阅读说明：本技术 具有优异的耐压缩性和减少的表面缺陷的核燃料烧结芯块 (Sintered nuclear fuel pellet with excellent compression resistance and reduced surface defects ) 是由 崔敏英 羅年洙 林光荣 郑泰植 李昇宰 睦用均 柳钟盛 于 2018-07-30 设计创作，主要内容包括：本发明旨在提供一种具有优异的耐压缩性和减少的表面缺陷的核燃料烧结芯块,该核燃料烧结芯块为圆柱状,该芯块的上表面和下表面包括：中央具有一定的曲率的球形凹槽形成的碟形凹陷(dish)；沿碟形凹陷的边缘形成的水平环面的脊部(land)以及沿脊部的边缘以一定角度切削棱角的形态形成的倒角(chamfer)。上述倒角包括靠近脊部边缘的第一倒角和沿第一倒角的边缘另外切削形成的第二倒角。(The present invention aims to provide a nuclear fuel sintered pellet having excellent compression resistance and reduced surface defects, the nuclear fuel sintered pellet having a cylindrical shape, the upper and lower surfaces of the pellet including: a disk-shaped recess (dish) formed of a spherical recess having a certain curvature at the center; a ridge (land) of a horizontal ring surface formed along the edge of the dish-shaped depression, and a chamfer (chamfer) formed in the form of an angular cut at an angle along the edge of the ridge. The chamfer includes a first chamfer near the ridge edge and a second chamfer formed by additional cutting along the edge of the first chamfer.)

1. A nuclear fuel sintered pellet excellent in compression resistance and reduced in surface defects, characterized in that it is cylindrical and comprises on the upper and lower surfaces thereof a dished recess formed by a spherical groove having a certain curvature at the center, a ridge portion which is a horizontal ring surface formed along the edge of the dished recess, and a chamfer formed in the form of an angular cut along the edge of the ridge portion,

the chamfer includes a first chamfer adjacent the edge of the ridge and a second chamfer additionally cut along the edge of the first chamfer.

2. A nuclear fuel sintered pellet having excellent compression resistance and reduced surface defects according to claim 1, characterized in that the first chamfer forms a smaller angle with the horizontal than the second chamfer.

3. A sintered pellet of nuclear fuel having excellent compression resistance and reduced surface defects according to claim 2, characterized in that the angle formed by the first chamfer and the horizontal plane is 1 ° to 4 °.

4. A sintered pellet of nuclear fuel having excellent compression resistance and reduced surface defects according to claim 2, characterized in that said second chamfer forms an angle of 14 ° to 25 ° with the horizontal plane.

5. A sintered pellet of nuclear fuel having excellent compression resistance and reduced surface defects according to claim 1, characterized in that the width of the ridge is 0.3 to 1.5 mm.

6. A method for preparing a nuclear fuel sintered pellet having excellent compression resistance and reduced surface defects, characterized in that the method for preparing a nuclear fuel sintered pellet comprises:

in the UO₂Adding a lubricant into the powder and mixing to prepare mixed powder;

compressing and molding the mixed powder at 2500-3000 kgf to prepare a molded body;

sintering the molded body in a reducing atmosphere at 1600-1850 ℃ for 2-4 hours;

in the step of preparing the molded body by compression molding, when the molded body is formed into a cylindrical fuel sintered pellet, on the upper surface and the lower surface of the fuel pellet: forming a dish-shaped recess in the shape of a spherical groove with a certain curvature at the center, forming a horizontal annular ridge along the edge of the dish-shaped recess, and forming a chamfer in the form of cutting an edge angle at a certain angle along the edge of the ridge;

the chamfer is composed of a first chamfer near the ridge edge and a second chamfer additionally cut along the edge of the first chamfer.

7. The method for producing a nuclear fuel sintered pellet having excellent compression resistance and reduced surface defects according to claim 6, characterized in that in the step of compression molding producing a molded body, the first chamfer is produced at a smaller angle with the horizontal plane than the second chamfer.

