Patent Number: 060884207
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A reactor core according to preferred embodiments of the present invention will be described hereunder with reference to the accompanying drawings. As mentioned above, FIG. 1 is a longitudinally sectional view theoretically showing a first embodiment of a reactor core according to the present invention, and FIG. 2 is a top plan view of the reactor core shown in FIG. 1. FIG. 1 and FIG. 2 each show an example in which a reactor core is applied to a water cooling reactor such as a light water reactor, for example, to a boiling water reactor. In the boiling water reactor, a reactor core 10 is formed in a reactor pressure vessel, not shown. In this reactor core 10, rectangular cylindrical fuel assemblies 11 are charged in a manner of being arranged at an regular pitch in a longitudinal and crosswise direction, for example, at a pitch interval of 300 mm or more. The fuel assembly 11 charged in the reactor core 10 is supported on a core support plate 12 as shown in FIG. 3. Further, the fuel assembly 11 is composed of at least two kinds, that is, a normal fuel assembly 13 having a normal fuel effective (exothermic portion) length LM and a partial fuel assembly 14 having a shorter fuel effective length LP. A plural kinds of the partial fuel assemblies 14 may be prepared in accordance with the fuel effective length LP. As shown in FIG. 1, shaft brackets 15, each having a length of about 20 to 30 cm, are provided on upper and lower portions of the normal fuel assemblies 13 and on a lower potion of the partial fuel assembly 14. The upper portion of the partial fuel assembly 14 is formed with a streaming path 16 and the shaft bracket is not provided thereon. In the reactor core 10 of the boiling water reactor, the normal fuel assemblies 13 and the partial fuel assemblies 14 are dispersively arranged alternately in a diametrical direction thereof at a predetermined arrangement pattern. These normal fuel assemblies 13 and partial fuel assemblies 14 may be also dispersively arranged alternately in the diametrical direction and the circumferential direction. Therefore, various arrangement patterns may be considered. However, it is desirable that four fuel assemblies 11, which should be arranged on the central portion of the reactor core 10, are composed of the partial fuel assemblies 14, and it is desirable that the fuel assemblies which should be arranged on the outermost circumference are the normal fuel assemblies 13. In the reactor core 10 of the boiling water reactor, the normal fuel assemblies 13 and the partial fuel assemblies 14 are charged in a state of being properly combined with each other. A control rod 18 having a cross-shaped traverse section is provided between four fuel assemblies 11 adjacent to each other so as to be taken in and out. Moreover, as shown in FIG. 4, the fuel assemblies 11 are arranged in a substantially square space defined by four control rods 18 adjacent to each other. A blade 18a constituting the control rod 18 is taken in and out so as to surround the fuel assembly 11. In the fuel assembly 11, the normal fuel assembly 13 and the partial fuel assembly 14 have the same plane structure as shown in FIG. 4. FIG. 4 is a view showing an upper surface of the fuel assembly 11. The fuel assembly 11 has an area size of several times, for example, four times, as much as the existing fuel assembly. Thus, the arrangement pitch of the fuel assembly 11 is wide, and for example, the fuel assembly having the four-times area size has an arrangement pitch of about 300 mm or more. The fuel assembly 11 itself is made larger than the existing fuel assembly, and thereby, the coolant in a gap between fuel assemblies 11 is reduced so as to lower a ratio of water serving as a coolant, with respect to the overall volume. Further, a ratio of water to fuel is lowered so that a breeding ratio is set to at least about 1, for example, to 1.0 to 1.1. As shown in FIG. 4, in the fuel assembly 11, a fuel bundle 21 is housed in a cylindrical channel box 20 as a rectangular outer housing. The fuel bundle 21 is a fuel element bundle which has, as a whole, a rectangular, e.g., square transverse section. A seal alloy or stainless steel material is used as a material for the channel box 20. In the fuel bundle 21, a number of fuel rods (fuel pins) 22 are closely arranged with a predetermined regularity, and is made into a bundle by means of a fuel spacer, e.g., a grid spacer 23 as shown in FIG. 5 so that an interval between fuel rods 22 is retained. A plurality of the grid spacers 23 are provided at a predetermined interval in a longitudinal direction of the fuel bundle 21. If the channel box 20 is formed of stainless steel, the channel box 20 has mechanical and physical strength larger than the channel box made of seal alloy used in the existing light water reactor. For this reason, the channel box 20 has high rigidity, so that the channel box 20 