JPH01229997A - Fuel assembly - Google Patents

Abstract

PURPOSE:To improve fuel economy by maximizing the passage resistance of a guide member of a moderator while preparing a passage resistant control mechanism for continuously reducing the passage resistance in accordance with the increase of the flow of a reactor core at the minimum flow of the reactor core of the moderator. CONSTITUTION:A lateral drain hole 23d is drilled in the side face of the upper end of a water rod 23 which is a moderator guide member and an orifice plate 27 is horizontally fixed on the upstream side thereof to form a passage resistant control mechanism. Since at the beginning of operation cycles the flow of cool ant is small and void of a reactor core is large, much plutonium is accumulated on the upper end of a long-sized, fuel rod 22a. On the other hand, at the end of the operation cycles the flow of the coolant is increased, passage resistance is reduced by the passage resistant control mechanism and the plutonium which is accumulated at the beginning of the operation cycles is burned. Therefore, the fuel economy can be planned.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a fuel assembly for a light water reactor such as a boiling water reactor (hereinafter referred to as BWR), and particularly relates to a fuel assembly for a light water reactor such as a boiling water reactor (hereinafter referred to as BWR), and in particular a fuel assembly such as a water rod or a water cross. The present invention relates to a fuel assembly capable of performing spectrum shift operation by controlling the flow path resistance of a moderator guide member.

(Prior 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.

In the fuel bundle 3, a plurality of fuel rods 4 are arranged in a square grid of, for example, 8 rows and 8 columns, and a water rod 5 having a larger diameter than the fuel rods 4 is arranged in the center thereof, and the water rod 5 is arranged in the axial direction. They are bound into a bundle by spacers 6 in the shape of flat rectangular cylinders arranged in multiple stages.

Further, an upper end plug 7 is fixed to the upper end of each fuel rod 4 and water rod 5, and a lower end plug 8 is fixed to the lower end thereof. They are each supported by a lower tie plate 10.

The lower tie plate 10 introduces reactor water, which functions as both a moderator and a coolant, into the interior from the opening 10a as shown by the arrow in the figure, and the gap between each fuel rod 4 is directed upward from below. At the same time, the heat emitted from each fuel rod 4 is removed, and the fuel rods are heated and flow to the upper part of the core, forming a gas-liquid two-phase flow.

The water rod 5 introduces reactor water into the interior from the water inlet 5a at its lower end, guides it upward in the axial direction, and guides the water to the water outlet 5a.
b and guided to the upper end of each fuel rod 4. Here, the reactor water mainly acts as a moderator, flows slowly from the lower part of the core to the upper part, and joins and mixes with the gas-liquid two-phase flow in the upper part of the core. Incidentally, the reactor water is also guided above the reactor core by the outer circumference of the channel box 2 and the water cross (not shown) in the same manner as the water rod 5.

By the way, in BWR, the reactor output is cold 1i11. ll
It is controlled by the flow M (recirculation l) flow rate) and the insertion of a control rod (not shown). In recent years, control rods have been used relatively infrequently due to the increase in reactor output, and are mainly used in cold IJs.
Output v control is performed by controlling the I-grind flow suspension.

This reduces the thermal impact on the fuel assembly 1,
It is well known that this is an excellent control means unique to BWR in ensuring the integrity of the fuel rods 4.

Furthermore, in the channel box 2, the number of voids (bubbles) increases as you move toward the upper part of the fuel assembly 1, and the void ratio may exceed 70% near the upper end of the heat generating part of the fuel assembly 1. Slightly above the bottom edge is the lowest area where voids occur.

In order to ensure the integrity of the fuel rods 4, the conventional fuel assembly 1 needs to have its output distribution as similar as possible in the axial direction throughout the entire period from the beginning to the end of the operation cycle. was there.

However, recently, by providing a barrier layer on the inner surface of the fuel liquid i-tube of the fuel rod 4, the integrity of the fuel rod 4 has been significantly improved. The need for maintenance has been significantly reduced.

In a BWR, the void ratio originally increases as one moves upwards in the core, so the power distribution is suppressed at the upper end of the fuel assembly at the beginning of the operating cycle, but is distorted toward the lower end of the fuel assembly.

On the other hand, at the end of the operating cycle, the concentration of fissile nuclides at the lower end of the fuel assembly is depleted by combustion, and at the upper end of the fuel assembly, depletion is delayed due to voids, and more plutonium is accumulated due to spectrum hardening due to voids. In contrast, the output decreases below the core and increases at the upper end of the fuel assembly.

Utilizing these inherent properties as much as possible is good for fuel economy, but in the past, more burnable poison was placed at the lower end of the fuel assembly in order to ensure or improve fuel integrity. The problem has been dealt with by increasing the fuel enrichment at the upper end of the fuel assembly. These caused a deterioration of the neutron economy, or caused a deterioration of the fuel economy due to unburned fuel.

This natural phenomenon can be adjusted over a fairly wide range by adjusting the recirculation flow rate; in the early stages of the operating cycle, voids occur lower in the core, resulting in high coolant pressure loss. As a result, the coolant core flow rate tends to decrease, and at the end of the operation cycle, exactly the opposite behavior occurs.

By the way, in the first half of the operation cycle, the density of water, which functions as a moderator, is reduced, hardening the neutron spectrum, and thereby promoting the production and accumulation of plutonium. It is well known that nuclear fuel can be used more effectively if nuclear fission can be caused by increasing the
As a means for implementing this, there is a cold 7J] material outflow control method for B to VR.

