JPH0682595A - Production of nuclear fuel element - Google Patents

Abstract

PURPOSE:To protect the outer face of pipe against corrosion by placing a nuclear fuel element in a high temperature, high pressure steam for a predetermined time to form an oxide film on the outer surface of a metal pipe while shrinking the cover pipe thereby suppressing emission of gaseous fission product (FP gas) and the phenomenon (PCl) for enlarging the cover pipe. CONSTITUTION:Nuclear fuel pellets 2 are contained in a cover metal pipe 1 to assemble a nuclear fuel element which is then placed in steam of a specific high temperature and high pressure for a predetermined time to subject the outer face of the cover pipe 1 to autoclave-treating and to form an oxide film thereon and then the cover pipe 1 is shrinked. When the pellets are contained in the cover pipe 1, the cover pipe 1 is held at first at a high temperature to be thermally expanded and then the pellets 2 are inserted while being kept at a low temperature to be thermally shrinked thus producing a narrow gap nuclear fuel element. This constitution suppresses emission of FP gas, PC1, and corrosion on the outer surface of the cover pipe 1 thus producing an excellent nuclear fuel element, i.e., a fuel having high degree of burn-up.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for manufacturing a nuclear fuel element.

[0002]

2. Description of the Related Art Most of the power generation reactors currently in practical use have a nuclear fuel pellet containing uranium oxide, plutonium oxide, thorium oxide, or a mixture thereof as a main component, sealed in a metal cladding tube. Structural nuclear fuel elements are used.

FIG. 2 shows a schematic vertical sectional view of an example of a nuclear fuel element. As shown in FIG. 2, the nuclear fuel element is a nuclear fuel obtained by sintering a plurality of uranium oxides, plutonium oxides, thorium oxides or a mixture thereof in a metallic cladding tube 1 made of zirconium or a zirconium alloy. The pellets 2 are stacked and housed, and both ends of the metal covered tube 1 are sealed with an upper end plug 3a and a lower end plug 3b, respectively. A plenum 4 serving as a gas reservoir is provided on the upper portion of the metallic cladding tube 1, and a spring 5 is attached to the plenum 4.

When the above-mentioned nuclear fuel pellets 2 are burned in a nuclear reactor, fission products are produced and accumulated in the nuclear fuel pellets 2. Of these, some of the gaseous fission products (FP gas) such as krypton and xenon are released to the outside of the nuclear fuel pellet 2 and cause an increase in the fuel rod internal pressure or an increase in the nuclear fuel pellet center temperature. Further, due to the difference in thermal expansion between the metallic cladding 1 and the nuclear fuel pellets 2, the nuclear fuel pellets 2 spread the metallic cladding 1 (P
CI) occurs.

On the other hand, in a water-cooled reactor, a metallic cladding tube 1
A zirconium alloy is used as a material for the above, but it is known that the zirconium alloy reacts with cooling water during the period of use to cause local corrosion called nodular corrosion.

[0006]

Problems to be Solved by the Invention FP gas release and PC
To reduce I, it is effective to lower the temperature of the nuclear fuel pellets. In the conventional example, a method of increasing the thermal conductivity of the nuclear fuel pellet by adding a trace element to uranium dioxide is adopted. However, in this case, there is concern that the inventory of uranium dioxide may be reduced and the performance of nuclear fuel pellets other than thermal conductivity may be reduced.

Further, it is possible to reduce the temperature of the nuclear fuel pellets by making the gap between the metal cladding tube and the nuclear fuel pellets small, but the narrowing of the gap entails a decrease in workability during fuel rod assembly. Become.

It is an object of the present invention to provide a method for producing a nuclear fuel element which has a small amount of FP gas emission and PCI and a small amount of outer surface corrosion of a metal cladding tube and which is excellent as a high burnup fuel.

[0009]

The above object can be achieved as follows.

(1) A nuclear fuel pellet containing uranium oxide, plutonium oxide, thorium oxide, or a mixture thereof as a main component is housed in a metal cladding tube, and the inner surface of the metal cladding tube and the outer surface of the nuclear fuel pellet. In a method of manufacturing a nuclear fuel element with a manufacturing gap of 100 μm or less, the nuclear fuel element is stored in a metal cladding tube and assembled into the nuclear fuel element, and then the nuclear fuel element is held in steam at high temperature and high pressure for a required time. , Forming an oxide film on the outer surface of the metal cladding tube and shrinking the outer diameter of the metal cladding tube.

