JPH01176981A - Fuel assembly - Google Patents

Abstract

PURPOSE:To largely change the average void rates in a fuel assembly with simple constitution by providing a solid moderator to a water rod in the position upper than the connecting part between a coolant rising flow passage and a coolant descending flow passage. CONSTITUTION:The steam region in the water rod 19A of double pipes is disposed in the position corresponding to fuel pellets 33 of fuel rods 11. A space is made in the upper region of the water rod 19A when the steam region of said rod is positioned in the lower part. Then, the coolant in the peripheral part within the fuel assembly 10 decreases and a thermal allowance decreases. The solid moderator 80 having a high neutron moderating effect, for example, a metal hydride such as ZrHX or CeH2 is installed in said upper space. The height l2 of the water rod 19A from a fuel rod supporting part 14 is set lower than the height l1 of the pellets 35 by taking the extension of the fuel effective part for about 5cm by an irradiation deformation into consideration. The height of the water rod 19A is thereby reduced to change the reactor core flow rate within the range which is not advantageous for the nuclear characteristics of fuel, by which a spectral shift operation is efficiently carried out.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a fuel assembly, and particularly to a fuel assembly suitable for application to a boiling water nuclear reactor to save consumption of nuclear fuel material. .

[Conventional technology]

As described in Japanese Patent Application Laid-Open No. 54-121389, a conventional boiling water reactor uses a fuel tank having pipes (hereinafter referred to as water rods) through which only cooling water flows in order to accelerate the deceleration of neutrons. The assembly is loaded into the reactor core. Under the operating conditions of a conventional boiling water reactor, the more hydrogen atoms there are with respect to uranium atoms, the higher the degree of reactivity, and the use of such water rods makes it possible to effectively utilize the nuclear fuel material loaded in the reactor core.

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

JP-A-57-125390 and JP-A-57-12
Publication No. 5391 shows one such method. In other words, these publications provide a slow neutron absorbing water rod and a medium speed neutron absorbing water rod made of stainless steel, which has a higher reactivity value than the water rod, and the introduction of these water rods into the reactor core. It states that the amount of cooling water in the core is adjusted by controlling the amount of water inserted. Water depletion is a means of changing the number of hydrogen atoms in the reactor core.

Increasing the amount of water thrusters inserted into the core will reduce the amount of cooling water in the core, and decreasing this amount will increase the amount of cooling water in the core. In the method described above, it is necessary to newly provide a different type of water pusher and operate the water pusher using a driving means, making the structure and operation complicated.

A fuel assembly using static means to solve this problem is disclosed in Japanese Patent Laid-Open No. 61-38589. As a means of changing the number of hydrogen atoms, this publication installs fuel rods with a low concentration of uranium-235 in the water ronds of fuel assemblies, and utilizes changes in the amount of voids in the water ronds before and after the disappearance of uranium-235 in these fuel rods. It describes what to do.

Furthermore, as a method that does not require the provision of new operating means such as a water push rod, there is a method of adjusting the flow rate of cooling water flowing through the reactor core. The flow rate of cooling water flowing through the core at the beginning of the fuel cycle is reduced, and the flow rate of cooling water is increased from the middle of the fuel cycle.

[Problem to be solved by the invention]

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

Figure 2 shows the characteristics of typical fuel assemblies used in boiling water reactors, with burnup plotted on the horizontal axis and infinite multiplication factor, which is an index of reactivity, plotted on the vertical axis. . 2
All lines are the same fuel assembly. However, the broken line shows the case where the volume ratio (void ratio) of vapor bubbles in the coolant flow path in the fuel assembly is kept constant (30% void ratio), and the solid line shows the case where the void ratio is initially high (50% void ratio). The figure shows the case where the vehicle was operated at 100% and the void rate was lowered (30% void rate). As is clear from FIG. 2, a higher burnup can be obtained by increasing the void fraction at the beginning of the combustion and lowering the void fraction later.

This is because the higher the void ratio and the smaller the ratio of the number of hydrogen atoms to the number of uranium atoms, that is, the smaller the number of hydrogen atoms, the higher the average velocity of neutrons and the easier it is to be absorbed by uranium-238. Nuclear fuel materials used in boiling water reactors contain 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, when uranium-235 is reduced by combustion, the reactivity decreases.

