JPH03210497A - Fuel assembly - Google Patents

Abstract

PURPOSE:To improve fuel economy by setting the maximum lateral cross-sectional area of a moderator guide member, along a whole length of an axial effective part of the fuel, and by decreasing volume ratio which removes water by the lateral cross-sectional area, at a certain range specified individually from the upper most part and the lower most part of a reactor core, and by increasing the volume ratio at the other parts. CONSTITUTION:The maximum cross-sectional area of a moderator guide member 22 is set along a whole length of an axial effective part of a fuel. Volume ratio which removes water by the lateral cross-sectional area is decreased at a certain area specified individually from the upper most part and the lower most part of a reactor core, and the volume ratio is increased, throughout an operational cycle, at comparatively upper position of the reactor core where concentration of fissile nuclides grows up relatively higher in fuel burning process. Accordingly, the fuel burning at the position is active at the position during an initial to an intermediate periods of an operational cycle, and an output peak is diminished in a last period of the operational cycle.

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 moderator such as a water rod. The present invention relates to a fuel assembly capable of performing spectrum shift operation by controlling flow path resistance of a 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.

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

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 of each fuel rod 4 and water rod 5. Furthermore, the upper end plug 7 is fixed to the upper tie plate 9, and the lower end plug 8 is fixed to the lower end of each fuel rod 4 and water rod 5. are supported by the lower tie plate 10, respectively.

The lower tie plate 10 introduces reactor water, which functions as both a moderator and a coolant, into the interior through its opening 10a as shown by the arrow in the figure, and the gap between each fuel rod 4 is directed upward from below. While the heat released from each fuel rod 4 is removed at this time, the fuel is heated and 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 intake port 5a at its lower end, guides it upward in the axial direction, and discharges it to the exhaust 605.
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 a BWR, the reactor output is controlled by the coolant flow rate (recirculation flow rate) and the insertion and removal of control rods (not shown) into the reactor core. In recent years, control rods have been used relatively infrequently to control reactor power, and power is primarily controlled by controlling the flow rate of coolant.

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.

In addition, the voids (bubbles) in the channel box 2 increase as they 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 is the lowest amount of void generation.

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 cladding tube of the fuel rod 4, the integrity of the fuel rod 4 has been significantly improved, so that the axial power distribution is kept as constant and flat as possible throughout the entire operating cycle. The need has eased.

In a BWR, the void ratio naturally increases as you move upwards in the core, so the power distribution is at the beginning of the operating cycle (hereinafter referred to as BO).
(referred to as C), the upper end of the fuel assembly is held down and distorted to the lower part of the fuel assembly.

On the other hand, at the end of the operating cycle (hereinafter referred to as EOC), the concentration of fissile nuclides in the lower part of the fuel assembly is depleted by combustion.
In the upper part of the fuel assembly, depletion is delayed due to voids, and more plutonium is accumulated (accumulated) due to spectral hardening due to voids, which reduces the power output below the core.
It shows the behavior that it becomes higher in the upper part of the fuel assembly.

(Problem to be solved by the invention) It is good for fuel economy to make use of the inherent properties of fuel assemblies as much as possible, but in the past, in order to ensure or improve fuel integrity, This has been addressed by placing more burnable poison and increasing the fuel enrichment in the upper part of the fuel assembly. These have led to a deterioration in the neutron economy, or a deterioration in the fuel economy due to unburned fuel. Furthermore, if the above-mentioned measures are not taken to improve fuel economy, there is a risk that fuel integrity will be affected due to the distortion of the output distribution at EOC, and it is necessary to avoid this problem. In order to do so, the reactor output must be lowered, and as a result, there is a problem that the economic efficiency of the nuclear power plant as a plant decreases.

The present invention has been made in consideration of the above circumstances, and its purpose is to provide a fuel assembly that can improve fuel economy and prevent a decline in the economic efficiency of a plant. .

