JPS6317036Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to the improvement of a nuclear fuel element for a nuclear reactor, and aims to obtain a nuclear fuel element that is safe and highly reliable, with fewer troubles in production and use.

At present, nuclear fuel elements used in power reactors are stacked with a large number of cylindrical fuel pellets 2' stacked in a thin-walled zirconium alloy cladding tube 1', as shown in Figure 1. The end plugs 3' and 4' are made of the same material as the cladding tube 1', and the abutting shoulders 3
a' is TIG welded and sealed all around.

During nuclear reactor operation, the fuel pellet 2' undergoes nuclear fission, and gaseous fission products are released from the fuel pellet 2' and accumulate in the fuel element. A gas reservoir chamber (plenum) is provided inside and above the cladding tube 1' to prevent the internal pressure in the fuel element from becoming excessively high due to objects.
5' is provided.

In addition, in the above-mentioned nuclear fuel element, in order to prevent the fuel pellets 2 from moving to the gas reservoir chamber 5' during transfer or handling and not returning to the predetermined position even after being loaded into the reactor, the gas reservoir chamber 5' is Coil spring 6 made of Inconel or stainless steel (free length 15
~30 cm) is arranged in a compressed state to suppress the fuel pellets 2'.

By the way, although this nuclear fuel element has been used for a long time, it has the following drawbacks.

When assembling the nuclear fuel element as shown in Fig. 1 and finally sealingly welding the upper end plug 3', the vicinity of the welded part of the end plug 3' and the cladding tube 1' becomes quite high temperature (1000~
1500℃), the zirconium alloy constituting the end plug 3' or the cladding tube 1' and the material of the coil spring 6' form a eutectic alloy with a lower melting point than the respective materials, and the properties of the alloy (heat resistance properties (corrosion resistance, mechanical properties) become unpredictable, significantly reducing reliability in terms of performance and safety of nuclear fuel elements.

Various attempts have been made to solve these difficulties, and one of them, as shown in FIG. A wound portion 6a' is formed, and the legs of a plug-shaped barrier member 7' made of the same material as the end plug 3' are inserted and attached to the tightly wound portion 6a'.
A fuel element in which the upper end of the coil spring 5' is not brought into contact with the end plug 3' has been proposed in Japanese Patent Publication No. 56-2679. However, in this case, when assembling the fuel element, it is necessary to compress the coil spring 6' more strongly by the amount of the barrier member 7' than in the case of the one in Fig. 1, so the coil spring 6' is There is a risk that the tightly wound portion 6a' will come into contact with the cladding tube 1' at a high temperature area near the welding area due to lateral displacement, and a eutectic alloy may be generated at that area. If the flange of 7' is made larger than the diameter of the coil spring 6', when the coil spring 6' with the barrier member 7' attached is pushed into the cladding tube 1', the flange of FIG. The welding part hits the open end of the weld and damages that part, causing a welding defect and reducing the reliability of the nuclear fuel element.

This idea was devised to improve the above-mentioned drawbacks, and involves forming a small-diameter tightly wound portion at the upper end of the coil spring disposed in the gum reservoir, and connecting this tightly wound portion to a cylindrical member made of the same material as the cladding tube. The upper end opening of the cylindrical member is pressed into contact with the spherical curved surface or conical surface formed at the bottom of the upper end plug, which not only prevents contact between the coil spring and the end plug, but also prevents the spring from being covered. The present invention provides a nuclear fuel element that does not come into contact with the hot welded part of the tube.

To explain this idea with reference to Figure 3,
Reference numeral 1 denotes a thin-walled cladding tube made of zirconium alloy, and as described above, inside the cladding tube 2 are cylindrical fuel pellets made of fissile material such as uranium dioxide.
A large number of plenums are arranged in a stacked manner, and a gas reservoir chamber (plenum) 5 with a required volume is provided above the plenum. A coil spring 6 is arranged in a compressed state, and the lower fuel pellet 2
This prevents the cladding tube 1 from moving.
End plugs 3 and 4 also made of zirconium alloy are fitted to the upper and lower ends of the butt shoulders 3a and 4a.
is TIG welded all the way around.

By the way, the circumferential surface of the bottom part 3b of the end plug 3 fitted to the upper end of the cladding tube 1 is spherical, and the upper end of the coil spring 6 is tightly wound with an outer diameter smaller than that of the lower part. The tightly wound portion 6a is made of a cylindrical member 7 made of the same material as the cladding tube 1 and the end plug 3 and having a required length (approximately 10 to 12 mm) whose outer diameter is slightly smaller than the outer diameter of the coil spring 6. It is fitted into the lower stepped hole 7a, and the inner peripheral edge of the upper end opening of the cylindrical member 7 is pressed against the spherical peripheral surface of the bottom part 3b of the upper end plug 3 by the spring force of the coil spring 6, thereby achieving its centering function. As a result, the axis of the cylindrical member 7 is aligned with the axis of the end plug 3.

