JPS62191792A - Nuclear fuel element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention 1 (Field of Industrial Application) The present invention relates to improvements in nuclear fuel elements used within the core of a nuclear reactor.

(Prior Art) Currently, Zircaloy-2, Zircaloy-4, and zirconium-niobium alloys are generally used as fuel cladding tubes constituting nuclear fuel elements. Fuel cladding tubes made of these materials have extremely excellent performance during normal operation. This is because zirconium has a small neutron absorption cross section, is highly ductile, extremely stable and non-reactive in demineralized water and steam, commonly used as reactor coolants and moderators.

However, as the proportion of nuclear power generation in all power generators increases, there is a demand for nuclear power generation to be operated not as a base load, but to be operated freely following the load. For this reason, mechanical interaction between cladding and nuclear fuel pellets (PCI) was developed as a fuel that can withstand load following.
This is a high-performance fuel that uses a zirconium liner cladding tube that is relaxed with a pure zirconium layer. Such a zirconium liner clad tube has a structure in which the inner surface of a zirconium alloy tube such as Zircaloy-2 or Zircaloy-4 is lined with a pure zirconium layer, and many patents have been published including the registration of Patent No. 1,264,727.

These high-performance fuels with zirconium liner cladding are
It shows good PCI resistance even in actual reactors and is being used successfully.

By the way, the above-mentioned PCI was a problem related to the inner surface of the cladding tube, but for long-term use of fuel, it is also necessary to solve the problem of corrosion on the outer surface. In the current Zircaloy-2 and Zircaloy-4 cladding tubes, nodular corrosion in the form of white spots may occur on the outer surface at the end of the fuel use. As a method to solve nodular corrosion, a method has been proposed to change the microscopic structure to martensite by β-quenching. However, since the above-mentioned zirconium liner tube is lined with a pure zirconium layer on its inner surface, there is a problem that oxidation becomes severe due to β-quenching. For this reason, Japanese Patent Application Laid-Open No. 58-207349 discloses a method in which only the outer surface is hardened by internal water cooling. Although such a method is currently widely used, it has problems such as being impractical.

On the other hand, the inventors of the present invention filed a patent application in 1983 for the purpose of widening the martensite region to prevent nodular corrosion.
In No. 185098, we proposed a single-structure cladding tube in which niobium, molybdenum, manganese, etc. were added to Zircaloy. This cladding tube is a two-phase alloy, that is, β phase (b, c, c) in alloys containing 0.1% to 17.5% niobium etc. to Zircaloy.
It becomes a mixed phase of and α phase (h, C, p), and the β phase can improve nodular corrosion resistance. However, there was a problem in that the PCI resistance could not 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 clad tube that has both nodular corrosion resistance and PCI resistance, and has excellent adhesion strength between the outer tube and the liner layer. The aim is to provide a nuclear fuel element having the following characteristics.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a composite coating consisting of a zirconium alloy tube to which 0.05 to 5.00% by weight of niobium is added and a metallic zirconium barrier inserted into the alloy tube. This nuclear fuel element is comprised of a tube and a nuclear fuel material body inserted into the composite cladding tube.

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 iday%, the effect of the addition (improving effect on nodular corrosion resistance) cannot be exhibited;
This is because when the weight ratio is exceeded, the neutron absorption capacity increases. A preferable addition amount is in the range of 0.1 to 2.5% by weight.

The metallic zirconium barrier consists of substantially pure zirconium.

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

First, a hollow liner billet of pure zirconium is produced by forging, heat treating, and drilling an ingot obtained by vacuum arc melting with low-oxygen zirconium to create a large billet.
Furthermore, it is made into a small billet by hot extrusion. A hollow billet of niobium-added Zircaloy 2.4 or the like is made into a billet by vacuum arc melting, forging, heat treatment, β-quenching, and hole machining.

The cladding tube body billet and the liner billet are both hollow, and are fitted together after the inner surface of the cladding tube body billet and the outer surface of the liner billet are cleaned.

Subsequently, the boundary portions between the main billet and the liner billet on both end faces of the fitted composite billet are welded in vacuum by electron beam welding or laser beam welding.

For this welding, the end face of the composite billet is placed on a rotary table installed in a vacuum chamber so that it is perpendicular to the incident beam of the electron or laser, and the incident beam is placed at the boundary between the main billet and the liner billet on the end face of the composite billet. Welding is performed by moving a rotary table from outside the vacuum chamber so that the beam enters accurately. That is, the entire circumference of the boundary between the main billet and the liner billet is welded so that the center of the incident beam passes through the boundary.

Next, the composite billet, which has been preheated to a temperature of approximately 600 to 700° C., is hot extruded to obtain an intermediate product. In this hot extrusion process, the main body billet and liner billet are completely integrated by the diffusion of niobium (Nb>) over the entire boundary surface in the longitudinal direction of the composite billet.After this, the composite billet, which is the intermediate product, is A composite cladding tube made of niobium-added Zircaloy with a pure zirconium lining of about 80 to 100 μm in thickness was obtained by repeating the same process as the manufacturing method of ordinary composite cladding tubes, that is, rolling with a pilger mill and annealing. It will be done.

(Function) Nodular corrosion caused by high-temperature water and water vapor on the outer surface of the fuel cladding tube in a nuclear reactor is difficult to occur when the microscopic structure is such that fine precipitates are arranged mainly at grain boundaries, and relatively large precipitates are This is likely to occur when the particles are uniformly dispersed both within the grain and at the grain boundaries. Furthermore, since the precipitates become fine even in a quenched structure rapidly cooled from a high temperature, nodular corrosion resistance is improved.

