Method for making Zr alloy nuclear reactor fuel cladding having excellent corrosion resistance and creep properties

Abstract

The invention provides a method for making Zr alloy nuclear reactor fuel cladding having excellent corrosion resistance and creep properties. The method includes performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling of a Zr alloy including, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr and 0.05 to 1.0% Nb, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon. The annealing is performed at a temperature of 550.degree. C. to 850.degree. C. for 1 to 4 hours such that the accumulated annealing parameter .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies relationships -20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb -0.2), wherein Ai represents the annealing parameter for the i-th annealing, ti represents the annealing time (hours) for the i-th annealing, Ti represents the annealing temperature (K) for the i-th annealing, and X.sub.Nb represents the Nb concentration (wt %).

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for making Zr alloy cladding for nuclear reactor fuel. The Zr alloy cladding has excellent corrosion resistance when it is exposed to hot high-pressure water or steam, and has excellent creep properties.

2. Description of the Related Art

A general type of nuclear reactor is a pressurized water reactor (PWR). A cladding tube for reactor fuel used in this type of reactor is made of a Zr alloy. A typical Zr alloy used in such a cladding tube is Zircaloy-4, which is composed of (hereinafter percentages are percent by weight) 1.2 to 1.7% Sn, 0.18 to 0.24% Fe, and 0.07 to 0.13% Cr, with the balance being Zr and incidental impurities.

Nb- or Nb/Ta-containing Zr alloys having excellent corrosion resistance have also been proposed for cladding tubes. The Nb-containing Zr alloy is composed of 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, and 0.05 to 1.0% Nb, with the balance being Zr and incidental impurities, wherein the nitrogen content as an incidental impurity is 60 ppm or less. The Nb/Ta-containing Zr alloy is composed of 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, 0.05 to 1.0% Nb and 0.01 to 0.1% Ta, with the balance being Zr and incidental impurities, wherein the nitrogen content as an incidental impurity is 60 ppm or less.

The profitability of a nuclear power plant can be increased by reacting fuel for longer periods of time. This requires increasing the residence time of fuel cladding tubes in reactors. However, the above-mentioned cladding tubes made of Nb-containing or Nb/Ta-containing Zr alloys do not have sufficient durability to meet long reactor residence time requirements because these alloys do not have sufficiently high corrosion resistance and sufficiently low creep properties. Thus, there is a need for high durability nuclear fuel cladding.

SUMMARY OF THE INVENTION

The present inventors have researched a method for making Zr alloy nuclear reactor fuel cladding tube having corrosion resistance and creep properties that are superior to those of conventional cladding tubes. The present inventors have discovered that the corrosion resistance and creep properties of Zr alloy cladding tube made from a conventional Nb- or Nb/Ta-containing Zr alloy can be improved by controlling heat treatment conditions, in particular annealing conditions, when making the cladding tube. The resulting Zr alloy cladding can have a long service life.

A first aspect of the present invention is a method for making Zr alloy nuclear reactor fuel cladding having excellent corrosion resistance and creep properties, comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold roiling of a Zr alloy including, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr and 0.05 to 1.0% Nb, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

the annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 and about 4 hours such that the accumulated annealing parameter .SIGMA.Ai represented by .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies the relationships: -20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb -0.2), wherein Ai represents the annealing parameter for the i-th annealing, ti represents the annealing time (hours) for the i-th annealing, Ti represents the annealing temperature (K) at the i-th annealing, and X.sub.Nb represents the Nb concentration (wt %).

A second aspect of the present invention is a method for making Zr alloy nuclear reactor fuel cladding having excellent corrosion resistance and creep properties, comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling of a Zr alloy including, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr and 0.05 to 1.0% Nb, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

the annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 and about 4 hours such that the accumulated annealing parameter .SIGMA.Ai represented by .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies the relationships: -20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb -0.2), wherein Ai represents the annealing parameter for the i-th annealing, ti represents the annealing time (hours) for the i-th annealing, Ti represents the annealing temperature (K) at the i-th annealing, and X.sub.Nb represents the Nb concentration (wt %); and the accumulated annealing parameter .SIGMA.Ai further satisfies the relationships: wherein 0.05.ltoreq.X.sub.Nb .ltoreq.0.5, -20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb -0.2); and when 0.5<X.sub.Nb, -20.ltoreq.log.SIGMA.Ai.ltoreq.-18-2.multidot.(X.sub.Nb -0.5).