This is achieved by lowering the coolant core flow resistance at the beginning of the operating cycle (BOC) to achieve high output in the lower part of the fuel assembly, while increasing the void ratio in the upper part of the fuel assembly to increase the
-Accumulates nium.

Also, the end of the driving cycle! 11 (EOC), by increasing the core outflow, high power is generated in the upper part of the fuel assembly, and the plutonium and residual uranium accumulated at the beginning of the operating cycle are burned. Such a driving method is called spectrum shift driving.

At the beginning of the BWR operation cycle, the water rod can be used as a void rod to remove water from the water rod, and if it can be used as a water rod at the end of the operation cycle in 14JI, spectral shift operation will be more effective and fuel economy will be improved. can be greatly improved. These characteristics are common in pressurized water reactors (PWRs).
The same is true for .

(Problem to be solved by the invention) Therefore, the present invention has been made in consideration of the above circumstances.
The objective is to provide a fuel assembly capable of improving fuel economy.

[Structure of the invention]

(Means for Solving the Problem) The invention according to claim 1 includes a plurality of fuel rods filled with nuclear fuel, and a moderator guide iu that guides the moderator from the bottom to the top of these fuel rods, In the fuel assembly loaded in the core immersed in the moderator, the flow path resistance of the moderator guide member is maximized when the moderator core outflow is at its minimum, and this flow path resistance is The invention according to claim 2 is characterized in that it has a flow path resistance control t2I1m structure that continuously reduces the resistance as the resistance increases. In a fuel assembly loaded in a reactor core immersed in the moderator, the flow path resistance of the moderator guide member is maximized when neutron irradiation growth is at a minimum; The present invention is characterized by having a flow path resistance control mechanism that continuously reduces this flow path resistance in accordance with the increase in neutron growth.

(Function) <Action of the invention according to claim 1> At the beginning of the operation cycle, the outflow of coolant from the core is relatively small and there are many voids in the core, so the flow path resistance of the moderator guide member is increased by the flow path resistance control mechanism. It is being

This reduces the flow rate of the moderator guided to the upper end of the fuel assembly by the moderator guide member, increases the void fraction in the upper part of the fuel assembly, and causes plutonium to enter the fuel assembly for spectral hardening. Generated and accumulated in large quantities.

On the other hand, at the end of the operating cycle, the core flow rate of the coolant is increased compared to the initial stage of the operating cycle, and voids in the core are reduced, so the flow path resistance of the moderator guide member is reduced by the flow path resistance control mechanism.

Therefore, the outflow of the moderator guided to the upper part of the fuel assembly by the moderator guide member is increased, the void fraction in the upper part of the fuel assembly is reduced, and the plutonium accumulated at the beginning of the operation cycle is combusted.

Therefore, according to the present invention, the plu-1 henium generated and accumulated at the beginning of the operating cycle can be combusted at the end of the operating cycle, so that the fuel economy can be improved accordingly.

<Action of the invention according to claim 2> At the beginning of the operation cycle, the amount of neutron irradiation is still relatively small, so neutron irradiation growth is small, and the flow path resistance of the moderator guide member is increased by the flow path resistance control mechanism.

For this reason, the outflow of the moderator guided to the upper part of the fuel assembly by the moderator guide member is reduced, the void fraction in the upper part of the fuel assembly is increased, and more plutonium is added to the fuel assembly due to spectral hardening. Generated and accumulated.

On the other hand, at the end of the operation cycle, the number of neutral dried fish increases more than at the beginning of the one-turn cycle, so neutron growth also increases.
The 'tc5 speed material guide material guide member resistance is reduced by the flow path resistance aill III machine h'4.

For this reason, the flow rate of the moderator guided to the upper end of the fuel assembly by the moderator guide member is increased, the void ratio at the upper end of the fuel assembly is reduced, and the plutonium accumulated at the beginning of the operation cycle is combusted. can do.

Therefore, according to the present invention, plutonium generated and accumulated at the beginning of the operating cycle can be combusted at the end of the operating cycle, so that fuel economy can be improved accordingly.

(Example) Examples of the present invention will be described below based on FIGS. 1 to 12. Note that in FIGS. 1 to 12, common parts are denoted by the same reference numerals, and explanations of the overlapping parts will be omitted.

FIG. 1 is a cross-sectional view of a first embodiment of the invention as claimed in claim 1, in which fuel assemblies (2) of this first embodiment are arranged in a square lattice shape in a rectangular cylindrical channel box 21. 'Ij
It accommodates a number of fuel rods 22 and a water rod 23 in the shape of a rectangular cylinder with a lid and a bottom arranged approximately in the center of these fuel rods 22, and the fuel rods 22 include an elongated type 22a, and A short type 22b of, for example, 3/4 of the total effective length of the fuel rod 22a is arranged.

The long fuel rods 22a and the water rods 23 have their upper ends supported by the upper tie plates 1 to 24, and their lower ends supported by the lower tie plate 25, and are supported by the short fuel rods 22b.
At the same time, a rectangular cylindrical channel box 21 is fitted around the outer periphery of these.