However, when uranium-238 absorbs high-energy neutrons produced by nuclear fission, it turns into plutonium-239.

Like uranium-235, plutonium-239 absorbs decelerated thermal neutrons and undergoes nuclear fission. The higher the void fraction, the higher the energy of neutrons and the greater the conversion rate from uranium-238 to plutonium-239.
Nuclear fission of 35 and plutonium 239 is suppressed.

Therefore, the higher the void fraction, the slower the total amount of uranium-235 and plutonium-239 decreases.

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 you lower the void rate at that point,
The effect of moderating neutrons increases, and the fission of uranium-235 and plutonium-239 increases compared to the case of combustion with a constant high void ratio, 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.

The method of providing static means in a water rod with a simple structure and the method of changing the number of hydrogen atoms in the core by changing the flow rate of cooling water flowing through the core (referred to as core flow rate) both improve the void ratio of the core. The problem is that the variation range cannot be very large, making it difficult to apply to actual nuclear reactors.

FIG. 3 shows the dependence of core average void fraction on core flow rate. Core flow is limited on the lower end by thermal limits and on the upper end by recirculation pump capacity and flow oscillations. Therefore, when the boiling water reactor is producing the rated thermal output, the void ratio can only be changed within a certain narrow range around the 100% rated core flow rate. For example, the range in which the core flow rate can be changed is 80
When it is up to 120%, the variation width of the void ratio is about 9%.

Furthermore, even with a structure in which a heating element (nuclear fuel material) whose calorific value decreases with the fuel is placed in the water rond as shown in Japanese Patent Application Laid-open No. 61-38589, the void ratio in the water rond is at most 30%. It only changes in degree. Since the water in the water rod does not contribute to cooling, the cross-sectional area of the water rod within the fuel assembly cannot be increased too much. Even if 30% of the cooling water flow path in the fuel assembly is devoted to the cross-sectional area of the water rond, a 30% change in void ratio will result in a change in the overall fuel assembly of 9.
% (30%X0,3). Furthermore, since a fuel rod with a low enrichment degree is used as one heating element, the structure is complicated and manufacturing is troublesome.

In order to achieve a larger range of void fraction changes, it is possible to change the flow rate in the water rod extremely greatly, or to change the calorific value of the nuclear fuel material in the water rod even more dramatically. It is impossible to change the flow rate and calorific value without moving parts. If a moving part is attached,
There are problems such as reliability problems and a complicated mechanism.

An object of the present invention is to provide a fuel assembly that has a simple structure and can significantly change the average void fraction within the fuel assembly.

[Means to solve the problem]

The above purpose is to provide a resistor at the lower end of the fuel assembly, and to connect the water rod to a coolant ascending flow path having a coolant inlet opening in a region below the resistor and a coolant ascending flow path. and a coolant downward flow path which has a coolant discharge port opened in a region above the resistor and which guides the coolant downward in the opposite direction to the flow direction of the coolant in the coolant upward flow path. This can be achieved by providing a solid moderator in the water rod at a position above the communication portion between the coolant ascending flow path and the coolant descending flow path.

[Effect]

When the flow rate of coolant passing through the core decreases, steam fills the coolant downflow path of the water rond, and when the coolant flow rate increases, the amount of steam in the coolant downflow path decreases significantly. Therefore, it is possible to increase the reactivity at the end of the fuel cycle.

〔Example〕

Before giving a detailed explanation of the present invention, a detailed explanation of the present invention will be given. FIG. 4 shows its structure.

Basically, there is a coolant ascending flow path 2 in which a coolant inlet 4 opens in a region below a resistor (for example, a lower tie plate) 6 provided at the bottom of the fuel assembly, and A water rod 1 having a coolant descending passage 3 in which the coolant flow flowing in the passage is reversed and guided downward, and in which a coolant discharge port 5 opens in an area above a resistor 6 is attached to a fuel assembly. It was established in The resistor 6 is provided with a plurality of coolant flow holes 7.

