JPH03183989A - Fuel assembly - Google Patents

Abstract

PURPOSE:To lower the pellet temp. of an MOX fuel rod and to improve the defectless property of the fuel rod by setting the spacing between the pellet of the MOX fuel rod and a cladding pipe smaller than the spacing between the pellet of a UO2 fuel rod and the cladding pipe. CONSTITUTION:The pellet 9a of the MOX fuel rod 2a in the central part of the fuel assembly is formed to the outside diameter D larger than the outside diameter C of the pellet 9b of the UO2 fuel rod 2b in the outer periopheral part of the fuel assembly. The heat transfer coefft. of the spacing between the pellet and the cladding pipe is improved by decreasing the spacing between the pellet 9a and the cladding pipe 10a, by which the surface temp. of the pellet 9a is lowered. The max. temp. of the pellet 9a is finally lowered and the amt. of the gaseous FP to be released is decreased and the fuel performance of the MOX fuel rod is improved.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a fuel assembly used in a light water reactor.

(Prior Art) A fuel assembly normally used in a light water reactor is one in which pellets of uranium dioxide are molded and sintered and loaded into a cladding tube to form fuel rods, which are arranged in a lattice pattern. Meanwhile, plans have been promoted in recent years to utilize plutonium obtained through spent fuel reprocessing in light water reactors. A common way to utilize Brittonium in a light water reactor is to use pellets made by mixing plutonium oxide with uranium dioxide, molding and sintering the mixture, and such fuel is called MOX fuel.

There are two types of MOX fuel: discrete type and island type. In the discrete type, all fuel rods are MOX
It is constructed using pellets made by mixing the fuel, plutonium oxide, with uranium dioxide, molding and sintering the mixture. On the other hand, the island type is a fuel rod loaded with the above-mentioned MOX fuel pellets (hereinafter referred to as MOX fuel),
Fuel rods loaded with ordinary uranium dioxide pellets (hereinafter referred to as
It is a mixture of UO2 fuel and UO2 fuel.

In MOX fuel rods, the neutron absorption cross section of plutonium-239 is larger than that of uranium-235, so the relative reactivity is lower than in U○2 fuel rods. In island-type MOX fuel, by arranging UO2 fuel rods near control rods where the reactivity is largely reduced, the nuclear properties of MOX fuel and UO2 fuel can be made almost the same. Furthermore, since the UO2 fuel rods on the outer periphery act as a barrier, there is also the effect of reducing the radiation dose of the MOX fuel. In this way, island type M
OX fuel is positioned as an extremely practical fuel for promoting the use of plutonium in light water reactors.

In this conventional island-type MOX fuel, MOX fuel rods and UO2 fuel rods differ only in the composition of Belem 1~, that is, whether or not plutonium oxide is mixed, but all other dimensions and shapes are the same. .

(Problems to be Solved by the Invention) It is known that the MOX fuel OX fuel pellets used in the island-type MOX fuel have a lower thermal conductivity than the UO2 pellets of the UO2 fuel rod. Therefore,
When compared with fuel rods experiencing the same linear power density, MOX
The maximum pellet temperature of the fuel rod will be higher than that of the U○2 fuel rod.

As the pellet temperature increases, the amount of gaseous fission products (%P gas) released from the pellet into the fuel rod increases, which increases the internal pressure of the fuel rod, which is unfavorable in terms of fuel rod performance.

In view of the above circumstances, an object of the present invention is to reduce the temperature of the pellets of MOX fuel rods by lowering the temperature of MOX fuel fuel O2.
MOX with the same fuel rod performance and excellent soundness
Our objective is to provide fuel assemblies.

[Structure of the invention]

(Means for Solving the Problems) The present invention provides a MOX fuel assembly that employs MOX fuel rods by reducing the gap between the MOX fuel pellet and the cladding tube.
By making the gap between the pellets of the U02 fuel rod and the cladding tube smaller, the temperature of the pellets of the MOX fuel rod is lowered.

