Patent Number: 052727416
Section: description

DESCRIPTION OF EMBODIMENTS OF THE INVENTION An embodiment of a fuel assembly according to the present invention will be described hereunder preferring to FIGS. 1 to 4. In FIG. 4 showing a side view of a nuclear fuel assembly for a BWR with a channel box being removed, the fuel assembly comprises a plurality of fuel rods 1, a plurality of spacer 6, a lower tie plate 12 supporting lower ends of the fuel rods 1, an upper tie plate 11 holding upper ends of the fuel rods 1 and a polygonal channel box 5 (shown in FIG. 1) surrounding the fuel rods 1. The spacers 6 (61 to 67) are of round cell type and arranged in several stages, for example seven stages in longitudinal direction, and named a first stage spacer 61 to a seventh stage spacer 67 from the top. Some of the spacer 6, for example second upper stage spacer 62 and a third upper spacer 63 each are constructed of cylindrical cells 7 each having vanes 8 on the surfaces thereof and cylindrical cells 7 without vanes. The other stage spacers 61, 64 to 67 are constructed of cylindrical cells 7 having no vanes. Each of the second and third stage spacers 6 (62, 63) will be described hereunder in detail referring to FIGS. 1 to 3. The spacer 6 (62 or 63) has a plurality of cylindrical cells 7 arranged in a grid fashion and a side band 610 (shown only in FIG. 4) enclosing the cells 7. Adjacent cylindrical cells 7 are joined together by welding to form rectangular configuration. The fuel rods 1 are inserted into the cylindrical cells 7, respectively, and each fuel rod 1 is supported by two projection 701 projecting inward and a spring 702 which is provided within the cell 7. In the fuel assembly for a BWR, power becomes higher around a position of fuel rods 1 which are disposed at sides of the spacer 6 which do not face a control rod 9, so that the position is severe in thermal conditions. In this embodiment, the cylindrical cells 7 with vanes 8 are used for the fuel rods at the above-mentioned position, and the cell 7 with no vanes for the other fuel rods 1. The position is a corner farthest from the control rod 9 which is cruciform and disposed adjacent the fuel assembly so that the two sides of the channel box 5 face the two sides of the control rod 9 as shown in FIG. 1, and around the corner. The corner and its vicinity are defined as a corner region here. An example of the cell arrangement is such that the cells 7 with vanes 8 are for several fuel rods 1 in two rows along each of the two sides farthest from the control rod 9 as shown in FIG. 1. In FIG. 1, 12 cells 7 each have 4 vanes 8. The vanes 8 each has a triangular configuration, as shown in FIG. 2, in which the sides 801 each are much longer than the base 802, one of the sides 801 is on the outer surface of the cylindrical cell 7, with the base 802 being at a downstream side, and a plane defined by the other side and the base projects substantially normally from the outer surface of the cell 7. The vane 8 is oblique to a plane in which the axis of the cell 7 lies. FIG. 5 shows flow condition of coolant between fuel rods 1 of a BWR. The flow is a two-phase flow. Namely, liquid film flow 2 is produced on the surface of the fuel rod 1 and a mixture of steam and liquid drops 3 flows in space between the fuel rods 1. Under the flow conditions, when conventional vanes previously mentioned are employed, the coolant is caused to be flows along the periphery of the fuel rod 1. The flow strips off the liquid film 2 on the fuel rod, so that the liquid film 2 is reduced and the boiling transition is apt to occur. Therefore, power at the boiling transition, that is, allowable power level decreases. The vanes 8 according to the present invention impart swirling motion to the coolant to generate swirling flows in the spaces 601 defined by adjacent opposite fuel rods 1 and in the spaces 602 defined by the side walls of the channel box 5 and the fuel rods 1 facing the side walls. The swirling flow generated between the fuel rods 1 is a shown by a reference numeral 4 in FIG. 6. By the swirling flow 4, liquid drops 3 in the steam are moved to the liquid layer or film 2 on the outer surface of the fuel rod 1 by centrifugal force, and adhered to the liquid film 2. Therefore, the thickness of the liquid film 2 increases, whereby heat transfer from the fuel rod 1 to the coolant (2) promoted, allowable power level is increases and thermal allowance to the boiling transition increases. The number of vanes 8 provided for the spacer 6 is