Method for detecting changes in preload on a tie rod installed as part of a core shroud repair in a boiling water reactor

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BACKGROUND OF THE INVENTION

1. Field Of the Invention

The present invention relates to boiling water nuclear reactors having a core shroud disposed concentrically within a reactor vessel and a plurality of tie rods installed in tension between vertically spaced sites on the core shroud at a respective plurality of angularly spaced locations about the periphery of the core shroud to axially compress the core shroud as part of a core shroud repair. In particular, the present invention relates to a method and apparatus for measuring the preload on such tie rods after installation in order to detect changes in the axially compressive preload applied to the core shroud by the tie rods.

2. Discussion of the Related Art

Boiling water reactor shrouds are disposed concentrically within reactor vessels to divide the flow of cooling water through the reactor vessels and to structurally support and align the fuel assemblies, steam separator assemblies and control rod guide tubes. The shrouds are generally cylindrical and are typically formed of multiple arcuate steel plates joined by welds along abutting vertical and horizontal edges. After periods of use, cracking of the shroud within heat affected zones of the welds tends to occur as a result of corrosion, radiation and stress. Cracking of the vertically oriented welds is expected to be minor and is considered acceptable because these welds are relatively short in length, relative to the overall shroud length, and because cracking along the vertically oriented welds does not adversely affect the function of the shroud. When excessive cracking of the horizontally oriented welds occurs, however, the shroud must either be replaced or repaired.

U.S. Pat. No. 5,402,570 to Weems et al, the disclosure of which is incorporated herein by reference, describes a method of repairing boiling water reactor core shrouds having horizontal cracks in heat affected zones of welds by securing plural tie rods in vertical orientation about the periphery of the cracked shroud to axially compress the shroud and thereby urge the opposing surfaces of the horizontal cracks toward one another. The method involves securing the tie rods between sites on the core shroud adjacent top and bottom portions of the shroud. Typically, the upper end of each tie rod is threaded and is made to pass through connection hardware, such as a bracket or beam, depending from the top of the shroud. A nut is then threaded onto the upper end of the tie rod and the tie rod tensioned to achieve a desired tie rod preload corresponding to an acceptable level of compression on the shroud. Tensioning of the tie rod is usually accomplished by threading a puller bar onto the upper end of the tie rod and using a hydraulic cylinder to lift the puller bar upwardly in a vertical direction. The nut is then tightened against the bracket so that, when the puller bar is released, an upwardly directed tensile force is exerted on the tie rod by the nut, and a corresponding downwardly directed compressive force is exerted by the nut on the bracket, thereby holding the shroud in axial compression along its length. This axial compression drives the opposing surfaces of any horizontal cracks toward one another, thereby urging the cracks together across all or part of the thickness of the shroud and preventing their adverse effects on the shroud structure. Axial compression of the shroud also improves the ability of the shroud to withstand axial or tensile loads caused by hydrostatic pressures and/or seismic events since such tensile loads are offset by the compressive preload and are thereby mitigated.

It is desirable for an axially compressive preload to be applied continuously to the shroud by the tie rods and for such a preload not to vary significantly from that originally applied by the tie rods at the time of installation; however, preloads can change after installation due, for example, to seismic events and/or thermal transients associated with the start-up and cool-down of the reactor. For example, changes in tie rod preload could occur due to cracking of the core shroud, settling and wear-in of threaded and other mechanical joints, thermal or neutron relaxation of the tie rods, or failure of the tie rods (for example, as a result of cyclic loading of the tie rods caused by thermal transients). To date, however, it has not been practical to detect changes in the axially compressive preload applied to the shroud by the tie rods due to the inaccessibility of the tie rods and the shroud and the highly radioactive and hostile environment within the reactor vessel.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to determine whether the preload on a tie rod has changed following installation of the tie rod in a boiling water reactor as part of a core shroud repair.

Another object of the present invention is to determine whether the preload on a tie rod installed as part of a core shroud repair in a boiling water reactor has changed by remotely measuring the existing preload on the tie rod after the occurrence of a seismic event and/or after a period of operation of the reactor following installation of the tie rod.

It is a further object of the present invention to apply a tensile load to a tie rod installed as part of a core shroud repair in a boiling water reactor and to remotely measure the load applied to the tie rod as well as the axial displacement or deflection of the tie rod in response to the applied load in order to determine the existing tie rod preload.

Yet another object of the present invention is to determine the existing preload on a tie rod installed as part of a core shroud repair in a boiling water reactor by identifying a change in the relationship between the tensile load applied to the tie rod and the resulting axial displacement of the tie rod which occurs when the applied load equals and exceeds the existing tie rod preload.

Some of the advantages of the present invention are that the method and apparatus can be used remotely to assure adequate axial compression of the shroud without having to detorque or unlock the nuts used to set the original tie rod preload, that the method and apparatus can be used upon initial installation of the tie rods and periodically thereafter as part of routine maintenance, and that the method and apparatus minimize reactor downtime and reduce the exposure of personnel to the potentially harmful effects of radiation.

The present invention is generally characterized in a method of measuring tie rod preload in a boiling water nuclear reactor having a core shroud disposed within a reactor vessel and a plurality of tie rods installed in tension between vertically spaced sites on the core shroud at a respective plurality of angularly spaced locations about the periphery of the core shroud as part of a core shroud repair. The method is performed after installation of the tie rods to determine changes in the preload on a tie rod and includes the steps of lowering a tool into the reactor vessel from a remote location positioned externally of the reactor vessel, attaching the tool to an upper portion of a tie rod, using the tool to apply a tensile load along a longitudinal axis of the tie rod, measuring the tensile load applied to the tie rod, measuring axial displacement of the tie rod as a result of the tensile load applied, plotting the load applied versus the axial displacement measured, and identifying a change in slope on the load-displacement plot occurring at a load corresponding to the tie rod preload.

A further aspect of the present invention is generally characterized in a method of measuring tie rod preload in a boiling water nuclear reactor having a core shroud disposed within a reactor vessel and a plurality of tie rods installed in tension between vertically spaced sites on the core shroud at a respective plurality of angularly spaced locations about the periphery of the core shroud as part of a core shroud repair. The method is performed after installation of the tie rods to determine changes in the preload on a tie rod and includes the steps of applying a tensile load along a longitudinal axis of the tie rod, measuring axial displacement of the tie rod in response to the applied tensile load, and remotely viewing the axial displacement measurement from a location externally of the reactor vessel.

Another aspect of the present invention is generally characterized in a tool for measuring tie road preload in a boiling water nuclear reactor having a core shroud disposed within a reactor vessel and a plurality of tie rods installed in tension between vertically spaced sites on the core shroud at a respective plurality of angularly spaced locations about the periphery of the core shroud as part of a core shroud repair, the tool including an elongate support member having an upper end for positioning externally of the reactor vessel and a lower end for positioning inside the reactor vessel adjacent the core shroud, a loading device suspended from the lower end of the support member and including a base for reacting against the core shroud and a puller movable relative to the base and having a configuration to couple with the upper end of a tie rod, a displacement indicator responsive to axial displacement of the puller relative to the base, and a load indicator responsive to axial loading of the tie rod by the puller.

Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments taken with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view, in elevation, of a tool for measuring tie rod preload in a boiling water reactor according to the present invention.

FIG. 2 is a fragmentary side view, partly in section, of the tie rod preload measurement tool shown in FIG. 1.

FIG. 3 is a fragmentary top view, partly in section, of the tie rod preload measurement tool of FIG. 1 showing a dial indicator mounted therein.

FIG. 4 is a fragmentary side view, partly in section, of a pole attachment for use with the tie rod preload measurement tool according