JPH0560076B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a cladding structure for loading nuclear fuel pellets, and more particularly to an improvement of a nuclear fuel composite cladding tube provided with a liner layer of pure zirconium on the inner surface.

[Technical background of the invention and its problems]

Conventionally, in nuclear fuel elements in which nuclear fuel pellets containing uranium oxide or plutonium oxide are coated with zirconium alloy, cladding failure accidents were thought to be mainly caused by hydrogen. This hydrogen is thought to be generated by the decomposition of latent moisture that was not removed during the production of nuclear fuel pellets, and the conventional method was to reduce hydrogen generation by loading a steam getter into the cladding tube. was taken. However, as research into nuclear fuel development progresses, it has become clear that in addition to damage caused by hydrogen embrittlement, stress corrosion cracking of the cladding due to iodine gas or cesium gas, which are fission products of the fuel, is also a major cause of cladding failure. came.

As a measure to prevent such stress corrosion cracking, conventionally, nuclear reactors have been operated at a reduced rate of power increase in the early stages of operation to prevent sudden stress from being applied to the cladding tubes. However, in recent years, as the proportion of nuclear power generation has increased, economical high-rate operation of nuclear reactors has been desired, and even under severe operating conditions such as rapid start-up and following load fluctuations, the mechanical interaction between nuclear fuel pellets and cladding Research is being conducted on structures that reduce stress corrosion cracking of the cladding caused by fission products.

For example, in Belgian Patent No. 835481,
A structure is shown in which pure zirconium is provided inside the outer tube to act as a cushion to reduce mechanical interaction with nuclear fuel pellets.

As shown in FIGS. 1 and 2, this composite cladding tube 1 has a structure in which a liner layer 3 made of pure zirconium is integrally bonded to the inside of an outer tube 2 made of a zirconium alloy. Inside the composite cladding tube 1, a plurality of nuclear fuel pellets 4, such as uranium oxide or plutonium oxide, formed into a pellet shape are stacked and filled, and the nuclear fuel pellets 4 are further packed into the upper end plug of the composite cladding tube 1. 5
It is fixed by a spring 6 whose one end is in contact with.

Further, Belgian Patent No. 870342 describes that the liner layer is formed of a metal zirconium layer with a high oxygen concentration, such as sponge zirconium.

A method for manufacturing a nuclear fuel composite cladding tube provided with such a liner layer is, for example, by inserting a pure zirconium sleeve for the liner layer into a hollow billet made of zirconium alloy, and then hot extruding the composite sleeve. A composite tube is manufactured by extrusion molding at the same time.

Next, this composite tube is reduced to a predetermined inner diameter and wall thickness by cold working through multiple passes using a device such as a Pilger tube squeezer to produce a composite cladding tube. Between each pass of cold working, the composite tube is typically annealed by heat treatment at a temperature and time sufficient to substantially completely recrystallize the zirconium alloy.

The heat treatment performed after the final tube drawing process plays an important role in determining the mechanical properties of the composite cladding tube, and in Japanese Patent Application Laid-open No. 164396/1983, it is stated that ``The pure zirconium layer that becomes the liner layer is substantially At the same time, the zirconium alloy that forms the outer tube needs to be heat-treated to simply remove the stress caused by cold working without causing complete recrystallization. It is said that this can be achieved by heat treatment at a low temperature of 440 to 510 degrees Celsius.

However, while conducting microscopic research on composite cladding, the inventor discovered that pure zirconium has a higher recrystallization temperature than zirconium alloy, and at the temperature described in the above publication, it forms a liner layer. In fact, it was discovered that pure zirconium was not recrystallized, but the zirconium alloy of the outer tube was recrystallized, which was actually the opposite of what was described in the above publication.

[Purpose of the invention]

The present invention was made based on the above findings, and
The pure zirconium that forms the liner layer is almost completely recrystallized to provide a cushioning effect.
The present invention provides a nuclear fuel cladding tube that maintains the strength of the zirconium alloy that forms the outer tube.

[Summary of the invention]

The gist of the present invention is a nuclear fuel composite cladding tube in which both the zirconium alloy that serves as the outer tube and the pure zirconium that serves as the liner layer that is metallurgically bonded to the inside of the zirconium alloy are in a substantially completely recrystallized state. .

Examples of the zirconium alloy used for the outer tube in the present invention include Zircaloy-2 and Zircaloy-4.

The nuclear fuel composite cladding tube according to the present invention is manufactured, for example, by the following method. First, a pure zirconium sleeve, which will become a liner layer, is inserted into a hollow billet of zirconium alloy, which will become an outer tube, and then the composite tube will be hot extruded and joined together.

Next, this composite tube is subjected to a plurality of passes and then drawn by cold working to form it into a predetermined inner diameter and wall thickness. A heat treatment is performed between each pass of this cold working to bring the zirconium alloy that forms the outer tube into a substantially complete recrystallized state, and to metallurgically join the outer tube and the liner layer together. In this case, heat treatment conditions include heating at 538 to 704°C for 1 to 15 hours, for example.

