Patent Number: 047284820
Section: description

DETAILED DESCRIPTION The present method provides a method for inservice inspection of a pressure vessel wall with the lower internals maintained in position within the pressure vessel. A pressurized water nuclear reactor 1 is illustrated in FIG. 1, having a pressure vessel 3 which is of a generally cylindrical shape having a cylindrical wall 5, closed at the bottom by a bottom wall 7 of a hemispherical contour. The vessel is closed at the top by a flanged dome shaped head 9, which is secured, such as by bolts, to the top edge 11 of the pressure resistance wall 5, through the flanged portion 13, and is removable for refueling and inspection. The pressure resistant wall 5 has a plurality of inlet nozzles 15 and outlet nozzles 17, only one of each being shown, distributed about its periphery, a pair of each of such nozzles usually being provided. A nuclear core 19 is supported in the lower region of the pressure vessel 3, the core being supported in spaced relationship to the bottom wall 7 by a core barrel 21. The core barrel has a flange 23 which rests on a ledge 25 in the top inner surface of the pressure resistant wall 5. A core former 27 is situated about the lower region of the core barrel 21. The core includes a series of fuel assemblies 29 and thimbles 31 for receiving control rods, not shown, with at least one such thimble 31 adapted for insertion therein of an instrument for monitoring the operation of the core. The fuel assemblies and thimbles are mounted between a lower core plate 33 and an upper core plate 35. The control rods, as is known, may contain rod clusters of high and lower absorption cross-section for neutrons, and serve to reduce the thermal power of the reactor, or otherwise control the same, through monitoring by use of the instrument in the dedicated thimble therefor, or to shutdown the reactor. A lower core support plate 37 is provided with support columns 39, and a secondary core support 40 is also provided, as illustrated. These components comprise the lower internals 41. In the upper region of the pressure vessel 3, vertical guides 43 for the control rods and vertical guides for water displacement rods are provided, which generally comprise the upper internals 45. The lower internals 41, containing the core 19, and upper internals 45 are mounted generally coaxially within the pressure vessel 3. An annulus 47 between the core barrel 21 and the pressure resistant wall 5 provides for communication between the inlet nozzles 15 and the lower end of the core 19. Drive rods 49 from the control rods extend through seals 51 in the head 9. Drive mechanisms (not shown) are used to properly position the control rods, axially. Affixed about the periphery of the core barrel 21 there are neutron shields 53, and a plurality of radiation specimen pockets 55, with specimens (not shown) for monitoring radiation, insertable into said pockets. In practice, the width of the annular chamber 47, between the pressure vessel wall 5 and the core barrel 21 is about six inches (15.24 cm), while the width is narrowed in the area of the specimen pockets 55, the distance between the outer wall of the specimen pocket and the pressure resistant wall being about four inches (10.16 cm). The outer circumference of the pressure vessel wall 5 is about 540 inches (13.716 m), which gives an indication of the size of the pressure vessel and area that requires periodic inspection. In operation of the pressurized water reactor, coolant enters through the inlet nozzles 15 and flows downwardly through the annulus 47 to the bottom wall 7 and then upwardly through the core 19, into upper internals 45 and then transversely to, and outwardly from, the outlet nozzles 17, as indicated by the arrows shown in FIG. 1. The pressure vessel 3 is constructed from a plurality of sectional units which are integrally connected together by welds. Such welds, illustrated as welds 57, may comprise welds between the bottom head and wall 7 and the cylindrical wall 5, and welds between sections of the cylindrical wall 5, such as about nozzles 15 and 17, and intermediate welds. It is to the internal inspection of these welds and of the inner surface 59 of the pressure vessel 3 to which the present method is directed. As best illustrated in FIG. 2, the core barrel 21 has a cylindrical wall section 61 from which flange 23 extends outwardly. The flange 23 has a plurality of apertures 63 therethrough which communicate with the annulus 47 when the core barrel is positioned within the pressure vessel 3. The apertures 63 are normally closed with a removable plug 65. The purpose of the apertures 63, which normally have a diameter of about 2.25 inches (5.72 cm), is normally to enable the removal of specimens from the radiation specimen pockets 55 which are located about the outer periphery of the core barrel cylindrical wall section, eight of such apertures being illustrated in FIG. 2, although more or less than this number of apertures may be provided. As illustrated in FIG. 3, the removal of the head of the pressure vessel and the upper internals can be affected while leaving the core barrel 21 and core 19 intact in the pressure vessel 3. The present invention enables inspection of the welds 57 and internal surface 59 of the pressure vessel wall 5 while the reactor is in such a partially dismantled condition. When the head of the pressure vessel and upper internals have been removed, the vessel is normally flooded with water to protect against radioactivity, and the present method can be carried out under such flooded conditions. In the present method, access to the annular chamber is provided by either removing a plug from an existing aperture through the flange of the core barrel or by forming an additional aperture, which could be about 3.5 inches (8.89 cm) in diameter, through said flange, which additional aperture is subsequently provided with a plug, which may also be removable. A means for inspecting a weld or the inner surface of the pressure vessel wall is inserted through the access into the annular chamber and positioned proximate the area to be inspected. The means for inspecting, which is inserted through the access, may be an ultrasonic testing device, a visual examining means such as an optical scanning device, or other inspecting means, dependent upon the type of inspection desired. Ultrasonic testing, which is normally used, involves the injection of pulses of high frequency sound into the component to be tested. Any internal defects reflect sound back to the transmitting transducer, which then acts as a receiver. Such ultrasonic testing has a high sensitivity for cracks and other planar defects and can measure both length and height of a defect. Such testing is conventionally used on various components. The method is schematically illustrated in FIG. 4, wherein inspection of a weld 71 in the pressure vessel wall 5 is effected. As illustrated, the head 9 and upper internals 45 have been removed from the reactor but the lower internals 41 remain in position. After removal of any plug in the aperture 63 illustrated, an inspecting means 69, on the end of a positioning tool 67, such as a sonic tester, is inserted through the aperture 63 into the annular chamber 47, and is positioned proximate the inner surface of the weld 71. The actual positioning of the inspecting means relative to the area to be inspected will vary dependent upon the particular inspecting means used. The inspecting means is positioned at a location that is proximate the area, i.e. at a location that is sufficient to enable the desired examination or testing of the area by the inspecting means. The sonic tester, or sensor, 69 is then activated to inspect the weld 71, with such inspection being effected while the lower internals 41, including the core barrel 21, are still positioned within the pressure vessel 3. After the inspection, the inspecting means is retrieved through the aperture and the aperture plugged. The inservice inspections, of predetermined selected areas of the inner vessel surface and welds, according to the present method, may be made during normal plant outages such as refueling shutdowns or maintenance shutdowns occurring during a scheduled interval without the need for removing the lower internals from the pressure vessel. Since removal of the lower reactor internals is not required, labor savings are achieved and a reduction of man REM exposure is also achieved, as compared with previous processes that require the removal of the lower reactor internals.