can be made thin. The thickness of the channel box made of stainless steel is set to about 3 mm to 5 mm, preferably, to about 3 mm. As described above, the channel box 20 is made thin, and a fuel volumetric ratio can be hence increased, and a ratio of water to the fuel can be lowered. The fuel bundle 21 housed in the channel box 20 is constructed in a manner that a number of fuel rods 22 are made into bundle. The grid spacer 23 bundling up the fuel rods 22 includes a thin hexagonal cylinder having a thickness of about 0.2 mm, or a honeycomb type grid lattice 25 which is constructed in a manner that many rectangular and cylindrical sleeves 24 as pipe-like grid structure are fixed and integrated in a state of being closely arranged in its plane. The rectangular cylindrical sleeve 24 is formed of stainless steel or inconell having high rigidity and high mechanical and physical strength. Moreover, the grid spacer 23 may be constructed in a manner of surrounding the outer peripheral side of the honeycomb type grid lattice 25 by a rectangular outer frame 26 which functions as a reinforcing frame 26, or the grid spacer 23 may be constructed by the honeycomb type grid lattice 25 having no outer frame. One corner portion of the rectangular cylindrical sleeve 24 of the honeycomb type grid lattice 25 is provided with a vibration preventive spring 27. The vibration preventive spring 27 comprises a spring member such as a flat spring or a rod spring, which is bent into a V-letter or arc shape and has the entire length of about 15 mm. Upper and lower ends of the vibration preventive spring 27 are welded to an inner wall surface of the corner portion of the rectangular cylindrical sleeve 24, and an intermediate portion of the spring elastically projects into the cylindrical sleeve 24. Further, the cylindrical sleeve 24 of the honeycomb type grid lattice 25 is provided with a pair of protrusions (projections) 28 on a symmetrical position separated from the vibration preventive spring 27 at an angle of 120.degree.. The protrusion 28 bulges like an arc from the corner portion of the rectangular cylindrical sleeve 24 to the inside thereof. Moreover, the protrusion 28 may be formed as a dimple which is recessed on a side face of the corner portion of the cylindrical sleeve 24 and projects into the sleeve. Further, the protrusion 28 may be formed in the following manner. Specifically, cut portions extending to the circumferential direction are formed to upper and lower portions of the corner portion of the rectangular cylindrical sleeve 24, and then, the cut portion having a up-and-down predetermined width is inwardly pressed and deformed so as to be bulged. The fuel rod 22, which is nuclear fuel element, is successively guided into each sleeve 24 of the honeycomb type grid lattice 25 constituting the grid spacer 23, and then, inserted fuel rod 22 is supported at three points by means of the paired protrusions 28 and the vibration preventive spring 27 so as to prevent fuel rods 22 from contacting with each other, thus constituting the fuel bundle 21. In the fuel bundle 21 thus constructed, each fuel rod 22 is held at a predetermined interval by means of the paired protrusions 28 and the vibration preventive spring 27, and a coolant channel is secured therein, whereby each fuel rod 22 is restricted from vibrating and a fuel can be previously and securely prevented from being broken down. As shown in FIG. 5 to FIG. 7, the honeycomb type grid lattice 25 has been constructed by integrally combining the cylindrical sleeve 24 and the vibration preventive spring 27. In place of the honeycomb type grid lattice 25, a grid spacer 23A as shown in FIG. 8 may be used. The grid spacer 23A comprises a spring-integral type grid lattice 30 which integrally combines a ring-like or tours-like upper and lower circular guide 31 constituting a grid and a vibration preventive spring 32. The circular guide 31 constitutes a grid of the grid spacer 23A. Further, a reference numeral 33 denotes a protrusion which is formed at the circular guide 31 of the spring-integral type grid lattice 30. It is desirable that the paired protrusions 33 and the vibration preventive spring 32 are provided at a 120.degree. angular interval. However, these paired protrusions 33 and vibration preventive spring 32 are not necessarily provided at the 120.degree. angular interval, and a degree of freedom of angle is given when providing them. In a number of fuel rods (fuel pin) 22 constituting the fuel bundle 21, as shown in FIG. 9, a fuel cladding tube 35 is filled with a nuclear fuel material 36 (fissionable material) which is a pellet or particle size fuel material. Plutonium and recovery uranium are used as the fuel material which is the nuclear fuel material. Further, as conventionally made, natural uranium or depleted uranium may be mixed with plutonium. However, in the case where the aforesaid plutonium and recovery uranium are used as the fuel material, the number of