(2) A nuclear fuel pellet containing uranium oxide, plutonium oxide, thorium oxide, or a mixture thereof as a main component is housed in a metal cladding tube, and the inner surface of the metal cladding tube and the outer surface of the nuclear fuel pellets. In the method of manufacturing a nuclear fuel element in which the manufacturing gap is 100 μm or less, the nuclear fuel element is stored in a metal cladding tube and assembled into the nuclear fuel element, and then the nuclear fuel element is heated to a temperature of 300 to 500
1-2 in steam at 1 to 110 atm (gauge pressure)
Hold for 4 hours to form an oxide film on the outer surface of the metal-coated tube and shrink the outer surface diameter of the metal-coated tube.

(3) In (1) or (2), when the nuclear fuel pellets are stored in the metallic cladding tube, the nuclear fuel pellets are inserted in a state in which the metallic cladding tube is kept at a high temperature to cause thermal expansion. thing.

(4) In (3), the temperature of the metal cladding tube at the time of inserting the nuclear fuel pellets is 50 to 300 ° C.

(5) In (1) or (2), when the nuclear fuel pellets are stored in the metal cladding tube, the nuclear fuel pellets are inserted in a state where the nuclear fuel pellets are kept at a low temperature to cause thermal contraction. .

(6) In (5), the nuclear fuel pellet temperature at the time of inserting the nuclear fuel pellets is −0 to −200 ° C.

[0016]

The present inventor analyzed the relationship between the manufacturing gap between the metallic cladding tube and the nuclear fuel pellets, the core temperature of the nuclear fuel pellets during use in the reactor, and the FP gas release. FIG. 3 is an explanatory diagram of the analysis result, and shows the temperature distribution in the radial direction of the nuclear fuel pellet in the conventional case and the present invention in which the manufacturing gap is different.

In the conventional example, the above-described manufacturing gap was about 200 μm, but if this gap is reduced to about 100 μm, the core temperature of the nuclear fuel pellets decreases by about 50 ° C., and if it is further reduced to about 50 μm, about 100 ° C. decreases. To do. Further, when the core temperature of the nuclear fuel pellets is lowered by about 100 ° C., the FP gas release rate is reduced to about ½ compared with the conventional case.

Although there is a concern that the PCI will become more severe when the manufacturing gap is reduced, the temperature of the nuclear fuel pellets decreases, and the thermal expansion amount of the nuclear fuel pellets decreases, so that the PCI can be relaxed. Is.

When the manufacturing gap is reduced,
There is a concern that the workability of storing the nuclear fuel pellets in the metal cladding tube may deteriorate. However, in the present invention, after the nuclear fuel pellets are inserted into the metal cladding tube having a sufficiently large inner diameter to assemble the nuclear fuel element, the diameter of the metal cladding tube is contracted by autoclaving in high temperature and high pressure steam. Since the narrow gap nuclear fuel element is manufactured, the workability is prevented from deteriorating and the narrow gap nuclear fuel element is easily manufactured.

Further, since a uniform oxide film having a thickness of several μm is formed on the outer surface of the metal-coated tube by the autoclave treatment, the protection against corrosion is enhanced.

[0021]

EXAMPLES Examples of the present invention will be described below.

In the first embodiment of the present invention, as described in the section of the operation, after the nuclear fuel pellets are inserted into the metallic cladding tube having a sufficiently large inner diameter to assemble the nuclear fuel element, steam at high temperature and high pressure is used. And autoclaved in to shrink the diameter of the metal cladding to produce a narrow gap nuclear fuel element.

The conventional autoclave treatment of the metallic cladding tube is carried out before the nuclear fuel pellet insertion-nuclear fuel element assembling step, but in this embodiment, it is carried out after the assembly as described above. Further, the autoclave conditions of the conventional example were a temperature of about 400 ° C. and a pressure of several atmospheres, but the autoclave conditions of this example were a temperature of 300 to 500 ° C., a pressure of 1 to 110 atmospheres (gauge pressure), and a treatment time of 1 to 24. It's time.

Under the autoclave conditions, the diameter of the metal cladding tube contracts under the external pressure of high-temperature high-pressure steam, and the contraction stops in the state of being in contact with the nuclear fuel pellets.
Further, it was found from the experiments by the inventors that the autoclave treatment forms an oxide film (thickness of several μm) having a uniform thickness and high protection against corrosion on the outer surface of the metal cladding tube.

FIG. 1 shows a comparison of corrosion resistance between the autoclave-treated metal-clad tube and the conventional metal-clad tube. FIG. 1 is a comparison of the amount of corrosion between the autoclave-coated tube of this example and the conventional tube under various conditions of a temperature of 400 ° C., a pressure of 105 atmospheres (gauge pressure), and a manufacturing time of 24 hours in high-temperature high-pressure steam. Although it is a figure, it can be seen from this figure that the amount of corrosion is significantly reduced by the autoclave treatment of this example.

