JPS62148885A - Fuel aggregate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a fuel assembly, particularly to a fuel assembly loaded into a light water nuclear reactor.

[Background of the invention]

In order to operate a nuclear reactor for a certain period of time, the reactor must have surplus reactivity at the beginning of operation equal to the amount of reactivity deterioration due to combustion of fissile material during operation. Also,
In order to keep the reactor critical, this excess reactivity must be controlled.

Conventional methods include inserting control rods made of neutron-absorbing materials into the reactor core, and adding gadolinia (Gd) to the fuel.
A method of loading fuel assemblies containing special fuel rods mixed with burnable poisons with a large neutron absorption cross section such as zOa) into the reactor core is often used in combination.

On the other hand, high burnup of fuel is currently being considered for light water reactors with the aim of improving fuel economy.6 Figures 2 and 3
The figure shows a vertical cross-sectional view and a cross-sectional view, respectively, of a fuel assembly of a conventional boiling water reactor. It is composed of a tie plate 3, a spacer 4, a lower tie plate 5, and a channel box 6 that houses these, and a control rod 7 is inserted between adjacent channel boxes 6.
is installed. Upper and lower tie plates 3, 5
fixes both ends of a plurality of fuel rods 1, and upper and lower tie plates 3 and 5 are each provided with a plurality of holes for cooling water passages.

A plurality of spacers 4 are provided at intervals in the axial direction within the channel box 6, and are arranged to support the plurality of fuel rods 1. Of the fuel rods 1, the fuel rod G indicated by 1a is filled with uranium dioxide (UO2) fuel pellets containing gadolinium (Gd), which is a neutron absorbing substance, in order to control the excess reactivity of the nuclear reactor. on the other hand,
Saturated water flows through the water rod W2, which is a hollow tube, making the amount of water at the center of the fuel assembly (see Figure 3) relatively large compared to the volume of the fuel, and causing neutrons to flow through the water rod W2. The deceleration effect increases the reactivity of the fuel and flattens the local power distribution within the fuel assembly. Also.

The water rod W2 also has the role of reducing changes in reactivity due to changes in void ratio within the fuel assembly.

In order to increase the extraction burnup using such a fuel assembly, it is necessary to increase the enrichment of fissile material in the fuel. Increasing the enrichment of the fuel increases the surplus reactivity that must be controlled in the early stages of combustion, and also increases the amount of burnable poison required for control. Therefore, the conventional G
In a fuel assembly using fuel rods loaded with fuel pellets containing a burnable poison containing dzOa as a burnable poison (hereinafter referred to as fuel rods containing a burnable poison), it is necessary to increase the number of fuel rods containing a burnable poison. be. Furthermore, as the fuel enrichment increases, the average energy of neutrons in the core increases, so the effect of controlling the surplus reactivity per fuel rod containing burnable poison decreases. For this reason, it becomes necessary to further increase the number of fuel rods containing burnable poison or to increase the concentration of poison.

Fuel pellets added with GdzOa (hereinafter referred to as GclzO)
(referred to as a-added fuel pellets) have a lower melting point and lower thermal conductivity than normal U 2 O2 pellets. Therefore, under the same linear power output, GdzOs-added fuel pellets have a higher fuel core temperature than normal UO2 fuel pellets.

In addition, in the fuel pellets containing 20 g of Qd, the initial heterogeneous fuel fine structure of the U Ox + G d aox mixture gradually becomes homogeneous because the solid solution of Gd in the U O2 aggregate structure progresses gradually under irradiation. be converted into

Due to the formation of a solid solution during combustion, surplus oxygen is generated within the fuel pellets and reacts with the inner surface of the Zircaloy cladding.
Compared to normal U O2 fuel, the oxidation rate on the inside of the cladding tube is higher. At the same time, high-energy gamma rays generated when Gd absorbs neutrons are thought to accelerate corrosion on the coolant side of the cladding. There is concern that this may be a cause of pipe breakage.

For these reasons, when the number of fuel rods loaded with fuel pellets containing burnable poison increases at high burnup, there is a problem that the integrity of the entire fuel assembly deteriorates.

In addition, in the case of a fuel assembly consisting of fuel rods loaded with fuel pellets containing burnable poisons, the fuel rods containing burnable poisons must be made separately, and the fuel manufacturing management from fuel pellet production to assembly is difficult. There are problems in that it becomes complicated and increases manufacturing costs.