limited, so that pressure loss increases as shown in FIG. 7. The vane 8 can be formed by simply cutting a part of the cell 7, the vane 8 and the cell 7 are formed of one piece and the cells are assembled integrally to be a spacer 6 by welding, so that the spacer 6 has an excellent reliability. The vane 8 as shown in FIG. 2 has a triangular configuration, however the vane having a rectangular configuration also can obtain similar effects to the above-mentioned one. Further, even by providing vanes for generating swirling flows at a portion independent of the round type spacer 6, for example, an inner surface of the channel box 5, similar effect can be obtained. The vanes can be provided on the side band 610. Preferable axial position at which the vanes 8 are disposed was studied. The boiling transition as previously mentioned takes place at the upstream sides of the second stage spacer 62 and the third stage spacer 63 in the upper region. In case the spacers 6 of seven stages are employed, the vanes 8 are provided on the second and third stage spacers 62 and 63. It is found that this construction makes the liquid film 2 thicker with small numbers of vanes 8. FIGS. 8 and 9 each show liquid film thickness at each axial position of spacers, wherein FIG. 8 is in case any spacers have no vanes and FIG. 9 is in case only the second and third stage spacers have vanes 8 at and around the corner farthest from the control rod 9. The nuclear fuel assembly shown in FIG. 9 has liquid film increased in thickness as compared with one shown in FIG. 8. Therefore, it is found that the thermal allowance increases. As mentioned above, the nuclear fuel assembly has relatively small numbers of vanes 8 used. Pressure loss increases as the number of vanes increases as shown in FIG. 7. Therefore, in the fuel assembly according to the present invention pressure loss little increases. Further thermal allowance is improved greatly. Therefore, a method of reducing the pressure loss by utilizing the increment of the thermal allowance was studied. 20% of the pressure loss of the fuel assembly takes place at the round cell type spacers 6. The pressure loss can be reduced drastically by reducing the number of stages of the spacers 6 to six stages from seven stages. However, when the number of spacers 6 is reduced, position intervals of the spacer 6, that is, pitches thereof becomes longer. As is apparent from the relation between the spacer pitches and allowable power, shown in FIG. 10 in which A is experimental value and B is analytical value, the allowable power, that is to say, power at the boiling transition decreases and thermal allowance becomes small. However, when the round cell-type spacers 6 (upper second and upper third stages) according to the present invention are employed, the thermal allowance increases, so that the above-problem is solved, the thermal allowance is beyond one in conventional fuel assemblies, and the pressure loss can be reduced drastically. Another embodiment of the present invention will be described referring to FIGS. 11 and 12. Parts the same as and corresponding to the previous embodiment are given the same reference numbers. In this embodiment, cylindrical cells 7 each have a rectangular cut out portion 710 or portions formed on the cylindrical wall of the cylindrical cells 7 and the cells 7 are inverted alternately and joined so that an axially opposite part 711 to the cut out portion 710 of a cell 7 is inserted to in the cut-out portion of an adjacent cell 7, and the walls of the adjacent cells 7 are overlapped partially in the longitudinal direction, as shown in FIG. 12, in other words, a part of the periphery of a cell 7 is in the periphery of adjacent cell 7. Therefore, projection area of the cells 7 is reduced and the pressure loss is reduced drastically. Vanes 8 are provided on cells 7 which are at and around the corner farthest to the control rod 9 as mentioned previously. The spacers can be used with 6 stages. In this case, the number of vanes is reduced greatly, so that allowable power level increases with little pressure loss. Further another embodiment is explained referring to FIG. 13. In FIG. 13, an example of an arrangement of vanes 8 on the spacer 6 is shown. The spacer 6 has one cylindrical cell 7 with 4 vanes 8, one cylindrical cell 7 with 3 vanes 8, 4 cell 7 with 2 vanes 8 and the other cell 7 with no vanes. Therefore the spacer has total 15 vanes. As shown in FIG. 7 there is little pressure loss with the number of vanes 15 or less.