The water rod 23 is a moderator guide member, and as shown in FIG. 2, which is a sectional view taken along the line I-II in FIG. However, the lower end 23a up to 1/4L from the lower end of the effective heat generation of the fuel assembly (2) is slightly reduced,
Further, the cylinder axis length is formed to be approximately equal to the entire effective length of the elongated fuel rod 22a.

The water rod 23 has a small (
In each case, one auxiliary intake port 23b, 23b is bored through the width direction, and a plurality of main intake ports 23c, 23c are provided at the upper end step of the narrow lower end portion 23a around 1/4L from the lower end.
are drilled in the axial direction to form auxiliary intake ports 23b and 23b.
At the same time, reactor water, which has the ability to act as a moderator, is introduced into the reactor.

A horizontal outlet 23d is bored in the side surface of the upper end of the water rod 23, and the water is disposed upstream of the outlet 23d in the flow direction of the reactor water (lower side in FIG. 2) and near the outlet 23d. An A-rifice plate 27 is fixed in parallel to the horizontal direction within the rod 23.

The orifice plate 27 has a plurality of orifices 28 bored through it in the thickness direction, and these orifices 28 have a hole diameter that has a large flow resistance against voids (bubbles) in the reactor water. It is set so that it is small for .

Therefore, when the void of reactor water passing through the orifice 28 increases, the orifice plate 27 continuously increases the flow path resistance and reduces the amount of reactor water flowing through the water rod 23. The passage resistance is continuously reduced in accordance with the decrease in voids, and the amount of reactor water flowing through the water rod 23 can be increased, and is configured as a passage resistance control mechanism.

Next, the effect when this embodiment is loaded into the core of a BWR type nuclear reactor, for example, will be explained.

The axial power distribution of the fuel assembly ■ loaded in the core of a BWR reactor is distributed as shown in Figure 3 (A), and the output peak is at +91 (hereinafter referred to as BOC) at the beginning of the operating cycle. It is located in the lower part of the fuel assembly from the lower end of the effective heat generation (hereinafter referred to as the lower end) to around 1/4L of the body weight, but as it progresses from the middle of the operating cycle (hereinafter referred to as MOC) to the end of the operating cycle (hereinafter referred to as EOC), it increases to the upper part of the fuel assembly. gradually transition to.

The main cause of such axial power distribution is shown in Figure 3 (B).
Due to the change in void ratio shown by
It gradually decreases as it progresses to EOC. This is because the core flow m of reactor water is minimum at BOC and increases as it progresses sequentially to MOC and EOC, increasing the reactor water pressure and improving the efficiency of crushing voids.

In addition, as shown in Figure 3 (B), the point where voids begin to occur is distributed between the bottom end of the fuel assembly ■ and 1/4L, so the main intake is located near this 1/4L. 2
3c and 23c, even if the orifice plate 27 is not provided, more voids can be taken into the water rod 23 by BoC and the void ratio in the water rod 23 can be increased. It is possible to take in reactor water with a lower void ratio.

In the BOC, since the recirculation flow rate is relatively small in the entire operation cycle and the core flow rate of reactor water is relatively small, each main intake port 23C of the water rod 23 is shown in FIG. 3(B). , 23c is high, and the voids and liquid-phase reactor water taken into the water rod 23 through these main intakes 23c and 23C rise upward as shown by the arrows in FIG. Bottom side (upstream side)
reach.

Since voids accumulate in each orifice 28 on the lower surface side of this orifice plate 27, the void ratio increases here, the flow path of liquid phase reactor water becomes narrower, and the flow path resistance of each orifice 28 increases continuously. The amount of liquid phase flowing through these orifices 28 is reduced, and the amount of reactor water flowing out from the discharge port 23d and guided to the upper end of the long fuel rod 22a is reduced.

Therefore, at the upper ends of these long fuel rods 22a, the neutron moderating effect of the liquid phase reactor water is reduced, and more plutonium is produced and accumulated at the upper ends of the long fuel rods 22a.

On the other hand, in EOC, the recirculation flow is increased compared to BOC, and the core flow rate of reactor water is also increased, as shown in Figure 3 (B).
As shown in the figure, each main intake hole 23C of the water rod 23,
The void rate around 23G is reduced.

For this reason, while the voids introduced into the water rod 23 from each main intake port 23G, 23G are reduced,
The liquid phase reactor water increases and reaches the lower surface side (upstream side) of the orifice plate 27.

Since the voids accumulated on the lower surface side of the orifice plate 27 are reduced, the flow path w1 resistance of each orifice plate 28 is continuously reduced by that amount, and the flow rate of the liquid phase passing through these orifices 28 is increased. The reactor water flowing out from 23d and guided to the upper end of the long fuel rod 22a increases.

Therefore, at the upper ends of these long fuel rods 22a, the neutron moderating effect by the liquid phase reactor water increases, and the plutonium accumulated at the upper ends of the long fuel rods 22a can be burned during BOC. .

Therefore, fuel economy can be improved by the amount of plutonium that can be burned.

FIG. 4 shows the main parts of the second to sixth embodiments of the invention as claimed in claim 1, not shown along the vertical axis at the left end of the figure, but corresponding to the effective lower end of the core. The water rod 23II of the embodiment is 1 from the effective lower end of the core (hereinafter referred to as the lower end).
The main intake port 23cIr is opened in the vicinity of the spacer 29, and the main intake port 23cIr is opened near the spacer 29 to drain the reactor water. The spacer 29 guides the water rod 23 from the main intake port 23cI to the main intake port 23cI.
The aim is to maximize the amount of water taken into II.