8. A method for producing a sintered pellet of nuclear fuel having excellent compression resistance and reduced surface defects according to claim 7, characterized in that the angle formed by the first chamfer and the horizontal plane is 1 ° to 4 °.

9. A method for producing a sintered pellet of nuclear fuel having excellent compression resistance and reduced surface defects according to claim 7, characterized in that the angle formed by the second chamfer and the horizontal plane is 14 ° to 25 °.

10. A method of producing a sintered pellet of nuclear fuel having excellent compression resistance and reduced surface defects according to claim 6, wherein the width of the ridge is 0.3 to 1.5 mm.

Technical Field

The present invention relates to a sintered pellet (fuel pellet), and more particularly, to a shape of a sintered pellet having excellent compression resistance and excellent surface defect reduction properties for use in a nuclear power plant.

Background

Nuclear fuel used in nuclear power plants is generally constituted by a form of stacked sintered pellets inside a cladding, wherein the sintered pellets are mainly formed using UO₂And (3) preparing powder. Recently, in order to improve the economy of nuclear power plants, high-combustion, long-term operation is considered, and therefore, the operating environment of nuclear power plants becomes more severe and the development demand for high-performance nuclear fuels is increasing.

In particular, in the manufacturing process or moving process of the sintered pellet, surface defects are generated due to various external impacts, contact, and the like, and stress on the defective portion is excessively concentrated, thereby causing cracking of the sintered pellet, and in the prior art, problems such as damage of the PCMI, breakage of the sintered pellet due to accumulation of fission gases (fission gas) on the defective portion of the sintered pellet, and the like, are prominent, which have become important problems in increasingly severe nuclear power plant operating environments.

Accordingly, a great deal of research has been conducted to improve the performance of reducing surface defects in nuclear fuel of nuclear power plants, and particularly, many studies have been conducted on the shape of sintered pellets that can ensure the integrity of nuclear fuel in high-combustion/long-cycle operation. The sintered pellets are composed of a dished recess (dish) in the center portion, a chamfer (chamfer) at the end portion, and a ridge (land) connecting the dished recess and the chamfer. Particularly, the chamfer, which is the end of the sintered pellet, is connected to the end by a certain angle, which serves to minimize the breakage of the sintered pellet, but there are still many surface defects in the existing commercial sintered pellets, and thus, a great deal of research has been conducted on the change of the chamfer angle.

However, it is difficult to improve the performance of reducing surface defects only by changing the angle of the chamfer, and considering that the loss of the sintered pellet is difficult to compensate for the power generation efficiency with the change of the angle, studies have also been made on the change of the dishing, the ridge.

Accordingly, the present applicant has aimed to provide a sintered pellet which exhibits superior surface defect reduction performance than ever before by combining the change in the angle of the chamfer and the length of the ridge in the study of surface defect reduction performance.

[ Prior art documents ]

Korean registered patent publication No. 10-0982664 (registration date: 2010.09.10)

Disclosure of Invention

Technical problem

Accordingly, the present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a sintered pellet having improved surface defect reducing performance, by which a double chamfer is designed to be used for a core of a nuclear power plant.

Means for solving the problems

In order to achieve the above object, designed as a double chamfer, the sintered pellet of the present invention has the following shape.

During sintering of the shaped body, the chamfer is flattened by the densification of the sintered core block, and a new chamfer with a small angle in the double chamfer migrates to the center of the core block, thereby reducing the risk caused by surface defects. At this time, a ridge having an inclination of 0 ° must be present between the dished recess and the first chamfer. In the prior double chamfer related patent, since there is no ridge, but the chamfer is directly transited to the dished recess, the stress is concentrated at the chamfer starting position in the dished recess at this time, and there is no obvious effect on reducing the surface defect performance. In addition, by adding the ridge, the loss of the sintered pellets is reduced, and the efficiency of the power plant is improved.

