JPS645661B2 - - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a spacer for a fuel assembly for a light water reactor, and more particularly to a spacer made of a zirconium-based alloy that simultaneously satisfies excellent mechanical properties and excellent corrosion resistance.

Technical background of the invention and its problems In general, a fuel assembly for a light water reactor has a structure as shown in FIG. 1 as a partially cutaway longitudinal sectional view. That is, the fuel rods 3 containing the nuclear fuel UO ₂ and the fuel rods 3 are aligned and supported between the upper tie plate 1 and the lower tie plate 2 to suppress their lateral movement, and to open the coolant flow path. A plurality of spacers 4 are arranged, and the entire spacers 4 are housed in a channel box 5. Several hundred fuel assemblies are loaded into one nuclear reactor.

A plan view showing a part of the spacer is shown in FIG. In the figure, 6 is an outer frame with a plurality of protrusions A1
1. It has a plurality of protrusions B12 and a plurality of spherical protrusions 13, and in a square plane surrounded by the outer frame, a plurality of bars 7 and a plurality of dividers 8 intersect with each other in a horizontal plane. The ends of each are fixed to the outer frame 6 by, for example, welding. Further, the intersections between the bars and between the bars and the dividers are fixed by welding. A lantern spring 9 is supported at the intersection of the bars. In this way, a plurality of small partitions are formed, into which the fuel rods (3 in FIG. 1) are inserted. The inserted fuel rod is connected to the lantern spring 9 and the S-shaped spring 1 machined into the center of the two dividers facing it.
0, is elastically held by the protrusion B12 or the spherical protrusion 13.

The materials that make up such a spacer include:
Usually, the lantern spring 9 is made of a nickel-based alloy called Inconel Alloy %, iron 0.18-0.24%, chromium 0.07-0.13
A zirconium-based alloy (referred to as Zircaloy-4) is used.

When a furnace is operated with such a spacer installed, speck-like white products are generated on the surface of the Zircaloy-based alloy due to a corrosion reaction called nodular corrosion. This white product gradually grows as the nodular collage develops, and may eventually peel off from the surface. When corrosion progresses to such a state, the spacer becomes thinner due to the flaking mentioned above, and as a result,
The mechanical strength of the spacer decreases and constitutes the cause of furnace operation accidents.

Under the current operating conditions of actual reactors, spacers made of zirconium-based alloys, of which Zircaloy-4 is a typical example, perform their functions satisfactorily.

However, recently there are plans to lengthen the residence time of the spacer in the furnace, and in that case, from the perspective of furnace safety, the material of the spacer is made of zirconium-based alloy, which has excellent corrosion resistance and therefore excellent mechanical strength. Development is strongly desired.

A zirconium-based alloy containing 1,600 to 3,000 ppm of oxygen has been proposed as a material that meets these demands, for example in Japanese Patent Laid-Open No. 16416/1983. The zirconium-based alloy disclosed here has high tensile strength and corrosion resistance (ASTM standard G2-
74 or B353-77a) is superior to currently used zirconium-based alloys containing 700 to 1600 ppm oxygen.
However, this prior art has the following problems. That is, first of all, in order to produce a spacer member made of the above-mentioned zirconium-based alloy, a zircaloy ingot containing 1,600 to 3,000 ppm of oxygen must be produced. This requires changes to the currently common ingot manufacturing process, which is not industrially desirable. Second, since the overall oxygen concentration of the zirconium-based alloy is increased, there is a risk that the ductility of the material will decrease and its deterioration will be accelerated.

In addition, in order to improve corrosion resistance against nodular corrosion, a method has been proposed in which the surface of a nuclear reactor core member made of a zirconium-based alloy is coated with a thin layer of an electronically conductive material ( (See Japanese Patent Application Laid-Open No. 52-5629). However, this method cannot be said to be a sufficient solution because it causes problems such as contact corrosion due to the coexistence of metals of a different type than the base alloy on the surface of the zirconium-based alloy member. Ta.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a spacer that solves the above-mentioned problems, particularly a spacer that has excellent corrosion resistance against nodular corrosion and excellent mechanical properties.

