JPS5920118B2 - nuclear fuel pellets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to columnar fuel pellets loaded into a fuel cladding tube. This relates to nuclear fuel pellets that are protected against direct attack by gamma rays and that also alleviate local stress in the cladding due to cracks in the pellets, thereby preventing damage to the fuel cladding when power increases. .

Conventionally, when constructing a fuel rod by filling a cladding tube with cylindrical fuel pellets, a gap of 0.15 to 0.30 mm in diameter is provided between the fuel pellets and the cladding tube.

When fuel pellets are sintered, they form an hourglass shape in which both ends are larger in diameter than the center.
Therefore, the gap value is adjusted to be uniform by performing outer periphery polishing.

However, when fuel rods whose cladding tubes are filled with conventional cylindrical fuel pellets are loaded into a nuclear reactor and put into operation, the fuel pellets develop cracks in the radial direction due to the large temperature gradient within the pellets. Pellet pieces that enter the pipe and are generated by cracks protrude outward and come into contact with the cladding tube.

As the power of the reactor increases, the pellet pieces deform due to thermal expansion and nulling, bending outward from their central axis as shown in the model in Figure 1, and come into strong contact with the cladding, causing interaction. arise.

Therefore, the stress on the inner surface of the cladding tube facing the opening of the pellet 1 increases locally, causing strain concentration.

This type of strain concentration alone does not necessarily cause damage to the fuel cladding tube, but thermal radiation, FP Ganu flow, and gamma rays from the high temperature part of the fuel pellet 1 core cause strain concentration along the crack. The high-temperature corrosive atmosphere generated by contact with the inner surface of the cladding tube promotes the occurrence and propagation of cracks on the inner surface of the cladding tube, which eventually leads to fuel failure.

FPP generated and accumulated in pellets due to nuclear fission
The 2O release rate depends on the temperature of the pellet.

In particular, when the reactor power increases rapidly, FP Ganu such as iodine emitted from the center of the pellet increases in a burst manner, and as shown in Figure 2, thermal radiation, FP Ganu flow, and gamma rays increase. 5 directly hits the inner surface crack opening 4 of the cladding tube 2 along the crack 3 of the fuel pellet 1, and locally increases the temperature of the inner surface of the cladding tube due to thermal radiation.

Areas where strain is locally concentrated on the inner surface of the cladding tube due to strong contact between pellet pieces caused by cracks will undergo stress corrosion when FP gas flow such as high concentration iodine and gamma rays are sprayed intensively. Even if the total circumferential stop that occurs in the cladding tube 2 is small, the cladding tube will be damaged at the strain concentrated portion.

The damaged part is indicated by the reference numeral 23.

The purpose of the present invention is to eliminate such drawbacks of the prior art,
An object of the present invention is to provide pellets that are less likely to cause stress corrosion on the inner surface of a cladding tube.

Hereinafter, the present invention will be explained in detail based on the drawings.

FIGS. 3 and 4 show an embodiment of the nuclear fuel pellet according to the present invention.

The fuel pellet 1 has a cylindrical shape in which the force at the height of the pellet is smaller than its outer diameter, and at corresponding positions on both end faces of the pellet 10, from the center region of the pellet toward the outer circumferential region, like the blades of a water wheel. Six arcuate V-shaped grooves 6 are formed on each side.

Further, both end faces of the pellet 10 are provided with a chamfered shape (chamfer shape) 8.

FIG. 5 is a plan view showing another embodiment of the nuclear thermal pellet according to the present invention.

In this embodiment, four polygonal V-shaped grooves 7 are formed on both end faces of the pellet 1, respectively.

Although there is no particular limit to the number of V-shaped grooves, it is meaningless to increase the number too much, and the number is about 4 to 16.

Especially when the diameter is IQ77117, like fuel for light water reactors.
When the number is about 11, it is preferably about 6 to 12.

Further, it is important that the shape of the cladding tube is such that the tip portion (outer peripheral region) faces obliquely with respect to the inner surface of the cladding tube.

In this way, damage to the inner surface of the cladding tube can be prevented.

The reason will be clear from the description below.

In the case of conventional cylindrical pellets, when the power of the reactor suddenly increases, high-temperature thermal radiation, FP Ganu flow, and γ-rays that are instantaneously released in large quantities in the central region of the pellet are transmitted along cracks to the outer periphery of the pellet. In contrast to the direct adverse effect on the inner surface of the cladding tube at the opening, according to the present invention, as shown in FIG. 6 or FIG. The high temperature thermal radiation, the high concentration FP Ganu flow, and the γ rays 11 that generate the shaped cracks 10 are significantly relaxed by passing through the curved Vanu.

