Patent Number: 052672867
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment according to the present invention will be described hereunder with reference to FIGS. 1 to 7. In this embodiment, a fuel assembly 10 is composed of four small fuel bundles 30, fuel assembly upper portion tie members 12b, channel boxes 17, a water cross 50 and fuel assembly lower nozzles 18. The water cross 50 is integrally welded to the channel box 17 to divide the inside of the channel box 17 into four coolant flow passages. The four small fuel bundles 30 are respectively provided with upper tie plates 12a and lower tie plates 13a and arranged in flow passages surrounded by the central water cross 50 and the channel boxes 17 with the lower tie plates 13a being mounted on the fuel assembly lower nozzles 18. The upper and lower end portions of the fuel rods 11 are supported by the upper and lower tie plates 12a and 13a. A plurality of spacers 16 are arranged along the axial direction of the fuel bundles 30 to properly keep the gaps between the mutually adjoining fuel rods 11. The fuel spacers 16 are supported in their axial directions by fuel rods provided with tabs, not shown. The channel boxes 17 are fastened to the fuel assembly lower nozzles 18 with fastening screws 22 to thereby surround the outer peripheries of the four fuel bundles 30, respectively, thus constituting one fuel assembly unit. The lower tie plate 13a mounted on the lower nozzle 18 has, at its upper end, a fuel rod support 14a which is provided with, a coolant inlet from space 15. As shown in FIG. 7, each of the fuel rod 11 is composed of a cladding tube 45 into which a plurality of fuel pellets 48 are charged and upper and lower ends of the cladding tube 45 are plugged with plugs 46 and 47. A gas plenum 49 is formed to the inner upper portion of the cladding tube 45. FIGS. 3 and 4 represent detailed structure of the water cross 50 providing one feature of the present invention. Referring to FIGS. 3 and 4, the water cross 50 is constituted by four plate members 51 each in substantially L-shape, the channel box 17 and, coolant rising and lowering passages 53 and 54 both surrounded by the L-shaped plate members 51 and the channel box 17. FIG. 4 is an illustration of sectional view taken along the line IV--IV of FIG. 3, and referring to FIG. 4, the upper and lower end portions of the cross water 50 are closed by upper and lower cover members 55 and 56, and partition plates acting as flow passage sectioning spacers 52 are arranged to keep gaps between the L-shaped plate members 51 to thereby form the coolant rising and lowering passages 53 and 54. A guide tube 24 for guiding a control element is fitted in the central portion of the water cross 50 and supported by the L-shaped plate members 51. The guide tube 24 penetrates the upper cover member 55 at its upper end. The upper end of the guide tube 24 is opened upward. The lower end of the guide tube 24 penetrates the lower cover member 56 and extends downward into a through hole 62, shown in FIG. 5E, of a fuel assembly lower nozzle 18. The flow passage sectioning spacers 52 are disposed at blade (or wing) portions of the water cross 50 and divides the flow passage of this blade portions into the coolant rising passages 53 and the coolant lowering passages 54. In the illustrated structure, the flow passage adjoining the control element guide tube 24 is referred to as the coolant lowering passage 54. The flow passage sectioning spacer 52 has a conjunction port 57 positioned below the upper cover member 55 to connect the coolant rising passage 53 with the coolant lowering passage 54. The sectioning spacer 52 extends towards the lower cover member 56 to divide the coolant passages. The lower portions of the blade portions of the water cross 50 are closed by the lower cover member 56 positioned below the fuel rod support portion 14a of the lower tie plate 13a. The coolant rising passage 53 has a coolant inlet 42a positioned above the fuel rod support portion 14a. The coolant lowering passage has a drain port 43a towards the control element guide tube at a portion near the lower end thereof. The details of the lower nozzle or nozzles 18 and its associated members are described hereunder with reference to FIGS. 5A to 5D. A fuel assembly weight is applied to the upper portion of the fuel assembly lower nozzle 18 because the lower tie plate 13a of the fuel bundle 30 is mounted on the fuel support seat 61. The lower end of the water cross 50 is inserted into grooves 64 formed to the fuel support seat 61 to thereby separate the coolant flow passages of the fuel bundle 30 so as not to be connected between them. Namely, as shown in FIG. 5A, the grooves 64 are also have a cross shape as a whole. A