neutrons generated from materials other than plutonium is increased, so that a breeding ratio can be made high. The fuel rods of the fuel bundle 21 is formed into a triangular arrangement so as to improve a filling density of fuel rods 22. For example, the fuel rods 22 are arranged by relatively shifting them by a fuel rod single pitch in a column direction in a manner such that that an even-column fuel rod 22 is positioned between odd-column fuel rods. In this manner, three fuel rods adjacent to each other are closely arranged so as to form an equilateral triangle. In this fuel bundle 21, the even-column fuel rod 22 is relatively shifted by only half pitch in the column direction with respect to the odd-column fuel rod 22. The relative shift serves to make small a fuel rod pitch in a line direction, further improving the arrangement density of the fuel rods 22. The odd-column fuel rod 22 has been relatively shifted by the half pitch with respect to the even-column fuel rod 22. In place of doing so, even if the odd-column fuel rod 22 is relatively shifted by only half pitch in the line direction with respect to the odd-column fuel rod 22, substantially the same effect as that described above will be obtainable. Since the fuel rods 22 constituting the fuel bundle 21 has a triangular arrangement structure, these fuel rods 22 can be closely arranged and the filling density of fuel can be improved as compared with a square arrangement structure in which fuel rods 22 are arranged in a lined-up form in longitudinal and traverse directions, i.e. column and line directions. The existing zirconium alloy (zircaloy) material or stainless steel material is used as the material for the fuel cladding tube 35 of the fuel rod 22. In the case where the stainless steel having high mechanical and physical strength is used in place of the existing zirconium alloy (zircaloy) material, a wall thickness of the fuel cladding tube 35 can be made thin. In the case of using the fuel cladding tube 35 made of stainless steel, when a fuel rod diameter is 10 mm.phi., the wall thickness of the fuel cladding tube is set to 0.25 mm to 0.4 mm, preferably, to a degree of 0.3 mm. Therefore, the wall thickness can be made thinner as compared with about 0.5 mm in the case of using zirconium alloy. As described above, the wall thickness of the fuel cladding tube 35 is made thin, which serves to increase the fuel volume ratio and the ratio of water to the fuel is lowered, and the ratio, thus being applicable to the reactor core 10 whose breeding ratio is at least about 1. The reactor core 10 is one applicable for a fast spectral reactor. Therefore, even if the stainless steel is used, the neutron absorption by the structural material is small like the case of zirconium alloy used in the existing light water reactor. The fuel assemblies 11 are composed of the normal fuel assemblies 13 and the partial fuel assemblies 14 as shown in FIG. 1 to FIG. 3. The control rod 18 having a cross-shaped traverse section is provided between the four fuel assemblies 11 adjacent to each other so as to be freely taken in and out. As shown in FIG. 4, a protrusion 38 is provided on the central portion at which the blade 18a of the control rod 18 does not reach in the gap defined between the fuel assemblies 11. The protrusion 38 is provided on the central portion of the outer side of the channel box 20 along the longitudinal direction thereof. Further, the protrusion 38 constitutes a water removal space to lower the ratio of water to fuel. The interior of the protrusion 38 may be formed into a hollow, and as described above, since the protrusion 38 is provided, or the interior of the protrusion 38 is formed so as to provide a hollow structure, the breeding ratio can be increased and a void reactivity can be reduced. The channel box 20 of the fuel assembly 11 is provided with a support pad 39 at an outer side on the upper portion thereof. The support pads 39 are provided at four portions on the outer side of the channel box 20. These support pads 39 serve to dispense an upper lattice plate which functions as a fuel assembly fixing frame used in the existing light water reactor and also serve to make small a gap defined between fuel assemblies 11. Therefore, it is possible to increase a fuel volume ratio and to lower the ratio of water to fuel. In FIG. 3, a reference numeral 40 denotes an orifice for guiding the coolant into the fuel assembly 11. Further, in the reactor core 10, the partial fuel assemblies 14 and the normal fuel assemblies 13 are arranged in combination with each other. According to this arrangement, if a reactor power output rises up and the void reactivity increases, neutrons generated in the reactor core 10 leak out at an upper portion of the reactor core through the streaming path 16 of the partial fuel assembly 14. In this manner, the neutron leakage effect is obtained in the core axial direction, and thereby, the void reactivity can be made negative. The partial fuel assembly 14 is a fuel assembly which has a fuel effective length LP of an exothermic portion shorter than the height of core. The normal fuel assembly 13 is an ordinary fuel assembly which has a predetermined fuel effective length LM and is substantially equal to a fuel effective height of core. In the case where the entire length of the fuel assembly 11 is approximately 4 m, the maximum length (fuel effective length) LM of the exothermic portion of the normal fuel assembly 13 is set to, for example, 2 m or less. On the other hand, the maximum length (fuel effective length) LP of the exothermic portion of the partial fuel assembly 14 is set to, for example, 1 m or less. According to this arrangement, the core diametrical direction size is made the same as the conventional light water reactor while the void reactivity being made negative. The control rod 18, which is inserted between the fuel assemblies in four groups so as to be freely taken in and out, is constructed as shown in FIG. 10. The control rod 18 has the entire length equal to the entire length of the fuel assembly 11 of, for example, about 4 m. A neutron absorption substance such as B.sub.4 C, hafnium or the like is stored in a lower half portion of the control rod 18. The lower half portion of the control rod 18 is constructed as a control rod absorber 43. A follower 45 is formed on the upper portion of the control rod absorber 43 through an intermediate partition plate 44 having a hollow structure. The intermediate partition plate 44 is formed of stainless steel, for example. A reference numeral 46 denotes a supporting portion of the control rod 18. The follower 45 is formed of a thin-walled steel plate and is formed with a flat hermetic (sealed) space 47 having an interior which is filled with by an inert gas such as helium or the like. This hermetic space 47 forms a water removal space. When the control rod 18 is withdrawn, as shown in FIG. 3, the follower 45 is positioned correspondingly to the fuel effective length portion (exothermic portion) LM of the fuel assembly 11 so as to prevent the coolant from flowing therein, while forming a coolant removal space. Thus, the ratio of water in the gap between the fuel assemblies 11 to the overall volume is lowered, and also, the ratio of water to fuel is lowered. The control rod absorber 43 of the control rod 18 has a length in the axial direction equivalent to the fuel effective portion (exothermic portion) LM of the normal fuel assembly 13. Moreover, in the reactor core 10 of the boiling water reactor, the shaft brackets 15 are provided at the upper and lower portions of the normal fuel assembly 13 and at the lower portion of the partial fuel assembly 14 so as to absorb the neutrons leaking from the reactor core 10. For example, stainless steel having a neutron absorbing ability is used as the material for the shaft bracket 15. Further, the shaft bracket 15 is not provided on the upper portion of the partial fuel assembly 14. According to such arrangement provided with no shaft bracket, a neutron leakage to the upper portion of the partial fuel assembly 14 is increased in the void increase, and the void reactivity is made negative even if the breeding ratio is at least about 1. Therefore, an inherent stability of the reactor core 10 can be ensured. Next, the operation of the reactor core 10 of the boiling water reactor of this embodiment will be described hereunder. The reactor core 10 has a volume ratio of water to fuel, which is less than 1, preferably, about 0.5 or less, and is remarkably smaller than the conventional reactor core of a light water reactor having a volume ratio of water to fuel, which is about 2.0 to 2.5. A ratio of coolant channel cross section to the fuel cross section of the reactor core 10 is set preferably to about 0.5 or less. Accordingly, in the reactor core 10, a fissionable material such as plutonium or the like in the fuel is subjected to a fissile reaction by a neutron, and the heat and neutrons are generated. A part of high energy neutrons (fast neutron) produced through the fissile reaction leaks outside the reactor core 10. However, most of high energy neutrons is moderated and scattered by the water as a coolant flowing between fuel rods 22, between these fuel rods 22 and the channel box 20, and between the channel box 20 and the control rod 18, and then, are again incident upon the fuel rod 22, thus contributing to the fissile reaction or the neutron absorption reaction. In the case where the volume ratio of water to fuel is about 0.5, a moderation (slow-down) effect by water is small, and an average neutron energy is an energy for a water cooling reactor close to sodium fast breeder reactor. For this reason, the ratio of neutron capture reaction by fissionable material is small like the existing light water reactor, and the neutron per neutron absorption is much generated, for example, two or more. Thus, the neutron absorbed in a parent material (element) such as uranium 238 (U-238) or the like is much increased, and it is possible to set the breeding ratio to about 1, preferably, to a range from 1.0 to 1.1. In the reactor core structure mentioned above, it is possible to set the breeding ratio to at least about 1, so that a utilization (capacity) factor of uranium resource can be greatly improved. More specifically, the utilization factor is about 100 times as much as in a case of the conventional utilization factor. Thus, even in the reactor core having the same dimension as the core diametrical direction size of the conventional boiling water reactor, the void reactivity can be made negative in the overall operating range. Therefore, it is possible to obtain negative reaction feedback characteristic and to secure inherent stability. Further, effective utilization of the fuel can be achieved, and also, environmental protection and economy can be simultaneously satisfied. FIG. 11 shows a second embodiment of a reactor core according to the present invention. The reactor core shown in this second embodiment is constructed in a manner that a rectangular and cylindrical hermetic container 50 is provided on an upper portion of the partial fuel assembly 14 charged in a reactor core 10A. The entire reactor core structure and the supporting structure of the fuel assembly 11 are substantially the same as those shown in FIG. 2 and FIG. 3. Therefore, like reference numerals are used to designate the same components as these of the first embodiment and their details are omitted. The reactor core shown in FIG. 11 is applied to a water cooling reactor such as light water reactor, for example, to a boiling water reactor. In the reactor core 10A, a number of rectangular cylindrical fuel assemblies 11 are charged in a state of being arranged at an equal pitch in longitudinal and traverse direction. The fuel assemblies 11 charged in the reactor core 10A are composed of at least two kinds, that is, normal fuel assemblies 13 each having a normal fuel effective (exothermic portion) length LM and partial fuel assemblies 14 each having a shorter fuel effective length LP as shown in FIG. 3. The upper portion of the partial fuel assembly 14 is formed with a streaming path 16. The streaming path 16 is formed by providing the cylindrical hermetic container 50 used as an empty can on the upper portion of the partial fuel assembly 14. The cylindrical hermetic container 50 is made of zirconium, zircaloy or aluminum material having a small neutron absorption cross section, and an inert gas such as helium, argon or the like is encapsulated or sealed as a seal gas in the interior thereof. As described above, the hermetic container 50 is made of zirconium, zircaloy or aluminum material, and it is therefore possible to make small a neutron collisional reaction of a neutron and the structural material of the hermetic container (hermetic container itself). Further, the hermetic container 50 is housed in the upper portion of the cylindrical channel box 20 of the partial fuel assembly 14 so as to form a water removal space. FIG. 12 and FIG. 13 are top plan views showing the normal fuel assembly 13 and the partial fuel assembly 14 charged in the reactor core of the water cooling reactor, respectively. The normal fuel assembly 13 and the partial fuel assembly 14 of the fuel assembly 11 are housed in the channel box 20 forming as rectangular cylindrical outer housing so as to form the fuel bundle 21 as a fuel element bundle therein. In the fuel bundle 21, a large number of fuel rods 22 is formed into a bundle by means of the fuel spacer, for example, the grid spacer 25 as shown in FIG. 5 to FIG. 7 so as to form a substantially square shape in its plane and are closely arranged in the channel box 20. The fuel bundle 21 is constructed in a manner that three fuel rods (fuel pin) adjacent to each other are arranged so as to provide an equilateral triangular shape, and then, is formed into a square bundle shape (rectangular shape), as a whole. A plurality of protrusions 51 for engagement are provided on an inner side of the channel box 20. These protrusions 51 are provided so as to correspond to unevenness on the outer side of the fuel bundle 21 and attains a function as a guide. Further, the protrusions 51 extend along the axial direction of the channel box 20 and form water removal space. Moreover, the protrusions 38 shown in FIG. 4 may be provided on the outer sides of the channel box 20, and if the protrusions 38 are provided, the ratio of water to fuel can be made smaller. The inner side of the channel box 20 is provided with the protrusions 51, each facing recess portion of the outer side of the fuel bundle 21. According to this arrangement, it is possible to make small a gap defined between the fuel bundle 21 and the channel box 20. Further, the protrusions 51 are provided so as to correspond to unevenness formed on the outer sides of the fuel bundle 21, and it is