Patent Number: 044366922
Section: summary

TECHNICAL FIELD This invention relates to the nuclear reactor field. More particularly, it pertains to method and apparatus for plugging a relatively large underwater nozzle in the sidewall of a reactor pressure vessel. BACKGROUND ART A boiling water reactor vessel is constructed in such a manner that it has a relatively large underwater nozzle extending through its sidewall which is used to recirculate water in the core shroud. This nozzle is relatively large--conventionally, for example, 28" in diameter. Quite often the piping and valving associated with the line leaving this nozzle require servicing, or even replacement. When such service is required, it would be desirable to be able to perform it without having to drain the reactor pressure vessel. In this manner, the necessity for removing the reactor fuel to a storage pool could be avoided. A smaller sized nozzle can often be plugged by means of a "freeze seal". This is a technique wherein liquid nitrogen is circulated around the nozzle to freeze the water within to effectuate a seal. However, the usual recirculating nozzle is too large to employ such a technique. In the past, it has been possible to plug the recirculation nozzle of a boiling water reactor vessel by lowering into the vessel a disk shaped plug which is curved to fit against the vessel sidewall and includes a relatively deep, conical extension which serves to position the plug in the nozzle opening. A hydraulic cylinder on the rear of the plug pushes against the core shroud to seat the plug in the nozzle opening. A circular pneumatic sealing member on the edge of the plug disk is then inflated to seal off the plug. The foregoing technique has been successfully applied in older-type reactor vessels. However, the plug which is used in that technique is relatively thick, approximately 61/2 inches. Nevertheless, it could be lowered straight down in the annular space between the core shroud and the reactor wall. In the newer reactors, however, there have been added a number of elements which form obstructions, thereby preventing the relatively thick prior art plugs from being employed. The primary obstructions are jet pumps which are positioned in the space between the reactor wall and the core shroud. Prior art plugs are too thick to pass between the jet pumps and the sidewall. Even if they could, they would be prevented from doing so because of intervening plumbing, such as core spray inlet pipes and feedwater spargers, and also by the reactor pressure vessel guide rod. Accordingly, it is a primary object of the present invention to provide improved method and apparatus for plugging the recirculation nozzle of a water-filled boiling water reactor pressure vessel. Other objects, features, and advantages will be apparent from the following description and appended claims. DISCLOSURE OF INVENTION There is disclosed herein a method of plugging an underwater nozzle in the sidewall of a cylindrical nuclear reactor pressure vessel which contains a core shroud radially displaced from the sidewall to form an annular space. The annular space contains jet pumps which are spaced from the sidewall. In accordance with the method, a substantially disk shaped plug is provided which has a diameter sufficient to cover the nozzle opening. The plug is curved to match the curve of the reactor sidewall. Means are provided on one surface of the plug for sealing against the sidewall around the nozzle opening. Cylinder/piston means are carried by the other side of the plug. The plug is lowered by means of a first cable into the annular space above the jet pumps. The cable is then pushed toward the sidewall until the plug is vertically aligned with the space between the sidewall and the jet pumps. The plug is lowered on the first cable until it enters the space between the sidewall and the jet pumps. The plug is then laterally shifted into vertical alignment with the nozzle to be plugged and the weight of the plug is simultaneously shifted to a second cable. The plug is then lowered by the second cable into alignment with the nozzle opening. The cylinder/piston means is then actuated to force the plug away from the core shroud and into sealing engagement over the nozzle opening.