A third aspect of the present invention is a method for making a Zr alloy nuclear reactor fuel cladding having excellent corrosion resistance and creep properties, comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling of a Zr alloy including, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, 0.05 to 1.0% Nb and 0.01 to 0.1% Ta, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

the annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 and about 4 hours such that the accumulated annealing parameter .SIGMA.Ai represented by .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies the relationships: -20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb+Ta -0.2), wherein Ai represents the annealing parameter for the i-th annealing, ti represents the annealing time (hours) for the i-th annealing, Ti represents the annealing temperature (K) at the i-th annealing, and X.sub.Nb+Ta represents the total concentration of Nb and Ta (wt %).

A fourth aspect of the present invention is a method for making Zr alloy nuclear reactor fuel cladding having excellent corrosion resistance and creep properties, comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling of a Zr alloy including, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, 0.05 to 1.0% Nb and 0.0 1 to 0.1% Ta, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

the annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 and about 4 hours such that the accumulated annealing parameter .SIGMA.Ai represented by .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies the relationships: -20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb+Ta -0.2), wherein Ai represents the annealing parameter for the i-th annealing, ti represents the annealing time (hours) for the i-th annealing, Ti represents the annealing temperature (K) at the i-th annealing, and X.sub.Nb+Ta represents the total concentration of Nb and Ta (wt %); and the accumulated annealing parameter .SIGMA.Ai further satisfies the relationships: when 0.05.ltoreq.X.sub.Nb+Ta .ltoreq.0.5, -20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and X.sub.Nb+Ta -18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-10.multidot.(X.sub.Nb+Ta -0.2); and when 0.5<X.sub.Nb+Ta -20.ltoreq.log.SIGMA.Ai.ltoreq.-18-2.multidot.(X.sub.Nb+Ta -0.5).

In these aspects, it is preferable that annealing prior to the final cold rolling be performed in a range of temperatures between about 650.degree. C. and about 770.degree. C. for a period of time between about 1 and about 10 minutes followed by quenching by argon gas.

In addition, in these aspects, it is preferable that the various steps are preformed sequentially.

A fifth aspect of the present invention is Zr alloy nuclear reactor fuel cladding, having excellent corrosion resistance and creep properties, made by one of the methods described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the range of annealing conditions in a method for making Zr alloy cladding in accordance with the present invention; and

FIG. 2 is a graph of the range of annealing conditions in a method for making Zr alloy cladding in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, Zr alloy cladding for nuclear reactor fuel is produced through steps of melting, ingot forging, solution heat treatment, hot extrusion, repeated cycles of annealing and cold rolling, annealing prior to final cold rolling, final cold rolling, and then final stress relief annealing.

The ingot forging is hot forging to decompose the cast structure and is performed in a range of temperature between about 800.degree. C. and about 1,100.degree. C. The solution heat treatment is performed by holding the forged alloy in a range of temperatures between about 1,000.degree. C. and about 1,100.degree. C. and then cooling the alloy with water so as to eliminate microscopic segregation of elements in the alloy. In the hot extrusion, the Zr alloy is heated in a range of temperatures between about 600.degree. C. and about 800.degree. C. and then extruded to form a seamless tube. The annealing is generally performed in a vacuum furnace, after the hot extrusion and before the subsequent cold rolling. The subsequent cold rolling is generally performed using a pilger mill in the case of the Zr alloy cladding. The final stress relief annealing is generally performed in a range of temperatures between about 450.degree. C. and about 500.degree. C. for a period of time between about 1 and about 4 hours so as to relieve the stress in the Zr alloy cladding.

The method for making the Zr alloy cladding in accordance with the present invention is characterized as follows. When the alloy has a composition of 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr and 0.05 to 1.0% Nb, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less, the annealings after the hot extrusion and before the final cold rolling are performed so as to satisfy the conditions described in the above-mentioned first or second aspect. When the alloy has a composition of 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, 0.05 to 1.0% Nb and 0.01 to 0.1% Ta, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less, the annealings after the hot extrusion and before the final cold rolling are performed so as to satisfy the conditions described in the above-mentioned third or fourth aspect.

The accumulated annealing parameter .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) must satisfy the relationship -20.ltoreq.log.SIGMA.Ai.ltoreq.-15. When -20.ltoreq.log.SIGMA.Ai, the Zr alloy cladding is fully annealed as a result of the repeated annealing steps. Since the repeated annealing steps must be performed within the .alpha.-zirconium phase region, log.SIGMA.Ai.ltoreq.-15 must be satisfied.

The accumulated annealing parameter .SIGMA.Ai is greatly affected by the Nb concentration (X.sub.Nb) and the total concentration of Nb+Ta (X.sub.Nb+Ta). Thus, the following conditions must also be satisfied:

-18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb -0.2) for the Nb-containing Zr alloy, or -18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb+Ta -0.2) for the Nb/Ta-containing Zr alloy, wherein the concentrations X.sub.Nb and X.sub.Nb+Ta are represented by percent by weight.