The coolant (
When the flow rate of cooling water changes, the differential pressure ΔP between the region below the resistor 6 and the region above the resistor 6 changes. Since the pressure difference due to contractile flow resistance is approximately proportional to the square of the cooling water flow rate, for example, if the cooling water flow rate passing through the resistor 6 is 80.
% to 120%, the differential pressure ΔP is approximately 2.
It becomes 25 times.

On the other hand, the relationship between the amount of cooling water in the water rod 1 and the differential pressure between the inlet and outlet of the water rod 1 (the differential pressure between the coolant inlet 4 and the coolant outlet 5) is shown in FIG. When the cooling water flow rate is increased from the cloud, the pressure difference between the entrance and exit of the water rod 1 reaches the maximum value, and when the cooling water flow rate is further increased, the pressure difference between the entrance and exit of the water rod 1 becomes minimum - and then becomes monotonous. To increase.

This is due to the development shown in FIG. Figure 6 (
a) shows the state inside the water rod 1 at the 8 points in Fig. 5, Fig. 6 (b) shows the state at the T point in Fig. 5, and Fig. 6 (Q) shows the state at the U point in the 5th @. The conditions inside the water rod 1 are shown respectively.

The cooling water in the water rod 1 is also affected by 0.5
It generates heat at a rate of ~2 W/, ffl.

When the flow rate of the cooling water flowing through the water rod 1 is very small (state at point 8 in Figure 5), the cooling water inside the water rod 1 generates heat and evaporates due to irradiation with neutrons, etc., and this steam is released into the water rod 1. As shown in FIG. 6(a), the upper portions of the coolant ascending channel 2 and the coolant descending channel 3 are filled with the liquid. A liquid level L1 is formed in the coolant ascending channel 2, and the differential pressure between the inlet and outlet of the water rod 1 is equal to the difference between this liquid level L1 and the coolant outlet 5 of the water rod 1 (the outlet of the coolant descending channel 3). This occurs due to the difference in hydrostatic head between the liquid levels L2. The flow rate of the cooling water flowing into the coolant ascending flow path 2 is balanced with the flow rate of the coolant flowing out from the coolant discharge port 5 in the form of steam.

When the cooling water flow rate is increased from point 8 in FIG. 5, the amount of cooling water flowing into the coolant ascending flow path 2 exceeds the amount of evaporation of the cooling water. In such a case (for example, point T in FIG. 5), the cooling water flows down in the coolant downward flow path 3 as shown in FIG. 6(b). At this time, a portion of the static water head in the cooling water ascending channel 2 is canceled by the weight of the cooling water flowing in the coolant descending channel 3, so that the differential pressure between the inlet and outlet of the water rod 1 is lower than the maximum value So. Decrease. However, when the cooling water flow rate is further increased, the unsaturated water that flows in from the coolant inlet 4 is suppressed from boiling in the coolant ascending flow path 2 and the coolant descending flow path 3 (the void ratio is significantly reduced). coolant flows out from the outlet 5 (state at point U in Figure 5, state at point C in Figure 6)
)). For this reason, the coolant ascending flow path 2 and the coolant descending flow path 3
Inside, the flow is almost single-phase. Therefore, in the state shown in FIG. 6(a), the static water heads at the level of the coolant outlet 5 in the coolant ascending flow path 2 and the coolant descending flow path 3 cancel each other out, and the difference between the static water heads becomes very large. becomes smaller. However, water rod 1
Since the flow rate of cooling water flowing therein is large, pressure loss due to friction and reversal of the cooling water flow increases, and the differential pressure between the inlet and outlet of the water rod 1 rises again.

Due to the development described above, even if the amount of change in the differential pressure between the inlet and outlet of the water rod 1 is small, the range of change in the flow rate of cooling water in the water rod 1 becomes very large, and the range of change in the void ratio also increases significantly.

Therefore, for example, when the pressure difference between the entrance and exit of the water rod 1 when the core flow rate is 80% is less than the pressure difference between the entrance and exit of the water rod 1 corresponding to the minimum value To in FIG. 5, and when the core flow rate is 120%, If the resistance of the resistor 6 is adjusted so that the differential pressure between the inlet and outlet of the water rod 1 exceeds the differential pressure between the inlet and outlet of the water rod 1 corresponding to the maximum value SO in FIG. 5, the fuel will flow inside the fuel assembly. Significant changes in void ratio can be achieved by changing the cooling water flow rate (core flow rate). In the above example, the core flow rate of 80% is to the left of the maximum value So, preferably to the left of the Q point (differential pressure between the inlet and outlet, which is the same as the minimum value To) in FIG. 5, and the core flow rate of 120% is to the left of the minimum value To 5, preferably to the right of point R in FIG. 5 (the differential pressure between the inlet and the outlet that is the same as the maximum value SO).

