JPH07218672A - Burnable poison aggregate - Google Patents

Abstract

(57) [Abstract] [Purpose] By reducing the diameter of the BPR to be inserted into the control rod guide pipe, the flow rate of the coolant in the control rod guide pipe is increased and the temperature rise in the guide pipe is reduced. [Structure] A BPR1 or a thimble plug 2 is also added so that the cross-sectional area of these axes becomes smaller than about 60% of the cross-sectional area of the inside of the control rod guide tube. The decrease in the neutron absorption effect due to the above is compensated by an increase in the amount of neutron absorbing elements contained in the BPR1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a burnable poison assembly which is inserted into a control rod guide tube of a PWR fuel assembly for a certain period of time and mainly suppresses excessive reactivity of fresh fuel by absorbing neutrons. Hereinafter referred to as BPRA).

[0002]

2. Description of the Related Art A fuel assembly loaded in a nuclear reactor is taken out after a predetermined energy is generated and replaced with a new fuel. The loading ratio of this new fuel is normally about 1/4 to 1/3 of the total number of loaded bodies, but since the new fuel has a higher reactivity than the fuel that is continuously used, When deciding the fuel arrangement in the reactor, it is necessary to reduce the output of the new fuel and to flatten the output. As one of the measures, there is a method of loading a new fuel and a burnable poison in combination.

A combustible poison is an element which has a very large absorption cross section for neutrons, and changes into a substance having a relatively small absorption cross section for neutrons after an absorption reaction.
It is mainly used in combination with new fuel to suppress the initial reactivity.

As a means for combining the burnable poison with the fuel assembly, the burnable poison is sealed in a tubular cladding tube to form a burnable poison rod (hereinafter referred to as BPR), and a plurality of BPRs are clustered. A method of bundling and inserting from the upper part of the fuel assembly (BPRA) and a method of premixing into fuel pellets that are oxides of fissionable material loaded in fuel rods (fuel with gadolinia) are currently commercial reactors. Has been put into practical use in.

Generally, an element having a large neutron absorption cross section, such as boron, is enclosed in the BPR as a glass-like or mixed sintered body with alumina. The concentration of the neutron absorbing substance such as the sintered body is optimized from the viewpoint of flattening the output of the fuel assembly, the viewpoint of the nuclear characteristics of the entire core, the required initial reactivity suppressing effect, and the like.

On the other hand, FIG. 3 shows the appearance of a BPRA used in combination with a 17 × 17 type PWR fuel assembly. This BPRA is composed of a plurality of long BPRs.
It has a cluster type structure in which 1'and a short thimble plug 2'for adjusting the flow rate in the control rod guide tube are bundled at the upper part.
The BPRA 8 is inserted into the fuel assembly N as shown in FIG. 4 from above so as to pass through the control rod guide tube of the fuel assembly. 5 and 6 show BPRA.
8 is a view of a state in which 8 is inserted in the fuel assembly N as seen from above and in the lateral direction.

[0007]

By the way, FIG. 7 is a horizontal cross-sectional view of the BPR inserted into the control rod guide tube, showing a BPR cladding tube 1a 'filled with a neutron absorbing material B'. A constant gap 9'is formed between the control rod guide tube S and the cooling water flows through the gap 9'when the BPRA is inserted.

That is, in the control rod guide pipe, a certain proportion of the coolant usually flows upward from the lower portion of the fuel assembly, but the pressure loss in the control rod guide pipe increases due to the insertion of the BPRA. , Its flow rate decreases. Also, BPR
The BPR constituting A itself generates heat by being irradiated with neutrons and γ rays in the fuel, and therefore, in combination with the above-described action of decreasing the flow rate, the temperature rise of the coolant in the control rod guide tube,
As a result, the control rod guide tube is heated, and the heating of the control rod guide tube promotes oxidative corrosion of the inner surface of the tube. If this oxidative corrosion progresses excessively, the integrity of the control rod guide tube may be impaired, and the mechanical strength of the fuel assembly may be lost.