Zircaloy-2 and Zircaloy-4 are in α phase (h,
C, p), and when a cladding tube is manufactured using conventional manufacturing techniques, relatively large precipitates are uniformly dispersed in the α phase. These Zircaloy-2 and Zircaloy-4 have β phases (b, c
, c) When niobium, which is a stable element, is added, it forms two phases, an α phase and a β phase, at room temperature, and the nodular corrosion resistance is improved by the inclusion of the β phase. In the present invention, the nodular corrosion resistance is improved by constructing the outer tube of the composite cladding tube from Zircaloy-2 and Zircaloy-4 to which a predetermined amount of niobium is added.

Furthermore, by providing the outer tube with a liner layer made of pure zirconium that acts as a barrier, the PCI resistance can be improved even if the outer tube of the cladding tube is made of niobium-added Zircaloy.

As mentioned above, by composing the outer tube with niobium-doped Zircaloy and the liner layer with pure zirconium, nodular corrosion resistance is achieved.
A highly reliable nuclear fuel element can be obtained by inserting a nuclear fuel material into a composite cladding tube that has both Johnston resistance and PCI resistance.

Furthermore, in the composite cladding constituting the nuclear fuel element of the present invention, the outer tube is made of niobium-doped zircaloy and the liner layer is made of pure zirconium, so that the niobium in the outer tube is made of pure zirconium in the liner layer during manufacture of the composite cladding. It diffuses into the zirconium, creating a diffusion layer with a niobium concentration gradient of about 5 μm, thereby dramatically improving the adhesion strength between the outer tube and the liner layer.

However, because the operating temperature inside the reactor is low, the niobium in the Zircaloy of the outer tube does not diffuse into the pure zirconium of the liner layer during use, and a deep diffusion layer is not generated in the liner layer. . Therefore, the present invention simply uses niobium-doped Zircaloy for the outer tube and pure zirconium for the liner layer to achieve nodular corrosion resistance and PC resistance that can be expected from these materials.
By combining the outer tube made of niobium-doped zircaloy and the liner layer made of pure zirconium, it is possible to obtain a composite cladding tube that not only improves the I properties but also improves the indentation strength between the outer tube and the liner layer described above. It is possible to achieve effects that are difficult to predict with conventional technology.

(Embodiments of the Invention) Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 and 2.

First, Zircaloy-2 (1,5% S ml -0,12
%1”e-0,05%Ni-0,1%Cr-Remainder Zr'
) with 0.2% niobium added and dissolved, β-quenched,
A hollow billet of niobium-added Zircaloy-2 was produced by forging and mechanical cutting.

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

Next, after hot extruding the composite tube, a pilger
Cold working was repeated using a tube drawing machine, and the finished shape was achieved through multiple passes. During this cold working, 580℃
Annealing was performed by heat treatment for 2 hours. Next, the cold-worked composite tube was heated at 600℃ for 2 hours.
A composite cladding tube 1 was manufactured by performing vacuum heat treatment. After that, the lower end plug 5b is inserted into the lower end opening of the composite cladding tube 1, the nuclear fuel pellets 4 are stored in the cladding tube 1, and the spring 6 is inserted into the upper opening of the cladding tube 1. Upper end plug 5a
A nuclear fuel element shown in FIGS. 1 and 2 was manufactured in which the

When the cross section of the composite cladding tube was subjected to line analysis using an X-ray microanalyzer, an explanatory diagram showing the elemental analysis as shown in FIG. 3 was obtained. In Fig. 3, the center line indicates the interface between the Zircaloy-2 outer tube and the pure zirconium liner layer, and the horizontal axis on the left side of the center line indicates the outer tube (Zry-2) made of Zircaloy-2, the right side The horizontal axis is the part of the liner layer (Zr) made of zirconium, and the vertical axis is the amount proportional to the concentration.

In the composite cladding tube manufactured in this example, as shown in FIG. 3, Nb in Zircaloy-2 forming the outer tube is replaced by MZr
It can be seen that diffusion progresses to the liner layer consisting of

In S n SF e 1Cr N N ', no diffusion was observed, and it seems that Zr and intermetallic compounds are formed at the interface, and there is no diffusion with the liner layer made of pure Zr. Nb
Due to the diffusion of Zr, the adhesion between the outer tube 2 and the liner layer 3 of the composite cladding tube of this example was significantly improved, and peeling of the pure Zr liner layer 3 did not occur in a normal bending test.

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

[Effects of the Invention] As detailed above, according to the present invention, the outer tube is made of niobium-doped zircaloy and the liner layer is made of pure zirconium, thereby achieving both nodular corrosion resistance and PCI resistance. It is possible to provide a highly reliable nuclear fuel element having an i-gold clad tube in which the adhesion strength between the tube and the liner layer is dramatically improved.

[Brief explanation of drawings]

Figure 1 is a longitudinal cross-sectional view showing a nuclear fuel element with nuclear fuel pellets inserted into a composite cladding tube, Figure 2 is an enlarged cross-sectional view of Figure 1, and Figure 3 is an X-ray at the interface between the outer tube and the liner layer. FIG. 3 is an explanatory diagram of elemental analysis using a microanalyzer. DESCRIPTION OF SYMBOLS 1... Composite cladding tube, 2... Outer tube, 3... Liner layer, 4... Nuclear fuel pellet, 5a, 5b... End plug, 6
···spring. Applicant's agent Patent attorney Takehiko Suzue Figure 1

Claims (3)

[Claims] (1) A composite cladding tube consisting of a zirconium alloy tube to which 0.05 to 5.00% by weight of niobium is added, 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 characterized in that it consists of: (2) A nuclear fuel element according to claim 1, characterized in that the barrier consists of substantially pure zirconium. (3) Zirconium alloy is zircaloy with niobium added.
The nuclear fuel element according to claim 1, characterized in that it is Zircaloy-2 or Zircaloy-4.