Patent Number: 048184764
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail, FIGS. 1 and 2 show one design of a typical nuclear reactor pressure vessel 10 having a removable closure head 13, the closure head 13 being sealingly engaged with the pressure vessel 10 by means of a plurality of stud bolts 16 (see FIG. 3). Each bolt 16 has a lower end 19 which is threadingly received in a portion of the reactor vessel flange 22. An upper end 25 of the bolt 16, which also has a threaded section, passes through a corresponding aperture in the vessel head 13 and projects thereabove. The upper end 25 is threadingly engaged with a nut 28 and washer 31 to compress vessel head closure seals 34 disposed between the vessel head 13 and the flange 22 to thereby sealingly engage the closure head 13 with the pressure vessel 10. In this configuration, the upper most end of the top threaded section 25 remains exposed to the environment. To prevent damage to the top threaded section 25 or the nut 28, which would prevent the nut 28 from being easily removed from the stud bolt 16, a reactor vessel stud thread protector 40 (see FIG. 4) of the present invention is utilized. The stud thread protector or cap 40 is comprised of a tubular or generally cylindrical wall portion 43 disposed over a single stud bolt which is opened at its lower end 46 and substantially closed at its upper end 49, the upper end 49 having a hole therethrough to allow passage of a closure or hold down screw 52 which is threadingly engaged with an internally threaded section 55 of vertical bore 58 within the stud bolt 16. On the lower end 46 of the wall portion 43 is a seal 61 to prevent coolant, typically water, from passing under the protector 40 into contact with the stud 16, nut 28, or washer 31. Preferably, this seal 61 is constructed of nuclear grade neoprene and is in the form of a replaceable O-ring or gasket. To prevent damage to the stud bolt 16, the hold down screw 52, which may also have a seal associated therewith, is sufficiently torqued within the reactor vessel stud bolt 16 to sufficiently compress the seal 61. In the embodiment shown in FIG. 4, the reactor vessel stud thread protector 40 further includes a drip pan 64 associated therewith; the drip pan 64 being radially inwardly disposed with respect to the outer periphery of the vessel head 13 whereby the drip pan 64 collects any fluid which may be emitted from the reactor vessel 10. This drip pan 64, at one end 67 thereof, is form fitting with the top surface 70 of the vessel head 13 to further prevent any damage by borated fluid to the stud bolts 16. If necessary, coolant may be removed from around the bolts 16 by a typical water vacuum system prior to removal of the reactor vessel head 13. With the inclusion of the drip pan 64 any possible accidental corrosive leakage from mechanical seals of the control rod drive mechanism housings or instrumentation ports 73 on top of the reactor vessel 16 can be controlled. The drip pan 64 can be an integral part of the protector 40, or a separate member which is placed upon the protector prior to the hold down screw 52 being threadedly engaged therein. A further advantage of the reactor vessel stud thread protector 40, in addition to protecting the threaded section 25 of the stud bolt 16 from accidental corrosive leakage impingement, is that the reactor vessel stud thread protector provides protection from inadvertent deformation due to operator maintenance around adjacent studs. Preferably the stud thread protector 40 is made of a strong material, such as stainless steel, the walls 43 being from 1/16 to 1/8 inch thick. By providing a seal 61, a close fit between the protector 40 and the elements of the stud 16 and nut 28 is not required. Thus, no problem arises with the protector 40 becoming stuck on the stud bolt 16 and thereby causing delay in removal of the head of refueling operations. In a second embodiment shown in FIG. 5, the reactor vessel stud protector 40 is designed to cover several studs with one device in the shape of a curved box having a generally rectangular cross-section. For example, five such studs 16 could be covered by one protector 40. In this manner, it is necessary only to engage a holddown screw 52 with alternating stud bolts 16 (see FIG. 5). Since there is essentially no vibration of the reactor vessel 10, the minimal force required to seal the protector 40 against the vessel head 13 with hold down screws 52, i.e., to compress the seal 61 which in this embodiment is oval-shaped in conformity with the multiple stud protector, is therefore readily provided for by such a configuration. In this design drip pans 64 of adjacent protectors 40 have overlapping sections such that, as a whole, said drip pans 64 combine to form a continuous channel around the circumference of the vessel head 13. Alternatively, if the drip pan 64 is a separate member apart from the protector 40, it can be constructed of a continuous, one-piece ring-like element which is positioned over the stud protector 40 and the vessel head 13. This configuration lends itself to even greater speed in removing the reactor vessel head 13, in that a substantially less number of hold down screws 52 need be removed from the stud bolts 16, prior to removal of the nuts 28 and washers 31 to remove the vessel head 13. Once the hold down or closure screws 52 have been removed from every other stud 16, the stud thread protectors 40 can easily be removed by engaging a lifting eye (not shown) on the protector 40 to lift it out of engagement with the vessel head 13. Since the risk of damage to the exposed upper threaded section 25 of the stud bolt 16 by either the inadvertent corrosive leakage impingement or accidental deformation is virtually eliminated, removal of the nuts 28 is greatly facilitated. Hence, the vast number of stud bolts 16 can be more quickly and easily removed thereby reducing man-rem exposure to maintenance personnel within a potentially hazardous environment, as well the amount of time necessary for refueling operations. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alterations would be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting to the scope of the invention which is to be given the full breadth of the appended claims and in any and all equivalents thereof.