[Structure of the invention]

(Means for solving the problem) The above-mentioned natural phenomena of the reactor can be adjusted over a fairly wide range by adjusting the recirculation flow rate, and in BOC, voids occur lower in the core. The pressure loss of the coolant becomes high, and as a result, the coolant core flow rate tends to decrease, while EOC exhibits exactly the opposite behavior.

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, thereby promoting the production and accumulation of plutonium, and using this plutonium in EOC to increase the density of the moderator (water). It is well known that if nuclear fission can be caused by this, nuclear fuel can be used effectively, and as a means of implementing this, there is a coolant flow rate control method in BWR.

This is done by lowering the coolant core flow rate with 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 generate and accumulate plutonium.

In addition, in EOC, by increasing the core flow rate, high output is achieved in the upper part of the fuel assembly, and the plutonium and residual uranium accumulated in BOC are burned. Such a driving method is called a spectrum shift driving method.

In the BWR BOC, the water rod is used as a void rod,
That is, if the void rod is used to remove water from the water rod, and if it can be used as a water rod in the EOC, spectral shift operation will be more effective, and fuel economy can be greatly improved. These characteristics are exactly the same in pressurized water reactors (PWRs).

However, even with this type of operation, the power distribution in the axial direction of the reactor core changes significantly as the operating cycle progresses from BOC to EOC, so the power output is extremely high and there is a risk that fuel integrity may be compromised. As a result of quantitative studies, it has become clear that there is. In addition, the increase in power in the upper part of the core in EOC results in a decrease in neutron economics (i.e., neutron (The multiplication factor does not increase as expected.) Therefore, the present invention regularly arranges a large number of fuel rods and spectrum-shift moderator guide members having a diameter larger than the fuel rods, and the axes of the fuel rods and the guide members are aligned. In a fuel assembly in which water, which acts as a moderator and coolant, flows in a direction parallel to the moderator, the maximum cross-sectional area of the moderator guide member is set to span the entire length of the axial fuel effective part, and water is removed using this cross-sectional area. The volume ratio is small in certain ranges determined from the upper and lower ends of the core, and is larger in other parts.

(Function) In the present invention having the above configuration, the maximum cross-sectional area of the moderator guide member is set over the entire length of the axial fuel effective section, and the volume ratio at which water is removed by this cross-sectional area is set at the upper end of the core and The output peak occurs at the relatively upper part of the core where the concentration of fissile nuclides becomes relatively high during the combustion process, so the power peak occurs at the EOC, and when the reactor shuts down, it becomes subcritical. As the volume ratio was increased throughout the operating cycle in the area where the pressure becomes shallow, combustion in this area progresses from BOC to MOC (middle of the operating cycle), and at EOC, the power peak is reduced, or lower (inside) the core. shift to

As a result of these phenomena, the axial power distribution of the reactor core becomes more stable throughout the operating cycle, power peaking is alleviated, or the reactor water with higher cooling capacity is used for cooling, which improves fuel health and operational stability. improves. In addition, the degree of subcriticality during reactor shutdown does not become too shallow, improving reactor shutdown margin. In addition, the scram characteristics at the time of emergency shutdown of the reactor are improved. Furthermore, the leakage rate of neutrons out of the core during operation is reduced, improving fuel economy. In addition, the pressure loss of the reactor water in the upper part is reduced, improving the economic efficiency and stability of the plant.

(Example) The present invention will be described below based on an illustrative example. FIG. 1(A) shows a fuel assembly according to a first embodiment of the present invention. As shown in FIG. 2A, the fuel assembly 20 has fuel rods 21 arranged in 9 rows and 9 columns, with nine fuel rods corresponding to a 3x3 lattice removed from the center, and a moderator guide member and ( , water rods 22 with large diameters are arranged, and a rectangular cylindrical channel box 25 is arranged on the outer periphery.The water rods 22 are bundled together with fuel rods 21 by fuel spacers, and the Water is used as a moderator to guide around the fuel rods 21. Also, water rods 22
The inner tube 24 is housed coaxially within the outer tube 23 to form a double tube structure.