The present invention addresses the above-mentioned actual situation, and the above B
By reducing the diameter of PR, the flow rate of the coolant in the control rod guide pipe is increased, and the temperature rise in the guide pipe is reduced.

[0010]

That is, the feature of the BPRA of the present invention which meets the above object is that the outer diameter of the BPR bundled together with the thimble plug is the inside of the control rod guide tube whose cross-sectional area is perpendicular to the axis. The diameter was reduced to about 60% or less of the cross-sectional area orthogonal to the axis, and the decrease in the neutron absorption effect due to this diameter reduction was compensated by the increase in the amount of neutron absorbing elements contained in the BPR. It is in. Further, in the BPRA of the present invention, it is also preferable to make the outer diameter of the thimble plug as thin as the combustible poison rod so as to keep the pressure loss balance.

[0011]

In the above-mentioned BPRA of the present invention, the temperature rise in the control rod guide tube can be reduced while maintaining the reaction suppressing effect of BPRA, thereby suppressing the oxidative corrosion of the inner surface of the control rod guide tube and reducing the fuel consumption. The mechanical integrity of the assembly can be ensured.

[0012]

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

FIG. 1 is an overall view of a BPRA according to an embodiment of the present invention,
FIG. 2 is a horizontal sectional view when the BPR of the same embodiment is inserted into the control rod guide tube.

In the BPRA of the above-mentioned embodiment, as shown in FIG. 1, some thimble plugs 2 are mixed in a plurality of BPRs 1 loaded with a neutron absorbing material B, and these are connected to the upper structure 3.
The upper structure 3 is fixed to the end plate 4 by a nut and, as shown in FIG. 1, a hold-down spring 6 is fitted and inserted into a sleeve 5 standing on the end plate 4, and A hold down bar 7 is fitted onto the upper part of the member 5, and the hold down bar 7 is elastically supported by the hold down spring 6. The neutron absorbing material B is generally formed by mixing a known element having a large neutron absorption cross section such as boron as a glass-like or alumina-sintered mixed body, and as shown in FIG. It is sealed in the coating tube 1a.

On the other hand, in the present invention, in order to secure a sufficient flow rate of the coolant in the control rod guide pipe S, the above B shown in FIG. 2 is used.
All of the above BPR1 are uniformly thinned so that the area of the cross section orthogonal to the axis of PR1 is about 60% or less of the cross sectional area of the hollow part inside the control rod guide tube S of the fuel assembly into which this BPR1 is inserted. It has become. As described above, by reducing the cross-sectional area of the BPR perpendicular to the axis from the conventional 70% to, for example, about 60%, it is expected that the flow rate in the control rod guide tube will increase by about 20% and the temperature rise can be suppressed. And these BP
The decrease in the neutron absorption effect accompanying the reduction in the diameter of R1
By increasing the amount of the neutron absorbing element such as boron contained in the neutron absorbing substance B of R1, the neutron absorbing effect as in the conventional case is compensated. In the figure, 9 indicates a gap between the BPR coating pipe 1a and the control rod guide pipe S.

Since the neutron absorbing action of the BPR1 is proportional to the surface area of the neutron absorbing substance B, the BP
In order to balance the reduction of the diameter of R and the compensation of the neutron absorbing action, the increase amount of the neutron absorbing element should be increased by the ratio of the surface area of the neutron absorbing substance B which is decreased by the reduction of the diameter. Good.

That is, Co is the concentration of the neutron absorbing element before the diameter reduction, C is the concentration of the same element after the diameter reduction, Do is the outer diameter of the neutron absorbing material B before the diameter reduction, and D is the diameter reduction. Assuming the outer diameter of the same substance B later, the above relationship is evaluated by C = Co · Do / D.

On the other hand, by reducing the diameter of BPR1, the BP
Although the pressure loss in the control rod guide tube S into which R1 is inserted is reduced, when the thimble plug 2 has the conventional outer diameter, the control rod guide into which the thimble plug 2 is inserted is reduced. The flow rate of the coolant in the pipe S is further reduced due to the reduction of the pressure loss of the other control rod guide pipe, and the coolant temperature in the pipe S becomes higher than usual.

As a result, the water density in the control rod guide tube S in which the thimble plug 2 is inserted is reduced, and the degree of deceleration of neutrons is reduced, thereby degrading the reactivity balance in the fuel assembly. In order to avoid this, the outer diameter of the thimble plug 2 is also changed so as to be approximately the same as the pressure loss due to BPR1. That is, it is desirable that the outer diameter of the thimble plug 2 is also made as thin as the outer diameter of the thinned BPR 1 as shown in FIG.

By this measure, as a whole, the flow rate of the coolant in the control rod guide tube S is increased as compared with that before the diameter reduction of the BPR 1 and the thimble plug 2, but on the other hand, it relatively flows through the fuel rod cells. Although the flow rate of the coolant decreases, the degree thereof can be suppressed to a sufficiently small level, and the present invention can be applied within the range in which the core characteristics of the fuel are not changed.

In the BPRA of the embodiment of the present invention, however, the flow rate of the coolant in each of the control rod guide tubes S in which the BPR1 and the thimble plug 2 are inserted is increased in a well-balanced manner, while the reactivity suppressing effect of the BPRA is suppressed. It is possible to secure the mechanical integrity of the fuel assembly by suppressing the inner surface of the control rod guide tube S while maintaining the above condition.

Although one embodiment of the present invention has been described above, the cluster shape of the BPRA or the number of BPRs and thimble plugs can be appropriately changed depending on the size of the fuel assembly to be used and other reasons. .

[0023]

As described above, the BPRA of the present invention
Is formed by adding BPR or thimble plug so that the cross-sectional area of these axes is about 60% or less of the cross-sectional area of the inside of the control rod guide tube. The decrease in the neutron absorption effect is compensated by the increase in the amount of neutron absorbing elements contained in the BPR, and the coolant flow rate in the control rod guide tube is increased while maintaining the reactivity suppressing effect of BPRA, It is possible to reduce the temperature rise in the control rod guide tube, thereby suppressing the oxidative corrosion of the inner surface of the control rod guide tube and ensuring the mechanical integrity of the fuel assembly.

[Brief description of drawings]

FIG. 1 is an overall view showing a BPRA of an embodiment of the present invention.

FIG. 2 is a horizontal sectional area showing a state in which the BPR of the same embodiment is inserted into a control rod guide tube.

FIG. 3 is an overall view showing a conventional BPRA.

FIG. 4 is a schematic diagram of a 17 × 17 type PWR fuel assembly.

FIG. 5 is a plan view showing a state in which BPRA is inserted into a fuel assembly.

FIG. 6 is a partial side view of the same.

FIG. 7 is a horizontal sectional view showing a state in which a conventional BPR is inserted into a control guide tube.

[Explanation of symbols]

 1 BPR 1a BPR cladding tube 2 thimble plug 3 upper structure 4 end plate 5 sleeve 6 hold down spring 7 hold down bar B neutron absorbing material S control rod guide tube

Claims (2)

[Claims] 1. A combustible poison assembly in which a plurality of combustible poison rods and thimble plugs are focused according to the arrangement of control rod guide tubes of a fuel assembly, wherein the outer diameter of the combustible poison rod is The combustible poison is formed by reducing the neutron absorption effect due to the reduction in diameter so that the cross-sectional area orthogonal to the axis is about 60% or less of the cross-sectional area orthogonal to the axis inside the control rod guide tube. A burnable poison aggregate, which is compensated by increasing the amount of neutron absorbing elements contained in the rod. 2. The burnable poison aggregate according to claim 1, wherein the outer diameter of the thimble plug is made as thin as the burnable poison rod.