The water rod 231 [[ of the third embodiment uses high-pressure reactor water located below the lower tie plate 25 as driving water, and increases the intake m of reactor water from the main intake port 23CI [[ by the jet pump principle. In other words, the water rod 23゛■ is 1/4L from the bottom end.
It is divided into an upper part 23111a and a lower part 23IIIb at a nearby point, and the upper end of this lower part 23mb in the figure is reduced in diameter, and the reduced diameter end is extended into the opening bottom of the upper part 23DIa in the same ring shape, and is inserted and fixed. There are multiple intake ports 23 on the outer periphery of the reduced diameter end.
cIff is opened at a required pitch, while the lower part 231
The bottom of 11b is entirely opened 3o below the lower tie plate 25.

Therefore, high-pressure water below the lower tie plate 25 is introduced into the bottom opening 30 of the lower part 231ffb of the water rod 23111, rises upward, and flows into the upper part 231ffb of the water rod 23111.
When introduced into the bottom of the 3 ma, the dynamic pressure causes the static pressure around each main intake port 23clI to be appropriately reduced.

Therefore, according to the jet pump principle, the main intake 23
It is possible to increase the amount of reactor water taken into the water rod 23TII from the water rod 23TII, and to increase the water intake, orifice plates 27 are provided in two stages, upper and lower, to ensure the orifice effect.

The water rod 231V of the fourth embodiment also uses the high-pressure reactor water below the lower tie plate 25 as driving water, and the water rod 231V is located inside the main intake port 23c, which has a frontage approximately 1/4L from the lower end of the water rod 23rV. A cylindrical rotary valve 31 with both upper and lower ends open is accommodated in the rotary valve 31.
Driving fans 32 and 32 are fixed to the lower end of a rotating shaft 318 that is coaxially fixed to the rotating shaft 318 and extends downward in two stages, upper and lower.
Stoppers 33, 33 are provided above and below the two stages of driving fans 32, 32 to restrict excessive movement of each driving fan 32, 32 in the vertical direction.

Therefore, the bottom opening 301 of the water rod 23rV
When the high-pressure reactor water that has flowed into the interior from the To control.

That is, when the rotational speed of the rotary valve 31 is high, the static pressure decreases by the amount that the surrounding dynamic pressure increases, and the amount of reactor water taken in from the main intake port 23CTV increases. When the rotational speed of the main intake port 23c is low, the opposite is true, and the intake of reactor water 8 from the main intake port 23c-2 decreases.

Therefore, from the main intake 23cTV to the water rod 2
The amount of reactor water taken into the 3IV can be continuously controlled.

Although the fourth embodiment has been described as a system in which the valve 31 is rotated, it is of course possible to operate as a valve even when the valve 31 is not rotated.

In addition, a plurality of partition plates 34, 34, 34 are arranged parallel to each other in the width direction of the water rod 23■ at the upper part of the water rod 23rV, and the required spacing is set for each partition plate. A flow path is formed, and, like the orifice plate 27, the flow path resistance is adjusted according to the amount of voids.

The water rod 23V of the fifth embodiment is provided with a streamlined valve 35 projecting downward above the orifice plate 27 (on the downstream side), and this valve 35 is connected to a hanging rod 36 and a coiled spring. It is suspended by 37.

Since the spring constant of the spring 37 may change due to neutron irradiation, the spring 37 is arranged as far as possible above the upper end of the core effective part.

On the other hand, the lower end of the water rod 23V is 1/
4L is removed to widen the main intake port 23CV, and a propeller 38 with a large rotational resistance is provided within the water rod 23V near the main intake port 23cV.

Therefore, when the core flow rate is relatively low, the flow rate of the reactor water is also low, so the rotational speed of the propeller 38 becomes low and the flow path resistance increases, so the reactor water is taken from the main intake port 23cV into the water rod 23V. The amount of water intake also decreases, and the interior tends to have a high void ratio, which leads to the production of plutonium,
This will be convenient for M-Ten.

On the other hand, when the core flow rate is relatively high, the flow rate of reactor water is also high, so the rotation speed of the propeller 38 also becomes high, and the flow path resistance decreases continuously, so water is taken into the water rod 23V from the main intake port 23cV. The amount of water intake will also increase, and the interior will have a low void ratio, which will be favorable for burning plutonium.

The water rod 23VI of the sixth embodiment is the orifice plate 2
7, a flow path resistor 39 is fitted and fixed slightly below (on the upstream side) of the discharge port 23d.

The flow path resistor 39 is constructed as shown in the plan view of FIG. 5(A) and the U cross-sectional view of FIG. Similarly to the orifice plate 27, the flow path resistance is continuously controlled depending on the amount of voids.

The through-hole 41 may include a plurality of ring-shaped through-holes 41a arranged concentrically to have a smaller diameter, as shown in the plan view of FIG. 6 (A>).

Furthermore, other flow path resistors 42 are shown in FIGS. 6(B) and 6(C).
．． 43, and the former is constructed by connecting the open lower end of a small-diameter spiral tube 42 with open ends to the main intake port 23c of the water rod 23.

Therefore, the flow path resistance of the small-diameter spiral tube 42 increases continuously as the void ratio of the reactor water flowing through it increases.
The flow path resistance can be continuously reduced as the void ratio is reduced.

