Patent Number: 
Section: description

The nuclear thermal-hydraulic stability of a BWR is more stable at lower reactor output if the core flow rate is the same. In view of that point, an embodiment of the present invention is based on the concept below. The liquid (coolant) surface varying function of the spectral shift rod is applied to the non-rated power operation to drop a level of the water surface in a rising pipe at a low core flow rate near the low end at the automatic flow control range. Correspondingly, an amount of the coolant (water) in fuel assemblies is relatively reduced and a moderating rate of neutrons is also reduced, thereby producing a lower reactor output than the case of the water surface being not formed. This lowering of the reactor output necessarily increases an allowance for the nuclear thermal-hydraulic stability of a reactor core at the low end at the automatic flow control range. A BWR core according to a preferred embodiment of the present invention will be described below with reference to FIGS. 1 to 7. The BWR includes a core shroud 3 disposed inside a reactor pressure vessel 8. Within the core shroud 3, a core 1 is formed of a number of fuel assemblies 2 arrayed in the form of a square lattice. Upper end portions of the fuel assemblies 2 are supported by an upper lattice plate 4, which is fixed to an upper portion of the core shroud 3, in such a manner that the fuel assemblies 2 are restrained from moving horizontally. A core lower portion supporting plate 6 is mounted to the core shroud 3 and is positioned at a lower end portion of the core 1. Control rods 11 each having a cruciform cross-section are inserted between the fuel assemblies 2 through control rod guide pipes 5, and are driven by control rod driving mechanism 9 provided below the reactor pressure vessel 8. Fuel support pieces 10, shown in FIG. 2, are provided respectively at the top portion of the control rod guide pipes 5 and penetrates the core lower portion supporting plate 6. Four fuel assemblies 2 are inserted in and held by four insertion holes 10a formed in the fuel support piece 10, respectively. A load of the fuel assemblies 2 is finally supported by a bottom plate 8a of the reactor pressure vessel 8 through the fuel support piece 10, the control rod guide pipe 5, and a housing 9a of the control rod driving mechanism 9. In a side surface of the fuel support piece 10, four orifices are formed to take in a coolant (cooling water) flowing externally. These orifices lob are communicated respectively with the corresponding insertion holes 10a. An inner diameter d of the orifice 10b differs depending on the type of the reactor. In this embodiment, the diameter d is about 6.2 cm that is primarily used in BWRs having electric power of 1.1 million KW class. The fuel support piece 10 has a cross-shaped hole 10c which is formed between the four insertion holes 10a and receives the control rod 11. A plurality of internal pumps 20 are provided at the bottom the reactor pressure vessel 8. Each of the internal pumps 20 comprises a pump portion 21 including an impeller, and a motor portion 22 including a motor coupled to the impeller. The pump portion 21 is arranged in an annular passage 23 formed between the reactor pressure vessel 8 and the core shroud 3. The motor portion 22 is positioned outside the reactor pressure vessel 8. The core 1 is constructed, as shown in FIG. 3, by arranging each four fuel assemblies 2 around one control rod 11. Each control rod 11 is positioned adjacent to two sides of each fuel assembly 2. A detailed structure of the fuel assembly 2 will be described below with reference to FIGS. 4, 5, 6 and 7. The fuel assembly 2 comprises 74 fuel rods 12 arrayed in the form of a 9-row, 9-column square lattice, two water rods (spectral shift rods) 13 arrange in an area capable of accommodating seven fuel rods 12, a plurality of spacers 14 for holding spacings between adjacent ones of the fuel rods 12 and the water rods 13 to set values, and an upper tie plate 15 and a lower tie plate 16 for holding respectively upper and lower end portions of the fuel rods 12 and the water rods 13. A channel box 17 surrounds an outer periphery of a bundle of fuel rods 12 held together by the fuel spacers 14. Each of the fuel rods 12 comprises, though not shown, a cladding pipe which is made of a zirconium alloy and is filled with uranium fuel pellets containing U-235, U-238, etc. The fuel rods 12 include full-length fuel rods 12a having a fuel effective length L (corresponding to a portion where the fuel pellets are present) equal to a normal full length, and partial-length fuel rods 12b having a fuel effective length L shorter than that of the full-length fuel rods 12a. The total length of the partial-length fuel rod 12b is shorter than that of the full-length fuel rods 12a. The fuel assembly 2 is made up of 66 full-length fuel rods 12a and 8 partial-length fuel rods 12b. The partial-length fuel rods 12b are arranged in the second layer counting from the outermost side, as shown in FIG. 5. The lower tie plate 16 comprises, as shown in FIG. 6, a lower end portion 16a and a fuel rod holding portion 16b. The lower end portion 16a has an opening formed therein for introducing the cooling water supplied through the insertion hole 10a in the fuel support piece 10. The fuel rod holding portion 16b has a plurality of through holes 18 for introducing the cooling water to a cooling water passage 19 formed between the fuel rods 12, and holds the lower end portions of the fuel rods 12. A flow passage area ratio r (=S1/S2) of a total cross-sectional area (total cross-sectional area as viewed in a section Bxe2x80x94B in FIG. 6) S1 of all the through holes 18 to a total cross-sectional area (total cross-sectional area as viewed in a section Vxe2x80x94V in FIG. 4) S2 of the cooling water passage 19 in the channel box 17 is 0.3. The water rod 13 comprises, as shown in FIG. 7, a rising pipe 13a for introducing upward the cooling water introduced from the interior of the lower tie plate 16, and a falling pipe 13b communicated with an upper end portion of the rising pipe 13a for introducing downward the cooling water introduced from the rising pipe 13a. The rising pipe 13a has a cooling water inlet 25 formed at its lower end portion. The cooling water inlet 25 is positioned at the same height as (or lower than) the fuel rod holding portion 16b. The falling pipe 13b has a cooling water outlet 26 formed at its lower end portion. A height (outlet height) h from an upper surface of the fuel rod holding portion 16b to the cooling water outlet 26 is set to satisfy the following relationship: xe2x88x922.1r2+2.2rxe2x88x920.3xe2x89xa6(h/L) less than xe2x88x922.2r2+1.8r+0.04xe2x80x83xe2x80x83(1) The upper surface of the fuel rod holding portion 16b lies substantially at the same level as the lower end position of the fuel effective length L of the fuel rod 12. The position at which the rising pipe 13a of the water rod 13 communicates with the falling pipe 13b thereof lies near (or above) the upper end of the fuel effective length L of the full-length fuel rod 12a. The height from the upper surface of the fuel rod holding portion 16b to the communicating portion between the rising pipe 13a and the falling pipe 13b is about 3.7 m (the fuel effective length L). The operation of this embodiment will be described below. Upon driving of the internal pumps 20, the cooling water is forced to flow in a lower plenum 24 through the annular passage 23. The cooling water flows into the interior of the fuel support piece 10 through the orifices 10b, and is then introduced to the interior of the lower tie plate 16 through a cooling water passage formed in the fuel support piece 10. Most of the cooling water flows into the cooling water passage 19 via the through holes 18 to cool the fuel rods 12a and 12b. Because of a pressure loss due to a throttling effect developed by the through holes 18, the pressure of the cooling water after having passed the through holes 18 is lower than the pressure of the cooling water before passing the through holes 18. The remainder of the cooling water introduced to the interior of the lower tie plate 16 flows into the rising pipe 13a of the water rod 13 through the cooling water inlet 25. After rising in the rising pipe 13a, the cooling water falls in the falling pipe 13b and then flows out externally of the water rod 13 through the cooling water outlet 26. Since the pressure loss caused by the through holes 18 is negligible for the flow of the cooling water passing the water rod 13, a surface of the cooling water (liquid) is formed in the rising pipe 13a at a level corresponding to a pressure difference resulted from the pressure loss due to the throttling effect developed by the through holes 18. More specifically, a height (water surface level) H from the upper surface of the fuel rod holding portion 16b to the water surface formed in the rising pipe 13a is expressed by; H=(P1xe2x88x92P2)/(xcfx81xc3x97g)xe2x80x83xe2x80x83(2) where P1 is the pressure at the cooling water inlet 25, P2 is the pressure at the cooling water outlet 26, xcfx81 is the density of the cooling water in the rising pipe 13a, and g is the acceleration of gravity. Since the pressure P2 is reduced as the outlet height h increases, the water surface level H rises as the outlet height h increases. In this embodiment, the problems in the rated power operation and the non-rated power operation are solved by utilizing such a property of the water surface level in the rising pipe 13a. Specifically, in the rated power operation, the cooling water is always filled in the rising pipe 13a, and therefore influences of a transient event can be suppressed with sufficient reliability. Also, in the non-rated power operation, the water surface level in the rising pipe 13a is lowered to reduce an amount of the cooling water in the fuel assemblies 2, whereby the reactor output can be suppressed to improve the nuclear thermalhydraulic stability sufficiently. These two types of functions will be described below one by one. First, the inventors studied influences of the outlet height h and the water surface level H in the rated power operation, and obtained results shown in FIG. 8. The results of FIG. 8 were obtained by employing a core (comparative core) having the same structure as the core 1 of this embodiment, and determining values of the water surface level H with analysis while the outlet height h was changed variously, on condition that the comparative core is at the rated power and the core flow rate is minimum, i.e., that the reactor output is 100% of the rated value and the core flow rate is 90% of the rated value. Incidentally, the flow passage area ratio r in the comparative core is fixed to 0.3 as with this embodiment. The rising pipe 12a in the comparative core has however no limitations in length. In the graph of FIG. 8, the horizontal axis represents the outlet height h in units of nodes scaled by 24-division of the fuel effective length L. The water surface level H represented by the vertical axis means the height (m) from the upper surface of the fuel rod holding portion 16b to the water surface. As is apparent from FIG. 8, as the outlet height h increases, the water surface level H also increases. For example, the water surface level H takes H≈2.6 (m) at h=1 (node), takes H≈3.7 (m) at h≈4.5 (node), and amounts to H≈7.2 (m) at h≈12 (node). Accordingly, assuming that the length of the rising pipe 13a is, e.g., 3.7 m corresponding to the fuel effective length L, if the coolant outlet 26 is positioned near the upper surface of the fuel rod holding portion 16b (namely, h≈0) as disclosed in the above-cited JP, A, 63-73187, U.S. Pat. No. 5,023,047, U.S. Pat. No. 5,640,435, and Hitachi Hyoron, Vol. 74, No. 10 (1992), the water surface is formed at a level corresponding to H≈2.5 m, and a portion of the rising pipe 13a above the water surface becomes a vapor zone. On the other hand, if the outlet height h is set to satisfy hxe2x89xa74.5, Hxe2x89xa73.7 (m) is obtained, thus meaning that the water surface is not formed in the rising pipe 13a and the rising pipe 13a is fully filled with the cooling water. In this case, a limit height h0 of the outlet height h capable of filling the rising pipe 13a with the cooling water is 4.5 (node). Additionally, the reason why the water surface level H does not change smoothly with respect to an increase of the outlet height h in FIG. 8 is that the cross-sectional area of the cooling water passage in the fuel assembly 2 is partially reduced at the positions of the fuel spacers 14 and the pressure loss is increased correspondingly. Such an h-H characteristic also varies depending on the flow passage area ratio r. Specifically, a larger flow passage area ratio r reduces the throttling effect developed by the through holes 18 and hence the pressure loss, whereby an increasing rate of the water surface level H with respect to the outlet height h lowers. As a result, the limit height h0 of the outlet height h capable of filling the rising pipe 13a with the cooling water is increased. Then, the inventors studied influences of the limit height h0 of the outlet height h in the rated power operation, and obtained results shown in FIG. 9. The characteristic of FIG. 9 was obtained by, as with the case of FIG. 8, employing a core (comparative core) having the same structure as the core 1 of this embodiment, and determining values of the limit height h0 of the outlet height h with analysis, at which H=3.7 is obtained, while the flow passage area ratio r was changed variously from 0.2 to 0.4, on condition that the comparative core is at the rated power and the core flow rate is minimum. Incidentally, in the graph of FIG. 9, the vertical axis represents the limit height h0 as a relative value with the fuel effective length L set to 100%. As is apparent from FIG. 9, as the flow passage area ratio r increases, the limit height h0 also increases. For example, the limit height h0 takes h0≈6 (%) at r=0.2, h0≈17 (%) at r=0.3, and h0≈24 (%) at r=0.4. The characteristic curve of FIG. 9 is expressed by: h0=xe2x88x92210r2+220rxe2x88x9230xe2x80x83xe2x80x83(3) In a region lying on and above the characteristic curve expressed by the formula (3), i.e., in a region meeting; h0xe2x89xa7xe2x88x92210r2+220rxe2x88x9230xe2x80x83xe2x80x83(4) Hxe2x89xa73.7 is obtained. This means that when the length of the rising pipe 13a is 3.7 m corresponding to the fuel effective length L, the rising pipe 13a can be kept fully filled with the cooling water. Accordingly, if the outlet height h (m) satisfies the following relationship on condition that the flow passage area ratio r is in the range of 0.2xe2x89xa6rxe2x89xa60.4; h/Lxe2x89xa7xe2x88x922.1r2+2.2rxe2x88x920.3xe2x80x83xe2x80x83(5) the rising pipe 13a of 3.7 m can be kept fully filled with the cooling water. In the core 1 of this embodiment, it is possible to always maintain the water surface level H not lower than 3.7 m and to keep the rising pipe 13a of 3.7 m fully filled with the cooling water during the rated power operation by satisfying the conditions of 0.2xe2x89xa6rxe2x89xa60.4 and xe2x88x922.1r2+2.2rxe2x88x920.3xe2x89xa6h/L. Accordingly, unlike the conventional structure wherein the water surface is formed in the rising pipe during the rated power operation, if there should occur a transient event such as an abrupt increase of the core flow rate due to, e.g., an abnormal condition occurred in the control system for the internal pumps 20, a rising rate of the reactor output would be small because the rising pipe 13a is originally fully filled with the cooling water. As a result, this embodiment can suppress influences of the transient event with sufficient reliability. In other words, safety of the reactor can be further improved. While the above description has been made, by way of example, in connection with the case under conditions of the core being at the rated power and the core flow rate being 90% of the rated value, when the core flow rate is larger than 90% of the rated value, a value of the right side (P1-P2) in the formula (2) becomes larger and the water surface level H rises as compared with that resulted in the above case of the minimum core flow rate. Next, the inventors studied influences of the outlet height h and the water surface level H in the non-rated power operation during which the core flow rate and the reactor output are lower than those in the rated power operation, and obtained results shown in FIG. 10. The results of FIG. 10 were obtained by employing a core (comparative core) having the same structure as the core 1 of this embodiment, and determining values of the water surface level H with analysis while the outlet height h was changed variously, under condition at the above-described low end at the automatic flow control range in the non-rated power operation (here, on condition that the reactor output is 90% of the rated value and the core flow rate is 65% of the rated value). Incidentally, as with the case of FIG. 8, the flow passage area ratio r is fixed to 0.3 and the rising pipe 12a has no limitations in length. The horizontal axis and the vertical axis represent the same parameters as those in the graph of FIG. 8. As is apparent from FIG. 10, similarly to the case of FIG. 8, as the outlet height h increases, the water surface level H also increases. However, the values of the water surface level H are smaller as a whole than those in FIG. 8. For example, the water surface level H takes H≈1.4 (m) at h=1 (node), H≈3.7 (m) at h≈9.5 (node), and H≈4.6 (m) at h≈12 (node). Accordingly, assuming that the length of the rising pipe 13a is, e.g., 3.7 m corresponding to the fuel effective length L, if the outlet height h is set to satisfy h less than 9.5, the water surface appears in the rising pipe 13a, and a vapor zone is formed in a portion of the rising pipe 13a above the water surface. In this case, a limit height h1 of the outlet height h capable of forming the water surface in the rising pipe 13a is 9.5 node. Then, the inventors studied influences of the limit height h1 of the outlet height h in the non-rated power operation, and obtained results shown in FIG. 11. The characteristic of FIG. 11 was obtained by, as with the case of FIG. 9, determining values of the limit height h1 of the outlet height h with analysis, at which H=3.7 is obtained, while the flow passage area ratio r was changed variously from 0.2 to 0.4. In the graph of FIG. 11, the vertical axis represents the limit height h1 as a relative value like that in FIG. 9. As is apparent from FIG. 11, similarly to the case of FIG. 9, as the flow passage area ratio r increases, the limit height h1 also increases. However, an increasing rate of the limit height h1 is smaller than in FIG. 9. The reason is that because the cooling water outlet 26 is positioned much above the upper surface of the fuel rod holding portion 16b, the pressure loss in the cooling water passage 19 corresponding to a level difference from the upper surface of the fuel rod holding portion 16b to the cooling water outlet 26 is increased and the influence of the flow passage area ratio r is reduced. Further, the values of the limit height h1 itself are larger as a whole than those in FIG. 9. For example, the limit height h1 takes h1≈32 (%) at r=0.2, h1≈37 (%) at r=0.3, and h1≈41 (%) at r=0.4. The characteristic curve of FIG. 11 is expressed by: h1=xe2x88x92220r2+180r+4xe2x80x83xe2x80x83(6) In a region below the characteristic curve expressed by the formula (6), i.e., in a region meeting; h1 less than xe2x88x92220r2+180r+4xe2x80x83xe2x80x83(7) H less than 3.7 is obtained. This means that when the length of the rising pipe 13a is 3.7 m corresponding to the fuel effective length L, the water surface can be formed in the rising pipe 13a.  Accordingly, if the outlet height h (m) satisfies the following relationship on condition that the flow passage area ratio r is in the range of 0.2xe2x89xa6rxe2x89xa60.4; h/L less than xe2x88x922.2r2+1.8r+0.04xe2x80x83xe2x80x83(8) the water surface can be formed in the rising pipe 13a of 3.7 m and a vapor zone can be formed in a portion of the rising pipe 13a above the water surface. In the core 1 of this embodiment, it is possible to set the water surface level H to be lower than 3.7 m and to form the water surface in the rising pipe 13a and a vapor zone above the water surface at least at the low end at the automatic flow control range in the non-rated power operation by satisfying the conditions of 0.2xe2x89xa6rxe2x89xa60.4 and h/L  less than xe2x88x922.2r2+1.8r+0.04. With this embodiment, therefore, the void fraction in the fuel assemblies 2 can be increased to reduce the neutron moderating effect, whereby the reactor output can be suppressed to improve the nuclear thermal-hydraulic stability of the core 1 sufficiently in the non-rated power operation. Additionally, it is a principal rule that operation of the reactor below the core flow rate range at the rated power is performed, as mentioned above, when the reactor is started and stopped. During such operation, a possibility that there occurs an event like an abrupt increase of the core flow rate, such as assumed in the above (1), is much lower than during the rated power operation. Therefore, the formation of the water surface in the rising pipe 13a, when the reactor is in the low core flow rate range as encountered in the starting and stopping periods thereof, hardly raises significant problems in comparison with the period of the rated power operation. According to this embodiment, as described above, since the outlet height h is set to satisfy the relationship of xe2x88x922.1r2+2.2rxe2x88x920.3xe2x89xa6h/L less than xe2x88x922.2r2+1.8r+0.04 under condition of 0.2xe2x89xa6rxe2x89xa60.4, influences of the transient event during the rated power operation can be suppressed with sufficient reliability, while the nuclear thermal-hydraulic stability of the core during the non-rated power operation can be sufficiently improved. This embodiment can also provide an advantage below. In the conventional structure wherein the water surface is formed in the rising pipe during the rated power operation, no problems occur in usual reactor output control because the level of the water surface in the rising pipe can be approximately calculated based on the core flow rate. However, when there is a need for achieving a core control ability with higher accuracy, it is required to actually