Patent Number: 046719202
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to ice condenser containments, also known as ice baskets, employed for condensing steam generated from the primary water of a nuclear reactor in the event of an accidential loss of coolant and, more particularly, to a maintenance screw drill/conveyor tool and method for maintenance of ice beds of ice condenser containments, for filling sublimated ice voids within each of the plurality of ice baskets, without having first to remove any of the existing charges of ice within the baskets. 2. State of the Relevant Art Ice condenser containments, also known as ice baskets, are employed with nuclear reactors for condensing the steam from the primary water of the reactor in the event of an accidental loss of coolant. In a typical installation, there are provided approximately 2,000 ice baskets, each of which is approximately one foot in diameter and 48 feet in height and is filled with approximately 1,500 pounds of ice. The sidewalls of the ice basket, substantially cylindrical in configuration, are perforated to permit rapid exposure of the steam to the ice and corresponding, rapid cooling. Each such ice basket incorporates, at spaced intervals therein, stiffening rings comprising structural elements which provide lateral rigidity and support, to accommodate not only the weight of ice contained therein but also seismic and other disturbances. Conventional ice baskets furthermore include elements known as cruciforms, comprising generally x-shaped metal straps which extend diametrically across the interior of the basket and are welded at the ends of the straps to the interior sidewalls of the basket, typically at or adjacent the stiffening rings. Typically, the cruciforms are formed of 1/8 inch thick metal, and are 11/2 inches in height. Further, typically, seven (7) such cruciforms are assembled within the basket, spaced at approximately six (6) foot vertical intervals. While not structural members of the baskets and thus not necessary to add radial stiffness to the ice basket cross-section, the cruciforms are necessary elements for satisfying various requirements of the ice bed within the basket. For example, cruciforms are necessary to assure that the geometry of the ice bed within the basket is contained during the entire meltout period of a postulated loss of coolant accident. Specifically, as the ice melts upwardly and away from the bottom of the ice baskets, ice contained therein at higher elevations cannot be permitted to fall down into the bottoms of the baskets, since this presents the potential of creating bypass flow routes for the emerging steam which would accompany the loss of coolant. The cruciforms also serve to prevent large masses of ice from falling during seismic disturbances; falling masses of ice could impose unacceptable impact loads on the ice basket and related, underlying supporting structure. The presence of the cruciforms, however, compounds a maintenance problem which exists in the operation of nuclear power systems employing such ice condenser containments. Particularly, due to sublimation of the ice, the initial volume and weight thereof decreases with time and thus the ice must be replenished, or replaced. Typically, a minimum of at least 1,200 pounds of ice is required for each ice basket, for assuring its effectiveness for counteracting the accidental loss of coolant, as above referenced. The cruciforms impose a substantial obstacle to replenishing the supply of ice, as the latter is depleted due to sublimation. For example, the successively lower compartments of the basket, as delineated by the cruciforms, may contain inadequate charges of ice due to sublimation loss, which charges remain spaced apart due to the presence of cruciforms. Thus, while access may be had to the upper, open end of the ice basket for recharging the first and possibly the second upper compartments, the lower compartments are inaccessible, yet may contain an inadequate charge of ice due to sublimation voids. The ice basket thus loses its required cooling capability, presenting a potentially serious deficiency. Known methods and related apparatus for replenishing sublimated ice voids in the ice baskets all effectively require the complete removal of all remaining ice and the cruciforms, as well, before the compartments may be successively reloaded with new charges of ice and replacement cruciforms installed, to the required capacity of the entire basket. There thus exists a significant requirement, for which no solution has appeared heretofore, of satisfying this critical maintenance operation without having to remove remaining ice or existing cruciforms. SUMMARY OF THE INVENTION The present invention affords a maintenance screw drill/conveyor tool, and related method of operation, for maintaining ice beds of ice condenser containments, in accordance with which an axially extending hole is drilled longitudinally down through the ice condenser containment, or basket, typically to a depth of from 42 feet to 43 feet from the open, upper end of the basket (i.e., for a typical basket of a height of 48 