Therefore, in the present invention, the ridge is interposed between the chamfer and the dished recess, and the performance of reducing surface defects is enhanced by the change in the angle of the chamfer and the change in the length of the ridge.

Effects of the invention

The sintered pellet of the present invention has excellent performance of reducing surface defects by controlling the chamfer angle and the ridge length, can withstand high compressive strength of the clad, improves the integrity of the sintered pellet, and can be effectively used even in a high-combustion, long-cycle operating environment.

Drawings

FIG. 1 is a top cross-sectional view of a sintered pellet of the present invention;

FIG. 2 is a schematic diagram of a compression fatigue test according to the present invention;

FIG. 3 is a cross-sectional view of a sintered pellet after a compression fatigue test in accordance with the present invention.

Detailed Description

The description of the specific structures and functionalities presented in the embodiments of the present invention are exemplary presented for purposes of explaining embodiments in accordance with the present inventive concepts, and other embodiments of the present inventive concepts may be embodied in different forms. Further, it should not be construed as limited to only the embodiments described in the present specification, but should be construed to include all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

The present invention will be described in detail below with reference to the accompanying drawings.

The sintered pellet of the present invention is composed of a dish-shaped depression, a ridge and a chamfer, the dish-shaped depression is the same as the existing sintered pellet, and the surface defect resistance is improved by changing the length of the double chamfers and the ridge.

In addition, the sintered pellet has double chamfers, thereby stabilizing the uranium loss amount of the sintered pellet and improving the surface defect resistance.

In addition, the sintered pellet has the first chamfer and the second chamfer as the double chamfers, thereby relieving the stress concentration phenomenon of the existing commercial sintered pellet from the ridge to the chamfer and improving the surface defect resistance.

In addition, in the double chamfer of the sintered pellet, the first chamfer has a more gentle slope than the second chamfer.

In addition, the angle of the first chamfer of the double chamfers of the sintered core block is 1-4 degrees, and the length is 0.3-1.5 mm.

In addition, the angle of the second chamfer of the double chamfers of the sintered core block is 12-25 degrees, and the length is 0.13-0.18 mm.

In addition, in the sintered core block, the ridge is arranged between the double chamfers and the disc-shaped depressions, so that abnormal stacking of the sintered core block is prevented, stress concentration between the first chamfers and the disc-shaped depressions is reduced, and the defect resistance is improved, wherein the length of the ridge is 0.3-1.5 mm.

The green pellet of the present invention has the same range of chamfer and ridge lengths as the green pellet because it does not change much in spite of shrinkage in length and angle in the process of becoming a sintered pellet after sintering.

In addition, if the first chamfer of the double-chamfer sintered pellet is less than 1 °, the second chamfer becomes completely flat after sintering, and the effect of improving the resistance to surface defects is not exhibited; if the first chamfer of the double-chamfered sintered pellet is greater than 4 °, it acts on an edge (edge) between the first chamfer and the second chamfer, and damage is easily caused at that portion.

In addition, if the double-chamfered sintered pellet does not have a ridge portion, a break occurs between the first chamfer and the dish-shaped recess, and therefore, the double-chamfered sintered pellet must include a ridge portion having a certain length.

The sintered pellets of the present invention were prepared by the following method.

Step 1, in UO₂Adding lubricant into the powder, mixing, pulverizing, drying, and sieving to obtain mixed powder.

Step 2 is to form a green pellet by adding the mixed powder prepared according to the preparation method of the above step 1 into a mold and under a certain pressure. At this time, the pressure is 2500 to 3000 kgf.

In addition, in the above-mentioned double-chamfered sintered pellet, when the above-mentioned pressure is less than 2500kgf, the molding density is low, and when it is more than 3000kgf, defects may be generated at the end of the molded body.

Step 3 is a step of preparing a sintered pellet having improved surface defect resistance by holding the molded body obtained by the preparation method of step 2 at a temperature of 1600 to 1850 ℃ for 2 to 4 hours in a reducing hydrogen atmosphere.

