NUCLEAR FUEL SINTERED PELLET HAVING EXCELLENT IMPACT RESISTANCE

Abstract

Proposed is a nuclear fuel pellet manufactured with UO.sub.2 powder and being in a cylindrical shape, the nuclear fuel pellet including: a dish (10) provided in a shape of a spherical groove having a predetermined curvature and a diameter of 4.8 to 5.2 mm at a center of each of top and bottom surfaces of the nuclear fuel pellet; a shoulder (20) provided in an annular plane along a rim of the dish (10); a first chamfer (310) provided along a rim of the shoulder (20) while being adjacent to the shoulder (20); and a second chamfer (320) provided along a rim of the first chamfer (310), wherein a width (SW) of the shoulder (20) is 0.4565 mm to 0.6565 mm, an angle between the first chamfer (310) and a horizontal plane is 2.0°, and an angle between the second chamfer (320) and the horizontal plane is 18.0°.

Claims

1. A nuclear fuel pellet manufactured with UO.sub.2 powder and having a cylindrical shape having a height of 9 mm to 13 mm and a horizontal sectional diameter of 8 mm to 8.5 mm, the nuclear fuel pellet comprising: a dish (10) provided in a shape of a spherical groove having a predetermined curvature and a diameter of 4.8 to 5.2 mm at a center of each of a top surface and a bottom surface of the nuclear fuel pellet; a shoulder (20) provided in an annular plane along a rim of the dish (10); a first chamfer (310) provided along a rim of the shoulder (20) while being adjacent to the shoulder (20); and a second chamfer (320) provided along a rim of the first chamfer (310), wherein a width (SW) of the shoulder (20) is 0.4565 mm to 0.6565 mm, an angle between the first chamfer (310) and a horizontal plane is 2.0°, and an angle between the second chamfer (320) and the horizontal plane is 18.0°.

2. The nuclear fuel pellet of claim 1, wherein the nuclear fuel pellet is manufactured using a powder in which at least one of PuO.sub.2 powder, Gd.sub.2O.sub.3 powder, and ThO.sub.2 powder is mixed with UO.sub.2 powder.

3. The nuclear pellet of claim 2, wherein the nuclear fuel pellet is a molded object comprising UO.sub.2 powder mixed with a pore former and a lubricant and being sintered.

4. The nuclear pellet of claim 1, wherein the dish (10) has a center depth of 0.22 mm to 0.26 mm, and a diameter of 4.70 mm to 4.80 mm.

5. The nuclear pellet of claim 4, wherein the dish (10) is configured to have a shape of double concentric circles provided by a second dish (120) having a predetermined diameter and a first dish (110) provided at a center of the second dish (120) and having a diameter smaller than the second dish (120).

6. The nuclear pellet of claim 1, wherein the width of the shoulder (20): a width of the first chamfer (310) is

0. 4565:0.7565 to 0.6565:0.5565.

Description

DESCRIPTION OF DRAWINGS

[0026] FIG. 1a is a photograph of a conventional fuel pellet.

[0027] FIG. 1b is a photograph showing that damage occurs in a state where the fuel pellet having a surface defect is loaded inside a fuel rod.

[0028] FIG. 2 shows photographs showing types of missing pellet surface of nuclear fuel pellet.

[0029] FIG. 3 shows conceptual views showing a simulated impact test.

[0030] FIGS. 4a and 4b are graphs showing a weight loss when an impact energy is a variable and a chamfer angle is a variable, respectively, in the simulated impact test of FIG. 3.

[0031] FIG. 5 is a table showing the graph of FIGS. 4a and 4b.

[0032] FIG. 6 is a graph showing the table of FIG. 5 into a relationship between the chamfer angle and the weight loss of nuclear fuel pellet.

[0033] FIG. 7 is a graph showing the weight loss of missing pellet surface (MPS) resistance fuel pellet with various impact angles by dropping impact test.

[0034] FIG. 8 is a vertical sectional view showing an upper portion of a nuclear fuel pellet in an embodiment of the present invention.

[0035] FIG. 9 is a vertical sectional view showing a double dish, which is a modified embodiment of FIG. 8.

[0036] FIG. 10 is a vertical sectional view showing a double chamfer, which is a modified embodiment of FIG. 8.

MODE FOR INVENTION

[0037] Specific structures or functional descriptions presented in embodiments of the present invention are exemplified for a purpose of describing the embodiments according to a concept of the present invention, and the embodiments according to the concept of the present invention may be implemented in various forms. In addition, the present invention should not be construed as being limited to the embodiments described herein but should be understood to include all modifications, equivalents, or substitutes included in the spirit and scope thereof.