[Purpose of the invention]

The present invention provides a fuel assembly that maintains the function of controlling excess reactivity caused by burnable poisons, ensures the soundness of the fuel, and avoids increases in the manufacturing cost of the fuel assembly and complication of manufacturing management. The purpose is to provide the following.

[Summary of the invention]

The present invention includes a cylindrical channel box, a tie plate fitted on the upper and lower sides of the channel box, a plurality of spacers installed at intervals in the axial direction of the channel box, and the tie plate. and fixed by the spacer and arranged in a grid pattern,
In a fuel assembly comprising a fuel rod having fuel pellets in an I pipe and a water rod, and containing a burnable poison having a large neutron absorption cross section, at least a majority of the burnable poison having a large neutron absorption cross section is The first feature is that the fuel rods are contained in a portion other than the fuel pellets of the fuel assembly, and are arranged in a lattice shape or a close-packed hexagonal lattice shape. In a fuel assembly comprising a rod guide tube and a plurality of spacers installed at intervals in the axial direction to fix these, and containing a burnable poison with a large neutron absorption cross section, the neutron absorption cross section is A second feature is that at least a majority of the large burnable poison is contained in a portion of the fuel assembly other than the fuel pellets.

That is, the present invention was made based on the following study results.

As mentioned above, one method for controlling excess reactivity is to load fuel assemblies containing special fuel rods in which fuel pellets are mixed with a burnable poison with a large neutron absorption cross section, such as Gd20a, into the reactor core. This method has disadvantages such as a decrease in the melting point and thermal conductivity of the fuel pellets, an increase in oxidation of the inner and outer surfaces of the cladding tube, a complication of the manufacturing process, and an increase in manufacturing costs due to the addition of burnable poisons. However, in order to achieve a high burnup to reduce fuel costs, initial excess reactivity must be controlled using burnable poisons. Therefore, in the present invention, we have investigated a method of controlling initial surplus reactivity using burnable poisons, improving the soundness of fuel pellets and fuel cladding, and not increasing manufacturing costs, and removing burnable poisons from within fuel pellets. It is used, for example, in the form of an alloy with zirconium in fuel rod cladding or water rond cladding. An example of use in a water rod cladding tube will be described below.

Generally, the neutron absorption cross section of burnable poisons such as G d 208 used in light water reactors increases as the energy of the neutrons decreases. Therefore, if a burnable poison is placed in an environment where the neutron energy is low, that is, the neutron spectrum is soft, the effect of controlling the excess reactivity can be enhanced. Therefore, in the present invention, the burnable poison is not added to the fuel pellets, but is used in the water rod cladding tube in the form of an alloy with zirconium, so that the average neutron energy spectrum within the fuel assembly is relatively soft. The neutron absorption reaction was caused by burnable poison, and the effect of controlling excess reactivity was enhanced.

Further, according to the present invention, there is no need to add burnable poison to the fuel pellets, so there are the following advantages.

The first point is that it is possible to prevent an increase in fuel core temperature due to the addition of burnable poison into fuel pellets. That is, by adding a burnable poison, the UOz sintered fuel becomes
When adding d2086% by weight, the melting point decreased by about 150°C from 2800°C, and the thermal conductivity was 2.84% at 1000°C.
It decreases by about 17% from X 10-2 (W/am'C). As a result, compared to regular U Ox pellets, Gd2
．． 0g additive fuel pellets.

The fuel center temperature changes with respect to the linear output as shown in FIG. The horizontal and vertical axes of this figure show the line power (W/
Cm) and fuel center temperature ('C) are 6% by weight.
The cases of GdzOa-added U○2 fuel and UO2 fuel are shown, and at a linear output of 200W/Cm, 6% by weight of G
The fuel center temperature of the dzOs-added U Ox fuel is approximately 130° C. higher than that of the normal U 02 fuel.

Figure 5 shows the linear output history of fuel rods containing burnable poison, and the horizontal and vertical axes show the average burnup (GWd/
T) and the relative output level when average linear output = 1.0, and the peak linear output and 2 and 3 weight% Gd
The relative power level of the rods is indicative. This figure shows that at the start of combustion, the linear output is about half that of normal U Ox fuel pellets due to the surplus reactivity control effect of burnable poison, and then the linear output gradually increases until the burnup exceeds 10 GWd/lU, which causes normal U Ox, It now has the same linear output as fuel pellets. Therefore, an increase in the fuel core temperature due to the addition of burnable poison becomes a problem after the burnable poison controls the initial surplus reactivity. According to the present invention, since the burnable poison exists as an alloy in the water rod cladding tube, the temperature increase at the center of the fuel pellet containing the burnable poison as described above does not occur, and all the fuel rods in the fuel assembly It is usually UO2 pellets and can maintain the fuel center temperature appropriately.