In addition, the latter flow path resistor 43 is fixed by opening the bottom opening end of the covered cylindrical body coaxially with the main intake port 23c■, and diverts reactor water taken from the main intake port 23C■ downward. By flowing downward, the flow path resistance is increased when the void ratio of the intake reactor water is high, and as the void ratio is reduced,
The purpose is to control the flow path resistance so as to continuously reduce it.

In FIGS. 5 and 6, four flow path resistors have been described, but there are various other possibilities.

For example, it is effective to make the inner surface of the water rod rough or to have a throat shape.

The water rod 23TX of the seventh embodiment is constructed as shown in the vertical sectional view of the main part in FIG.
), in BOC, the axial power distribution of the core is higher at the bottom of the core, and the neutron flux and gamma ray flux (prompt gamma rays) are higher, which is the bottom peak, which shows a bottom beak, and in EOC, the opposite is true. be.

That is, the water rod 23IX accommodates therein, in the vicinity of the main intake port 23crX, an elevator valve 50 which is a hollow cylindrical body having an inverted truncated conical longitudinal section and is movable up and down. As shown in the partially enlarged view of Figure (C), the lifting rod 5
1 and is supported by a coiled bimetal spring 52.

The lift valve 50 has a plurality of small holes 50a in its lower bottom, drains reactor water from the upper end of the opening, and connects the bimetal spring 5.
The bimetal spring 52 moves up and down in accordance with the expansion and contraction of the lift valve 50 , and when the temperature rises above a required set temperature, the bimetal spring 52 contracts and lowers the lift valve 50 . The reactor water flow path, which is the gap between the surface and the main intake port 23crX, is narrowed to continuously increase the flow resistance.

Also, when the temperature of the bimetal spring 52 is below the set temperature, the bimetal spring 52 expands and the lift valve 5
0 rises, and its reduced diameter lower end outer peripheral surface is the main intake 23Cr
It faces the inside of X, expands the flow path in the opposing gap, and continuously reduces the flow path resistance.

Around the bottom peak of the outer periphery of the bimetal spring 52 is a heating ring 54 containing a radiation absorbing heating element 53.
is fitted externally with play, and the water rod 231'X
7(B), an auxiliary intake port 23brX of a horizontal hole is opened at the outer periphery of the upper end thereof.

As the radiation absorbing heating element 53, natural uranium, slightly enriched uranium, depleted uranium, etc., which generate heat by absorbing neutrons, are suitable.

The essential reason for this is that the sum of the fissile nuclides of residual uranium and accumulated plutonium does not change much throughout the entire operation cycle.

If this changes throughout the entire cycle, the position of the lift valve 50 will change depending on the operating cycle and depending on the timing of the operating cycle even at the same neutron (γ ray) level.
This is because it is necessary to take sufficient measures to deal with changes in core operating characteristics.

In addition, other radiation absorbing heating elements 53 include boron carbide (B4C), hafnium ()I, which are neutron absorbing materials.
f), europium oxide (Eu2o3), etc., which absorb neutrons and generate heat.

B4C absorbs neutrons and becomes 1e and li, and since these have small neutron absorption characteristics, the position of the lift valve 50 will shift slightly in the latter half of the entire cycle, resulting in poor reversibility.

On the other hand, Hf, EuO, DyO, 1lfo2, etc. are long-life absorbing materials and are generally convenient because they allow the reversible lift valve 50 to be raised and lowered.

Furthermore, as another heating element 53, there is a gamma ray absorber. If the neutron flux is high, the prompt gamma rays generated by nuclear fission etc. will increase proportionally, so the gamma rays may be absorbed to generate heat. Elements with a large number of extranuclear electrons (and therefore a large atomic number) are excellent as such substances.

However, those with low melting points and those that tend to be highly radioactive should be avoided. The above-mentioned 1-1f', Hfo2, etc. are also excellent as gamma ray absorbers. Zircaloy, which is the same material as the water rod, can also be used.

Although the above example describes the case of absorbing γ rays irradiated from the outside, it may also absorb neutrons and generate heat. In this case, it is best to use a material with an atomic number that is not very large and is close to iron. However, it is useless if the neutron absorption cross section is small. The above-mentioned Hf and HfO,2 are also excellent in these respects.

Next, the operation of the seventh embodiment will be explained.

In BOC, the core 21Lffi is relatively small compared to other operating cycles, but the axial power distribution in the core is high in the lower part of the core as shown in Figure 7 (Δ), and the neutron flux, γ The line also shows an increasing bottom beak.

For this reason, the amount of heat generated by the radiation heating element 53 that absorbs neutron flux, gamma rays, etc. increases, which heats the reactor water flowing through the inner circumference of the heating ring 54 and promotes the generation of voids. The bimetallic spring 52 is heated above the set temperature and contracted.

For this reason, the R,14 valve 50 is lowered, and the outer circumferential surface of its large diameter upper end faces the main intake port 23c IX inwardly, so the reactor water flow path, which is the opposing gap, becomes narrower. The flow resistance increases continuously.

On the other hand, in EOC, the core flow rate increases compared to BOC, but the axial power distribution in the core is low in the lower part of the core, as shown in Figure 7 (A), and the neutron flux and gamma rays are low. Decrease.

