JPH0812259B2 - Nuclear fuel element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to improvements in nuclear fuel elements used in the core of a nuclear reactor.

(Prior Art) Currently, zircaloy-2 or zircaloy-4, zirconium-is commonly used as a fuel cladding tube constituting a nuclear fuel element.
Niobium alloy is used. The fuel cladding tube made of these materials has extremely excellent performance when operated under normal conditions. This is because zirconium has a small neutron absorption cross section, has strong ductility in demineralized water or steam commonly used as reactor coolant and moderator, and is extremely stable and non-reactive.

However, as the ratio of the reactor power generation to the total power generation increases, the reactor power generation is not required as a base load, but a free load following operation is demanded. Therefore, the mechanical interaction between the cladding and the nuclear fuel pellets (PCI) was developed as a fuel that can withstand load following.
Is a high-performance fuel that uses a zirconium liner cladding tube that is relaxed with a pure zirconium layer. Such a zirconium liner cladding tube has a structure in which a pure zirconium layer of a zirconloy-2 or zirconloy-4 zirconium alloy tube is lined, and many patents have been published including the registration of Japanese Patent No. 1264727. These high-performance fuels using zirconium liner cladding tubes show good PCI resistance even in actual furnaces and are being used successfully.

By the way, although the above-mentioned PCI is a problem related to the inner surface of the cladding tube, it is necessary to solve the corrosion of the outer surface when the fuel is used for a long time. The current Zircaloy-2 and Zircaloy-4 cladding tubes may have white spotted nodular corrosion on their outer surfaces at the end of fuel use. As a method of solving nodular corrosion, a method of making a microstructure into martensite by quenching β has been proposed. However, in the above-mentioned zirconium liner pipe, since the pure zirconium layer is lined on the inner surface, there is a problem that the β quenching causes severe oxidation. For this reason, JP-A-58-207349 discloses a method of quenching only the outer surface with internal water cooling. Although such a method is most often adopted at present, it has a problem that it is not practical.

On the other hand, the inventors of the present invention have disclosed a Japanese Patent Application No. 55-
In 185098, we proposed a cladding tube with a single structure in which niobium, molybdenum, manganese, etc. were added to zircaloy. This cladding tube is a two-phase alloy of zircaloy containing 0.1% to 17.5% niobium, that is, β phase (b, c, c) and α phase (h, h,
Being a mixed phase of c and p), the β phase can improve nodular corrosion resistance. However, there is a problem that PCI resistance cannot be improved with such a single-structured cladding tube.

(Problems to be Solved by the Invention) The present invention has been made in view of the above circumstances, and is a composite coating tube having both nodular corrosion resistance and PCI resistance, and excellent adhesion strength between the outer tube and the liner layer. It is intended to provide a nuclear fuel element having

[Structure of the Invention] (Means for Solving Problems) The present invention relates to a zirconium alloy tube containing 0.05 to 5.00% by weight of niobium, a composite cladding tube including a metal zirconium barrier inserted into the alloy tube, and A nuclear fuel material body inserted into the composite cladding tube, wherein a niobium diffusion layer is formed by niobium diffusion from the alloy tube in the barrier portion located near the interface between the alloy tube and the barrier. It is characterized by being That is,
Near the outer surface of the metal zirconium barrier that contacts the inner surface of the zirconium alloy tube, niobium diffusion having a concentration gradient is formed by diffusion of niobium of the alloy tube into the barrier, and the alloy tube and the barrier are integrated. It

Examples of the zirconium alloy include niobium-added Zircaloy-2 and Zircaloy-4.

The reason why the amount of niobium added to the zirconium alloy tube is limited to the above range is as follows. That is, if the amount of niobium added is less than 0.05% by weight,
The addition effect (the effect of improving the nodular corrosion resistance) cannot be exhibited, and the addition amount is 5.00% by weight.
This is because the neutron absorption capacity becomes higher when the value exceeds. The preferred addition amount is in the range of 0.1 to 2.5% by weight.

The metal zirconium barrier consists essentially of pure zirconium.

 Next, a method for manufacturing the composite cladding tube will be described.