The second point is that increased oxidation on the outer and inner surfaces of the fuel rod cladding can be prevented. When fuel containing GdzO3 is irradiated in the reactor, 12I"G d (n + "I) lll8G d
Nuclear reactions such as 111'G d (n + 7) 15''G d generate up to 0.103 M e for each reaction.
γ rays of V, 0, 36 M e V are generated. The γ rays cause radiolysis of the cooling water near the outer surface of the cladding tube, increasing the oxygen concentration in the cooling water. On the other hand, water scale (crud) adheres to the outer surface of the cladding tube, and the presence of this water scale makes the flow of cooling water extremely slow in the immediate vicinity of the cladding tube. Therefore, the above-mentioned state of increased oxygen concentration continues.

Corrosion of the outer surface of the fuel cladding tube under high-temperature underwater tron irradiation may be further accelerated.

According to the present invention, all burnable poisons are included in the water rod cladding as an alloying component.

Increased corrosion on the outer surface of the fuel cladding tube as described above does not occur. Furthermore, an increase in oxidation of the inner surface of the fuel cladding tube due to promotion of solid solution formation of GdzOa can also be avoided.

The third point is that fuel production management from production to assembly of fuel pellets becomes easier, and production costs can be reduced. In the case of a fuel assembly consisting of fuel rods loaded with burnable poison fuel pellets I, it is necessary to separately produce the fuel rods containing the burnable poison, but this is no longer necessary according to the present invention. Note that it is necessary to make two types of fuel rod cladding tubes and water rod cladding tubes: Zircaloy-2 and zirconium-gadolinium (Zr-Gd) alloys, but manufacturing Zr-Gd alloy is relatively easy. .

Next, the composition of the Zr-Gd alloy used in the present invention will be explained.

The current design specifications for GdzOs-added fuel pellets are New 8.
×8 type fuel assembly burnup 45 G W d/t
When Uwo target is set, GdzOa'a degree is 4.0 to 4.5
Loading fuel rods into 1 by weight fuel rods per assembly, 1
3 to 14 pieces are required. Therefore, in this case, the required Cd
The amount will be 1430 g to 1740 g per aggregate. FIG. 6 shows the weight percent of Gd in the water rod cladding tube when this amount of Gd is contained in the water rod cladding tube as an alloy component of Zr. Take the number of water rods on the horizontal axis,
If we take the Gd weight percent of the required water rod cladding tube on the vertical axis, the Gd weight percent will be 29 to 34 weight percent in the case of four water rods, and 25 to 29 weight percent in the case of five water rods.

Further, when the minimum number is one, the Gd weight % is 67.0 weight %, and when the maximum number is eight, the Gd weight % is 17.0 weight %.

The phase state of the Zr-Gd alloy having such a composition can be determined from the Zr-Gd phase diagram shown in FIG. When the Gd weight percentage is 17 to 67 weight % (Gd atomic percentage is 11 to 54%), the Zr-Gd alloy is in the α-Zr and α-Gd states near the cladding temperature (approximately 300°C). exist. Since α-Zr and α-Gd both have hexagonal close-packed crystal structures, and Gd is ductile and malleable like other rare earth metals, Zr-ad gold alloy can be used as a coating material.

The number of fuel rods and water rods in an assembly using the Zr-Gd alloy of the present invention is determined as follows. The water rond containing the burnable poison of the present invention functions as a neutral moderator in the same way as the conventional water rond after the burnable poison is combusted. If the fuel rods in the fuel assembly are replaced with water rods containing burnable poison and water rods free of burnable poison, the moderator-to-fuel ratio increases.

This increase in moderator-to-fuel ratio.

When the enrichment of the fuel is kept the same, the infinite neutron multiplication factor increases, and the amount of natural uranium required to ensure a certain amount of extraction burnup can be saved as shown in FIG. Figure 8 shows the effect of saving natural uranium to obtain a unit burnup when some of the fuel rods are replaced with water rods containing burnable poison according to the present invention. , the horizontal axis shows the number of water rods containing burnable poison, and the vertical axis shows the amount of natural uranium saved (%). As the number of water rods increases, the uranium saving effect increases, reaching a peak value at 4 rods, and when increasing the number of rods to 10 or more, fuel economy deteriorates. On the other hand, the number of burnable poison-containing water rods required is determined by the Gd weight percent of the water cladding tube, the amount of excess reactivity that must be controlled, and the like. Considering these, the number of water rondos containing necessary Od is about 4 to 5.

