AUTOMATICALLY CLOSING SWING GATE CLOSURE ASSEMBLY

Abstract: A swing gate closure assembly for nuclear reactor wherein the swing gate (120) is cammed open by a fuel element or spacer but is reliably closed at a desired closing rate primarily by hydraulic forces in the absence of a fuel charge.

Description: The present invention relates to a swing gate closure assembly and more particularly to a swing gate closure assembly for a nuclear reactor which uses hydraulic forces coupled with the weight of the swing gate to provide a foolproof closure mechanism. When nuclear fuel is depleted, it must, of course, be removed from the reactor core so that it may be replaced with fresh fuel. One current method for effecting removal involves introducing water under pressure into the tube holding the fuel charge. This causes the fuel charge to move through the tube to the end of the tube, where it is expelled. A fuel charge generally compresses about sixteen fuel elements arranged in the tube by spacers at either end. When a charge/discharge operation is not in effect, the reactor tubes are typically sealed by an arrangement such as that shown in Figure 1. In Figure 1, the nozzle end of the process tube is indicated by numeral 10. The arrow in outline indicates a direction of flow. Nozzle cap 40 is threaded onto the nozzle 10. The nozzle 10 is also sealed by a seal plate 20 and graphoil seal 30. The seal plate 20 is loaded by three loading bolts 50, two of which are visible in Figure 1. When it is desired to effect fuel discharge, it is necessary for an operator to remove nozzle cap 40 and seal plate 20. This clears the way for the fuel charge to leave the process tube nozzle 10. A problem which exists, however, is that after the last spacer of the fuel charge has cleared the tube, water which has just traversed the radioactive core gushes out and sprays the operator. This imposes an undesirably high risk of radioactive contamination. It is therefore the object of the present invention to provide a mechanism by which the nozzle can automatically close after the final fuel spacer has cleared the tube and which functions extremely reliably given the high safety standards required for nuclear reactor applications. With this object in view the present invention resides in a swing gate closure assembly for a nuclear reactor, comprising a closure assembly body having an inlet, an outlet, and a passage extending between said inlet and outlet, said inlet, outlet and passage being dimensioned to permit passage of a fuel charge there-through, and a swing gate in said body and movable between a first position in which said swing gate is removed from said passage and a second position in which said swing gate is disposed in, and seals, said passage, said swing gate being movable to said first position by said fuel charge moving through said passage, characterized in that said body includes operating means in fluid communication with said inlet for hydraulically moving said swing gate to said second position in the absence of said fuel charge in said closure assembly. The swing gate closure assembly of the present invention does not obstruct egress of fuel elements or spacers, and is capable of ready, rapid, and reliable closure in the absence of a fuel element or spacer. Closure is not effected through the provisions of mechanical closing means, but instead is effected by forces generated by the fluid itself. These forces, coupled with the weight and movements of the swing gate, provide a virtually foolproof closing mechanism. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only in the accompanying drawings, in which: Figure 1 is a cross-sectional view of a prior art nozzle discharge cap arrangement, described above; Figure 2 is a perspective, particularly cut away view of an automatically closing swing gate closure assembly according to one embodiment of the present invention; Figure 3 is a cross-sectional view of the closure assembly shown in Figure 2; Figure 4 is a view of the closure assembly shown in Figure 3, the right side being a view taken along line BB of Figure 3, and the left side being a view taken along line CC of Figure 3; Figure 5 is a view taken along staggered line AA in Figure 4; Figure 6 is a view taken along line HH of Figure 4; Figure 7 is a view showing engagement of a locking plunger with a swing gate as will be described; Figure 8 is a view showing a typical application of the closure assembly shown in Figure 2; Figure 9 is a schematic diagram of a nuclear reactor incorporating the closure assembly according to the present invention; and Figure 10 is a diagram showing flow patterns in a closure assembly according to the present invention. Nozzle cap 40 and seal loading bolts 50 as shown in Figure 2, are essentially the same as those shown in Figure 1. Interposed between the nozzle cap 40 and nozzle 10, however, is the housing 60 of an automatically closing swing gate closure assembly according to the present invention. This closure assembly includes an inlet 70, an outlet 80, and a through bore 90 which provides a path of fluid communication between the inlet 70 and outlet 80. The housing 60 also defines an upper chamber 100 disposed above the through bore 90. This upper chamber 100 is in fluid communication with inlet 70 through orifice 110. Orifice 110 is located in swing gate 120. Swing gate 120 includes an upper planar surface 130 which defines a bottom surface