JPH03273189A - Fuel assembly - Google Patents

Abstract

PURPOSE:To flatten the output distribution in axial direction by forming the upper part of a 1st combustible poison containing fuel rod to concentration different from the upper part of a 2nd combustible poison-containing fuel rod and making the concentration difference between their upper parts larger than the difference between the lower parts. CONSTITUTION:Gadolinium-containing rods G2 are arranged in an area 83, where fuel rods arranged in the 3rd layer from the outermost periphery of the fuel assembly 1 are positioned, adjacently to a large-diameter water rod 7. The fuel rods G2 are 5.0wt.% in gadolinium concentration within a range from the lower end of the fuel effective length to 12/24 as large as the reactor core effective length and 4.5wt.% above it in a gadolinium concentration 2 area. In an area 82 positioned outside the area 83 where the fuel rods G2 are arranged, gadolinium-containing fuel rods G1 which are 0.5 wt.% increased in gadolinium concentration at the lower part as compared with the fuel rods G2 and 1.0wt.% decreased at the upper part are arranged.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fuel assembly used in a boiling water reactor (BWR), and particularly to a fuel assembly with a high burnup to improve fuel economy. .

[Conventional technology]

The BWR fuel assembly has a structure in which fuel rods loaded with nuclear fuel and fuel rods containing burnable poison (hereinafter referred to as "Gadolinian fuel rods") are arranged in a square grid. have. Normally, the reactivity and thermal characteristics of the core are appropriately controlled by adjusting the concentration of gadolinia mixed into the fuel rods and the number of gadolinia fuel rods to rAu. That is, the Gadolinian fuel rod is designed to satisfy the following four items.

■It is possible to operate the specified operation cycle, and the change in surplus reactivity combustion during the operation cycle satisfies the design conditions.

■At the end of the driving cycle (
EOC), and no burnable poison remains in the reactor core.

■The reactor shutdown margin must be satisfied.

■Thermal design conditions must be satisfied.

The gadolinia concentration and the number of gadolinium fuel rods were determined to satisfy the above condition ■■, and at the same time, the condition
It is sufficient if you can satisfy the following. For boiling water reactors, usually 1
One operating cycle is 12 months, with an average concentration of 3% by weight.
The degree is low. Therefore, the surplus reactivity to be suppressed is small, and the amount of gadolinia, which is the product of the gadolinia concentration and the number of gadolinia fuels, may be small. One driving cycle.

The period from reactor startup to reactor shutdown for fuel exchange.

However, in recent years, fuel assemblies have been developed that can improve fuel economy by increasing the enrichment and extending the fuel extraction burnup (lengthening the inversion cycle period). There is. Regarding this type of fuel assembly, for example, Japanese Patent Application Laid-Open No. 106391/1982 can be cited. In the prior art shown in FIG. 8, two or more types of gadolinia-containing fuels 71 and 72 with different gadolinia concentrations are used, and when these are divided into a first group and a second group according to the gadolinia concentration, the first group Gadolinia reactivity value disappears in one operating cycle,
adjusting the gadolinia concentration such that the gadolinia reactivity value of the second group persists for more than one operating cycle;
The first group is arranged outside the second group.

[Problem to be solved by the invention]

In a fuel assembly for a boiling water reactor, a difference occurs in the neutron spectrum between the fuel rods at the periphery of the fuel assembly and the fuel rods at the center of the fuel assembly. Therefore, just as the burnable poison added to a new fuel assembly burns out during the first operation cycle of the fuel assembly, in other words, the burnable poison remains unburned and wasteful thermal neutron absorption occurs during the next operation cycle. To prevent this from occurring, the burnable poison concentration must be designed taking into account the position within the cross section of the fuel assembly.

However, in high burnup fuel assemblies with a larger extraction burnup, as shown in Figure 9, in order to improve the homogeneity of the neutron spectrum in the cross section within the fuel assembly, the central part of the fuel assembly is A large-diameter water rod 7 is placed in the area.

