JPH04296691A - Fuel assembly - Google Patents

Abstract

PURPOSE:To obtain a fuel assembly capable of freely performing spectrum shift operation due to the void fraction in a water rod when operated at high output in the vicinity of rated output regardless of the flow quantity at a reactor core and the output distribution of the fuel assembly. CONSTITUTION:A fuel assembly has the water rod 19 arranged between fuel rods, and the water rod 19 is provided with a cooling material rising flow passage 40 having a cooling material inlet port in the lower area than a fuel rod supporting section 14a and a cooling material outlet port 43 which is connected to the cooling material rising flow passage and opened in the area upper than the fuel rod supporting section 14a, and a cooling material lowering flow passage 41, which introduces cooling material downward opposite to the cooling material flowing in the cooling material lowering passage. The lower end opening of the rising flow passage is separated from the fresh water flow for cooling fuel rods.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a boiling water reactor (hereinafter referred to as B
The present invention relates to a fuel assembly (referred to as WR), and particularly relates to a fuel assembly that can perform spectrum shift operation by controlling the amount of voids in water rods or water crosses within the fuel assembly.

0002

2. Description of the Related Art An example of a conventional fuel assembly loaded into a BWR core is one constructed as shown in FIG. It houses the fuel bundle 3.

[0003] The fuel bundle 3 includes a plurality of fuel rods 11,
For example, the fuel rods 11 are arranged in a square grid of 8 rows and 8 columns, and a large diameter water rod 5 is arranged in the center thereof.
The water rods 5 are bound together by spacers 16 arranged in multiple stages in the axial direction. Also, each fuel rod 11
and an upper end plug 46 at the upper end of the water rod 5.
However, lower end plugs 47 are respectively fixed to the lower ends, and furthermore, the upper end plugs 46 and the lower end plugs 47 are supported by the upper tie plate 12 and the lower tie plate 13, respectively.

The lower tie plate 13 introduces reactor water, which functions as both a moderator and a coolant, into the interior through its opening as shown by the arrow in the figure, and flows through the gaps between the fuel rods 11 from below to above. At the same time, the heat released from each fuel rod 11 is removed and the fuel flows to the upper part of the core, becoming a gas-liquid two-phase flow.

The water rod 5 introduces reactor water into the interior through the opening 5a at its lower end, guides it upward in the axial direction, flows out through the discharge port 5b, and guides it to the upper end of each fuel rod 11. . Here, the reactor water flowing through the water rod 5 mainly acts as a moderator, flows slowly through the water rod, joins and mixes with the gas-liquid two-phase flow at the upper part of the reactor core. Incidentally, FIG. 17 shows an example in which a water cross 4 having a cross-shaped channel shape is used instead of the water rod. The water cloth 4 also has a coolant intake (not shown) at the bottom and an open end that remains cross-shaped at the top.

[0006] The conventional BWR is disclosed in Japanese Patent Application Laid-Open No. 54-12138.
As described in Japanese Patent No. 9, a fuel assembly having a water rod through which only a coolant flows is loaded in a reactor core in order to promote deceleration of neutrons. The use of such water rods allows for effective utilization of the nuclear fuel material loaded in the reactor core, since under conventional BWR operating conditions, the greater the number of hydrogen atoms relative to uranium atoms, the higher the degree of reactivity.

However, in order to make more effective use of nuclear fuel material, it is better to change the number of hydrogen atoms in the reactor core as the nuclear fuel material burns.

The advantages of changing the number of hydrogen atoms in the core as the nuclear fuel material burns will be explained below.

FIG. 15 shows burnup on the horizontal axis and infinite neutron multiplication factor on the vertical axis for a typical fuel assembly used in BWR. Both of the two lines in the figure are the same fuel assembly, but the dashed line shows the case where the void ratio in the coolant flow path in the fuel assembly is kept constant (40%) and the solid line shows the case where the void ratio is initially high (40%). 50%) and the void ratio was lowered (30%) midway through the operation. As is clear from Fig. 15, if the void ratio is initially burnt at a high value and then the void ratio is lowered, a higher infinite multiplication factor, that is, a higher extraction burnup can be obtained at the end of the fuel life. Can be done.

