As seen in FIG. 1, a conventional boiling water reactor has a reactor pressure vessel 10 and a core shroud 12 arranged concentrically in the reactor pressure vessel with an annular region, namely, the downcomer annulus 14, therebetween. The core shroud 12 is a stainless steel cylinder surrounding the nuclear fuel core. In particular, the core shroud 12 comprises a shroud head flange 12a for supporting the shroud head (not shown); a circular cylindrical upper shroud wall 12b having a top end welded to shroud head flange 12a; an annular top guide support ring 12c welded to the bottom end of upper shroud wall 12b; a circular cylindrical middle shroud wall welded assembly 12d welded to the top guide support ring 12c; and an annular core plate support ring 12e welded to the bottom of the middle shroud wall 12d and to the top of a lower shroud wall 12f. As seen in FIG. 1, the shroud 12 is vertically supported by a plurality of shroud support legs 16, each of the latter being welded to the bottom head of the reactor pressure vessel 10. The shroud is laterally supported by an annular shroud support plate 18, which is welded at its inner diameter to the shroud 12 and at its outer diameter to the reactor pressure vessel 10. The shroud support plate 18 has a plurality of circular apertures 20 in flow communication with the diffusers of a plurality of jet pump assemblies (not shown),
The fuel core of a BWR consists of a multiplicity of upright and parallel fuel bundle assemblies 22 arranged in 2.times.2 arrays, each assembly consisting of an array of fuel rods inside a fuel channel made of zirconium-based alloy. Each array of fuel bundle assemblies is supported at the top by a top guide 24 and at the bottom by a core plate 26. The core top guide 24 provides lateral support for the top of the fuel assemblies; the core plate 26 provides lateral support for the bottom of the fuel assemblies. This lateral support maintains the correct fuel channel spacing in each 2.times.2 array to permit vertical travel of a cruciform control rod blade 28 in between the fuel channels.
The power level of the reactor is maintained or adjusted by positioning the control rods 28 up and down within the core while the fuel bundle assemblies 22 are held stationary. Each control rod 28 has a cruciform cross section consisting of four wings at right angles. Each wing consists of a multiplicity of parallel tubes welded in a row, each tube containing stacked capsules filled with neutron-absorbing material. Each control rod is raised or lowered inside a control rod guide tube 30 by a control rod drive (not shown) which is releasably coupled by a spud (not shown) at its top to a socket in the bottom of the control rod.
Control rod drives are used to position control rods in BWRs to control the fission rate and fission density, and to provide adequate excess negative reactivity to shutdown the reactor from any normal operating or accident condition at the most reactive time in core life. Each control rod drive is mounted vertically in a control rod drive housing 32 which is welded to a stub tube 34, which in turn is welded to the bottom head of the reactor pressure vessel 10. The control rod drive is a double-acting, mechanically latched hydraulic cylinder. The control rod drive is capable of inserting or withdrawing a control rod (not shown) at a slow controlled rate for normal reactor operation and of providing rapid control rod insertion (scram) in the event of an emergency requiring rapid shutdown of the reactor.
The control rod drive housing 32 has an upper flange that bolts to a lower flange of the guide tube 30. Each guide tube 30 sits on top of and is vertically supported by its associated control rod drive housing 32. The uppermost portion of the guide tube penetrates a corresponding circular aperture in the core plate 26. There are typically 140 guide tubes penetrating an equal number of circular apertures in the core plate, each aperture having a diameter slightly greater than the outer diameter of the guide tube.
Referring to FIG. 2, each guide tube 30 has a machined step at the top edge thereof which forms a flange 30a. A pair of lugs 30b and 30c project radially outward at diametrally opposite positions on flange 30a. The guide tube lug 30b has a slot (not shown) of width slightly greater than the diameter of a vertical guide pin 36 mounted on the core plate. During installation, the guide tube 30 must be rotated until the lug slot lines up with the guide pin 36. A fuel support casting 38 sits on top of the guide tube and has a 2.times.2 square array of openings. The fuel bundle assemblies 22 of each array are lowered through the square opening in the top guide and onto the fuel support casting. The fuel bundle assemblies are vertically supported by the fuel support casting during reactor operation.
The control rod drive housings and guide tubes have two functions: (1) to house the control rod drive mechanisms and the control rods, respectively, and (2) to support the weight of the fuel. The fuel weight is reacted at the orifice fuel support casting 38 which sits in the top of the guide tube 30. The control rod drive guide tubes and housings act as columns carrying the weight of the fuel.
The top guide 24 provides lateral support to the upper end of the fuel bundle assemblies 22, neutron monitoring instrument assemblies (not shown) and installed neutron sources (not shown), and maintains the correct fuel channel spacing to permit control rod insertion. The top guide 24 is designed so that during periodic refueling operations, the fuel bundle assemblies 22 can be lifted out of and lowered into the core without removing the top guide. One type of top guide installed in certain types of BWRs has a fabricated design comprising a lattice of interlocking upper and lower beams held together by a large circular ring. The circular ring of the top guide sits on the top guide support ring 12c of the shroud 12, and is provided with radially inwardly directed flanges that capture the distal ends of the beams.
The core plate 26 is bolted to and supported by the core plate support ring 12e. The core plate of a BWR has two functions: (1) to act as a flow barrier directing the flow of coolant water through the fuel channels containing the fuel rods to maximize heat transfer; and (2) to provide lateral restraint for the fuel channels by restraining horizontal movement of the control rod guide tubes 30. The pressure across the core plate 26 results in an upward load that is carried by the core plate and its underlying support structure 40.
During operation of the reactor, water is continuously recirculated down the downcomer annulus 14, into the lower plenum 42 and then up through the core. This flow is induced by a multiplicity of jet pumps (not shown) located in the downcomer annulus and driven by recirculation pumps (not shown) outside the reactor pressure vessel 10. The water in the lower plenum 42 enters the core via a plurality of flow inlets 44 in the guide tube. Each flow inlet 44 (see FIG. 2) is in flow communication with an opening at the bottom of a corresponding fuel channel via a corresponding opening formed in the fuel support casting 38. The fuel support casting has four such openings in a square array with a cruciform opening for passage of the control rod blade therebetween. The alignment of the lug slot with the guide pin 36 ensures that the fuel support casting will be properly oriented relative to the corresponding square opening in the top guide 24 to allow the fuel channel openings to align with the fuel support casting openings when the fuel bundle assemblies are lowered through the top guide and into position. During reactor operation, water flows upwardly through the fuel channels and acts as both a coolant for removing heat and a moderator for stopping neutrons.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as the core plate support structure, which are exposed to the high-temperature water environment inside a BWR. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding or cold working. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
Postulated cracking of the core plate support structure could result in loss of core plate support, resulting in uncontrolled loads on the control rod drive housings. Loss of core plate support would result in upward movement of the core plate caused by pressure under the plate. Thus, there is a need for a remotely installable means for repairing damaged core plates.