top head. Within the RPV will be arranged a generally cylindrically shaped core shroud100that surrounds the reactor core in which fuel bundles will be supported by a core plate102having receiving recesses106arranged on a top surface. The shroud100will be supported at one end by a shroud support104and will typically include a removable shroud head at the other end. An annular gap110is formed between shroud100and the core plate102into which a series of core plate wedges108, which may include handling loops108a, are partially inserted into the annular gap to control the relative positioning of the shroud and the core plate.

The amount of heat generated in the reactor core is regulated by inserting and withdrawing control rods of neutron absorbing material such as, for example, hafnium. To the extent that control rods are inserted into the fuel bundles, the control rods will absorb neutrons that would otherwise be available to promote the chain reaction which generates heat in the reactor core. Control rod guide tubes may be used to maintain the vertical motion of control rods during insertion and withdrawal and control rod drives may be used for selectively inserting and withdrawing the control rods during the operation of the reactor. The fuel bundles may be aligned using the core plate102and the receiving recesses106or other alignment structure to control the spacing and stabilize the fuel bundles. A top guide plate may also be used for aligning the fuel bundles as they are lowered into the reactor core.

Welds between the shroud, the shroud support and the core plate may encompass a variety of different “standard” configurations depending on the particular generation of BWR, the equipment vendor and the contractor. In certain BWR installations, both the shroud100and the core plate102will be welded to the shroud support104with welds112that are positioned at the lower end of the annular gap110formed between the shroud and the core plate as illustrated inFIG. 1B.

As illustrated inFIGS. 1A–C, the small annular space or gap110formed between the shroud and the core plate, which may have a width of only about 13 mm (0.5 inch), and the presence of the core plate wedges108, hampers access to the welds112and renders the necessary installation and in service inspections of the welds a serious challenge.

FIGS. 2A–2Cillustrate the operation some of the basic components of an exemplary apparatus according to the present invention. As illustrated, a track support200supports an arcuate track202that, in turn, supports a scanning assembly on its outer face that includes a carriage204, a sensor support206and a sensor208and mechanisms that provide for the relative movement of the various elements (not illustrated). As illustrated inFIG. 2B, the track support200may be arranged and configured for repositioning the arcuate track in an offset or cantilevered position relative to the track support. Similarly, as illustrated inFIG. 2C, the scanning assembly may be moved along the arcuate track202in order to position the sensor208for scanning and to conduct the scan.

The operation of the basic components illustrated inFIGS. 2A–Cand described above are substantially duplicated inFIGS. 3A–C. As reflected inFIG. 3D, however, the sensor support206may also be moved vertically relative to the arcuate track202utilizing a track or guide210for the purpose of inserting the sensor208into the annular gap110and positioning it in sufficient proximity to the welds112or one of the side surfaces of the shroud100or core plate102to allow for the desired scanning operation to be conducted.

An exemplary scanning operation is illustrated inFIGS. 4A–C(with the shroud removed). As shown inFIG. 4A, the apparatus will be positioned adjacent the inner surface of the shroud100with the arcuate track202generally parallel the top surface of the core plate102. The carrier204will be positioned above a portion of the annular gap110that is not obstructed by a core plate wedge108. The sensor208portion of the apparatus will then be lowered into the annular gap110and positioned adjacent the weld or surface of interest. The sensor element or elements provided within the sensor208are then activated as the sensor is moved along the scanned feature to scan an area, generally suggested by area300, and generate the desired scan data. This data is then analyzed to evaluate the condition of the scanned feature so that corrective action, if required, may be taken in a timely fashion. Additional guide and/or resilient elements (not shown) may be incorporated in the sensor support to allow a limited degree of passive “float” to accommodate minor irregularities in the scanned feature without damaging the sensor or requiring active vertical repositioning of the sensor support.

As will be appreciated, the scanning portions of the apparatus may be embodied in a wide variety of configurations, but will generally include an arcuate track202that is constructed to have a radius of curvature that this substantially identical to that of the inner surface of the shroud100. Depending on the sizing of the annular gap110, the lower portions of the sensor support206that will be inserted into the annular gap and one or more of the surfaces of the sensor208may also be configured with similar curvatures. By adapting these elements to better correspond to the annular gap110, the likelihood of mechanical interference during the insertion into and movement along the gap may be reduced and the positioning mechanisms may be simplified. The apparatus may also include one or more stand-off elements216that will contact the inner surface of the shroud and positively establish a known offset for the arcuate track202relative to the shroud.

