JPH05273369A - Fuel rod and fuel assembly - Google Patents

Abstract

PURPOSE:To improve radial peaking of a fuel assembly body and obtain a fuel rod and a fuel assembly capable of reducing axial peaking over whole combustion cycle. CONSTITUTION:Pellets (gadolinium pellet 1, 2) containing combustible poison and the other normal pellets 3 containing no combustion poison are charged in a fuel rod coated tube, and combustible poison containing pellets are arranged to disperse in the axial direction. The distribution density of the combustible poison containing pellets and combustible poison concentration and different at axial positions of the fuel rod, in the end thereof (for instance, 15% from the upper and lower ends of effective length) 3% concentration gadolinium pellets are arranged at a rate of 1 to 9 pellets and in the center thereof (for instance, 40% from the center of the effective length) 8% concentration gadolinium pellets are set at a rate of 1 to 5.7 pellets.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel rod and a fuel assembly containing a burnable poison such as gadolinia (Gd ₂ O ₃ ) as a neutron absorber, and in particular, it significantly improves axial peaking of a nuclear reactor. It is a thing.

[0002]

2. Description of the Related Art FIG. 4 shows a conventional pressurized water reactor (hereinafter referred to as P
It is an explanatory view showing an arrangement of fuel rods of a fuel assembly (denoted as WR), and shows a 17 × 17 type fuel assembly. As shown in the figure, the PWR uses gadolinia-containing fuel (hereinafter referred to as gadolinium fuel) in which gadolinia which is a burnable poison is mixed with fuel. In the gadget fuel assembly, a few to a dozen or more gadget fuel rods 104 are dispersed and inserted in a normal fuel rod 105. In addition, an instrumentation guide tube 107 is arranged in the center of the fuel assembly, and control rod guide tubes 106 into which control rods are inserted are arranged in a dispersed manner. It should be noted that the Gadri fuel rods 104 used in the conventional PWR fuel assembly are
Is a mixture of gadolinia almost uniformly in the axial direction, and the neutron absorption effect by gadolinia has not been changed in the axial direction of the fuel rod.

[0003]

However, the conventional PWR fuel assembly as described above has the following problems. First, the output of a burnable poison-bearing fuel rod changes significantly with the burning of burnable poison, and the relative output of other fuel rods also changes significantly. There is a problem that the output peaking (local peaking) in the inner diameter direction of the assembly becomes large as compared with the fuel assembly that is configured. In order to solve this problem, the applicant of the present application filed a previous patent application (filed on January 22, 1992) by loading combustible poison-containing pellets in the axial direction of the fuel rods in a dispersed manner, and It is proposed that some fuel rods be burnable poison-bearing fuel rods. With such a configuration, it is possible to reduce local peaking of the fuel assembly.

However, in the PWR fuel assembly, there is a problem that the output distribution in the axial direction is not uniform in addition to the deviation of the output in the radial direction of the assembly. That is, (a) of FIG.
As shown in (c), early combustion cycle (BOC)
In the center part, axial peaking occurs, and in the end of cycle (EOC), conversely, axial peaking occurs near the end part. However, in the past, the only means of suppressing the axial output peaking of the PWR fuel assembly was so-called constant axial offset (AO) control operation. This A. O constant control operation is (Φ ^U −Φ ^L ) /
The value of (Φ ^U + Φ ^L ) (where Φ ^U is the upper thermal neutron flux, Φ
^L is to operate while simply monitoring the in-core neutron flux monitor so that the lower thermal neutron flux) is within ± 5% of the target value, and this method cannot suppress the peak generation itself. It was

The present invention has been made in view of the above points, and it is possible to improve the peaking in the inner diameter direction of the fuel assembly and to reduce the axial peaking over the entire combustion cycle of the fuel rod and the fuel assembly. The purpose is to obtain.

[0006]

In order to achieve the above object, a fuel rod according to a first aspect of the present invention is a fuel rod in which a plurality of pellets formed by fissile material are loaded in a fuel cladding tube. A part of the pellets forming the fuel rod is a burnable poison-containing pellet containing a burnable poison, and the distribution density of the burnable poison-containing pellet and the burnable poison concentration in the pellet are determined by the axial position of the fuel rod. It is different. The fuel rod of the invention of claim 2 has a distribution density of the burnable poison-containing pellets that is dense in the central portion of the fuel rod, roughened at both end portions, and of the burnable poison-containing pellets arranged at the end portions. The concentration of the burnable poison is lower than that of the pellet containing the burnable poison in the central portion.

