Method of manufacturing a corrosion-resistant zirconium alloy for a nuclear fuel cladding tube

Abstract

A method of manufacturing a zirconium alloy for a nuclear fuel cladding tube includes melting a mixture of 0.5 wt % of Nb, 0.4 wt % of Mo, 0.1 to 0.15 wt % of Cu, 0.15 to 0.2 wt % of Fe, and a balance of zirconium to prepare a melted ingot; heat treating the melted ingot at 1,000 to 1,050° C. for 30 to 40 min. followed by quenching in water to prepare a heat-treated ingot; preheating the heat-treated ingot at 630 to 650° C. for 20 to 30 min. to prepare a preheated ingot followed by hot rolling the preheated ingot at a reduction ratio of 60 to 65% to provide a hot-rolled material; thrice performing vacuum annealing followed by cold-rolling; and vacuum annealing a third cold-rolled material in a final vacuum annealing at 510 to 520° C. for 7 to 9 hrs. to provide the zirconium alloy as a cold-rolled material.

Claims

1. A method of manufacturing a zirconium alloy having corrosion resistance for a nuclear fuel cladding tube, the method comprising steps of: (1) melting a mixture consisting of: 0.5 wt % of Nb, 0.4 wt % of Mo, 0.1 to 0.15 wt % of Cu, 0.15 to 0.2 wt % of Fe, and a balance of zirconium to prepare the zirconium alloy as a melted ingot; (2) solution heat treating the melted ingot at a temperature ranging from 1,000 to 1,050° C. for a time ranging from 30 to 40 min. followed by quenching in water to prepare a heat-treated ingot; (3) preheating the heat-treated ingot at a temperature ranging from 630 to 650° C. for a time ranging from 20 to 30 min. to prepare a preheated ingot followed by hot rolling the preheated ingot at a reduction ratio of 60 to 65% to provide a hot-rolled material; (4) vacuum annealing the hot-rolled material in a first vacuum annealing at a temperature ranging from 570 to 590° C. for a time ranging from 3 to 4 hrs. to prepare an annealed hot-rolled material followed by a first cold rolling at a reduction ratio of 30 to 40% to provide a first cold-rolled material; (5) vacuum annealing the first cold-rolled material in a second intermediate vacuum annealing at a temperature ranging from 560 to 580° C. for a time ranging from 2 to 3 hrs. followed by a second cold-rolling at a reduction ratio of 50 to 60% to provide a second cold-rolled material; (6) vacuum annealing the second cold-rolled material in a third intermediate vacuum annealing at a temperature ranging from 560 to 580° C. for a time ranging from 2 to 3 hrs. followed by a third cold-rolling at a reduction ratio of 30 to 40% to provide a third cold-rolled material; and (7) vacuum annealing the third cold-rolled material in a final vacuum annealing at a temperature ranging from 510 to 520° C. for a time ranging from 7 to 9 hrs. to provide the zirconium alloy as a cold-rolled material for a nuclear fuel cladding tube.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 illustrates the process of manufacturing a zirconium alloy for a nuclear fuel cladding tube according to the present invention; and

(3) FIGS. 2A and 2B are graphs illustrating the weight gain over time in corrosion testing of the zirconium alloy according to the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

(4) Hereinafter, a detailed description will be given of the present invention.

(5) The present invention addresses a zirconium alloy for a nuclear fuel cladding tube, comprising: 0.5 to 1.2 wt % of Nb, 0.4 to 0.8 wt % of Mo, 0.1 to 0.15 wt % of Cu, 0.15 to 0.2 wt % of Fe, and the balance of zirconium.

(6) Alternatively, the present invention addresses a zirconium alloy for a nuclear fuel cladding tube, comprising: 0.5 to 0.6 wt % of Nb, 0.4 to 0.5 wt % of Mo, 0.1 to 0.15 wt % of Cu, 0.15 to 0.2 wt % of Fe, and the balance of zirconium.