therefore possible to lower the ratio of water to nuclear fuel and to increase the breeding ratio of the nuclear fuel, while the void reactivity can be reduced. The fuel bundle 21 housed in the channel box 20 is constructed as shown in FIG. 14. More specifically, a coolant removal rod 52 is arranged on the center in each gap between three fuel rods 22 which have a triangular arrangement structure in its fuel rod arrangement. The coolant (water) removal rod 52 is made of zirconium, zircaloy or aluminum material having a small neutron absorption cross section. Preferably, the coolant removal rod 52 is formed into a shape of a hollow tube so as to restrict a neutron absorbing moderation by the structural material of the coolant removal rod 52. The coolant removal rod 52, which does not contain a nuclear fuel, is provided in each gap between fuel rods 22 to reduce the amount of coolant, and a change in the coolant amount after and before the void becomes small. In a water cooling reactor of a fast spectral system, a positive void reactivity occurs. The principal factor is as follows. More specifically, a moderation of neutron is reduced due to void effect of coolant, and the neutron spectrum is hardened, and thus, a neutron per absorption reaction of fissionable material (nuclear fuel material) is much generated and increase in its number. In the cooling water reactor, a factor of the positive void reactivity is eliminated by decreasing an amount of the coolant existing in the reactor core 10A, and the neutron leakage effect which is a factor of negative void reactivity, is unchanged, so that the void reactivity can be reduced as a whole. Next, the operation of the reactor core of the second embodiment will be described hereunder. The reactor core 10A is applied to a water cooling reactor, and during normal operation, in the reactor core 10A, a nuclear fissionable material (a nuclear fuel) such as plutonium or the like is mainly subjected to the fission reaction by means of neutrons so that heat and neutrons (mainly, fast neutrons) are generated. A part of generated neutrons leaks outside the reactor core 10A. However, most neutrons are moderated and scattered by water serving as a coolant flowing between the fuel rod 22 and the channel box 20 or between the channel boxes 20, and then, are again incident upon the fuel rod 22 so as to cause a fission reaction or neutron absorption reaction. If the ratio of water to fuel is small, the neutron moderation effect by water is small, and an average neutron energy is close to a sodium water cooling type fast breeder reactor. In the reactor core 10A, the hermetic container 50, which is filled with a sealed gas, is provided on the upper portion of the partial fuel assembly 14. Thus, the hermetic container 50 serves to remove the water as the coolant, and the ratio of water to fuel is small, and further, the number of generated neutrons per neutron absorption is two or more. Therefore, the number of neutrons absorbed in the parent material such as U-238 is much, and as a result, the breeding ratio can be increased. An empty space in the hermetic container 50 is filled with a gas, and for this reason, an atomic abundance density is lower than a state that water is boiled, and scattering reaction of neutrons is hard to occur. Therefore, the neutrons are easy to pass through the hermetic container 50, so that the neutrons can easily leak from the reactor core 10A in the core axial direction. For this reason, the coolant of the core fuel portion or the coolant of the streaming channel portion constituting the streaming path 16 can facilitate a leakage of voided neutron in the core axial direction, and it is possible to lower the void reactivity and to make it negative value. In this case, aluminum, zirconium or zirconium alloy (zircaloy) is used as the material of the hermetic container 50 having an empty space, and it is therefore possible to make small neutron collision reaction of the neutron and the structural material of the hermetic container 50. On the other hand, if the hermetic container 50 is made of a high strength material such as stainless steel or the like, iron or nickel (Ni) is contained in the stainless steel. Thus, a neutron absorption cross section of iron or the like is relatively large, and an exothermic reaction will be caused by the neutron absorption. However, aluminum has a neutron absorption cross section smaller about one place in figure than that of iron. Further, zirconium or zircaloy has a neutron absorption cross section which is about a half of the neutron absorption cross section of nickel, and the exothermic reaction caused by neutron absorption is decreased. If aluminum, zirconium or zirconium alloy (zircaloy) is used as the material of the hermetic container 50, neutron absorption by the hermetic container 50 is reduced. On the other hand, neutron absorption by the parent material (U-238) in nuclear fuel material is relatively increased. The