As described above, in the fuel assembly of the present invention, instead of adding burnable poison to the fuel pellets, Zr-
By including it as a Gd alloy in the water rond M pipe,
Fuel rod integrity can be maintained throughout the entire irradiation period. Furthermore, there is no need to mix burnable poisons such as GdxOs into the fuel rods, and fuel economy is also improved.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 1 is a cross-sectional view of the first embodiment, and the same parts as in FIG. 3 are given the same reference numerals. This example uses an 8x8 lattice fuel assembly with 5 burnable poison water rods 8 and 59
It is composed of fuel rods 9 that do not contain burnable poisons. The numbers 1.2 and 3.4 in the circle indicating the fuel rod 9 in the figure indicate the uranium enrichment level of the fuel rod.

The uranium enrichment in the fuel rod numbered 1 is 3.1% by weight.
, 2 is 2.8% by weight, 3 is 2.4% by weight, and 4 is 1.8% by weight, and the average concentration of 59 bottles is 2.77% by weight! be. The water rod 8 containing the burnable poison is a cladding tube with the same dimensions as a conventional water rond, and is fused with 25 to 29% by weight of G.
d as an alloy composition.

The water rods 8 containing the burnable poison are arranged symmetrically with respect to a diagonal line connecting the corner of the fuel assembly into which the control rod 7 is inserted and the corner into which the control rod is not inserted. This is to maintain the property that the power distribution and neutron flux distribution within the fuel assembly are symmetrical with respect to the diagonal line when inserting and withdrawing the control rods, and to reduce variations in fuel burnup caused by asymmetry. Also, water rondos containing burnable poisons are placed so that they are not next to each other.

When water rods containing burnable poisons approach each other, their interaction hardens the nearby thermal neutron energy spectrum due to neutron absorption by the burnable poisons, resulting in
This is to avoid reducing the surplus reactivity control effect of the water rod containing burnable poison.

One of the effects of the present invention is the effect of preventing deterioration of the characteristics of fuel pellets by including burnable poison in the water rod cladding tube. It can also be obtained by including it in a rod cladding tube.

FIG. 9 shows a second embodiment of the present invention, in which a water rod 8 containing a burnable poison and a fuel rod 1a containing Gd 208 loaded with GdxOs-added fuel pellets are used together to control the required reactivity. This is what we do. The water rods 8 containing the burnable poison are arranged in a region closer to the center of the fuel assembly than the Gd2O3-filled fuel rods 1a, and the GdxOs-filled fuel rods 1a are arranged on the outside closer to the water gap surrounding the fuel assembly. be.

The reason for this arrangement is that the GdzO3-containing fuel rod 1
By placing a in a region with low neutron energy,
On the other hand, the neutron energy in the water rod 8 containing the burnable poison is low due to the neutron moderating effect of the moderator (light water), so the excess reactivity can be controlled sufficiently. This is because the effect is enhanced. According to this embodiment, by employing the water rods 8 containing burnable poison, it is possible to reduce the number of GdzOs-containing fuel rods 1a that would be required to be increased in a conventional fuel assembly. As a result, the soundness of the fuel assembly is improved, and at the same time, in terms of fuel economy, the uranium saving effect is achieved by the number of water rods 8 containing burnable poison.

As described above, the first and second embodiments have shown examples in which water rods containing burnable poisons are used. Even if it contains a toxic substance, it acts in the same way and the same effect can be obtained.

In the above embodiment, the present invention was applied to a fuel assembly for a boiling water reactor, but it is also possible to apply the present invention to a fuel assembly for a pressurized water reactor.

That is, even in pressurized water reactors, high burnup is aimed at, and for this reason, it is necessary to use a large amount of fuel containing GdzOs, and when this is used, the same problems as in the case of boiling water reactors described above arise. To address this, GdzOs is separated from the fuel pellets.

It is conceivable to include Cd necessary for reactivity control in a fuel cladding tube using a Zr-Gd alloy, or to use a Zr-Gd alloy in a structural material such as a control rod guide tube.