As illustrated inFIGS. 5A–D, the apparatus may be arranged within a casing or frame212that is configured to correspond to a fuel bundle assembly so that it may be inserted into the reactor core using the openings provided in the top guide and the corresponding alignment structures provided on the frame212aand core plate102to fix the position of the frame relative to the inner surface of the shroud. During insertion and positioning of the frame212, the scanning apparatus may be maintained in a protected or retracted position within the frame both to ease the insertion through the top guide and protect the more delicate elements of the apparatus. Once the frame has been positioned within the core, typically using a peripheral fuel bundle location, the scanning apparatus may be reconfigured to extend from the frame212and toward the inner surface of the shroud.

The arcuate track202, which may be positioned in a substantially vertical orientation to fit within the frame212,FIG. 5B, may then be rotated about an axis218to place it in a substantially horizontal configuration,FIG. 5C, i.e., substantially parallel to the top surface of the core plate, in preparation for the scanning operation. The extending portion of the scanning apparatus may be connected to the frame212using a variety of mechanical elements214and positioning elements including, for example, hydraulic pistons, pneumatic pistons, stepper motors, four-bar linkages that may be selected and configured to position the scanning apparatus adjacent the inner wall of the shroud100and above the annular gap110as illustrated inFIG. 5D.

As illustrated inFIGS. 6A–D, the sensor support may be configured in a “T” shape,206a, or an “L” shape,206b, with sensors208,208aand208b, provided on the extended portion(s) of the sensor supports. The use of such modified sensor supports in the scanning operation illustrated inFIGS. 4A–D, would allow the sensor head(s) to be positioned below the core plate wedges that limit the horizontal travel of the upper portion of the sensor support and increase the circumferential area that could be scanned using the exemplary apparatus. Further, as illustrated inFIGS. 6B and 6D, the sensor supports206a,206band/or the sensors208,208a,208b, may, depending on the relative sizing also be configured to correspond to the radius of curvature of the annular gap110.

As illustrated inFIG. 8, the sensor208may comprise a frame or main body302arranged for fastening to the sensor support and for holding a plurality of sensors304,306that may have different configurations and/or be suitable for analyzing different properties of the scanned feature(s) or surfaces. For example, the sensor208may include a series of ultrasonic transducers configured to generate different frequency ranges and focused on different regions relative to the frame position. For example, the individual sensors in an exemplary sensor may include a 45° shear transducer operating at 2.25 MHz, a 60° refracted longitudinal (RL) transducer operating at 2.25 MHz, an outer diameter (OD) creeper transducer operating at 2.25 MHz and a 0° longitudinal transducer operating at 5 MHz. In order to accommodate the relatively narrow spacing of the annular gap110, these transducers will typically have dimensions on the order of 5 mm to 21 mm, but, as will be appreciated, the actual sizing may be adapted to and will be determined by the particular application and the particular sensors being utilized in the sensor head. The sensor is not limited to ultrasonic devices and may include eddy current, electrical resistance, optical and other sensors as desired, provided that they can be configured to meet the space requirements. The frame302will also typically include a plurality connections or ports308for both power and sensing, communication or data lines that may include both conductive wires or cables and optical fiber lines.

Other exemplary embodiments of apparatus according to the invention are illustrated inFIGS. 7A–C. As illustrated inFIG. 7A, the arcuate track202may be provided with holding devices arranged near the ends of the tracks. One embodiment of a holding device includes a cylinder220or other actuator that can be used to extend a positioning device such as a wedge222, resilient member (not shown), cam (not shown) or other element. As illustrated inFIG. 7B, the positioning device(s) may be configured and may be positioned to temporarily fix the position of the track202with respect to the annular gap110by contacting one or more of the sidewalls of the annular gap, the inner surface of the shroud100and the core plate wedges108(not shown). Once the scanning operation has been completed, the positioning devices222may be released or withdrawn to allow for the repositioning of the scanning apparatus adjacent a different circumferential portion of the inner shroud100.

As illustrated inFIG. 7C, the scanning apparatus may be provided on a pole226that may support a frame or assembly224and allow for a wider range of movement within the reactor core. The frame224may also be provided with alignment structures224afor positioning in fuel bundle locations on the core plate or may include other assemblies (not shown) for temporarily fixing the position of the scanning apparatus relative to the inner surface of the shroud100. The other assemblies may be arranged and configured to connect to other hard points or attachment fixtures typically provided at various locations within the reactor vessel for conducting periodic maintenance. Alternatively, the other assemblies may be configured to interact with other structures typically arranged near the periphery of the core plate whereby the scanning apparatus may be temporarily fixed in position relative to the inner surface of the shroud. For example, vacuum assemblies may be provided for fixing the scanning apparatus to the shroud100or the core plate102by creating a pressure differential sufficient to hold the apparatus in place.