A fuel assembly according to a third aspect of the present invention is a fuel assembly in which a plurality of fuel rods formed by loading a plurality of pellets formed of fissile material into a fuel cladding tube are bundled into a bundle. In order to achieve the object of (1), at least a part of the fuel rods constituting the fuel assembly is a burnable poison-containing fuel rod containing a burnable poison-containing pellet in a part in the axial direction, and the burnable poison-containing fuel rod is included. The distribution density of the burnable poison-containing pellet of the fuel rod and the burnable poison concentration in the pellet are varied depending on the axial position of the fuel rod.

[0008]

[Function] Generally, even with gadolinium fuel rods, those having an equivalent reactivity suppressing effect (those having a low gadolinia concentration to increase the number of gadolinia fuel rods and those having a high gadolinia concentration to decrease the number) As shown in FIG. 2, the reactivity combustion characteristics differ depending on the gadolinia concentration. That is,
The low concentration A type has a smaller self-shielding effect than the high concentration B type, and therefore burns immediately. In the present invention, by utilizing the difference in the reactivity combustion characteristics due to the difference in the concentration of the burnable poison, the type A and the type B shown in FIG. 2 are selectively used depending on the axial position of the fuel rod to suppress axial peaking. Is. Specifically, increase the number of burnable poison-containing pellets at the center of the fuel rod, decrease the number of burnable poison-containing pellets at the end, and further reduce the gadolinia concentration of the end pellets lower than the center pellets. To do.

As described above, by increasing the concentration of the pellet containing burnable poison in the central portion of the fuel rod and increasing the number of pellets to improve the neutron absorption effect, the peaking of the central portion at the beginning of the combustion cycle is shown in FIG. ) Is reduced. Further, as described above, by suppressing the output in the central portion in the first half of the cycle, the output in the central portion is increased in comparison with the end portion in the latter half of the cycle, whereby the output distribution is as shown in FIG. 5 (d). Is flattened to. Furthermore,
By lowering the concentration of combustible poisons in the end pellets, the combustion of combustible poisons at the end, which is relatively difficult to burn, is promoted, and unnecessary combustible poisons remain unnecessarily in the latter half of the combustion cycle. It is possible to prevent the neutron economy from deteriorating by absorbing.

Here, the output distribution in the normal operation state is expressed by the equation 1
The heat flux is limited by the heat flow channel coefficient (F _Q ). F _Q = MAX (F _XY ^N × F _Z ) × _FU ^N × F _Q ^E Formula 1 However, F _XY ^N : radial output peaking at the Z direction position (axial direction), F _Z : axial output peaking , F _U ^N : Nuclear uncertainty factor (= 1.05) F _Q ^E : Engineering heat flux heat channel coefficient (= 1.03)

In Formula 1, since F _U ^N and F _Q ^E are constant values, it is necessary to reduce F _XY ^N and F _Z in order to reduce F _Q. In the present invention, as described above, the axial output peaking F _Z can be significantly reduced. or,
The burnable poison is not added to the entire fuel rod, but the pellets containing the burnable poison are dispersed in the axial direction and added to one fuel rod while maintaining the necessary neutron absorption effect. You can increase the number of burnable poison-containing fuel rods if necessary by reducing the amount of burnable poison.
This also significantly reduces radial output peaking F _XY ^N. That is, according to the present invention, the heat flux heat channel coefficient F _Q can be significantly reduced by reducing F _XY ^N and F _Z. Further, by reducing F _Q by improving F _Z , it becomes possible to relax the limit value of F _XY ^N , and the degree of freedom of the fuel loading pattern increases. As a result, it is possible to realize a fuel loading pattern that emphasizes economy.

[0012]

Embodiments of the present invention will be described with reference to FIGS. 1 (a) and 1 (b) are schematic vertical cross-sectional views of an end portion and a central portion of a gadget fuel rod 4 used in this embodiment, and FIG. 3 is a 17 × 17 fuel assembly according to this embodiment. FIG. 3 is an explanatory view showing the arrangement of the fuel rods of FIG. In this embodiment, 264
Of the fuel rods, 128 are Gadori fuel rods 4 and 136 are ordinary fuel rods 5 without gadolinia, and the Gadri fuel rods 4 are arranged dispersedly in the cross section of the fuel assembly.
Further, the instrumentation guide tube 7 is provided at the center of the assembly, and the 24 control rod guide tubes 6 are arranged in a dispersed manner, which is the same as in the prior art.