Summary of the Invention The present inventors conducted the following experiments to study countermeasures against nodular corrosion in zirconium-based alloys.

First, from the spacer divider to Zircaloy.
4 specimen of zirconium-based alloy (20mm x 7mm
Cut out a small piece of
Kg/cm ² of pressurized steam. Note that this test environment is an accelerated test of nodular corrosion that simulates a boiling water environment at 290°C and 76 kg/cm ² .

A few hours later, a speckled white product appeared on the surface of the specimen, which gradually grew larger over time. This tendency is shown as curve a in FIG. 3 as a relationship between the weight increase per unit area of the specimen (corrosion increase: mg/dm ² ) and time.

Then, the white product on the surface of the sample was peeled off and collected, and its structure and composition were investigated by infrared absorption spectroscopy and X-ray diffraction. Monoclinic _ZrO2
It was hot.

On the other hand, in order to investigate the influence of water on the zirconium-based alloy, quantitative analysis of oxygen and hydrogen contained in the specimen was conducted in the above nodular corrosion acceleration test. In the analysis of these two components, it goes without saying that the oxidation products on the surface of the sample were removed before the analysis. As a result, it was found that as the corrosion progressed, the oxygen content hardly changed, but the hydrogen content increased significantly. The relationship between corrosion time (hr) and hydrogen content (ppm) is shown as curve a in Figure 4. This increase in hydrogen content indicates that hydrogen generated by the reaction between the specimen and water due to nodular collage is absorbed into the interior of the specimen.

Furthermore, when the internal structure of the specimen was observed using an optical microscope after 40 hours, it was found that zirconium hydride was present.
It was found that the black substance belonging to ZrHx was almost uniformly distributed throughout the specimen, including near the surface.

The same results as described above were obtained for specimens cut from the outer frame and bar of the spacer.

From the above, it is inferred that the nodular collosis of zirconium-based alloy is characterized by the reaction shown in the following formula.

( _{1 +} 4 / I found out.

Therefore, the present inventors came up with the idea that the occurrence of nodular corrosion could be prevented or suppressed by preventing hydrogen from penetrating into the interior of the zirconium-based alloy, and thus completed the present invention. Ivy.

The spacer of the present invention includes a lantern spring, a divider, a bar, and an outer frame, in which at least one of the divider, the bar, and the outer frame has an oxygen diffusion layer on the surface with a thickness of 0.1 μm to 0.1 mm. It is characterized by being composed of a zirconium-based alloy having the following properties.

The zirconium-based alloy according to the present invention is characterized in that an oxygen diffusion layer is formed only on the surface or near the surface. This diffusion layer functions as a kind of barrier against the penetration of hydrogen generated by the corrosion reaction into the interior of the base alloy. The thickness of this layer is between 0.1 μm and 0.1 mm. If it is less than 0.1 μm, the mechanical strength of the zirconium-based alloy will not be sufficiently high, and if it exceeds 0.1 mm, it will lead to a decrease in ductility, which is not preferable.

In addition to Zircaloy-4, Zircaloy-2, Zr-2.5%Nb, Zr-1%Nb, and Auzenite can also be used as effective zirconium-based alloys in the present invention.

The zirconium-based alloy according to the present invention can be manufactured as follows. A zirconium-based alloy plate with a predetermined composition or each spacer member already processed into a predetermined shape is heated in an electric furnace at 500 to 1000°C.
Using an appropriate heating means such as an induction heating furnace or an infrared furnace, the temperature is heated under an oxygen partial pressure of 1×10 ^-3 to 1×10 ⁴ mmHg for 1 second to
Heat treatment is performed for 1 hour to form an oxygen diffusion layer on the surface. At this time, if the heating temperature is less than 500℃, a suitable oxygen diffusion layer will not be formed, and if it exceeds 1000℃, the grain size of the alloy structure will become coarse in the zirconium-based alloy, resulting in a decrease in the overall mechanical strength. It happens. Further, if the heating time is less than 1 second, a sufficient oxygen diffusion layer will not be formed, and if it exceeds 1 hour, the oxygen diffusion layer will become too thick and the overall ductility will decrease.
Furthermore, if the oxygen partial pressure is out of the above range, it is not only difficult to form an appropriate oxygen diffusion layer, but also economically disadvantageous.