That is, the water rod 22 has a fuel effective lower end (B A F ) as shown in FIG. 1(B).
The diameter is reduced at approximately 1/4L (L is the effective fuel length of approximately 3.7 m) from f Active Fuel, and a flow path resistor 30 is placed at the outer lower end of the double pipe configuration, and the active fuel TAF: Top of Ac
About 1/4L from tive fuel), it is a large diameter water rod with single layer structure, and from TAF to a certain range (1/12
L) is considered a small diameter water rod. That is, in this embodiment, the maximum cross-sectional area of the water rod 22 is set over the entire length of the axial fuel effective section, and the rate of water removal with this cross-sectional area is set at the upper end of the core, for example, about 30 to 90 AO, and at the lower end of the core. For example, it is small in the range of about 15 to 90 cm.

Reactor water flowing through the water rod 22 has two paths branched into two paths. That is, one side flows in from the water passage hole 27 in the downwardly extending portion of the inner pipe 24, and the fuel flows into the effective lower end BAF.
The cross-sectional area of the flow is expanded around 3/4L to form a large-diameter water rod, and then the diameter is reduced mainly to reduce cooling water pressure loss, and a drainage hole is provided near the upper end of the fuel effective part TAF. It is discharged from 28. The other flow passes through a flow path resistor 30 provided near the lower end of the outer tube 23 between the outer tube 23 and the inner tube 24, and fills the gap between the outer tube 23 and the inner tube 24, which have a reduced diameter. Flow, approximately 1 from the lower effective fuel end BAF
/4 From above, the outer pipe 23 and inner pipe 24 have a larger diameter.
The outer fitting 23 flows around 3/4L.
It is discharged from the microhole 29 provided in the.

On the inner pipe 24 side, reactor water that is kept in a non-boiling state or with a low void ratio always flows throughout the operation cycle, and in the gap between the outer pipe 23 and the inner pipe 24, from BOC to MOC, the reactor water is voided or has a high void content. Reactor water flow holes, discharge holes, and flow path resistors 3 so that water flows, and in EOC, reactor water that is in a non-boiling state or suppressed to a low void ratio flows.
0 is designed.

Next, the flow path resistor 30 will be explained based on FIG. The flow path resistor 30 disposed between the outer tube 23 and the inner tube 24 shown in FIG. 2(A) is divided into two parts in the axial direction due to manufacturability. As shown in )(C) and (D), for example, four water holes 31 are provided symmetrically. This water passage hole 31 is machined into a female thread shape as shown in FIG. 2(A). The axial length of the flow path resistor 30 is usually 15 to 30 cm, which may cause some processing problems when providing the water passage hole 31. For this reason, FIG. 2 shows an example of dividing into upper and lower halves.

Each flow path resistor 30 is provided with a ring-shaped water passage groove 32 as shown in FIG. 2(B) on the upper and lower end surfaces, and the outer and inner peripheries are carved except for the vicinity of both end surfaces. A flow path resistor 3 is provided between the inner circumferential surface of the outer tube 23 and the outer surface of the flow path resistor 30 and between the inner surface of the flow path resistor 30 and the outer circumferential surface of the inner tube 24.
A heat insulating layer 33 is formed to insulate this when no heat is generated.

In the above configuration, when the power around the flow path resistor 30 is low, the reactor water branches and flows in two directions: inside the inner pipe 24 and between the outer pipe 23 and the inner pipe 24 (Fl).

F2). Furthermore, when the output increases, the flow path resistor 30 absorbs gamma rays and neutron rays and generates heat. Then, the outer tube 23
Between the inner surface and the outer surface of the flow path resistor 30 and the flow path resistor 30
The water between the inner surface and the outer surface of the inner tube 24 evaporates, forming a steam insulation layer from which the water or steam escapes through the micropores 29.