A preferred embodiment of the present invention utilizing the principle described above, that is, a fuel assembly applied to a boiling water reactor is shown in FIG.
This will be explained based on FIG. 7, FIG. 8, and FIG. 9.

As shown in FIG. 7, the fuel assembly 10 of this embodiment includes fuel rods 11. Upper tie plate 12. Lower tie plate 13
．． Fuel spacer 16. It consists of a channel box 17 and a water rod 18.

The upper and lower ends of the fuel rods 11 are held by an upper tie plate 12 and a lower tie plate 13. Water rod 19 also 1
Both ends are the upper tie plate 12 and the lower tie plate 1
3. The fuel spacer 16 is attached to the fuel assembly 10
Several fuel rods are arranged in the axial direction of the fuel rods 11 to maintain an appropriate gap between the fuel rods 11. The fuel spacer 16 is held by a water rod 19. The channel box 17 is
Attached to the upper tie plate 12, the fuel spacer 16
It surrounds the outer periphery of the bundle of fuel rods 11 held by the fuel rods 11. The lower tie plate 13 has a fuel rod support section 14 at its upper end, and further has a space 15 below the fuel rod support section 14 . The fuel rod support section 14 supports the fuel rods 11 and the water rods 1.
It supports the lower end of 9. As shown in FIG. 8, the fuel rod 11 has a large number of fuel pellets 33 loaded in a cladding tube 3o whose both ends are sealed with an upper end plug 31 and a lower end plug 32. A gas plenum 34 is formed at the upper end within the cladding tube 30. The diameter of the water rod 19 (the outer diameter of the outer tube 21 described later) is larger than the 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.

The detailed structure of the water rod 19 will be explained with reference to FIG. The water rod 19 has an inner tube 20. It is composed of an outer tube 21 and a spacer 22. The outer tube 21 and the inner tube 20 are arranged concentrically, and the outer tube 21 surrounds the outer periphery of the inner tube 20. Outer tube 2
The upper end of the tie plate 1 is sealed with a cover part 23, and the upper part of the cover part 23 is inserted into the upper tie plate 12 and held therein.

The cover portion 23 covers the upper end of the inner tube 2o so as to form a gap with the upper end of the inner tube 20. The upper end of the inner tube 2o is fixed to the inner surface of the outer tube 21 via plate-shaped spacers 22 arranged radially from the axis of the water rod 19.

The lower end of the outer tube 21 is sealed with a sealing part 24. Inner tube 20
The lower end portion of the sealing portion 24 passes through the sealing portion 24 and protrudes below the sealing portion 24 . The lower end of the inner tube 20 is connected to the lower tie plate 1
It penetrates through the fuel rod support part 14 of No. 3. A cooling water inlet 28 formed at the lower end of the inner tube 20 is connected to the lower tie plate 1.
It opens into the space 15 of No. 3. The inside of the inner tube 20 is a cooling water ascending passage 25 . The annular passage formed between the inner tube 2o and the outer tube 21 is the cooling water downward flow path 26. A plurality of cooling water discharge ports 2 are provided in the tube wall at the lower end of the outer tube 21 in the circumferential direction.
9 is formed. These cooling water discharge ports 29 are provided at equal intervals in the circumferential direction. The cooling water discharge port 29 opens in an area above the fuel rod support portion 14 . In this embodiment, the fuel rod support portion 14 has the function of the resistor 6 shown in FIG. The cooling water ascending flow path 25 and the cooling water descending flow path 26 are formed at the inverted portion 2 formed at the upper end of the water rod 19.
Contacted by 7. In this way, the water rod 19 has a cooling water ascending channel 25. It has an inverted U-shaped cooling water flow path consisting of a cooling water downward flow path 26 and an inverted portion 27 .