In this way, the final tube drawing step is carried out, and the composite tube, which has reached the finished dimensions, is subjected to a final heat treatment, resulting in virtually complete re-treatment of both the outer tube made of zirconium alloy and the liner layer made of pure zirconium. yield crystals and produce nuclear fuel composite cladding.

In this case, the final heat treatment recrystallizes the liner layer, but the heat treatment conditions are
Heating is carried out at 586-720°C, preferably 590-620°C for 1-15 hours. The final heat treatment conditions are determined within a range in which pure zirconium is substantially completely recrystallized. There are various methods for determining the completion point of recrystallization, but for example, the crystal grain size, dislocation density, and Witzker's hardness are measured, and the determination is made from a state where these are low and saturated.

Figure 3 shows pure zirconium A with an oxygen concentration of 50 to 100 ppm and pure zirconium B with an oxygen concentration of about 500 ppm.
2 is a graph showing changes in dislocation density, Witzker's hardness, and grain size depending on annealing temperature. As is clear from this graph, it is at 586° C. or higher that the dislocation density rapidly decreases, the Witzkers hardness becomes low and saturated, and recrystallization is completed. When heat treated at a higher temperature than this, the grain size gradually grows larger. The upper limit of the heat treatment temperature was set at 720°C because if the temperature exceeds this temperature, the grain size of the zirconium alloy constituting the outer tube may exceed the specified 80 μm and the strength may decrease.

In the thus obtained composite cladding of the present invention, the liner layer is substantially completely recrystallized and has a fine equiaxed crystal structure, which alleviates mechanical interaction with nuclear fuel pellets. Acts as a cushion and increases resistance to stress corrosion cracking.

In addition, the zirconium alloy that makes up the outer tube is similarly recrystallized to suppress the growth of grain size and make it finer, so it has greater resistance to stress corrosion cracking and is also strong and has excellent properties as a nuclear fuel cladding tube. It is something that you have.

[Embodiments of the invention]

A zirconium alloy hollow billet that becomes the outer tube,
After cleaning the surface of the pure zirconium sleeve that will become the liner layer, it is inserted and assembled. Next, the boundaries of the combined composite tubes are welded in vacuum by electro beam welding.

Next, this composite tube was hot extruded and then cold worked repeatedly using a Pilger tube drawing machine to give it a finished shape through multiple passes. In between cold workings, heat treatment was performed at 580°C for 2 hours for annealing.

The composite tube that had undergone the final cold working was then subjected to vacuum heat treatment at 600°C for 2 hours to produce a nuclear fuel composite cladding tube.

The thickness of the liner layer of the composite cladding tube thus obtained is approximately 70±20 μm, and the dislocation density of both the pure zirconium forming the liner layer and the zirconium alloy forming the outer tube is 2×10 ⁹ cm. ^-2 , and recrystallization was essentially completed in both cases. Furthermore, the particle size of the liner layer was about 10 μm, and the particle size of the outer tube was about 3 μm, which was fine and had excellent strength.

In addition, for comparison with the present invention, the conventional
When the final heat treatment was performed at 500℃ for 2 hours using the method described in Publication No. 164396, the dislocation density of the liner layer was 7×10 ¹⁵ cm ^-2 , and it was confirmed that there was no recrystallization. .

〔Effect of the invention〕

As explained above, according to the nuclear fuel composite cladding tube according to the present invention, the pure zirconium forming the liner layer is substantially completely recrystallized to provide a cushioning effect, and the strength of the zirconium alloy forming the outer tube is increased. At the same time, the stress corrosion cracking of the cladding tube can be reduced.

[Brief explanation of the drawing]

Figure 1 is a vertical cross-sectional view showing a nuclear fuel element with nuclear fuel pellets installed in the nuclear fuel composite cladding tube, Figure 2 is an enlarged cross-sectional view of Figure 1, and Figure 3 is the dislocation density of pure zirconium and Witzkarts with respect to the final heat treatment temperature. It is a graph showing changes in hardness and particle size. DESCRIPTION OF SYMBOLS 1... Composite cladding tube, 2... Outer tube, 3... Liner layer, 4... Nuclear fuel pellet, 5... Upper end plug,
6...Spring.

Claims (1)

[Scope of Claims] 1. In a nuclear fuel composite cladding tube in which pure zirconium is provided as a liner layer inside an outer tube made of a zirconium alloy, and the two are metallurgically joined, the zirconium alloy serving as the outer tube and the liner layer are A nuclear fuel composite cladding tube characterized in that it is made of pure zirconium and is in a substantially completely recrystallized state. 2. The nuclear fuel composite cladding tube according to claim 1, wherein the crystal grain size of the pure zirconium forming the liner layer is 80 μm or less.