As a result, the heat generated from the flow path resistor 30 is efficiently transferred to the water passage hole 31. The reactor water passing through this water passage hole 31 is effectively heated and voids are generated. Many of these voids are trapped in the troughs of the internal threads of the water passage hole 31 and grow, until eventually the water passage hole 31 is almost completely blocked by the voids, and the flow of reactor water F2 is almost eliminated or is reduced to a small amount. Therefore, the reactor water on the downstream side of the flow path resistor 30 absorbs gamma rays and neutron rays and generates heat, and eventually vaporizes (boils) and becomes voids or has a high void ratio. Become.

On the other hand, in EOC, the output around the flow path resistor 30 generally decreases and the reactor water flow rate increases.
0 heat generation is reduced, the generation of voids in the water passage holes 31 is reduced, and the generated voids are also washed away by the increase in flow rate, so that reactor water is supplied to the downstream side of the flow path resistor 30, and the outside The space between the pipe 23 and the inner pipe 24 is filled with reactor water. Such a process can be controlled either by inserting a control rod or by reducing the heat generated by the flow path resistor 30.

On the other hand, the flow path resistor 30 has a built-in water passage hole 3 and a radiation absorbing heating element, and the output distribution reaches a bottom peak from BOC to MOC, so the heat generation rate of the heating element increases and the water passage hole 31 flow path resistance increases. As a result, the inner tube 24
Almost no reactor water flows between the and outer tube 23,
The reactor water in that space is heated and becomes void, and the water level in that space decreases in balance with the slight discharge from the micropores 29, and eventually the entire space can be made void.

However, at EOC, the flow rate of the reactor water increases, and together with the relative increase in the concentration of remaining fissile material at the upper end of the active fuel zone, the power distribution becomes top-peak type. As a result, the heat generation rate of the heating element decreases, the flow path resistance of the water passage hole 31 decreases, and reactor water easily flows between the outer tube 23 and the inner tube 24.

In order to accelerate this phenomenon, the control rod that has been pulled out may be inserted so that its insertion tip is located at a height near the flow path resistor 30. By using the control rods in this manner, the flow path resistance of the flow path resistor 30 can be controlled not only depending on the reactor water flow rate but also only for a specific fuel assembly. When the gap between the outer tube 23 and the inner tube 24 is filled with reactor water,
The water rod 22 of this embodiment has almost the same effect as a general full-length, large-diameter water rod. In other words, the water rod 22 is E
It has almost the same effect as a general large diameter water rod in OC, and BO
From C to MOC, the power peak occurs at the EOC in the upper part of the core, where the margin for reactor shutdown is small, where the lack of moderator is alleviated by using large diameter water rods, and at other locations, the moderator is reduced to stabilize the power distribution. , contributes to flattening, etc.

A typical example of the flow path resistor 30 is a radiation-absorbing heating element. Various types of radiation-absorbing heating element can be considered. natural uranium, depleted uranium, neutron absorbing materials, etc.

Next, the operation of this embodiment will be explained.

The water rod 22 acts as a large-diameter water rod in the upper part of the core (around 3/4L to 11/12L) from BOC to MOC, and acts as a small-diameter water rod in other parts.
4L to 3/4L acts as a large diameter void rod, and 0 to 1/4L acts as a small diameter void rod. The upper part of the reactor core is generally a fuel assembly where the relative distribution of fissile nuclide concentration in the axial direction of the reactor is high due to high void ratio and combustion delay, and the degree of subcriticality becomes particularly shallow during reactor shutdown. In the example fuel assembly 20, unlike other parts, this part is always a large-diameter water rod, so plutonium is generated due to the high void ratio outside the water rod 22, and the water rod 22 is used as a large-diameter water rod. The combustion of plutonium produced by this action is relatively easy compared to other parts, so the phenomenon in which the degree of subcriticality becomes particularly shallow at the time of reactor shutdown is alleviated.