for upper chamber 100. It also includes an arcuate surface 140 facing outlet 80 and a concave surface 150 facing the inlet 70. In the embodiment shown in Figure 2, the concave surface 150 is partially convolute to form an upward projection 155. The convolution is about an axis parallel to the pivot axis of the swing gate 120. As will be explained more fully below, the flow in the area under swing gate 120 while it is closing is largely vortexing. The concave surface 150 establishes boundary conditions tending to support such flow. The swing gate is arranged to pivot about a pivot 160 arranged on a horizontal axis above through bore 90, adjacent inlet 70, and perpendicular to a central axis of through bore 90. As will be described in more detail below, the center of curvature of arcuate surface 140 is displaced from pivot 160. The inside of the closure assembly housing includes a seat 170 which receives the surface 140 of swing gate 120. Seat 170 may be in the form of an insert, as shown. The sealing surfaces of the closure assembly's gate and seat 170 are preferably precision fit with metal-to-metal arc contact. Sleeve 180 is upstream from seat 170. The arrangement shown in Figure 2 also includes a locking plug or plunger 175 which is received in an aperture in swing gate 120 and locks it in the position in Figure 2. It also includes a nozzle seal 190, a collar 200, an access cover 210, an omega seal 220, and attachment bolts 230. Shown in Figure 3 is a fuel charge 240 passing out of nozzle 10 and just encountering swing gate 120. Swing gate 120, however, cannot be moved into the position shown in phantom due to the engagement of locking plunger 175 therewith. Figure 4 is an end-on view of the arrangement of Figure 3. The right hand side of Figure 4 is along crooked line BB of Figure 3, i.e., the line passing through the access cover 210 and upper chamber 100, veering right through the pivot 160, and then veering back left to pass through seat 170. The left hand side of Figure 4 is a view taken along line CC in Figure 3. Considered as a whole, Figure 4 thus shows the upper access cover 210, omega seals 220 and the placement of attachment bolts 230. Figure 4 also shows the relative vertical positions and dimensions of upper chamber 100 and pivot 160. The left hand side shows web 250 of swing gate 120. Web 250 imparts structural rigidity to swing gate 120 and is also the element which engages a fuel spacer entering through bore 90 and transmits the force from the fuel charge which moves or "cams" swing gate 120 out of through bore 90. This function requires that web 250 have a continuous edge running from the top right hand part to the bottom left hand part through bore 90 under swing gate 120. In the presently preferred embodiment, web 250 has a simple linear edge. It will be apparent, however, that other shapes could be used to obtain satisfactory results. Figure 5 is in essence a partially broken away plan view of the embodiment of Figure 3. The relative lengthwise positioning of elements is visible in figure 5. It can be seen in Figure 5 that web 250 is centrally disposed length-wise on swing gate 120. After the locking plunger has been disengaged from the swing gate, the swing gate is free to move to its upper position in the upper chamber, as shown in Figure 6. Also as shown in Figure 6, the swing gate center of curvature 260 is preferably displaced upward from the axis of horizontal pivot 160. This is advantageous in that the motion of the gate does not become obstructed by particulate contaminants, and in that the gate, when in the lower position, forms a superior seal with minimal scrubbing. Figure 7 is included to show the position of the locking plunger just before it enters swing gate 120, or just after it has left it. As can be seen, seal loading bolts 50 in the swing gate closure assembly according to the present invention are used to load the locking plunger 175 and keep it securely engaged with the swing gate 120. Figure 8 shows the swing gate closure assembly according to the present invention as part of an overall tip off assembly, including top off chute 270. Fuel elements and spacers comprising a fuel charge discharged from nozzle 10 pass through the swing gate closure assembly, through the chute, and into a proper receptacle. Figure 9 is a schematic diagram included for the purposes of illustrating the application and positioning of a swing gate closure assembly according to the present invention in a nuclear reactor. The nuclear reactor shown in Figure 9 includes a process area 280. Inside process area 280 is a process tube 290. Water is introduced into one end of the process tube, and flows through the other, to set up a flow within the tube. This flow tends to drive a fuel charge contained in the process tube 290 out through either one of the closure assembly bodies 60. As mentioned above, the primary function of the closure assembly is to provide a pressure boundary seal during reactor operation, and to prevent the spray of radioactive water during the fuel charging process. During the low pressure reactor refuelling mode, nozzle cap 40 and locking plunger 175 may be manually removed from the closure assembly. The swing gate 120 preferably remains tightly closed until cammed open by the chain action of a new fuel charge. After the last fuel spacer of the fuel charge has exited the tube, the gate automatically closes against a pressure head of up