SE526648C2 - Zirconium Niobium Tin Alloy for Use in Nuclear Reactors and Method of Preparation - Google Patents

Abstract

A corrosion resistant zirconium based alloy for use in nuclear fuel cladding is made of a low tin content zirconium alloy consisting essentially of: by weight percent, 0.60-2.0 Nb; when Sn is 0.25, then Fe is 0.50; when Sn is 0.40, then Fe is 0.35 to 0.50; when Sn is 0.50, then Fe is 0.25 to 0.50; when Sn is 0.70, then Fe is 0.05 to 0.50; when Sn is 1.0, then Fe is 0.05 to 0.50 (area 10 of FIG. 1); where the weight percent of Fe plus Sn is greater than 0.75, with no more than 0.50 additional other component elements and with the remainder Zr.

Description

526 648 Corrosion of zirconium alloys in aqueous solution is a complex, eg step process.

Corrosion of the alloys in reactors is further complicated by the presence of intense radiation fields that can affect every step of the corrosion process. In the early stages of oxidation, a thin compact black oxide film develops which is protective and prevents further oxidation. This thick layer of zirconia is rich in the tetragonal phase, which is normally stable at high pressure and high temperature. As the oxidation proceeds, the compressive stresses in the oxide layer cannot be counterbalanced by the tensile stresses in the metal substrate and the oxide undergoes a transition. Once the transition has taken place, only part of the oxide layer remains protective. The dense oxide layer is then renewed under the transformed oxide. A new 'dense oxide layer grows' under the porous oxide. Corrosion of zirconium alloys is characterized by this' repeated process of growth and transition. Finally, the process results in a relatively thick outer layer of non-protective, porous oxide. A large variety of studies have been performed on corrosion processing of zirconium alloys. These studies range from field measurements of oxide thickness on irradiated fuel rods to detailed microcharacterization of oxides formed in well-controlled laboratory environments.

However, corrosion of zirconium alloys inside a reactor is an extremely complicated am parameter process. Not a single theory has yet been able to thwart it.

Corrosion is accelerated by the presence of lithium hydroxide. Since refrigerants in pressurized water reactors contain lithium (added for pH control and / or present due to the degradation of chemical control means Bm via (n, the cÛ reaction), extreme acceleration of corrosion due to lithium concentration must be avoided.

U.S. Patent Specifications with no. Nos. 5,112,573 and 5,230,758 (both Foster et al.) Show an improved ZIRLO composition which was manufactured more economically and provided a more easily controlled composition while maintaining corrosion resistance similar to that of previous ZIRLO compositions. It contained 0.5-2.0% by weight of Nb; 0.7-1.5% by weight Sn; 0.07-0.14% by weight of Fe and 0.03-0.14% by weight of at least one of Ni and Cr, with the residue Zr. This alloy had a weight gain at a high temperature of 520 ° C for 15 days of not more than 633 mg / dmz.

I "In-Reactor Corrosion Performance of ZIRLO and Zircaloy-4" Zircgnium ig the Ngglgar Industry: Tenth International Symposium, A.M. Garde and E.R. Bradley ed., American Society for Testing and Materials, Philadelphia 1994, p. 724-744, demonstrates Sabol m. Fl. that ZIRLO materials, in addition to improved corrosion performance, also have better dimensional stability than ZIRLO-4. 526 648 More recently, the U.S. patent specification with no. 5,560,790 (Nikulina et al.) On high tin zirconium based materials where the microstructure contained Zr-Fe-Nb particles. The composition contained: 0.5-1.5% by weight of Nb; 0.9-1.5% by weight Sn; 0.3-0.6 wt.-Wo Fe, with minor amounts of Cr, C, O and Si, with the residue Zr. The U.S. patent specification with no. No. 5,940,464 (Mardon et al.) Showed zirconium alloy tubes forming the entire or outer portion of a nuclear fuel gauge or control rod assembly having a low tin composition: 0.8-1.8% by weight Nb; 0.2-0.6% by weight Sn; 0.02-0.4% by weight of Fe, with a carbon content of 30-180 ppm, a silicon content of -120 ppm and an oxygen content of 600-1800 ppm, with the residue Zr. Mardon m. Fl. showed a wide range of Sn-to-Fe contents, that is, at 0.02 wt% Sn, Fe is 0.2 wt% to 0.4 wt%, and at 0.6 wt% -% Sn, Fe is 0.02% by weight to 0.4% by weight of Qin; with a g preferred range for Sn of 0.25% to 0.35% by weight and of Fe 0.2% to 0.3% by weight.