In addition, the part at the height of the large-diameter water rod mentioned above is a normal fuel assembly, and in BOC-MOC, the output is significantly reduced, and the EO
Although the fuel assembly 2 of this example increased significantly in C.
At 0, BOC-M acts as a large diameter water rod.
Since the decrease in output at OC is alleviated and the increase in output at EOC is alleviated, changes in the axial power distribution throughout the cycle are reduced, the output distribution is stabilized and flattened, and the health of the fuel is improved. This will improve reactor operability and stabilize reactor operability.

Furthermore, the fact that the decrease in the degree of subcriticality in the part where the degree of subcriticality is particularly shallow (near the upper end of the core to 3/4L) is alleviated means that the degree of subcriticality decreases slightly toward the center of the core, that is, toward the lower side than before. This means that the region where the criticality becomes shallow shifts. As shown in FIG. 1(B), the water rod 22 in 2/4L to 3/4L is BOC-MOC and has a high void ratio.

Water rod 22 in this and channel box 25
The average value of the void fraction distribution with the outside (larger in the upper part) becomes large in 2/4L to 3/4L, and as a result, the axial distribution of fissile nuclide concentration shifts downward compared to the conventional case. This is the reason why the region where the degree of subcriticality becomes shallow shifts downward.

Among these functions of the fuel assembly of this embodiment, the void ratio distribution and power distribution will be explained based on FIG. 3 in particular. Figure 3 (A) shows the axial distribution of void fraction,
In a conventional fuel assembly using a large-diameter water rod of full-length type, the result is as shown by the dotted line.

On the other hand, in the fuel assembly 20 of this embodiment, the BOC is as shown by a solid line, and the EOC is as shown by a broken line. The small trapezoidal bulge near the bottom of the core is water rod 2.
This is due to the reactor water removal effect of the flow path resistor 30 provided in No. 2. This part and the water rod 22 near the upper end of the core
Except for the diameter-reduced portion (FIG. 1(B)), in the EOC, there is almost no difference in void fraction distribution between the conventional type and the fuel assembly of this example. On the other hand, in BoC, the area is slightly below the top of the core (
3/4L to 5/6L) are the same as the conventional type due to the large diameter water rod, but the height is higher in other parts.

Figure 3 (B) shows the core axial power distribution (relative value),
The basic axial power distribution is not much different from the conventional type because the void ratio distribution is mainly controlled by the void ratio of the water rod 22, but due to the action of the water rod 22, the BOC
Although the void fraction distribution is almost the same in EOC, it becomes like a broken line because the characteristics of plutonium production and combustion in BOC-MOC change. The remarkable feature is that the output from 3/4L to 5/6L is relatively low due to the action of the large diameter water rod from 3/4L to 5/6L, and the action of the small diameter water rods and large diameter void rods in other parts. It is at a point where it rises and fuel can easily flow. Accordingly, the output peak at EOC is suppressed, and the peak location shifts downward from the top of the core (.

ing. Since the heat removal ability of the cooling water is higher on the lower (upstream) side than in the upper part of the core, the integrity of the fuel is improved. In addition, since the power peak position shifts to the inside of the core, neutron leakage outside the core is reduced, improving neutron economy.
In EOC, the reactivity of the core is improved and the fuel economy is improved.

BOC-MO in the upper part of the core (3/4L to 5/6L)
As mentioned above, the progress of combustion in C is that the area where the subcriticality becomes shallow at the time of reactor shutdown shifts to the bottom of the core and becomes shallower, which improves the margin for reactor shutdown and increases the The effectiveness of control rods during control rod scram is accelerated in time.

In other words, the scrum characteristics are improved.

By the way, light water reactors, especially BWRs, are operated with the flow rate of cooling water lowered to some extent from the cycle average flow rate from BOC to MOC. Then, the void fraction distribution rises at a relatively lower part, and the power distribution tends to become higher at the lower part of the core. When the fuel assembly 20 of this embodiment is used, the flow rate decreases, and as a result, when the output increases in the lower part of the core, the resistance of the flow path resistor 30 provided at the lower part of the water rod 22 increases, and the water rod 22 The flow rate of water through this is reduced. Of those provided in a double tubular shape, the flow path resistor 3
On the side where 0 is provided, voids are generated due to heat generation due to absorption of gamma rays, etc. Further, on the side where the flow path resistor 30 is provided, the cross-sectional area is small in a certain range at the upper end and lower end of the core, and becomes large in other parts.