3 (Function) By reducing the gap between the pellet and the cladding tube, the heat transfer coefficient of the gap between the pellet and the cladding tube can be improved. This improvement in the gap heat transfer coefficient allows the pellet surface temperature to be reduced, and ultimately the pellet maximum temperature to be reduced. As a result, it is possible to reduce the amount of gas released outside the FP, reduce the internal pressure of the fuel rod, and
The integrity of the fuel rods of the OX fuel assembly can be improved.

(Example) An example of the present invention will be described below with reference to FIGS. 1 to 4.

The island-type MOX fuel of this example has MOX fuel rods in the central part of the fuel assembly, and U-shaped fuel rods in the outer peripheral part of the fuel assembly.
O2 fuel rods are installed. FIG. 1 shows cross sections 1 and 4 of a fuel assembly showing an example of an Islandid-type MOX fuel, in which 2a shows a MOX fuel rod and 2b shows a UO2 fuel rod. Further, 3 is a water rod, and 10 is a control rod.

4- An example in which the present invention is applied to such an island-type MOX fuel is shown in FIG. MOX fuel rod 2a, UO2 fuel rod 2
The upper and lower ends of b are sealed with an upper end plug 5 and a lower end plug 6. The MOX fuel rods 2a, UO, and fuel rods 2b are entirely surrounded by an upper end plug 5, a lower end plug 6, and a via 1. In addition, due to the spacers 4 provided at seven locations in the axial direction,
The mutual spacing of the fuel rods 2 is maintained. The expansion spring 9 is configured to absorb the difference in thermal expansion of the fuel rods 2 in the axial direction.

Figure 3 is a cross-sectional view of UO and fuel rod 2b, Figure 4 is MOX
FIG. 2 is a cross-sectional view of a fuel rod 2a.

UO on the outer periphery of the fuel assembly, pellet 9b of the fuel rod 2b
According to the gap A between the fuel rod and the cladding tube 10b, both the pellets 9a of the MOX fuel rod in the central part of the fuel assembly and the cladding tube 10a are made of Zircaloy. Since the MOX fuel rods 2a and the UO2 fuel rods are arranged in a grid pattern, it is not preferable to change the outer diameter of the cladding tube.

Therefore, in order to change the gap as described above, Belen 1-
The outer diameter of the That is, the outer diameter of the pellets 9a of the MOX fuel fuel a in the central portion of the fuel assembly is made larger than the outer diameter C of the pellets 9b of the UO□ fuel fuel b in the outer peripheral portion of the fuel assembly.

By reducing the gap between the pellets 9a and the cladding tube 10a, the heat transfer coefficient of the pellet-to-cladding gap can be improved. This allows the surface temperature of the pellets 9a to be reduced, and ultimately the maximum temperature of the pellets 1 to 98 to be reduced. Reducing the temperature of the pellets 9a can reduce the amount of FP gas released and can improve the MOX fuel performance.

Note that reducing the gap between the pellets and the cladding tube is due to the mechanical interaction between the pellets and the cladding tube during operation.
11. et Cladding Interaction
n: PCI) amount. PC
As the amount of I increases, the stress generated in the cladding increases,
This may lead to damage to the cladding. However, for the following two reasons, in island-type MOX fuel assemblies, MOX fuel rods have a higher P than UO and fuel rods.
CI characteristics are relaxed in a comfortable direction.

(t) MOX fuel pellets have a larger creep amount than UO2 fuel pellets. Since pellet creep has the effect of mitigating PCI, MOX fuel rods are considered to have relatively better PCI characteristics than UO and fuel rods.

■ As shown in Figure 1, there is a U on the outermost periphery on the control rod side.
O2 fuel rods are located. In this position, when output fluctuation occurs due to control rod operation, the range of output fluctuation is generally the largest. In terms of operating conditions, the MOX fuel rod located in the center of the fuel assembly has a lower P than the UO2 fuel rod.
The upper side of the CI is stacked against the upper side.

