JP2504474B2 - Initially loaded core of boiling water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to an initial-loaded core of a boiling water reactor.

(Prior Art) An initially loaded core of a boiling water reactor has used one type of fuel having an average fuel assembly enrichment of about 2.2% by weight. (Hereinafter, this is referred to as a uniform core.) However, in recent years, in order to improve the efficiency of the fuel cycle, a multi-enrichment core using three or more types of fuel assemblies with different average enrichment of fuel assemblies has been considered. There is.

In such a core composed of fuel assemblies having different enrichments, a fuel assembly of a low enriched fuel after the first cycle, a medium enriched fuel after the second cycle, and a highly enriched fuel after the third cycle is taken out, and the initial loading is performed. Residual U in the extracted fuel assembly
^The amount of ²³⁵ can be remarkably reduced compared to the conventional uniform core, and the efficiency of the fuel cycle is greatly improved.

By the way, as for the axial design of such a fuel assembly, the conventional design concept of the fuel assembly of the uniform core has been proposed as it is. That is, in the axial design of the initially loaded fuel assemblies of each type, when there is a distribution of enrichment, each fuel assembly of each type has a boundary of enrichment classification at the same axial height position.

Further, regarding the fuel assembly arrangement in the radial direction of the core, in order to reduce the leakage of neutrons from the core during the first cycle and improve the neutron economy, the outermost core of the core has the lowest enrichment of each type. Fuel assemblies are arranged.

For example, a case of a core composed of three types of fuel assemblies will be described. The fuel assemblies are referred to as Type I, Type II, and Type III fuel assemblies in descending order of average enrichment. Then, as compared with the case of the uniform core, the average power (radial power distribution) of the type I fuel assemblies arranged in the central part of the core becomes larger. Therefore, due to the difference in enrichment in the axial direction due to the same axial design as in the case of the uniform core, that is, by increasing the enrichment in the upper part and lowering the enrichment in the lower part, type I and II
Even if the axial power distribution control of the fuel assembly is performed, type I
The lower output of the fuel assembly tends to be larger than that of the uniform core as the radial power peaking coefficient increases. In other words, the linear power density (kw / ft) is the limit value for driving (for example, 13.4kw / ft).
ft) tends to run out of room. By the way, since the type III fuel assembly has a low enrichment, the linear power density has a sufficient margin with respect to the operation limit value. Further, for simplification of the design, there is no axial enrichment distribution for power distribution control and no combustible poisons, so there is a margin for the operation limit value.

(Problems to be solved by the invention) In order to improve the margin for the above-mentioned limit value of operation, the difference in vertical enrichment between the type I and type II fuel assemblies is increased,
The difference in the amount of combustible poisons is made higher or lower (increasing the concentration of combustible poisons at the bottom, or increasing the number of fuel rods with burnable poisons added at the bottom). By doing so, the reactivity difference above and below the axial zone division boundary increases, so that the portion immediately above this boundary becomes the power peak, and the core axial distribution becomes the shape of the central peak. Therefore, the linear output density of this portion becomes severe.

Further, during most of the first cycle, the type I fuel assembly burns in the axial peak power distribution state of the central peak, and the fuel assembly with the maximum reactivity at the end of the cycle is the type I fuel assembly, The burnup distribution is shown by the solid line L in FIG. 7, and as a result, the output distribution at the end of the cycle is 8th.
The distribution is the solid line M in the figure. In addition, since voids are generated from the lower part of the core, the average void fraction in the core becomes large, and the reactivity at the end of the cycle is lost accordingly.

Furthermore, in order to improve fuel economy, a blanket part of natural uranium or depleted uranium is provided at the upper and lower ends of the active fuel length part of the type I, II, and III fuel assemblies to increase the enrichment of the region between the blanket parts. . With this design, even if the average enrichment of the fuel assembly is the same, the fuel assembly has a high reactivity. However, with that design, the line output density at the central portion in the axial direction becomes stricter than in the past. Further, the linear power density of the type I fuel assembly becomes strict in the design in which the boundaries of the axial sections of the axial enrichment design are the same as described above. In addition, a type III fuel assembly with the lowest enrichment is placed in the outermost periphery of the initially loaded core, and type I and II fuels with higher enrichment are provided from the second layer (one inside the outermost periphery fuel) to the inside. When the aggregates are mixedly arranged, the outputs of the outermost periphery and the second layer of the initially loaded core are further reduced as compared with the conventional uniform core. Therefore, the combustion amount of the outermost peripheral fuel in the first cycle is small, and if the lowest enrichment fuel in the re-outer periphery is taken out after the first cycle, the fuel with a small amount of power generation has the same maximum processing cost as the fuel with a large amount of power generation. This is a disadvantage in terms of nuclear fuel cycle efficiency.

It is an object of the present invention, in an initially loaded core using a plurality of fuel assemblies having different enrichments, to determine the axial enrichment distribution division of the fuel assemblies in the axial power distribution design as described above. Solve the problem caused by the same boundary,
Furthermore, by improving the fuel arrangement in the radial direction of the core, the linear power density, M
The object is to provide a core that fully satisfies the requirements for core characteristics such as CPR and reactor shutdown margin and has a high degree of effective fuel utilization.

[Structure of Invention]

(Means for Solving Problems) In the present invention, assuming that a type I fuel assembly, a type II fuel assembly, and a type III fuel assembly are arranged in descending order of average enrichment, a difference in vertical enrichment between the type I fuel assemblies. Of the type II fuel assembly
The boundary of the II fuel assembly is set 1/12 to 1/6 of the active fuel length above that of the type I fuel assembly. In addition, 1/12 to 1/1 above the enrichment region of type I and II fuel assemblies.
Provide a low flammable poison zone of length 6. In addition, the enrichment of the low combustible poison region of the type I fuel assembly is made smaller than that of the region immediately below it, and the enrichment of the low combustible poison region of the type II fuel assembly is set immediately below it. Same as. An initially loaded core having such type I, II, and III fuel assemblies has a type III fuel assembly on the outermost periphery, a type I fuel assembly on the second layer from the outermost periphery, reactivity compensation during operation, Type II is used for the positions (control cells) of the four fuel assemblies around the control rod that are inserted to control the power distribution.
I fuel assembly, type I, II, III in other positions regularly
Preferably, the fuel assemblies are regularly arranged in the order of Type I, II, III fuel assemblies.

(Operation) By shifting the boundary between the type I fuel assembly and the type II fuel assembly, it is possible to prevent an output peak from appearing at the central portion in the axial direction of the type I fuel assembly.

Furthermore, by reducing the amount of combustible poisons on the upper part of the fuel assembly, the output peak of the upper part is increased, and the type I fuel assembly is provided with a relatively flat lower peak power distribution. Can be burned. In addition, by making the enrichment of the low burnable poison region of the type I fuel assembly smaller than that of the lower portion thereof, it is possible to suppress the output distribution of the extreme upper peak at the cold temperature, and to make the reactor shutdown margin stricter. Even in the replacement core, the reactor shutdown margin can be improved.

Since the low enrichment type III fuel assemblies are arranged on the outermost circumference, the radial output (average assembly output) of the type I fuel assemblies in the central region tends to be too large. This can be improved by placing a type I fuel assembly in the eye and increasing the power around the core. Further, by arranging the outermost type III fuel assembly at the same position as in the first and second cycles, the take-out burnup is promoted and the nuclear fuel cycle efficiency of the outermost type III fuel assembly is improved.

(Embodiment) FIG. 1 shows an example of fuel loading arrangement of a core according to the present invention. Fuel flow assembly Three types of fuel assemblies with different average enrichment (type I fuel assembly 1 for high enrichment fuel, type II fuel assembly 2 for medium enrichment fuel, type III fuel assembly for low enrichment fuel) 3) is used.

In the above-mentioned core arrangement example, the type III fuel assembly 3 is arranged on the outermost periphery, and the control cell C (the output of the control cell C is composed of the type III fuel assembly 3 around the four control rods in the central area of the core). , A dedicated control rod cell for controlling the reactivity control and the output distribution control, and the fuel around the control rod is a fuel assembly having a low reactivity.

The type I fuel assembly 1 having the highest enrichment is arranged on the second layer from the outermost circumference, and the type I, II, and III fuel assemblies 1, 2, and 3 are basically I, I, III at the remaining positions. It is arranged regularly in the order of II and III.

Fig. 2 shows examples of axial enrichment distributions and burnable poison axial distributions of Type I, Type II, and Type III fuel assemblies. The type I fuel assembly 1 and the type II fuel assembly 2 have an axial distribution of enrichment, and the enrichment of the type III fuel assembly 3 is uniform in the axial direction. The type I fuel assembly 1 has a partition boundary I of enrichment at a position of about 1/3 to 1/2 from below the active fuel length L, and the enrichment above and below the boundary I is higher. The boundary II of the type II fuel assembly 2 is 1/12 or more above the effective length of the type I fuel assembly 1 and about 2/3 below the effective length. Type I and II fuel assemblies 1 and 2 have burnable poisoned fuel rods. Gd ₂ O ₃ is considered here as an example of a burnable poison, but other burnable poisons may be used. The design of the axial distribution of burnable poison is such that the burnable poison exists in the concentrated area within the active fuel length L, and the burnable poison amount is uniform within that area or at the same position as the boundary I, II of the enrichment classification. The difference is that the top is small and the bottom is large. Furthermore, either or both of the type I and II fuel assemblies 1 and 2 are combustible with a length of about 1/12 to 1/6 of the active fuel length L from the upper end of the enrichment region above the boundaries I and II. It has a low-flammability toxicant area with a low amount of toxicant. As a means for reducing the amount of combustible poison, the concentration of Gd ₂ O _{3 for} the type I fuel assembly 1 is made smaller than that of the region immediately below the low combustible poison region. For example, the concentration is set as low as about 1.5 to 3%. Or Gd ₂
Reduce one O ₃ fuel rod. Or, there are means such as both.

Type II fuel assembly 2 is Gd ₂ O _{3 in the} low flammable poison range
Reduce the number of added fuel rods by 2 to 3, and simultaneously increase the Gd ₂ O ₃ concentration to 1.5 to ₃
Use a low concentration of about%.

Here, the Gd ₂ O ₃ concentration and the number of Gd ₂ O ₃ -added fuel rods in the low-burnable poison area are whether the maximum linear power density due to the axial power distribution satisfies the limit value, and the reactor shutdown margin exceeds the design target value ( Usually 1% Δk or more). If this is satisfied, the number of Gd ₂ O ₃ -added fuel rods may be further reduced, and the Gd ₂ O ₃ concentration may be lower, for example 1 to 1.5%. The enrichment of the low burnable poison region of the Type I fuel assembly should be less than that of the region immediately below. The enrichment may be the same as the enrichment below the boundary I of the type I fuel assembly 1 for the purpose of streamlining the production, or may be even lower than this.

The axial power distribution of each type of fuel assembly at the initial stage of the first cycle combustion in the initially loaded core according to the embodiment of FIG. 2 is the distribution shown in FIG. That is, the type I fuel assembly 1 has a flat power distribution and has a slightly lower peak solid line 2-
a, the type II fuel assembly 2 shifts the boundary I of the type I fuel assembly 1 upward, and the peak position shifts upward due to the effect of the low combustible poison region and the broken line 2-b of the upper peak. Furthermore, the type III fuel assembly 3 has a lower peak curve, but it is a void curve because the output is low at low enrichment, but the output is lower peak at the axial direction due to voiding because the output is low at low enrichment. Is weaker than the one-dot chain line 2-c
Becomes The axial power distribution of the entire core is the curve H in FIG.

By the way, if both Type I and II fuel assemblies 1 and 2
When I and II are at the same height, the output distribution of the type I fuel is output above the boundary I as indicated by the broken line 2-ab due to the synergistic effect of the axial reactivity distributions of the adjacent type I and type II fuels. The peak becomes prominent, and the margin becomes smaller than the operation limit value of the line power density.

Further, since the low-combustible poison region is provided, the output at the upper end becomes large, the peak at the center becomes relatively low, and the linear power density at the peak of both type I and II fuel assemblies 1 and 2 decreases. A margin for the operation limit value can be secured.

According to the calculation, it is known that the effect of reducing the peak of the axial power distribution does not become large unless the distance between the boundary I and the boundary II is shifted by about 30 cm or more. The design example shifted by about 45 cm is effective in reducing the maximum line power density of about 0.5 kw / ft (about 5%).

By the way, in the example of FIG. 2, if the difference in enrichment of uranium above and below the boundary is set to be too large without providing the low-combustible poison region, for example, if it is 0.3 to 0.5% by weight, the first cycle to the final stage of the first cycle The axial output distributions of the type I fuel assembly 1 and the type II fuel assembly 2 over the course of time are the distributions shown by the curve J in FIG. With such an output distribution, the output distribution at the end of the cycle is the distribution shown by the curve K in FIG. This is because the Type I and II fuel assemblies 1 and 2 burn with the power distribution having a peak in the center during most of the cycle, so combustion near the lower end of the fuel does not proceed, and even at the end of the cycle, Type I , II Fuel assembly 1,2
The reactivity in the vicinity of the lower end is still high and the axial power distribution has a downward peak. Then, voids are generated from the lower part of the core, the void amount of the core is increased, the reactivity is lost due to the voids, and the cycle burnup is reduced. In order to improve this defect, the difference in enrichment above and below the boundary is not made too large,
It should be about 0.2% by weight. However, this causes a lower output peak at the beginning of the cycle. Therefore, if both the type I and II fuel assemblies are provided with the low-combustible poison region, the reactivity at the upper part of the fuel assembly is increased, so that the lower output peak can be reduced and the deterioration of the output distribution at the beginning of the cycle can be improved.

In the low combustible poison region of the type I fuel assembly 1, since the type I fuel assembly 1 has a high enrichment and the reactor shutdown margin is not so large, the number of Gd ₂ O ₃ added fuel rods is set to be a low combustible poison. One less than the area just below the area, Gd ₂ O
₃ Decrease the concentration to around 1.5 to 3.0, or reduce the number of Gd ₂ O ₃ fuel rods and reduce the Gd ₂ O ₃ concentration.

On the other hand, the low burnable poison region of the type II fuel assembly 2, the Gd ₂ O ₃ added number of fuel rods because a large reactor shutdown margin at medium enrichment type II fuel assembly 2, the low burnable poison region immediately Reduced 2-3 from the lower part, and the Gd ₂ O ₃ concentration is 1.5 to 3.0.
% To a low concentration.

The type I fuel assembly 1 is a highly concentrated fuel and is a fuel extracted in the third and subsequent cycles. Therefore, the void fraction of the type I fuel assembly 1 can be increased by increasing the average fuel enrichment and setting the output distribution of the lower peak from the early stage to the middle stage of the first cycle. Increase the amount of Pu accumulated from the central part to the upper part,
It can contribute to the improvement of reactivity at the end of the cycle and the second cycle. From this point of view, it is desirable that the boundary of the type I fuel assembly be located below the axial power distribution in a permissible range. That is, by making the boundary as low as possible, the region of the high enrichment portion becomes wider, so that the average enrichment of the type I fuel assembly can be increased and the ejection burnup of the type I fuel assembly can be improved.

Also, if the difference in enrichment between the low burnable poison region in the upper part of the type I fuel assembly and the region immediately below it is made too small, there is little effect, and the enrichment in the low burnable poison region is reduced by about 0.2% by weight or more. As a result, it was found that the linear power density in the low burnable poison region of the type I fuel assembly could be prevented from increasing too much, and that the effect of improving the reactor shutdown margin in the replacement core was great. In other words, the core shutdown margin of the replacement core largely depends on the reactivity of the four fuels adjacent to the control rods at low temperature in the upper 1/6 region, and the higher the reactivity, the core shutdown margin. Less is. Of the lower flammable poison area
Gd almost burned in the first and second cycles and did not remain in the third cycle. Therefore, if the enrichment in this region is high, it is necessary to increase the Gd ₂ O ₃ concentration in the upper part of the replacement fuel to reduce the reactivity and secure a reactor shutdown margin in the replacement core. This is disadvantageous in terms of neutron economy because the amount of residual Gd at the end of the core cycle becomes large. Therefore, as in the present invention, the type I fuel assembly 1 of the initially loaded fuel is
Reducing the concentration of the low-burnable poisonous substance region in the can improve the reactor shutdown margin and is advantageous in neutron economy.

Further, such type I, II, III fuel assemblies 1, 2, 3 are arranged in the core as shown in FIG. 1, and the outermost type III fuel assembly 3 is used for the first, second and second cycles. If left as it is, the burnup of the outermost type III fuel assembly will be taken in the central region where the burnup is taken out in the first cycle.
It extends to the level of the III fuel assembly 3, which is advantageous in terms of the efficiency of the nuclear fuel cycle. Further, since the type I fuel assembly 1 having high output is arranged on the second layer from the outermost periphery, the radial peak of the type I fuel assembly 1 in the central region is mixed with each type on the second layer from the outermost periphery. Can be smaller than when loaded.

The above embodiment has been described with reference to an example of a core composed of fuel assemblies having three types of average enrichment. In this example, another fuel assembly is provided between the type II fuel assembly and the type III fuel assembly. Prepare and use it as a Type IIIA fuel assembly 3A,
The fuel assembly with the lowest enrichment is called the type IV fuel assembly 4, and it has the same enrichment as the type III fuel assembly 3 described above, or a lower enrichment (may be natural uranium or depleted uranium). It may be the core used as fuel. In that case, the type IIIA, IV fuel assemblies 3A, 4 do not contain burnable poisons, the type I, II fuel assemblies 1, 2 are the axial enrichment of the type I, II fuel assemblies 1, 2 described above, Combustible poison distribution design, type IIIA fuel assembly 3A enrichment is type II fuel assembly 2
The axial design of the same idea as the type IV fuel assembly 4 is designed to be uniform in the axial direction.

The core using these four types of fuel assemblies is a design which is convenient for improving the take-out burnup in the case of an initially loaded core simulating an equilibrium core with about 25% replacement.

〔The invention's effect〕

As described above, according to the present invention, in the initially loaded core of the boiling water reactor using a plurality of fuel assemblies having different enrichments, the operation limit of the maximum line power density is stably protected from the initial cycle to the middle cycle. However, it is possible to effectively use the fuel while maintaining the output distribution of the lower peak to increase the burnup and improve the shutdown margin of the replacement core.

[Brief description of drawings]

FIG. 1 is a diagram showing a fuel loading pattern in the first cycle of the embodiment according to the present invention, and FIG. 2 is a type I, II, III of the present invention.
FIG. 3 is a diagram showing an axial burnable poison and enrichment design of a combustion assembly, FIG. 3 is a diagram showing an axial power distribution of a fuel assembly according to the present invention, and FIG. 4 is a core average axial power distribution according to the present invention. FIG. 5 is a diagram showing the output distribution at the end of the first cycle of the type I fuel assembly when the difference in vertical enrichment of the boundary according to the present invention is large, and FIG. 6 is a type according to the present invention. FIG. 7 is a diagram showing the axial end-cycle axial power distribution of the I fuel assembly, FIG. 7 is a diagram showing a conventional axial burnup distribution,
FIG. 8 is a diagram showing a conventional axial output distribution. DESCRIPTION OF SYMBOLS 1 ... Type I fuel assembly 2 ... Type II fuel assembly 3 ... Type III fuel assembly C ... Control cell, L ... Active fuel length ... Enrichment area ... Low combustible poison area I, II ... Boundary

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Michihiro Ozawa 3-1, 1-1 Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. Hitachi Factory (72) Inventor Takaaki Mochida 3-chome, Saiwaicho, Hitachi, Ibaraki No. 1 in Hitachi factory, Hitachi, Ltd. (56) References JP-A-62-80586 (JP, A)

Claims (3)

(57) [Claims] 1. In an initially loaded core of a boiling water reactor that uses a plurality of types of fuel assemblies having different average enrichments, the fuel assemblies are type I fuel assemblies and type II fuels in order of increasing average enrichment, respectively. Assuming an assembly and a type III fuel assembly, these fuel assemblies have an enrichment region inside the active fuel length, and the type I fuel assembly and the type II fuel assembly have an axial enrichment distribution in the enrichment region. It has a boundary of the axial section of its enrichment distribution at a position of about 1/3 to 2/3 from the lower part of the active fuel length, and the upper part above and below this boundary has a higher enrichment than the lower part. , The type II fuel assembly and the type III fuel assembly, at least the boundary of the section of the type II fuel assembly is at least 1/12 of the active fuel length above the boundary of the section of the type I fuel assembly, I fuel assemblies and type II fuel assemblies are enriched regions Of a type I fuel assembly having a burnable poisonous substance and a low burnable poisonous substance region having an axial effective length of 1/12 to 1/6 at the upper end of the concentration region above the boundary. Boiling water, characterized in that the enrichment in the low burnable poison region is less than that immediately below it, and the enrichment in the low burnable poison in the type II fuel assembly is the same as that immediately below it. -Type reactor core for the first reactor. 2. A type III fuel assembly at the outermost periphery of the core,
The type I fuel assembly is located in the second layer from the outermost circumference, the type III fuel assembly is located in the control cell, and the type I fuel assembly, the type II fuel assembly, and the type are located at other positions in principle.
III. Initially loaded core of a boiling water reactor according to claim 1, characterized in that fuel assemblies are regularly arranged in this order. 3. The type I fuel assembly and the type II fuel assembly have an enrichment of about 0.2 wt% above and below the boundary above and below the boundary, and the boundary of the type II fuel assembly is above the boundary of the type I fuel assembly. Claims characterized by being above 1/12 of the active fuel length and having a enrichment in the region of low burnable poisons of the type I fuel assembly less than about 0.2% by weight less than the enrichment immediately below. Initially loaded core of a boiling water reactor according to claim 1 or 2.