Therefore, the core axial voidage distribution is from BOC to M
Although the upper (downstream) side becomes higher as a whole toward OC, it is raised except for certain areas at the upper and lower ends of the core. As a result, the hardening of the neutron spectrum promotes the accumulation of plutonium in areas with increased void fraction.

On the other hand, in EOC, the flow rate of the cooling water is increased to some extent from the cycle average flow rate. Then, the void fraction distribution curve rises higher (downstream) than in the case of BOC or MOC, and the core power distribution tends to be higher at the upper part of the core rather than at the lower part. In this embodiment, when the flow rate of cooling water in the core increases, the power in the upper part of the core increases and the power in the lower part decreases, the flow path resistance of the flow path resistor 30 decreases, and the water rods 22 The flow rate of water through 30 is increased. Therefore,
Among those provided in a double tube shape, the voids generated on the side where the flow path resistor 30 is installed are washed away, and the amount of void generation is reduced, so that the side where the flow path resistor 30 is installed also contains almost no voids. It becomes like a water rod.

That is, in EOC, the water rod 22 becomes a water rod.

Since the flow path resistor 30 is not water, it becomes a void for neutrons locally, but in other parts, a certain range from the top and bottom of the core is a small diameter water rod, and the other part is a large diameter water rod. It becomes a water stick.

There are several reasons for reducing the diameter in the upper part of the core, one of which is to reduce the pressure loss of the coolant, and the other is the development of highly economical fuel that uses natural uranium in the upper part of the core. However, in that case, if a large-diameter water rod is used, neutrons will recede, or there will be an excess of water. Excess water phenomenon may occur in the lower part of the reactor core, and the void ratio increases from the BOC to the MOC (if the diameter is large), increasing the leakage of neutrons downward, which tends to cause neutron loss. has been made into If the water rod 22 is made from a large diameter to a small diameter or is made from a large diameter water rod at the upper part of the reactor core, the void ratio is relatively reduced from the BOC to the MOC, so that combustion progresses. Plutonium is also relatively easy to generate and burn.

Therefore, the concentration of fissile nuclides at the EOC does not become particularly high as a relative distribution in the axial direction. As a result, subcriticality does not become too shallow when the reactor is shut down. In other words, the reactor shutdown margin is improved.

In addition, the fact that the concentration of fissile nuclides does not become particularly high at the above location is consistent with the fact that the shallow subcriticality shifts to the lower part of the reactor core, which means that the negative reaction to the reactor caused by the control rods in the event of a reactor emergency. The application of energy can be accelerated. In other words, the scram characteristics are improved, which also contributes to improving the safety of the nuclear reactor.

FIG. 4 shows a second embodiment of the present invention, and the same parts as in the first embodiment will be described with the same reference numerals. The main difference between this embodiment and the first embodiment is that the inner tube 24 is also BOC.
- Voiding occurs in the MOC, the flow of reactor water in the outer tube 23 is downward, and 3/4L
-L (upper end) has a smaller diameter. The action of the water rod 22 on neutrons in this embodiment throughout the operation cycle is substantially the same as in the first embodiment.

In this embodiment, reactor water flows in from the lower side of the inner pipe 24,
It passes through the flow path resistor 30 and rises, and the outer tube 2 near 3/4L.
3 and the inner tube 24, and is discharged from around 1/6L. In BOC-MOC, the flow after the flow path resistor 30 is mainly void, and in EOC, the flow is mainly non-boiling water or reactor water with a low void ratio.
This is the same as in the embodiment. The lower end of the large diameter outer tube portion extends slightly downward than in the first embodiment. The part of the fuel assembly 3/4L-L (upper end) from the lower end of the core effective length is a water rod without an outer tube, and is separated from the lower part by a plug 40, so the reactor water is separated from the water rod. It makes up the flow. This part is not intended to improve the characteristics against neutrons, but its main purpose is to prevent the water rod 22 from floating due to the reactor water flow.