Therefore, in the cross section of the fuel assembly, the neutron spectra are almost the same, and as will be described later, the output portion 41 in the cross section is Xfl flat.

Therefore, in a fuel assembly for high burnup in which a large-diameter water rod is placed in the center of the fuel assembly, it is necessary to optimize the distribution of burnable poisons by considering the void fraction distribution in the axial direction of the fuel assembly. The inventors discovered something.

An object of the present invention is to flatten the axial power distribution of the reactor core and improve core safety in a high burnup fuel assembly having large diameter water rods, without reducing fuel economy. The objective is to provide a fuel assembly that can.

[Means to solve the problem]

In order to achieve the above objective, in the case of a high void ratio in the upper part of the core, the output near the large diameter water rod inserted into the center of the fuel assembly is lower than in other fuel areas, so burnable poison The concentration should be small compared to other fuel regions.

Furthermore, in the case of a low void fraction corresponding to the lower part of the core, the homogeneity of the aggregate is enhanced, thereby reducing the difference in burnable poison concentration. By configuring the axial burnable poison distribution of the fuel assembly as described above, it is possible to flatten the axial power distribution, ensure sufficient scram reactivity, and improve core safety.

That is, the present invention provides a fuel assembly for a boiling water reactor, which has one or more large-diameter water rods in the center of the fuel assembly, and in which a plurality of fuel rods are arranged in a lattice pattern. The first burnable poison fuel rods are mixedly arranged in the fuel rod group in the area facing the diameter water rod, and the second fuel rod group is arranged in the area outside the area where the first burnable poison fuel rods are arranged. The fuel rods containing burnable poison are arranged in a mixed manner, and the first and second fuel rods containing burnable poison are formed with different concentrations of burnable poison at the top and bottom, and the flammable poison in the upper part of the first fuel rod containing burnable poison is different. The concentration of the poison is lower than that in the upper part of the second burnable poison-filled fuel rod, and the difference in the burnable poison concentration between the upper parts of the first and second burnable poison-filled fuel rods is greater than the difference between the lower parts. This fuel assembly is characterized by:

In the fuel assembly, the fuel rod group in the region facing the large-diameter water rod has a third fuel rod in which the concentration of burnable poison is uniformly formed in the upper and lower part of the fuel rod group.
It is better to have fuel rods containing burnable poison mixed together.

Further, it is preferable that a third burnable poison-containing fuel rod, in which the burnable poison concentration is lower in the lower part than in the upper part, is mixedly arranged in the fuel rod group in the area facing the large-diameter water rod. Also,
It is preferable that the burnable poison concentration in the upper part of the first burnable poison-containing fuel rod is zero or less than half of the minimum burnable poison concentration in other parts. Alternatively, the burnable poison concentrations in the lower portions of the first and second burnable poison-containing fuel rods may be equal. Further, it is preferable that the fuel rod is divided into multiple regions along the fuel enrichment axis of the fuel rod.

In addition, the concentration difference of the burnable poison between the upper parts of the first and second burnable poison fuel rods is changed so that the first burnable poison fuel rod side is higher than the second burnable poison fuel rod side. It may be something that has been done.

[Effect]

Burnable poisons with large diameter water rods (e.g. gadolinia)
Table 1 compares the average output in each region shown in FIG. 9 for a fuel assembly that does not include.

Table 1 When the void ratio is O%, which corresponds to the lower part of the core, the presence of large-diameter water rods increases the homogeneity within the assembly, so the average power P+ in the region 83 around the large-diameter water rods is It is next highest after the region 81 facing the water region 8. Further, the ratio PI/PM of the average output PI of the area 83 to the average output PM of the area 82 that is adjacent to the area 83 and does not face the gap water area 8 is 1.010.
, and the power distribution is flattened in the cross section of the fuel assembly. In addition, the void rate corresponding to the upper part of the core is 70%.
In the case of , the ratio of the average power PI in the region 83 to the average power PM in the region 82 is 0.977, and the power distribution in the cross section of the fuel assembly is flattened.