(7) Alternatively, the present invention addresses a zirconium alloy for a nuclear fuel cladding tube, comprising: 1.1 to 1.2 wt % of Nb, 0.4 to 0.5 wt % of Mo, 0.1 to 0.15 wt % of Cu, 0.15 to 0.2 wt % of Fe, and the balance of zirconium.

(8) Alternatively, the present invention addresses a zirconium alloy for a nuclear fuel cladding tube, comprising: 0.5 to 0.6 wt % of Nb, 0.7 to 0.8 wt % of Mo, 0.1 to 0.15 wt % of Cu, 0.15 to 0.2 wt % of Fe, and the balance of zirconium.

(9) The preparation of the zirconium alloy having the above composition according to the present invention is described below.

(10) The present invention addresses a method of manufacturing a zirconium alloy for a nuclear fuel cladding tube, comprising the steps of:

(11) (1) melting a mixture of zirconium alloy elements, thus preparing an ingot; (2) subjecting the ingot prepared in step (1) to solution heat treatment at 1,000 to 1,050° C. (β) for 30 to 40 min and then to β-quenching using water; (3) preheating the ingot, annealed in step (2), at 630 to 650° C. for 20 to 30 min and subjecting the ingot to hot rolling at a reduction ratio of 60 to 65%; (4) subjecting the material hot-rolled in step (3), to primary intermediate vacuum annealing at 570 to 590° C. for 3 to 4 hr and then to primarily cold-rolled at a reduction ratio of 30 to 40%; (5) subjecting the material primarily cold-rolled in step (4), to secondary intermediate vacuum annealing at 560 to 580° C. for 2 to 3 hr and then to secondarily cold-rolled at a reduction ratio of 50 to 60%; (6) subjecting the material secondarily cold-rolled in step (5), to tertiary intermediate vacuum annealing at 560 to 580° C. for 2 to 3 hr and then to tertiarily cold-rolled at a reduction ratio of 30 to 40%; and (7) subjecting the material tertiarily cold-rolled in step (6), to final vacuum annealing.

(12) A better understanding of the present invention may be obtained through the following examples.

<Examples 1 to 9> Preparation of Zirconium Alloys 1 to 9

(13) Zirconium alloys were prepared using components in the amounts and through the annealing shown in Table 1 below, and the alloys were manufactured into alloy sheets through the following method.

(14) The chemical compositions for the zirconium alloys and the final annealing temperatures are summarized in Table 1 below.

(15) TABLE-US-00001 TABLE 1 Chemical Composition (wt %) Final Annealing Temp. Nb Sn Mo Fe Cu Cr Zr (° C.) Ex. 1 0.5 — 0.4 0.2 0.1 — Balance 460 Ex. 2 0.5 — 0.4 0.2 0.1 — Balance 520 Ex. 3 0.5 — 0.8 0.2 0.1 — Balance 580 Ex. 4 0.5 — 0.8 0.2 0.1 — Balance 460 Ex. 5 0.5 — 0.8 0.2 0.1 — Balance 520 Ex. 6 0.5 — 0.8 0.2 0.1 — Balance 580 Ex. 7 1.2 — 0.4 0.2 0.1 — Balance 460 Ex. 8 1.2 — 0.4 0.2 0.1 — Balance 520 Ex. 9 1.2 — 0.4 0.2 0.1 — Balance 580 C. Ex. 1 — 1.5 — 0.2 — 0.1 Balance Commercially available

(16) (1) Formation of Ingot

(17) In step (1), zirconium alloy elements are formed into an ingot using VAR (Vacuum Arc Remelting).

(18) In order to prevent the segregation of impurities and the non-uniform distribution of the alloy composition, this process is repeated about three times, and the alloy is melted under the condition that the chamber for VAR is maintained at a vacuum level of 10.sup.−5 torr or less, thus forming an ingot.

(19) To prevent the surface of the sample from being oxidized during the cooling, cooling is performed in the presence of an inert gas such as argon.

(20) The Zr that was used is zirconium sponge (Reactor Grade ASTM B349), and the added elements, such as Nb, Mo, Fe, Cu and the like, have a high purity of 99.99% or more.

(21) (2) β-Solution Heat Treatment and β-Quenching

(22) In step (2), the ingot is annealed in the β-range and then quenched using water so that the alloy composition in the prepared ingot is made homogenous and fine precipitates are obtained.

(23) In order to prevent the oxidation of the ingot, the ingot is clad with a stainless steel plate having a thickness of 1 mm and then spot welded. The annealing is performed at 1,000 to 1,050° C. for about 30 to 40 min.

(24) Also, β-quenching is performed so as to uniformly distribute the size of SPP (Secondary Phase Particles) in the matrix and to control the size thereof, and is carried out through water cooling at a cooling rate of about 300° C./sec or more.

(25) (3) Annealing and Hot Rolling

(26) In step (3), the β-quenched sample is subjected to hot rolling.

(27) To this end, the sample is preheated at 630 to 650° C. for about 20 to 30 min, and then rolled at a reduction ratio of about 60 to 65%.

(28) If the processing temperature falls out of the above range, it is difficult to obtain the rolled material suitable for use in subsequent step (4).

(29) If the reduction ratio of hot rolling is less than 60%, the tissue of the zirconium material becomes non-uniform, undesirably deteriorating hydrogen brittleness resistance. On the other hand, if the reduction ratio is higher than 80%, subsequent processability may become problematic.

(30) The material hot-rolled, is treated as follows: the clad stainless steel plate is removed, an oxide film and impurities are removed using a pickling solution comprising water, nitric acid and hydrofluoric acid at a volume ratio of 50:40:10, and the remaining oxide film is completely mechanically removed using a wire brush in order to facilitate subsequent processing.

(31) (4) Primary Intermediate Annealing and Primary Cold Rolling

(32) In order to remove residual stress after hot rolling and prevent damage to the sample upon primary cold processing, primary vacuum annealing is performed at about 570 to 590° C. for about 3 to 4 hr.

(33) To prevent oxidation during the annealing, the sample is covered with a piece of stainless steel foil and the vacuum level is maintained at 10.sup.−5 torr or less.

(34) The intermediate vacuum annealing is preferably carried out at a temperature elevated to a recrystallization annealing temperature. If the temperature falls out of the above range, corrosion resistance may deteriorate.

(35) After completion of the primary intermediate vacuum annealing, the rolled material is subjected to primarily cold-rolled at a reduction ratio of about 40 to 50% at an interval of about 0.3 mm for each pass.

(36) (5) Secondary Intermediate Vacuum Annealing and Secondary Cold Rolling

(37) After completion of the primarily cold-rolled, the rolled material is subjected to secondary intermediate vacuum annealing at 560 to 580° C. for about 2 to 3 hr.

(38) If the intermediate annealing temperature falls out of the above range, corrosion resistance may deteriorate.

(39) After completion of the secondary intermediate vacuum annealing, the rolled material is subjected to secondarily cold-rolled at a reduction ratio of about 50 to 60% at an interval of about 0.3 mm for each pass.

(40) (6) Tertiary Intermediate Vacuum Annealing and Tertiary Cold Rolling

(41) After completion of the secondarily cold-rolled, the rolled material is subjected to tertiary intermediate vacuum annealing at 560 to 580° C. for 2 to 3 hr.

(42) If the intermediate annealing temperature falls out of the above range, corrosion resistance may deteriorate.

(43) After completion of the tertiary intermediate vacuum annealing, the rolled material is subjected to tertiarily cold-rolled at a reduction ratio of about 30 to 40% at an interval of about 0.3 mm for each pass.

(44) (7) Final Vacuum Annealing

(45) After completion of the tertiarily cold-rolled, the rolled material is finally annealed in a high vacuum of 10.sup.−5 torr or less.

(46) Final annealing is performed for about 8 hr in three temperature ranges, including SRA (Stress Relief Annealing) at 460 to 470° C., PRXA (Partial Recrystallization Annealing) at 510 to 520° C., and RXA (Recrystallization Annealing) at 580 to 590° C.

