JPH0868886A - Mox fuel assembly for pwr - Google Patents

Abstract

(57) [Abstract] [Purpose] In the MOX fuel assembly for PWRs, the number of types of fuel rods is reduced, and the cost of fuel forming is reduced. A MOX fuel rod 1 having a constant Pu enrichment is disposed on the outer peripheral portion of the fuel assembly excluding each corner portion, and a constant Pu richness higher than that of the outer peripheral fuel rod is provided on the central portion of the fuel assembly. A MOX fuel rod 2 having a degree of chemical conversion is provided, and a uranium fuel rod 3 containing gadolinia is provided at each of the corners.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a MOX fuel loaded in a PWR core mixed with uranium fuel, that is, a MOX fuel assembly containing fissile plutonium.

[0002]

2. Description of the Related Art When MOX fuel is loaded in a PWR core, it is not possible to use all the fuel in the core as MOX fuel from the viewpoint of nuclear characteristics. Therefore, it is mixed with normal uranium fuel mainly containing UO _2. Be loaded.

When the MOX fuel is mixed with the uranium fuel, thermal neutron inflow from the uranium fuel increases the output peaking at the outer peripheral portion of the fuel assembly as a boundary, particularly at the corner portion. Three types of Pu enrichment of the MOX fuel rods constituting the fuel assembly are manufactured to reduce the Pu enrichment of the outer peripheral portion and the corner portion.

For example, as an example of the current situation, it is as shown in FIG. 4, and a low enrichment of 0.2 wt% is applied to each of the fuel rods at the corners of the fuel assembly and the outer peripheral portion adjacent thereto.
^{2 3 5} disposed U + 3 wt% Puf fuel rods 4, the outside of the corner portion of the outer peripheral portion remaining with its one column inside and control rod guide thimbles, 0.2 wt% of Nakatomi degree ^{2 3 5} U + 4 wt%
A Puf fuel rod 1 is arranged. Then, 0.2 wt% ^{2 35} U + 7 wt% Puf fuel rod 2 with high enrichment is formed in the remaining central portion.
Is arranged.

The percentage of Puf referred to here is the percentage of Pu that is fissile only, and Pu that does not have fissile properties is also not shown in this figure, but some is mixed.

[0006]

However, in the above-mentioned conventional MOX fuel assembly, MO of the above three types of enrichment is used.
Since X fuel rods must be manufactured, there is a problem in that the cost of fuel forming processing becomes high.

The present invention copes with the above-mentioned actual situation, and by finding a novel structure in some fuel rods and their arrangement, the output peaking is suppressed to the same level as the conventional one, and the fuel assembly as a whole is suppressed. The purpose of the present invention is to reduce the number of types of the MOX fuel rods and to reduce the cost of the fuel forming process without substantially reducing the Pu loading amount.

[0008]

That is, the feature of the MOX fuel assembly of the present invention is that it contains M containing fissile Pu.
In a MOX fuel assembly for PWR formed by focusing OX fuel rods, MOX fuel rods having a constant Pu enrichment are arranged in the outer peripheral portion of the fuel assembly except for each corner portion, and the central portion of the fuel assembly is also provided. And a constant Pu higher than the outer peripheral fuel rods
MOX fuel rods having a high degree of enrichment are provided, and uranium fuel rods containing gadolinia are provided at the respective corners.

[0009]

In the above MOX fuel assembly of the present invention, the conventional 3
There are two types of enriched MOX fuel rods, and in order to suppress output peaking, a total of four fuel rods with gadolinia that absorb neutrons are used at each corner.

That is, according to the present invention, the MOX fuel rod has the above-mentioned structure while suppressing the output peaking of the fuel assembly to the same level as the conventional one, and at the same time, substantially reducing the fissile Pu loading of the entire assembly. It is possible to reduce the number of types to the above two types and reduce the cost of fuel forming processing.