In order to shrink the diameter of the metal coated tube, it is desirable to set the autoclave temperature to 400 ° C. or higher. However, if the temperature is 500 ° C. or higher, the outer oxide film becomes too thick, and the corrosion resistance deteriorates. Higher pressure is desirable, but considering safety during autoclave work,
110 atmospheres (gauge pressure) with a proven record in corrosion resistance evaluation tests
Below.

According to the present embodiment, a desired narrow gap nuclear fuel element can be manufactured, and a nuclear fuel element excellent in external corrosiveness can be provided.

In the second embodiment of the present invention, the workability was secured by inserting the nuclear fuel pellets while keeping the temperature of the metal cladding tube at 50 to 300 ° C. As an example, by setting the temperature of the metal cladding tube to about 300 ° C., the inner surface diameter of the metal cladding tube is increased by about 30 μm, which facilitates insertion of nuclear fuel pellets. When the temperature of the metal cladding tube exceeds about 300 ° C, the surface of the metal cladding tube may be oxidized, so
The holding temperature is 300 ° C or lower.

In the third embodiment of the present invention, the workability is ensured by inserting the nuclear fuel pellets while maintaining the temperature of the nuclear fuel pellets at −0 ° C. to about −200 ° C. (approximately the liquid nitrogen temperature). ing. The temperature of nuclear fuel pellets is about 200
℃ lower, the diameter of nuclear fuel pellets is about 20μm
Is decreasing.

In the fourth embodiment of the present invention, the temperature of the metallic cladding tube is set to 50 to 300 ° C. during the nuclear fuel pellet storage operation.
In addition, the temperature of the nuclear fuel pellets is maintained at −0 to −200 ° C. to facilitate the insertion of the nuclear fuel pellets.

[0031]

According to the present invention, FP gas release and PCI
It is also possible to manufacture a nuclear fuel element excellent as a high burnup fuel because it has less fuel consumption and less corrosion on the outer surface of the cladding tube.

[Brief description of drawings]

FIG. 1 is a comparison diagram of the amount of corrosion between an autoclaved cladding tube and a conventional cladding tube according to a first embodiment of the present invention.

FIG. 2 is a schematic vertical sectional view of a nuclear fuel element.

FIG. 3 is an explanatory view of a radial temperature distribution in each nuclear fuel pellet of the present invention and a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Metal cladding tube, 2 ... Nuclear fuel pellet, 3a ... Upper end plug, 3b ... Lower end plug, 4 ... Plenum, 5 ... Spring.

Claims (6)

[Claims] 1. A nuclear fuel pellet containing uranium oxide, plutonium oxide, thorium oxide, or a mixture thereof as a main component is housed in a metallic cladding tube, and the inner surface of the metallic cladding tube and the nuclear fuel pellets are In a method of manufacturing a nuclear fuel element having a manufacturing gap with an outer surface of 100 μm or less, the nuclear fuel element is housed in the metal cladding tube and assembled to the nuclear fuel element, and then the nuclear fuel element is heated in high-temperature, high-pressure steam. A method for producing a nuclear fuel element, which is maintained for a required time to form an oxide film on the outer surface of the metallic cladding tube and shrink the outer surface diameter of the metallic cladding tube. 2. A nuclear fuel pellet containing uranium oxide, plutonium oxide, thorium oxide, or a mixture thereof as a main component is housed in a metallic cladding tube, and the inner surface of the metallic cladding tube and the nuclear fuel pellets are In a method of manufacturing a nuclear fuel element having a manufacturing gap with an outer surface of 100 μm or less, the nuclear fuel element is housed in the metal cladding tube and assembled into the nuclear fuel element, and then the nuclear fuel element is heated to a temperature of 300 to 500 ° C. Holding in water vapor of ~ 110 atm (gauge pressure) for 1 to 24 hours to form an oxide film on the outer surface of the metal-coated tube and shrink the outer surface diameter of the metal-coated tube. Method for manufacturing a nuclear fuel element. 3. The nuclear fuel pellets are inserted into the metallic cladding tube when the nuclear fuel pellets are stored in the metallic cladding tube in a state in which the metallic cladding tube is kept at a high temperature to cause thermal expansion. Of manufacturing nuclear fuel elements of. 4. The method for producing a nuclear fuel element according to claim 3, wherein the temperature of the metallic cladding tube at the time of inserting the nuclear fuel pellets is 50 to 300 ° C. 5. The nuclear fuel pellet according to claim 1 or 2, wherein, when the nuclear fuel pellet is stored in the metallic cladding tube, the nuclear fuel pellet is held at a low temperature to cause thermal contraction. Manufacturing method of nuclear fuel element. 6. The method for producing a nuclear fuel element according to claim 5, wherein the nuclear fuel pellet temperature at the time of inserting the nuclear fuel pellets is −0 to −200 ° C.