measure the water surface level by a detector or the like. Manufacture of such a detector or the like pushes up a cost and reduces economy of the reactor. Also, a difficulty arises in installing the detector in a limited space inside the fuel assembly. According to this embodiment, the rising pipe 13a is fully filled with water during the rated power operation. Thus, since the reactor output control is performed on condition that the rising pipe 13a is always fully filled with water, highly accurate control can be achieved in this embodiment without providing such a detector or the like. As a result, this embodiment can improve economy of the reactor in comparison with the conventional structure. The above-mentioned advantages can also be provided in the case of using fuel assemblies having the fuel effective length L other than 3.7 m. Specifically, neutrons leak in a considerable amount to the outside of the core near at the upper end of the fuel effective length L, and change of the fuel effective length L at the upper end of the fuel effective causes no significant influence upon the pressure loss characteristic in the lower portion of the fuel assembly 2. Thus, even if the fuel effective length L is changed on the order of 0.1 m, the same advantages as described above can be obtained. The through holes 18 in the fuel rod holding portion 16b are simply cylindrical in shape, and the total cross-sectional area S1 of the through holes 18 as viewed in the section taken along line Bxe2x80x94B in FIG. 6 gives a minimum value of the total cross-sectional area S1. When the through holes 18 have, for example, any other complex shape, the total cross-sectional area S1 is given by a minimum total cross-sectional area of the through holes 18 in the direction of height thereof. In this embodiment, the orifices 10b in the fuel support piece 10 are formed in the side surface of the fuel support piece 10. However, so long as the same pressure loss characteristic is obtained by the through holes 18 which restrict an expanded flow of the cooling water, the orifices 10b may be formed in a lower surface, an oblique surface as viewed from the horizontal surface, or a curved surface. Further, the shape of each orifice 10b is not necessarily required to be circuit, but may be, e.g., elliptic, rectangular or triangular so long as the same pressure loss characteristic is obtained based on the restriction of an expanded flow of the cooling water. The water rod 13 is not necessarily required to have the structure shown in FIG. 7. So long as the conditions of the above formulae (5) and (8) are satisfied, the water rod 13 may have the structure of a water rod having a rising passage and a falling passage, as disclosed in JP, A, 63-73187 and JP, B, 7-89158, for example. Also, the overall length of the water rod 13 is not limited to 3.7 m, but may be longer than 3.7 m. Further, the position at which the rising pipe 13a and the falling pipe 13b communicate with each other is not necessarily limited to the vicinity of the upper end of the water rod 13. It is just essential that the length of the communicating position between both the pipes to the lower end of the water rod is about 3.7 m or more. These modifications can also provide similar advantages as described above. While this embodiment uses the fuel assembly having a 9-row, 9-column array of fuel rods, the present invention is also applicable to a fuel assembly having a 10-row, 10-column array of fuel rods. While the inner diameter d of the orifices 10b in the fuel support piece 10 is set to about 6.2 cm in this embodiment, the inner diameter d may have other value in some cases. In an advanced boiling water reactor (ABWR), for example, the fuel support piece 10 having the orifices 10b with the inner diameter d of about 5.6 cm is primarily employed. A BWR core including such a fuel support piece will be described below as another embodiment of the present invention. In another embodiment, the construction except the fuel support piece is the same as that of the above embodiment (first embodiment) having been described in connection with FIGS. 1 to 7. (A) Case of d≈5.6 (cm) For a core which has the same structure as the first embodiment and includes the fuel support piece 10 having the orifices 10b with the inner diameter d of about 5.6 cm, the inventors studied influences of the limit height h0 of the outlet height h in the rated power operation, and obtained results shown in FIG. 12. Also, the inventors studied