It is preferred that the accumulated annealing parameter .SIGMA.Ai for the Nb-containing Zr alloy be defined in more detail as follows, because it significantly depends on the concentrations X.sub.Nb.

When 0.05.ltoreq.X.sub.Nb .ltoreq.0.5, the annealing is performed so as to satisfy the relationships:

-20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-10.multidot.(X.sub.Nb -0.2), or when 0.5<X.sub.Nb, the annealing is performed so as to satisfy the relationships: -20.ltoreq.log.SIGMA.Ai.ltoreq.-18-2.multidot.(X.sub.Nb -0.5).

Similarly, it is preferred that the accumulated annealing parameter .SIGMA.Ai for the Nb/Ta-containing Zr alloy be defined in more detail as follows.

When 0.05.ltoreq.X.sub.Nb+Ta .ltoreq.0.5, the annealing is performed so as to satisfy the relationships:

-20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and -18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-10.multidot.(X.sub.Nb+Ta -0.2), or when 0.5<X.sub.Nb+Ta, the annealing is performed so as to satisfy the relationships: -20.ltoreq.log.SIGMA.Ai.ltoreq.-18-2.multidot.(X.sub.Nb+Ta -0.5).

FIG. 1 is a graph showing a preferred annealing range for producing Nb-containing Zr alloy cladding for nuclear reactor fuel. The Nb-containing Zr alloy has a composition of 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr and 0.05 to 1.0% of Nb, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less. In FIG. 1, the horizontal axis indicates log.SIGMA.Ai and the vertical axis indicates the Nb concentration (X.sub.Nb). The coordinates of dots A, B, C, D, E, F, G, H, and I are set as follows:

A(log.SIGMA.Ai=-15, X.sub.Nb =0.05), B(log.SIGMA.Ai=-15, X.sub.Nb =0.2), C(log.SIGMA.Ai=-18, X.sub.Nb =1.0), D(log.SIGMA.Ai=-20, X.sub.Nb =1.0), E(log.SIGMA.Ai=-20, X.sub.Nb =0.2), F(log.SIGMA.Ai=-18.5, X.sub.Nb =0.05), G(log.SIGMA.Ai=-18, X.sub.Nb =0.5), H(log.SIGMA.Ai=-19, X.sub.Nb =1.0), and I(log.SIGMA.Ai=-20, X.sub.Nb =0.5)

The annealing range in the method for making the Nb-containing Zr cladding in accordance with the first aspect of the present invention is defined by the area surrounded with lines AB, BC, CD, DE, EF, and FA in FIG. 1.

The annealing range in the method for making the Nb-containing Zr cladding in accordance with the second aspect of the present invention is defined by the area surrounded with lines AB, BG, GI, IE, EF, and FA for 0.05.ltoreq.X.sub.Nb .ltoreq.0.5, or the area surrounded with lines GH, HD, DI, and IG for 0.5<X.sub.Nb.

FIG. 2 is a graph showing a preferred annealing range for producing Nb/Ta-containing Zr alloy cladding for nuclear reactor fuel The Nb/Ta-containing Zr alloy has a composition of 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, 0.05 to 1.0% Nb and 0.01 to 0.1% Ta, with the remainder being Zr and incidental impurities, and the incidental nitrogen impurity content being 60 ppm or less. In FIG. 2, the horizontal axis indicates log.SIGMA.Ai and the vertical axis indicates the total concentration of Nb and Ta (X.sub.Nb+Ta). The coordinates of dots J, K, L, M, N, O, P, Q, and R are set as follows:

J(log.SIGMA.Ai=-15, X.sub.Nb+Ta =0.05), K(log.SIGMA.Ai=-15, X.sub.Nb+Ta =0.2), L(log.SIGMA.Ai=-18, X.sub.Nb+Ta =1.0), M(log.SIGMA.Ai=-20, X.sub.Nb+Ta =1.0), N(log.SIGMA.Ai=-20, X.sub.Nb+Ta =0.2), O(log.SIGMA.Ai=-18.5, X.sub.Nb+Ta =0.05), P(log.SIGMA.Ai=-18, X.sub.Nb+Ta =0.5), Q(log.SIGMA.Ai=-19, X.sub.Nb+Ta =1.0), and R(log.SIGMA.Ai=-20, X.sub.Nb+Ta =0.5)

The annealing range in the method for making the Nb/Ta-containing Zr cladding in accordance with the third aspect of the present invention is defined by the area surrounded with lines JK, KL, LM, MN, NO, and OJ in FIG. 2.