[0038] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0039] A nuclear fuel pellet according to the present invention is prepared with UO.sub.2 powder and is a cylindrical fuel pellet with a height of 9 to 13 mm and a horizontal sectional diameter of 8 to 8.5 mm.

[0040] Specifically, as shown in FIG. 1a, the nuclear fuel pellet according to the present invention includes: a dish 10 provided in a shape of a spherical groove having a predetermined curvature and a diameter of 4.8 to 5.2 mm at a center of each of a top surface and a bottom surface of the nuclear fuel pellet; a shoulder 20 provided in an annular plane along a rim of the dish 10; a first chamfer 310 provided along a rim of the shoulder 20 while being adjacent to the shoulder 20; and a second chamfer 320 provided along a rim of the first chamfer 310, wherein a width SW of the shoulder 20 is 0.4565 mm to 0.6565 mm, an angle between the first chamfer 310 and a horizontal plane is 2.0°, and an angle between the second chamfer 320 and the horizontal plane is 18.0°. At this time, when a cylindrical shape of the fuel pellet is vertically arranged, a ratio of a width CW of the chamfer 30 to a height CH of the chamfer 30 is 0.1 to 0.4, wherein the CH is a difference between a top end height and a bottom end height of the chamfer 30.

[0041] Here, when the angle between the chamfer 310 and the horizontal surface is 2.0°, the angle between the chamfer 320 and the horizontal surface is 18.0°, and the width SW of the shoulder 20 is 0.4565 mm to 0.6565 mm, weight loss of the nuclear fuel pellet due to impact damage becomes minimal. A relationship between the width SW of the shoulder 20 and the mass loss will be described in detail with reference to FIG. 7 and Table 1.

[0042] In addition, the nuclear fuel pellet according to the present invention is manufactured using a powder in which at least one of PuO2 powder, Gd2O3 powder, and ThO2 powder is mixed with UO2 powder. In addition, the nuclear fuel pellet is manufactured by sintering UO2 in a state of being mixed with a pore-former and a lubricant into a molded object by molding equipment.

[0043] As shown in a photograph of FIG. 1a, there is provided a fuel pellet, the pellet including: a dish 10 provided recessed on a center of each of a top surface and a bottom surface; a shoulder 20, being an annular plane and perpendicular to a body of the fuel pellet along the rim of the dish 10; and a chamfer 30 provided into a plane that is a circular shape along the rim of the shoulder 20, wherein the plane is provided by a corner at which the body of the fuel pellet and the shoulder 20 meet along the rim of the shoulder 20, wherein the corner is provided in a predetermined angle due to a chamfering process.

[0044] The reason why the dish 10 is provided in a recessed shape is that a space in which thermal expansion may be accommodated is required when thermal expansion occurs in the axial direction in the center of the fuel pellet during reactor operation. Therefore, as the dish 10 is provided, the growth of the fuel rod in the longitudinal direction is limited.

[0045] A reason why the shoulder 20 is needed is that it is necessary to provide a surface on which a stacking load between the plenum spring and the fuel pellets is applied when the fuel pellets are stacked inside a nuclear fuel rod. Therefore, in the absence of the shoulder 20, there is a high risk of local damage occurring on the contact surface between the fuel pellets due to the stacking load.

[0046] The chamfer 30 serves to reduce a phenomenon that local stress is concentrated on an inner wall of a cladding due to pellet-cladding interaction occurring during the nuclear fuel rod is burned in a reactor and to reduce the missing surface pellet due to the impact generated during preparing the fuel pellets.

[0047] On the other hand, in the present invention, under an assumption that a role of the chamfer 30 is intensively and highly exerted at a specific chamfer 30 angle, a simulation of an impact simulation test of the fuel pellet as shown in FIG. 3 was conducted to attempt preliminary analysis.

[0048] As a result, when the simulation was performed as shown in graphs of FIGS. 4a, 4b, and 6 and a table of FIG. 5, it was confirmed that, when the impact energy was set as a variable, the larger the impact energy, the greater the amount of weight loss due to breakage of the fuel pellet, and, when the chamfer 30 angle, which is the angle between the chamfer 30 and the horizontal plane in a state where the fuel pellet is vertically erected state, was set as a variable, the amount of weight loss of the fuel pellet due to the impact converged at a specific angle.

[0049] In the simulation of FIG. 3, as shown in the graph of FIG. 4b and the table of FIG. 5, when the fuel pellet was impacted, it was confirmed that the weight loss caused by scattering of debris at the corner was the smallest when the chamfer 30 angles are about at least a 14.0° in the graph of FIG. 4b and a 14.0° to 18.0° in the table of FIG. 5, respectively.