[0010] This is because the void ratio is high and the ratio of the number of hydrogen atoms to the number of uranium atoms is small, that is, the fewer the number of hydrogen atoms, the higher the average velocity of neutrons and the easier they are to be absorbed by uranium-238. The nuclear fuel material used in BWR contains uranium-235 and uranium-238, with uranium-235 accounting for several percent of the total nuclear fuel material and uranium-238 accounting for the majority. Among these, only uranium-235 absorbs neutrons and causes nuclear fission, while uranium-238 hardly causes nuclear fission. Therefore, uranium 2
When 35 is reduced by combustion, the reactivity decreases.

However, when uranium-238 absorbs high-energy neutrons generated by nuclear fission, it becomes plutonium-23.
Changes to 9. Like uranium-235, plutonium-239 absorbs decelerated thermal neutrons and undergoes nuclear fission. The higher the void fraction, the higher the energy of neutrons, the higher the rate at which uranium-238 is converted to plutonium-239, and the more nuclear fission of uranium-235 and plutonium-239 is suppressed. Therefore, the higher the void rate, the more uranium-23
The total amount of plutonium-239 and plutonium-239 decreases slowly.

However, when the void ratio is high, the absolute value of the reactivity is low. Therefore, if the void fraction remains high, the reactivity reaches the minimum level at which criticality can be maintained sooner than when the void fraction is low. Therefore, if the void fraction is lowered at that point, the neutron moderating effect will increase, and the fission of uranium-235 and plutonium-239 will increase compared to the case of combustion with a constant void fraction, resulting in higher reactivity. Therefore, the fissile material contained in the nuclear fuel material can be burned for a longer time until the minimum reactivity required for criticality is reached.

What has been described above is the principle of effectively utilizing nuclear fuel material by changing the void ratio as the fissile material burns, and is called spectral shift operation.

[0014] Due to such spectral shift operation,
As a method to change the number of hydrogen atoms in the reactor core as nuclear fuel material burns, it is possible to significantly change the average void fraction within the fuel assembly with a simple structure, and was proposed by the Atomic Energy Society of Japan's 1986 Meeting ( 1988.4/4-4/6) Presentation N
o. F15 “Significant spectral shift BWR core concept (
1)" and Japanese Patent Application Laid-Open No. 63-73187, a resistor (
A fuel rod support portion) 14 is provided, the outer tube 36 of the water rod 9 has a coolant inlet 42 opened in a region below the resistor, and an inner tube in which a coolant upward flow path 40 is formed. 35 is arranged through a spacer 37, and the coolant is connected to the coolant ascending passage through a connecting port 34 bored in the upper part of the inner tube, and is opened in an area above the resistor 14. It is proposed to provide a coolant downflow channel 41 with a discharge opening 43. In FIG. 19, 38 indicates an end plug, and 39 indicates an annular end.

As shown in FIG. 20 in a fuel assembly constructed in this way, when the flow rate of coolant passing through the core decreases, the differential pressure at the entrance and exit of the water rod decreases, and the flow path is filled with steam. When the coolant flow rate increases, the differential pressure between the inlet and outlet increases, and the amount of steam in the flow path decreases significantly. Therefore, it becomes possible to significantly change the average void fraction within the fuel assembly, and it becomes possible to increase the reactivity at the end of the operation cycle. That is, in the first half of the operation cycle when the coolant flow rate is reduced, the moderator density is high in the lower part of the core where the liquid phase flow exists in the water rod flow path, and the moderator density is lower in the upper part of the core where the vapor phase exists. Therefore, in the first half of the operating cycle, the lower part of the reactor mainly burns, and in the upper part of the core, uranium-238 is converted to plutonium-239, and in the second half of the operating cycle, the plutonium-239 in the upper part of the core, which was converted in the first half of the cycle, is Since it mainly contributes to combustion, fuel combustion efficiency increases due to the spectral shift effect.