feet). The tool is then selectively operated to feed ice downwardly through the drilled hole and into the voids, while gradually being retracted, or withdrawn, from the hole as the voids are filled, from the lowermost to the uppermost compartment of the basket. In more detail, each ice basket initially is weighed to determine the net weight, or content of the borated ice therein and thus establish whether the ice content thereof must be replenished. Suitable equipment for accomplishing that weighing function is disclosed in the copending application, entitled "COMPACT ICE BASKET WEIGHING TOOL," of which the coinventors herein are among the coinventors thereof and which application is assigned to the common assignee hereof. Once an ice basket is identified as requiring replenishment of the ice, it is isolated and enclosed by installing and inflating elongated plastic inflatible bags about the exterior surface of, and between, the selected ice basket and those of adjacent rows and columns, to prevent spillage of the ice out of the holes in the sidewalls of the ice basket. This isolation technique is a part of the ice loading procedure as typically is employed both in initial charging of ice and subsequent replenishment of ice, in accordance with known procedures. The screw drill/conveyor tool of the present invention includes a support frame which is mounted on the lattice frame surrounding a selected ice basket and locked in place thereon. A drill is mounted for selectively controlled, vertically reciprocating movement relative to a support frame by an electrically driven winch which selectively raises and lowers the drill in a vertical direction. An auger shaft of plural sections interconnects the drill and a rotary bit. Initially, the winch raises the drill to its uppermost position and a first auger shaft section is connected to the drill at its upper end and to a rotary bit at its other end. The drill is turned on and the auger and bit rotated thereby and the winch turned on to permit the drill to progress downwardly in a parallel axial relationship within the ice basket, until approximately one foot of the upper end of the shaft section remains above the top of the ice basket. A clamping device pivotally mounted on the support frame then is rotated into position to capture the protruding end of the drill shaft section to prevent it from dropping into the ice basket. The drill then is disconnected and raised by the winch to the uppermost position of its travel in the support frame. A subsequent drill shaft section then is coupled to the first, and connected to the drill. The operation then is repeated until the coupled drill shaft sections have advanced the bit to the aforementioned desired depth, typically some 42 feet to 43 feet below the top of the basket. At the conclusion of the drilling operation, a final shaft section is coupled in place, so as to dispose the electrically driven drill at the uppermost travel position relative to the platform and as controlled by the winch. The clamping device then is disengaged to release the drill shaft section, and a funnel pivotally mounted on the frame is rotated into position adjacent the protruding drill shaft section. The winch motor is maintained in an off condition, thereby locking the drill and the coupled shaft sections at the established elevation. The drill then is switched to rotate in a reverse direction and granular ice chips or flakes are fed into the funnel. By virtue of a continuous, helical spiral fin on the auger shaft sections, the reverse rotation thereof pulls the loose ice downwardly through the drilled hole and fills the lowermost, and the successive, next higher voids. As the voids become full, the ice compacts about the rotating drill shaft section, as evidenced by a tendency of the rotational speed of the drill to decrease and of the auger to attempt to back out of the ice basket. When these conditions are observed, further ice conveying operations are halted and one or more drill shaft sections are removed, by a reverse sequence of the drilling operation, and then the ice feeding, or conveying, operation is repeated. These operations continue until the successive compartments of the ice basket are filled. Upon completion of filling a given ice basket, the plastic blowout bags are deflated and removed and the ice basket is lifted and weighed to verify the new weight and confirm that it satisfies the requisite content of ice. If not, the foregoing procedures are repeated for that ice basket. If the measured weight confirms the adequacy of the ice replenishing operations, the operations are performed for a next successive ice basket. Accordingly, the screw drill/conveyor tool and maintenance method in accordance with the present invention overcomes significant limitations of prior art mechanisms and procedures, greatly facilitating the important maintenance function of replenishing sublimation voids in ice baskets employed with the nuclear reactor systems. These and other advantages of the apparatus and method of the present invention will be more apparent from the following drawings and detailed description.