Patent Number: 048266540
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, some preferred embodiments of the invention will be described in detail, referring to the drawings. Embodiment 1 An embodiment of a fuel assembly for use in a boiling water reactor according to the present invention will be described referring to FIGS. 1 and 2. In FIG. 1, the fuel assembly 1 comprises a plurality of fuel rods fixed at both end portions by upper and lower tie plates 3, 4, water rods 6 disposed among the fuel rods 2, a plurality of fuel spacers 5 disposed in an axial direction to keep the fuel rods 2 and the water rods 6 in a predetermined laterally spaced relation, and a channel box 7, fitted to the upper tie plate 3, extending downwards so as to encompass the bundle of the fuel rods 2 and mounted on the lower tie plate 4. In FIG. 2, a space region 8 is formed at the center on the cross-sectional plane of the fuel assembly 1. The lower end of this space region 8 is positioned on the upper surface of the lower tie plate 4, and its upper end on the lower surface of the upper tie plate 3. The water rods 6 are disposed radially so as to extend outwards from the space region 8 every two of the water rods 6 (towards the four sides of the channel box 7). The space region 8 is encompassed by four water rods 6 and is represented by a square of dotted line in FIG. 2 for convenience' sake. In practice, however, any structure expressed by the square of the dotted line does not exist. When the fuel assemblies 1 are outside the reactor, the space region 8 is a mere space but when they are loaded in the reactor, it serves as a passage of cooling water. Four water rods 6 are directly adjacent to the space region 8, that is, the four water rods 6 face to the space region 8. The size of the space region 8 is such that one fuel rod 2 or one water rod 6 can be disposed. The fuel rods 2, the water rods 6 and the space region 8 are arranged in a grid form as shown in FIG. 2. Four kinds of fuel rods 21-24 are used as the fuel rods 2 as shown in Table 1. Though the fuel rods 21-23 do not contain gadolinium which is an burnable poison the fuel rod 24 does. The enrichment is 1.8 wt % for the fuel rod 21, 2.5 wt % for the fuel rod 22 and 3.3 wt % for the fuel rods 23 and 24. TABLE 1 ______________________________________ Fuel Rod No. 21 22 23 24 ______________________________________ enrichment (wt %) 1.8 2.5 3.3 3.3 gadolinium (wt %) -- -- -- 3.5 number of rods 4 8 52 8 ______________________________________ After the fuel assembly 1 of this embodiment 1 is loaded into the core of a boiling water reactor, the reactor is started to operate. During the operation of the boiling water reactor, cooling water flows into the fuel assembly 1 from the lower tie plate 4. This cooling water rises through the gaps formed among the fuel rods 2 and through the water rods 6 and the space region 8. During this elevation process, the cooling water is heated while it cools the fuel rods 2 and part of the cooling water changes to a vapor. Since the cooling water flows into the water rods 6 in the proximity of the upper surface of the lower tie plate 4, hardly any vapor is contained in the cooling water flowing inside the water rods 6. The vapor rises through the gaps among the fuel rods 2 and through the space region 8. In this embodiment, since the water rods 6 are adjacent to the space region 8, the void fraction of the space region 8 is lower than the mean void fraction of the fuel assembly 1 for the following reason. FIG. 3 shows the influences of the difference of relative position between the space region 8 and the water rod 6 upon the number of hydrogen atoms in the space region 8 by the use of the mean void fraction of the fuel assembly as a parameter. The reference value uses the number of hydrogen atoms in a unit cell for the water rod when the mean void fraction of the fuel assembly is 0%. The term "unit cell" hereby means one of square defined by orthogonally intersecting grid plates in a grid-like fuel spacer constituted by mutually crossing and different grid plates disposed in x and y directions, that is, the unit cell corresponds to a space 8 defined by the dotted line. A characteristic line 11 represent characteristics when the fuel rods 2 are disposed in all the unit cells adjacent to the space region 8 and a line 12 represent characteristics when a half of the unit cells adjacent to the space region 8, that is, 4, are filled with water rods 6 as shown in FIG. 2. The number of hydrogen atoms in the space region 8 at a low void fraction is greater than that in the water rod 6 represented by characteristics 13 by the number that corresponds to the absence of a hollow pipe of the water rod 6, irrespective of the relative position to the water rod. Under the high void fraction state where nonhomogeneity in both the axial and radial directions in the fuel assembly becomes the greatest, however, the number of hydrogen atoms in the space region 8 in which the fuel rods are disposed in all of the adjacent unit cells drops remarkably when compared with the number of hydrogen atoms of the water rod 6 (see the characteristics 11). On the other hand, in the space region 8 in which the water rods 6 are disposed in the half of the adjacent unit cells (see FIG. 2), the number of hydrogen atoms at a void fraction of up to 40% is greater than that of the water rod 6, and even at a void fraction higher than 40% the number of hydrogen atoms does not drastically drop when compared with that of the water rod 6. This results from the phenomenon that in the fuel assembly 1 having the water rods 6 disposed