Apparatus and method to inspect, modify, or repair nuclear reactor core shrouds

This invention generally concerns robotic systems and is particularly concerned with improved apparatus and methods for remotely inspecting, modifying or repairing a core shroud in a nuclear reactor. The apparatus of the invention includes a partial upper track which horizontally movable along the core shroud, a head and frame assembly which is horizontally movable along the partial upper track, a lower track which is connected to the head and frame assembly and is horizontally movable along the core shroud, and a carriage and arm assembly which extends downward into an annulus formed by the reactor pressure vessel and the core shroud, wherein the arm includes at least one sensor for inspecting the core shroud.

FIELD OF THE INVENTION

This invention generally concerns robotic systems and is specifically concerned with an improved apparatus and method for remotely inspecting, modifying or repairing a core shroud in a nuclear reactor.

BACKGROUND OF THE INVENTION

A nuclear reactor produces electrical power by heating water in a reactor pressure vessel that contains a nuclear fuel core in order to generate steam which is used in turn to drive a steam turbine. The reactor pressure vessel includes a cylinder surrounding the nuclear fuel core. This cylinder is the core shroud. Feed water is admitted into the reactor pressure vessel and flows through an annular region which is formed between the reactor pressure vessel and the core shroud. Within the annular region, jet pump assemblies are circumferentially distributed around the core shroud. The core shroud includes various welds which are later discussed in detail herein. A core shroud head is positioned atop the core shroud. The material of the core shroud and associated welds is austenitic stainless steel having reduced carbon content. The heat-affected zones of the shroud welds have residual weld stresses. Therefore, the mechanisms are present for the shroud welds to be susceptible to stress corrosion cracking.

Stress corrosion cracking in the heat affected zone of any shroud weld diminishes the structural integrity of the shroud. In particular, a cracked shroud increases the risks posed by a Loss-of-Coolant Accident (LOCA) or seismic loads. During a LOCA, the loss of coolant from the reactor pressure vessel produces a loss of pressure above the shroud head and an increase in pressure inside the shroud, i.e., underneath the shroud head. The result is an increased lifting force on the shroud head and on the upper portions of the shroud to which the shroud head is bolted. If the core shroud has fully cracked girth welds, the lifting forces produced during a LOCA could cause the shroud to separate along the areas of cracking, producing undesirable leaking of reactor coolant. Also, if the shroud weld zones fail due to stress corrosion cracking, there is a risk of misalignment from seismic loads and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.

Thus, the core shroud is examined periodically to determine its structural integrity and the need for repair. Ultrasonic inspection is a known technique for detecting cracks in nuclear reactor components. The inspection area of primary interest is the outside surface of the core shroud and the horizontal mid-shroud attachment welds. However, the core shroud is difficult to access. Installation access is limited to the annular space between the outside of the shroud and the inside of the reactor pressure vessel, between adjacent jet pumps. Scanning operation access is additionally restricted within the narrow space between the shroud and jet pumps. The inspection areas are highly radioactive, and are located under water 50 to 80 feet below the operator's work platform. Thus, inspection of the core shroud in operational nuclear reactors requires a robotic device which can be installed remotely and operated within a narrowly restricted space.

Remote operation is mandatory due to safety risks associated with radiation in the reactor. During reactor shutdown, servicing of components requires installation of inspection manipulators or devices 30 to 100 feet deep within reactor coolant. The inspection equipment typically consists of manually controlled poles and ropes to manipulate servicing devices and/or positioning of these devices. Relatively long durations are required to install or remove manipulators and can impact the plant shutdown duration. In addition, different inspection scopes can require several manipulator reconfigurations requiring additional manipulator installations and removals. The long durations cannot only impact plant shutdown durations, but also increase personnel radiation and contamination exposure.

Plant utilities have a desire to reduce the number of manipulator installations and removals to reduce radiological exposure as well as cost and plant outage impact. This invention allows the number of reconfigurations, installations and removals to be minimized. In addition, plant utilities have relatively small working areas near the access point of the reactor cavity. Therefore, the size of the manipulators can impact other activities during plant shutdown. The design of this invention allows the elimination of large track ring typically utilized on similar equipment. Plant utilities also desire flexible and effective coverage on the reactor core shroud. This invention allows the manipulator operations to position end effectors in various locations on the shroud which are often inaccessible to currently designed tooling. The small profile and flexible axes system of this invention provide efficient core shroud coverage which can be significantly greater than the coverage provided by currently existing equipment.