[0011]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a 17 × 17 array P of the embodiment of the present invention.
FIG. 3 is a plan view showing the arrangement of fuel rods in a WR MOX fuel assembly, in which the MOX fuel rods containing fissile Pu are converged into a substantially square matrix except for the four corner portions, and the outer peripheral portion is formed. Higher enrichment M in the center than
An OX fuel rod is provided.

In the embodiment of the present invention, the MOX
In the fuel assembly, a medium enrichment degree of 0.2 wt% ² 35 U + 4 wt% Puf fuel rod 1 is arranged on the outer peripheral portion excluding the above four corner portions and on the outer side of the outer peripheral control rod guide thimble. High enrichment of 0 in the remaining center of the fuel assembly.
Disposed to ^{2 wt% 2 3 5 U +} 7wt% Puf fuel rods 2, and respectively to said each ^{corner, 2.6 wt% 2 3 5 U} + 6wt% G
A d ₂ O ₃ fuel rod 3 is provided.

A comparison of the infinite multiplication factors of the MOX fuel assembly according to the embodiment of the present invention and the conventional MOX fuel assembly shown in FIG. 4 is shown in FIG. 2 and also in FIG. A comparison of peaking is shown.

The results are also shown in Table 1 below.
The present invention and the conventional example are almost the same in each burnup, and the degree of freedom of the loading pattern is increased by reducing the local peaking at 0 GWd / t. Local peaking is suppressed to the same degree as the degree of chemical conversion.

[0016]

[Table 1]

Further, in each of the fuel assemblies of the embodiment of the present invention and the conventional example, the fissile Pu loading amount per assembly is only reduced by about 1% in the embodiment of the present invention as compared with the conventional example. is there.

That is, in the embodiment of the present invention, as described above, the output peaking of the fuel assembly is suppressed to the same level as in the conventional case, and the fissionable Pu loading of the entire assembly is hardly reduced, and the MOX fuel is produced. It is possible to reduce the types of rods to two types of general-purpose, which are the same as the conventional ones, and to significantly reduce the cost of fuel molding.

[0019]

As described above, in the MOX fuel assembly for PWR according to the present invention, MOX fuel rods having a constant Pu enrichment degree are arranged in the outer peripheral portion of the fuel assembly excluding each corner portion. A MOX fuel rod having a constant Pu enrichment higher than that of the outer peripheral fuel rods is arranged at the center of the fuel assembly, and a uranium fuel rod containing gadolinia is arranged at each of the corners. Yes, the output peaking of the fuel assembly is suppressed to the same level as that of the conventional one, and the fissionable Pu loading of the entire assembly is hardly reduced.
This has a remarkable effect of reducing the number of types of X fuel rods from the conventional 3 enrichment to 2 and reducing the cost of fuel forming processing.

[Brief description of drawings]

FIG. 1 is a 17 × 17 array MO for PWR according to an embodiment of the present invention.
FIG. 6 is a plan view showing the arrangement of fuel rods in an X fuel assembly.

FIG. 2 is a MOX fuel assembly according to an embodiment of the present invention and a conventional MO.
6 is a graph comparing infinite multiplication factors of X fuel assemblies.

FIG. 3 is a MOX fuel assembly according to an embodiment of the present invention and a conventional MO.
7 is a graph comparing local peaking of X fuel assemblies.

FIG. 4 is a plan view showing an arrangement of fuel rods in a conventional 17 × 17 array MOX fuel assembly for PWR.

[Explanation of symbols]

 1 Medium enrichment fuel rods 2 High enrichment fuel rods 3 Uranium fuel rods with gadolinia 4 Low enrichment fuel rods

Claims (1)

[Claims] 1. A MOX fuel assembly for a PWR, which is formed by focusing MOX fuel rods containing fissionable Pu, wherein MOX fuel rods having a constant Pu enrichment are arranged at the outer peripheral portion of each fuel assembly except for each corner portion. In addition, at the center of the fuel assembly, a MO having a constant Pu enrichment higher than that of the outer peripheral fuel rods is installed.
An MOX fuel assembly for a PWR, characterized in that X fuel rods are provided, and uranium fuel rods containing gadolinia are provided at the respective corners.