JPH04220595A - Fuel aggregate for boiling-water reactor and reactor core of reactor - Google Patents

Abstract

PURPOSE:To obtain a fuel aggregate for boiling-water reactor and a reactor core of reactor for enabling fuel to be utilized effectively by using a spectral shift operation effectively. CONSTITUTION:A concentrate level of a lower part in axial direction of a nuclear fission material is set to a higher level than that of a higher part and a content of a combustible poison at a lower part in axial direction is set to a higher level than that at a higher level, a border of the concentrate of these nuclear fission materials is allowed to differ from that of the content of the combustible poison, and furthermore MLHGR is kept to a constant height, thus achieving nearly a constant output profile and at the same time enabling a spectral shift operation to be efficient.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to fuel assemblies and reactor cores for boiling water reactors, and particularly in the early stages of combustion, the power peak on the axially lower side is increased to accumulate fissile plutonium, and in the final stages of combustion, In this case, it is suitable for boiling water reactors that perform so-called spectral shift operation, which makes effective use of nuclear fuel material by reducing the core average void fraction and increasing reactivity by effectively using accumulated fissile plutonium. It relates to fuel assemblies and reactor cores.

0002

2. Description of the Related Art FIGS. 8 and 9 are explanatory diagrams showing the axial enrichment and gadolinia distribution of a conventional two-stream fuel assembly for a boiling water nuclear reactor. Two-stream fuel is a core management method in which two types of fuel with different designs are prepared and their core loading ratios are adjusted to obtain desired core characteristics.

[0003] The enrichment level in the figure is expressed as % by weight of the total uranium amount of 235U, and the amount of gadolinia is UO2.
It is expressed as the weight of Gd2O3 in +Gd2O3. In addition, the fuel lattice shape is a 9x9 structure with a large water channel placed in the 3x3 position in the center of the 9 rows x 9 columns of fuel rods.
x9 fuel, and the total number of heat generating fuel rods is 72. For example, in the example of Figure 8, 3.60%e+11G3.5 is 2
The 35U concentration is 3.60% and the gadolinia is 3.5W.
It shows that there are 11 t% fuel rods. In all figures, 1/24 of the upper and lower ends of 0.71%e represent natural uranium blankets.

As shown in each figure, the natural uranium part and the upper part 2
Except for the nodes, both the enrichment and the gadolinia concentration are uniform in the axial direction in the aggregate in Fig. 9, and the enrichment is uniform in the axial direction in the aggregate in Fig. 8, but the gadolinia concentration is lower in the axial direction. That's how big it is.

The fuel assembly shown in FIGS. 8 and 9 is used as a two-stream fuel, and is used as a high-gadolinia fuel and a low-gadolinia fuel. The purpose of this is to maintain appropriate surplus reactivity, adjust the power distribution during high-temperature operation with only a small number of deep-inserted control rods, and ensure margin for reactor shutdown during cold temperatures. In other words, if the cycle length becomes shorter than the planned cycle length for some reason, when the next cycle is operated, the excess reactivity at the beginning of the cycle (BOC) tends to increase, so the normal equilibrium Load more high gadolinia fuel than low gadolinia fuel compared to the cycle, and if for some reason the cycle length becomes longer than planned, load low gadolinia fuel to ensure 100% output. It was necessary to increase the surplus reactivity by loading a relatively large amount.

[0006]

[Problems to be Solved by the Invention] The fuel assemblies shown in FIGS. 8 and 9 have the following drawbacks. In the assembly shown in Fig. 8, since both the enrichment and the gadolinia concentration are uniform in the axial direction, a significant lower output peak occurs at the beginning of the cycle due to the difference in void reactivity between the upper and lower parts in the axial direction, which is unique to BWR in the early stage of combustion. There was a possibility that the maximum linear power density (MLHGR) would exceed the limit value.

In addition, in the assembly shown in FIG. 9, the difference in multiplication factor K∞ between the upper and lower parts at the early stage of combustion becomes smaller and relatively flat due to the enhancement of gadolinia in the lower part, but the spectral shift effect cannot be obtained accordingly. . In addition, if the amount of gadolinia is adjusted (reducing the amount of gadolinia in the lower part) in order to achieve a sufficient spectral shift effect, the combustion in the lower part of the axis at the end of the cycle will progress and in the next cycle. Since the infinite multiplication factor K∞ in the axially lower part of the axial direction decreases, even if a spectrum shift operation is attempted, it is no longer possible to perform the spectrum shift operation.

Similar to the assembly shown in FIG. 9, there is a design in which the concentration of the axial lower part is lower than that of the upper part in order to lower the infinite multiplication factor K∞ of the axial lower part. This has the disadvantage that it becomes difficult to perform spectrum shift operation from the next cycle onwards.

[0009] For safety reasons, the present invention is based on the maximum linear power density (Max.Linear
Heat Generation Rate (kW/
ft) :hereinafter referred to as MLHGR) is set at the lower core, and a nearly constant MLHGR is achieved throughout the cycle, and the level of the energy spectrum of the neutron flux in the core is adjusted. Increased to 238U (non-fissionable) 239P
It has a fuel assembly and core configuration that effectively utilizes Gd2O (which promotes conversion to fissionable Gd2O and contributes to the reactivity of the core), and the reactivity of the conventional axial lower part is reduced to Gd2O.
3. Fuel assemblies and nuclear reactors for boiling water reactors are designed to solve the problem that effective use of fuel cannot be achieved because sufficient spectral shift operation cannot be achieved with fuel that is lowered only by increasing or reducing enrichment. The purpose is to obtain a reactor core.

0010

[Means for Solving the Problems] In the fuel assembly for a boiling water nuclear reactor according to the invention as set forth in claim 1, a plurality of fuel rods each having a fissile material loaded in a fuel cladding tube are arranged in a bundle. In the fuel assembly for a boiling water reactor, the enrichment of the fissile material is higher in the lower part in the axial direction than in the upper part, and the burnable poison content is higher in the lower part in the axial direction than in the upper part, The boundaries of the concentration of fissile material and the boundaries of burnable poison content are made different.

[0011] In the boiling water reactor core according to the invention as set forth in claim 2, a boiling water reactor core is constructed in which a plurality of fuel assemblies are arranged in parallel in a pressure vessel to constitute a fuel assembly portion. In the reactor core, the enrichment of fissile material in the fuel assembly is higher in the axial lower part than in the upper part, and the burnable poison content is higher in the axial lower part than in the upper part, and the enrichment boundary and flammable The boundaries of toxic substance content are different.

[0012] In the boiling water reactor core according to the invention as set forth in claim 3, in the boiling water reactor core as set forth in claim 2, at least two types of fuels having different burnable poison contents are used. It uses aggregates.

In the boiling water reactor core according to the invention set forth in claim 4, in the boiling water reactor core set forth in claim 2, the first boiling water reactor core has a higher enrichment degree in the lower part in the axial direction than in the upper part. This is a fuel assembly in which a second fuel assembly in which the burnable poison content in the axially lower part is higher than that in the upper part are arranged side by side.

[0014] In the boiling water reactor core according to the invention set forth in claim 5, in the boiling water reactor core set forth in claim 4, at least two types of fuels having different burnable poison contents are used. It uses aggregates.

[0015]

[Operation] The present invention makes the concentration of fissile material higher in the axial lower part than in the upper part, and the burnable poison content in the axial lower part than in the upper part, and the boundary between the enrichment levels of these fissile materials. In addition to different burnable poison content boundaries, the MLHGR is maintained at a certain level and a substantially constant power distribution is achieved while enabling spectrum shift operation.

That is, as a specific example, the amount of gadolinia is increased and the degree of enrichment is increased on the lower side in the axial direction of the aggregate, and furthermore, the boundary between the amount of gadolinia and the degree of enrichment is not made to coincide in the axial direction. . If the boundaries of both the enrichment level and the content of gadolinia, which is a burnable poison, coincide near the center of the axis, this will be far below the lower part of the axis (5 nodes below the lower part when dividing the axis into 24 parts). This is to reduce the output peak that occurs in the vicinity. In other words, if both boundaries are made different, as schematically compared in FIGS. 2 and 3, which will be described later, the reactivity will only be increased in the central part in the axial direction where the output peak is not so large. , MLHGR does not become so large.

Note that FIG. 2 shows a comparison of the infinite multiplication factors K∞ when the boundaries are different, and FIG. 3 shows a comparison of the infinite multiplication factors K∞ when the boundaries are the same. Incidentally, the infinite multiplication factor K∞ is based on an actual reactor core taking into consideration the axial void distribution.

In a reactor core loaded with a fuel assembly according to the present invention, the output in the lower axial direction is suppressed at the beginning of the cycle (BOC), but the output in the central portion increases, and overall the output in the lower axial direction is relatively low. This increases the core average void fraction. This hardens the neutron spectrum and
Accumulation of fissile Pu is promoted (as the void ratio increases, the neutron moderation ability decreases and the neutron spectrum becomes hardened) (
The energy level of the neutron flux increases), the resonance absorption of non-fissile 238U neutrons increases, and the conversion to fissile 239U is promoted).

Furthermore, from the mid-cycle (MOC) to the end-cycle (
EOC), the reactivity value of gadolinia is small and almost equal in both the upper and lower parts of the axis, and the enrichment in the lower part was high in the early stage of combustion, so the output in the lower part of the axis remains high, and the same This promotes the accumulation of fissile Pu.

[0020] Therefore, even when moving to the next cycle due to spectrum shift operation, it is not possible to obtain the same axial power distribution as in the previous cycle because the reactivity on the lower side is large in the amount of fissile 235U. can. In other words, in each cycle, it is possible to achieve a nearly constant axial power distribution, and also to perform sufficient spectral shift operation, keeping the MLHGR at a certain level and making effective use of fuel. will be possible.

Note that the present invention can be applied within a fuel assembly, or can be applied, for example, between two types of fuel assemblies. In other words,
One aggregate can have the characteristics shown in Figure 2, or one aggregate can have the characteristics shown on the left side of Figure 2, and the other aggregate can have the characteristics shown on the right side of Figure 2. It is also possible to load almost the same number of both assemblies into the reactor core so that the core as a whole has the characteristics shown in FIG.

[0022]

[Example] Fig. 1 is an explanatory diagram showing the axial enrichment and gadolinia distribution of a two-stream fuel assembly for a boiling water reactor according to an embodiment of the present invention, and Fig. 2 is a configuration of the fuel assembly of the present invention. FIG. 3 is an explanatory diagram schematically showing the configuration of a conventional fuel assembly.

As shown in the figure, the concentration of the lower part in the axial direction of the aggregate is made higher than that of the upper part, and the content of gadolinia, which is a burnable poison, in the lower part of the aggregate is made higher than that of the upper part, and the boundary between the enrichment levels and The boundaries of burnable poison content are different. That is, on the lower side in the axial direction of the aggregate, the amount of gadolinia is increased and the degree of enrichment is also increased, and furthermore, the boundary between the amount of gadolinia and the degree of enrichment is not made to coincide in the axial direction. Therefore, the spectral shift operation is made effective while flattening the output distribution, and since both boundaries are made different, the MLHGR does not become so large.

FIG. 1 of this embodiment and the conventional comparative examples 1 and 2
Each of the fuel assemblies shown in Figures 8 and 9 of , and Figure 10 of Comparative Example 4 with the same boundaries has the characteristics shown in Table 1 below by averaging the high gadolinia fuel and the low gadolinia fuel, and has almost the same characteristics. It has a certain amount of substance.

[0025]

[Table 1]

[0026] Also, in the table, Σ(WGi・NGi・NNi
) is an index representing the total amount of gadolinia, WGi,
NGi,NNi are the gadolinia weight (wt%) in the axial division area i, the number of fuel rods containing gadolinia, and the area i
The number of nodes in the axial direction of , the reactivity gain depends on the fuel design.

Although the fuel assembly shown in FIG. 10 has the best design for spectrum shift operation, a very large lower output peak occurs at the beginning of the cycle, making it impossible for the design to meet the limit value. It is something that is.

[0028] In each case, the control rod (C/R)
A Hering calculation, which calculates the core burn-up assuming that the reactor is operated at a constant power throughout the cycle without inserting a C/R operation, and a control rod plan, which calculates the core burn-up taking into account actual C/R operations, were carried out. The core is a 1100 MWe class BWR/
5, consisting of 764 fuel assemblies,
The number of replacement units is assumed to be 232 units, assuming a 15-month operation. The calculation results are shown in Table 2 below.

[0029]

[Table 2]

In this characteristic analysis, the same local peaking coefficient and the same control rod plan were used in all cases in order to prevent the calculation results from depending on the detailed design of the assembly. As for the characteristics in Table 2, it can be seen that while there is almost no difference in the life and other characteristics of the Hering calculation results, there are considerable differences in the cycle life and MLHGR in the control rod plan in each case.

FIG. 4 is a diagram plotting the relationship between MLHGR and cycle life in a control rod plan in order to make this point easier to understand. As shown in Figure 4, the MLHGR
It can be seen that the life span in this example is increased without any increase.

Furthermore, FIGS. 5, 6, and 7 are diagrams showing the output distribution at the MLHGR generation handle for BOC, MOC, and EOC. In this example, indicated by the solid line in the figure, the change in output at the lower axial direction is small from BOC to EOC, and from this it is possible to perform the same spectral shift operation as in this control rod plan in the next cycle as well. It turns out that it is possible. Comparative example 1 of broken line and two-dot chain line,
3, with this control rod plan, a significant gain in cycle life was seen as shown in Figure 4, but in the next cycle, it was difficult to perform spectrum shift operation, and overall it was not possible to achieve that much gain in life. Furthermore, as shown in FIGS. 5 and 6, the lower output peak is large from BOC to MOC, and the MLHGR is high.

As described above, it can be said that the present invention is effective as a method for continuously performing sufficient spectrum shift operation while lowering the MLHGR to some extent.

[0034]

As explained above, the present invention makes the concentration of fissile material higher in the axial lower part than in the upper part, and the burnable poison content in the axial lower part is higher than in the upper part. By making the boundaries of the concentration of toxic substances and the boundaries of burnable poison content different, the output distribution is made flat, and in particular, without increasing the MLHGR significantly,
Lifetime gains can be obtained through spectral shift operation.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the axial enrichment and gadolinia distribution of a two-stream fuel assembly for a boiling water nuclear reactor according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram schematically showing the configuration of a fuel assembly of the present invention.

FIG. 3 is an explanatory diagram schematically showing the configuration of a conventional fuel assembly.

FIG. 4 is a diagram plotting the relationship between MLHGR and cycle life in a control rod plan.

[Figure 5] BOC output distribution at MLHGR generation handle
FIG.

[Figure 6] MOC the output distribution at the MLHGR generation handle
FIG.

[Figure 7] EOC output distribution at MLHGR generation handle
FIG.

FIG. 8 is an explanatory diagram showing the axial enrichment and gadolinia distribution of a conventional two-stream fuel assembly for a boiling water reactor.

FIG. 9 is an explanatory diagram showing the axial enrichment and gadolinia distribution of another conventional two-stream fuel assembly for a boiling water reactor.

FIG. 10 is an explanatory diagram showing the axial enrichment and gadolinia distribution of a two-stream fuel assembly for a boiling water reactor for comparison.

Claims (5)

[Claims] Claim 1: In a fuel assembly for a boiling water reactor in which a plurality of fuel rods each having a fissile material loaded into a fuel cladding tube are bundled together, the enrichment of the fissile material is adjusted to the lower part in the axial direction. is higher than the upper part, and the burnable poison content is higher in the axial lower part than in the upper part, and the boundary of the enrichment degree of the fissile material and the boundary of the burnable poison content are different. Fuel assembly for boiling water reactors. [Claim 2] In a boiling water reactor core in which a plurality of fuel assemblies are arranged in parallel in a pressure vessel to constitute a fuel assembly section, the enrichment of fissile material in the fuel assembly section is controlled. A boiling device characterized in that the lower part in the axial direction is higher than the upper part, and the burnable poison content is higher in the lower part in the axial direction than the upper part, and the boundary of the enrichment level and the boundary of the burnable poison content are different. Water reactor core. 3. The boiling water reactor core according to claim 2, characterized in that at least two types of fuel assemblies having different burnable poison contents are used. . 4. The boiling water reactor core according to claim 2, wherein the first fuel assembly has a higher enrichment in the axial lower part than the upper part, and a burnable poison content in the axial lower part. A boiling water reactor core characterized in that a second fuel assembly is installed in parallel with the second fuel assembly which is higher than the upper part. 5. The boiling water reactor core according to claim 4, characterized in that at least two types of fuel assemblies having different burnable poison contents are used. .