In addition, in the case of the double-chamfered sintered pellet, if the sintering time is less than 2 hours, the first chamfer is hardly flattened due to insufficient densification caused by sintering, and if the sintering time is more than 4 hours, the first chamfer is completely flattened, and the function thereof is hardly exhibited.

The present invention constituted by the above-described steps will be described in more detail below by taking each embodiment as an example.

Specific details of the sintered pellets of the present invention are listed in table 1 below.

[ Table 1]

< comparative example 1>

In comparative example 1, commercial sintered pellets were used in a nuclear power plant. The commercial sintered pellet is a single chamfer with no second chamfer.

< comparative example 2>

In comparative example 2, a double-chamfered sintered pellet without ridges was prepared and used.

< Experimental example 1> test for reducing surface defect Performance

To investigate the reduction of the surface defect performance of the sintered pellets of the invention, they were subjected to a low cycle compressive fatigue test by simulating the stacking of the sintered pellets.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. After the sintered pellets of examples 1 to 6 were prepared according to the above preparation method, the situation of stacking the sintered pellets in the clad was simulated, and the pressure was increased to 500 to 1500N with a cycle of 50 (cycle). The stacking of the sintered pellets is performed in both the normal state and the abnormal state.

After the low cycle compression fatigue test, the sintered pellets that did not break were removed, and the weight loss of the sintered pellets that did break was measured, and the results are shown in table 2.

[ Table 2]

	Before testing	After the test	Amount of change in weight
				Example 1	5.8679	5.7521	0.1158
Example 2	5.8075	5.4637	0.3438
				Example 3	5.8101	5.8101	0.0000
Example 4	5.8524	5.6533	0.1991
				Example 5	5.8309	5.6084	0.2225
Example 6	5.8428	5.7069	0.1359
				Comparative example 1	5.4564	4.9744	0.4820
Comparative example 2	5.7866	5.0537	0.7329

As shown in the above table 2, the sintered pellets of examples 1 to 6 produced by the present invention had a smaller number of broken sintered pellets and a smaller weight loss than the commercial sintered pellets shown in comparative example 1. In addition, the weight loss thereof was also smaller than that of the ridge-free double-chamfered sintered pellet shown in comparative example 2. Further, when the sintered pellet was observed for a broken cross section: the breakage of the double-chamfered sintered pellets is smaller. Thus, it can be seen that the surface defect resistance of the sintered pellets of examples 1 to 6, which simulate an in-clad sintered pellet, is improved.

< Experimental example 2> compression Strength test

Compression tests were performed to determine the maximum compressive strength applied to the sintered pellets of the invention. The compression test was performed at a rate of 1mm/min until fracture after approximately 10 seconds at 50N. It can be seen that higher maximum compression strength was exhibited in the double-chamfered sintered pellets of examples 1 to 6, indicating improved compressive strength properties, relative to the commercial sintered pellets shown in comparative example 1.

[ Table 3]

	Maximum compressive Strength (MPa)
		Example 1	105.0
Example 2	115.8
		Example 3	123.8
Example 4	123.1
		Example 5	119.7
Example 6	120.2
		Comparative example 1	83.0
Comparative example 2	76.6

The double chamfer reduces the risk of surface defects during sintering of the compact by the densification of the sintered pellet, with the new chamfer at a small angle moving towards the center of the pellet and becoming flattened. At this time, a ridge having an inclination of 0 ° must be present between the dished recess and the first chamfer. In the prior double chamfer related patent, since there is no ridge but the chamfer is directly transited to the dished recess, stress is concentrated at the chamfer starting portion in the dished recess at this time, and there is no significant effect on reducing the surface defect performance. In addition, the increase of the ridge also has the effects of reducing the loss amount of the sintered pellets and improving the efficiency of a power plant.

Therefore, in the present invention, the ridge is interposed between the chamfer and the dished recess, and the resistance to surface defects is improved by the change in the angle of the chamfer and the change in the length of the ridge.