control element guide tube support tube 68 is arranged at the central portion of the grooves 64 to form a through hole 62 to be inserted by the control element guide tube 24. Extended portions 65 constituting the grooves 64 extend to the lower portion of the lower nozzle 18 to thereby divide into sections four lower nozzle flow passages 63. Orifice plates 23 are disposed at the lower portions of the lower nozzle flow passages 63, respectively, to form orifices 67. The lower end portion of the control element guide tube 24 is connected to the control element guide tube 24a secured to a fuel support fitting 20. The fuel support 20 is fitted into an upper opening of the control rod guide tube 71 and is provided with an opening 25 at its central portion so as to act as a guide for the insertion of the cruciform control rod blade into the water gap between the fuel assemblies 10 as shown in FIG. 1. Surrounding the cross shaped through hole 25, there are arranged four openings 73 for receiving the fuel assembly lower nozzles 18, respectively, to form a coolant flow passage of the fuel support 20. One such coolant flow passage is formed to one fuel assembly surrounding the cruciform control rod blade. In the central portion of the opening 73, the control element guide tube 24a is disposed. The guide tube 24a is connected to the control element guide tube 24, penetrating the fuel support 20, having a control element insertion hole 26 at the bottom of the support 20 and then supported by a support plate 69. The support plate 69 has an opening 27 through which the coolant passes. The fuel support 20 is provided with a lower side surface to which orifices 21 for the coolant flow passage conjunction with the openings 73 are formed at a portion facing the openings 72 of the control rod guide tube 71. FIG. 6 shows an appearance of the control rod 6 utilized in combination with the fuel assembly according to the present invention. The control rod 6 corresponds to that shown in FIG. 23 but additionally provided with columnar control elements 7 and mainly composed of sheaths 117 control rod blade provided inside with poison tubes 118 and the control elements 7. The control element 7 may be composed of a hollow tube made of SUS into which neutron absorbing poison such as B.sub.4 C, Hf is packed, or composed of mere a hollow tube or SUS rod. Further, in the illustrated example, a speed limiter and a control rod connection-separation handle of conventional structure are not located. The operation and function of the fuel assembly of the present embodiment will be described hereunder by assuming that the fuel assembly is charged in the core of the BWR. As shown in FIG. 1, the coolant is guided into the coolant flow passage between the fuel rods 11 of the fuel bundle 30 through the opening 72 of the control rod guide tube 71, the coolant passing orifice 21 of the fuel support 20, the opening 67 of the lower nozzle 18 of the fuel assembly, and the through hole formed to the fuel rod support member 14a of the lower tie plate 13a. One part of the coolant flown in the coolant guide port 15 of the lower tie plate 13a flows towards a bypass flow passage, which is outside a channel flow passage, through a leak hole formed to the lower nozzle 18. One part of the coolant flown in the upper portion of the fuel rod support member 14a is flown, as shown in FIGS. 3 and 4, into the coolant rising passage 53 through the coolant inlet 42a of the water cross 50, and then drained inside the control element guide tube 24 through the conjunction port 57, a flow lowering passage 54 and the drain port 43a at a portion near the lower end of the flow passage of the water cross. In a case where the front (upper) end of the control element 7 is positioned below the drain port 43a, the coolant flown through the coolant drain port 43a becomes the liquid phase and/or steam phase in accordance with the flow rate of the coolant flown through the coolant inlet port 42a in response to the core flow rate (refer to curve A in FIG. 8). In the present embodiment, the coolant inlet port 42a is disposed slightly above the fuel rod support member 14a for the reason that the pressure in the control element guide tube 24 is equal to that in the bypass flow passage, and accordingly, the coolant inlet port 42a is disposed above the fuel rod support member 14a for preventing the coolant from excessively flowing. This further has advantage for easily designing the openings diameter of 42a and 43a. So, depending on desighs, the coolant inlet port 42a is disposed under the fuel rod support member 14a. In a case where the front end of the control element 7 is positioned above the drain port 43a, there causes a large flow resistance because the drain port 43a is closed by the control element 7. A steam void is caused in the inside of the water cross 50 by the heating and heat transfer due to neutron and .gamma.