therein, the distribution of the void fractions inside the channel box 7 changes greatly depending upon the position. In other words, when the fuel rods 2 are disposed in all the unit cells adjacent to the space region 8, the void fraction of the space region 8 increases to a level substantially equal to the mean void fraction of the fuel assembly, and when the water rods 6 are disposed in the unit cells adjacent to the space region 8, the void fraction of the space region 8 becomes by far smaller than the mean void fraction of the fuel assembly 1 in the same way as in the region inside the water rod 6. In FIG. 3, when the mean void fraction of the fuel assembly 1 is 70%, the number of hydrogen atoms of the characteristics 12 increases by about 70% in comparison with that of the characteristics 11. This is a decrease of only about 10% in comparison with the case of the water rod 6. Furthermore, when the reducing effect of the void fraction in the water rods 6 adjacent to the space region 8 due to disposition of the space region 8 is taken into consideration, the space region 8 of the characteristics 12 has the number of hydrogen atoms substantially equal to that of the water rod 6. FIG. 4 shows the relation between the number of hydrogen atoms and reactivity in order to represent the nuclear effect of the characteristics 11 and 12. For example, the reactivity at a mean void fraction of 70% of the assembly rises by about 0.04% .DELTA.K.infin. owing to the change from the point A to the point B in FIG. 3. In other words, it can be understood that when the water rods are disposed in the unit cells adjacent to the space region 8, the space region 8 exhibits substantially the same nuclear function as the water rods 6 at a high void fraction. At a low void fraction, the reactivity of the space region 8 increases by about 0.05% .DELTA.K.infin. (per space region for one unit cell) in comparison with the water rod 6 due to the increase of the homogenization effect in the radial direction of the fuel assembly resulting from the increase in the number of hydrogen atoms and due to the decrease of the neutron absorption quantity resulting from the absence of the hollow pipe. In this manner, since the water rods 6 and the space region 8 are adjacent to one another in the fuel assembly 1 of this embodiment, the reactivity can be improved and hence, fuel cycle economy can be improved, too. On the other hand, the pressure drop reducing effect of the space region 8 is not dependent upon its relative position with the water rod 6 but is dependent upon the area of the space region 8. FIG. 5 shows a relation between the area of the space region 8 and the pressure drop. In this embodiment, as the cross-sectional area of the pipes disposed in the fuel assembly and the wetted area decrease when compared with the case where the water rods 6 are disposed inside the space region 8, too, the pressure drop can be reduced. Obviously, the fuel assembly 1 of this embodiment is superior to the fuel assembly using only the water rods 6 from the aspects of the pressure drop and economical efficiency of fuel. Though only reduction of the pressure drop has been described as the effect of increasing the flow path area by the space region 8, the following can also be accomplished by limiting the reduction of the pressure drop when there is a sufficient margin for the pressure drop: (i) promotion of uniformity of fuel assembly structure by the increase in the number of water rods; and (ii) extension of life of fuel assembly by increasing uranium loading quantity. The effect described above is not limited to the case where the water rods 6 are disposed in the half of the unit cells adjacent to the space region 8 as in the fuel assembly 1 shown in FIG. 2. FIG. 6 shows a relation between the number of water rods 6 occupying the unit cells adjacent to the space region 8 and the number of hydrogen atoms in the space region 8. It can be understood from FIG. 6 that if the water rod 6 is disposed in at least one of adjacent unit cells, the effect described above can be obtained. The enrichment distribution which shows the effect of this embodiment (shown in FIG. 2) in comparison with the fuel assembly 30 shown in FIG. 7(A), which is obtained by replacing the space region 8 of this embodiment by the water rod 6, is the same as that of this embodiment. The void fraction of the flow passage in the region A shown in FIG. 7(B) is compared with respect to this embodiment and the fuel assembly 30 in FIG. 7(A). The result at the upper end of the fuel assembly is shown in Table 2. In this embodiment, the void fraction drops not only in the region f which is the space region 8 but also in the regions d and e which are adjacent unit cells. As a result, the number of hydrogen atoms in the region A is substantially equal in the fuel assembly 1 of this embodiment and in the fuel assembly 30 in FIG. 7(A). TABLE 2 ______________________________________ Region Embodiment 1 FIG. 7(A) ______________________________________ a 67.4% 67.9% b 62.8 62.5 c 60.1 59.9 d 55.4 56.4 e 51.0 53.2 f 45.4 49.6 ______________________________________ Therefore, the neutron infinite multiplication factor is equal to that of the fuel assembly 30 at the upper part of the fuel assembly 1 and is higher by about 0.05 .DELTA.K.infin. at its lower part than that of the fuel assembly 30. On the other hand, the pressure drop in the fuel assembly 1 is lower by about 3% (about 0.021 Kg/cm.sub.2) than that of the fuel assembly 30. In the embodiment described above, the arrangement of the fuel rods and the like is 9 rows by 9 columns inside the fuel