JP3237922B2 - Fuel assemblies and cores for boiling water reactors - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high burnup fuel assembly having improved thermal margin and economy, and a boiling water reactor core loaded with the fuel assembly.

[0002]

2. Description of the Related Art In recent years, higher burnup of fuel has been promoted in order to improve the economic efficiency of nuclear power generation. An example of such a high burn-up fuel assembly will be described with reference to FIG. FIG. 12A is an elevational view showing the fuel assembly in a partial cross section.
(B) is a cross-sectional view taken along the line BB of FIG. (A), and a cross-sectional view taken along the line CC of (C) and (A) of FIG.

[0003] In FIG. 12 (A), a fuel assembly 1 includes a long fuel rod 2, a short fuel rod 3 and a large water rod 6.
Are bundled in a square lattice by spacers 8 and fixed to the upper tie plate 4 and the lower tie plate 5 to form a fuel rod bundle. The fuel rod bundle is surrounded by the channel box 7. Further, an external spring 9 is interposed between the long fuel rod 2 and the upper tie plate 4.

[0004] The fuel assembly for high burnup constructed as described above has the following features, for example, as compared with the conventional fuel assembly for low burnup disclosed in JP-A-2-296192. are doing.

[0005] That is, in order to achieve high burnup, it is necessary to increase the fuel enrichment, which further increases the axial output peaking due to the void distribution.
Further, since more kinds of fuels are mixed in the core than in the different furnace stay periods, the radial output peaking also increases.

As a result, thermal margins such as a maximum linear power density and a minimum limit power ratio are reduced. In order to improve this, in the fuel assembly 1 of FIG. 12, the number of fuel rods is increased by changing the fuel rod arrangement from 8 rows and 8 columns of conventional fuel to 9 rows and 9 columns.

However, as the number of fuel rods increases, the pressure loss increases, and the stability of the reactor is impaired. Therefore, in the fuel assembly 1 of FIG. 12, a short fuel rod 3 in which a part of the fuel rod is shorter than the long fuel rod 2 is used, and the pressure loss is large because the coolant is a two-phase flow. The flow path at the top of the fuel is enlarged to counteract the increase in pressure loss due to the increase in the number of fuel rods. Fig. 12 shows the length of the short fuel rod 3.
As is clear from (B) and (C), about 2
/ 3.

In addition, when the output becomes excessive, the output of the fuel assembly when the heat transfer from the fuel rods to the coolant undergoes a boiling transition from efficient nucleate boiling to inefficient film boiling becomes a critical output. It is.

Since the boiling transition is highly likely to occur at the upper part of the fuel rod, the short fuel rod 3 can improve the limit output. For this purpose, the position of the short fuel rod 3 is selected at a place where the cooling efficiency of the fuel rod is low, thereby increasing the minimum limit power ratio.

[0010] The short fuel rods 3 also have the effect of improving the furnace stop margin. When the reactor is shut down, the neutron flux forms a peak at a location one-fourth to one-third below the total length from the upper end of the core.

When the reactor is shut down, the coolant has a low temperature and a high density, so that it acts as a neutron absorbing material. Therefore, the number of fuel rods is reduced and the amount of coolant is increased in the upper part in the axial direction to improve the reactor shutdown margin. Can be.

[0012]

[Problems to be solved by the invention]

(First problem) As described above, the long fuel rod 2 and the short fuel rod 3
In the fuel assembly 1 composed of the above, an axial upper region where the short fuel rods 3 do not exist (a cross section taken along the line BB in FIG. 12B) and an axial lower region where the short fuel rods 3 exist (FIG.
12 (C), the number of fuel rods is different, so that the reactivity characteristics during operation are significantly different between the upper and lower parts of the fuel assembly.

That is, high-energy neutrons generated by nuclear fission are more likely to be decelerated in the upper part where the moderator-to-fuel ratio is large, so that the infinite multiplication factor in the upper part is larger than that in the lower part.

Although a boiling water reactor originally tends to cause a lower power peak due to void distribution during power operation, the number of fuel rods in a core loaded with the fuel assembly 1 including the short fuel rods 3 is different. Has a favorable effect of mitigating this difference and flattening the axial power distribution over a long period of combustion.

However, at the beginning of the driving cycle,
The vertical difference in the number of fuel rods, on the other hand, causes a problem of increasing output peaking generated in the lower part. Generally, in a fuel assembly used for a boiling water reactor, burnable poison such as gadolinia is mixed into some fuel rods for controlling reactivity.

As a result, the infinite multiplication factor is reduced in the early stage of combustion, the combustion change of the excess reactivity of the reactor core is flattened, and the operability and safety of the reactor are improved. The reactivity control amount of gadolinia in the initial stage of combustion is substantially proportional to the number of gadolinia-containing fuel rods, and the duration of the reactivity control is substantially proportional to the gadolinia concentration.

However, in the fuel assembly 1 including the short fuel rods 3 shown in FIG. 12, even if the number of gadolinia-containing fuel rods is equal at the top and bottom, the reactivity control amount at the initial stage of combustion is the number of fuel rods per cross section. Is larger in the upper part with less moderator than in the lower part.

As a result, especially at the beginning of the operation cycle of the reactor, the infinite multiplication factor at the upper part becomes smaller than that at the lower part,
The power peaking in the lower part of the core will increase.

As an example, curves 9 and 10 in FIG. 13 show infinite multiplication factors when the void ratio of the fuel assembly 1 in FIG. 12 is 40% when the average enrichment is about 4% and no gadolinia is included. Curve 9 shows the infinite multiplication factor at the bottom, and curve 10 shows the infinite multiplication factor at the top.

The void fraction in the core is 40 at the bottom.
% And more than 40% at the top, it is more rigorous to compare the infinite multiplication factor at the top and bottom void fractions. However, here, since the relative magnitude relationship between the upper and lower infinite multiplication factors is important, comparison is made with the same void ratio.