influences of the limit height h1 of the outlet height h at the low end at the automatic flow control range (where the reactor output and the core flow rate are respectively about 70% and about 50% of those in the rated power operation), and obtained results shown in FIG. 13. The FIGS. 12 and 13 correspond respectively to FIGS. 9 and 11 for the first embodiment. As is apparent from FIG. 12, for example, the limit height h0 takes h0≈12 (%) at r=0.2, h0≈=24 (%) at r=0.3, and h0≈28 (%) at r=0.4. The characteristic curve of FIG. 12 is expressed by: h0=xe2x88x92420r2+340rxe2x88x9240xe2x80x83xe2x80x83(9) In a region lying on and above the characteristic curve expressed by the formula (9), i.e., in a region meeting; h0xe2x89xa7xe2x88x92420r2+340rxe2x88x9240xe2x80x83xe2x80x83(10) Hxe2x89xa73.7 is obtained. This means that when the length of the rising pipe 13a is 3.7 m corresponding to the fuel effective length L, the rising pipe 13a can be kept fully filled with the cooling water during the rated power operation. Accordingly, if the outlet height h (m) satisfies the following relationship on condition that the flow passage area ratio r is in the range of 0.2xe2x89xa6rxe2x89xa60.4; h/Lxe2x89xa7xe2x88x924.2r2+3.4rxe2x88x920.4xe2x80x83xe2x80x83(11) the rising pipe 13a of 3.7 m can be kept fully filled with the cooling water. As is apparent from FIG. 13, for example, the limit height h1 takes h1xe2x88x9254 (%) at r=0.2, h1≈56 (%) at r=0.3, and h1≈58 (%) at r=0.4. The characteristic curve of FIG. 13 is expressed by: h1=xe2x88x9253r2+50r+46xe2x80x83xe2x80x83(12) In a region below the characteristic curve expressed by the formula (12), i.e., in a region meeting; h1 less than xe2x88x9253r2+50r+46xe2x80x83xe2x80x83(13) H (the water surface level in the rising pipe)  less than 3.7 is obtained. This means that when the length of the rising pipe 13a is 3.7 m corresponding to the fuel effective length L, the water surface can be formed in the rising pipe 13a.  Accordingly, if the outlet height h (m) satisfies the following relationship on condition that the flow passage area ratio r is in the range of 0.2xe2x89xa6rxe2x89xa60.4; h/L less than xe2x88x920.53r2+0.5r+0.46xe2x80x83xe2x80x83(14) the water surface can be formed in the rising pipe 13a of 3.7 m and a vapor zone can be formed in a portion of the rising pipe 13a above the water surface. With this embodiment wherein the inner diameter d of the orifices 10b in the fuel support piece 10 is set to about 5.6 cm, similar advantages as those in the first embodiment can also be obtained by setting the outlet height h so as to satisfy the condition of: xe2x88x924.2r2+3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x920.53r2+0.5r+0.46 (B) Case of Inner Diameter d being Larger Than 5.6 cm but Smaller Than 6.2 cm Comparing the first embodiment with the embodiment (second embodiment) described in the above (A), it is seen that the relationship between the outlet height h and the water surface level H is changed depending on the inner diameter d of the orifices 10b formed in the side surface of the fuel support piece 10. Accordingly, there exists some range of the outlet height h which can be used in one of the first and second embodiments, but cannot be used in the other. To cope with such a range of the outlet height h, two types of water rods 13 each comprising the rising pipe 13a and the falling pipe 13b require to be fabricated depending on the inner diameter d of the orifices 10b, thus resulting in an increased production cost. In view of the above, by taking the logical product of the conditions expressed by the formulae (5) and (8) and the conditions expressed by the formulae (11) and (12), the outlet height h can be used for any of d≈5.6 cm, d≈6.2 cm, and 5.6 cmxe2x89xa6dxe2x89xa66.2 cm. The resulting logical product is given by a region between the characteristic curve of FIG. 12 and the characteristic curve of FIG. 11, the region being expressed by: xe2x88x924.2r2+3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x922.2r2+1.8r+0.04xe2x80x83xe2x80x83(15) Similar advantages as those in the first embodiment can also be obtained by a core constructed by loading fuel assemblies including the water rods 13 for each of which the outlet height h is set to satisfy the condition of the above formula (15). Further, since those fuel assemblies can be loaded in cores provided with fuel support pieces having orifices with its inner diameter d ranging from about 5.6 cm to about 6.2 cm, fuel economy can be improved.