SUMMARY OF THE INVENTION

The invention provides apparatus and methods for inspecting a core shroud in a reactor vessel. In one aspect, the invention provides an apparatus for remotely inspecting a core shroud of a nuclear reactor. The apparatus includes a partial upper track positioned on an annular rim of the core shroud and horizontally movable along the rim, an assembly which includes a head movably connected to the partial upper track such that the head is horizontally movable along the partial upper track, a lower track, a frame having a first end and a second end, the first end being mounted to the head and the second end being connected to the lower track such that the lower track is horizontally movable along the core shroud, a carriage movably connected to the lower track, a scan arm connected to the carriage and extending vertically downward along the core shroud, and at least one sensor connected to the scan arm for inspecting the core shroud.

The partial upper track can be positioned on a steam dam of the core shroud as a result of its center of gravity. The head can include a positioning motor and gear combination to move the head along the partial upper track. A track brake system can be connected to the partial upper track. In certain embodiments, when the track brake system is activated, the partial upper track is stationary on the rim of the core shroud and when the track brake system is released, the partial upper track is horizontally movable along the rim of the core shroud.

The head can include a positioning pin which is vertically extendable. In certain embodiments, when the positioning pin is extended and the track brake system is released, the head is stationary relative to the core shroud and the partial upper track is horizontally movable along the rim of the core shroud.

Bearing wheels can be attached to the frame such that the frame is horizontally movable along the lower track. The carriage can include at least one motor/gear combination and a pneumatic/hydraulic cylinder such that the carriage is horizontally movable along the lower track and the scan arm is pivotally movable relative to the carriage. The sensor can be an ultrasonic transducer.

In another aspect, the invention provides a method for inspecting a core shroud of a nuclear reactor. The method includes positioning an inspection tool on an annular rim of the core shroud such that the tool at least partially vertically extends into an annulus formed between the core shroud and the nuclear reactor. The inspection tool includes a partial upper track, a head and frame assembly, a lower track, a carriage and arm assembly, and at least one sensor connected to the scan arm. The method further includes moving horizontally at least one of the partial upper track, the head and frame assembly, the lower track, and the carriage along the annular rim of the core shroud such that the sensor scans the core shroud.

The partial upper track can be horizontally moved from a first position to a second position along the annular rim of the core shroud and the head and frame assembly remains stationary. The carriage and arm assembly can be moved from a first position to a second position along the lower track and the lower track can be moved from a first position to a second position relative to the head and frame assembly. The method can further include assessing the inspection results and determining if modification or repair of the core shroud is needed.

In another aspect, the invention provides a method for inspecting a core shroud of a nuclear reactor. The method includes positioning an inspection tool on an annular rim of the core shroud such that the tool at least partially vertically extends into an annulus formed between the core shroud and the nuclear reactor. The inspection tool includes a head and frame assembly, a lower track movably connected to the head and frame assembly, a carriage and arm assembly movably connected to the lower track, and at least one sensor connected to the carriage and arm assembly. The method further includes moving horizontally at least one of the head and frame assembly, the lower track, and the carriage along the annular rim of the core shroud such that the sensor scans the core shroud.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It should be understood that when an element of component is referred to as being “on”, “connected to,” “coupled to,” or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or components may be present.

The invention relates to robotic devices for remotely inspecting, modifying or repairing a core shroud of a reactor pressure vessel in a nuclear power plant. In certain embodiments, the nuclear power plant includes a light water reactor, such as a boiling water reactor or a pressurized water reactor. The reduced thickness and the two-axis positioner of the device allow for positioning and operation of the device in the narrow annulus space provided between the core shroud and a wall of the reactor pressure vessel. Further, these features allow the device to be maneuvered and configured circumferentially and vertically around jet pumps (and other obstructions) that are located in the annulus space.

Referring toFIG. 1A, there is illustrated a core shroud2in a reactor pressure vessel (RPV)4of a conventional boiling water reactor (BWR). Feedwater is admitted into the RPV4via a feedwater inlet (not shown) and a feedwater sparger6, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV4. The feedwater from the sparger6flows downwardly through a downcomer annulus8, which is an annular region formed between the core shroud2and the RPV4.

The core shroud2is a stainless steel cylinder surrounding the nuclear fuel core, the location of which is generally designated by numeral10inFIG. 1. The core is made up of a plurality of fuel bundle assemblies (not shown). Each array of fuel bundle assemblies is supported at the top by a top guide and at the bottom by a core plate (neither of which are shown). The core top guide provides lateral support for the top of the fuel assemblies and maintains the correct fuel channel spacing to permit control rod insertion.