For this reason, the heat generation m of the radiation heating element 53 is reduced, and the bimetal spring 52 is lowered to below the set temperature.
The water rod 2 is extended and the lifting valve 50 is raised.
31X flow path resistance can be continuously reduced.

FIG. 8 shows a longitudinal section of the main part of the eighth embodiment, which shows a water rod 23
It is housed in a double tube, with outer and inner tubes 100 and 101.
An annular flow path narrower than the inner diameter of the inner tube 100 is formed between them.

The inner tube 101 has a ring-shaped orifice plate 102 on its outer periphery.
are fitted and fixed on the outside at the required intervals in the vertical direction,
Each orifice plate 102 has high resistance to voids,
Orifice hole 10 having a hole diameter with low resistance to the liquid phase
2a are drilled at the required pitch.

A portion corresponding to the bottom beak above the opening lower end 101a of the inner tube 101, for example, from the opening lower end 101a to 40
A heating element 103 is arranged near 90cjI.

This heating element 103 is formed by forming the lower end of the inner tube 101 made of Zircaloy with a thick wall, for example, and generates heat by absorbing neutrons, gamma rays, etc., and has grooves repeated in the axial direction on its outer peripheral surface. It is formed into a wave shape to increase the area of the outer surface.

A flow path resistor 39 (see FIG. 4) is internally fitted into the upper end of the internal information 101 in the vicinity of a plurality of small holes 101b opened at the upper end of the blockage, and the flow path resistor 39 is fixed on the lower surface side (upstream side). ), and the flow path resistance of the flow path resistor 39 is continuously controlled depending on the size of the voids.

On the other hand, the outer tube 100 has an upper end plug 104 fixed to its upper end.
A discharge passage 105 and a discharge hole 106 are bored in the outer pipe 10.
0 and the outside, and the inner end of the upper end plug 104 is provided with a plurality of protrusions 107 or ring-shaped protrusions 107 that protrude inward on both sides of the inner open end of the discharge passage 105. are doing.

Further, the outer tube 100 has a lower end plug 108 fixed to its lower end, and an intake port 109 is opened on the outer peripheral surface of the lower end.

Next, the operation of this embodiment will be explained.

Since the BOC shows a bottom peak, the calorific value of the heating element 103 that absorbs neutrons and gamma rays increases, and the outer tube 1
Reactor water taken into the interior from the intake port 109 at the lower end of 00 is heated by the inside and outside of the heating element 103, but the annular flow path between the outer pipe 100 and the inner pipe 101 is The reactor water passing through is heated to a higher temperature, generates more voids than the reactor water flowing into the interior through the opening lower end 101a of the inner tube 101, and rises as shown by the arrow in the figure.

The reactor water flowing up through the annular flow path absorbs neutrons and generates heat, and is further heated by the inner tubes 100 and 101, generating more voids, and flowing along the outer periphery of the outer tube 100. Together with the voids of the rising reactor water, this forms a so-called heat insulating layer.

For this reason, the reactor water rising inside the internal pipe 101
And when the reactor water is heated by its own absorption of neutrons, etc.
Since the heat is insulated by the heat insulating layer, the inner pipe 10
More voids are generated from the reactor water rising inside 1,
It reaches the upstream side of the flow path resistor 39, and many voids accumulate there, so that the flow path resistance increases continuously.

Furthermore, when the reactor water that has passed through the flow path resistor 39 flows upward from the small hole 101b at the closed upper end of the inner pipe 101, it collides with the reactor water that has flowed up through the annular flow path above it from the side. As a result, the flow is braked and the flow path resistance increases, and in addition, the flow is obstructed by the protrusions 107, 107, further increasing the flow path resistance.

On the other hand, since the EOC does not show a bottom peak, the heat generated by the heating element 103 is reduced compared to the case of the BOC, and the rate of occurrence of voids in the reactor water in the water rod 23XI is correspondingly reduced, reducing the void rate. .

Therefore, the flow path resistance of the orifice plate 102, the flow path resistor 39, the small hole 101b of the inner tube 101, and the discharge path 105, which had increased the flow path resistance due to voids, is reduced, and the flow path is guided to the outside through the discharge hole 106. The void fraction of the reactor water decreases and the liquid phase increases.

As explained above, in the second to eighth embodiments, similarly to the first embodiment, water rods 23 to 23■, 2
The flow path resistance in 3XI is 0N-OF i! IJ 11
Since it can be controlled continuously instead of 1
For the same reason as described in the embodiment, fuel economy can be safely improved.

FIG. 9 shows a vertical section of a main part of the first embodiment of the invention as claimed in claim 2 in correspondence with the length in the axial direction of the reactor core. The purpose is to continuously control the flow path resistance of the water rod 23X.

In the method that utilizes the difference in irradiation growth, it must be noted that it is irreversible.

Generally, burnable poisons are often contained within fuel assemblies. In this case, if the total number of cycles is 4, the first
In the cycle, the infinite multiplication factor (■) reaches its maximum at EOC, and the EOC (■) of the second cycle is equal to or slightly smaller than that of the first cycle BOC.

Therefore, it is preferable to design the intake port so that the flow path resistance can be continuously adjusted so that the void ratio in the guide rod is reduced in the second cycle of EOCl or the third cycle of EOC. In the third cycle BOC, since the koo of other new fuels is large, there is no need to reduce the void ratio in the water rod to increase koo. When comparing the second cycle EOC and the third cycle EOC, the latter is generally more desirable. This is to allow more plutonium to be used for combustion in the accumulated plutonium, which has a great effect on the effective use of fuel.

