JPH0368895A - Fuel cladding tube for nuclear reactor - Google Patents

Abstract

PURPOSE:To enable a stress release by a deformation of a grooved part which is formed in a surface, and to prevent a breaking-down even if a tensile stress works on a fuel cladding tube by providing grooves which are parallel to an axial direction, in the inner surface of the fuel cladding tube. CONSTITUTION:A fuel cladding tube is made of a zirconium alloy 1, a ductile material 2 is lined on its inner surface and grooves 3 are formed in its surface. When a stress is generated on a fuel cladding tube due to a change of the reactor output, a circumferential tensile stress higher than that in the circumference works on the inner surface. In this situation, as the grooves are separated each other, the grooves themselves can be easily deformed and thereby the stress can be easily mitigated as well. Also, the grooved parts 3 can be easily and plastically deformed only at their surfaces and therefore the stress can be absorbed and propagation of cracks into an inner part of the cladding tube can be prevented. Consequently, a possibility of a ductile breaking-down is rather small even if an excess stress is generated by a PCI.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to fuel cladding tubes used in nuclear reactors.

[Prior Art] FIG. 5 shows a longitudinal cross-sectional view of a nuclear fuel element used in current light water reactors. As shown in this figure, the nuclear fuel elements are
A cylindrical nuclear fuel pellet 12 is loaded into a zirconium-based alloy cladding tube 11, and the cladding tube 11 is sealed with an upper end plug 13 and a lower end plug 14. In FIG. 5, 15 is a plenum part, 1G is a spring that presses down the nuclear fuel pellet 12, and 17 is a gap between the pellet and the cladding tube.

When these fuel elements are burned in a nuclear reactor, a large temperature gradient occurs in the individual nuclear fuel pellets in the radial direction, causing the upper and lower ends to bulge outward, causing the pellets to deform into an hourglass-like shape. In addition, since the fuel element is filled with nuclear fuel pellets stacked, the deviations in parallelism between the upper and lower surfaces of individual pellets are accumulated, and each pellet group is tilted in the axial direction. In particular, the end face of the pellet, which has a large amount of deformation and deviation in parallelism, comes into contact with the cladding tube (PCI).

(pellet-cladding interaction), resulting in high stress and strain on the cladding. Then, the pellet or the fission products (FP gas) released from the pellet act on the stress and strain concentrated location of the cladding tube corresponding to the end of the pellet, increasing the possibility of stress corrosion cracking.

To address these problems, a proposal was made to line the inside of the cladding with a zirconium liner layer to absorb interaction with fuel pellets and prevent damage to the zirconium alloy (Japanese Patent Application Laid-Open No. 63-61989).

[Problems to be Solved by the Invention] The fuel cladding provided with this liner layer has improved PCI resistance compared to the fuel cladding made only of zirconium alloy because the liner layer is softer than the zirconium alloy. .

However, higher burnup of nuclear reactors is currently being considered, and as a result, even higher reliability of fuel cladding is required. In other words, as burn-up increases, the elongation in both the axial and circumferential directions may increase, so even if tensile stress is applied to the inner surface of the cladding, defects such as cracks will not occur on the surface of the liner layer. is necessary.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fuel cladding tube with higher reliability by further improving PCI resistance in response to higher burnup.

[Means for Solving the Problems] The above object is achieved by providing the inner surface of the fuel cladding tube with unevenness parallel to the axial direction.

[Function] In the present invention, the unevenness provided on the inner surface of the fuel cladding tube can be easily deformed in response to stress in the circumferential direction,
Moreover, since the stress concentration caused by PCI is absorbed by the plastic deformation of the convex portion, the PCI resistance is significantly improved. Therefore, in the fuel cladding tube of the present invention, failure such as ductile cracking does not occur in the normal stress range.

[Embodiment] Hereinafter, an embodiment of the present invention will be explained using FIGS. 1 to 4.

FIG. 1 shows a cross section of a fuel cladding tube according to an embodiment of the present invention. This fuel cladding tube is made of zirconium alloy 1 and is lined with a ductile material 2.
Irregularities 3 are formed on its surface.