-rays, which results in the increasing of the pressure drop at portions of the drain port 43a, the coolant rising passage 53 and the coolant lowering passage 54. Accordingly, the water levels of the coolant rising and lowering passages 53 and 54 fall until the time when the pressure difference between the coolant inlet 42a and the drain port 43a is balanced to the water head and pressure drop in the water cross 50, thus the steam being filled up in the water cross 50. Furthermore, since the coolant is less supplied in the control element guide tube 24, the steam void is also caused and the inside of the water cross is hence almost filled up with the steam (refer to the curve C in FIG. 8). In a case where the control rod 6a is inserted into the upper portion, the control element 7 is also inserted into the upper portion of the control element guide tube arranged centrally in the water cross 50. In this case, the upper end of the flow passage 58 is opened, and hence, the coolant is discharged through the upper end opening without increasing its pressure. According to such structure, the control rod can be easily inserted, thus preventing the fuel assembly from jumping at the control rod insertion operation. In a case where the control rod is withdrawn downward, the pressure in the control element guide tube is reduced, but since the steam occupies the inside of the control rod guide tube during power operation state and its steam is expanded, the degree of such pressure reduction is small. In the shut-down state, since non-boiling water occupies the inside of the control element guide tube, the flow resistance becomes small and the non-boiling water is counterflown through the upper end opening, thus easily withdrawing the control rod. The function of the fuel assembly 10 of the present invention charged in the BWR will be described hereunder. An example is taken to a case where 100% rated power is kept between the core flow rate of 80 to 115%. The core flow rate is kept to 80% during the almost period (about 70 to 80%) of the operating cycle, thereby compensating against the reactivity change due to the burning of the fuel by adjusting the reactivity by means of the control rod. In the fuel assembly with the control rod being drawn out, the axial direction of the control rod is set so as to position the upper end of the control element 7 to a position above the coolant drain port 43a of the water cross 50 and below the fuel active region of the fuel assembly. The number of the fuel assemblies, in which the upper ends of the control elements 7 are set to portions below the drain port 43a by further drawing downward the control rods from the time when the rated power cannot be kept even by entirely drawing out the all control rods from the core fuel active regions, is increased in response to the reduction of the reactivity. Furthermore, the core flow rate is finally increased so as to obtain the core maximum flow rate of 115% at the end of the operating cycle, whereby the core reactivity at the end of the cycle is increased and the cycle life can thus be expanded. When the pressure difference (between the inlet and outlet ports of the water cross of the present invention)--the void factor characteristic is set to the curve A in FIG. 8 with respect to a case where the upper end of the control element 7 is positioned below the drain port 43a, the inside of the water cross is kept with the void factor less than 10% in a core flow rate range (in this example, 80 to 115% rated core flow rate) utilized in the rated power operation period. Accordingly, there causes no dispersion of the void fraction inside the water rod between the fuel assembly due to the power distribution (curve B in FIG. 8) caused by the water rod of the conventional design of the fuel assembly. Further, in a case where the upper end of the control element 7 is positioned above the drain port 43a, the void fraction inside the water cross is kept more than 80%, as shown by the curve C in FIG. 8, in the core flow rate range operated with the rated power. Accordingly, at the power operation period, for the control rods except for those inserted into the fuel active region of the fuel assemblies for controlling the excessive reactivity and the core power distribution, the void fraction inside the water cross can be kept more than 80% by setting the control rods at axial portions at which the upper ends of the control elements 7 are positioned just above the drain ports 43a and below the fuel active region in the core flow rate range which is utilized at