Patent Number: 041486854
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS According to the present invention the total or long term shutdown of the nuclear reactor will be effected by a first shutdown system, which comprises absorber rods, insertable from the top into the fuel element pile, and the partial or rapid shutdown of the reactor will be effected by a second shutdown system, independent on the first, which comprises absorber rods which are movable in the top reflector and in the space formed between the top reflector and the fuel element pile. The fine adjustment and rapid load adjustment of the nuclear reactor is effected by an adjustment system, which consists of absorber rods which are inserted and movable in the side reflector. Thus, in the process according to the invention, there are used two shutdown systems. One is represented by the shutdown device known from the prior art having absorber rods which are freely insertable into the fuel element pile. The second shutdown system comprises absorber rods, which are movable only above the fuel element pile. Both systems are independent from each other and are equipped with different operating mechanisms. Furthermore, there is arranged in the side reflector a ring of absorber rods, which accepts adjustment tasks. These absorber rods are--apart from an exception described later--always inserted in the side reflector and thus, up to a depth of 70%. They decrease the neutron flux in the side reflector and thereby protect the graphite from receiving too high of a neutron dose. The upper final position of the absorber rods of the first shutdown system lies approximately 50 cm above the lower edge of the top reflector. For the total or long term shutdown, these absorber rods extend deeply into the combustion element pile. They are released either automatically or manually. Due to the large amount of force to be applied during the insertion and withdrawal of the absorber rods into or out of the reactor core, it is most favorable to use a pneumatic drive for these rods. The partial or rapid shutdown system is arranged in the top reflector and is equipped with electric drives. As a result of the axial power distribution prevailing in nuclear reactors characterized by a single pass of the combustion elements, namely, those having a maximum in the upper third of the core, more than 4% .DELTA.K/K efficiency can be reached by movement of the absorber rods of that shutdown system (.DELTA.K/K is the deviation in percent from the multiplication factor K, measured as a percentage of this factor K). The efficiency of the second shutdown system is dependent on the surface area of each absorber rod, on the number of rods, on the rod pattern and on the height of the free space above the fuel element pile. In the partial or rapid shutdown, it must be possible to compensate for at least the reactivity existing due to a malfunction. In the case of rod accidents and water entry a malfunction excess reactivity of maximum approx. 1.5% .DELTA.K/K arises. Therefore, altogether a total efficiency of 2.5% .DELTA.K/K of the second shutdown system is sufficient for the partial or rapid shutdown. The devices for the after-heat removal are designed accordingly, so that the nuclear reactor will remain subcritical for more than an hour, also under unfavorable circumstances (e.g., no Xenon-135 formation to support the shutdown). The efficiency of the second shutdown system increases with larger nuclear reactors, since nuclear reactors with spherical fuel elements are constructed with constant core depth (approx. 5-6 m), due to the high load of the rods inserted directly in the core, and therefore an increase in power will be realized by using a larger radius. In these reactors, the proportion of the neutrons which escape laterally above the fuel element pile into the side reflector which is equipped with absorber rods becomes increasingly smaller as the radius becomes larger. As already mentioned, the absorber rods of the adjustment system are nearly always inserted in the side reflector. In case of a full load, they are always in their normal position, inserted approximately 1/2-3/4 of the depth of the openings in the side reflector. They thereby bind to the Xenon-135 compensation the excess reactivity necessary for the rapid load change from full load to partial load and decrease the neutron flux in the side reflector, and thus, protect it from too high a dose. During the rapid loading change from full load to partial load they are moved out only temporarily; however, since this is effected with a decreased load in such cases of operation the reflector is likewise not exposed to a higher neutron radiation, because the neutron flux decreases proportionally with the power. Also in large nuclear reactors up to 1500 MW, it is still possible to attain reflector efficiencies of approximately 2-2.5% .DELTA.K/K, so that the absorber rods of the adjustment system can still take over the fine adjustment. Since the reflector rods in their normal position are inserted at most up to 3/4 of their length in the side reflector, it is possible for fuel elements, which are exposed at first to small neutron flux near the side reflector, to afterburn in the lower part of the reactor core, whereby a constant radial outlet profile of the gas temperature will be maintained. If the efficiency of the absorber rods of the adjustment system is not sufficient, it is possible for additional selected absorber rods of the second shutdown system to take over adjustment tasks, e.g., in the rapid loading adjustment. Since the second shutdown system can supply enough efficiency in case of appropriate design, some absorber rods of that system can be reserved for the adjustment task. Advantageously, these reserved rods of the second shutdown system are applied for the rapid loading increase from partial load to full load, whereby a selected ring of absorber rods is used, which ring is advantageously situated at half of the radius of the reflector core. For the rapid loading decrease from full load to partial load, the absorber rods of the adjustment system are combined advantageously in groups and are always operated in such a way that all absorber rods have nearly the same insertion depth. This is effected in such a manner that a first group of absorber rods is at first moved upwardly approximately 10 cm, afterwards a second group so on until all groups have been moved upwardly by this distance. Then, it will start over again with the first group, until all groups have moved the same distance in the same direction. In this manner a quasi-rod-bank-movement of all absorber rods of the adjustment system can be achieved. The