[0016]

[Problems to be Solved by the Invention] However, in the previously proposed fuel assembly, in order to significantly change the average void fraction within the fuel assembly, the differential pressure at the entrance and exit of the proposed water rod must be controlled by the core flow rate. There is a need. By the way, in a BWR, the coolant flow rate of the fuel assembly depends on the output of the fuel assembly and the axial output distribution. As the aggregate output increases, the amount of voids increases and the two-phase pressure drop increases, causing the coolant flow rate of the aggregate to decrease. Furthermore, even if the aggregate output is the same, the amount of voids is larger when the axial power distribution has a downward peak, and the coolant flow rate of the aggregate decreases due to an increase in two-phase pressure loss. The range of variation in the aggregate coolant flow rate due to this output distribution is as much as 20%. As shown in FIG. 20, the average void fraction of this water rod changes greatly depending on subtle changes in the differential pressure at the entrance and exit of the water rod. Therefore, for example, when operating with a core flow rate of 110% rated, the water rods of all fuel assemblies are approximately 10% of the low void fraction.
, when the core flow rate is rated at 70%, the high void rate is approximately 70%
Even if it were possible to reliably control the core flow rate, there would be large variations between 10% void and 70% void depending on the output of the assembly at core flow rates in between. As a result, the error between the evaluation results of the three-dimensional nuclear thermal-hydraulic calculation program that monitors and simulates the output of the assembly online and the signals of the nuclear instrumentation in the reactor becomes large, and the accurate thermal limit (MCPR) inside the reactor increases.
, MLHGR) had disadvantages for evaluation. In addition, with conventionally proposed water rods, in order to perform spectrum shift operation that significantly changes the void ratio within the water rod, it is necessary to make such a significant change in the core flow rate, and due to limitations such as MCPR, the flow rate cannot be reduced. In this case, there was a drawback that the spectrum could not be shifted by the water rod.

An object of the present invention is to eliminate the above-mentioned drawbacks, and to enable spectrum shift operation by freely changing the void ratio in the water rod at high power near the rated power, regardless of the core flow rate and the power distribution of the fuel assembly. Our objective is to provide fuel assemblies.

[0018]

[Means for Solving the Problems] The above object is to provide an upper tie plate, a lower tie plate, an upper end portion held by the upper tie plate, a lower end portion held by the lower tie plate, and a plurality of fuel pellets filled inside. In the fuel assembly, the fuel assembly has a plurality of fuel rods disposed between the fuel rods and is housed in a rectangular cylindrical channel box constituting a coolant flow path. A rod includes a coolant ascending passage having a coolant inlet opening in a region below the fuel rod support, and a coolant ascending passage communicating with the coolant rising passage and opening in a region above the fuel rod support. It includes a coolant discharge port and a coolant downward flow path that guides the coolant downward in the opposite direction to the flow direction of the coolant in the coolant upward flow path, and the lower end opening of the upward flow path is for cooling the fuel rods. This is achieved by a fuel assembly that is characterized by being isolated from the reactor water flow.

[0019]

[Operation] According to the present invention, when the temperature of the coolant taken in from the opening at the lower end of the water rod is low, the amount of voids generated within the water rod is small, and the void ratio inside the water rod is less than 5%; When the water temperature is high, the amount of voids generated is large and the void ratio inside the water rod is over 80%.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be explained with reference to the drawings.

The nuclear reactor according to this embodiment has a reactor core 7 surrounded by a core shroud 63 in a pressure vessel 62. Hold the body 10. A cross-shaped control rod 6 surrounded by a fuel assembly 10 is inserted from the bottom of the reactor core 7 and driven up and down. As shown in FIG. 2, the drive is located below the core 7 and receives the weight of the fuel assembly 10.
This is done by a control rod drive unit 65 that operates a drive shaft that passes through the inside of a control rod guide tube 61 that accommodates the control rod when the control rod 6 is pulled out downward from the control rod. The control rod drive section 65 is cooled by cooling water (or purge water is injected) that is lower temperature than the reactor water in the lower plenum 66, and the cooling water is guided to the reactor core 7 through the control rod guide tube 61. The system for supplying the control rod drive unit cooling water includes a control rod drive water pump 80, a flow control valve 82, a heat exchanger 83,
It is composed of a flow control valve 84 and a temperature sensor 85. The hot water supplied to the heat exchanger 83 may be branched from the intake pipe of the reactor coolant purification system.