the rated power without lowering the power at the lower portions of the fuel assemblies. As this result, according to the present invention, the steam void can be caused inside the water cross 50 by the axial position control of the control rod without being influenced with the core flow rate, and the power level and the axial power distribution of the fuel assembly throughout the almost operation period of cycle with the core flow rate being less than 100%, whereby the production of the plutonium 239 can be facilitated under the suppression of the neutron moderating. Furthermore, in a case where the core flow rate is largely reduced at the reactor starting period or shutdown operation period, for example, at the time of less than 65% rated core flow rate, the void fraction inside the water cross can be kept high regardless of the position of the control rods, so that the inclination of the curve, representing the reactor core flow rate--power curve, become large and the core power control can be hence easily done, which is the same merit as that in the conventional design of the fuel assembly with the water rod 9. Still furthermore, since the void fraction in the water cross can be precisely controlled by the control rods, the evaluation accuracy of the thermal limitation, the power distribution, the exposure distribution and the reactivity can be remarkably improved, with the three dimensional nuclear-thermal-hydraulic simulation code, as well as the improvement in the monitoring of the core performance. In addition, since the control elements can be inserted into the inside of the central control element guide tube of the water cross of the fuel assembly, the core shut-down margin can be increased in comparison with the fuel assembly of the conventional structure. Other embodiments according to the present invention will be described hereunder with reference to FIGS. 10 to 20. First, referring to FIG. 10 representing a second embodiment of the fuel assembly according to the present invention, the L-shaped plate members 51 constituting the water cross are press bent and welded to form coolant flow sections. In the first embodiment, the flow passage sectioning spacers of the water cross are utilized for forming the coolant rising passage and the coolant lowering passage. According to the structure of this second embodiment, the spacers 52 of the first embodiment can be eliminated for forming the coolant rising and lowering passages 75 and 76, thus simplifying the structure. Next, FIG. 11 represents a third embodiment according to the present invention, in which a hollow tube 52b is arranged as flow passage sectioning member at an outside of the control element guide tube 24 concentrically therewith and in which the blade portions of the water cross are formed as coolant rising passages 77 and an annular portion is formed as coolant lowering passage 78. In this embodiment, a plurality of support spacers are arranged in a hollow tube 52b along the axial direction of the fuel assembly and support the control element guide tube 42, not shown in FIG. 11, having structure capable of passing the coolant through the annular portion. According to this structure of the fuel assembly, the control element guide tube and the water cross can be made simple, thus being easily manufactured. In the fuel assembly of FIG. 11, when the water cross has thin thickness, the hollow tube of the central control element guide tube is made too slender and, hence, only small contribution is achieved to the increasing of the control rod worth, a countermeasure is such that the outer diameter of the control element guide tube may be made large by removing the fuel rods adjoining the central portion of the water cross to increase the control rod reactivity. As described above, such improved structure may be utilized in combination with the water cross of the structure shown in FIG. 2 or 10. FIGS. 12 and 13 further represent a fourth embodiment according to the present invention, in which a plurality of control element guide tubes 24 are arranged in the water cross 50. In this structure, the fuel bundle 30 has a lattice structure larger than that, i.e. 4.times.4 lattice structure, of the embodiment shown in FIG. 2. For example, this structure may be preferably adapted for the large-sized fuel assembly 78 having a water cross constituted by fuel bundles each in 6.times.6 lattice structure. By increasing the number of the control element guide tubes in this manner, the control rod reactivity can be increased and the reduction of the reactor shut-down margin in the enlargement of the fuel assembly can be improved. Furthermore, since the control elements 7 are inserted into the control element guide tubes disposed