JPS6321589A - Fuel aggregate - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an improvement of a fuel assembly for a boiling water nuclear reactor (hereinafter referred to as BWR).

(Prior Art) A fuel assembly used in BliV R generally has the following configuration. In other words, a channel box with an approximately square cross section contains multiple lines (for example, 8 lines x 8 lines).
A row of 64 fuel rods are loaded in a lattice pattern, and their upper and lower ends are fixed by an upper tie plate and a lower tie plate. Further, spacers are installed at the same number of locations in the axial direction, and these spacers maintain the spacing between the fuel rods. The fuel rod has a cladding tube loaded with a plurality of fuel pellets made of sintered uranium dioxide (UO3), and its upper and lower ends are sealed with an upper end plug and an upper end plug.

In order to ensure the integrity of the fuel rods mentioned above during nuclear reactor operation, it is necessary to suppress the output per unit length of the fuel rod (hereinafter referred to as power density) to below the design limit value. It is designed as shown.

■First case: The thermal neutron flux distribution in the cross section of the fuel assembly is highest at the outermost fuel rods facing the water gap, and lower at the inner side.

Therefore, in order to flatten the power distribution and reduce power peaking, a water rod is placed in the center of the fuel assembly to allow coolant to flow through it, thereby increasing the thermal neutron flux at the center of the fuel assembly.

In addition, four types of fuel rod enrichment are used, and fuel rods with low enrichment are placed in positions where thermal neutron flux is high (corners and outermost periphery of the fuel assembly), and fuel rods with low enrichment are placed in positions where thermal neutron flux is high. Fuel rods with higher spa degrees are arranged in a stepwise manner toward the inner side.

(2) Next, in the case of BWR, since voids occur, the axial output distribution has a characteristic that it has a downward peak. Therefore, in order to flatten the axial output distribution, an axial one degree distribution design and a gadolinia design as shown in FIG. 5 are performed. That is, the fuel enrichment is made higher in the upper part, and the gadolinia concentration is made to be higher in the lower part. This promotes combustion upward in the axial direction, thereby reducing the linear power density (10 to 20%). Furthermore, the fifth
In the diagram L (till), the horizontal axis represents 'fA contraction degree, the vertical axis represents the core axial equivalent i! (the length of the shaded part in the figure is the effective fuel length), and the enrichment axis 5 is a diagram showing the directional distribution.The diagram on the right side of FIG. In this case, a complex core design with low power peaking was performed in order to suppress the linear power density below the design limit value.

■Instead of (8 rows x 8 columns) fuel assemblies, it is being considered to use (9 rows x 9 columns) fuel assemblies. This is a method of reducing the output per fuel rod by increasing the number of fuel rods, that is, reducing the linear power density. According to this method, the linear power density can be reduced by about 20%.

Nowadays, there is a tendency for fuel reprocessing to increase and cycles to become longer, and there is a demand for improving fuel cycle costs by increasing the concentration of replacement fuel and reducing the number of replacement fuels. In addition, with the development of zirconium liner fuel rods (zirconium is placed inside the cladding of the fuel rod to absorb the difference in thermal expansion between the pellet and the cladding), the linear power density can be as low as possible within the limits of linear power density. afiii! There is a need to design fuels with high reactivity and low fuel cycle costs. In response to these recent design requirements, a nuclear design concept (8 rows x 8 columns) fuel assembly as shown in FIG. 6 has been put into practical use. However, for a (9 rows x 9 columns) fuel assembly, there is a problem in that an optimal concept in combination with the above-mentioned zirconium liner fuel rods has not been established. To put this more concretely, for example, as a method of operating a nuclear reactor to improve fuel cycle costs, the power distribution is operated at a downward power distribution until the linear power density limit is reached from the beginning to the middle of the cycle, and at the end of the cycle, the power distribution is reduced to the central peak. Alternatively, an operation method has been adopted in which the upper peak is set, thereby reducing the core void fraction and obtaining the core reactivity (hereinafter referred to as BSO operation). however(
In the case of the 8x8) fuel assembly, as described above, the axial design is complicated and the axial power distribution is controlled, so the reactivity gain due to the BSO operation is small. In contrast, the (9×9) ffi toll collection assembly meets the power density limitation without complicating the axial design.