When the fuel assembly 1 of this embodiment is loaded into the core of a boiling water reactor (all fuel assemblies are fuel assemblies 1) and the boiling water reactor is operated, most of the cooling water is in the lower type. rate 1
The fuel assembly 1 is loaded into the reactor core through the space 15 of 3 and the through hole 18 (FIG. 9) provided in the fuel rod support part 14.
o fuel rods 11 are introduced directly between each other. The remaining part of the cooling water that has flowed into the space of the lower tie plate 13 flows into the cooling water ascending passage 25 of the water rod 19 from the cooling water inlet 28 and further passes through the reversing part 27 and the cooling water descending passage 26. The cooling water is discharged from the cooling water discharge port 29 to an area above the fuel rod support portion 14 . The cooling water discharged from the cooling water outlet 29 is either liquid or gas (steam) as described above, depending on the flow rate of the cooling water flowing into the water rod 19 from the cooling water inlet 28. In this example, the core flow rate is 100
% (in the water rod 19, the state of flow at the maximum value So shown in FIG. 5), the state shown in FIG. The pressure loss of the fuel rod support part 14 and the inner pipe 2 are reduced so that the condition shown in FIG. 6(c) occurs in the water rod 19 at
0 and the specifications of the outer tube 21 are set in advance.

In order to increase the effect of the spectral shift operation, it is desirable to arrange the steam region in the double pipe water rod according to this embodiment at a position corresponding to the fuel pellets 33, as shown in FIG.

This is because the effect of spectral shift operation is enhanced by arranging the steam region in the fuel pellet 33 part where nuclear fission occurs than by arranging the steam region in the gas plenum part where nuclear fission does not occur. As a countermeasure, it is possible to increase the double backwater rondo height rA4, but Q4
In order to increase this, it is necessary to increase the core flow rate, and it is necessary to increase the pump driving capacity of the recirculation system. By locating the steam region of the water rod at the bottom, a space is created in the upper region of the water rod, allowing the coolant in the assembly to flow into the upper part of the water rod, reducing the amount of coolant in the periphery of the fuel assembly and reducing the thermal margin.

In order to increase the effect of this embodiment, it is preferable to use a solid moderator having a large moderating effect. The substance with the greatest neutron moderation ability is hydrogen. Therefore, materials with high hydrogen content are preferred as moderators.

Examples of materials that meet the above conditions include zirconium hydride (ZrHx) and cesium hydride (C
There are metal hydrides such as eHz). In this embodiment, the water rod height Q2 from the fuel rod support is set lower than the fuel pellet height Ql.

The effective fuel portion of the fuel rod 11 is elongated by about 5 ann due to irradiation deformation. The water rod height Q2 and the fuel pellet height Ql are determined in consideration of the above-mentioned points.

In the above-mentioned embodiment, the entire fuel effective area is filled with concentrated fuel pellets, but in order to improve fuel economy and increase the shutdown margin of the reactor core, natural gas is added at the upper end of the active fuel area. It is effective to install a uranium blanket area.

Below, another embodiment of the present invention will be shown in which the upper end of the fuel effective portion is a natural uranium blanket region.

FIG. 10 is a schematic diagram showing the height position of the upper end of the water rod when the upper end of the fuel effective portion is in the natural uranium blanket region. In this embodiment, the fuel effective part is
It is composed of an enriched fuel effective part 33 and a natural uranium blanket region 81 above it, and the plenum part 34 is located above the natural uranium blanket region 81. The height position of the upper end of the water rod 19A is not made to coincide with the upper end of the effective fuel part but with the upper end of the enriched effective fuel part 33, that is, the lower end of the natural uranium blanket region 81. The reason why the upper end of the water rod is not extended to the natural uranium blanket region is due to consideration of the nuclear characteristics of the natural uranium blanket region. This will be explained below.

Generally, the infinite multiplication factor of fuel has a value that becomes maximum when the amount of moderator is increased, and tends to decrease when the amount of moderator is increased beyond that value. FIG. 11 shows the relationship between infinite multiplication factor and moderator. The horizontal axis is the hydrogen to uranium atomic ratio (
H/U ratio), and as the moderator water increases, the H/U ratio increases.