From the above, in high burnup fuel assemblies with large diameter water rods, the void fraction distribution in the axial direction of the fuel assembly is considered rather than the cross section of the fuel assembly, and the upper and lower parts of the fuel assembly are It may be advantageous to determine the gadolinia concentration in each cross section.

10 and 12, the amount of gadolinia added to the fuel assembly is constant, and the gadolinia fuel is added to the region 8.
2. Place 8 and 7 rods in region 83, respectively, and set the gadolinia concentration of the gadolinian fuel specimen placed in region 82 as GM (wt%), and the gadolinia concentration of the gadolinian fuel specimen placed in region 83 as (3+ (wt%) and shows the combustion change due to the difference in the infinite neutron multiplication factor of the fuel assembly.
The broken line in Figure 1 shows the difference in infinite multiplication factor between case B and case A in Figure 10 [i.e. (case B) - (case A)], and the solid line in Figure 11 shows the difference between case C and case A in Figure 10. The difference in infinite multiplication factor from A [i.e. (Case C) - (Case A
)). In FIG. 13, the broken line shows the infinite multiplication factor difference between case B and case A [i.e. (case B) - (case A)], and the solid line shows the infinite multiplication factor difference between case C and case A [i.e. (case C)-(Case A)) are shown respectively. Table 2 shows the combinations of gadolinia concentrations GM and Gx. Case A is a reference case in which the gadolinia concentrations in the region 83 and the region 82 outside it are equal; Case B is a case in which the gadolinia concentration is larger in the region 83; and Case C is the opposite case. shall be.

10 and 11 show the case where the void ratio is 70% (upper part of the fuel assembly), and FIGS. 12 and 13 show the case where the void ratio is 0% (lower part of the fuel assembly). When the void ratio is 70%, which corresponds to the upper part of the fuel assembly, cases B and C in which the gadolinia concentrations in the region 83 and region 82 (the second layer of fuel rods from the outside) are not equal are more likely to have gadolinia. Compared to case A (standard) where the concentration is the same, it is
/ s t ), the infinite neutron multiplication factor can be maintained at a maximum of about 0.3%Δ. Also.

Even when the void fraction is 0%, which corresponds to the lower part of the fuel assembly, the infinite neutron multiplication factor can be made larger in cases B and C than in case A from the beginning of the operation cycle to the middle thereof.

The effect when the gadolinia concentration difference is increased between the central part and the outer region in each cross section of the upper and lower parts of the fuel assembly has been explained above. In other words, by creating a large gadolinia concentration difference between the central part of the fuel assembly (area 83) around the large-diameter water rod and the outer area (area 82) as in Cases B and C, it is possible to The neutron infinite multiplication factor up to the end of the operating cycle becomes large, and the neutron infinite multiplication factor from the middle of the operating cycle to the end of the operating cycle becomes small. In addition, if there is no difference in gadolinia concentration between the two regions as in case A, the infinite neutron multiplication factor up to the middle of the operating cycle will be smaller, and the infinite neutron multiplication factor after the middle of the operating cycle will be approximately 0.9. %Δ. Therefore, in the upper part of the fuel assembly, there is a difference in gadolinia concentration between the center and outer regions in the cross section, and in the lower part of the fuel assembly, the difference in gadolinia concentration between the center and outer regions in the cross section is made equal to the difference in gadolinia concentration in the upper part. In a fuel assembly with a gadolinia concentration distribution that is smaller than the difference (preferably zero), the neutron multiplication rate between the upper part of the fuel assembly and the lower part is infinite from the middle of the operation cycle to the end of the operation cycle. This makes it possible to reduce the difference in fuel assembly average axial power distribution. in this case,
The difference in the infinite multiplication factor between the upper part and the lower part of the fuel assembly from the beginning of the operating cycle to the middle of the operating cycle also becomes smaller. Furthermore, it is possible to correct the upward distortion in the axial power distribution at the end of the operation cycle.