The annealing range in the method for making the Nb/Ta-containing Zr cladding in accordance with the fourth aspect of the present invention is defined by the area surrounded with lines JK, KP, PR, RN, NO, and OJ for 0.05.ltoreq.X.sub.Nb+Ta .ltoreq.0.5, or the area surrounded with lines PQ, QM, MR, and RP for 0.5<X.sub.Nb+Ta.

In the more preferred embodiments of the method for making the Zr alloy cladding in accordance with the present invention, satisfying the annealing conditions shown in FIG. 1 or FIG. 2, the annealing prior to the final cold rolling is performed in a range of temperatures between about 650.degree. C. and about 770.degree. C. for a significantly short time period of between about 1 and about 10 minutes, and then the cladding is quenched with argon gas. This annealing is accomplished by introducing a reduced volume of cladding into a vessel maintained at a high temperature in order to rapidly heat the cladding and then introducing high-purity argon gas into the vessel to rapidly cool the cladding.

The ground for limiting the range of the composition of the Zr alloy in accordance with the present invention are now described.

(A) Sn

Tin (Sn) improves the strength of the alloy when the content is 0.2% or more. On the other hand, the corrosion resistance of the alloy significantly decreases when the content is higher than 1.7%. Thus, the Sn content is set to 0.2 to 1.7%.

(B) Fe and Cr

A combination of these components contributes to improvements in corrosion resistance and creep properties of the alloy when the Fe content is 0.18% or more and the Cr content is 0.07% or more. The corrosion resistance, however, significantly decreases when the Fe content is higher than 0.6% and the Cr content is higher than 0.4%. Thus, the Fe content is set to 0.18 to 0.6%, and the Cr content is set to 0.07 to 0.4%.

(C) Nb and Ta

These components contribute to further improvements in corrosion resistance and creep properties of the alloy when the Nb content is 0.05% or more or the Ta content is 0.01% or more. The corrosion resistance, however, significantly decreases when the Nb content is higher than 1.0% or the Ta content is higher than 0.1%. Thus, the Nb content is set to 0.05 to 1.0%, and the Ta content is set to 0.01 to 0.1%.

(D) N as an incidental impurity

Nitrogen (N) is a significantly harmful component and deteriorates corrosion resistance of the alloy. Since the corrosion resistance significantly decreases when the N content is higher than 60 ppm, the upper limit of the N content is set to 60 ppm.

The method for making the Zr alloy cladding for a nuclear reactor fuel in accordance with the present invention will now be described in more detail based on the following examples.

EXAMPLES

Zr alloy ingots 1 to 64 having compositions shown in Tables 1 to 6 are prepared as follows. A Zr sponge with a purity of 99.8%, and granular Sn, Fe, Cr, Nb and Ta each having a purity of 99.9% or more are compounded based on the compositions to form electrodes. Electrodes are melted in a consumable electrode-type vacuum arc furnace to form Zr alloy ingots.