The maximum value of the infinite multiplication factor for the moderator is H/U.
As the ratio increases, the moderation of neutrons is promoted, and the probability P of escaping from resonance absorption increases, but the thermal neutron absorption of the moderator increases, so the thermal neutron utilization rate f decreases, so P
This is because both and f cancel each other out. In the normal operating range, the fuel for boiling water reactors falls within the range of insufficient deceleration shown in area A in Figure 11, in order to maintain a negative output reactivity coefficient and easy self-control. .

However, in the case of natural uranium, since the amount of uranium-235 is 1/3 to 1/4 of that of normal fuel, the thermal neutron absorption cross section of uranium-235 decreases, and the thermal neutron utilization rate f decreases. In addition, the overall thermal neutron absorption cross section also decreases, so
The proportion of the thermal neutron absorption cross section of the moderator increases relatively. Therefore, when the H/U ratio changes, the rate of change in the thermal neutron utilization rate f is large. Therefore, as shown by the broken line in FIG. 12, f in natural uranium has a steep downward slope to the right compared to normal fuel, and the maximum value of the infinite multiplication factor shifts to the left. From the above, natural uranium is in a state of excessive deceleration in the normal operating range, and as the H/U ratio increases, the infinite multiplication factor decreases.

Due to the above nuclear characteristics, extending the water rods to the natural uranium blanket region would result in a slight loss of core reactivity, so the upper ends of the water rods were determined as shown in Figure 10. As a result, spectrum shift operation can be efficiently performed by shortening the water rod height and changing the core flow rate within a range that is not disadvantageous in terms of the nuclear characteristics of the fuel.

FIG. 15 shows another embodiment of the invention.

The fuel assembly 51 of this embodiment is disclosed in Japanese Patent Application No. 60-12610.
This invention is applied to the fuel assembly shown in FIG. 15 of No. 9. This embodiment differs from the previous fuel assembly 57 in that the water rod 19A of the fuel assembly 57 is replaced with a water rod 19H consisting of an inner tube 2OA and an outer tube 21B; It is the same as body 57. Further, the water rod 19H is structurally the same as the water rod 19A, except that the lengths of the inner tube 20A and the outer tube 21B are shorter than those of the water rod shown in FIG. In this embodiment, water rod 1
9H does not penetrate through the fuel support portion 14.

Therefore, the fuel assembly 51 is easier to manufacture than the fuel assembly 57, and can obtain the same effects as the fuel assembly 57.

In the above fuel assembly 57.51, the lower tie plate has the function of changing the pressure loss like the orifice S2, but if you do not expect that effect, - Insert the opened orifice plate into fuel assembly 5.
It can also be used in place of the orifice 52 of Nos. 7 and 51. It is also applicable to the fuel assembly 10 of the orifice 52 shown in FIG.

Although the coolant flow in the water rond described above is an example of a water rond in which the flow is from the inner pipe 20 to the outer pipe 21, the present invention also applies when the coolant flow in the water rond is reversed. Applicable. An example will be explained below. FIG. 19 shows an example thereof.

In this embodiment, the coolant flows from the outer pipe to the inner pipe as shown in the sixth example, which shows an example when the flow of the coolant in the water rod flows from the outer pipe 21 to the inner pipe 20. By making the cross-sectional area of 25 smaller than the cross-sectional area of coolant descending flow path 26, it is possible to shorten the duration of the phenomenon shown in FIG. becomes possible.

FIG. 20 shows another embodiment combining a water rod and a partial length fuel rod according to the present invention. This figure shows the water rod as fuel rod 1.
This is an example in which the present invention is applied to a fuel assembly consisting of a short fuel rod 11'. In general, removing and shortening the most thermally demanding upper part of the most thermally demanding fuel rod in a fuel assembly significantly increases the thermal margin. This is because in a normal fuel assembly, the output becomes abnormally high and insufficient heat removal occurs at the top of the fuel rod or near the second spacer, so it is necessary to remove the fuel rods in these areas. If so, the thermal problems of the short fuel rods themselves would be eliminated.

At the same time, the heat and margin of the fuel rods adjacent to the shorter fuel rods also increases. This is because removing the upper part of the fuel rod increases the amount of cooling water per surrounding fuel rod. That is, the limit output of a fuel rod tends to increase as the amount of cooling water increases, and the limit output of a normal long fuel rod adjacent to a short fuel rod is improved.