drives of the absorber rods are preferably designed as electric drives. For a load decrease from 100%-40%, an efficiency of 1.8% .DELTA.K/K is necessary. As already mentioned, an efficiency of 2-2.5% can be achieved with the absorber rods of the adjustment system; thus, there is still enough efficiency remaining for the fine adjustment. If more efficiency exists, the rods of the adjustment system can also be applied for the rapid loading increase. For the excess reactivity of approximately 3.5% .DELTA.K/K from Pa-U-233 conversion, which arises after a long term shutdown and which can be reduced again only over a long period of time (during months), a ring of absorber rods of the first shutdown system is inserted into the reactor core. As already described, the absorber rods of the second shutdown system are equipped with electrical drives. In this regard, several absorber rods (up to three rods are suitably coupled in each case by a chain or a rope to a drive. The absorber rods are made advantageously of boron steel and have a Y-shaped cross-section. Other cross-section configurations are also usable; but a rod with a Y-shaped cross-section has the advantage of a large surface with a small cross-sectional area, so that a sufficient efficiency of the top reflector will be kept, if the absorber rods are in their upper final position (approx. 50 cm above the upper reflector edge). It is advantageous to provide the absorber rods of the second shutdown system at their upper end with a projection which is coupled with a shock absorber. The projection is formed in such a way that the absorber rods will be caught in their lower end position, in case of a rope- or chain failure, and thus, they cannot sink into the fuel element pile. Turning now to the drawings, FIG. 1 shows a gas cooled nuclear reactor, comprising essentially a prestressed concrete vessel 1 which encloses a reactor core 2. The core 2 consists of a pile of spherical fuel elements 3, which are introduced into the core 2 by means of a feeding device (not shown in the drawing) and which leave the core through a discharge duct 5. The nuclear reactor is operated in the so-called single pass process, i.e., only fresh fuel elements are introduced into the core 2, and they are completely consumed after one pass through the core, whereupon they are removed from the core. The core 2 itself is surrounded by a reflector which is formed by a top reflector 6, a cylindrical side reflector 7 and a bottom reflector 8. The pile of fuel elements 3 is traversed from the top to the bottom by coolant gas, which is marked by arrows (the introduction and the removal of the coolant gas is not further shown). The shutdown of the nuclear reactor is effected by means of two shutdown systems. The first shutdown system, used for the total- or long-term shutdown, comprises a number of absorber rods 9 which are inserted directly into the fuel element pile 3. The second shutdown system, provided for the partial or rapid shutdown, consists of absorber rods 10 which are arranged in the top reflector 6 and can only be moved within that reflector and within the space 11 above the fuel element pile 3. The absorber rods 9 are pneumatically operated, and the absorber rods 10 electrically operated. The drives of the both shutdown systems are independent of each other. The adjustment and control of the nuclear reactor is effected by means of an adjustment system, comprising a number of absorber rods 12 which are arranged in the wall of the side reflector 7. These rods are moved in special openings 13 in the side reflector 7. In their normal position they are inserted approximately 70%. They are also equipped with electrical drive means. The absorber rods 12 take over the fine adjustment task and the load reduction from full load to partial load. During the load increase from partial load to full load, they are assisted by some absorber rods 10, located in the top reflector 6, as will be described hereinbelow. In FIG. 2, the positions of the absorber rods 9 of the first shutdown system as well as those of the absorber rods 12 of the adjustment system are shown. The absorber rods 9 are distributed across the total cross-section of the core 2 according to the illustrated pattern. The absorber rods 12 are arranged on a circle in the side reflector 7, spaced equidistantly. In FIG. 3 the positions of the absorber rods 10 of the second shutdown system are shown. They are located between the absorber rods 9 and are distributed over the cross-section of the top reflector 6 according to a similar pattern. These absorber rods have a Y-shaped cross-section and move in openings 14 of corresponding shape in graphite blocks 15 of which top reflector 6 is formed. Openings 16 with round cross sections are provided for the absorber rods 9 in the top reflector 6. For example, considering a 1000-MWe-nuclear reactor with a power density of 10 W/cm.sup.3, the core has a radius of 3.63 m and a depth of 6.0 m. Seventy-two absorber rods 9 of the first shutdown system, having a diameter of 10 cm. are distributed across the core 2. Between them, there are seventy-two absorber rods 10 of the second shutdown system in the top reflector 6; these rods have an edge length of 10 cm and a slab thickness of 1 cm. The space 11 which is below the top reflector 6 and is free of fuel elements is 1.25 m high. In the side reflector 7 are provided forty-eight absorber rods 12 of the adjustment system. They are combined in groups of six absorber rods each and have a diameter of 10 cm. The absorber rods 12 of the adjustment system produce 3.5% .DELTA.K/K. The absorber rods 9 of the first shutdown system produce 24% .DELTA.K/K and the absorber rods 10 of the second shutdown system produce 5% .DELTA.K/K shutdown reactivity. For the rapid loading adjustment from full load to partial load (100%-40%) 1.8% .DELTA.K/K are necessary. To this, there is added a need for reactivity of 0.5% .DELTA.K/K for the fine adjustment and a further 0.5% .DELTA.K/K for compensation of fluctuations in the feeding process. Thus, there is available a shutdown reactivity of only 0.7% in the absorber rods 12 of the adjustment system. That reactivity is applied for the rapid loading adjustment from partial load to full load (40%-100%), for which there is altogether necessary approximately 1.3% .DELTA.K/K. In order to be able to compensate the remaining 0.6% .DELTA.K/K, there is reserved a ring of six absorber rods 10 of the second shutdown system. Since a total efficiency of the second shutdown system of 2.5% .DELTA.K/K will be sufficient for the partial shutdown, a reservation of absorber rods 10 of this shutdown system for the rapid loading increase is possible.