Note that a gas-liquid mixture flows inside the small-diameter water rod in the 3/4L-L portion during reactor operation, and reactor water occupies the interior when the reactor is stopped. In addition, this embodiment has a smaller diameter in the upper part of the core, so it has the feature of being able to significantly reduce the pressure loss of the coolant. However, compared to the first embodiment, there is a problem that causes a slight shortage of moderator in the upper part of the core. be. In order to avoid this, the large-diameter portion of the lower core heliotrorad 22 is extended. The other configurations and operations are the same as those of the first embodiment, so their explanation will be omitted.

Incidentally, there are many possible configurations of the fuel assembly in which the water rods serving as the moderator guide members used in the present invention can be arranged. For example, the fuel assembly 50 shown in FIG. 5 corresponds to the first embodiment, but the fuel assembly 50 shown in FIG.
A book water rod 22 is arranged. Eight water rods 22 are arranged in the fuel assembly 60 shown in FIG. 6, and nine water rods 22 are arranged in the fuel assembly 70 shown in FIG. In addition, the fuel assembly 80 shown in FIG. 8 is divided into four subassemblies in which fuel rods 81 are arranged in a 5×5 arrangement, and between each subassembly there is water extending from the central water rod 22 in a flat plate shape. A plate 82 is provided. Here, the configuration of the water rod 22 is the first and second water rods.
The structure is similar to that of the embodiment.

〔Effect of the invention〕

As described above, according to the present invention, the maximum cross-sectional area of the moderator guide member is set over the entire length of the axial fuel effective section, and the rate of water removal using this cross-sectional area is determined separately from the upper and lower ends of the core. This makes it possible to suppress large changes in the power distribution throughout the operation cycle and ensure fuel integrity without reducing reactor power through a stable power distribution. can.

Further, by moving the output peak position in the EOC to the lower (upstream) side and improving the neutron economy, there is an effect of improving the fuel economy.

[Brief explanation of drawings]

FIG. 1(A) is a plan view showing a fuel assembly according to a first embodiment of the present invention, FIG. 1(B) is a cross-sectional view showing the water rod of FIG. 1(A), and FIG. A) is a vertical cross-sectional view of the flow path resistor provided in the water rod, and FIG.
2(C) is a sectional view taken along line C-C in FIG. 2(A), and FIG. 2(CD) is a sectional view taken along line BB in FIG. 2(A). Cross-sectional view along line D, Figure 3 (A)
9(B) is the axial distribution of the void fraction of the water rod,
FIG. 4 is a graph showing the power distribution in the core axial direction; FIG. 4 is a vertical cross-sectional view showing the water rod in the second embodiment of the present invention;
6, 7, and 8 are plan views showing other arrangements of water rods, and FIG. 9 is a longitudinal sectional view showing a conventional fuel assembly. 20...Fuel assembly, 21...Fuel rod, 22...
Water rod (moderator guide member), 2...Outer tube, 2
4...Inner tube, 25...Channel box, 30.
...Flow path resistor. (,4) Fig. 1<8) Fig. 1 Fig. 4 Fig. 5 Small 'zudo rod (%) (A) 1st output (1 @ vs.) (B) Fig. 3

Claims (1)

[Claims] A large number of fuel rods and spectrum-shift type moderator guide members having a larger diameter than the fuel rods are regularly arranged, and water, which acts as a moderator and coolant, is arranged in a direction parallel to the axes of the fuel rods and the guide members. In the flowing fuel assembly, the maximum cross-sectional area of the moderator guide member is set over the entire length of the axial fuel effective section, and this cross-sectional area determines the volume ratio from which water is removed from the upper and lower ends of the core, respectively. A fuel assembly characterized by being smaller in a certain range and larger in other parts.