In addition to the above, a power surge test conducted in a test reactor confirmed that MOX fuel rods have better PCI resistance than UO2 fuel rods and can maintain fuel integrity up to higher power. Furthermore, in a similar power surge test for 7-UO2 fuel rods, even when the gap between the pellets and the cladding tube was reduced to about 1/2, there was no difference in the power range in which the integrity of the fuel could be maintained.

From the above, even if the gap between the pellets and the cladding tube of the MOX fuel rod is reduced in the UO2 fuel rod as in the present invention, the gap between the pellets and the cladding tube of the MOX fuel rod is reduced to 1/2 that of the UO2 fuel rod. Even if you do so, there will be no PCI problems.

According to this embodiment, the difference in the CI characteristics of the MOX fuel UO2 fuel rods is taken into consideration, and by lowering the MOX fuel pellet temperature without causing PCI problems, the island type MOX fuel assembly can be improved. Fuel performance can be improved.

Normally, during use of a fuel rod, the pressure of the coolant outside the fuel frame is higher than the pressure inside the fuel rod, so under irradiation, the diameter of the fuel cladding tube decreases, or so-called creep down occurs. According to recent irradiation data of MOX fuel rods, the amount of creep down of MOX fuel rods is larger than that of UO2 fuel rods. In the present invention, the gap between the pellets of the MOX fuel rod and the cladding tube is kept small, so that when the creepdown progresses, the belenium 1- and the cladding tube come into contact and the pellets prevent the creepdown of the cladding tube. , and the amount of creep down is also UO,
It can be made equivalent to a fuel rod.

In this example, the outer diameter of the MOX fuel rod pellets is made larger than the conventional one without changing the outer diameter of the cladding tube, thereby making the gap between the MOX fuel rods smaller than the gap between the UO2 fuel rod pellets and the cladding tube. are doing. Therefore, since the fuel of this embodiment can be loaded with a larger weight of fuel pellets than conventional fuels, the fuel cycle cost can be reduced and an island-type MOX fuel assembly with excellent economic efficiency can be provided.

Note: The gap between the MOX fuel rod pellet and cladding tube is
As another means of making the pellets and cladding smaller than those of two fuel rods, the outer diameter of the fuel rods may be changed or the wall thickness of the cladding may be changed.

Furthermore, the arrangement of the MOX fuel rods within the fuel assembly is not limited to this embodiment.

〔Effect of the invention〕

According to the present invention, since the gap between the MOX fuel pellet 1- and the cladding tube is made smaller than the gap between the UO2O2 fuel pellet and the cladding tube, the temperature of the MOX fuel pellet can be lowered, and the MOX fuel assembly can improve the integrity of fuel rods.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of an island-type MOX fuel assembly according to one embodiment, FIG. 2 is a diagram showing the overall configuration of a fuel assembly according to one embodiment, and FIG. 3 is a cross-sectional view of a UO2O2 fuel assembly according to one embodiment. , FIG. 4 is a cross-sectional view of an example MOX fuel rod. 2...Fuel rod, 2a...MOX fuel fuel 2b...UO2 fuel rod, 5...Soil end plug, 6
...lower end plug, 7..." two-part tie plate, 8...lower tie plate, A...UO2 fuel rod pellet-standing tube gap B.
・・MOX fuel rod pellet 1--cladding gap Fig. 11- N 2b Fig. Fig.

Claims (1)

[Claims] (1) A first fuel rod is made by stacking a plurality of sintered uranium dioxide pellets in a cladding tube, and sealing both ends with an upper end plug and a lower end plug, and a mixture of plutonium oxide and uranium dioxide and sintering. In a fuel assembly formed by arranging a plurality of second fuel rods in which a plurality of bonded pellets are stacked in a lattice shape, and holding the lower ends of the second fuel rods by an upper tie plate and a lower tie plate, the second fuel rod is A fuel assembly characterized in that the gap between the pellets of the rod and the cladding tube is smaller than the gap between the pellets of the first fuel rod and the cladding tube.