assembly by taking into consideration the fact that since a large number of water rods are disposed, the number of fuel rods decreases and the linear output density increases. However, the concept of this embodiment can of course be applied to fuel assemblies having the arrangement of 10 rows by 10 columns and 11 rows by 11 columns. In recent years, a fuel assembly capable of increasing the linear power density has been developed by bonding a Cu or Zr thin film on the inner wall of a cladding pipe to decrease the interaction between fuel pellets and the cladding pipe. In the case of such a fuel assembly, the arrangement of the fuel rods of 8 rows by 8 columns can be employed. A fuel assembly 35 shown in FIG. 8 is another application example of Embodiment 1. The fuel assembly 35 increases the mean enrichment of the fuel assembly 1 in Embodiment 1 by about 4 wt %. As fuel rods 3 in this fuel assembly 35, four kinds of fuel rods 31-34 as shown in Table 3 are used. TABLE 3 ______________________________________ Fuel rod number 31 32 33 34 ______________________________________ enrichment (wt %) 3.0 3.6 4.5 3.3 gadolinium (wt %) -- -- -- 4.5 number of rods 4 8 46 14 ______________________________________ If the enrichment is increased in a conventional fuel assembly, the moderation effect of neutrons drops so that the worth of the control rod decreases and the void coefficient (absolute value) increases. On the other hand, when a large number of water rods 6 are disposed to obtain a synergistic effect with the space region 8 as in this embodiment, hardly any increase in the pressure drop in the fuel assembly occurs and in addition, the moderation effect of neutrons can be increased. As a result, fuel cycle economy can be improved by about 7% in comparison with the conventional fuel assembly and a surplus linear power density at the stop of reactor can be limited to substantially the same level as the one in the case where the enrichment is about 3 wt %. Embodiment 2 FIG. 9(A) is a cross-sectional view of a fuel assembly 36 in another embodiment of the invention. In the fuel assembly 36 of this embodiment, the positions of the water rods 6 are different from those of the fuel assembly 1 in Embodiment 1. The fuel rods 2 are the same as in the Embodiment 1, using the fuel rods 21-24. In this embodiment, the water rods 6 are disposed in all the unit cells adjacent to the space region 8. Since the water rods 6 and the space region 8 are adjacent to one another in this embodiment, too, the same function as that of Embodiment 1 develops. The void fraction of the flow passage of the region B of FIG. 9(B) in this fuel assembly 36 is compared with fuel assembly obtained by replacing the space region 8 of this fuel assembly 36 by the water rods 6. The effect at the upper end of the fuel assembly 36 is shown in Table 4 and the effect is greater than in Embodiment 1. At the lower part of the fuel assembly 36, that is, in the low void region, however, the improvement in reactivity is less than at the lower part of the fuel assembly 1. On the other hand, the pressure drop in the fuel assembly 36 decreases by about 3% (about 0.021 Kg/cm.sup.2) in comparison with the fuel assembly 30 shown in FIG. 7(A). TABLE 4 ______________________________________ Fuel Assembly having water rods in Region Embodiment 2 place of space region of FIG. 9(A) ______________________________________ d 55.7% 56.7% e 50.5 53.1 f 42.5 48.1 ______________________________________ Embodiment 3 FIG. 10 is a cross-sectional view of a fuel assembly 37 in accordance with still another embodiment of the present invention. This embodiment has a space region 15 which is five times greater than the space region of Embodiment 1 in order to improve the reducing effect of the pressure drop in the fuel assembly. The water rods 6 are disposed in four of unit cells 16 adjacent to the space region 15. The void fraction of the flow passage in the region A shown in FIG. 7(B) is compared between the fuel assembly of this embodiment and that of FIG. 7(A) in the same way as in Embodiment 1. The result at the upper end of the fuel assembly is shown in Table 5. Though the effect is smaller than in Embodiment 1, the void fraction is lower. At a low void fraction, since the neutron absorption quantity by the hollow pipe decreases, the neutron infinite multiplication factor is improved by about 0.2% .DELTA.K.infin. than that of fuel assembly shown in FIG. 7(A). TABLE 5 ______________________________________ Region Embodiment 3 FIG. 7(A) ______________________________________ a 68.4% 67.9% b 62.5 62.5 c 59.2 59.9 d 54.9 56.4 e 51.4 53.2 f 48.3 49.6 ______________________________________ As to the pressure drop in the fuel assembly, on the other hand, it decreases by about 13% (about 0.084 Kg/cm.sup.2) in comparison with that in the fuel assembly shown in FIG. 7(A). Though each of the embodiments described above uses the water rod as the moderator rod, the same effect can be obtained when a moderator rod having sealed therein a solid moderator material having a high hydrogen density and a small neutron absorption cross-section is used in place of the water rod. When a burnable poison is mixed in the solid moderator rod, the void fraction distribution of the fuel assembly becomes further greater. In accordance with the present invention, since the space region and the moderator rods are disposed adjacent to each other, the reactivity of the fuel assembly can be improved, the fuel cycle economy can be improved and the sectional area of the coolant flow passage can be increased so that the pressure drop in the fuel assembly can be reduced.