[0050] In particular, in FIG. 6 graphically showing the table of FIG. 5, it was confirmed that when the chamfer 30 angle was a 16.0° to 18.0°, the weight loss was the smallest.

[0051] In view of this, it was confirmed that the weight losses of the fuel pellet according to a relation of the chamfer 30 angle and the center depth of the dish 10, and of the width of the shoulder 20 and the height of the chamfer 30 were correlated.

[0052] Here, in the simulation shown in FIGS. 3 to 6, the shape and size of the dish 10 are given conditions that the center depth DD of the dish 10 is provided in 0.22 mm to 0.26 mm while the diameter DW of the dish 10 is provided in 4.70 mm to 4.80 mm.

[0053] At this time, as illustrated in FIG. 9, a first dish 110 having a predetermined diameter may be formed at the center of the dish 10. In this embodiment, the dish 10 may be referred to a second dish 120.

[0054] When the first dish 110 and the second dish 120 are provided as described above, the principle that damage due to the impact of the fuel pellet may be suppressed is as follows.

[0055] When the fuel pellet is burned due to the operation of the reactor, fuel pellet damage may occur due to lateral stress caused by the axial growth of the fuel pellet due to combustion heat. Because the growth of the fuel pellet may be further suppressed when the first dish 110 is provided, as shown in FIG. 9, in the central portion of the second dish 120 that has a predetermined diameter and is provided to suppress the axial growth of the fuel pellet, damage to the fuel pellet may be further prevented.

[0056] On the other hand, as shown in FIG. 10, the chamfer 30 may be configured to include: a first chamfer 310 provided along the rim of the shoulder 20 while being adjacent to the shoulder 20; and a second chamfer 320 provided by a corner, at which the first chamfer 310 and a flank of the fuel pellet meet, the corner chamfered along the rim of the first chamfer 310.

[0057] That is, the chamfer 30 is divided into two chamfers 310 and 320 having different angles from one another.

[0058] At this time, in the case that the fuel pellet in the cylindrical shape is vertically disposed, and when an angle C1A of the first chamfer, which is the angle between the first chamfer 310 and a horizontal plane, is 2.0°, and an angle C2A of the second chamfer, which is the angle between the second chamfer 320 and the horizontal plane, is 18.0°, the weight loss is the smallest as shown in the graph of FIG. 6 and, therefore, the impact resistance is the strongest, as is confirmed.

[0059] In addition, in this case, the shoulder 20 width may be 0.4565 mm to 0.6565 mm.

[0060] By synthesizing the graph of FIG. 7 and the data in Table 1 below, it may be confirmed that a certain combination of the variables minimizes the weight loss of the fuel pellet in the case that the chamfer is separated into the first chamfer 310 and the second chamfer 320, and the weight loss is minimized when the width of the shoulder 20 is 0.4565 mm to 0.6565 mm. Table 1 below shows a weight mass loss rate (%) in an impact test for each angle on the specimen.

TABLE-US-00001 TABLE 1 First UO.sub.2 weight loss rate after chamfer Shoulder impact test (%) width width Impact angle (mm) (mm) 5.0° 25.0° 45.0° 75.0° 85.0° Specimen 1 1.02 0.1930 1.5 0.83 0.04 1.06 2.04 Specimen 2 0.8561 0.3569 1.23 0.71 0.04 0.95 1.73 Specimen 3-1 0.8115 0.4015 1.12 0.69 0.04 0.79 1.72 Specimen 3-2 0.7565 0.4565 0.89 0.51 0.03 0.71 0.98 Specimen 3-3 0.6565 0.5565 0.62 0.35 0.03 0.35 0.42 Specimen 3-4 0.6118 0.6012 0.63 0.39 0.02 0.41 0.39 Specimen 3-5 0.5565 0.6565 0.76 0.51 0.02 0.47 0.67 Specimen 3-6 0.5111 0.7019 1.09 0.71 0.04 0.91 1.39 Specimen 4 0.4774 0.7356 3.09 0.91 0.04 1.37 4.81 Specimen 5 0.378 0.8350 3.21 0.88 0.085 1.48 6.02 Specimen 6 1.213 0.00 2.68 0.76 0.06 1.48 5.41 Reference (Single chamfer) 4.67 1.43 0.67 1.59 8.93 specimen (Conventional fuel pellet)

[0061] The specimens 1 to 6 are double chamfers divided into a first chamfer 310 and a second chamfer 320.

