Patent Number: 043127045
Section: summary

The invention concerns a shut-off device for tubular conduits, particularly for the downwards-leading quick core emptying of nuclear reactors of the bulk type, especially the pebble bed reactor type, designed for spontaneous discharge of the fuel elements when the conduit is not shut off. In the operation of nuclear reactors, it is extremely important to assure the afterheat removal in the case of a failure of reactor cooling. Therefore, high-temperature reactors utilizing spherical fuel elements (like in pebble bed reactors) are provided with a quick core-discharge system to prevent excessive core heating in the case of a failure of all available apparatus for removal of heat developed in the core. Such systems lead the spherical fuel elements in such an emergency to emptying conduits that are distributed around the periphery of the core bottom, inclined downwards, and running generally radially, leading the fuel elements out of the core into a cooled reception cavity which may particularly be a ring chamber surrounded by water and located at a level beneath that of the core. These emptying conduits could even lead perpendicularly downward, but they are preferably inclined about 30.degree. to the horizontal and usually branch out from the conventional fuel element extracting conduits of the core. In such quick spontaneous core discharge systems, the mechanical shut-off devices that are located in the downward leading discharge conduits must be sure to function. Repeated testing of their function in every case under full load is, therefore, required without producing a substantial discharge or emptying of the reactor core and without resulting in a noticable modification of the original condition of the shut-off device in question. These requirements are not met from a practical standpoint by the known shut-off devices. In process engineering technology, cut-off devices such as diaphragms, insertion discs, flaps, and rupture-type safety discs operated by levers are known. These are either are not capable of, or accessible for, repeated testing (rupture discs) or are insufficient even in a combination of several similar devices, one behind the other, for effectively providing repeated functional testing without appreciable modification of the original condition as above mentioned. If the first shut-off device directly adjacent to the core is opened, a substantial volume of fuel elements drops into the space between the first and second shut-off device of a series of successive gate discs or blocking flaps, as the result of which a precise closing of the open first shut-off device is either completely prevented or is possible only at the price of a higher rate of breakage of the spherical fuel elements. On the otherhand, elimination of the quantities of fuel elements getting into the region between the shut-off devices in functional testing produces difficulties. It is an object of this invention to develop a new shut-off device to avoid the difficulties described, which will permit repeated functional testing, without resulting in an appreciable modification of the initial state and without compromising or making doubtful, by a modification of the system, the readiness condition established as a result of the functional test. SUMMARY OF THE INVENTION Briefly, two interfitting rod combs, each movable into the conduit cross-section, are individually actuatable and combine to form a load-bearing wall running transversely across the conduit and the gap between the individual rods of each set of comb rods (which rods may be constituted either as solid or as hollow rods of suitable cross-section) is smaller than the diameter of the fuel elements (balls). Preferably, the respective sets of rods of the two interfitting combs or grids combine to form a load-bearing wall with ball-fitting grooves on the upstream side of this gate wall, running in a direction aligned with the rods, with the bottom portion of each groove being formed by the upstream face of a rod of one of the combs and the side portions of the groove being completed in adjoining portions of the two rods adjacent on either side and belonging to the other of the combs. Preferably, a symmetrical forming of the ball-fitting grooves is provided by correspondingly formed longitudinal sides of the rods. Apart from the difference in the cross-sectional shape of the rods of the respective combs necessary to provide the grooves, the interfitting rods of the two combs could have basically similar configurations. A particularly favorable arrangement mechanically is obtained by approximately I-shaped cross-sections of the rods of the first comb which provide the bottoms of the ball-fitting grooves, while the complementary cross-sectional shapes of the rods of the second comb fitting in between are provided by hollow rods, as further described below with reference to FIG. 3. In this preferred configuration, the width of the rods of both combs, as particularly seen on the (downstream) side of the structure opposite the side with the ball grooves, is substantially equal. The ball grooves formed by the cooperation of the rods of the two combs of the shut-off device prevent a change of position of the balls, which are fuel element balls of uniform diameter, upon the operation of one or the other of the combs, particularly if care is taken upon the initial complete loading of the system, to pack the first three layers of balls adjoining the shut-off device as closely and as free of voids as possible. It can, accordingly, be useful to pack these first layers of balls by hand, but even if that is not done, the spontaneous ordered arrangement of the balls is favored by the grooves and, also, by a cooperating profiling of the rods of the second comb, so that the connecting surfaces between the surfaces forming the edges of the ball grooves is, preferably, roof-shaped. Furthermore, the ordered gapless ball filling in the region of the conduit adjacent to the shut-off device can be aided by a shaping of the joining conduit wall section on the "upstream" side of the combs, which shaping should favor the gapless filling of each of the grooves with fuel balls. By means of the present invention, it is basically possible to perform repeated functional tests of the shut-off device by successively opening and again closing the first and then the second of the combs. The direction of the rods has practically no special significance in this connection. If one proceeds, however on the basis that the system should remain unimpairedly capable of operation even after a serious accident that previously has been only simulated, without requiring any entry from outside, it is useful to have the generally I-shaped rods of the first comb arranged to be movable from above downwards and the complementary rods of the second comb from below upwards to reach their interfitting closed positions. In this manner the opening up of the comb rod guideways can be prevented so that the balls could fall into them when the shut-off device is opened. This is particularly so if the rod lengths and the closed-off end surfaces of the rods of the second comb are so constituted that when the second comb is moved downward into its open position, a flush closing of the guideways in the conduit walls is provided. Furthermore, in order to prevent the possibility that balls could be pressed upwards into corresponding holes, the second comb preferably includes a slotted cover plate of half-ring-shaped configuration so that it is complementary to the inner conduit wall, as is further show, for example, in FIG. 5. The approximately I-shaped rods of the first comb that is moved down from above into its closed position preferably have at their free ends extensions that engage, in the closed position, into corresponding cavities in the duct wall. In this manner, a supplementary supporting of the I-shaped rod is obtained which raises the load-bearing ability of the rods with respect to the column of balls lying against them. Upon opening this first comb, it is moved so far upward that these extensions no longer project into the duct cross-section. The above-described oppositely directed movements of each of the combs and the closing off of the passages can be readily constituted mechanically if the connecting webs or bridges of the combs are offset from the closure plane, which can be provided by having the rods of the two combs fastened by extensions projecting away at right angles to the closure plane over to the connecting webs or bridges. Hydraulic, pneumatic, or electrically operating systems can be provided that work by means of pistons, spindles, or other mechanical drives for the separate actuation of the individual rod combs. A spindle drive is particularly suitable in which the connecting bridges of the combs are guided, for example, by four roller bearings on corresponding rails that run in the direction of movement of the combs and are mounted laterally with respect to the tubular conduit, or else, in particular, are fastened to the inside of a pressure container that serves as a supporting construction. The entire shut-off equipment is enclosed in a double pressure container with pressure-tight spindle guides leading thereto. Within the emptying or discharge duct, three gas-blocking valves, particularly in the form of wedge valves are connected to the spindle guides.