First, a hollow zirconium purenlinet is obtained by forging, heat-treating, and hole-forming an ingot obtained by melting low-oxygen zirconium with a vacuum arc to form a large billet, and further hot extruding the billet into a small billet. Hollow billets such as zircaloys 2 and 4 with niobium added are subjected to vacuum arc melting, forging, heat treatment, β quenching, and hole processing to obtain billets.

Both the cladding tube body billet and the liner billet are hollow, and the inner surface of the coating tube body billet and the outer surface of the liner billet are cleaned and then fitted. Subsequently, the boundary portions between the main body billet and the line navilet on both end surfaces of the fitted composite billet are welded in vacuum by electron beam welding or laser beam welding. In this welding, the composite billet was installed on the turntable installed in the vacuum chamber so that the end surface of the composite billet was perpendicular to the incident beam of the electron or laser, and it was incident on the boundary part between the main billet and the linenavilet on the end surface of the composite billet. Welding is performed by moving the rotary table from the outside of the vacuum chamber so that the beam is accurately incident. That is, the entire circumference of the boundary is welded so that the boundary between the main body billet and the linenavit passes through the center of the incident beam.

Then, the composite billet preheated to a temperature of about 600 to 700 ° C. is hot extruded to obtain an intermediate product. In this hot extrusion process, the main billet and the linen billet are completely integrated by diffusion of niobium (Nb) over the entire boundary surface in the length direction of the composite billet.
After that, the intermediate product, the composite, was subjected to the same steps as those for producing a conventional composite cladding tube, that is, rolling by a Pilger mill and annealing were repeated, and pure zirconium having a thickness of about 80 to 100 μm was lined on the inner surface thereof to obtain niobium added. A composite cladding tube made of Zircaloy is obtained.

(Function) Nodular corrosion due to high-temperature water and steam on the outer surface of the fuel clad tube in the nuclear reactor is difficult to occur when fine precipitates are mainly present in the grain boundaries in the microstructure, and relatively large precipitates Is likely to occur when the particles are uniformly dispersed within the particles and without passing through the grain boundaries. In addition, since the precipitates become finer even in a quenched structure that is rapidly cooled from a high temperature, the nodular corrosion resistance becomes good.

Zircaloy-2 and Zircaloy-4 form an α phase (h, c, p) at room temperature, and when a cladding tube is manufactured by a conventional manufacturing technique, relatively large precipitates are uniformly dispersed in the α phase. . When niobium, which is a β-phase (b, c, c) stable element, is added to such Zircaloy-2 and Zircaloy-4, it becomes two phases of α phase and β phase at room temperature, and the inclusion of β phase results in nodular corrosion resistance. Be improved. In the present invention, the nodular corosion resistance is improved by forming the outer tube of the composite clad tube with Zircaloy-2 and Zircaloy-4 containing a predetermined amount of niobium.

Further, by providing the outer tube with a liner layer made of pure zirconium as a barrier, the PCI resistance can be improved even if the outer tube of the cladding tube is formed of zircaloy containing niobium.

As described above, by forming the outer tube with niobium-added zircaloy and forming the liner layer with pure zirconium, it has a composite cladding tube that has both nodular corrosion resistance and PCI resistance. A reliable nuclear fuel element can be obtained.

Further, in the composite cladding tube constituting the nuclear fuel element of the present invention, the outer tube is made of niobium-added Zircaloy and the liner layer is made of pure zirconium, so that the niobium in the outer tube becomes a pure liner layer during the production of the composite cladding tube. It diffuses into zirconium, and a diffusion layer having a niobium concentration gradient is generated at about 5 μm, and the adhesion strength between the outer tube and the liner layer can be dramatically improved. However, since the operating temperature in the reactor is low, the niobium in the outer tube zircaloy does not diffuse into the pure zirconium in the liner layer during use, and the liner layer does not form a deep diffusion layer. . Therefore, the present invention is not only to improve the outer tube of niobium-added zircaloy, the nodular corrosion resistance that can be expected from those materials with a pure zirconium liner layer, to improve the PCI resistance, the outer tube of those niobium-added zircaloy and By combining with the liner layer of pure zirconium, it is possible to obtain an effect which is difficult to predict in the conventional technique that the composite cladding tube with improved adhesion strength between the outer tube and the liner layer is obtained.

Embodiments of the Invention Embodiments of the present invention will be described below with reference to FIGS. 1 and 2.

First, Zircaloy-2 (1.5% Sn-0.12% Fe-0.05% N
A hollow billet of niobium-added Zircaloy-2 was prepared by adding 0.2% niobium to (i-0.1% Cr-remainder Zr) and dissolving it, and performing β-quenching, forging, and machine cutting.

Next, after cleaning the surfaces of the hollow billet (outer tube) 2 and the pure zirconium sleeve (liner layer) 3 previously manufactured, these were inserted and combined. Subsequently, the boundary line between the outer tube 2 and the liner layer 3 was welded in a vacuum by electron beam welding.

Next, after hot-extruding the composite pipe, cold working was repeated with a Bilger pipe squeezing machine to obtain a finished shape through a plurality of passes. In between the cold work, 580
Annealing was performed by heat treatment at 2 ° C. for 2 hours. Next, the cold-worked composite pipe was heated at 600 ° C for 2 hours,
A vacuum heat treatment was performed to manufacture the composite cladding tube 1. The resulting composite cladding tube had an overall wall thickness of about 860 μm and a liner layer thickness of about 80-90 μm. After this, a lower end plug 5b is inserted into the lower end opening of the composite cladding tube 1, the nuclear fuel pellet 4 is housed in the cladding tube 1, and a spring 6 is inserted in the upper opening of the cladding tube 1. The nuclear fuel element shown in FIGS. 1 and 2 with the upper end plug 5a inserted was manufactured.

Then, line analysis of the cross section of the composite cladding tube was performed by an X-ray microanalyzer, and an explanatory diagram showing the elemental analysis of FIG. 3 was obtained. In FIG. 3, the center line indicates the interface between the outer tube of Zircaloy-2 and the liner layer of pure zirconium, and the horizontal axis on the left side of the center line is the outer tube made of Zircaloy-2 (Zry-2), the right side. The abscissa represents the liner layer (Zr) part made of zirconium, and the ordinate represents the amount proportional to the concentration.

In the composite cladding tube produced in this example, as shown in FIG. 3, it can be seen that Nb in Zircaloy-2 forming the outer tube has diffused into the liner layer made of pure Zr. Sn,
In Fe, Cr, and Ni, no diffusion was observed, and it seems that Zr and intermetallic compound were formed at the interface, respectively, and did not diffuse into the liner layer made of pure Zr. Due to the diffusion of Nb, the adhesion between the outer tube 2 and the liner layer 3 was markedly improved in the composite cladding tube of this example, and the pure Zr liner layer 3 was not peeled off in the usual bending test.

Moreover, the composite cladding tube of this example had both nodular corrosion resistance and PCI resistance.

[Effects of the Invention] As described in detail above, according to the present invention, the outer tube is made of niobium-added zircaloy and the liner layer is made of pure zirconium, so that it has both nodular corrosion resistance and PCI resistance. It is possible to provide a highly reliable nuclear fuel element having a composite cladding tube in which the adhesion strength between the tube and the liner layer is dramatically improved.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view showing a nuclear fuel element in which a nuclear fuel pellet is inserted in a composite cladding tube, FIG. 2 is an enlarged transverse sectional view of FIG. 1, and FIG. 3 is an X-ray at an interface between an outer tube and a liner layer. It is explanatory drawing of the elemental analysis by a microanalyzer. 1 ... Composite cladding tube, 2 ... Outer tube, 3 ... Liner layer, 4 ...
… Nuclear fuel pellets, 5a, 5b …… End plugs, 6 …… Springs.

Claims (3)

[Claims] 1. A composite cladding tube comprising a zirconium alloy tube containing 0.05 to 5.00% by weight of niobium and a metal zirconium barrier inserted into the alloy tube, and a nuclear fuel material body inserted into the composite cladding tube. A nuclear fuel element, wherein a niobium diffusion layer is formed by niobium diffusion from the alloy tube in the barrier portion located near an interface between the alloy tube and the barrier. 2. A nuclear fuel element according to claim 1, wherein the barrier is composed of substantially pure zirconium. 3. The nuclear fuel element according to claim 1, wherein the zirconium alloy is zircaloy-2 or zircaloy-4 containing niobium.