In the lower part of the fuel assembly, it is not necessarily the case as in case A.

It is not necessary to set GM=G+, that is, if we look at the gadolinia concentration difference, if the gadolinia concentration difference in the upper half of the fuel assembly is considered as a reference, it is smaller in the lower half of the fuel assembly than in the upper half of the fuel assembly.

It is possible to flatten the fuel assembly axial power distribution by reducing the difference in the infinite neutron multiplication factors between the upper half and the lower half of the fuel assembly.

Furthermore, in order to flatten the axial power distribution, in each region of the upper and lower halves of the fuel assembly, the maximum value of the representative gadolinia concentration, which affects the infinite neutron multiplication factor during the operation cycle for the longest time, must be It is also necessary to spout. That is, as described above, the void fraction distribution in the core axis direction is low at the bottom of the fuel assembly and high at the top of the fuel assembly. Therefore, since the neutron spectrum is softer in the lower part of the fuel assembly, gadolinia, which has a larger thermal neutron absorption cross section, is consumed more in the lower part of the fuel assembly. Similar to how gadolinia burns, fissile materials such as U-235 also react more at the bottom of the fuel assembly where the neutron spectrum is softer, so the axial power distribution becomes larger at the bottom of the fuel assembly. It tends to be. Therefore, in order to flatten the axial power distribution, it is sufficient that the maximum value of the one representative Gatlinium concentration is larger at the lower part of the fuel assembly.

In addition, the combination of gadolinia concentrations Gs and Gl was
FIG. 14 shows the combustion change due to the difference in the infinite neutron multiplication factor difference Δk swing during cold operation when the conditions are as shown in the table.

As shown in FIG. 14, the void ratio during operation corresponds to the upper part of the fuel assembly. 70%, case E where the gadolinia concentration difference is 1.0% by weight or more in the upper part of the fuel assembly;
F corresponds to 10 to 15, which corresponds to the first half after the middle of the driving cycle.
Difference in reactivity at cold temperature during operation at GWd/st Δk swing
can be made smaller. Thereby, it is possible to improve the reactor shutdown margin after the middle of the operation cycle.

In this way, by creating a gadolinia concentration difference of 1.0% by weight or more between the central part and the outer region in the cross section of the upper part of the fuel assembly, in addition to the effects described above, the margin for reactor shutdown can be improved.

〔Example〕

An example of the present invention is shown below.

FIG. 1(a) shows a first embodiment of the present invention.

The fuel assembly 1 has a large diameter water lot 7 in the center of its horizontal section.
The fuel rods 2 are arranged in a 9x9 square lattice. Symbol A in the figure.

B, C, and D are fuel rod symbols, and symbols G and G2 are gadolinia fuel rod symbols. Fuel rod A.

The enrichment levels of B, C, and D are shown in FIG. 1(b), respectively. In the figure, 21 is an area filled with natural uranium pellets,
22 indicates an area filled with enriched uranium pellets. In FIG. 1(b), the numbers with G indicate the gadolinia concentration (% by weight), and the other numbers indicate the degree of concentration (% by weight).

The total length in the axial direction of the region including the natural uranium filling region 21 and the enriched uranium filling region 22 is referred to as the effective fuel length. On the other hand, the gadolinia-filled fuel rod G2 is placed adjacent to the large-diameter water rod 7 in a region 83 where the fuel rods placed in the third layer from the outermost periphery of the fuel assembly 1 are located.

The gadolinia fuel supply G2 has a gadolinia concentration of 56% in the range from the lower end of the fuel effective length to 12/24 of the core effective length.
0 (wt%), so that at the top it is 4.5 (wt%)
), this is a fuel rod with a gadolinia concentration in the 2 range. In the region 82 located outside the region 83 where the Gadolinian fuel rod G2 is placed, compared to the Gadolinian fuel rod G2, the gadolinia concentration is 0.5 (wt%) l<L in the lower part and 1. 0 (wt%) lighter. Gadolinian Fuel Offering G
Place the yo. That is, in this example, the burnup is 0 GWd/
st, the representative gadolinia concentration difference between the lower half of the fuel assembly and the upper half of the fuel assembly is 0.5 (wt%) and 1.0, respectively.
(% by weight).

A comparative example is shown in FIG. In FIG. 15, the representative gadolinia concentration difference is O (wt%) in the upper half of the fuel assembly and 0.5 (wt%) in the lower half. Comparing the core operating characteristics between the first embodiment and the comparative example shown in FIG. 15, it is possible to flatten the axial power distribution at the end of the operation cycle in this embodiment. Looking at this in terms of the output sharing ratio of the lower half of the fuel assembly, this example increases by about 0.2% to about 50.1%. By increasing the power sharing ratio of the lower half of the fuel assembly, sufficient scram reactivity can be ensured and the safety of the reactor core can be improved.

On the other hand, when comparing the reactor shutdown margins of the cores, the core according to this embodiment has an increased margin of 0.3 to 0.4% Δ.

In addition, in the embodiment shown in Fig. 1, even in another embodiment in which the upper and lower two regions where the Gadolinian fuel supply G□ and G2 are inserted are swapped, the reactor shutdown margin is 0.2 to 0.3%.
The degree margin increases in Δ.

In addition, as shown in Figure 2, in addition to using two types of Gadolinian fuel supply G, G2 in the upper and lower two areas, in area 110.
It may be a fuel assembly that includes the Gadolinian fuel unit G3 in the lower middle area. Said Gadolinian Fuel Offering G
By inserting Gadolinia fuel in the upper and lower 1 regions instead of 2, the gadolinia abundance ratio in the upper and lower halves of the fuel assembly can be increased, the core axial power distribution is flattened, and the reactor Stop margin is further improved.

In the upper and lower gadolinia fuel rods inserted into the assembly, the effects of the present invention can be obtained even if the gadolinia concentration is very low, as shown below.

That is, as shown in FIG. 3, in order to suppress the maximum linear power density at the beginning of the operation cycle, the
The lower portion of the fuel assembly may contain several gadolinium fuels G having a very light gadolinia concentration of 150 (wt%) in the upper and lower two regions.

Furthermore, as shown in fJS4 diagrams (a) and (b), the gadolinia concentration corresponding to the upper part of the upper and lower two regions arranged in the region 83 is set to zero or 1.0 (wt%). By making it extremely diluted, the gadolinia concentration difference between regions 83 and 82 in the upper part of the core may be taken. A fuel assembly having two ranges of power may also be used. In this embodiment, the fuel rod B located at the outermost periphery of the fuel assembly is a fuel rod having two regions of fuel enrichment, upper and lower. In this embodiment, by making the average enrichment in the upper part of the fuel assembly higher than in the lower part of the core, it is possible to flatten the axial power distribution at the beginning of the operation cycle.

In this example, flattening of the axial power distribution can be promoted by making the average enrichment in the upper part of the fuel assembly, where the void ratio is large, higher than that in the lower part of the fuel assembly. The effect of flattening the output distribution in the axial direction can be promoted.

As shown in FIG. 6, the degree of concentration may be in multiple regions in the axial direction. In this embodiment, the enrichment of the fuel rods A located inside the outermost peripheral part of the assembly is set to three regions in the axial direction. In this embodiment, 20 of the effective fuel length is calculated based on the lower end of the effective fuel length.
The average enrichment in the range of /24 to 23/24 is 8/24 to 20/24 of the central fuel effective length (from the lower end of the fuel effective length).
By combining this with lowering the concentration below the average concentration in the range of , it is possible to improve the reactor shutdown margin at the beginning and end of the operating cycle.

As in this example, by increasing the enrichment from the lower part of the fuel assembly to the central part of the fuel assembly and the upper part of the fuel assembly,
Promotes flattening of axial power distribution.

The effect of the gadolinia distribution shown in the present invention can be promoted.

As shown in FIG. 7, a part of the fuel rod may be a partial length fuel rod having a short effective fuel length. This embodiment is a partial length fuel rod P in which eight of the fuel rods A are set to 15/24 of the effective fuel length of the remaining fuel rods in the embodiment shown in FIG.
That is. In this embodiment as well, by using the partial length fuel rods P, the reactor shutdown margin can be improved. Incidentally, even if the fuel assembly is configured to include the partial length fuel rods 1], the above-mentioned effects of the present invention will not be lost.

In addition, when plutonium is sufficiently supplied and MOX fuel is used, the fuel of the first embodiment can be used as MOX fuel.
One possibility is to replace it with X fuel.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to reduce the difference in the infinite neutron multiplication factors between the upper part of the fuel assembly and the lower part of the fuel assembly after the middle stage of the operation cycle, thereby flattening the axial power distribution of the fuel assembly. By the way.

1 minute scram reactivity can be secured and core safety can be improved.

[Brief explanation of drawings]

1(a), (b) to FIG. 7(a), (b) are horizontal sectional views of a fuel assembly according to an embodiment of the present invention (each time (a))
FIG. 8 shows a horizontal cross-sectional view of a conventional fuel assembly, and FIG.
The figure shows a horizontal cross-sectional view of a high burnup fuel assembly.
11 and 13 are characteristic diagrams showing the relationship between the difference in the neutron infinite multiplication factor and the fuel degree. Figure 14 is a characteristic diagram showing the change in reactivity difference at cold temperature during operation with respect to burnup, and Figure 15 (a) and (b) are horizontal cross-sectional views of the fuel assembly of the comparative example (Figure (a)) and axial direction. It is a concentration distribution map ((b) of the same figure). 1.5... Fuel assembly, 2... Fuel rod, 4... Water rod, 6... Cruciform control method, 7... Large diameter water rod.

Claims (1)

[Claims] 1. A fuel assembly for a boiling water reactor, which has one or more large-diameter water rods in the center of the fuel assembly, and in which a plurality of fuel rods are arranged in a lattice pattern, A first burnable poison-containing fuel rod is mixedly arranged in a fuel rod group in an area facing the large-diameter water rod, and a fuel rod group in an area outside the area where the first burnable poison-containing fuel rod is arranged is The second burnable poison-containing fuel rods are arranged in a mixed manner, and the first and second burnable poison-containing fuel rods are formed with different burnable poison concentrations at the top and bottom, and the upper part of the first burnable poison-containing fuel rod is The burnable poison concentration is formed at a different concentration from the upper part of the second burnable poison fuel rod, and
A fuel assembly characterized in that the difference in burnable poison concentration between the upper parts of the burnable poison-containing fuel rods is larger than the difference between the lower parts. 2. The fuel assembly according to claim 1, wherein third burnable poison-containing fuel rods are mixedly arranged in the fuel rod group in the area facing the large-diameter water rod, and the third burnable poison-containing fuel rods are formed to have a uniform burnable poison concentration vertically. 3. The fuel according to claim 1, in which a third burnable poison-containing fuel rod is mixedly arranged in the group of fuel rods in the area facing the large-diameter water rod, in which the burnable poison concentration is lower in the lower part than in the upper part. Aggregation. 4. The fuel assembly according to claim 1, wherein the burnable poison concentration in the upper part of the first burnable poison-containing fuel rod is zero or less than half of the minimum burnable poison concentration in the other parts. 5. The fuel assembly according to claim 1, wherein the burnable poison concentration in the lower part of the first and second burnable poison-containing fuel rods is equal. 6. A fuel assembly according to any one of claims 1 to 5, in which the fuel enrichment of the fuel rods is divided into regions in the axial direction.