In the first embodiment of the second invention shown in FIG. 9, a water rod 23X is horizontally divided in the vicinity of 1/4L from its lower end to form an upper portion 23Xa and a lower portion 23Xb, respectively.

The upper end of the lower part 23Xb is reduced in diameter and extended and inserted into the opening lower end of the upper part 23Xa, and the inner intake port 60, 60 is shifted below the main intake port 23cX of the upper part 23Xa and opened at this inserted reduced diameter end. The flow path resistance is controlled by the degree of deviation between each main intake port 23cX and each inner intake port 60.

Further, an auxiliary intake hole 23b is provided at the lower part of the lower part 23Xb, and a discharge port 23d is provided at the upper end of the upper part 23Xa.

Furthermore, the upper part 23Xa is fixed to the upper tie plate 24 via the upper end plug 61, and the lower part 23CX is fixed to the lower tie plate 25 via the lower end plug 62.

On the other hand, for example, the upper and lower ends of the conventional twin Talorado 5 shown in FIG.
Each is firmly attached to the other.

Generally, fuel rod cladding or water rods 5, 2
3X is made of a zirconium alloy, but this zirconium is a hexagonal crystal, which shrinks when exposed to C@ force-directed neutron irradiation, resulting in a.

It is well known that it has the property of elongating in the b-axis direction.

Therefore, the conventional water rod 5 shown in FIG. 9 is manufactured so that the C@ is approximately 45° with respect to the axial direction in order not to extend it much in the axial direction or reduce it in the diametrical direction.

On the other hand, the water rod 23X is manufactured with the C-axis directed in the axial direction of the guide rod, or in a perpendicular direction (radial direction), or in the same direction. Therefore, a difference in growth occurs between the conventional water rod 5 and the r quarter rod 23X due to neutron irradiation.

Next, the operation of this first embodiment will be explained.

In BOC, the neutron irradiation M in the reactor core is less than that in the BOC cycle, so the neutron irradiation growth period between both water rods 5゜23X is also relatively small, and one water rod 23
Since the elongation in the axial direction of X is small, the difference in elongation in the axial direction of both 5.23X is also small.

Also, the elongation of one water rod 23X is such that the upper part 23Xa of this water rod 23X is connected to the upper tie plate 2.
Since the lower part 23Xb is fixed to the lower tie plate 25, the free ends of the upper part 23Xa and the lower part 23Xb extend in the axial direction, thereby being absorbed.

As a result, the main intake 23cX of the upper part 23Xa and the lower part 23
Although the inner intake ports 60 of Xb are close to each other, there is almost no common opening area where both the multiple inlets 23cX and 60 overlap with each other, so the flow path resistance is large.

Therefore, in BOC, the upper liquid phase in the fuel assembly is reduced, the void fraction is further increased, and more plutonium is produced and accumulated.

On the other hand, in the EOC, the neutral dried fish 1:) J m in the core is BOC
Since the increase is greater than when the water rod is 23
The elongation of the water rod 5 in the axial direction is large, and the difference in elongation between the water rod 5 and the other water rod 5 becomes large.

As a result, the upper part 23Xa of one water rod 23X
and each free end of the lower part 23Xb extends greatly in the axial direction, and the multiple inlets 23CX and 60 overlap each other almost concentrically as shown in FIG. 9, and the common opening area of both the inlets 23CX and 60 increases, The flow path resistance increases continuously, the liquid phase in the upper part of the fuel assembly increases, and the void ratio decreases.

Therefore, more plutonium produced and accumulated in the BOC can be combusted in the EOC, thereby improving its fuel economy.

FIG. 10 shows a longitudinal section of a second embodiment of the invention as claimed in claim 2, in which the water rod 23 is placed in an outer tube 70 with a high neutron irradiation growth rate and into a thin tube with a small neutron irradiation growth rate. It is characterized by having a double tube structure by coaxially accommodating inner tubes 71 of different diameters.

Further, a pair of annular upper and lower orifice plates 72 and 73 are provided in the annular flow path between the outer tube 70 and the inner tube 71 at a required interval in the vertical direction, and both orifices 7
2.73 of the orifices 72a and 73a control the flow path resistance according to the void ratio in the reactor water. Note that the outer tube 70 and the inner tube 71 are fixed to each other at the lower end of the outer tube.

A required valve body 74 is fixed to the upper end of the inner pipe 71,
The valve body 74 moves forward and backward into the inner end of the discharge passage 76 that communicates the discharge lower 5a bored on the upper outer surface of the upper end plug 75 with the open upper end of the outer tube 70, expanding and contracting the flow passage as appropriate. The flow path resistance is continuously controlled.

In other words, when the neutron irradiation rate of the core is low in the BOC, the neutrons enter the inner end of the discharge passage 76 of the upper end plug 75 largely as shown in FIG. It's increasing.

Then, when the neutron irradiation dose increases in EOC, the outer tube 70
extends greatly in the axial direction, creating a large difference in elongation between the inner tube 71 and the inner tube 71, which has less elongation, expanding the flow path at the inner end of the discharge path 76 around the valve body 74, and reducing the flow path resistance. .

