JPS5920119B2 - nuclear fuel pellets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to columnar fuel pellets (K) loaded into a fuel cladding tube.
More specifically, when used in a nuclear reactor, it suppresses direct attack of thermal radiation and gaseous fission products (hereinafter abbreviated as rFPJ) from the high-temperature central part of the fuel to the inner surface of the cladding tube. This invention relates to nuclear fuel pellets that prevent damage to the fuel cladding during fluctuations in the output of a nuclear reactor.

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 inside the pellets. The pellet fragments generated by the cracks protrude outward and come into contact with the cladding tube.

As the power of the nuclear reactor increases, the pellet pieces come into contact with the cladding tube and interact due to thermal expansion and swelling.

When the output is further increased, the pellet piece is deformed by its end surface effect so as to warp outward from its central axis, as shown schematically in FIG.

Therefore, the tensile stress in the circumferential direction of 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 lead to damage to the fuel cladding tube, but thermal radiation and FP gas flow from the high temperature part of the fuel pellet 1 core are generated along the cracks on the inner surface of the cladding tube where strain concentration has occurred. The high-temperature corrosive atmosphere created by the exposure to water promotes the occurrence and propagation of cracks on the inner surface of the cladding pipe, eventually leading to fuel failure.

F'P generated and accumulated in the pellet by nuclear fission
The rate of gas release depends on the temperature of the pellet.

In particular, when the reactor power increases rapidly, the FP gas released from the center of the pellet increases in bursts, and the second
As shown in the figure, the thermal radiation and the F'P gas flow 5 directly hit the inner surface crack opening 4 of the cladding tube 2 along the crack 3 of the fuel pellet 1, and the thermal radiation locally causes the inner surface temperature of the cladding tube to rise. raise the level.

Areas where strain is locally concentrated on the inner surface of the cladding tube due to strong contact between pellet pieces caused by cracks are exposed to high concentration of F.
When P gas flow is sprayed intensively, stress corrosion occurs,
Even if the total weight in the circumferential direction generated in the cladding tube 2 is small, the cladding tube will break at the strain concentrated portion.

The damaged part is indicated by the reference numeral 30. An object of the present invention is to eliminate such drawbacks of the prior art and to provide pellets that are less susceptible to stress corrosion on the inner surface of the cladding tube.

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

FIG. 3 is a perspective view showing one embodiment of the present invention.

The fuel pellet 6 has a cylindrical shape in which the height of the pellet is smaller than its outer diameter, and a concentric trap is provided at approximately the center of the radius between the center region and the outer peripheral region of the upper and lower end surfaces of the pellet 6. If the depth exceeds the pellet height, the mechanical strength of the pellet will be damaged, and the radial V-shaped The number of grooves 8 and 9 is preferably about 8 to 16, although it depends on the outer diameter of the pellet.

It is preferable that the groove has a V-shaped cross-section.

Furthermore, both upper and lower edges of the fuel pellet 6 are chamfered (chamfered) 10, and a circular recess 11 is provided approximately at the center.

The chamfered shape 10 is, for example, approximately 0.3 to 1.0 ml in the radial direction from the virtual edge in a 9 to 16 mm pellet.
, slopes are formed on both the upper and lower end surfaces at the position of the line connecting the depths of approximately 0.3 to 1.5 m.

The diameter of the circular recess 11 is about 2 to 6 m.

Such pellets 6 are manufactured by filling a molding die with raw material powder, compressing it with upper and lower punches, and then sintering it.

In particular, since there are no irregularities on the outer peripheral surface of the pellet,
It is possible to mass-produce green pellets by high-speed punching without paying attention to the direction of the punch-forming die.

The pellets 6 are loaded into a fuel cladding tube 13 to which a first end plug 12 is welded, and assembled into a fuel element by inserting a spring 14 and welding a second end plug 15, as shown in FIG. .

When the fuel pellet according to the present invention is used, cracks will occur along the V-shaped grooves 7, 8.9 when the reactor power increases, and 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 (see Figure 2), when the output increases, the high-temperature FP gas flow and thermal radiation, which are instantly released in large quantities in the central region of the pellet, flow along the crack 3 to the opening of the pellet at the outer periphery. In contrast to the direct adverse effect on the inner surface of the cladding tube in section 4, according to the present invention, as shown in FIG. The high-temperature thermal radiation and high-concentration F'P gas flow generated in the central region pass through the radial crack 20 on the central side, but are significantly relaxed by hitting the circumferential crack wall 21, resulting in the passage of the high-temperature thermal radiation and the high-concentration F'P gas flow into the circumferential crack 22. It is then bent again and flows along the radial cracks 23 on the outer circumferential side.

Therefore, the crack opening 24 on the inner surface of the cladding tube does not form a high-temperature corrosive environment with adverse conditions that cause stress corrosion (the inner surface of the cladding tube 13 is not damaged).

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

By using such pellets, the amount of deflection outward from the central axis as shown in Figure 1 can be suppressed to a negligible extent, and contact and interaction with the cladding tube can be reduced accordingly. This helps prevent damage.

In particular, when the height of the pellet is much lower than that of the conventional one as in the above embodiment, the outer periphery polishing for adjusting the gap between the pellet 6 and the cladding tube 13 is not necessarily performed on all the pellets. In both cases, the process only needs to be performed on the necessary number of pellets, which have been selected in advance using a go/no-go gauge, which reduces the loss of nuclear material due to polishing and increases the economic efficiency of pellet production. be able to.

Furthermore, in the embodiment of the present invention, since the upper and lower end surfaces of the pellet are chamfered 10, it is possible to prevent chips and cracks from occurring during the pellet manufacturing process.

Since the pellet of the present invention has a well-shaped shallow circular recess in its center, interaction with the cladding tube at the upper and lower end surfaces of the pellet can be further reduced.

Especially in the case of pellets with high burnup, this circular recess 1
, 1 can absorb the expansion due to swelling of the pellet 6, so it is better to provide a large circular recess.

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 that occur in the pellet will not occur between the central region and the outer peripheral region. Because there is a difference in the radial direction at the boundary, high-temperature thermal radiation, high-concentration FP gas flow, and gamma rays from the high-temperature fuel part at the center of the pellet are significantly relaxed and attenuated by the time they reach the inner surface of the cladding tube. This has excellent effects such as being able to prevent localized stress corrosion on the inner surface of the cladding tube and improving fuel permeability.

[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. FIG. 4 is an explanatory diagram of an example of a fuel element assembled using the pellets, and FIG. 5 is an explanatory diagram of the crack structure of the pellets. 1.6...Fuel pellets, 2,13...Fuel cladding tube, 7...Concentric V-shaped grooves, 8,9...Radial V-shaped grooves,
10... Chamfered shape, 11... Circular recess.

Claims (1)

[Scope of Claims] 1. In a nuclear fuel pellet whose height is smaller than its outer diameter, the pellet is located between the central region and the outer peripheral region of the upper and lower end surfaces of the pellet and at a position approximately at the center in the mid-diameter direction, concentrically with the pellet outer periphery. A nuclear fuel characterized by having a plurality of radial V-shaped grooves arranged in a radial direction with the concentric V-shaped groove as a boundary and formed at corresponding positions on the upper and lower end surfaces. pellet. 2. The nuclear fuel pellet according to claim 1, wherein the pellet has a circular recess approximately at the center of its upper and lower end surfaces. 3. The nuclear fuel pellet according to claim 1 or 2, wherein both upper and lower edges of the pellet are chamfered. is the nuclear fuel pellet described in Section 3.