The feedwater flows through the downcomer annulus8, into the jet pumps18, and into the core lower plenum12. The feedwater subsequently enters the fuel assemblies, wherein a boiling boundary layer is established. A mixture of water and steam enters a core upper plenum14under a shroud head16. The steam-water mixture than flows through vertical standpipes (not shown) atop the shroud head16and enters steam separators (not shown), which separate liquid water from steam. The liquid water then mixes with feedwater in the mixing plenum, which mixture then returns to the reactor core via the downcomer annulus8. The steam is withdrawn from the RPV via a steam outlet.

The BWR also includes a coolant recirculation system which provides the forced convection flow through the core which is necessary to attain the required power density. A portion of the water is removed from the lower end of the downcomer annulus8via a recirculation water outlet (not visible inFIG. 1) and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies18(two of which are shown inFIG. 1A) via recirculation water inlets20. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The jet pump assemblies are circumferentially distributed around the core shroud2.

Referring toFIG. 1B, there is illustrated the core shroud2in detail. There is a shroud head flange2afor supporting the shroud head16, a circular cylindrical upper shroud wall2bhaving a top end welded to shroud head flange2a, an annular top guide support ring2cwelded to the bottom end of the upper shroud wall2b, a circular cylindrical middle shroud wall having a top end welded to top guide support ring2cand consisting of upper and lower shell sections2dand2ejoined by mid-shroud attachment weld, and an annular core plate support ring2fwelded to the bottom end of the middle shroud wall and to the top end of a lower shroud wall2g. The entire shroud is supported by a shroud support22, which is welded to the bottom of lower shroud wall2g, and by annular jet pump support plate24, which is welded at its inner diameter to shroud support22and at its outer diameter to RPV4.

The material of the shroud and associated welds is austenitic stainless steel having reduced carbon content. The heat-affected zones of the shroud girth welds, including the mid-shroud attachment weld, have residual weld stresses. Therefore, the mechanisms are present for mid-shroud attachment weld W and other girth welds to be susceptible to stress corrosion cracking.

The apparatus of the invention is in contact with the core shroud and operated in the annulus formed between the reactor pressure vessel and the core shroud to perform an inspection of the core shroud and any welds associated therewith. The apparatus includes an upper partial track which is positioned, e.g., placed on or connected to, a portion of the core shroud, such as an upper annular rim of the core shroud. In one embodiment, the upper partial track is placed on the steam dam of the core shroud and is supported thereon by its center of gravity. The upper partial track guides a precision head and rigid frame structure which is movably coupled to the upper partial track. The rigid frame structure extends vertically downward from the precision head. The precision head and frame structure includes an electric motor and ball bearings or the like which allows the structure to travel horizontally along the upper partial track. Further, the upper track contains motors and brakes which are systemically configured to allow the use of this apparatus without a complete track ring. The head and frame structure houses a sensor positioner for performing inspections or repairs on upper barrel regions of the core shroud. The precision head and frame are also operable to position a moveable lower track. The lower track region houses a lower arm and positioner to perform inspections or repairs on middle and lower reactor core shroud barrel regions. The positioner is a two-axis positioner which allows displacement of the arm vertically and circumferentially along the core shroud. At least one sensor, such as an ultrasonic transducer, is connected to the lower arm for inspecting the core shroud. In certain embodiments, the lower arm can include multiple sensors in a spaced apart relationship to each other.

Referring toFIG. 2, there is illustrated a core shroud inspecting apparatus generally referred to by reference character100for inspecting a core shroud in a nuclear reactor, in accordance with certain embodiments of the invention. The apparatus100includes a head110, a frame112and a partial upper track114. The frame112has an upper end116and an opposite lower end118. The upper end116of the frame112is mounted to the head110. The head110is connected to the partial upper track114for suitably moving in a horizontal direction relative to the partial upper track114. The lower end118of the frame112is mounted to a lower track120for suitably moving the lower track120relative to the frame112. A carriage122is coupled to the lower track120for suitably moving horizontally relative to the lower track120. In certain embodiments, the partial upper track114and the lower track120are curved to suitably conform to the cylindrical shape of the core shroud in the nuclear reactor.