As shown in FIG. 13, the infinite multiplication factor is larger in the upper part (curve 10) than in the lower part (curve 9), and the difference is maximum at the beginning of combustion and decreases with combustion. Generally, the axial power distribution of a boiling water reactor has the lowest peak at the beginning of the operation cycle, and the lower combustion progresses from the upper part with the combustion, so it gradually shifts upward toward the end of the operation cycle. Go.

The vertical difference of the infinite multiplication factor shown in FIG. 13 is suitable for correcting the above-mentioned combustion change of the power distribution and providing a flat axial power distribution throughout the operation cycle.

On the other hand, curves 11 and 12 in FIG. 13 show changes in combustion at an infinite multiplication factor when gadolinia having a concentration of 3.5% is added to the 14 long fuel rods 2 over the entire length thereof. A curve 11 indicates the lower infinite gain, and a curve 12 indicates the upper infinite gain.

Since the reactivity control ability by gadolinia is larger in the upper part than in the lower part, the infinite multiplication factor is reversed upside down at the initial stage of combustion, and the lower peak at the beginning of the operation cycle is increased.

The characteristics of the core loaded with such a fuel assembly include a core loaded with a first fuel assembly composed of 66 long fuel rods 2 and eight short fuel rods 3, and 74. (A) axial power peaking of a core loaded with a second fuel assembly composed of only the long fuel rods 2,
FIG. 14 shows the (B) maximum linear power density and the axial power distribution at the beginning of the (C) operation cycle and at the end of the (D) operation cycle.

In each of the fuel assemblies, a natural uranium region is provided at the upper and lower ends of all the long fuel rods 2, and the 14 long fuel rods 2 have a concentration in the entire internal region except the natural uranium region. 3.5% gadolinia is added.

In FIG. 14, the axial power peaking of the first and second fuel assemblies is represented by curves 13 and 14, the maximum linear power density is represented by curves 15 and 16, and the axial power distribution is represented by curves 17 and 14. 19 and curves 18,20.

In the core loaded with the first fuel assembly, the axial power distribution peaks downward from the beginning to the middle of the operation cycle, compared with the core loaded with the second fuel assembly, so that the axial power peaking is reduced. The maximum linear power density has increased by up to 0.6 kW / ft at the beginning of the driving cycle. At the end of the driving cycle, the difference in the axial power distribution is small, and the maximum linear power density is almost the same.

By the way, the feature of the axial power distribution described above has an effect of improving economic efficiency.
It is described in 2-296192. That is, the operation is performed with the power distribution of the lower peak from the beginning to the middle of the operation cycle, whereby the average void fraction of the core is increased, and the neutron spectrum is hardened particularly in the upper part of the core.

As a result, the production of plutonium is promoted, and the output distribution reaches an upper peak at the end of the operation cycle, and the plutonium accumulated in the upper portion is efficiently burned. Such an effect is called a spectrum shift effect .

In the infinite multiplication characteristic of FIG. 13 in the fuel assembly 1 shown in FIG. 12, the vertical difference of the infinite multiplication factor caused by the difference in the number of fuel rods corrects the lower peak at the beginning of the operation cycle and reduces the power distribution. Reduce the spectral shift effect for flattening.

On the other hand, the vertical difference of the infinite multiplication factor due to the difference in the reactivity control amount of gadolinia in the early stage of combustion further enhances the lower peak, and thus has the effect of increasing the spectrum shift effect.

A first object of the present invention is to provide a fuel assembly having short fuel rods, in particular, which suppresses an increase in the maximum linear power density at the beginning of an operation cycle and has a sufficient thermal margin for a high burnup fuel assembly. Is to provide the body.

Further, simply flattening the axial power distribution reduces the spectrum shift effect. Therefore, it is necessary to utilize the infinite multiplication factor characteristic caused by the difference between the number of fuel rods and the control amount of gadolinia reactivity. Accordingly, it is also an object of the present invention to provide a highly burnable fuel assembly for high burn-up which sufficiently exhibits a spectrum shift effect.

(Second Problem) By preparing in advance two types of fuel assemblies, Type 1 having a large number of fuel rods containing gadolinia and Type 2 having a small number of fuel rods containing gadolinia, and appropriately changing the number ratio thereof. A so-called two-stream core technology for coping with fluctuations in the operation cycle period is described in, for example, Japanese Patent Application Laid-Open No. 2-296192.

That is, the change in the operating cycle length before or after the change may necessitate a change in the number of fuel replacement bodies from the initial schedule. In this case, if the number of replacements is smaller than expected, more type 1 fuel assemblies are loaded, and if the number of replacements is greater than expected, more type 2 fuel assemblies are loaded. . Thus, the excess reactivity can be set in an appropriate range of 1 to 2% Δk.

In such a two-stream core, generally,
A type 1 fuel assembly having a large number of gadolinia-containing fuel rods and a small infinite multiplication factor is disposed at the center of the core, and a type 2 fuel assembly is disposed at the periphery of the core.

Since the power distribution in the radial direction of the core is higher in the center and lower in the periphery, the number of gadolinia-containing fuel rods differs.
By arranging the two types of fuel assemblies as described above, the radial power distribution is flattened, and the maximum linear power density and the minimum critical power ratio are improved.

However, in such a core, the maximum linear power density may appear in a fuel assembly disposed around the core. In general, when the burnup becomes higher, the enrichment becomes higher and the infinite multiplication factor after gadolinia burns out increases, but the number of gadolinia-containing fuel rods per fuel assembly increases. Decrease.

Therefore, at the beginning of the operation cycle, the maximum linear power density tends to appear in the second cycle fuel having the largest infinite multiplication factor. The concentration of gadolinia is set so that it burns out at the end of the operation cycle, but the output is lower at the periphery of the core than at the center and the progress of combustion is delayed, so the infinite increase in the second cycle fuel placed around the core Magnification may just peak.

Further, when the combustion proceeds at the lower peak, the infinite multiplication factor at the upper part becomes larger than that at the lower part, but the lower part of the core remains at the lower peak because the burnup is smaller than that at the center part. Therefore, in the two-stream core, an axial design corresponding to the radial position where the fuel assembly is loaded is required.