In this second embodiment, the valve body 74 at the upper end of the inner tube 71 is omitted to form a nozzle-shaped frontage, and the lower end of the inner tube 71 is made to extend vertically downward from the lower end of the outer tube 70. The reactor water taken in from the lower end of this opening is spouted from the nozzle-shaped upper end opening to the inner end of the discharge passage 76 of the upper end plug 75, and the flow resistance around the inner end of the discharge passage 76 is controlled by Bernoulli's theorem. Alternatively, a flow path resistor 39 shown in FIG. 4 may be brought into close contact with at least one of the orifice plates 72, 73 on the reactor water upstream side.

The eighth point is that placing the heating element at the bottom of the core improves the effectiveness.
This is the same as the case shown in the figure.

Further, the present invention also provides a water cloth 8 shown in FIG. 11(A).
For example, the flow path resistance of the water cloth 80 can be continuously controlled by utilizing the difference in the neutron irradiation growth rate between the channel box 81 and the sub-bundle fuel 82, and the flow path resistance can be reduced. It may be configured such that it is increased when the amount of neutron irradiation is low and is continuously reduced as the amount of neutron irradiation increases.

Furthermore, the present invention can also be applied to a fuel assembly having two large-diameter water rods 90.90 and two small-diameter water rods 91.91, as shown in FIG. 11(B). Note that in FIG. 11(B), reference numeral 92 is a rectangular cylindrical channel box, and 93 is a fuel rod.

Furthermore, the present invention may be configured as shown in FIG. That is, this has an intake port 110a of a horizontal hole in the lower side surface of the water rod 110 made of zirconium, and an outlet 111a in the upper end plug 111 fixed to the upper end of the water rod 110, and this outlet 111a is connected to the water outlet. Discharge path 1 communicating with the internal space of the rod 110
1b and connect the discharge port 111a to the upper tie plate 11.
The support fitting hole 112a is opened in the second support fitting hole 112a.

The water rod 110 is manufactured with the C-axis of the zirconium crystal oriented in the axial direction of the water rod 110, and with the a and blml oriented in the direction perpendicular to the axis of the water rod 110. By axially contracting the rod 110, the upper end plug 11
1 discharge port 111a is exposed below, that is, outwardly, from the support fitting hole 112a of the upper tie plate 112,
It is configured to reduce the outflow resistance of reactor water flowing out from this discharge port 111a, thereby reducing the flow path resistance. In addition, in FIG. 12, the reference numeral 113 is a lower tie plate.

〔Effect of the invention〕

As explained above, the invention according to claims 1 and 2 can continuously control the flow path resistance of the straight material guide members such as water rods and water crosses by the core flow rate and neutron irradiation m, so that the spectrum shift You can drive safely.

As a result, for example, more plutonium can be produced and stored in BC) and this plutonium can be combusted in EOC, thereby improving fuel economy.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a first embodiment of the invention as claimed in claim 1;
Figure 2 is a sectional view taken along the If-If line in Figure 1, Figure 3 (A>,
(8) is a graph showing the axial power distribution and void fraction distribution of a general fuel assembly, respectively; FIG. 4 is a vertical cross-sectional view of each water rod of Examples 2 to 6; A) is a plan view of an example of the flow path resistor shown in FIG. 4, and FIG. 5(B) is l111! of FIG. 5(A). Ii side view, Figure 6 (A), (
B) and (C) are diagrams showing other examples of the flow path resistor shown in FIG. 4, and FIG. Figure (B) is a longitudinal sectional view of the water rod of the seventh embodiment, Figure 7 (C)
is an enlarged view of the main part of FIG. 7(B), FIG. 8 is a longitudinal sectional view of the water rod of the eighth actual droplet example, and FIG. 10 is a vertical sectional view of the main part of the second embodiment, FIGS. 11(A) and (B) are cross sectional views of other actual droplet examples, and FIG. 12 is a longitudinal sectional view of the third embodiment. A vertical cross-sectional view of the water rod, and FIG. 13 is a vertical cross-sectional view of a conventional fuel assembly. 21...Channel box, 22...Fuel rod, 2
3.23I[, 23n1.23rV, 23V, 23VI
. 23■, 23■, 23TX, 23X, 23X1.23 size...water rod, 27...orifice plate (flow path resistance control mechanism), 28...orifice hole, 31...
Rotary valve, 50... Lifting valve. Figure 1 Figure 2 Figure 3 41a (C) (A) (B) Figure 7 Figure 7 Figure 8 Figure 10 Figure 11 Figure 12 Figure 13

Claims (1)

[Scope of Claims] 1. A reactor core comprising a plurality of fuel rods filled with nuclear fuel and a moderator guide member that guides a moderator from the bottom to the top of these fuel rods, and is immersed in the moderator. In the loaded fuel assembly, the flow path resistance of the moderator guide member is maximized when the core flow rate of the moderator is at its minimum, and this flow path resistance is continuously reduced as the core flow rate increases. A fuel assembly characterized by having a flow path resistance control mechanism. 2. A fuel assembly that includes a plurality of fuel rods filled with nuclear fuel and a moderator guide member that guides the moderator from the bottom to the top of these fuel rods, and is loaded into the reactor core immersed in the moderator. The method further includes a flow path resistance control mechanism that maximizes the flow path resistance of the moderator guide member when the neutron irradiation growth is at its minimum, and continuously reduces this flow path resistance as the neutron growth increases. A fuel assembly featuring: