Patent Number: 062051962
Section: description

DESCRIPTION OF EMBODIMENTS OF THE INVENTION Embodiments of the present invention will be explained hereunder, referring to the drawings. FIG. 1 shows an embodiment of the present invention. In this embodiment, in a boiling water type nuclear reactor in which a plurality of fuel assemblies 2 each surrounded by a channel box 1 are loaded, and a plurality of control rods having control rod blades each disposed between the channel boxes, long blade control rods 6 each having control rod blades extending latintudinal in four directions, respectively, are arranged between the channel boxes on a diagonal of each of square bundle regions each formed of a plurality of (four in this embodiment) the fuel assemblies 2, and short blade control rods 7 each are arranged between channel boxes of each of the square bundle regions at the center of the region, each of which short blade control rods 7 has a blade length (in a lateral or latitudinal direction) of about one half of the width of one of the square bundle regions, for example, substantially the same as the width of each of the above-mentioned fuel assemblies. With this construction, as mentioned above, in the long blade control rod 6 arranged between the channel boxes on the diagonal, the control rod worth per one rod increases and the number of control rods and the number of control rod driving devices can be reduced by the number corresponding to an increment of the control rod worth, so that the cost can be reduced. Quantitatively, the number of the control rods can be reduced by 25% as compared with the conventional lattice. FIG. 2 shows an arrangement of the control rods over the whole reactor core. Symbols .omicron. denote the latitudinal long blade control rods 6 and symbols .circle-solid. denote the latitudinal short blade control rods 7. Moreover, as clearly shown in both FIGS. 1 and 2, the long blade control rods each have a blade length in a latitudinal direction which is about twice as long the blade length in a latitudinal directional of the short blade control rods. It is found that the number of control rods and the number of the control rod driving devices can be reduced largely as compared with the conventional arrangement and the control rod system can be simplified. Further, as explained previously in the summary of the invention, the reactor shutdown margin can be secured easily and the number of control rods is reduced. As a result, Gd for securing a reactor shutdown margin does not remain and low inventory fuel is not loaded, whereby economy is improved greatly. Further, in this embodiment, by sharing the role of the control rods such that the long blade control rods on the diagonal serve for reactor shutdown and the short blade control rods at central portions are for controlling reactivity during operation and at time of scram, the system can be rationalized and simplified, and the cost of the whole plant can be reduced. Further, in the short blade control rods for controlling reactivity, by using a neutron absorber of material (B.sup.10) which has a high reactivity effect, the control rod worth of the short blade control rods increases, and scram characteristic and reactivity control characteristic can be increased. Further, since the long blade control rods on the diagonal are not used for scram, a control system of high speed scram, etc. can be omitted, which enables use of a hydraulic driving system of a low cost, whereby a cost is reduced largely. Further, in the above-mentioned embodiment, it is possible to share the role of the control rods such that the long blade control rods 6 on the diagonal are used for controlling reactivity during operation and for shutdown of the reactor and the central short blade control rods 7 are used for scram. In this case, the system is rationalized and simplified as mentioned above, so that reduction of the cost can be expected. Further, in the arrangement as shown in FIG. 1, another embodiment, in which the reactivity worth of a control rod is improved at an upper region thereof, is explained hereunder with respect to a neutron absorber used in a control rod. In this embodiment, in particular, enrichment of B.sup.10 in the short blade control rod arranged at the central portion of the square bundle is made relatively high at the upper region. In general, in a boiling water type nuclear reactor, since a void ratio is higher at an upper region of the reactor during operation, neutron spectrum is hardened, and production of Pu.sup.239 by neutron absorption is promoted. Therefore, the enrichment of fissionable materials becomes high at an upper portion of the reactor and the reactor shutdown margin in the region decreases relatively. In this embodiment, the enrichment of B.sup.10 in the upper region of the length of the control rod is increased for the upper region of the nuclear core in which a reactor shutdown margin decreases relatively, whereby the reactor shutdown margin can be increased, as shown in FIG. 4 which is a graph showing the ratio of the high B.sup.10 enriched region of the control rod to the reactor shutdown margin. Further, since an amount of used B.sup.10 can be reduced, a manufacturing cost can be reduced. Therefore, a cost of the whole plant can be reduced in total. FIGS. 5 and 6 show a conventional core in part and a core in part according to the present invention, each of which is adopted for fuel assemblies of fuel rod lattice structure of 9.times.9. FIG. 6 shows an embodiment of the present invention in which a large-sized fuel assembly is formed by 4 mini-bundles each of which has a bundle width of about 12 inches (30.5 cm) as used in BWR and ABWR at present. As in the previous embodiments, in this embodiment in FIG. 6, a cost of the plant can be reduced largely by reduction of the number of control rods. Further, by role sharing of the control rods, a control rod system can be simplified, and a cost the whole plant can be reduced largely. Although not illustrated, fuel rod lattice structures of 8.times.8 and 10.times.10 also can be applied. FIGS. 7A and 7B show another embodiment of the present invention. In this embodiment, fuel assemblies 2 constituting a square bundle region as shown in FIG. 1 each have nine (9) water rods 11 as shown in FIG. 7A. As shown in FIG. 7B, each water rod 11 has an ascending flow path 12 and a descending flow path 13, the ascending and descending flow paths 12, 13 are connected to an inflow hole 14 and an outflow hole 15, respectively, and the inflow hole 14 is positioned at a portion lower than the outflow hole 15. The density of water in each water rod in this embodiment changes largely according to a flow rate of water passing through the fuel assembly. That is, under the condition that a flow rate of water in the core is small, since an amount of steam generated in the water rod becomes larger than an amount of water flowing in the control rod, the water rod inside is filled with steam. When the flow rate of water increases, an amount of water flowing in the water rod goes beyond an amount of steam generated therein, so that the water rod inside is filled with water. Therefore, the water rod inside is filled with steam in operating at a low flow rate in an initial burning stage, whereby an average density of water inside the fuel assembly decreases, so that neutron spectrum is hardened, whereby production of Pu.sup.239 is promoted. On the other hand, since the water rod inside is filled with water in operation at a high flow rate in a final burning stage and the average density of water inside the fuel assembly increases, the neutron spectrum is and it is possible to effectively burn Pu.sup.239 produced in the operation at a low flow rate, and fuel economy is raised. That is, since excessive neutrons in the initial burning stage can be used for production of Pu.sup.239, the number of short blade control rods can be reduced by a decrease in reactivity control by absorption of excessive neutrons during operation, so that further cost reduction is realized. Further, since a lot of the water rods can be arranged by making the fuel assembly large in size, an effect of effective use of Pu.sup.239 increases and fuel economy can be improved greatly. In this invention, since the fuel assemblies each are made large in size as the fuel assemblies in FIG. 1, it is effective from a viewpoint of fuel economy to provide, inside each fuel assembly, water rods the cross-sectional area of each of which corresponds to that of several fuel rods. Further, the reactor shutdown margin can be improved by using such material that control rod worth becomes high at a portion facing central side portions of the fuel assembly 2, as a neutron absorber arranged inside the control rod blades. According to the present invention, the number of control rods can be drastically reduced without decreasing control rod worth. Further, since the role of the control rod can be shared, the control system can be simplified and rationalized and a cost of the plant can be reduced.