Furthermore, the initial surplus reactivity control method using the Zr-Gd alloy according to the present invention is applicable not only to light water reactors, but also to high conversion light water reactors (H
(high Conversion Reactor). A high-conversion light water reactor is a reactor that improves the conversion ratio of fissile material by narrowing the spacing between fuel rods and increasing the average neutron energy from the thermal neutron region to the epithermal neutron region, in order to effectively utilize nuclear fuel.

As described above, in high-conversion light water reactors, the outer surface corrosion of fuel rod cladding becomes more of a problem than in conventional light water reactors due to the narrowing of the fuel rod spacing and the longer loading period of fuel assemblies in the reactor in order to achieve high burnup. Therefore, if the need to add burnable poisons to fuel pellets is eliminated by using Zr-Gd alloys in cluster control rod guide tubes, corrosion on the outer surface of fuel rod cladding tubes will be reduced compared to before. The integrity of the fuel rods will improve.

As described above, the fuel assembly of the example is made by using combustible poison in the form of an alloy with zirconium in the fuel rod cladding tube or water rod cladding tube, etc., instead of adding the combustible poison into the fuel pellet. Initial surplus reactivity control using toxic substances can be implemented and improve the integrity of fuel pellets and fuel cladding, preventing increases in manufacturing costs.

〔Effect of the invention〕

The fuel assembly of the present invention retains the function of controlling excess reactivity caused by burnable poisons, ensures the soundness of the fuel, and makes it possible to avoid increases in fuel assembly manufacturing costs and complication of manufacturing management. This is a major industrial effect.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of an embodiment of the fuel assembly of the present invention;
FIG. 2 is a longitudinal sectional view of a conventional fuel assembly. Figure 3 is a cross-sectional view of a conventional fuel assembly, Figure 4 is a Gcl
A diagram showing a comparison of the center temperature of zOs + added U O additive and normal U Ox fuel, Figure 5 is a diagram showing the output history of GdzOa added fuel, and Figure 6 is a diagram showing the number of water rods and water rond cladding. A diagram showing the relationship between Gd weight percent, and FIG. 7 is a phase diagram of a Zr-Gd alloy. FIG. 8 is a diagram showing the relationship between a water rod containing burnable poison and the effect of saving natural uranium, and FIG. 9 is a cross-sectional view of another embodiment of the fuel assembly of the present invention. 1...Fuel rod, 1a...Fuel rod (filled with fuel pellets containing Gd), 1b...Fuel rod (not containing burnable poison), 2...Water rod ,6・
... Channel box, 7... Control rod, 8... Flammable front (1 other person) Me 1 Leap 2nd meter 4th meter 50 ′Average 稺nichitō-1,0) 目1=(2) ＼＼-−1nt(舛１ ＧO

Claims (1)

[Claims] 1. A cylindrical channel box, tie plates fitted to the top and bottom of the channel box, and a plurality of spacers installed at intervals in the axial direction of the channel box. and fuel rods and water rods fixed by the tie plate and the spacer and arranged in a grid pattern, each having fuel pellets in a cladding tube,
In a fuel assembly containing a burnable poison having a large neutron absorption cross section, at least a majority of the burnable poison having a large neutron absorption cross section is contained in a portion of the fuel assembly other than the fuel pellets. Characteristic fuel assembly. 2. Claim 1, wherein the portion other than the fuel pellet containing the burnable poison having a large neutron absorption cross section is a part of the fuel rod or a cladding tube of the water rod.
Fuel assembly as described in section. 3. The burnable poison with a large neutron absorption cross section is 17
．． The fuel assembly according to claim 1 or 2, which is a zirconium-gadolinium alloy containing gadolinium in an amount of 0% by weight or more and 67.0% by weight or less. 4. Fuel rods having fuel pellets in cladding tubes arranged in a lattice shape or a dense hexagonal lattice shape, a control rod guide tube, and a plurality of spacers installed at intervals in the axial direction to fix these. in a fuel assembly containing a burnable poison having a large neutron absorption cross section, at least a majority of the burnable poison having a large neutron absorption cross section being contained in a portion of the fuel assembly other than the fuel pellets. A fuel assembly characterized by: 5. Claim 4, wherein the portion other than the fuel pellets containing a burnable poison with a large neutron absorption cross section is a part of the fuel rod and the cladding tube of the control rod guide tube. fuel assembly. 6. The burnable poison with a large neutron absorption cross section is 17
．． The fuel assembly according to claim 4 or 5, which is a zirconium-gadolinium alloy containing gadolinium in an amount of 0% by weight or more and 67.0% by weight or less.