Regarding the results of the prototype test, the outer diameter is 12.52.
Fuel pellets 2 are stacked and loaded inside a zirconium alloy cladding tube 1 with an inner diameter of 10.8 mm, and 40 mm
A plenum part 5 of cm is provided, and the free length is 46.5 cm.
A thin diameter (8 mm) tightly wound part 6a with a length of about 5 mm is formed at the upper end of the stainless steel coil spring 6 with an outer diameter of 10.4 mm, and this tightly wound part 6a is made of zirconium with an outer diameter of 9 mm, an inner diameter of 7 mm, and a length of 10 mm. The coil spring 6 and the cylindrical member 7 are fitted into the lower stepped hole 7a (diameter 8 mm) of the cylindrical member 7 made of alloy, and the coil spring 6 and the cylindrical member 7 are inserted into the plenum part 5. The bottom part 3b is processed to have a spherical surface with a radius of 11 mm, and the end plug 3 is inserted into the cladding tube 1 while the bottom part 3b is brought into contact with the upper end opening of the cylindrical member 7 and the coil spring 6 is pushed in. It was fitted onto the upper end, and the butt shoulder was hermetically welded around the entire circumference.

After that, an X-ray inspection revealed that the cylindrical member 7 was aligned along the axis of the cladding tube 1 by the bottom spherical surface of the end plug 3, and that the coil spring 6 was aligned in the vicinity of the weld with the end plug 3. No contact with the cladding tube 1 occurred, there was no abnormality in the welded portion, and the position where the spring 6 curved and contacted the cladding tube 1 was 50 mm or more away from the welded portion. This position is an area where there is no need to worry about the effects of welding.

If performance equivalent to this is to be achieved with the barrier member, the length of the barrier member would need to be 50 mm or more, which would compress the coil spring more than necessary and deteriorate its performance.

In addition, the peripheral surface of the bottom part 3b of the end plug 3 in this invention is plated with hard chrome, titanium carbide, etc., or a counterbore is formed on the inner peripheral edge of the open end of the cylindrical member 7 as shown in FIG. If the end plug 3 and the cylindrical member 7 are aligned better, the end plug 3
The bottom part 3b of the bottom part 3b does not need to be completely spherical, but it is sufficient that the circumferential side that the cylindrical member 7 comes into contact with has a spherical surface.
Equivalent consistency can be obtained even with a conical surface as shown in FIG. Note that such a curved surface can be easily formed by melting treatment.

As described above, in this invention, a coil spring with a small-diameter tightly wound portion formed at the upper end is arranged in the gas reservoir chamber inside the cladding tube, and the tightly wound portion is made of the same material as the cladding tube and has an outer diameter of the coil spring. It fits inside the lower part of a cylindrical member with a slightly smaller diameter, and an end plug made of the same material as the cladding tube and having a spherically curved or conical bottom circumferential surface is fitted and welded to the upper end of the cladding tube. Since the upper end opening is brought into pressure contact with the circumferential surface of the bottom of the end plug, the upper end of the coil spring can be easily aligned with the axes of the end plug and the cladding tube without coming into contact with them. Not only can it prevent the formation of eutectic alloy, but it can also reliably prevent eutectic melting with the welded high temperature part of the cladding tube, and also prevent damage to the welded open end of the cladding tube when inserting the coil spring into the gas reservoir chamber. Never.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of a conventionally known fuel element, Fig. 2A is a sectional view of another conventional fuel element, Fig. 2B is a sectional view showing the lateral deviation of the upper end of the coil spring, and Fig. 2C is a sectional view of the coil spring. 3rd cross-sectional view showing the pressed state of
The figure is a cross-sectional view of an embodiment of the invention, and FIG. 4 is a cross-sectional view of the upper part of the element showing a modification of the upper end plug. In the figure, 1, 1'... cladding tube, 2, 2'... fuel pellets, 3, 3'... upper end plug, 3a, 3a'...
...Butt shoulder, 3b, 3b'...Bottom, 5, 5'
...Gas reservoir chamber, 6,6'...Coil spring, 6a,
6a′...Tightly wound portion, 7...Cylindrical member.

Claims (1)

[Claims for Utility Model Registration] (1) A coil spring 6 having a small diameter tightly wound portion 6a formed at the upper end is disposed in the upper gas reservoir chamber 5 in the cladding tube 1, and the coil spring 6 is tightly wound. The part 6a is fitted inside the lower part of a cylindrical member 7 made of the same material as the cladding tube 1 and having a slightly smaller diameter than the outer diameter of the coil spring 6, and the upper end of the cladding tube 1 is made of the same material as the cladding tube 1 and fitted around the bottom part 3b. A nuclear fuel element characterized in that an end plug 3 whose side surface is a spherical curved surface or a conical surface is fitted and welded so that the upper end opening of the cylindrical member 7 is brought into pressure contact with the bottom circumferential surface of the end plug 3. (2) The nuclear fuel element according to claim 1, wherein the bottom peripheral surface of the end plug 3 is surface-treated with hard chromium, titanium carbide, or the like. (3) The nuclear fuel element according to claim 1, wherein a counterbore is formed on the inner peripheral edge of the upper end opening of the cylindrical member 7.