in the water cross, the number of the fuel rods per one fuel assembly is not reduced and the fuel packing amount in the fuel pellet is not reduced, thus being advantageous in the fuel economy. Furthermore, in such large-sized fuel assembly, since it is intended that the thermal neutron flux at the central portion of the fuel bundle is increased to make flat the power distribution, the water rod 5 of the conventional structure shown in FIG. 21 or the water rod 9 capable of having spectrum shift of the conventional structure shown in FIG. 25 may be utilized in combination. FIGS. 14 and 15 represent a fuel assembly of a fifth embodiment as a modification of the first embodiment, in which a further coolant inlet 42b is formed to the lower portion of the control element guide tube 24, and an opening is formed to a corresponding position of the hollow tube 68, shown in FIG. 5B, disposed at the central portion of the flow passage sectioning member 65 of the lower nozzle 18, so that the coolant passing the flow passage 63 can be taken inside the control element guide tube 24 through the inlet opening 42b. In this fifth embodiment, the coolant inlet opening 42b is formed above the orifice 23 acting as the flow passage resisting means in a view point of reducing the pressure difference between the openings 42b and 32 shown in FIG. 15 and preventing the extremely large coolant flow rate. As shown in FIGS. 14 and 15, in a case where the upper end of the control rod element 7 is positioned below the inlet opening 42b, the coolant through the lower nozzle 18 flows in the control element guide tube through the opening 42b and then flows out through the upper opening 32 in the upper plenum. As this result, non-boiling water flows inside the control element guide tube. This fact increases the amount of the moderator at the end of cycle and the area of the control element guide tube is occupied with the non-boiling water like the case of the water cross when it is required to facilitate the moderation of the neutron, thus increasing the moderating effect. In a case where the upper end of the control element 7 is positioned above the drain opening 43a, the opening 43a is closed by the control element 7 and the flow resistance at the drain port becomes large, so that the coolant flow amount flown out is reduced, whereby the steam void is caused in the water cross as described hereinbefore due to the heating and the heat transfer by the neutron and .gamma.-rays and the liquid surfaces of the flow passages 53 and 54 are depressed. In thus manner, the liquid levels in the flow passages 53 and 54 are depressed downward until the time when the pressure difference between the inlet port 42a and the drain port 43a has been balanced to the pressure drop and water head in the flow passage in the water cross. As this result, the inside of the water cross is filled up with the steam. Further, a large amount of the steam void is also caused in the control element guide tube because of the reduced coolant flow rate through the openings 42b and 43a. When the upper end of the control element 7 is moved downward below the coolant inlet port 42b by the downward withdrawal of the control element 7, the coolant flows into the control element guide tube through the coolant inlet 42b and rises upward, and at this time, the steam flow is sucked through the drain port 43a. Accordingly, the flow mode change, i.e. transformation from steam filling state to liquid single phase flow state, in the water cross is accelerated, and the power increasing of the reactor can be speedily changed in comparison with the case of locating no coolant inlet port 42b. FIGS. 16 to 18 represent a sixth embodiment of the fuel assembly 80 of the present invention, which is different from the embodiment of FIGS. 1 and 2 in which the control element guide tube is incorporated in the water cross. Namely, in the sixth embodiment, the fuel assembly 80 is provided with a water rod 19 in which a control element guide tube is incorporated, and FIG. 16 shows an example in which one polygonal, substantially square in illustrated cross section, water rod 19 is centrally disposed in the fuel assembly 80, but it may be possible to dispose, in alternation, a circular cylindrical water rod or a plurality of water rods. Furthermore, the water rod 5 of the conventional structure or the water rod 9 having the spectrum shift function may be utilized in combination. The water rod 19 is composed, as shown in FIGS. 17 and 18, of an inner tube 35, an outer tube 36, spacers 37 and a control element guide tube 24. The inner tube 35 and the control element guide tube 24 are supported by the spacers 37, and the upper end openings of the inner tube 35 and the outer tube 