TABLE 1______________________________________Composition (% by weight)(Balance: Zr and incidental impurities)Type Sn Fe Cr Nb Ta N (ppm)______________________________________Zr 1 1.13 0.22 0.11 0.053 -- 32alloy 2 1.02 0.22 0.10 0.11 -- 29ingot 3 1.66 0.21 0.11 0.21 -- 25 4 1.21 0.21 0.11 0.22 -- 30 5 0.95 0.183 0.11 0.21 -- 34 6 0.97 0.37 0.11 0.20 -- 28 7 0.99 0.58 0.11 0.19 -- 31 8 1.01 0.21 0.075 0.20 -- 26 9 0.92 0.21 0.20 0.22 -- 33 10 0.97 0.21 0.38 0.21 -- 29 11 0.98 0.44 0.23 0.22 -- 30______________________________________ TABLE 2______________________________________Composition (% by weight)(Balance: Zr and incidental impurities)Type Sn Fe Cr Nb Ta N (ppm)______________________________________Zr 12 0.94 0.58 0.39 0.20 -- 35alloy 13 0.75 0.20 0.10 0.19 -- 32ingot 14 0.49 0.187 0.11 0.22 -- 33 15 0.49 0.39 0.11 0.21 -- 27 16 0.49 0.57 0.11 0.19 -- 41 17 0.49 0.21 0.078 0.20 -- 30 18 0.49 0.21 0.22 0.21 -- 55 19 0.49 0.21 0.39 0.20 -- 28 20 0.49 0.41 0.22 0.20 -- 25 21 0.49 0.58 0.38 0.22 -- 31 22 0.24 0.21 0.11 0.20 -- 34______________________________________ TABLE 3______________________________________Composition (% by weight)(Balance: Zr and incidental impurities)Type Sn Fe Cr Nb Ta N(ppm)______________________________________Zr 23 0.85 0.20 0.09 0.36 -- 26alloy 24 0.77 0.184 0.11 0.49 -- 33ingot 25 0.81 0.38 0.11 0.48 -- 29 26 0.80 0.57 0.11 0.47 -- 30 27 0.80 0.19 0.075 0.49 -- 57 28 0.79 0.19 0.20 0.48 -- 40 29 0.77 0.19 0.36 0.49 -- 31 30 0.78 0.41 0.20 0.47 -- 28 31 0.81 0.58 0.37 0.49 -- 33 32 0.49 0.20 0.11 0.49 -- 30 33 0.21 0.183 0.11 0.48 -- 28______________________________________ TABLE 4______________________________________Composition (% by weight)(Balance: Zr and incidental impurities)Type Sn Fe Cr Nb Ta N (ppm)______________________________________Zr 34 0.22 0.38 0.11 0.49 -- 27alloy 35 0.20 0.57 0.11 0.48 -- 31ingot 36 0.21 0.20 0.076 0.49 -- 30 37 0.22 0.20 0.19 0.49 -- 35 38 0.23 0.20 0.38 0.48 -- 27 39 0.21 0.40 0.21 0.49 -- 29 40 0.22 0.59 0.38 0.49 -- 32 41 0.77 0.21 0.11 0.75 -- 28 42 0.52 0.21 0.11 0.75 -- 27 43 1.01 0.19 0.10 0.98 -- 33 44 0.23 0.19 0.10 0.99 -- 35______________________________________ TABLE 5______________________________________Composition (% by weight)(Balance: Zr and incidental impurities)Type Sn Fe Cr Nb Ta N (ppm)______________________________________Zr 45 1.15 0.21 0.10 0.051 0.013 25alloy 46 1.65 0.20 0.10 0.15 0.06 29ingot 47 0.95 0.183 0.11 0.16 0.05 28 48 0.97 0.38 0.12 0.12 0.09 35 49 0.97 0.57 0.09 0.18 0.02 31 50 0.99 0.20 0.074 0.15 0.05 56 51 1.01 0.20 0.19 0.13 0.08 45 52 0.96 0.21 0.39 0.17 0.02 34 53 0.98 0.43 0.21 0.14 0.06 27 54 0.97 0.59 0.38 0.11 0.09 30 55 0.25 0.20 0.10 0.15 0.05 31______________________________________ TABLE 6______________________________________Composition (% by weight)(Balance: Zr and incidental impurities)Type Sn Fe Cr Nb Ta N (ppm)______________________________________Zr 56 0.79 0.182 0.11 0.48 0.01 28alloy 57 0.77 0.37 0.12 0.43 0.05 26ingot 58 0.81 0.58 0.12 0.41 0.08 33 59 0.80 0.20 0.073 0.47 0.02 52 60 0.79 0.21 0.21 0.44 0.05 28 61 0.80 0.20 0.37 0.40 0.08 40 62 0.78 0.43 0.19 0.45 0.03 29 63 0.79 0.57 0.36 0.43 0.06 30 64 0.26 0.20 0.11 0.93 0.05 32______________________________________

Example 1

Zr alloy ingots 1 to 64 shown in Tables 1 to 6 are subjected to forging at 1,010.degree. C. to decompose the cast structure, heating at 1,010.degree. C., quenching with water for solution heat treatment, machine working to remove oxide scales, hot extrusion at 600.degree. C., machine working to remove oxide scales, three cycles of annealing and cold rolling under the conditions of log.SIGMA.Ai shown in Tables 7 to 12, and final stress relief annealing at 450.degree. C. for two hours. Zr alloy cladding tubes 1 to 64 having a thickness of 0.5 mm are produced. The resulting cladding tubes are subjected to the following tests.

Corrosion Test

A test piece with a length of 50 mm is cut out from each Zr alloy cladding, washed with acetone, and immersed into pure water of a saturated vapor pressure of 190 atmospheres at 360.degree. C. for 900 days in a static autoclave. An increased weight of each test piece per unit surface area ((weight after testing-weight before testing)/surface area of test piece) (unit: mg/dm.sup.2) is shown in Tables 7 to 12.

Creep Test

Each test piece from Zr alloy cladding is pressurized internally and maintained at a temperature of 400.degree. C. for 15 days under a stress of 12 kg/mm.sup.2. The outer diameter of the test piece is measured with a laser diameter gauge. The creep strain (%) is calculated from the increase of the outer diameter using the equation ((outer diameter after testing-outer diameter before testing)/outer diameter before testing).times.100. The results are also shown in Tables 7 to 12.

Example 2

Zr alloy ingots 1 to 64 shown in Tables 1 to 6 are subjected to forging at 1,010.degree. C. to decompose the cast structure, heating at 1,010.degree. C., quenching with water for solution heat treatment, machine working to remove oxide scales, hot extrusion at 600.degree. C., machine working to remove oxide scales, two cycles of annealing and cold rolling under the conditions of log.SIGMA.Ai shown in Tables 13 to 21, heating and cooling with argon gas under the conditions shown in Tables 13 to 21, final cold rolling, and then final stress relief annealing under the same conditions as in Example 1. Zr alloy cladding tubes 65 to 128 having a thickness of 0.5 mm are produced. The resulting cladding tubes are subjected to the corrosion test and the creep test as in Example 1. The results are also shown in Tables 13 to 21.

As described above, the Zr alloy cladding obtained by the method in accordance with the present invention has excellent corrosion resistance and creep properties, hence it can be used as a nuclear fuel cladding for a long period of time.

TABLE 7______________________________________ Increased weight per unit area of test piece after Zr corrosion Creep strain alloy test after creepType ingot log.SIGMA.AI (mg/dm.sup.2) test (%)______________________________________This 1 1 -16.8 294 2.66invention's 2 2 -16.8 198 2.49method 3 3 -16.0 234 1.88 4 4 -15.1 233 1.71 5 5 -17.9 185 2.93 6 6 -16.8 171 2.44 7 7 -16.0 171 1.86 8 8 -15.1 202 1.82 9 9 -17.9 189 2.85 10 10 -16.8 191 2.40 11 11 -16.0 189 1.86______________________________________ TABLE 8______________________________________ Increased weight per unit area of test piece after Zr corrosion Creep strain alloy test after creepType ingot log.SIGMA.AI (mg/dm.sup.2) test (%)______________________________________This 12 12 -15.1 206 1.65invention's 13 13 -16.8 162 2.62method 14 14 -16.0 150 2.22 15 15 -16.8 141 2.67 16 16 -16.8 135 2.63 17 17 -16.8 149 2.71 18 18 -16.8 159 2.66 19 19 -16.8 163 2.64 20 20 -16.8 151 2.61 21 21 -17.9 152 2.94 22 22 -16.0 133 2.33______________________________________ TABLE 9______________________________________ Increased weight per unit area of test piece after Zr corrosion Creep strain alloy test after creepType ingot log.SIGMA.AI (mg/dm.sup.2) test (%)______________________________________This 23 23 -17.9 246 2.84invention's 24 24 -17.9 251 2.69method 25 25 -17.9 243 2.63 26 26 -17.9 240 2.57 27 27 -17.9 255 2.73 28 28 -17.9 262 2.66 29 29 -17.9 267 2.61 30 30 -17.9 249 2.60 31 31 -17.9 266 2.54 32 32 -17.9 234 2.75 33 33 -17.9 212 2.81______________________________________ TABLE 10______________________________________ Increased weight per unit area of test piece after Zr corrosion Creep strain alloy test after creepType ingot log.SIGMA.AI (mg/dm.sup.2) test (%)______________________________________This 34 34 -17.9 208 2.77invention's 35 35 -17.9 202 2.77method 36 36 -17.9 216 2.80 37 37 -17.9 221 2.77 38 38 -17.9 223 2.74 39 39 -17.9 204 2.79 40 40 -17.9 210 2.77 41 41 -17.9 291 2.66 42 42 -17.9 290 2.72 43 43 -18.8 294 2.50 44 44 -18.8 250 2.72______________________________________ TABLE 11______________________________________ Increased weight per unit area of test piece after Zr corrosion Creep strain alloy test after creepType ingot log.SIGMA.AI (mg/dm.sup.2) test (%)______________________________________This 45 45 -16.8 284 2.65invention's 46 46 -17.9 241 2.65method 47 47 -17.9 180 2.89 48 48 -17.9 177 2.83 49 49 -17.9 171 2.81 50 50 -17.9 180 2.91 51 51 -17.9 193 2.91 52 52 -17.9 201 2.82 53 53 -17.9 192 2.85 54 54 -17.9 188 2.81 55 55 -16.8 129 2.83______________________________________ TABLE 12______________________________________ Increased weight per unit area of test piece after Zr corrosion Creep strain alloy test after creepType ingot log.SIGMA.AI (mg/dm.sup.2) test (%)______________________________________This 56 56 -17.9 248 2.72invention's 57 57 -17.9 233 2.72method 58 58 -17.9 252 2.61 59 59 -17.9 261 2.81 60 60 -17.9 260 2.67 61 61 -17.9 264 2.71 62 62 -17.9 252 2.75 63 63 -17.9 281 2.51 64 64 -17.9 248 2.70______________________________________ TABLE 13______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 65 1 -17.8 720 5.2 289 2.98invention's 66 2 -17.8 725 3.0 226 2.93method 67 3 -19.5 670 2.2 291 2.83 68 4 -18.8 675 5.0 239 2.85 69 5 -18.8 680 6.1 220 2.91 70 6 -18.8 700 3.0 212 2.84 71 7 -18.8 720 1.2 207 2.81______________________________________ TABLE 14______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 72 8 -17.8 730 2.5 188 2.88invention's 73 9 -17.8 690 8.3 191 2.89method 74 10 -17.8 660 5.5 234 2.82 75 11 -18.8 740 2.3 192 2.84 76 12 -17.8 770 1.0 194 2.88 77 13 -17.8 710 9.5 175 2.99 78 14 -17.8 725 3.0 159 2.98______________________________________ TABLE 15______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 79 15 -17.8 720 5.5 143 2.94invention's 80 16 -17.8 760 1.8 133 2.92method 81 17 -17.8 700 9.8 155 2.97 82 18 -18.8 680 4.5 194 2.93 83 19 -17.8 730 5.0 169 2.99 84 20 -17.8 735 4.5 155 2.98 85 21 -17.9 720 5.0 161 2.93______________________________________ TABLE 16______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 86 22 -17.8 720 5.0 133 2.97invention's 87 23 -17.8 700 8.8 243 2.85method 88 24 -17.8 710 8.0 260 2.72 89 25 -17.8 730 3.2 251 2.64 90 26 -17.8 725 3.5 250 2.71 91 27 -17.8 760 1.0 259 2.77 92 28 -17.8 750 1.2 277 2.75______________________________________ TABLE 17______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 93 29 -17.8 710 5.0 275 2.67invention's 94 30 -17.8 700 9.0 266 2.71method 95 31 -17.8 730 2.5 281 2.73 96 32 -17.8 720 5.0 241 2.81 97 33 -17.8 740 1.1 222 2.88 98 34 -17.8 730 2.6 223 2.82 99 35 -17.8 720 4.8 201 2.89______________________________________ TABLE 18______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 100 36 -17.8 725 3.3 223 2.81invention's 101 37 -17.8 710 5.0 225 2.82method 102 38 -17.8 700 7.5 220 2.79 103 39 -17.8 740 1.2 207 2.81 104 40 -17.8 720 4.0 221 2.83 105 41 -18.8 650 8.1 282 2.94 106 42 -18.8 680 2.0 249 2.85______________________________________ TABLE 19______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 107 43 -18.8 690 1.0 299 2.82invention's 108 44 -18.8 660 5.0 262 2.91method 109 45 -17.8 720 4.0 283 2.94 110 46 -17.8 725 3.0 242 2.66 111 47 -17.8 730 2.5 182 2.91 112 48 -17.8 710 7.0 174 2.81 113 49 -17.8 730 3.0 176 2.79______________________________________ TABLE 20______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 114 50 -17.8 735 2.0 182 2.72invention's 115 51 -17.8 710 5.2 188 2.88method 116 52 -17.8 715 4.5 201 2.76 117 53 -17.8 720 3.8 197 2.79 118 54 -17.8 700 9.0 189 2.84 119 55 -17.8 705 5.5 142 2.98 120 56 -17.8 725 2.0 247 2.76______________________________________ TABLE 21______________________________________ Annealing Increased condition weight per before final unit area of Creep cold rolling test piece strain Heat- Hold- after after Zr ing ing corrosion creep alloy temp. time test testType ingot Log[A1 (.degree. C.) (min) (mg/dm.sup.2) (%)______________________________________This 121 57 -17.8 730 2.0 232 2.62invention's 122 58 -18.8 670 3.5 190 2.81method 123 59 -17.8 695 9.0 255 2.72 124 60 -17.8 715 4.0 249 2.79 125 61 -17.8 730 2.2 272 2.66 126 62 -17.8 720 3.0 272 2.73 127 63 -17.8 725 2.6 287 2.75 128 64 -18.8 680 2.5 267 2.94______________________________________

The disclosures of the priority documents, application numbers 09-278935 and 10-287800, which were filed in Japan on Oct. 13, 1997 and Oct. 9, 1998, respectively, are incorporated herein by reference in their entireties.

While the present application has been described with reference to specific embodiments, it is not confined to the specific details set forth, but is intended to convey such modifications or changes as may come within the skill in the art.