A fuel assembly according to this embodiment is shown in FIGS. 3 and 4. The fuel assembly 10 includes fuel rods 11, an upper tie plate 12, a lower tie plate 13a, a fuel spacer 16,
It consists of a channel box 17, a water rod 19, and a fuel support fitting 20. The upper and lower ends of the fuel rods 11 are connected to an upper tie plate 12 and a lower tie plate 1.
It is held at 3a. The water rod 19 also has both ends connected to the upper tie plate 12 and the lower tie plate 13a.
is maintained. A plurality of fuel spacers 16 are arranged in the axial direction of the fuel assembly 10 and maintain appropriate gaps between the fuel rods 11 and the water rods 19. The axial position of the fuel spacer 16 is maintained by a water rod 19. The channel box 17 is attached to the fuel support fitting 20 with screws 22, surrounds the outer periphery of the four bundles of fuel rods 11 held by the fuel spacer 16, and constitutes a large fuel assembly surrounding the control rods (FIG. 4). reference). The lower tie plate 13a rests on the fuel support fitting 20, has a fuel rod support part 14a at its upper end, and has a space 15 below the fuel rod support part 14a. The fuel rod support portion 14a supports the fuel rod 11 and the water rod 1.
It supports the lower end of 9. The fuel rod 11 has a large number of fuel pellets 48 loaded in a cladding tube 45 whose both ends are sealed with an upper end plug 46 and a lower end plug 47, as shown in FIG. A gas plenum 49 is provided at the upper end within the cladding tube 45 . The diameter of the water rod 19 is larger than the outer diameter of the fuel rod 11, and the water rod 19 is arranged at the center of the cross section of the fuel assembly 10.

Water rod 19 which is a feature of the present invention
The detailed structure will be explained with reference to FIG. water rod 1
9 consists of an inner tube 35, an outer tube 36, and a spacer 37. The inner tube 35 is held by a spacer 37, and the upper end of the outer tube 36 is sealed with an end plug 38. The upper end of the end plug 38 is inserted into and held within the upper tie plate 12. Inner tube 3
5 has a connecting port 34 below the end plug 38, which connects a flow path 40 (coolant upward flow path) in the inner tube and a flow path 41 (coolant downward flow path) in the annular portion. The spacer 37 has an opening so that the coolant descending flow path 41 in the annular portion can be secured. The lower end of the outer tube 36 is sealed with an annular end 39 located above the fuel rod support portion 14a, and has a coolant discharge port 43 at the lower portion. The lower end of the inner tube 35 has a coolant inlet 42a that penetrates the fuel rod support portion 14a of the lower tie plate 13a and opens into the space 15. The opening 42a is expanded into a trumpet shape.

The fuel support fitting 20 is fitted onto the control rod guide tube 61 and has a coolant inlet 21 on the lower side face at a position facing the opening 25 of the control rod guide tube. One coolant inlet 21 is provided for each of the four fuel bundles. A cooling water guide pipe 24 is fixed to the fuel support fitting 20 by the bottom of the fuel support fitting 20 and a guide pipe support plate 23, and the upper end of the cooling water guide pipe 24 is connected to the water rod 1 of the present invention.
9 into the opening 42a at the lower end. The guide tube support plate 23 is provided with an opening through which a coolant passes. A cross-shaped opening is provided in the center of the fuel support fitting, and serves as a guide for inserting a cross-shaped control rod 6 into the center of the fuel assembly, as shown in FIG.

The fuel assembly according to this embodiment is loaded into the core of a BWR and operated. The water that cools the fuel assemblies in the core is a mixture of feed water injected from the feed water sparger 75 and high-temperature water obtained by separating the two-phase flow of steam and water from the core by a steam separator 68 and a steam dryer 69. The downcomer (the annulus between the pressure vessel 62 and the core shroud 63) is then pressurized by the recirculation pump 64 and enters the lower plenum 66. The water entering the lower plenum 66 passes through the opening 25 in the upper part of the control rod guide tube 61 and the coolant inlet 21 of the fuel support fitting 20, and then passes through the lower tie plate 13.
The cooling water is guided to the cooling water flow path between the fuel rods 11 through a through hole (omitted in FIG. 3) provided in the fuel rod support portion 14a of a. A portion of the cooling water flows from the leak hole provided in the fuel support fitting to the bypass flow path.