36 are closed by an annular end plug 38a. The control element guide tube 24 is connected with a control element guide tube 24a, having an upper portion extending beyond the end plug 38a and inserted into the upper tie plate 12 and supported thereby with an upper end opening 32 being opened above the upper tie plate 12. The inner tube 35 is provided with a conjunction hole 34 positioned below the end plug 38a so as to connect an annular flow passage 41 (coolant lowering passage), between the inner tube 35 and the control element guide tube 24, with an annular coolant rising passage 40, between the outer tube 36 and the inner tube 35. Each of the spacers 37 has an opening for ensuring the spaces for the coolant rising and lowering passages 40 and 41. The lower ends of the inner and outer tubes 35 and 36 are closed by an annular end plug 39a positioned above the fuel rod support member 14, and the end plug 39a is provided with a coolant inlet opening 42a connecting with the annular coolant flow passage 40. The control element guide tube 24 has a drain port 43a positioned above the annular plug 39a and the lower end portion of the guide tube 24 penetrates the fuel rod support member 14 and is supported by a guide tube support plate 23a. The lowermost end of the guide tube 24 is formed as a control element insertion opening 26 opened at the lower end portion of the lower tie plate 13. The fuel support 20 is fitted to the upper end opening of the control rod guide tube 71 and is provided, at its lower side surface, with coolant inlet ports 21 facing the openings 72 of the control rod guide tube 71 as shown in FIG. 16. The coolant inlet port 21 is formed to each of the four fuel bundles. The control element guide tube 24a is secured to the fuel support fitting 20 by means of a guide tube support plate 69 and the bottom portion of the fuel support 20. The upper end portion of the control element guide tube 24a is engaged with the lower end portion of the control element guide tube 24 of the water rod 19 in this embodiment. To the central portion of the fuel support 20 is formed a cross shaped opening into which the cross shaped control blades are guided and inserted so that the control blades are positioned at the central portion of the four fuel assemblies such as shown in FIG. 2. The coolant is guided, as shown in FIG. 16, to the coolant passage between the fuel rods 11 of the respective fuel rod bundles through the opening 72 formed to the side surface of the control element guide tube 71 and the coolant inlet orifice 21 formed to the side surface of the fuel support 20 and then through the through hole, not shown in FIG. 16, formed to the fuel rod support member 14 of the lower tie plate 13 of the fuel assembly. One part of the coolant flown in the coolant guide inlet 15 of the lower tie plate 13 flows into the bypass flow passage, which is outside the channel flow passage, through the leak hole, not shown in FIG. 16, formed to the lower tie plate 13. As shown in FIG. 18, the coolant flows into the coolant rising passage 40 through the coolant inlet 42a of the water rod 19 and then is drained into the inside 58 of the control element guide tube 24 through the conjunction hole 34, the coolant lowering passage 41 and finally the drain port 43a positioned near the lower end portion of the water rod 19. In a case where the front (upper) end of the control element 7 is positioned below the drain port 43a, the coolant flown through the drain port 43a is becomes the liquid phase or steam phase in accordance with the flow rate of the coolant flown through the coolant inlet 42a in accordance with the core flow rate (refer to the curve A in FIG. 8). In a case where the upper end of the control element 7 is positioned above the drain port 43a, the drain port 43a is closed by the control element 7 and the drain port resistance is hence increased. The steam void is caused in the inside of the water rod 19 due to the heating and heat transfer by the neutron and .gamma.-rays, whereby the pressure drop is increased at the drain port 43a and in the coolant rising and lowering passages 40 and 41, and the water levels in the passages 40 and 41 are lowered until the time when the pressure difference between the coolant inlet 42a and the drain port 43a is balanced to the pressure drop and the water head in the water rod passage. As this result, the inside of the water rod 19 is filled up with the steam, and furthermore, since the coolant is less supplied into the inside 58 of the control element guide tube 24, the void is caused in the inside 58 of the guide tube 24 and the steam is hence filled up therein (refer to the curve C in FIG. 8). When the control rod 6a is inserted into the upper portion, the control element 7 is also inserted into the upper portion of the central control element guide tube of the water rod 19. In this case, since the upper end of the inside passage 58 is opened, the inner pressure does not increase and the coolant is then discharged through this upper end opening. As this result, the control rod is smoothly inserted without jumping the fuel assembly at the control rod insertion time. On the contrary, when the control rod is drawn downwardly outward, the inner pressure in the control element guide tube 24 is reduced. However, the steam occupies the inside of the control element guide tube during the power operation state and is expanded therein, so that the degree of this pressure reduction is small. At the reactor shut-down state, the non-boiling water occupies its inside to thereby make small the flow resistance and counterflows through the upper end opening, thus smoothly drawing downward the control rod. Accordingly, the void fraction can be surely controlled by the control elements 7, as described with respect to the first embodiment, in the fuel assembly provided with the water rod 19. Furthermore, according to the structure in which the opening is formed to the lower portion of the control element guide tube 24 at a portion below the fuel rod support member 14 to thereby guide the coolant into the inside of the control element guide tube 24, substantially the same functions as those attained by the fourth embodiment of FIGS. 14 and 15 can be attained as well as the controlling of the axial position of the upper end of the control element 7. It may be better to secure the water rod 19 integrated with the control element guide tube 24 to the lower tie plate 13 to prevent the water rod from vertically shifting at the control rod insertion or withdrawal time. In FIG. 17 there is the water rod in one fuel assembly, but a plurality of water rods design case is considered also. FIGS. 19 and 20 represent a seventh embodiment as a modification of the first embodiment, in which four fuel assemblies 10 are combined to form a large fuel assembly unit 81. In such fuel assembly unit 81, the lower nozzles of the fuel assemblies 10 and the fuel support are integrated and a channel box 17a is directly fastened to the fuel support by means of screws. According to the provision of such large-sized fuel assembly unit, the four fuel assemblies surrounding the control rod are integrated, so that the control elements 7 and their guide tube 24 can be stably mounted at their combined portions, thus being advantageous. It is to be understood that four fuel assemblies of the other embodiments can be combined into a fuel assembly unit in the like manner to thereby achieve substantially the same advantages. In a case where an abnormal transition phenomenon or accident be caused during the reactor power operating state, the control rod should be rapidly inserted to rapidly change the reactor to a subcritical state or low power operating state to thereby protect the nuclear reactor or power plant. In such case, it is better for a scram control rod, i.e. rapidly inserting control rod, to have a weight as light as possible and insertion resistance as small as possible. In this meaning, it is better for the scram control rod to have a conventional cross shape (such as structure shown in FIG. 6 but not provided with the control element 7). In such case, there causes a coolant flow directing from the coolant inlets 42a and 42b, towards the control element insertion inlet or towards the upper end opening 32 of the control element guide tube, and the coolant flow rate cooling the fuel rods 11 is reduced, so that it is better to insert, from the lower portion of the guide tube 24, and then attach thereto an inserting member, as a flow rate limiter, having a shape corresponding to a upper end of the control element at a portion below the opening 43a so as to close the opening 42b. In such case, the spectrum shift function can be also realized by the control of the core flow rate. Furthermore, this can be realized by the number of the scram control rods less than one fourth of the number of the total control rods in the core, so that the effects attained by the spectral shift operation is less lowered. In the foregoing embodiments, the cross shaped control rod has a B.sub.4 C poison tube, but it may be possible to provide a control rod formed by Hf rods or Hf plates in shape of cross form to attain substantially the same effects. Furthermore, the water rod of the structure shown in FIG. 18 integrated with the control element guide tube may be utilized in a fuel assembly used in combination of a cluster type control rod also to attain substantially the same effects described above.