The partial upper track114includes a track brake system124. When the track brake system124is activated, the partial upper track114remains stationary and the head110(and frame112mounted thereto) is horizontally movable along the partial upper track114. When the track brake system124is deactivated or released, the partial upper track114can be driven into a different position along the rim of the core shroud. The track brake system124allows the head110and frame112to walk along the shroud without requiring a complete guide track ring. Thus, the head110and frame112are horizontally movable to drive along the partial upper track114, or alternatively, the partial upper track114is horizontally movable to be driven into a different position along the rim of the core shroud.

Referring toFIG. 3, there is illustrated a fixed gear rack mechanism150for moving the partial upper track114along the core shroud. The fixed gear rack mechanism150interfaces with a positioning motor155and gear combination157located within the head110. When the motor155is driven, the head110is moved relative to the partial upper track114. If the track brake system124is applied, the partial upper track114will remain stationary relative to the core shroud and the head110moves relative to the partial upper track114and the core shroud. Alternatively, a positioning pin160can be pneumatically or hydraulically extended from the head110to react with reactor hardware positioned on the rim of the core shroud. If the position pin160is extended and the track brake system124is released, the head110remains stationary relative to the core shroud and the partial upper track will move relative to the core shroud. This provides for relocation of the entire apparatus100relative to the core shroud. The head motor155provides full position feedback so that global positioning of the entire apparatus is maintained and monitored within a tight tolerance.

Referring toFIG. 4, there is illustrated a movable bearing system170which allows the lower track120to be driven relative to the frame112and to reach positions along the core shroud which are outside of the typical boundaries and obstructions exhibited by known apparatus. The frame112contains bearing wheels172that roll along guides174coupled to the lower track120.

Referring toFIGS. 4 and 5, the frame112houses a fixed motor175and pinion gear177which can be driven and react against a rack gear178coupled to the lower track120. Upon rotation of the frame motor175, the lower track120moves relative to the frame112.

As shown inFIG. 5andFIGS. 6A-6D, the lower track120houses additional vertical and horizontal precision positioners180to provide precision position of tooling sensors or end effectors182. The carriage system122on the lower track120houses two motor/gear combinations184A,B and one pneumatic/hydraulic cylinder185. One of the motor/gear combinations184A interfaces with the rack gear200coupled to the lower track120which allows the carriage system122to move along the lower track120. The other motor/gear combination184B is coupled to a linear lead screw190which drives the pivoting cylinder185vertically in the general areas relative to the lower track120. The pivoting cylinder185provides pivoting motion for the attached arm134and end effectors182for positioning the end effectors182away from reactor obstructions. Overall, the apparatus contains seventeen axes of motion to position sensor and end effectors182in an efficient method to minimize size, plant shutdown schedule impacts, and personnel radiological exposure, and to maximize end effector coverage on the reactor core shroud around obstructions.

Referring toFIG. 7, there is illustrated the core shroud inspecting apparatus100(shown inFIG. 2) which is positioned on an annular rim130of a core shroud132. The apparatus100extends vertically downward into an annulus space formed between the core shroud132and a reactor pressure vessel (not shown).FIG. 7includes the head110, frame112, partial upper track114, lower track120, and the carriage122(as shownFIG. 2). Further,FIG. 7includes an arm134connected to the carriage122and extending vertically downward therefrom along the core shroud132. At least one sensor (not shown) is attached to the arm134. The sensor is capable to detect and analyze the surface of the core shroud132including any welds contained therein. Suitable sensors for use in this invention can include those devices, such as but not limited to ultrasonic sensors, which are known in the art for inspections. In certain embodiments, multiple sensors can be positioned in a spaced apart relationship to each other along a vertical length of the arm134. Placement, e.g., spacing, of the sensors can be determined by and correspond to specific areas of the core shroud132to be inspected, such as the middle and lower barrels (not shown). InFIG. 7, the lower track120is offset from the head110and the frame112, and the carriage122with the arm134is offset from the lower track120.

In certain embodiments, the apparatus and method of the invention does not include the partial upper track. In this embodiment, the head and frame assembly is positioned on the annular rim of the core shroud and the frame portion vertically extends into the annulus formed between the core shroud and the reactor pressure vessel. The lower track is movably connected to the frame portion to allow the lower track to travel horizontally relative to the frame portion. Further, the carriage and arm assembly is movably connected to the lower track to allow the carriage and arm to travel horizontally relative to the lower track. At least one sensor is connected to the arm for the purpose of performing a scan to inspect the core shroud.