[0062] The angle between the second chamfers of the specimens 1 to 6 and the horizontal plane is the 18.0°, and the angle between the first chamfers and the horizontal plane is a 2.0°.

[0063] The reference specimen has a single chamfer, and the angle between the single chamfer of the reference specimen and the horizontal plane is a 14.0°.

[0064] The specimen height of the specimens 1 to 6 is 9.8 mm, the horizontal cross-section diameter is 8.192 mm, the dish diameter is 4.75 mm, and the second chamfer width is 0.408 mm.

[0065] The specimen 6 does not have the shoulder 20, the dish 10 is configured to have a shape of double concentric circles, with reference to FIG. 9, the diameter D1W of the first dish 110 provided at the center is 1.9474 mm, a depth D1D of the first dish is 0.2 mm, the diameter D2W of the second dish 120 surrounding the first dish 110 is 4.75 mm, and the depth D2D of the second dish is 0.3 mm.

[0066] Table 1 and the graphs of FIG. 7 above show the weight loss caused by the breakage due to an impact by dropping the fuel pellet at various angles.

[0067] The specimens notated on the upper left of the graph in FIG. 7 are all 7 in the order from top to bottom, and this order corresponds to the specimen order in Table 1. The lowermost specimen of Table 1 and the lowermost reference specimen (hereinafter referred to as a □reference specimen□) of the specimens on the upper left of the graph of FIG. 7 are each a conventional nuclear fuel pellet.

[0068] The first thing that may be noticed is that the weight loss at a 45.0° is similarly good for all specimens, but the deviation is greater as it deviates from the 45.0° and becomes severe at a 5.0° and 85.0°. At this time, in the reference specimen, the angle between the single chamfer and the horizontal surface is the 14.0°, whereas in the specimens 1 to 6, the angle between the second chamfer and the horizontal surface is the 18.0°. As previously mentioned, it may be seen that the weight loss is much smaller in the impact test of the 5.0° and 85.0° in the case where the chamfer angle is the 18.0° than in the case where the chamfer angle is the 14.0° of the reference specimen.

[0069] In addition, with reference to FIG. 7 and Table 1 above, it may be seen that in the range of the shoulder width of 0.4565 mm to 0.6565 mm, the weight loss is significantly less than that of the reference specimen.

[0070] Since the impact angle generated on the specimen varies depending on the situation where the impact occurs, the weight loss due to impact should be minimized at all angles, considering the impact angle is almost random. Therefore, even at the 5.0° and 85.0°, the weight loss needs to be significantly reduced compared to the conventional art.

[0071] In particular, even at the impact angles of the 5.0° and 85.0°, the weight loss is extremely small in the specimen 3, and then the weight loss gradually increases in the order of the specimen 2 and the specimen 1. Therefore, it may be seen that specimens 3-2 to 3-5 having a particularly small weight loss correspond to cases where the shoulder width is 0.4565 mm to 0.6565 mm, that is, the shape with the lowest weight loss.

[0072] In addition, with reference to Table 1, when the shoulder width is 0.4565 mm to 0.6565 mm, the most preferable width ratio range of the shoulder 20 width: the first chamfer 310 width is 0.4565:0.7565 to 0.6565:0.5565.

[0073] Therefore, the shoulder width at which the weight loss is minimized at almost every angle is 0.4565 mm to 0.6565 mm, and in this case, the most preferred first chamfer 310 width size is 0.5565 mm to 0.7565 mm.

[0074] The present invention described above is not limited by the above-described embodiments and accompanying drawings. It will be obvious to those who have the ordinary knowledge in the art that various substitutions, modifications, and changes are possible within the scope of the present invention without departing from the technical spirit of the present invention.

[0075] The present disclosure is a result of “Development of Technology Customized for Global Market for Nuclear Power Plant Industry” sponsored by the Ministry of Trade, Industry and Energy” of Republic of Korea. [Task name: Development of Safety Enhanced Nuclear Core Technology for APR/Task unique number: 20217810100050]

TABLE-US-00002 <Description of the Reference Numerals in the Drawings> C1A: Angle of first chamfer C2A: Angle of second chamfer CH: Height of chamfer C1H: Height of first chamfer C2H: Height of second chamfer CW: Width of chamfer C1W: Width of first chamfer C2W: Width of second chamfer DD: Center depth of dish D2D: Depth of second dish D1D: Depth of first dish DW: Diameter of dish D2W: Diameter of second dish D1W: Diameter of first dish SW: Width of shoulder 10: Dish 20: Shoulder 30: Chamfer 110: First dish 120: Second dish 310: First chamfer 320: Second chamfer