FIG. 2 shows the manufacturing process of the fuel cladding shown in FIG. 1. The order of the steps is indicated by the numbers in circles. Process ■
By repeating steps (1) to (2), a conventional zirconium alloy tube can be manufactured, and a fuel cladding tube is almost completed in steps (1) to (2). Furthermore, irregularities are provided on the inner surface of the fuel cladding tube by the steps from step (1) to [phase]. In step (2), the cross-sectional shape of the mandrel is set to be the opposite shape to that shown in FIG. In the process [phase], the strain caused when forming the unevenness on the inner surface is released by vacuum annealing, and the fuel cladding tube is completed.

When stress occurs in the fuel cladding due to fluctuations in the furnace output, a higher tensile stress in the circumferential direction acts on the inner surface than on the outer circumference. At this time, since the unevenness on the inner surface is separated from each other, it can be easily expanded and contracted, and stress can be alleviated.

In addition, since only the surface of the convex portion 3 is susceptible to plastic deformation, it can absorb stress and prevent cracks from propagating inside the cladding tube, and even if excessive stress is generated due to PCI, there is a possibility of ductile fracture. is small.

An example of the cross-sectional structure of the uneven portion is shown in FIGS. 3 and 4.

In FIG. 3, the surface edge is chamfered and the bottom of the recess has a smooth spherical shape. In this case, even if tensile stress is applied, the stress can be distributed uniformly, and cracking due to stress concentration can be prevented. Further, in FIG. 4, the bottom of the recess has a teardrop shape. Since the shape of the bottom is larger than the opening width of the recess, the cross-sectional area of the bottom of the fourth part is 3.
It is larger than the one shown in the figure, and the tensile stress acting in the circumferential direction can be uniformly distributed, making it difficult for cracks to occur. Note that it is preferable that the depth of the recess is several tens of μm and that the number of grooves is as large as possible. The ductile material 2 may be zirconium, which has been used conventionally.

According to this example, even if tensile stress is applied to the fuel cladding tube in the circumferential direction, the convex portion formed on the inner surface will plastically deform and absorb the stress, so defects such as cracks from the inner surface will not occur. Therefore, the integrity of the fuel cladding can be maintained.

[Effects of the Invention] According to the present invention, by providing unevenness on the inner surface of the fuel cladding tube, even if tensile stress is applied to the cladding tube, the unevenness formed on the surface easily deforms and releases the stress. be able to. Therefore, the inner surface does not undergo ductile fracture (it becomes deformable, and defects such as microcracks do not occur in the cladding tube, which has the effect of further improving the integrity of the fuel cladding tube. Even if excessive stress is locally generated on the inner surface due to PCI, the convex portion plastically deforms and absorbs the stress, thereby preventing damage to the cladding tube and maintaining the safety of the reactor.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a fuel cladding tube according to an embodiment of the present invention, FIG. 2 is a diagram showing the manufacturing process of the fuel cladding tube of FIG. 1, and FIGS. FIG. 5 is a longitudinal sectional view of a conventional fuel cladding tube, which shows the cross-sectional shape of the uneven inner surface of the fuel cladding tube. 1...Zirconium alloy 2...Ductile material 3...Surface unevenness (8733) Agent: Patent attorney Yoshiaki Inomata (and others)
1 person) Kaya 2 Kaya 5 column

Claims (4)

[Claims] (1) A fuel cladding tube for a nuclear reactor, characterized in that an uneven groove parallel to the axial direction of the cladding tube is formed on the inner surface. (2) The fuel cladding tube for a nuclear reactor according to claim 1, wherein the cross-sectional structure of the bottom of the concave portion of the uneven groove has a constant curvature. (3) The fuel cladding tube for a nuclear reactor according to claim 1, wherein the cross-sectional structure of the bottom of the concave portion of the uneven groove is in the shape of a teardrop. (4) The fuel cladding tube for a nuclear reactor according to claim 1, wherein the uneven grooves are formed in a liner layer made of a ductile material provided on the inner surface of the fuel cladding tube.