The thus prepared zirconium-based alloy having an oxygen diffusion layer on its surface is applied to one, preferably all, of the divider, bar, and outer frame, and combined with a lantern spring according to a conventional method. Thus, the spacer of the present invention is produced.

Note that the oxygen diffusion layer only needs to be formed on the surface of the zirconium-based alloy member, so the heating means may be a laser beam that can maintain the surface temperature at 500 to 1000°C by irradiating only the surface;
An electron beam or the like can also be applied. Also,
The formation of this oxygen diffusion layer can be achieved by subjecting the product to autoclave treatment, which is commonly used in the finishing stage of the product, for a fairly long period of time. Furthermore, an oxide layer formed on the surface by normal autoclave treatment can be diffused into the interior to form the oxygen diffusion layer according to the present invention.

Example of the invention Sn1.2-1.7%, Fe0.18-0.24% by weight ratio,
Strips were manufactured according to ASTM standard B353-73 from a zirconium-based alloy consisting of 0.07-0.13% Cr and the balance substantially Zr.

Each member of the divider, bar, and outer frame for the spacer was punched out from this strip.

An oxygen diffusion layer with a thickness of 0.1 μm was formed on the surfaces of these members in an electric furnace under the following conditions. temperature
900℃, oxygen partial pressure 2×10 ^-3 mmHg, treatment time 5 minutes.

A specimen of 20mm x 7mm x 0.5mm was taken from the divider and kept in pressurized steam at 500℃ and 107Kg/cm ² to observe changes in the condition of the specimen surface, increase in corrosion of the specimen, changes in hydrogen content, and The internal organization was observed.

Even after 40 hours of holding time, no white products were observed on the surface, and as shown by curve b in Figure 3, the change in corrosion weight was extremely small, indicating good corrosion resistance against nodular corrosion. It was hot. Furthermore, as shown by curve b in Figure 4, the hydrogen content hardly increased. Microscopic observation of the internal structure also revealed that the amount of black material belonging to the hydride ZrHx was extremely small.

In terms of mechanical properties, the specimens according to the present invention have a 0.2% proof stress of 385 to 395 MPa and a tensile strength of 385 to 395 MPa.
549~553MPa, elongation 31~32% (roll direction)
It was hot. A conventional zirconium-based alloy without an oxygen diffusion layer had a 0.2% yield strength of 376 MPa, a tensile strength of 530 MPa, and an elongation of 33% (in the roll direction).
From this, it was confirmed that the zirconium-based alloy according to the present invention has mechanical properties that are generally equivalent to or better than those of the conventional alloy.

Effects of the Invention As is clear from the above description, the spacer of the present invention is made of a zirconium-based alloy member that has excellent corrosion resistance against nodular corrosion and excellent mechanical properties. During operation, it can fulfill its functions for a long period of time.

Moreover, the production of the zirconium-based alloy requires only the formation of an oxygen diffusion layer of 0.1 μm to 0.1 mm on the surface, and there is no need to add a new large-scale process for its production. It is economical and has great industrial value.

[Brief explanation of drawings]

Figure 1 is a partially cutaway vertical sectional view of the fuel assembly;
The figure is a plan view of a portion of the spacer. FIG. 3 shows the change in corrosion weight over time in a corrosion test of a zirconium-based alloy, and FIG. 4 shows the change in hydrogen content over time. 1... Upper tie plate, 2... Lower tie plate, 3... Fuel rod, 4... Spacer, 5... Channel box, 6... Outer frame, 7... Bar, 8... Divider, 9... Lantern spring, 10... S-shaped spring, 11... Protrusion A, 12... Protrusion B, 13... Spherical protrusion.

Claims (1)

[Claims] 1. In a spacer consisting of a lantern spring, a divider, a bar, and an outer frame, at least one of the divider, the bar, and the outer frame is made of a zirconium-based alloy having an oxygen diffusion layer with a thickness of 0.1 μm to 0.1 mm on the surface. A spacer comprising: