JP2966877B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a fuel assembly used in a boiling water reactor.

(Prior Art) In a boiling water type (BWR) nuclear power plant, a fuel assembly in which a large number of fuel rods are arranged in a 7 × 7 or 8 × 8 square grid and accommodated in a channel box is used. The core is loaded into the furnace together with the control rods.

As the above-mentioned fuel rod, one in which a large number of fuel pellets obtained by sintering low-enriched uranium dioxide of 2 to 3% are filled in a cladding tube has been widely used. In addition, in BWR fuel assemblies, burnable poisons are generally mixed into some fuel rods in order to adjust core reactivity.

In order to improve the economic efficiency of such a nuclear power plant, it is necessary to reduce the fuel cycle cost by increasing the burnup of fuel while responding to the improvement of the capacity factor by extending the operation period. It is effective.

In order to increase the burnup of the fuel, it is conceivable to use a conventional fuel and increase the enrichment.

(Problems to be Solved by the Invention) However, simply using a conventional fuel and simply increasing its enrichment results in hardening of the neutron spectrum,
The following phenomena occur in the core characteristics.

Increase in absolute value of void coefficient Increase in reactivity of core at cold temperature Decrease in ability to control reactivity of burnable poisons such as gadolinia This may cause thermal margin and furnace shutdown margin.

Further, in the case of achieving high burnup simply by increasing the enrichment, natural uranium cost per fuel, enrichment cost, etc. increase, so that the reduction in fuel cycle cost due to the high burnup becomes small. .

An object of the present invention is to solve the above-mentioned problems in core performance and to provide a high-economic high-burnup fuel capable of achieving a target high burnup with the lowest possible enrichment.

[Constitution of the Invention] (Means for Solving the Problems) A fuel assembly according to the present invention includes a plurality of fuel rods each having a plurality of fuel rods filled with a fuel substance. At least some of the fuel rods
A fuel rod to which a burnable poison is added by varying the content in the axial direction, and one or two fuel rods among the fuel rods containing the burnable poison are located in an axial position including enriched uranium. Contains the burnable poison, and the unit length and the amount of burnable poison per fuel rod are increased or increased from the top to the middle, and the unit length and fuel It has a region where the content of burnable poison per rod is set to be smaller in the middle, and the remaining fuel rods among the fuel rods containing the burnable poison have a unit length per unit fuel rod. It is characterized in that the content of the burnable poison is gradually increased from the upper part to the lower part.

As a result, the average content of burnable poisons in the axial direction of the entire fuel assembly is gradually increased from the upper part to the lower part.

(Operation) In the fuel assembly of the present invention, the distribution of burnable poisons in the fuel rods is made different in the axial direction of the distribution of burnable poisons in the fuel rods in the fuel assembly of the present invention. The contents are different in 2 to 5 regions in the axial direction.)

In BWRs, the lower part of the core, which generally has a lower void fraction, has a greater neutron moderating effect, a higher reactivity, and therefore a higher output. For example, if the content of burnable poisons is different between the upper and lower regions, Increasing the amount of burnable poison in the region can suppress the lower output peak at the beginning of the cycle, and keep the maximum linear power density, which is the operation control value of the fuel, within the design standard.

However, when such a burnable poison distribution is given,
For the most part of the cycle, combustion occurs in the axial power distribution with the power in the lower part of the core suppressed, and an increase in core reactivity cannot be expected.

Conversely, this effect can be expected if the peak below the core is reached from the beginning of the cycle to the middle of the cycle.

That is, the axial power distribution has a lower peak from the beginning of the cycle to the middle of the cycle, and when the average void fraction of the core increases, the neutron spectrum becomes harder, plutonium Pu can be accumulated, and the core reactivity can be increased. it can. Also, at the end of the cycle, the burnable poison burns out, the combustion in the lower region progresses from the early stage to the middle of the cycle, and the enrichment distribution is higher in the upper region than in the lower region. The distribution becomes the upper peak.

As a result, the core average void fraction is reduced, and the core reactivity can be increased.

Further, the core reactivity at the time of cold temperature can be reduced by the effect of the fuel rod, so that the reactor shutdown margin can be improved.

In the fuel assembly of the present invention, the lower peak at the beginning of the cycle is entirely suppressed in the lower region where there is a large amount of burnable poison, and one or two fuel rods containing the burnable poison having a low concentration are arranged in the lower region. As a result, the core reactivity at the end of the cycle can be obtained as a lower peak after the burnable poison has burned out and past the early stage of the cycle.

(Embodiment) Next, an embodiment of the fuel assembly of the present invention will be described with reference to FIG. 1 to FIG.

The fuel assembly 10 of the present embodiment includes a large number of fuel rods 20, a water rod 21 formed at the center of the fuel rods 20, and a channel box 22 for accommodating them.
Further, a lower tie plate, an upper tie plate and a spacer (all not shown) are attached to the fuel assembly 10.

The upper and lower ends of the fuel rod 20 and the water rod 21 are held by an upper tie plate and a lower tie plate. A number of spacers are arranged in the axial direction of the fuel rod 20 to maintain a proper gap between the fuel rod 20 and the water rod 21. A channel fastener (not shown) is attached to the upper tie plate.

The fuel rod 20 has a lower end plug and an upper end plug (not shown) in which a number of fuel pellets are loaded in a cladding tube having both ends sealed.

Fuel pellets are composed of uranium dioxide UO ₂ is a fuel material includes uranium-235 is a fissionable substance.
Further, a spring is disposed in the gas plenum in the cladding tube, and presses the fuel pellet downward.

The water rod 21 is formed of a cladding tube of the same material as the fuel rod 20, but does not carry a fuel substance, and is provided with holes through the upper and lower side surfaces of the cladding tube (not shown).
It has a structure in which coolant that does not boil inside makes money.

A control rod 30 having a cross-shaped cross section is inserted into the core of the BWR at a ratio of one to four fuel assemblies 10.

In this core, the width of the water gap formed on the side wall of the fuel assembly 10 facing the inserted control rod 30 is different from the side wall side of the fuel assembly 10 not facing the control rod 30 on the opposite side. The width of the water gap formed on the side of the fuel assembly 10 facing the control rod 30 and the width of the core (D lattice core) wider than the width of the water gap formed at Thus, there is a core (C lattice core) having a width equal to the width of a water gap formed on the side wall of the fuel assembly 10 not facing the control rod 30.

The fuel assembly 10 of this embodiment is a fuel assembly loaded on the C lattice core.

As shown in FIG. 1, there are eight types of fuel rods 20 constituting the fuel assembly 10, that is, fuel rods 11 to 18. These fuel rods
11 to 18 are arranged in a cross section of the fuel assembly in the channel box 22 as shown in FIG.

The diameter of the water rod 21 in this embodiment is sized to occupy the area of the four fuel rods 20 as shown in FIG. 1, and is located at the center of the cross section of the fuel assembly 10.

As a result, the cost of water to fuel becomes larger than that of conventional fuel, and the deterioration of the core characteristics due to the high burnup is solved, and at the same time, the reactivity is improved.

Eight types of fuel rods 11 to 18 constituting the fuel rod 20 include a natural uranium bracket region N1 in which fuel pellets made of natural uranium are filled at the lower end and the upper end of the fuel material filling region.
N2 is formed.

As shown in FIG. 2, these natural uranium brackets N1 and N2 have an axial length (hereinafter referred to as an effective fuel length H) of the fuel material filling region from the lower part 0 and the upper end 24 of the fuel material filling region. Occupies up to 1/24 position.

The fuel material filling region means a region filled with fuel pellets, and the fuel material filling regions of the fuel rods 11 to 18 have the same axial length. In each fuel rod, the range from 1/24 to 23/24 of the active fuel length H from the lower end of the fuel material filling region is the enriched uranium region C filled with enriched uranium.

As shown in FIG. 2, in the fuel rods 11, 12, 16, 17 and 18, the enriched uranium region C has a uniform enrichment in the axial direction, and in the fuel rods 12, 14 and 15, The uranium region has three regions C1, C2 with different enrichments in the axial direction.
Consists of C3.

The enrichment in the enriched uranium region of each fuel rod is 4.
3% by weight and 3.3% by weight for fuel rods 13, 16, 17 and 18.

In the enriched uranium region, the enrichment C1 in the range of 1/24 to 10/24 of the active fuel length H is 3.3% by weight, and 10/24 of the active fuel length H in the enriched uranium region. ~ 20
/ 24 range C2 enrichment 3.8% by weight, active fuel length H 20/2
The enrichment C3 in the range from 4 to 23/24 is 3.3% by weight.

Similarly, in the fuel rod 14, C1 = 2.9, C2 = 3.3, C3 = 2.9
% For fuel rod 15, C1 = 2.1, C2 = 2.5, C3 = 2.
1% by weight.

The fuel rods 16, 17, and 18 contain gadolinia, which is a burnable poison, in the fuel pellets in the enriched uranium region.

The gadolinia concentration in the axial direction of the enriched uranium region in the fuel rod 16 is 5.0% by weight in the range of 1/24 to 10/24 of the active fuel length H and 10/10 of the active fuel length H based on the lower end of the fuel material filling region. 4.0% by weight in the range of 24-20 / 24, 20 / 24-23 / 2
In the range of 4, it is 3.0% by weight.

The gadolinia concentration of the fuel rod 17 is within a range of 1/24 to 20/24 of the active fuel length H, with the lower end of the fuel material filling region as a base point.
4.0% by weight, and 3.0% by weight in the range of 20/24 to 23/24 of the active fuel length H.

The gadolinia concentration of the fuel rod 18, which is a feature of the present invention, is 2.0% by weight in the range of 1/24 to 10/24 of the active fuel length H, and 1/2 to 20/20.
4.0% by weight in the range of 24, 3.0% by weight in the range of 20/24 to 23/24
It is.

The fuel rods 11 to 15 do not contain gadolinia.

Fuel rods 11 to 18 having the axial enrichment distribution as described above
By arranging the fuel assemblies 10 as shown in FIG.
As shown in the right end of FIG. 2, the cross-sectional average enrichment distribution of each part in the axial direction is a range C1 of 1/24 to 10/24 of the active fuel length H based on the lower end of the fuel material filling region of the fuel assembly.
(Lower region of enriched uranium region) and effective fuel length H of 20
3.C3 in the range / 24-23 / 24 (upper region of enriched uranium region).
44% by weight, and 3.64% by weight in the range C2 (control of the central part of the enriched uranium region) in the effective fuel length H of 10/24 to 20/24. Also,
The natural uranium blanket regions N1, N2 formed at the upper and lower ends of the fuel material filling region contain 0.71% by weight of uranium 235.

In the fuel assembly according to the present embodiment, the cross-sectional average enrichment of the lower region C1 of the enriched uranium region is reduced, the cross-sectional average enrichment of the central portion C2 of the enriched uranium region is increased, and burning is performed between those regions. An average concentration difference of 0.2% by weight is provided.

In the BWR, the density of water, which is a neutron moderator, decreases at the upper part of the core because there are many vapor bubbles (voids) toward the upper end of the core. For this reason, when a fuel assembly having a uniform axial enrichment distribution is loaded into the core, the output distribution tends to have a lower strain in which an output peak occurs at the lower portion of the fuel assembly.

Therefore, as described above, by increasing the enrichment in the central region than in the lower region of the fuel assembly, the axial power distribution of the fuel assembly can be flattened.

The difference in average enrichment between the central region C2 and the lower region C1 in the fuel assembly of the present embodiment, about 0.2% by weight, and the central region C
The boundary position (position of 10/24 of the active fuel length H) between 2 and the lower region C1 is selected so as to maximize the effect of flattening the axial power distribution.

The average weight% of the gadolinia of the ten gadolinia-containing fuel rods 16 and 17 of the fuel assembly 20 in the present embodiment is the second gadolinia.
As shown in the figures and FIG. 3, the lower region is generally higher and the central region is lower, with about 0.5
Weight% difference.

As described above, the density of water, which is a neutron moderator, is low in the upper part of the core and high in the lower part of the core because the voids increase toward the upper end of the core in the BWF. For this reason, if the average gadolinia weight% in the axial direction is the same, the neutron spectrum in the lower part of the core is softer than that in the upper part, so that gadolinia combustion proceeds quickly. This increases the rate of increase in the reactivity of the lower part of the core when gadolinia combustion proceeds, so that the axial power distribution tends to have a lower peak.

In the present invention, in order to appropriately control this, the average concentration of gadolinia in the lower region is increased by 0.5% by weight. This concentration difference of 0.5% by weight was selected so that the effect of flattening the axial power distribution was optimized.

In this embodiment, there are three types of gadolinia-containing fuel rods. This is for the sake of fuel production, and the difference in gadolinia concentration in the same fuel is about 1% by weight or more.

Further, in the present invention, the lower region of the fuel rod 18 containing gadolinia is
The average weight of gadolinia in C1 is 2.0% by weight and the central area C2
Before the gadolinia in the lower region burns out, the cycle power after the axial power distribution at the beginning of the cycle has suppressed to the lower peak, and after the gadolinia in the lower region has burned. The core reactivity is obtained by setting a moderate lower peak after the middle stage.

The margin of output peaking of the fuel assembly caused by the axial enrichment distribution and gadolinia concentration difference obtained by such a selection is about 15% to 20%. In the fuel assembly of the present invention, the margin of the output peaking is alleviated to the increase of the peaking by increasing the burnup and the fuel economy is improved by allocating the margin to the reactivity gain as described above.

As shown in FIG. 3, the upper region C3 of the enriched uranium region in the fuel assembly (20/24 to 23 /
The cross-sectional average enrichment of 24) is the lower area C1 (effective fuel length H).
The low enrichment area is almost equal to the average cross-sectional enrichment of 1/24 to 10/24) and corresponds to the low-concentration area of burnable poisons on the fuel rods 16 and 17, compensating for the reduction in the margin of furnace shutdown. doing.
The axial length (3/24 of the active fuel length H) of the upper region C3 of the enriched uranium region is determined so that the economic effect due to the reduction of burnable poisons is maximized.

As described above, according to the fuel assembly of the present invention, fuel economy can be significantly improved.

That is, the fuel assembly of the present invention can achieve the target high burnup with the lowest possible enrichment while ensuring a sufficient safety margin of the reactor, thereby reducing the cost required for the fuel cycle. It can be significantly reduced. Further, the amount of spent fuel assemblies generated can be significantly reduced.

[Effects of the Invention] According to the present invention, deterioration of core performance due to high burn-up can be eliminated, and the margin of output peaking can be used for improving fuel economy, and the extraction energy can be significantly increased.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a fuel assembly according to an embodiment of the present invention,
FIG. 2 is an explanatory diagram showing the enrichment and gadolinia distribution of the fuel rods constituting the fuel assembly shown in FIG. 1, and FIG. 3 is a graph showing the enrichment and the distribution of burnable poisons constituting the fuel assembly of the present invention. FIG. 10 Fuel assembly 20 (11 to 18) Fuel rod 21 Water rod 22 Channel box 30 Control rod

Claims (2)

(57) [Claims] 1. A fuel assembly having a plurality of fuel rods filled with a fuel substance therein, wherein at least a part of the plurality of fuel rods constituting the fuel assembly has different contents in the axial direction. And one or two of the fuel rods containing the burnable poison contain the burnable poison at an axial position containing enriched uranium. The unit length, the content of burnable poison per fuel rod from the top to the middle is equal or sequentially increased, and the unit length below the middle, the content of burnable poison per fuel rod It has a region where the amount is set smaller in the middle, and the remaining fuel rods among the fuel rods containing the burnable poison have a unit length, the content of the burnable poison per fuel rod, Note that it is gradually increased from top to bottom. Fuel assembly to be featured. 2. The fuel assembly according to claim 1, wherein the average content of the burnable poison in the axial direction of the entire fuel assembly is gradually increased from the upper part to the lower part.