However, by setting (9X9), resonance absorption increases, and the void coefficient becomes negative and its absolute value becomes large (8X8).
) is disadvantageous in terms of reactivity compared to the case of Note that resonance absorption here means that increasing the number of fuel rods shortens the distance between the fuel rods (because the dimensions of the fuel assembly remain the same even if the number increases), and as a result, the distance between the fuel rods decreases. It refers to a phenomenon in which comrades interfere with each other. Therefore, in order to make the fuel cycle cost more advantageous than the (8x8) fuel assembly, it is necessary to
In SO operation, the final axial power distribution should be peaked higher than in the case of an (8×8) fuel assembly to reduce void reactivity. However, in the case of a (9X9) fuel Fl assembly, the void coefficient increases in the negative direction as described above, so the axial power distribution becomes (8X8)
The peak peaks higher than in the case of fuel assemblies, resulting in a worse scram curve. In other words, even if a control rod is inserted, the combustion part is above, so it takes time to scram. At the same time, in the event of a pressure rise transient event (e.g. generator load shedding or turbine trip), ΔMCP
R (change amount of critical power ratio) is larger than that of the (8x8) fuel assembly, so the CPR (critical power ratio) limit value (○CM'CPR) during normal operation must be increased. There's a problem.

(Problems to be solved by the invention) In this way, the conventional (8X8) fuel assembly and (9X9)
) There are various problems with fuel assemblies, and the present invention was made based on these points, and its purpose is to solve problems within the linear power density limit in (9X9) fuel assemblies. The object of the present invention is to provide a fuel assembly with higher fuel economy.

[Configuration of the Invention (Means for Solving the Problems) In other words, the fuel assembly according to the present invention has fuel rods arranged in a grid pattern in 9 rows x 9 columns in a channel box, and the upper and lower ends of the fuel rods are arranged in a grid pattern in 9 rows and 9 columns. In a fuel assembly that is fixed by a tie plate and a lower tie plate, and spacers are installed at multiple locations in the axial direction to maintain the spacing of the fuel rods, the fuel rods with high fuel enrichment are placed at the outermost positions, and It is characterized in that the fuel enrichment in the lower part in the axial direction is higher than the fuel enrichment in the upper part in the axial direction.

(Function) First, focusing on the fact that the adoption of a (9x9) fuel assembly increases the thermal margin compared to the case of an (8x8) fuel assembly,
An attempt is made to improve fuel economy by arranging fuel with high fuel enrichment at the outermost circumferential position.

In other words, the (9x9) fuel assembly has an approximately 20% increase in the number of fuel rods compared to the (8x8) fuel assembly, and as a result, the average linear power density decreases by approximately 20%. This is as described above. Therefore, if the output peaking coefficient (local peaking coefficient) is designed in the same way as before, (
9X9) The maximum linear power density of the fuel assembly decreases by about 20%, and the thermal margin increases. In other words, if the same maximum linear power density is used, the local cubbying coefficient can be increased by up to 1.2 times. Furthermore, by making the fuel enrichment in the lower part of the axis higher than that in the upper part in the axial direction, the axial power distribution in the (9x9) fuel assembly is made to have a lower peak.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 4. FIG. 1 is a cross-sectional view of a fuel assembly 101 according to this embodiment. Reference numeral 102 in the figure represents a channel box, and the channel box 102 has a substantially square cross section.

There are multiple channels (9x9) in this channel box 102.
The fuel rods 103 are arranged in a grid pattern.

These plurality of fuel rods 103 have their upper and lower ends fixed by an upper tie plate and a lower tie plate (not shown). Spacers are installed at multiple locations in the axial direction to prevent vibration. Reference numeral 104 in the figure indicates a control rod, and one control rod 104 is arranged at the center of four fuel assemblies 101 to form a unit cell.

The fuel rods 103 are made of a plurality of types, each of which is arranged at a predetermined position. First, fuel rod 1 indicated by the symbol G in the figure
03 is made by stacking fuel pellets containing gadolinia (Gd203) as a burnable poison in uranium dioxide (UO2).This fuel rod 103 is made of (2,
3), (2,7>, (3,2>, (3,5), (3,8
), (5°3), (5,7>, (7,2), (7,5)
, (7, 8), (8, 3), (8, 7> position.In order to indicate the position in Fig. 1, numbers are attached to the horizontal and vertical axes, and the vertical axis The position is specified by the number on the horizontal axis.

The fuel rod 103 also has a blanket region 135 (N) at its upper end, as shown in FIG. This is because the fuel pellet containing gadolinia has a slightly low thermal conductivity, so the temperature of the pellet increases, resulting in fission product gas (
This is because the release of more FP gas (hereinafter referred to as FP gas) than in fuel rods that do not contain gadolinia tends to increase the internal pressure. As for the lower blanket 11,
Hit Gus Blenheim. If the internal pressure is high, the upper blanket area will also be exposed to the gas blemish. The enrichment region is separated into two parts; the upper enrichment region (1/12 to 1/6 of the effective length of the fuel) has an enrichment level of 3,000 wt%, and the gadolinia concentration is 4.5 wt%; The enriched region has an enrichment degree of 4.6 wt% and a gadolinia concentration of 4.5*t%.

Further, the fuel rods indicated by the symbol P in the figure are partial length fuel rods. This partial length fuel rod 103 is connected to another fuel rod 1 as shown in FIG.
Its axial length is shorter than that of 03. This partial length fuel rod 103 is (2°2), (2,5), (2,
8>, (5,2), (5,8>, (8,2), (8,5
), (8, 8>).In addition, it is desirable that the partial length fuel rods 103 protrude slightly above the spacer grid in consideration of hydraulic motion.Therefore, the fuel of the partial length fuel rods is The length of the missing portion depends on the spacing of the spacer grid.

The partial length fuel rods 103 have the effect of improving reactor shutdown margin, reducing coolant pressure loss, and improving void coefficient, and in a design with seven spacer grids like the current BWR, one spacer It is desirable that the spacing be one spacer or two spacers apart. In this embodiment, the spacing is two spacers. Also, its concentration is 4.85*t%.

Also, a water rond is indicated by the symbol W in the figure, and is hollow, through which a coolant flows.

The fuel rod 103 indicated by reference numeral 1 in the figure is 1/2 of the effective length of the fuel.
It has a natural uranium or depleted uranium blanket region Fa (indicated by the symbol N in Fig. 2) with a length of 4 to 1/12,
Further, the enrichment region is divided into two stages, upper and lower, and has a border at a location approximately 1/3 to 1/2 from the upper end of the effective fuel length.

And while the concentration in the upper part is 4.60wt%, the concentration in the lower part is as high as 4.85wt%. Further, the position of the boundary is made to coincide with the upper end of the effective fuel length of the partial length fuel rod 103.

The fuel rod 103 designated by the reference numeral 2 in the figure has natural uranium or depleted uranium blanket regions N at the upper and lower ends, similar to the above-mentioned reversible fuel rod 103 shown by the reference numeral 1, but the enrichment region is not separated. , its concentration is 4.00 wt%. The same applies to the fuel rod 103 indicated by reference numeral 3, and its enrichment level is 3.00 wt%.

Each type has been explained above, but the number of each type is explained in the second section.
As shown in the figure, the number of fuel assemblies and each enrichment level are determined depending on the operating length in which (9×9) fuel assemblies 101 are used, and in this example, 15 to 18
Monthly effective output (EFP~l: ENactive Fu
ll Power Per Month). The cladding tube of each fuel rod 103 is made of zirconium alloy, and has a thin PCIC collar 9 fin layer on the inside.

According to the above configuration, the following effects can be achieved.

■ First, in the case of the fuel assembly 101 according to this embodiment, the outermost fuel rod 1 facing the water gap with a high neutron flux distribution
Due to the high enrichment of 03, the thermal neutron utilization rate improves,
Reactivity gains can be obtained and fuel economy is improved.

■Next, the fuel rods 103 indicated by symbols 1, 2, and 3 have blanket regions (N) installed at their upper and lower ends, so leakage of neutrons in the vertical direction can be reduced, and neutron inbortance is high. Since the enrichment in the central part is increased, the area of fuel assembly 1011/6 is a low enrichment area (the upper end of the fuel rod indicated by G), so there is less unburned fuel and the margin for reactor shutdown is improved. be done. That is, (9X9
) In the case of the fuel assembly ``l''-, the resonance absorption is larger than that of the (8x8) fuel assembly, and the P IJ at the upper part is larger.
Since there is a large accumulation of
There are many remains of 35. As a result, the axial reactivity distribution tended to peak upward in low-temperature conditions where combustion was promiscuous. This point could be improved by this embodiment.

■For the fuel rod indicated by code 1, the enrichment in the enrichment region below the border of about 1/3 to 1/2 from the upper end of the effective fuel length is about 0% lower than that in the 'aN region above it. .1~0
．． It is increased by about 3 wt%. This concentration range is 0 to 40
% of the coolant void ratio, and since the water-to-fuel ratio is larger than that of the 40-10% void skewer portion above it, the reactivity also tends to be large. This further increases the concentration of this region, so U23 is added to the region of high inbortance.
5 is arranged, which is extremely effective in improving the reactivity gain.

■Also, since the (9X9) fuel assembly has a larger void coefficient than the (8X8) fuel assembly, the axial power distribution is naturally lower than that of the (8X8) fuel assembly, as shown in Figures 3 and 4. The degree of peak is strong. Note that the shaded area in Figures 3 and 4 is the control rod, and Figure 3 (C) and Figure 4 (C)
is in a fully controlled state. Furthermore, since the degree of enrichment is higher in the lower part of the enrichment region, a lower peak power distribution can be obtained without changing the control rod pattern to a newly inserted control rod pattern when performing BSO operation. Therefore, in order to secure a margin for reactivity adjustment using control rods, 1/1/2 of the core effective length is required.
A control rod pattern can be assembled with about 12 to 1/6 of the control rods pulled out, making operation of the control rod plan easier.

Also, the concentration at the bottom of the enrichment area is about 0.3 w higher than that at the top.
t%, and even if BSO operation is performed, the reactivity in the lower part is still high at the end of the cycle, so the shape of the axial power distribution becomes a gentler upper peak shape than before. As a result, the scram curve is improved and the ΔMCPR in the event of an abnormal transient event is lower than conventional axial-component compression designs.

■Since the axial power distribution at the end of the cycle has a gentler upward peak than in the conventional axial-structural contraction design, the core average void fraction is slightly larger, and the core Keff has a slightly larger void reactivity, resulting in loss. However, by directing some of the U235 to the part of the core where the void ratio is high and the invotance is high rather than the part where the inbotance is low, it can be burned effectively from the beginning of the cycle to the middle of the cycle, and the combustion of the U235 in the upper part is Since the remaining amount is less than the conventional charge design, there is no loss in cycle burn-up increase. Furthermore, Fig. 3 shows the change in the axial power distribution during cycle operation of a conventional (9x9) fuel assembly, and Fig. 4 shows the change in the axial power distribution in the case of this embodiment. It is a diagram.

Note that the present invention is not limited to the one embodiment described above, and various embodiments are possible. For example, instead of making the partial length fuel rod including the cladding tube as in the above embodiment, the cladding tube may be the same as usual, and only the contents may be made into a partial length.

[Effects of the Invention] As detailed above, according to the fuel assembly according to the present invention,
(9X9) fuel assembly, which can improve fuel economy and make the axial power distribution have a downward peak, making it easy to apply BSO operation and effectively reducing the deterioration of the scram curve. The effect is great, such as being able to suppress the

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a fuel assembly according to an embodiment of the present invention, FIG. 2 is a diagram showing the configuration of fuel rods in the axial direction, and FIG.
), (b), and (c) are the axial power distribution when the conventional (9x9) fuel assembly is operated at 830, and the fourth
Figure (a). (b) and (C) are characteristic diagrams showing the axial power distribution when BSO operation is performed in a fuel assembly according to an embodiment of the present invention. FIG. 3 is a characteristic diagram showing the configuration of the fuel assembly in the axial direction. 101...Fuel assembly, 102...Channel box, 103...Fuel t! =. Applicant's agent Patent attorney Takehiko Suzue Figure 5 e! , :, 6 evil 8 ≠ entertainment crushing bonito

Claims (3)

[Claims] (1) Fuel rods are arranged in a grid pattern in 9 rows x 9 columns inside the channel box, and their upper and lower ends are fixed with upper tie plates and lower tie plates, and the fuel rods are maintained at intervals in multiple locations in the axial direction. In the fuel assembly formed by installing spacers, fuel rods with high fuel enrichment are arranged at the outermost circumferential position, and the fuel enrichment in the axially lower part is higher than the fuel enrichment in the axially upper part. fuel assembly. (2) The fuel assembly according to claim 1, wherein the fuel rod employs a liner cladding tube in which the inside of the cladding tube is lined with zirconium. (3) A patent claim characterized in that the fuel rod has a fuel rod whose axial length is shorter than the effective fuel length, the fuel rod is located in the lower part in the axial direction, and the fuel rod has a region of low enrichment in the upper part in the axial direction. The fuel assembly according to item 1.