Next, the arrangement of the pellets in the gadget fuel rod 4 in this embodiment will be described. First, at the fuel rod ends (15% each from the upper and lower ends of the effective length of the core),
As shown in (a), gadolinia pellets 1 having a gadolinia concentration of 3.0 w / o were arranged at a ratio of one to nine pellets (in the figure, 3 is a normal pellet not containing gadolinia).

Further, in the central portion of the fuel rod (central portion of the effective core length 40%), gadolinium pellets 2 having a gadolinia concentration of 8.0 w / o were arranged at a ratio of 1 to 5.7.
In addition, in the middle portion between the end portion and the central portion, gadolin pellets were arranged at a ratio of one to 6.5 pellets having a gadolinia concentration of 6.0 w / o.

The uranium enrichment in the fuel assembly of this example is 4.10 w / o for normal pellets and 2.60 w / o for gadolin pellets, and the average of assembly is 4.0.
2 w / o.

For comparison, the same 17 × 17 fuel assembly (average enrichment 4.02 w / o) in which gadolin pellets were uniformly arranged in the axial direction was prepared. I checked _Z. As a result, the results shown in Table 1 were obtained.

[0017]

As shown in Table 1, the fuel assembly of this example in which the number and concentration of gadolin pellets were changed depending on the axial position was compared with the fuel assembly of the comparative example in which the gadolin pellets were uniformly arranged in the axial direction. Axial peaking is reduced by about 10% both in the early and late stages of the combustion cycle. Further, the axial peaking of the conventional fuel assembly using the gadolinium fuel rods to which gadolinia is added throughout the axial direction is equivalent to the above-mentioned comparative example, and according to the present invention, the axial peaking is significantly increased as compared with the conventional case. Can be lowered to.

[0019]

As described above, according to the present invention, since the distribution density and the concentration of the burnable poison in the pellet containing the burnable poison are changed depending on the axial position of the fuel rod, the axial peaking is performed over the entire period of the combustion cycle. Can be drastically reduced. In addition, the improvement of the axial peaking makes it possible to relax the limit value of the radial peaking. This increases the flexibility of the fuel loading pattern,
It is also possible to design with an emphasis on economy.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view of a gadget fuel according to an embodiment of the present invention, in which (a) shows a fuel rod end portion and (b) shows a central portion configuration.

FIG. 2 is a graph for explaining a difference in combustion characteristics depending on gadolinia concentration.

FIG. 3 is an explanatory view showing an arrangement of fuel rods of a fuel assembly according to an embodiment of the present invention.

FIG. 4 is an explanatory view showing an arrangement of fuel rods of a conventional PWR fuel assembly.

5A to 5D are explanatory views showing a state in which axial peaking is suppressed by the present invention.

[Explanation of symbols]

1 ... Gadolinia-containing pellets (gadolinia concentration 3w /
o), 2 ... Gadolinia-containing pellets (gadolinia concentration 8
w / o), 3 ... normal fuel pellets (without gadolinia),
4 ... Fuel rod containing gadolinia, 5 ... Normal fuel rod (without gadolinia), 6 ... Control rod guide tube, 7 ... Instrumentation guide tube.

Claims (3)

[Claims] 1. A fuel rod in which a plurality of pellets molded of a fissile material are loaded in a fuel cladding tube, and a part of the pellets constituting the fuel rod contains a burnable poison. A fuel rod, which is a pellet, wherein the distribution density of the burnable poison-containing pellet and the burnable poison concentration in the pellet differ depending on the axial position of the fuel rod. 2. The distribution density of the burnable poison-containing pellets is dense at the center of the fuel rod and coarse at both ends, and the burnable poison-containing concentration of the burnable poison-containing pellets arranged at the ends. Is less than the concentration of the burnable poison pellets located in the center. 3. A fuel assembly in which a plurality of fuel rods formed by loading a plurality of pellets of a fissionable material into a fuel cladding tube are bundled into a bundle, and the fuel rods constituting the fuel assembly are A burnable poison-containing fuel rod at least a part of which contains a burnable poison-containing pellet, wherein the burnable poison-containing pellet distribution density and the burnable poison-containing pellet concentration of the burnable poison-containing fuel rod are A fuel assembly characterized by being different depending on an axial position.