Patent Number: 039792579
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a boiling-water reactor 1, a removable cover 2 closing the top of a pressure vessel 3 by means of a releasable flange connection 4. In the lower portion or lower third of the pressure vessel the reactor core 5 is positioned, this core comprising any of the prior art arrangements of individual fuel rod assemblies. Feed water is fed in through a feed-water inlet 6 and pumped outwardly through the reactor core 5 by a pump 7 having a motor 8 outside of the vessel. Above the reactor core 5 there is a steam collection dome 10 by which steam generated from the feed-water flow upwardly through the reactor core is collected, the steam discharging into water separators 12 which are structurally connected with or combined with a control rod guide arrangement 11 for a control element or assembly 13. An extension of the control rod guide 11 goes through the steam space 14 in the upper part of the vessel 3 where steam dryers 15 are additionally accommodated. The steam flows through the latter before leaving the pressure vessel via its steam outlet 16. The drive 17 for the control rod assembly 13 is arranged outside of the vessel and above its removable cover 2. This drive may be hydraulic or electric and in particular it may be an electro-magnetic mechanism. FIG. 1 shows that in spite of the pump motor 8 only little space is required underneath the reactor core 5 so that the reactor, as a whole, may have a low center of gravity when positioned in its usual concrete containment. This provides high stability and, therefore, greater safety against earthquakes occurring at the reactor installation. In FIGS. 2 and 3 two possibilities for guiding the control rods in the separator 12 are shown. In FIG. 2 a control rod assembly 13 with a cross-shaped profile is shown seated in the cylindrical separator 12. The cross is guided at the free ends of all of its arms by two parallel flanges 18, in each instance, so that it is accurately fixed insofar as its position in the reactor core is connected. In spite of this accurate guidance, which ensures the correct engagement of the control rod elements in the gaps normally formed in the fuel assemblies of the core 5, the external space required for the separator 12 is not increased at all, while the clear cross section of its interior is not affected appreciably by the control rod assembly. In FIG. 3 a cluster rod assembly is shown comprising four symmetrically distributed, so-called finger rods 21 associated with a central drive rod 20. The finger rods consist of neutron-absorbing material and are fastened at the drive rod end 20, at the upper finger rods ends via arms 22. The finger rods 21 are guided in tracks 23 which have cutouts fitted to the circular cross section of the finger rods 21 and are attached to the inside of the separator 12. Again only little space is, therefore, required inside of the separator 12 and no additional space on the outside is required at all. A side view on a larger scale is shown for the lastmentioned embodiment, in FIG. 4. The finger rods 21 run above the steam dome 10 in a standpipe 25 firmly connected with the steam dome 10. The standpipe 25 encloses the previously mentioned tracks 23 which extend down and end just above the reactor core 5 and are held together by a ring 26. The standpipe 25 also serves as an outlet for the steam, which is conducted into water separators 12 enclosing the standpipe 25. By means of baffles 27 provision is made here for a flow-wise favorable deflection, the separated water being returned downwardly into the core 5, while the steam leaves in an upward direction into the steam space 14. Above the separator 12 a steam dryer 15 is further provided which is also structurally combined or connected with a guide tube 30 for the control rod drive. The individual finger rods 21 can be associated with a single fuel assembly or with several adjoining fuel assemblies. Their length, which is provided with absorbing material, is advantageously shorter than the length of the tracks 23, so that the effect of the neutron-absorbing material, which can also be capable of burnoff, can be utilized locally targeted. In FIG. 5 the guidance of the control rod 13 in the interior of the core 5 is indicated, this figure showing on the left side the guidance for a finger control rod, and on the right, for a so-called cross rod as shown by FIG. 2. It can be seen that in the fuel assembly 32 one guide tube 33 is provided for each of the fingers 21. The guide tube 33 consists preferably of Zry 4 or a zirconium-niobium alloy; it extends almost to the lower core plate 35 and is part of the fuel assembly support structure. For this reason, it may be possible in some cases to dispense with the otherwise customary fuel element case. A shock absorber 34 is shown there. The control guide tube 33 progressively reduces in diameter as at 37 and 38 and is filled with the water. If the control rod is dropped, it successively displaces decreasing amounts of water providing a progressively increasing resistance to its fall, providing a shock-absorbing action.