Patent Number: 052727343
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear reactors and, more particularly, to a method of repairing a boiling-water nuclear reactor. Nuclear reactors are conservatively specified to minimize any risks from the hazardous materials involved in their use. Reactor vessel walls are several inches thick and the strongest materials are used for reactor components. Nonetheless, contingencies are required for failure as components are subjected to extreme stress for decades. These contingencies involve not only many layers of preventive systems, but also procedures for rectifying problems that arise. Of concern herein are incore-housing-related defects. Incore instrumentation housings, referred to more simply as "incore housings", house the links between instrumentation used to monitor the core and the host system used to analyze the data collected by instrumentation. The housings are tubular and penetrate the reactor vessel bottom, to which they are welded. Incore-housing-related defects include defects in the housing itself, in the weld bonding the housing to the vessel bottom and in the vessel bottom in the vicinity of the housing. These defects can cause or lead to leaks from the reactor vessel. Accordingly, a method is required to address such defects. Drastic approaches involve long-term shutdown of the reactor. The reactor could be replaced or removed, repaired, and reinstalled. These approaches are extremely costly, and alternatives are highly desirable. Some incore housing defects can be addressed by inserting a sleeve onto the incore housing to cover the defects and stop whatever leaking might occur. However, such patch approaches do not really address the defect, which can continue to grow due to further fatiguing. Furthermore, the weld between the sleeve and the housing can be a new source of defects. More recently, a method has been developed for replacing an incore housing that minimally impacts reactor components other than those being replaced. This method involves removal of all fuel from the reactor and performing most steps under water to minimize radiation exposure. While providing a relatively permanent repair and while being relatively economical, this method involves a series of over 100 procedures, several of which could introduce new defects. Such defects may be serious enough to force repetition of the replacement method. This repetition is undesirably time consuming and expensive. What is needed is an improved method of effecting a relatively permanent repair of an incore housing defect that is relatively economical and, preferably, minimally disruptive of the reactor system. SUMMARY OF THE INVENTION In accordance with the present invention, an improved method for replacing an incore housing includes an integral series of ultrasonic inspections. The ultrasonic inspections minimize the amount of re-repairing required in case the repair procedure introduces a defect. Also in accordance with the present invention, most of the steps, including the ultrasonic inspections, are done underwater to minimize radiation exposure. There are three major stages to the method, each having a major structural objective and an associated ultrasonic inspection. The first stage involves removal of the original housing and inspection of remaining weld material and exposed cladding at the reactor vessel bottom. The second stage involves forming a weld buildup and ultrasonically inspecting the weld buildup. The third stage involves welding a new housing to the weld buildup and ultrasonically inspecting the new housing and its attachment to the weld buildup. The method begins with the removal of the original incore housing. Initially, all fuel is removed from the reactor, leaving a vacated core region. The reactor vessel remains filled to its normal level with water. An ultrasonic scanner with a first ultrasonic probe is installed over the aperture through which the incore housing penetrated the reactor bottom. The scanner is installed beneath the core region in the space vacated by the removed housing. The attached first probe has a disk-shaped probe head that is free to tilt relative to the vertical shaft between it and the scanner. When the probe head is lowered to contact the reactor bottom, the probe head orientation conforms to the local contour of the bottom. An ultrasonic examination of cladding on the reactor bottom, and of unremoved weld material, is conducted while rotating the diskshaped probe head about the shaft axis. The probe head continuously reorients as required to maintain conformity with the local contour of the bottom. The scanner and the first probe are removed from the reactor vessel. If no defects are found, or if detected defects are appropriately corrected, the procedure continues as follows. Water is removed from the vessel. A weld buildup is formed over the aperture. The vessel is refilled with water, submerging the core region. In addition, the exterior of the weld buildup is machined smooth to aid in subsequent ultrasonic inspection. An aperture is machined through the weld buildup to provide access through the original bottom aperture. Once the weld buildup is finished, a second ultrasonic probe is attached to the scanner; the scanner is secured in the space vacated by the removed incore housing. The head of the second probe has a relatively large diameter so that it fills most of the aperture through the weld buildup, except for an annular portion next to the inner wall of the weld buildup. The probe head is scanned vertically along the weld buildup aperture to inspect the weld buildup for defects. Between vertical scans, the head is stepped circumferentially in appropriate, e.g., 2.degree.-6.degree., increments. The increments are chosen to ensure overlap of successive scans so that no inspection volume is skipped. Thus, a cylindrical raster scan of the weld buildup is performed. The scanner and attached second probe are removed from the reactor. Once again, if no defects are found, or if detected defects are appropriately corrected, the procedure continues. An annular groove having a "J"-shaped cross section is machined in the top of the weld buildup adjacent to the buildup aperture. This "J-prep" is in preparation for the weld between the weld buildup and a replacement incore housing. This replacement incore housing is inserted up through the vessel bottom. The new housing extends only several inches above the bottom contour of the vessel. Once the replacement incore housing is installed, water is removed from the reactor and the housing is "J-welded" to the weld buildup at the J-prep. The reactor is refilled. A third probe is attached to the scanner, and the scanner with third probe attached is secured just above the inserted housing. The head of this third probe is a relatively small diameter cylinder so that it fits with clearance through the interior of the replacement housing. This third head is lowered into the housing and a cylindrical raster scan of the housing and J-weld is performed. The scanner with third probe attached is removed. If no defects are detected or if detected defects are corrected, no more ultrasonic testing is required. A shrink coupling assembly is attached to the replacement housing and to the associated incore guide tube. Steam is applied to secure the shrink coupling. Repair is essentially complete. While the resulting structure differs from the original by the presence of the shrink coupling, it is essentially "like new" where the housing is attached to the weld buildup and the vessel bottom. This repair is effected without disruption of nearby components, and the reactor can be returned to operation with minimum delay. The repair is more permanent than various patches, and more economical and convenient than radical approaches that require major reactor disassembly. These and other features and advantages of the present invention are apparent from the description below with reference to the following drawings.