Further, when shifting from an equilibrium core loaded with only a low enrichment fuel assembly for low burnup to an equilibrium core loaded with only a high enrichment fuel assembly for high burnup, Each time refueling is performed, the fuel assemblies for low burnup are taken out, and the fuel assemblies for high burnup are sequentially loaded.

In the first or second cycle of the transition, a small number of high enrichment fuel assemblies for high burnup are loaded into the core containing many low enrichment fuel assemblies for low burnup. Radial output peaking occurs in the burnup fuel assembly.

Further, the fuel with higher enrichment has a further lower peak in the axial power distribution, so that there is a problem that the maximum linear power density is more likely to increase in the transition cycle than in the equilibrium core.

A second object of the present invention is to provide a two-stream core,
In particular, it is an object of the present invention to provide a boiling water reactor core having a sufficient thermal margin by reducing the maximum linear power density at the beginning of an operation cycle.

The invention according to claim 1 is characterized in that a long fuel rod,
Grid with short fuel rods whose effective part is shorter than long fuel rods
Burnable poisons in a fuel assembly
As a fuel rod containing
Most areas except burners contain burnable poisons.
A group of fuel rods, a long or short fuel rod,
Axial lower area corresponding to the height where the fuel rod is located
Contains at least part of burnable poisons
And a second group of fuel rods.
Of flammable toxins at sites containing flammable toxins
Contains the burnable poison of the first group of fuel rods.
It is noted that the concentration is set to 1/3 or less of the burnable poison concentration of the site.
It is a fuel assembly to be characterized. The invention according to claim 2 is a contract
2. The fuel rod of claim 1, wherein said second group of fuel rods is a water rod.
Characterized by being located adjacent to the fuel
It is an aggregate.

The invention according to claim 3 is the invention according to claim 1.
Type 1 fuel assembly and type 2 fuel as fuel assemblies
A plurality of fuel assemblies, and the first group of fuel rods
The number is from the type 1 fuel bundle from the type 2 fuel assembly.
The number of fuel rods in the second group is larger than that of the
The type 2 fuel assemblies are more numerous than the type 1 fuel assemblies.
And the type 1 fuel assembly and the type 1
At least fresh fuel and two fuels in each of the two fuel assemblies
It is characterized by being loaded with cycling fuel
It is a core for a boiling water reactor. The invention according to claim 4 is
A long fuel rod and a short rod whose effective part is shorter than this long fuel rod
Fuel rods are bundled in a grid and contain burnable poisons.
All fuel rods are long fuel rods except their upper and lower ends.
Group 1 fuels containing burnable poisons in most areas
2. The fuel assembly of claim 1, wherein said fuel assembly is a type 1 fuel assembly.
A plurality of IP2 fuel assemblies, and the first group
The number of fuel rods of the type 2
The fuel set of the fuel cell 1 is set more and the type 1 fuel
Each of the fuel assembly and the type 2 fuel assembly
At least new fuel and second cycle fuel loaded
This is a core for a boiling water reactor.

[0048]

[Action]

(Operation according to the first invention) In the fuel assembly according to the first invention, since the number of burnable poison-containing fuel rods is larger in the lower part than in the upper part, the infinite multiplication factor of the lower part is higher than that of the upper part especially in the initial stage of combustion. It is possible to reduce the lower peak of the axial power distribution at the beginning of the operation cycle due to the difference between the upper and lower gadolinia reactivity control amounts. As a result, the maximum linear output density at the beginning of the operation cycle is reduced, and a sufficient thermal margin can be secured.

Further, when the concentration of the burnable poison contained in the long or short fuel rods of the second group is set lower than the concentration of the burnable poison contained in the long fuel rods of the first group. The upper and lower infinite multiplication factors are as follows.

That is, in the very early stage of the combustion, the lower infinite multiplication factor becomes smaller than the upper one. However, if the burn proceeds and the low-concentration burnable poison burns, the difference between the upper and lower infinite multiplication factors is reduced, and the combustion is further reduced. , The infinite multiplication factor at the lower portion becomes smaller than that at the upper portion again due to the difference in the number of fuel rods.

As an example, in a fuel assembly including 14 long fuel rods containing 3.5% gadolinia as described with reference to FIG. 13, 1.5% gadolinia is added below two long fuel rods. The infinite multiplication factor in the case is shown by a curve 22 in FIG.

On the other hand, in the case of adding gadolinia having a concentration of 3.5%, the infinite multiplication factor indicated by the curve 23 in FIG. 15 lowers the infinite multiplication factor in the lower portion during the entire period until the gadolinia burns out. Curve 12 in FIG. 15 shows the infinite multiplication factor at the top when gadolinia having a concentration of 3.5% was added.

In the fuel assembly having such an infinite multiplication factor, the axial power distribution can be flattened to a desirable degree at the beginning of combustion. On the other hand, in the middle period of combustion when the vertical difference of the infinite multiplication factor is reduced, the power distribution becomes a lower peak, so that it is possible to improve the economy by realizing the spectrum shift effect and reduce the lower peak at the beginning of the next operation cycle. it can.

That is, when the combustion proceeds at the lower peak, the infinite multiplication factor gradually becomes larger in the upper part than in the lower part.
Even in the initial operation cycle, fuel assemblies other than fresh fuel have a property of flattening the axial power distribution. This effect is more effective the longer the lower peak combustion has been experienced.

In the middle stage of the operation cycle, increasing the lower peak does not excessively increase the maximum linear power density for the following reason.

The radial power peaking of the core is shown in FIG.
(A) As shown by the middle curve 21, there is a time when it becomes smaller in the middle of the operation cycle. For the sake of explanation, reference numerals I and II are attached to the upper left of FIG. 13, and the transition of the infinite multiplication factor of the new fuel during one operation cycle is shown in section I, and the transition of the infinite multiplication factor of the fuel in the second cycle is shown. Shown in section II.

Although the radial output peaking appears in the second cycle fuel having the largest infinite multiplication factor at the beginning of the operation cycle, the output of the second cycle fuel decreases with combustion, while the output of the new fuel increases. It appears in new fuel at the end of the driving cycle.

In the middle stage of the operation cycle during this period, the difference in the infinite multiplication factor between the new fuel and the fuel in the second cycle is reduced, so that the radial output peaking is reduced. Since the maximum linear power density depends on both the axial and radial power peaking, during the middle of the operation cycle in which the radial power peaking is small, the axial power distribution has a downward peak without excessively increasing the maximum linear power density. Can be

(Operation of the Second Invention) In a two-stream equilibrium core, a short fuel rod exists because a type 2 fuel assembly is loaded at the periphery of the core and a type 1 fuel assembly is loaded at the center of the core. The number of long or short fuel rods in the second group in which the burnable poison is contained in at least a part of the portion corresponding to the axial lower region is larger in the type-2 fuel assembly than in the type-1 fuel assembly. Thereby, the lower peak of the axial power distribution around the core can be suppressed, and the maximum linear power density can be reduced.

Further, the present invention can cope with variations in the operating cycle length. That is, the type 2 fuel assembly is usually arranged in the periphery of the core, but when the number of replacement bodies increases due to a variation in the operation cycle length, the type 2 fuel assembly may be arranged in the center of the core.

The type 2 fuel assembly has a smaller number of gadolinia-containing fuel rods and a larger infinite multiplication factor than the type 1 fuel assembly. Density may increase.

However, according to the present invention, the axial output peaking of the type 2 fuel assembly can be suppressed even when it is disposed at the center of the core.

Next, the operation in the transition cycle from the equilibrium core composed of only the fuel assemblies for low burnup to the equilibrium core composed of only the fuel assemblies for high burnup will be described.

The number of replacements of the high burn-up fuel assemblies in the transition cycle is limited to only the high burn-up fuel assemblies due to the low enrichment of the low burn-up fuel assemblies staying in the core. More than the configured equilibrium core.

In this case, in order to set the excess reactivity to an appropriate value, a large number of type 2 fuel assemblies are loaded on the core,
It will also be located in the center of the core. Since the type 2 fuel assembly has a larger infinite multiplication factor than the type 1 fuel assembly, the output is high and the axial output peaking is large. Therefore, even in such a transition cycle, an increase in the maximum linear output density can be suppressed by applying the present invention.

In the equilibrium core, the lower peak is not so remarkable as in the transition cycle.
It is important to keep the number of long or short fuel rods in the second group small. Otherwise, the equilibrium core would suppress the lower peaks too much, reducing the spectral shift effect and reducing economics.

[0067]

【Example】

(First Embodiment) A high burn-up fuel assembly which is a first embodiment of the fuel assembly according to the present invention will be described with reference to FIGS. FIG. 1 is a schematic cross-sectional view of the fuel assembly 1 and the enrichment and gadolinia concentration of the fuel rods in the fuel assembly 1 are shown in the axial distribution, and are circled in the upper fuel assembly. The numbers correspond to the numbers on the lower fuel rods.

The fuel assembly according to the first embodiment is shown in FIG.
The fuel rods have the same structure as the fuel assembly shown in FIG. 1, and as shown in the upper part of FIG. 1, the arrangement of the fuel rods is a square lattice arrangement of 9 rows and 9 columns, and the bundle of fuel rods is 66 long. It comprises a fuel rod 2, eight short fuel rods 3, and two large water rods 6. The length of the effective portion filled with fuel pellets is about 370 cm for long fuel rods 2 and about 370 cm for short fuel rods 3.
220 cm.

The effective portion of the long fuel rod 2 has an upper end of about 30 cm.
The natural uranium pellets are filled at the bottom and about 15 cm at the bottom.The enrichment at about 325 cm inside is uniform in the axial direction, and the enrichment of the fuel pellets inside each fuel rod is Are higher in the order of p>q>r> s.

The enrichment of the short fuel rod 3 is equal to the average cross-sectional enrichment of the fuel assembly, and is 4.1%. Therefore, the average enrichment of the cross-section of the fuel assembly except for the upper and lower end natural uranium portions is uniform in the axial direction. is there. The average enrichment including the upper and lower natural uranium parts is about 3.7%.

The long fuel rod No. 6 contains gadolinia with a concentration of 3.5% in the entire internal region except for the upper and lower ends of the natural uranium portion, and forms the first group of long fuel rods in this embodiment. .

The long fuel rods No. 7 and No. 8 are the second group of fuel rods in which gadolinia is contained only in the axial lower region where the short fuel rods are present. Either one of the long fuel rods or the long fuel rod No. 2 not including gadolinia.

In the long fuel rod A, gadolinia having a concentration α% is contained in a portion corresponding to the entire length of the effective portion of the short fuel rod, and in the long fuel rod B, gadolinia having a concentration β% is included. Is contained in a portion corresponding to one half of the effective portion of the short fuel rod.

As the power peaking in the axial direction of the equilibrium core loaded with the fuel assembly of this embodiment, No. 7 and No. 8
FIG. 2 (A) shows the relationship between the rate of increase in the axial power peaking and the burnup of a conventional equilibrium core loaded with a fuel assembly in which each of the fuel rods is a long fuel rod containing no gadolinia. It is shown in (B).

Further, in order to investigate the effect of the spectrum shift in this embodiment, the effective multiplication factor at the end of the operation cycle is shown in Table 1 as an increase with respect to the conventional fuel assembly.

FIG. 2A shows that the fuel rod of No. 7 is a long fuel rod of No. 2 and the fuel rod of No. 8 is a long fuel rod of A, and the gadolinia concentration α is 0.5% to 0.5%. This is the case where it is changed in the range of 3.5%.

According to the present embodiment, the axial output peaking is reduced particularly at the beginning of the operation cycle, and the effect becomes more remarkable as the gadolinia density becomes lower. When the gadolinia concentration is low, the axial output peaking in the middle of the driving cycle slightly increases. However, as shown in FIG. Growth is not a problem.

The lower peak in the middle of the driving cycle also promotes the spectral shift effect and increases the effective multiplication factor at the end of the driving cycle.

[0079] On the other hand, when the gadolinia concentration is dark, because the lower peaks in the operation cycle metaphase is insufficient, it does not proceed bottom of the combustion, so that the axis Direction power peaking increases in the operating cycle early and late.

