JPS6125117B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a test nuclear fuel element capable of detecting properties of nuclear fuel material such as exothermic temperature, amount of neutron production, and pressure of released gas.

When examining the various properties of nuclear fuel elements, it is usually done through irradiation tests, and it is necessary to measure the exothermic temperature of the nuclear fuel material, the amount of neutrons produced, and the pressure of fission products and gases within the cladding tube. For example, such a test nuclear fuel element has the configuration shown in FIG. 1. That is, in Figure 1,
After compacting oxide nuclear fuel powder into a cylindrical shape in a stainless steel cladding tube 1, a plurality of sintered fuel pellets 2 are loaded in the axial direction, and both ends of the cladding tube 1 are connected with end plugs 3, It is sealed at 4. Furthermore, the first
In Figure a, an elongated member 6 such as a temperature measuring element or a neutron detection element is inserted into a through hole 5 provided in the center of the upper end plug 3, and in Figure 1b, a side surface of the upper end plug 3 is inserted. An elongated member 6 is fixed to. Note that 7 in the figure is a plenum chamber, and a spring 8 is inserted in this plenum chamber 7 to prevent the pellet 2 from moving. The elongated member 6 also measures the temperature, pressure, amount of radiation, coolant temperature, etc. inside the pellet. Here, the cladding tube 1 in FIG.
The end plugs 3 and 4 have the function of preventing radioactive substances produced by nuclear fission from leaking to the outside. Therefore, if a member 6 is provided that penetrates the end plug 3, the insertion portion must be hermetically sealed by some means. Moreover, in the case of FIG. 1b, it is necessary to firmly fix the elongated member 6 so that it does not break and damage the coolant passage element.

As described above, possible means for fixing the end plug 3 and the member 6 include directly welding them, brazing them, or mechanically tightening them. For example, in a nuclear fuel element in which the temperature at the center of a pellet 2 is to be measured, means are employed to mechanically tighten and seal the inserted portion. However, with mechanical sealing means, internal radioactive substances may leak from the insertion part or external coolant may leak when thermal stress cycles such as temperature rises and falls or external mechanical forces are applied. The disadvantage is that troubles such as contamination of fuel elements may occur.

Further, when brazing is used, special brazing equipment must be used to perform brazing in an airtight chamber burner, and the brazing temperature must be delicately controlled. Therefore, the work becomes very complicated and the reliability of the product is also reduced.

On the other hand, as the welding means, for example, laser welding or electron beam welding, which is suitable for welding thin members such as thermocouples, is used. However, with these welding methods, high energy density is concentrated in a narrow portion, and in materials that are prone to hot cracking, such as austenite structures, cracks often occur in the welded portion. On the other hand, since fuel cladding tubes and end plugs are used at high temperatures for long periods of time, they are manufactured using austenitic stainless steel and controlled to reduce ferrite in consideration of strength. actually,
Austenitic stainless steel SUS316 for nuclear reactors
There are cases where when laser welding was performed, many microscopic cracks occurred and airtight welding was not possible. Furthermore, such cracks significantly reduce the reliability of the strength of the welded portion.

The present invention has been made in view of the above circumstances, and does not employ mechanical means as a means for inserting and fixing the elongated member to be inserted into the fuel element, but uses commonly used welding to prevent cracking. An object of the present invention is to provide a test nuclear fuel element that can be tightly sealed or fixed without causing any damage.

That is, the present invention relates to a test nuclear fuel having both ends of a cladding tube loaded with nuclear fuel material sealed with end plugs, and having an elongated member inserted through the end plugs or on the outer surface of the end plugs. In the element, an intermediate member that can be welded to the end plug and the elongated member and contains at least 1.0% of δ-ferrite is used between the end plug and the elongated member, and the elongated member is welded by laser welding or electron beam welding through the intermediate member. This test nuclear fuel element is characterized by being welded by energy beam welding.

Embodiments of the present invention will be described below with reference to FIGS. 2 and 3. Note that FIGS. 2 and 3 show only the main parts of the present invention in an enlarged manner, and other components are omitted because they are the same as those of the conventional example shown in FIG. 1. Therefore, only the main parts will be explained, and other explanations will be omitted.

In Figure 2 a, the outer diameter is 6.5 mm and the wall thickness is 0.47 mm.
An end plug 3 made of the same material is fitted into a cladding tube 1 made of 316 stainless steel. The tip 11 of this end plug 3 is made of SUS316 welding rod material (JIS standard) containing about 2% ferrite.
D316) is welded by plasma welding.
After the fitting portion of the end plug 3 with the intermediate member 11 and the cladding tube 1 are airtightly connected by TIG welding, the end plug 3
An elongated member, that is, a thermocouple protection tube 9 coated with 316 stainless steel with a diameter of 1.6 mm and a wall thickness of 0.19 mm, is inserted into the hole 5 drilled through the intermediate member 11 in the center of the The tip portion 12 and the thermocouple protection tube 6 were hermetically welded together by laser welding. When laser welding a welding rod material containing approximately 2% ferrite and steel with an almost completely austenitic structure, no cracks were observed even under a microscope, and the diameter was 1.6 mm.
It has been confirmed that the airtightness of laser welded thin-walled tubes with a wall thickness of 0.19 mm is sufficient in He leak tests.

Similarly, FIG. 2b shows an example in which the elongated member 6 is welded to the side surface of the end plug 3 via the intermediate member 11.

It has been confirmed that the intermediate member used in the example shown in FIG. 2 according to the above explanation contains about 2% ferrite, and the diagram shown in FIG. The amount of ferrite was confirmed to be approximately 2%.

As explained above, according to the present invention, elongated members such as thermocouple protection tubes can be welded without causing cracks in austenitic steel, which is generally used as a material for nuclear reactors, using commercially available laser welding equipment. It becomes possible to do so.

In addition, in the above embodiment of the present invention, a welding rod containing about 2% ferrite was used as the intermediate member, but this was done by taking advantage of the fact that the presence of δ-ferrite in the austenitic weld metal has the effect of reducing the tendency for cracking to occur. , a material containing at least 1.0% of δ-ferrite may be used as the intermediate member.

Incidentally, instead of the cylindrical pellets, powdered, granular, or lumpy nuclear fuel material may be used as the nuclear fuel material. In addition, for irradiation tests, reactor components are sometimes housed in cladding tubes and irradiated inside the reactor, but it can also be applied to attaching elongated members for characteristic measurement to such irradiation test elements. Of course.

[Brief explanation of the drawing]

Figures 1a and b are longitudinal cross-sectional views showing a conventional nuclear fuel element for testing with some parts omitted, and Figures 2a and b are enlarged views of only the main parts of the nuclear fuel element for testing according to the present invention. Longitudinal sectional view showing an example, No. 3
The figure is a Schiffler structure diagram showing the relationship between Ni equivalent and Cr equivalent. 1...Claying tube, 2...Fuel pellets, 3, 4...
...End plug, 5...Through hole, 6...Elongated member, 7...
Plenum, 8... Spring, 11... Intermediate member, 12... Welded portion.

Claims (1)

[Claims] 1. Test equipment that seals both ends of a cladding tube loaded with nuclear fuel material with end plugs and has an elongated member passing through the end plugs or on the outer surface of the end plugs for detecting the characteristics of the nuclear fuel element. In nuclear fuel elements,
between the elongated member and the end plug holding the elongated member, δ-ferrite is weldable to both;
The end plug and the intermediate member are welded by inserting an intermediate member containing 1.0% or more as a separate member, and the intermediate member and the elongated member are further welded together by high energy beam welding. Test nuclear fuel elements.