<Comparative Example 1> Preparation of Zirconium Alloy

(47) As a commercially available zirconium alloy for use in nuclear power plants, Zircaloy-4 was used.

<Test Example 1> Corrosion Resistance Testing

(48) In order to evaluate the corrosion resistance of the zirconium alloy composition according to the present invention, corrosion testing was performed as follows.

(49) Each of the zirconium alloys of Examples 1 to 9 was manufactured into a sheet sample through the above manufacturing process, which was then formed into a corrosion test sample having a size of 20 mm×20 mm×1.0 mm, followed by stepwise mechanical polishing using #400 to #1200 SiC abrasive paper.

(50) After completion of the surface polishing, the sample was pickled using a solution comprising water, nitric acid and hydrofluoric acid at a volume ratio of 50:40:10, sonicated with acetone, and then completely dried in an oven for 24 hr or longer.

(51) In order to determine the extent of corrosion of the alloy, the surface area and the initial weight of the alloy were measured before the alloy was loaded in an autoclave.

(52) The loaded sample was subjected to corrosion testing for 100 days using a static autoclave at 360° C. in an 18.6 MPa pure water atmosphere and a 70 ppm Li atmosphere.

(53) In the corrosion testing, the samples of Examples 1 to 9 and the Zircaloy-4 sample of Comparative Example 1 were placed in the autoclave together.

(54) The samples were taken out a total of three times, i.e. 50 days, 75 days, and 100 days after the corrosion testing, and the weights thereof were measured, and the weight gains were calculated so as to quantitatively evaluate the extent of corrosion. The results are shown in Table 2 below.

(55) TABLE-US-00002 TABLE 2 Weight Gain (mg/dm.sup.2) 360° C., 2700 psi, Water 360° C., 2700 psi, 70 ppm Li 50 days 75 days 100 days 50 days 75 days 100 days Ex. 1 17.0171 17.3227 18.5474 25.3438 31.7743 41.9875 Ex. 2 15.0506 16.0417 17.5433 20.5044 22.7827 26.2001 Ex. 3 14.2910 14.8125 18.5156 20.4824 23.1375 31.8615 Ex. 4 14.7509 16.8371 18.1544 25.5202 39.0309 51.7910 Ex. 5 14.1838 15.9509 18.3740 24.6221 31.1623 31.5470 Ex. 6 12.7171 15.7901 16.9807 22.5986 26.1053 33.1187 Ex. 7 14.1545 18.1708 21.2821 23.0809 23.8376 27.2430 Ex. 8 15.5565 18.1167 20.1212 17.1323 22.0816 29.6960 Ex. 9 14.0958 18.4966 20.3069 25.3782 28.4544 31.1460 C. Ex. 1 26.3268 33.1276 46.0908 27.7393 54.3227 71.6597

(56) As is apparent from Table 2, the zirconium alloys of Examples 1 to 9 according to the present invention were low in weight gain in both the water atmosphere and the 70 ppm Li atmosphere, compared to Zircaloy-4 of Comparative Example 1.

(57) For corrosion properties in a pure water atmosphere after 100 days, Examples 1 to 9 manifested the weight gain ranging from 17 to 21 mg/dm.sup.2, and Comparative Example 1 exhibited a significant weight gain, as large as 46 mg/dm.sup.2. Hence, corrosion resistance was greatly improved when using the compositions of Examples 1 to 9 under the annealing conditions of Examples 1 to 9.

(58) For corrosion properties in a Li atmosphere, the weight gain was greatly increased after 75 days in Comparative Example 1. After 100 days, the weight gain of Comparative Example 1 approximated 72 mg/dm.sup.2, which is regarded as significantly different from 26 to 51 mg/dm.sup.2, which was the weight gain of Examples 1 to 9 after 100 days.

(59) Particularly in the Sn-free alloy containing Mo and Cu, corrosion resistance was high in both a pure water atmosphere and a high-concentration Li atmosphere under annealing conditions of 520° C. and 580° C.

(60) Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.