On the other hand, if some of the fuel rods are made short, the amount of fissile material in the fuel assembly decreases, resulting in an increase in the output load per remaining long fuel rod. Therefore, in this embodiment.

The water rod according to the present invention is combined with a fuel rod in which short fuel rods are arranged at effective positions to reduce the number of shortened fuel rods and to make the thermal margin uniform. According to this embodiment, the thermal margins at various locations in the horizontal cross section are made uniform, the overall thermal margin is increased, the heat removal characteristics are excellent, and the spectrum shift is achieved within a range that does not become disadvantageous in terms of the nuclear characteristics of the fuel. Driving becomes possible.

〔Effect of the invention〕

According to the present invention, the range of variation in the average void fraction of the fuel assembly is significantly increased with a simple structure, and the effective use of nuclear fuel material is achieved by spectral shift operation within a range that does not disadvantage the nuclear characteristics of the fuel. be able to.

[Brief explanation of the drawing]

Fig. 1 is a diagram of a water rod and a fuel rod, which are preferred embodiments of the present invention, and Fig. 2 is a characteristic showing the change in infinite multiplication factor with respect to burnup in the case where spectrum shift operation is not performed and when it is performed. 3 is a characteristic diagram showing the relationship between core flow rate and core average void fraction, FIG. 4 is an explanatory diagram showing the origin of the water rod used in the present invention, and FIG. A conceptual diagram of the differential pressure characteristics occurring between the entrance and exit of the rod, Figure 6 (a
). (b) and (C) are explanatory diagrams showing the flow state in the water rond at points S, T, and U in Figure 3, and Figure 7 is an example of an implementation in which the present invention is applied to a high burnup core. For example, FIG. 8 is a partial cross-sectional view of the fuel rod shown in FIG. FIG. 9 shows the detailed structure of the water rondo in FIG. 1, FIG. 9(a) is a local vertical cross-sectional view of the water rondo, and FIG. FIG. 10 is a schematic diagram showing the position of the water rod height of the present invention, FIGS. 11 and 12 are conceptual diagrams showing H/U dependence such as infinite multiplication factor, and FIG.
The figure is an overall view of the fuel assembly in Figure 1, and Figure 14 is the overall view of the fuel assembly in Figure 13.
FIG. 15 is a longitudinal sectional view of a fuel assembly according to another embodiment of the present invention, and FIG. 16 is a cross-sectional view taken along section nn in the figure. FIG. 15 is a local plan view of the orifice, and FIG. 17 is a local plan view of the orifice.
6, x and -x cross-sectional views, FIG. 18, and FIG. 19 (a) are longitudinal cross-sectional views of a fuel assembly that is another embodiment of the present invention, and FIG. 19 (b) is FIG. 19 is a cross-sectional view taken along the line ■-■ in FIG. be. to, 35, 51, 57... fuel assembly, 11...
Fuel rod, IIA...Short fuel rod, 12...Upper tie plate, 13...Lower tie plate, 14...
Fuel rod support part, 19, 19A to 191... water rod,
20゜2OA...inner pipe, 21.21A, 21B...
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Claims (1)

[Scope of Claims] 1. A plurality of fuel rods, a resistor provided at the lower end, and a fuel rod that opens below the resistor and extends above the resistor and is disposed between the fuel rods. Water having a coolant ascending channel and a coolant descending channel that is connected to the coolant ascending channel, opens above the resistor, and guides the coolant guided in the coolant ascending channel downward. What is claimed is: 1. A fuel assembly comprising: a rod; on top of the rod a layer made of a solid moderator material having a large hydrogen content per unit volume and a small thermal neutron absorption cross section; 2. The fuel assembly according to claim 1, wherein the resistor is a holding portion of the fuel rod of the lower tie plate. 3. The fuel assembly according to claim 1, wherein the lower end of the upper solid moderator of the resistor is approximately equal to or below the upper end of the fuel pellet filling area. 4. The fuel rod has a natural uranium blanket region at the upper end of the fuel effective portion, and the upper end of the water rod is arranged at a position approximately equal to or below the lower end of the natural uranium blanket region. A fuel assembly according to claim 1.