U-238 is made into .sup.239 Pu (Pu-239) by neutron absorption, and is used as a nuclear fuel, thus increasing the breeding ratio. Moreover, a scattering cross section of neutron is substantially the same as that of an element such as aluminum or zirconium. However, aluminum or zirconium has a metallic atomic density smaller than that of stainless steel, and therefore, a neutron scattering is hard to be caused. Thus, the neutron is easy to pass through the hermetic container 50 in the axial direction, that is, streaming is easy to be made, so that the void reactivity can be further lowered. In the fuel assembly 11, the coolant removal rod 52 is provided between fuel rods 22 which form a triangular arrangement, and the protrusion 51 is provided in the channel box 22. According to this arrangement, an amount of water which is a coolant is reduced, so that the void reactivity can be further lowered. At this time, the protrusions 38 as shown in FIG. 4 are additionally provided on the central portions of the outer sides of the channel box 20, thus an amount of coolant being further reduced. Therefore, the protrusion 38 performs a function of lowering the void reactivity. In the fuel assembly 11 shown in FIG. 12 and FIG. 13, although there is shown an example in which the protrusion 51 has been provided in the channel box along the longitudinal direction of the channel box 20, the protrusion 51 may be constructed as shown in FIG. 15. The protrusion 51 shown in FIG. 15 is formed so as to have an inner hollow structure. Therefore, the structural material (protrusion itself) reacting with a neutron is reduced in its amount so as to further increase the breeding ratio. The neutron leakage to the core axial direction is easy to be caused, so that the void reactivity can be lowered. Next, a third embodiment of a reactor core according to the present invention will be described hereunder with reference to FIG. 16 and FIG. 17. In the reactor core of this third embodiment, the fuel assembly 11 charged in a reactor core 10B is improved. The entire construction of the reactor core of this embodiment substantially the same as that of FIG. 1 and FIG. 2, and therefore, the details are omitted. In the reactor core 10B, the fuel assemblies 11 are provided with support pads 55 at upper portions on the outer peripheries thereof. The support pad 55 serves to dispense an upper plate lattice which functions as a fuel assembly fixing frame used in the existing light water reactor. The support pad 55 has an L- or V-letter shape in plane as shown in FIG. 17 and is provided at each of four corners on the upper portion of the channel box 20 of the fuel assembly 11. The support pads 55 are provided at the corner portions of the upper side on the outer periphery of the fuel assembly 11 so as to make it possible to dispense or eliminate a fuel assembly fixing frame for supporting the top portion of the fuel assembly 11 in the horizontal direction. Thus, a gap between fuel assemblies 11 is made narrow, and the fuel assembly is much charged in the reactor core, so that the breeding ratio can be increased. The L-shaped support pad 55 is attached so as to ride on each corner portion of the cylindrical channel box 20 of the fuel assembly 11 from the side portion. Thus, the fuel assemblies 11 adjacent to each other are stably supported by means of two support pads 55 which contact with each other at two places on both sides in the widthwise direction of the channel box 20. Therefore, it is possible to surely prevent the fuel assemblies 11 from directly contacting to each other and to ensure a gap for inserting the control rod 18 between adjacent fuel assemblies 11 so that the control rod 18 can be stably withdrawal. FIG. 18 shows a fourth embodiment of a reactor core according to the present invention. In the reactor core of in this fourth embodiment, a fuel assembly 56 charged in the reactor core has an improved structure. The fuel assembly 56 of this embodiment is not constructed in a manner that the normal fuel assembly 13 and the partial fuel assembly 14 are combined. The fuel assembly 56 of this embodiment includes a normal fuel element region 57 and a partial fuel element region 58. The normal fuel element region 57 and the partial fuel element region 58 are formed through a coolant channel partition wall 59 and are housed in the rectangular cylindrical channel box 20. In FIG. 18, there is shown an arrangement example such that the partial fuel element region 58 is formed on the central portion of the fuel assembly 56, and the normal fuel element region 57 is formed at the peripheral portion of the partial fuel element region 58. The normal fuel element region 57 is formed in a manner that a normal fuel element having an ordinary fuel effective (exothermic portion) length LM is arranged, and on the other hand, the partial fuel element region 58 is formed in a manner that a short-dimension fuel element having a short fuel effective