The control rod drive section 65 is cooled by cooling water (or purge water is injected) that is lower temperature than the filtered water in the lower plenum 66, and the cooling water is guided into the core 7 through the control rod guide tube 61. It will be destroyed. A portion of the cooling water that has flowed into the space below the fuel support fitting located at the upper end of the control rod guide tube passes through the guide tube 24 and flows into the coolant ascending passage 40 from the coolant inlet 42 of the water rod 19. , and is further discharged from the discharge port 43 via the downward flow path 41 to the cooling water flow path located above the fuel rod support portion 14a. The fuel flows from the central cross-shaped opening of the remaining fuel support fittings 20 to the bypass flow path.

The cooling water discharged from the coolant discharge port 43 becomes a liquid phase or a vapor phase depending on the flow rate of the cooling water flowing in from the coolant inlet 42a and the water temperature.

As shown in FIG. 13, when the temperature of the control rod drive unit cooling water is high, voids are generated inside the water rod 19 due to heating by neutrons and gamma rays and heat conduction, and the discharge port 43 and flow paths 40, 41 pressure drop increases,
The water level in the inner pipe 40 decreases until it balances with the differential pressure between the inlet 42a and the outlet 43, the pressure loss in the water rod flow path, and the water head. As a result, the inside of the water rod is filled with steam.

As shown in FIG. 14, when the temperature of the control rod driving unit cooling water is low, non-boiling cooling water flows inside the water rod 19 with less void generation due to heating by neutrons and gamma rays and heat conduction.

The operation when the fuel assembly 10 of this embodiment is loaded into the core of a BWR will be described. An example will be explained in which 100% rated output is secured at a core flow rate between 80% and 115%. The core flow rate is maintained at 80% for most of the operating cycle (approximately 70-80%), and reactivity changes due to fuel combustion are responded to by adjusting reactivity using control rods. At this time, from the point at which the rated output cannot be maintained even if all control rods are completely withdrawn from the fuel active part of the reactor core, the core flow rate is increased and the control rod drive cooling water temperature is lowered until the maximum power is reached at the end of the cycle. Make it 115% of the core flow rate.

The control rod drive unit cooling water is supplied to the control rod drive water pump 80 at the rotational speed or the flow control valve 82,
The temperature of the driving unit cooling water can be controlled by the opening degree of the opening 84.

In the water rod of the present invention, the control rod drive cooling water isolated from the core cooling water flow by the recirculation pump is used instead of the inlet/outlet differential pressure-void ratio characteristic depending on the core flow rate as shown in FIG. Since it is used, it can be controlled independently of the core flow rate or the fuel assembly cooling channel flow rate, and the variations in the presence or absence of voids in the water rods between the assemblies due to the power distribution that occurred with the conventionally proposed water rods are eliminated. As a result, according to the present invention:
Most of the cycle period when the core flow rate is below 100% (approximately 80%
(above), voids are generated inside the water rods 19 by controlling the cooling water temperature of the control rod drive section, without being affected by the core flow rate, the output level of the fuel assembly, or the axial power distribution. This suppresses the deceleration effect of plutonium-239 and promotes the production of plutonium-239.

[0033] Furthermore, by maintaining a high void ratio in the water rod even when the reactor output is changed significantly depending on the core flow rate during reactor startup and shutdown, the slope of the reactor flow rate-output curve is increased. Therefore, the advantage of easy control of core power can be maintained as in the conventional proposal.

Furthermore, since the void fraction in the water rod can be accurately controlled by the control rod drive cooling water temperature, thermal limitations, power distribution, burnup distribution, and reactivity can be easily controlled by the three-dimensional nuclear thermal-hydraulic simulation code of the reactor core. The accuracy of evaluation will improve and the accuracy of core performance monitoring will increase.

The water rod shown in the first embodiment of FIG. 4 has a coolant upward flow path and a coolant downward flow path, as shown in FIG. This is an example of only the material ascending flow path. In this case, the upper opening 43a is a sufficiently narrowed hole with a small diameter. In this case, a flow path resistor such as an orifice may be provided just above the opening 42a (not shown) so as not to reduce the diameter of the exit hole too much.

The fuel assembly 10a shown in FIG. 8 is different from the example shown in FIG. 4 in which the outer tube has one square tube-shaped water rod 19, and the water rod 19 is a fuel assembly having a round water rod. This is an example of a body 10a (the figure shows a quarter of the aggregate).

FIG. 9 shows an example in which a water cross 50 is used instead of the water rod (the figure shows four points in the fuel assembly).
). In this example, the channel box 17a is coupled to the fuel support section 20. 4
Each of the small fuel bundles has an upper and lower tie plate, and is arranged in a space surrounded by a water cloth 50 and a channel box 17a, with a lower tie plate 13a resting on a fuel support fitting 20. The coolant flow paths 53 and 54 in the portion surrounded by the four L-shaped plates 51 and the channel box wall constituting the water cross function in the same way as the water rods described above.

FIG. 10 shows a view taken along the line B--B in FIG. 9. The upper and lower ends of the water cloth 50 are sealed, and a spacer 52 maintains the distance. An inner tube 35 is fitted into the center of the water cloth, and the inner tube 35 is held by an L-shaped member 51. The inner tube 35 has a communication port 34 below the upper end cover material 55, which connects a flow path 53 (coolant upward flow path) in the inner tube and a flow path 54 (coolant downward flow path) in the wing-shaped portion. The lower end of the wing-shaped portion is sealed with a lower end cover member 56 located below the fuel rod support portion 14a, and has a coolant discharge port 43 above the fuel rod support portion 14a. The lower end of the inner tube 35 passes through the fuel support fitting 20 to constitute the coolant guide tube 24, and is fixed to the bottom of the fuel support fitting 20 and the guide tube support plate 23. The coolant guide pipe 24 has an opening 42b at its lower end.

The fuel support fitting 20 is fitted onto the control rod guide tube, and has a coolant inlet 21 on its lower side facing the opening 25 of the control rod guide tube. The coolant inlet 21 is provided in each of the four small fuel bundles surrounding the water cloth.
They are provided one by one. The guide tube support plate 23 is provided with an opening through which the coolant passes. Further, a cross-shaped opening is provided in the center of the fuel support fitting 20 to guide the insertion of the cross-shaped control rod 6 into the center of the fuel assembly as shown in FIG.

The cooling water passes through the opening 25 of the control rod guide tube and the coolant inlet 21 of the fuel support fitting 20, and then through the through hole (not shown in FIG. 10) provided in the fuel support fitting 14a of the lower tie plate 13a. The cooling water is guided to the cooling water flow path between the fuel rods 11. A portion of the cooling water flows from the leak hole provided in the fuel support fitting to the bypass flow path.

The control rod drive section 65 is cooled by cooling water (or purge water is injected) that is lower temperature than the filtered water in the lower plenum 66, and the cooling water is guided into the reactor core 7 through the control rod guide tube 61. . A portion of the cooling water that has flowed into the space below the fuel support fitting located at the upper end of the control rod guide tube flows through the guide tube 24 into the coolant upward flow path 53 of the water cross 50, and further into the downward flow path. outlet 4 via 54
3 to the cooling water flow path located above the fuel support fitting 14a. The remainder flows from the cross-shaped opening in the center of the fuel support fitting 20 to the bypass flow path.

The cooling water discharged from the coolant discharge port 43 becomes a liquid phase or a vapor phase depending on the flow rate of the cooling water flowing in from the coolant inlet 42b and the water temperature. As a result, even in fuel assemblies with water cross, the first
As in the embodiment, the void ratio can be reliably controlled by controlling the temperature or flow rate of the control rod drive cooling water.