Therefore, the classic opening 12 on the inner surface of the cladding tube does not create a high-temperature corrosive environment with adverse conditions that cause stress corrosion, and the inner surface of the cladding tube is not damaged.

In addition, since the tips 13 and 14 of the pellet pieces caused by the arcuate cracks 9 or the polygonal cracks 10 come into contact with the inner surface of the fuel cladding tube in an oblique state, when the reactor power suddenly increases, this part will move to the outside of the pellet. Even if the pellet begins to bulge out toward the cladding tube, a small rack 15, 16 is additionally formed at the acute angle portion of the tip of the pellet, before excessive stress is generated on the inner surface of the cladding tube. will be released.

This is also one of the reasons why damage to the inner surface of the fuel cladding tube can be prevented.

This will be understood from the following explanation.

In general, many metal phase photographs from post-irradiation tests show that where the cracked part of a pellet is in contact with the inner surface of the fuel cladding at an angle, there is a damaged area on the inner surface of the fuel cladding. There are almost no examples.

FIG. 8 schematically shows an example of a post-irradiation test gold phase photograph of a fuel rod whose cladding tube is filled with conventional cylindrical pellets.

In the same figure, on the inner surface of the cladding tube at the crack opening of the crack 18 that occurred perpendicularly to the inner surface of the cladding tube 17, there is a radial crack surrounded by large pellet pieces 19.20 or 21.22 on both sides of the crack 18. Damage 23 to the fuel cladding tube can be seen inside the cladding tube at the crack opening, but there is a wedge-shaped crack 24 that occurs obliquely to the inner surface of the cladding tube (in this figure, there are no microcracks in the wedge-shaped crack, but This is probably because no excessive stress was originally generated in this part.

) and on the inner surface of the cladding tube facing the cracks 25 that were finely generated on the outer periphery.

The effectiveness of the present invention is also clear from such practical examples.

Further, the present invention uses nuclear fuel pellets whose height is smaller than their outer diameter.

By using such pellets, it is possible to suppress the warping deformation outward from the central axis as shown in Figure 1, and the contact and interaction with the cladding tube can be reduced accordingly.
Contributes to damage prevention.

Furthermore, in the embodiment of the present invention, the upper and lower end surfaces of the pellets are chamfered 8, which prevents chips and cracks from occurring during the pellet manufacturing process. Since the holes are provided in a well shape, interaction with the cladding tube at the edges of the upper and lower end surfaces of the pellet can be further reduced.

Since the present invention is a fuel pellet configured as described above, even if interaction occurs between the pellet and the cladding tube during nuclear reactor operation, cracks in the pellet will not cause the pellet to break down like the blades of a water wheel. Since the energy is generated in an arc shape or a polygonal shape from the center toward the periphery, the high temperature heat radiation, high concentration FP gas flow, and gamma rays from the deep fuel part in the center of the pellet reach the inside of the cladding tube. It is reduced to a significantly relaxed and attenuated state, and has excellent effects such as being able to prevent local stress build-up on the inner surface of the cladding tube and improving the integrity of the fuel.

[Brief explanation of the drawing]

Figure 1 is a model diagram of the deformation of a conventional cylindrical pellet in a nuclear reactor, Figure 2 is an explanatory diagram of a crack opening in a conventional cylindrical pellet, and Figure 3 is an example of a fuel pellet according to the present invention. 4 is a longitudinal sectional view thereof, FIG. 5 is a plan view showing another embodiment of the present invention, FIGS. 6 and 7 are explanatory views of crack openings, and FIG. 8 is a conventional FIG. 2 is an explanatory diagram of a post-irradiation test gold phase photograph of a fuel rod using cylindrical pellets. 1... Fuel pellets, 2, 17... Cladding tube, 6... Arc-shaped V-shaped groove, 7...... Broken line ■-shaped groove, 8... ...Changfa (chamfer).

Claims (1)

[Claims] 1. In a nuclear fuel pellet whose pellet height is smaller than its outer diameter, 4 to 16 arcuate or polygonal V-shaped grooves are formed at corresponding positions on the upper and lower end surfaces of the pellet from the center of the pellet toward the periphery. Nuclear fuel pellets characterized by being arranged like the blades of a water wheel.