Claims

1. A method for making Zr alloy nuclear reactor fuel cladding, the method comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling, ending in a final cold rolling, of a Zr alloy comprising, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr and 0.05 to 1.0% Nb, with a remainder being Zr and incidental impurities, and an incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

said annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 hour and about 4 hours such that an accumulated annealing parameter .SIGMA.Ai represented by

.SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies relationships as follow:

-20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and

-18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb -0.2),

wherein Ai represents an annealing parameter for an i-th annealing,

ti represents annealing time (hours) for the i-th annealing,

Ti represents annealing temperature (K) for the i-th annealing, and

X.sub.Nb represents Nb concentration (wt %).

2. A method for making Zr alloy nuclear reactor fuel cladding, the method comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling, ending in a final cold rolling, of a Zr alloy comprising, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr and 0.05 to 1.0% Nb, with a remainder being Zr and incidental impurities, and an incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

said annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 hour and about 4 hours such that an accumulated annealing parameter .SIGMA.Ai represented by .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies relationships as follow:

-2.ltoreq. log.SIGMA.Ai.ltoreq.-15, and

-18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb -0.2),

wherein Ai represents an annealing parameter for an i-th annealing,

ti represents annealing time (hours) for the i-th annealing,

Ti represents annealing temperature (K) for the i-th annealing, and

X.sub.Nb represents Nb concentration (wt %); and

the accumulated annealing parameter .SIGMA.Ai satisfies further relationships as follow:

when 0.05.ltoreq.X.sub.Nb .ltoreq.0.5,

-20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and

-18-10.multidot.X.sub.Nb .ltoreq.log.SIGMA.Ai.ltoreq.-15-10.multidot.(X.sub.Nb -0.2); and

when 0.5<X.sub.Nb,

-20.ltoreq.log.SIGMA.Ai.ltoreq.-18-2.multidot.(X.sub.Nb -0.5).

3. A method for making Zr alloy nuclear reactor fuel cladding, the method comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling, ending in a final cold rolling, of a Zr alloy comprising, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, 0.05 to 1.0% Nb and 0.01 to 0.1% Ta, with a remainder being Zr and incidental impurities, and an incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

said annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 hour and about 4 hours such that an accumulated annealing parameter .SIGMA.Ai represented by .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies relationships as follow:

-20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and

-18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb+Ta -0.2), and

wherein Ai represents an annealing parameter for an i-th annealing,

ti represents annealing time (hours) for the i-th annealing,

Ti represents annealing temperature (K) for the i-th annealing, and

X.sub.Nb+Ta represents a total concentration of Nb and Ta (wt %).

4. A method for making Zr alloy nuclear reactor fuel cladding, the method comprising: performing hot forging, solution heat treatment, hot extruding, and repeated cycles of annealing and cold rolling, ending in a final cold rolling, of a Zr alloy comprising, by weight, 0.2 to 1.7% Sn, 0.18 to 0.6% Fe, 0.07 to 0.4% Cr, 0.05 to 1.0% Nb and 0.01 to 0.1% Ta, with a remainder being Zr and incidental impurities, and an incidental nitrogen impurity content being 60 ppm or less, and then performing final stress relief annealing thereon; wherein

said annealing is performed in a range of temperatures between about 550.degree. C. and about 850.degree. C. for a period of time between about 1 hour and about 4 hours such that an accumulated annealing parameter .SIGMA.Ai represented by .SIGMA.Ai=.SIGMA.ti.multidot.exp(-40,000/Ti) satisfies relationships as follow:

-20.ltoreq.log.SIGMA.Ai.ltoreq.-15, and

-18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-3.75.multidot.(X.sub.Nb+Ta -0.2),

wherein Ai represents an annealing parameter for an i-th annealing,

ti represents annealing time (hours) for the i-th annealing,

Ti represents annealing temperature (K) for the -th annealing, and

X.sub.Nb+Ta represents a total concentration of Nb and Ta (wt %); and

the accumulated annealing parameter .SIGMA.Ai satisfies further relationships as follow:

when 0.05.ltoreq.X.sub.Nb+Ta .ltoreq.0.5,

-20.ltoreq.log.SIGMA.Ai<-15, and

-18-10.multidot.X.sub.Nb+Ta .ltoreq.log.SIGMA.Ai.ltoreq.-15-10.multidot.(X.sub.Nb+Ta -0.2); and

when 0.5<X.sub.Nb+Ta,

-20.ltoreq.log.SIGMA.Ai.ltoreq.-18-2.multidot.(X.sub.Nb+Ta -0.5).