Furthermore, by eliminating the inner pipe 35 and using only the coolant ascending flow path 53 as shown in FIG. As in the embodiment, the void ratio can be reliably controlled by controlling the flow rate or temperature of the control rod drive section cooling water.

Furthermore, in the fuel assembly of the present invention, the four fuel bundles surrounding the control rods are surrounded by a large channel box, and the channel box and fuel support fittings are fixed with screws.
This is the lower end coolant intake port 42a of the water rod.
This is because it is suitable for smooth engagement of the coolant guide pipe 24 and the coolant guide pipe 24.

[0045]

According to the present invention, it is possible to reliably control the large range of variation in the void ratio within the water rod or water cross inside the fuel assembly, and the effective use of nuclear fuel material can be achieved. Furthermore, as a result, a more accurate simulation evaluation of a spectrum-shift operating core that controls the void fraction in water rods or water crosses in a BWR core can be performed.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing an embodiment of a nuclear reactor according to the present invention.

FIG. 2 is a diagram showing a cooling water system of a control rod drive section according to the present invention.

FIG. 3 is a longitudinal sectional view showing one embodiment of a fuel assembly according to the present invention.

FIG. 4 is a sectional view taken along line A-A in FIG. 3;

FIG. 5 is a partial cross-sectional view of the fuel rod shown in FIG. 3;

FIG. 6 is a longitudinal sectional view showing the detailed structure of the water rod shown in FIG. 3;

FIG. 7 is a longitudinal sectional view showing a second embodiment of the water rod.

FIG. 8 is a partial cross-sectional view showing a different example of the shape of the water rod in FIG. 3;

FIG. 9 is a partial cross-sectional view of a fuel assembly with a water cross.

FIG. 10 is a longitudinal sectional view of the fuel assembly shown in FIG. 9;

FIG. 11 is a vertical cross-sectional view showing one embodiment of the water cloth.

FIG. 12 is a vertical cross-sectional view showing one embodiment of the water cloth.

FIG. 13 is a diagram explaining the action of the water rod.

FIG. 14 is a diagram explaining the action of the water rod.

FIG. 15 is a characteristic diagram showing the change in the infinite neutron multiplication factor with respect to the burnup when spectrum shift operation is not performed and when it is performed.

FIG. 16 is a longitudinal sectional view showing a conventional fuel assembly.

FIG. 17 is a cross-sectional view showing a conventional fuel assembly having a water cross.

FIG. 18 is a longitudinal cross-sectional view showing a conventional fuel assembly with water rods for spectral shift operation.

19 is a longitudinal sectional view of a water rod of the fuel assembly shown in FIG. 18.

FIG. 20 is a diagram showing the relationship between the void ratio and the differential pressure at the inlet and outlet of the water rod shown in FIG. 19;

[Explanation of symbols]

6 Control rod 7 Reactor core 10 Fuel assembly 11 Fuel rod 12 Upper tie plate 13 Lower tie plate 14a Fuel rod support part 17 Channel box 19 Water rod 21 Coolant intake 24 Coolant guide pipe 25 Opening 35 Inner pipe 36 Outer tube 38 End plug 40 Coolant ascending flow path 41 Coolant descending flow path 42 Coolant inlet 43 Discharge port 46 Upper end plug 47 Lower end plug

Claims (1)

[Claims] 1. An upper tie plate, a lower tie plate, a plurality of fuel rods having upper ends held by the upper tie plate, lower ends held by the lower tie plate, and filled with a plurality of fuel pellets; In a fuel assembly consisting of water rods arranged between fuel rods and a rectangular cylindrical channel box surrounding the bundle of fuel rods and water rods and forming a coolant flow path, the water rods a coolant ascending passage having a coolant inlet opening in a region below the rod support; and a coolant discharge port connected to the coolant ascending passage and opening in a region above the fuel rod support. and a coolant downward flow path that guides the coolant downward in the opposite direction to the flow direction of the coolant in the coolant upward flow path, and a lower end opening of the upward flow path is provided with a coolant downward flow path for guiding the coolant for cooling the fuel rods. A fuel assembly characterized by being isolated from the flow.