length LP is arranged. Both the normal and short-dimension fuel elements comprises a fuel rod, for example. Flow rates of the coolant guided into the normal fuel element region 57 of the fuel assembly 56 and the partial fuel element region 58 thereof are suitably distributed by means of an orifice 60, which is provided on the upper portion of the fuel assembly 56. According to this structure, the upper portion of the partial fuel element region 58 is voided and a neutron streaming path 61 is formed on the voided portion. At this time, the partial fuel element region 58 facilitates the leakage of neutron in the core direction, so that the void reactivity can be lowered. An empty can-like hermetic container as shown in FIG. 11 and FIG. 13 may be provided on the upper portion of the partial fuel element region 58. In the above fourth embodiment of the arrangement mentioned above, the partial fuel element region 58 is formed on the central portion of the fuel assembly 56, and the normal fuel element region is formed at the periphery of the partial fuel element region 58. In this fourth embodiment, the normal fuel element region 58 and the partial fuel element region 57 may be arranged in the manner reverse to the above arrangement, or various modifications may be made. Further, it may be possible to divide the region into three or more regions other than two regions, and each region may be properly selected as a normal fuel element region or as a partial fuel element region. Each of the above embodiments shows the example of the reactor core in which fuel rods as fuel elements are arranged in the fuel assembly 11 so as to form a triangular structure. These fuel rods 22 may be arranged so as to form a square, like the existing fuel assembly as shown in FIG. 19. In this case, in order to decrease the volume ratio of water to nuclear fuel of the fuel assembly charged in the reactor core, the coolant removal rod 52 is provided at the central portion between four adjacent fuel rods 22 which are mutually arranged so as to provide a square shape. Preferably, the coolant removal rod 52 is formed with an inner hollow structure so as to restrict moderation of neutrons. The coolant removal rod 52 is provided at the central portion between fuel rods 22 which are arranged so as to form a square, and the coolant having a large neutron moderation is removed. The coolant removal rod 52 is formed with the inner hollow structure, and the moderation of neutrons by the structural material (coolant removal rod) is restricted. Therefore, the breeding ratio can be increased. In this case, it is possible to facilitate a leakage of neutron in the axial direction of the coolant removal rod 52, and neutron leakage effect is enhanced. Therefore, the void reactivity lowering effect is slightly caused, thus contributing to lowering of the void reactivity. FIG. 20 shows a modified example of FIG. 19, in which the coolant removal rod 52, which is provided at the central portion between fuel rods 22 arranged so as to provide a square shape, is made of aluminum, zirconium or zircaloy having a low neutron absorption cross section. As described above, since the coolant removal rod 52 is made of a material having a low neutron absorption, the neutron absorption is restricted and the breeding ratio can be increased. In the embodiments of the present invention described above, although the reactor core is applied to the boiling water reactor, the arrangements or structures of the normal fuel assembly and the partial fuel assembly are applicable to a reactor core of a pressurized water reactor. In the case of being applied to the pressurized water reactor, a position where the fuel effective portion of the partial fuel assembly is formed is not specially limited, and the fuel effective portion may be in line with the upper side of the fuel effective portion of the normal fuel assembly. Further, a cluster type control rod is used as the control rod. According to the cluster type control rod, the control rod is taken in and out of the fuel assembly from the upper portion thereof. For this reason, in the cluster type control rod, a follower forming a water removal space is provided on the lower portion of the control rod absorber. In this manner, the present invention is applicable to a water cooling reactor which uses water as a coolant. As is evident from the above explanation, in the reactor core according to the present invention, the fuel volume ratio is increased so as to lower the ratio of water to fuel, and it is therefore possible to increase a breeding ratio and to improve the utilization factor of uranium resource. Thus, the uranium resource can be effectively utilized, and environmental protective, stability and economy can be greatly improved. It is to be noted that the present invention is not limited to the described embodiments and many other changes and modifications may be made without departing from the scopes of the appended claims.