As shown by the curve 21 in FIG. 14 (A), the radial output peaking becomes the smallest when about 1/3 of the operation cycle length has elapsed. Is preferably about 1/3 or less of the gadolinia concentration of the first group of fuel rods, and is preferably as low as possible within the range in which it can be manufactured.

FIG. 2B shows the rate of increase in axial power peaking for various combinations of fuel rods numbered seven and eight. The gadolinia concentration contained in the lower part of the long fuel rod of the code A or B is always 0.5%.

If the fuel rod of No. 7 in addition to the fuel rod of No. 8 is also a long fuel rod of the symbol A, the axial output peaking at the beginning of the operation cycle is greatly improved, but the spectrum shift effect is slightly reduced. .

When the fuel rod of No. 8 is replaced with the long fuel rod of the reference character A instead of the long fuel rod of the reference character A, the axial output peaking at the beginning of the operation cycle is slightly reduced. The spectral shift effect has been halved.

As described above, the lower the gadolinia concentration, the more simultaneously the thermal margin and the economic efficiency can be improved. However, in other combinations, the improvement in thermal margin and the improvement in economy are in conflict, so that a preferable combination must be appropriately selected as needed.

[0085]

[Table 1]

As the second group of fuel rods containing low-concentration gadolinia, a fuel rod indicated by reference numeral 36 in FIG. 1 that is adjacent to the two large-diameter water rods 6 is preferable. This code 36
Since the thermal neutron flux is high at the position indicated by, the control amount of gadolinia reactivity is large and burns quickly. Therefore, the number of fuel rods in the second group is substantially increased and the gadolinia concentration is lowered, which is effective in controlling the axial power distribution.

Further, since gadolinia has a low thermal conductivity and the temperature of the fuel rod tends to rise, the output of the fuel rod containing gadolinia is reduced by lowering the enrichment, and the degree depends on the gadolinia concentration. . High neutron flux sign
In the case where gadolinia is contained at a high concentration in the position indicated by 36, the enrichment of this fuel rod must be sufficiently reduced.

As a result, the enrichment of fuel rods that do not contain gadolinia must be increased, thus increasing the local power peaking of the fuel assembly. On the other hand, when the gadolinia density is low, it is not necessary to lower the concentration so much, and the increase in local output peaking does not matter.

(Second Embodiment) A two-stream fuel assembly according to a second embodiment of the present invention will be described with reference to FIG.
3A shows a type 1 fuel assembly, and FIG. 3B shows a type 2 fuel assembly. Like the first embodiment, the second embodiment has the same structure as the high burn-up fuel assembly shown in FIG. Therefore, the description of the same part is omitted.

The effective portion of the long fuel rod 2 has an upper end of about 30 cm.
The natural uranium pellet is filled at the bottom and about 15 cm at the lower end, but the enrichment at about 325 cm inside is uniform in the axial direction. The enrichment of the fuel pellets filled in each fuel rod is higher in the order of a>b>c> d, and the enrichment of the short fuel rod is c.

As a result, the average enrichment in the cross section of the assembly is approximately higher at the upper end than at the lower end of the short fuel rod 3 as a boundary.
0.1% higher. The average enrichment including the upper and lower natural uranium parts is about 3.7%.

In the type 1 fuel assembly, gadolinia is contained in the entire inner region of the fifteen long No. 6 fuel rods except for the upper and lower natural uranium parts, and the upper part is lower than the center of the shorter fuel rod. The gadolinia density is higher than that. With such a gadolinia distribution, a lower peak in the latter half of the operation cycle can be particularly suppressed.

Further, in one long fuel rod of No. 7, gadolinia having a concentration of 1% is included only below the vicinity of the center of the short fuel rod, and particularly, a lower peak at the beginning of an operation cycle is suppressed.

In the type 2 fuel assembly, in the case of twelve long fuel rods of number 6 and two long fuel rods of number 7,
Each has the same gadolinia distribution as the fuel rods of the same number in the type 1 fuel assembly.

FIG. 4 shows a conventional example for comparison with the second embodiment, in which (A) is a type 1 fuel assembly and (B) is a type 2 fuel assembly. Although the configuration is almost the same as that of FIG. 3, the enrichment of the fuel pellets filled in each fuel rod is higher in the order of e>f>g> h, and the enrichment of the short fuel rod is h which is the lowest. .

As a result, the average cross-sectional enrichment of the assembly is approximately higher at the upper end of the short fuel rod 3 than at the lower end thereof.
0.2% higher. 15 in type 1 fuel assemblies
The number 6 long fuel rods and the twelve number 6 long fuel rods in the type 2 fuel assembly are the same as the numbered long fuel rods of the second embodiment of the present invention shown in FIG. They have the same gadolinia distribution.

FIG. 5 shows the fuel layout and the maximum linear power density of the equilibrium core loaded with these type 1 and type 2 fuel assemblies. FIG. 5A is a 1/4 plan view of the core, and one square 24 represents one fuel assembly. The location of the new fuel is indicated by 1 or 2, the former being Type 1
A fuel assembly, the latter being a type 2 fuel assembly.

At other positions, the fuel in the fifth cycle is disposed on the outermost periphery in the control cell 25 in which a part of the fuel in the fourth cycle is surrounded by a thick frame, and the other fuels are substantially disposed in the remaining positions. They are arranged uniformly. FIG. 5B shows the relationship between the maximum linear output density and the burnup in FIG.

FIG. 4 shows the maximum linear power density (curve 26 in FIG. 5B) when the fuel assembly according to the second embodiment of the present invention shown in FIG. 3 is loaded in this core. Linear power density when a conventional fuel assembly is loaded (same curve
Compared to 27), it is significantly reduced at the beginning of the operation cycle. In the latter half of the operation cycle, the maximum linear power density increases, but the maximum value throughout the operation cycle is improved, indicating the effectiveness of this embodiment.

Next, the fuel arrangement diagram of the first cycle of the transition in which the high burnup fuel assembly of FIG. 3 or 4 is first loaded into the equilibrium core loaded with only the low burnup fuel assembly, and FIG. 6 shows the maximum linear power density, and FIG. 7 shows the fuel arrangement diagram and the maximum linear power density of the second transition cycle.
(A) is a 平面 plan view of the core, where 1 and 2 are new fuels of the high burnup fuel assemblies of type 1 and type 2, respectively.

In the first transfer cycle, a fuel assembly similar to the fuel assembly for low burnup disclosed in FIGS. 1 to 4 of JP-A-2-296192 is loaded at a position not shown. Have been.

This is an 8-row, 8-column fuel grid having a large diameter water rod for four fuel rods at the center.
3.3%. In the second transition cycle, the fuel in the second cycle of the fuel assembly for high burnup is loaded in addition to the fuel assembly for low burnup.

In FIGS. 6B and 7B showing the maximum linear power density, curves 28 and 30 correspond to the second embodiment of the present invention.
Curve shows a case where the fuel assembly according to the embodiment of the present invention is loaded.
29 and curve 31 show the case where the fuel assembly of the conventional example is loaded.

The new fuel of the fuel assembly for high burnup loaded in the first cycle of the transition has higher enrichment than the fuel assembly for low burnup, but has an infinite multiplication factor because the number of gadolinia-containing fuel rods is large. Is relatively small.

Therefore, in the first cycle of the transition, the maximum linear power density appears in the low burn-up fuel assembly in the second cycle, so that even when the conventional high burn-up fuel assembly is used, the maximum linear power density is increased. Is sufficiently small that there is little improvement using embodiments of the present invention.

In the second cycle of the transition, the maximum linear power density appears in the fuel assembly for high burnup in the second cycle. In particular, since the infinite multiplication factor reaches a peak at the beginning of the operation cycle, the maximum linear power density is greatly increased.

Further, as shown in FIG. 7A, in the transition cycle, a type 2 fuel assembly having a small number of gadolinia-containing fuel rods is arranged up to the center of the core.

Therefore, according to the present invention, the axial power distribution in the central part of the reactor core is flattened by lowering the infinite multiplication factor particularly at the lower part of the type 2 fuel assembly in the early stage of combustion, and the fuel of the second cycle fuel having a high power is obtained. By suppressing the direction peak, the maximum linear power density at the beginning of the driving cycle can be greatly reduced.

In both the second embodiment of the present invention shown in FIG. 3 and the conventional example shown in FIG. 4, the enrichment inside the long and short fuel rods except for the upper and lower ends of the natural uranium portion is not limited. Uniform in the axial direction.

In these fuel assemblies, by making the enrichment of the short fuel rods lower than the average enrichment of the long fuel rods, the cross-sectional average enrichment of the assembly has a vertical difference.
This flattens the axial power distribution.

As means for giving a vertical difference to the aggregate cross-sectional average enrichment, for example, there is a means for distributing the enrichment in the axial direction inside the fuel rod No. 1 in FIG. .

However, in the fuel rod manufacturing process, if the enrichment changes while fuel pellets are being filled into the cladding tube, the manufacturing process becomes complicated, and the inspection after the fuel rod is completed requires time and labor. Take it. These results can lead to increased manufacturing costs.

Therefore, as employed in the second embodiment of the present invention and the conventional example, if the enrichment of the cross section of the assembly is made to have a vertical difference by lowering the enrichment of the short fuel rods, the production and inspection can be performed. The process can be greatly simplified.

However, as in the conventional fuel assembly shown in FIG. 4, in order to flatten the axial output distribution only by the upper and lower enrichment distributions, it is necessary to extremely reduce the enrichment of the short fuel rods. is there. As a result, the enrichment of the other fuel rods must be increased to maintain a predetermined average enrichment, increasing the local power peaking of the fuel assembly.

On the other hand, if the axial power distribution is flattened by the optimum combination of the upper and lower enrichment distribution and the gadolinia distribution according to the present embodiment, it is not necessary to make the enrichment of the short fuel rods excessively low. , Local output peaking can be reduced.

As a result of appropriately determining the enrichments a to h in FIG. 3 and FIG. 4, the local output peaking at the burnup at which the infinite multiplication factor reaches a peak in the second embodiment of the present invention, as compared with the conventional example. For both Type 1 and Type 2 fuel assemblies
% Reduction. The maximum linear output densities shown in FIGS. 5 to 7 already include this effect.

In the fuel assembly according to the second embodiment of the present invention shown in FIG. 3, the arrangement of the fuel rods is further improved in order to improve the safety of the nuclear reactor. When the reactor is stopped, all control rods are inserted into the reactor core, and are gradually pulled out when starting operation. At this time, it is assumed that the control rod, which should have been pulled out, remains inserted for some reason, and at some point, it suddenly falls and is pulled out.

It is known that, at the time of such a control rod fall accident, the temperature rise of the fuel rod due to the application of the reactivity is the largest for fresh fuel. In particular, in a two-stream core, the temperature of a type 2 fuel assembly having a large infinite multiplication factor is more likely to rise than that of a type 1 fuel assembly.

Therefore, in order to secure a sufficient margin against a control rod falling accident, it is effective to lower the local output peaking of the type 2 fuel assembly. In addition, when the reactor is stopped, it is 1/4 to 1/3 from the upper end of the core.
Since the neutron flux forms a peak only at the lower portion, in the high burn-up fuel assembly 1 shown in FIG. 12, it is necessary to reduce the local output peaking in the upper cross section (B) where the short fuel rod 3 does not exist. is there.

In the type 2 fuel assembly of the prior art shown in FIG. 4B, long fuel rods having the highest enrichment No. 1 are arranged at a part of the outermost periphery of the fuel assembly in order to improve economy. I have.
During output operation, the void ratio above the short fuel rod 32 is 70%.
Since the fuel rods are close to each other, the output of the fuel rod 34 disposed adjacent to the outermost circumference is not so large.

However, when the reactor is stopped, short fuel rods
Above 32 is filled with high-density low-temperature water, so that the thermal neutron flux is high, and the output of the highest enriched fuel rod 34 located adjacent to the outermost periphery increases.

In order to improve this, in the type 2 fuel assembly according to the second embodiment of the present invention shown in FIG. 3 (B), the long fuel rod 32 disposed on the outermost periphery adjacent to the short fuel rod 32 is used. Fuel rod 33
Is the second concentration.

As a result, type 2 when the reactor was shut down
The local output peaking of the fuel assembly can be reduced by 3% in the second embodiment of the present invention compared to the conventional example, thereby increasing the safety against a control rod drop accident. Since the infinite multiplication factor of the type 1 fuel assembly is smaller than that of the type 2 fuel assembly, there is no problem even if the local output peaking at the time of reactor shutdown is somewhat large.

Therefore, the second embodiment of the present invention shown in FIG.
In the type 1 fuel assembly of the embodiment, the position 35 corresponding to the fuel rod 33 arranged at the outermost periphery of the type 2 fuel assembly is
The fuel rod with the highest enrichment of No. 1 is arranged to improve economy.

When a two-stream fuel is formed, it is desirable to use the same number of fuel rods of the same enrichment in the two types of fuel assemblies as in this embodiment. With this configuration, only the gadolinia content is adjusted in the fuel manufacturing plant even if the loading ratio between the type 1 fuel assembly and the type 2 fuel assembly is changed due to a change in the operation cycle period or the like. This makes it easy to respond. However, enriched uranium has been arranged well before its manufacture, so changes in enrichment cannot be easily accommodated.

As described above, when two types of fuel assemblies are configured by using the same number of fuel rods having the same enrichment,
The only difference between the two types of fuel assemblies is the number of fuel rods containing gadolinia and fuel rods having the same enrichment and no gadolinia. Because gadolinia has low thermal conductivity, gadolinia is usually contained in the second or third enriched fuel pellets.

In the type 2 fuel assembly having a small number of gadolinia-containing fuel rods, the number of fuel rods not including the second or third enrichment gadolinia is large. Can be used.

(Third Embodiment) A type 2 fuel assembly used in a two-stream core according to a third embodiment of the present invention will be described with reference to FIG. The type 1 fuel assembly in the present embodiment is the same as that shown in FIG.

That is, in the third embodiment, the second group of fuel rods containing gadolinia only in the lower portion is used only for type 2 fuel assemblies having a small number of gadolinia-containing fuel rods. As a result, the maximum linear output density at the beginning of the operation cycle is reduced as compared with the conventional example shown in FIG.

Next, various application forms of the present invention will be described for the fourth to sixth embodiments with respect to a fuel assembly applied to a one-stream core. Obviously, these embodiments are equally applicable to a two-stream core.

(Fourth Embodiment) A fuel assembly according to a fourth embodiment of the present invention will be described with reference to FIG. The long fuel rods Nos. 6 and 7 are the first and second groups of fuel rods, respectively.

In the present embodiment, the enrichment of the fuel pellets filled in each fuel rod is higher in the order of u>v>w> x, and the enrichment of fuel rods No. 2 and No. 3 is increased in the axial direction. Are distributed.

As a result, the average cross-sectional enrichment of the aggregate is about 0.2% higher in the upper part than in the lower part. Thus, although the production and inspection of the fuel is somewhat complicated, local power peaking can be reduced since the enrichment of the short fuel rods does not need to be excessively low.

(Fifth Embodiment) A fuel assembly according to a fifth embodiment of the present invention will be described with reference to FIG. In the first embodiment shown in FIG. 1, the long fuel rods are the second group of fuel rods. However, in the present embodiment, the concentration of 0.5% is applied to the entire area of the effective portion of the two short fuel rods No. 7. The second group of fuel rods contains gadolinia.

In this embodiment, except for the natural uranium portions at the upper and lower ends, the composition of the fuel pellets in all the fuel rods does not change on the way, so that the production and inspection of the fuel are greatly simplified.

(Sixth Embodiment) A fuel assembly according to a sixth embodiment of the present invention will be described with reference to FIG. In this embodiment,
As compared with the first embodiment shown in FIG. 1, in the first group of fuel rods of No. 6, the gadolinia concentration above the upper end of the short fuel rod 3 is lower. This reduces the unburned gadolinia at the end of the driving cycle and improves the economy.

In the high burn-up fuel assembly 1 having the short fuel rods 3 shown in FIG. 12, when the reactor is stopped, the high-density low-temperature water acts as a neutron absorbing material in the upper part where there is a lot of coolant. Even if the gadolinia concentration in the region is lowered, a sufficient margin for stopping the furnace can be secured. If the furnace shutdown margin seems to decrease excessively, the enrichment in the upper part is set to 0.
It may be lower by about 1 to 0.3%.

In the sixth embodiment, the gadolinia concentration at the lower part of the first group of fuel rods of No. 6 is increased. The boundary position is different from the upper end of the region where gadolinia is contained in the second group of fuel rods of No. 7.

As described above, various combinations of the boundary positions of the gadolinia concentration in the first group and the second group and the boundary positions of the enrichment when the enrichment is distributed in the axial direction can be considered.

[0140]

According to the present invention, in a fuel assembly for high burn-up composed of long fuel rods and short fuel rods, a low fuel level is reduced only in at least a part of the axial lower region where the short fuel rods are present. By including gadolinia in concentration, the axial power distribution can be optimally controlled.

That is, since the output distribution at the beginning of the operation cycle is flattened, the maximum linear output density can be reduced. Further, since the output distribution can have a lower peak in the middle stage of the operation cycle, the spectrum shift effect can be promoted and the economic efficiency can be improved.

In the two-stream core, the number of fuel rods containing gadolinia in at least a part of the axial lower region where the short fuel rods are present is set to be smaller than that of the type 1 fuel assembly having more gadolinia-containing fuel rods. By increasing the number of fuel rods with gadolinia in the type 2 fuel assembly with a small number of fuel rods, the maximum linear power density can be sufficiently reduced not only when the equilibrium core is operating as planned but also when the operating cycle length fluctuates. be able to.

Further, in the transition cycle from the equilibrium core composed of the fuel assemblies for low burnup to the equilibrium core composed of the fuel assemblies for high burnup, the maximum linear power density can be sufficiently reduced. it can.

[0144]

As described above, by applying the present invention, it is possible to provide a fuel assembly for high burnup which has high safety during operation and shutdown of the nuclear reactor and is excellent in economy. .

[Brief description of the drawings]

FIG. 1 is an axial distribution diagram showing enrichment and gadolinia concentration of each fuel rod corresponding to a cross section of a fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an increase rate of an axial power peaking in a core loaded with a fuel assembly according to a first embodiment of the present invention with respect to a conventional example, wherein (A) is included in a second group of fuel rods; (B) shows the case where the gadolinia concentration is changed, and (B) shows the case where the number of the fuel rods of the second group or the length of the region containing gadolinia in the fuel rods of the second group is changed.

3A and 3B are axial distribution diagrams of enrichment and gadolinia of each fuel rod of a two-stream fuel assembly according to a second embodiment of the present invention, wherein FIG. 3A shows a type 1 fuel assembly, and FIG. Indicates a type 2 fuel assembly.

FIG. 4 is an axial distribution diagram of enrichment and gadolinia of each fuel rod of a conventional two-stream fuel assembly,
(A) shows a type 1 fuel assembly, and (B) shows a type 2 fuel assembly.

FIG. 5A is a plan view showing a quarter of an equilibrium core loaded with only a two-stream fuel assembly according to the second embodiment of the present invention or a conventional example, and FIG. FIG. 4 is a characteristic diagram showing a linear output density.

6A is a fuel arrangement diagram showing 示 す of the core in the first cycle of the transition from the equilibrium core loaded with only the low burn-up fuel assemblies to the equilibrium core of FIG. 5, and FIG. FIG. 4 is a characteristic diagram showing a maximum linear output density in A).

7A is a fuel arrangement diagram showing 示 す of the core in the second cycle of the transition from the equilibrium core loaded only with the low burn-up fuel assembly to the equilibrium core of FIG. 5, and FIG. FIG. 4 is a characteristic diagram showing a maximum linear output density in A).

FIG. 8 is an axial distribution diagram of enrichment and gadolinia of each fuel rod of a type 2 fuel assembly used in a two-stream core according to a third embodiment of the present invention.

FIG. 9 is an axial distribution diagram of enrichment and gadolinia of each fuel rod of a fuel assembly according to a fourth embodiment of the present invention.

FIG. 10 is an axial distribution diagram of enrichment and gadolinia of each fuel rod of a fuel assembly according to a fifth embodiment of the present invention.

FIG. 11 is an axial distribution diagram of enrichment and gadolinia of each fuel rod of a fuel assembly according to a sixth embodiment of the present invention.

12A is an elevational view showing a conventional fuel assembly for high burn-up in a partial cross-section, FIG. 12B is a cross-sectional view taken along the line BB of FIG. 12A, and FIG. CC sectional view taken on the line.

FIG. 13 is a characteristic diagram showing a combustion change at an infinite multiplication factor when the void ratio of the fuel assembly in FIG. 12 is 40%.

FIG. 14A is a first fuel assembly composed of 66 long fuel rods 2 and eight short fuel rods 3 or a second fuel assembly composed of only 74 long fuel rods 2; The characteristic diagram showing the power peaking of the core loaded with the fuel assembly of (a), (B) is the characteristic diagram showing the maximum linear power density of each, (C) is the axial power distribution diagram at the beginning of the operation cycle, (D) is the operation Axial power distribution chart at the end of the cycle.

FIG. 15 is a characteristic diagram showing a combustion change at an infinite multiplication factor when two gadolinia are additionally added only in the lower part of the fuel assembly shown in FIG. 12 ;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Long fuel rod, 3 ... Short fuel rod, 4
... upper tie plate, 5 ... lower tie plate, 6 ... large diameter water rod, 7 ... channel box, 8 ... spacer, 9 ... external spring.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-56-157890 (JP, A) JP-A-4-58191 (JP, A) JP-A-4-223295 (JP, A) JP-A-3- 179293 (JP, A) JP-A-2-103951 (JP, A) JP-A-2-2977 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 3/30 G21C 5 / 00

Claims (4)

(57) [Claims] A fuel assembly comprising a long fuel rod and a short fuel rod having an effective portion shorter than the long fuel rod in a lattice shape, wherein the fuel rod containing a burnable poison is and fuel rods of the first group of burnable poison to most areas <br/> is contained except for its upper and lower ends a long fuel rod, elongated
It said height of length fuel rods are present or a length fuel rods
At least a portion of the axial lower area which corresponds to the position
Only it comprises a second group of fuel rods the burnable poison is contained, burnable poison is contained in the second group of fuel rods
The concentration of burnable poisons in the portion of
Of the concentration of burnable poisons at the site containing
A fuel assembly characterized by being set to 1/3 or less . 2. The fuel rod of the second group is connected to a water rod.
Claims characterized in that they are arranged at adjacent positions
2. The fuel assembly according to 1 . 3. The fuel assembly according to claim 1,
Plural fuel assemblies and type 2 fuel assemblies are loaded
The number of the first group of fuel rods is
Type 1 fuel assemblies more and before fuel assemblies
The number of fuel rods in the second group is from the above-mentioned type 1 fuel assembly.
More type 2 fuel assemblies are set, and
Type 1 fuel assembly and type 2 fuel assembly
At least new fuel and second cycle fuel
A reactor core for a boiling water reactor, wherein the reactor core is constituted by: 4. A long fuel rod and more effective than the long fuel rod
Combustible by combining short fuel rods with short parts in a grid
All fuel rods containing toxic poisons are long fuel rods.
Most areas except the upper and lower ends contain burnable poisons.
Claims 1. A type 1 fuel assembly, which is a first group of fuel rods,
A plurality of the type 2 fuel assemblies described in item 1 are loaded.
And the number of fuel rods in the first group is
The type 1 fuel assembly is set more than the body,
The type 1 fuel assembly and the type 2 fuel assembly
At least each new fuel and second cycle fuel
Boiling water reactor characterized by being loaded and configured
Core .