to 2,1 x 10⁵ Pa (30 psig). This automatic closing feature permits maintenance operations to be performed without subjecting personnel to radioactive water spray. As mentioned above, the pivot point for the gate's rotation is located upstream from the seating surface and offset from the common arc construction centers. This geometry allows the gate 120 to separate from its seat 170 during opening and to operate without jamming in the presence of foreign particle build up, and to engage the seat 170 during closing with a minimum amount of scrubbing. The offset also provides a hydraulic torque on the gate which is necessary for final closing as well as for maintaining a static force and thus sealing. Figure 10 shows dynamic flow patterns in a swing gate closure assembly according to the present invention. A unique feature is that gate 120 can swing closed against a flow stream. In essence, the swing gate and upper chamber together are analogous in operation to the operation of a piston in a cylinder. The driving forces that initiate and sustain automatic closing are derived from a combination of the gate's weight and hydraulic forces that act on the gate. The hydraulic forces are developed by a vortex flow pattern underneath the swing gate 120 and fed by the flow stream. This vortex tends to expel fluid into upper chamber 100, thus developing pressure therein which pushes downward against upper planar surface 130. These forces tend to accelerate the gate at a continuous rate until the gate is almost closed. The closing motion of the gate increases the volume of the upper chamber above the gate and tends to create a negative pressure therein. The pressure drop across orifice 110 is controlled by selecting the size of orifice 110 to provide the damping which is required to slow the gate to a desired closing speed. An excessively fast closure of the gate would not only damage closure assembly components, but would also induce a tremendous water hammer pressure surge that would potentially damage upstream piping components. Thus, closure is effected rapidly and reliably through the controlled interaction of at least three effects. First, the offset of the center of hydraulic pressure at 260 from the pivot center 160 creates a closing force. Second, the pressure drop across the top surface of swing piston 130, as regulated by orifice 110, results in a force which urges the gate closed. Finally, the weight of gate 120 also urges the gate into the closed position, assuming, as will normally be the case, that the closure assembly is oriented as shown in the drawings. The upward projection 155 on the center of the web of concave surface of swing gate 120 is included to engage a maintenance tool used to close gate 120 if it should ever stick open. The present invention was generated under Government Contract DE-AC06-76FF02170.

Claims: 1. A swing gate closure assembly for a nuclear reactor, comprising a closure assembly body (60) having an inlet (70), an outlet (80), and a passage (90) extending between said inlet (70) and outlet (80), said inlet (70), outlet (80) and passage (90) being dimensioned to permit passage of a fuel charge therethrough, and a swing gate (120) in said body (60) and movable between a first position in which said swing gate (120) is removed from said passage (90) and a second position in which said swing gate (120) is disposed in, and seals said passage (90), said swing gate (120) being movable to said first position by said fuel charge moving through said passage (90), characterized in that said body (60) includes operating means (100) in fluid communication (110) with said inlet (70) for hydraulically moving said swing gate (120) to said second position in the absence of said fuel charge in said closure assembly (60). 2. A swing gate closure assembly as claimed in claim 1, characterized in that said operation means comprises a chamber (100) formed within said body (60) and having a greater volume when said swing gate (120) is in said second position than when said swing gate (120) is in said first position. 3. A swing gate closure assembly as claimed in claim 1 or 2, characterized in that said swing gate (120) has an arcuate surface (140) facing said outlet ( 80 ) when said swing gate (120) is, in said second position, said arcuate surface (140) having an axis of curvature parallel to and displaced from the pivot axis of said swing gate (120). 4. A swing grate closure assembly according to claim 1, 2 or 3, characterized in that said swing gate (120) has an upper substantially planar surface (130) defining a bottom surface of said chamber (100), an arcuate surface (140) facing said outlet (80) and having a first center of curvature, and a substantially concave surface facing said inlet (70), and that orifice means (110) are provided in one of said swing gate (120) and body (60) for providing fluid communication between said upper chamber (100) and said inlet (10), and a closure assembly seat is formed in said closure assembly body (60) adapted to form a seal with said swing gate (120). 5. A swing gate closure assembly as claimed in any of claims 1 to 4, characterized in that a plunger (175) is received in said outlet (80) for locking said swing gate (120) in said second position.

Inventors: CHANG, SHIH-CHIH, GILMORE, RICHARD FRANCIS, SCHUCK, WILLIAM JOHN
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1987
Main classification: G21C19
IPC codes: G21C19