While these modified zirconium-based compositions claim to provide improved nuclear resistance as well as improved fabrication properties, the economy has driven nuclear power plants to higher coolant temperatures, higher combustion, higher concentrations of lithium in the coolant, longer cycles, longer cycles, and longer durations. in increased corrosion requirements on the enclosure. Continuation of this trend as burnout approaches and exceeds 70,000 MWd / MTU will require further improvement in the corrosion properties of zirconium-based alloys. The alloys for this invention provide such corrosion resistance, even in lithium-containing water at 360 ° C.

SUMMARY OF PPFINNI EN It is therefore a principal object of this invention to provide even more corrosion resistant zirconium based alloys for use in building materials for the radiant environment, such as fuel capsules, grids, guide rods and the like.

Another object of the invention is to provide zirconium-based alloys especially resistant to accelerated corrosion in lithium-containing water. 526 648 These and other requirements are met by providing a low tin zirconium alloy which consists essentially of, in weight percent: 0.60-2.0 Nb; and with the ratio of the Sn to Fe content such that, when Sn is 0.25, then Fe is 0.50; when Sn is 0.40, then Fe is 0.35 to 0.50; when sn is 0.50, then Fe is 0.25 and 0.50; when sn is 0.70, then Fe is 0.05 to 0.50; when Sn is 1.0, Fe is 0.05 to 0.50, where these Sn-to-Fe ranges define the area within the solid lines of the area 10 in fi g. A; where the weight percentage of Fe plus Sn is greater than 0.75, with not more than 0.50 of additional component components and with the remainder Zr. This composition range improves the corrosion resistance of the Zr-Nb-Sn-Fe alloys, both with respect to uniform corrosion resistance in water and steam, and especially lithium-containing aqueous environment. Such alloys are important for both encapsulation of fuel rods and structural components for fuel assembly (i.e., gratings and guide rods) for designs with high corrosion requirements, herein referred to as "radiant environment building materials". , 1% by weight Sn, 0.1% by weight Fe, the remainder Zr), the proposed composition allows a reduction of tin to reduce the uniform corrosion rate and the composition has a minimum iron plus tin content to maintain the corrosion resistance in lithium-containing aquatic environments. .

The development of advanced fuel assemblies, made of alloys such as those shown in this invention, will provide increased operating margins and will improve fuel reliability at high burnouts. The performance of the fuel assembly is usually limited by the decomposition of the fuel housing and the structural elements. The intense radiation environment inside the core causes degradation of these components by accelerating the rate of corrosion and hydrogenation.

The extension of the nuclear fuel cycles to higher burnout will result in reductions in fuel cycle costs.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference is made to exemplary embodiments shown in the accompanying drawings, in which: Fig. A is a graph of relative corrosion rates for samples exposed to 360 ° C water and 427 ° C steam against Sn. concentration; '526 648 Fig. 2 is a graph of relative corrosion rates for samples exposed to 360 ° C water containing 70 ppm lithium against the Fe- plus Sn concentration; and Fig. 3 is a block diagram showing the steps of this invention.

Fig. A is a diagram of tin versus iron concentration with the intention of showing the general area where an alloy of this invention provides corrosion resistance in water and steam at high temperature and in lithium-containing aquatic environments; THE ALLEGED DESCRIPTION OF THE PREFERRED Q TFQ RING SHAPES The zirconium alloy for this invention is a low tin alloy consisting essentially of, by weight percent; ~ 0.60-2.0Nb; with the following amounts of Sn and Fe: when Sn is 0.65, then Fe is 0.10 to 0.50; when Sn is 0.70, then Fe is 0.05 to 0.50; when sn is 0.85, then Fe is 0.05 now o, so; and when sn is 0.90, then Fe is 0.05 rm o, so; where sn begs slg in the range between 0.65 and 0.90% by weight, and the weight percentage of Fe plus Sn is higher than 0.75.

This composition (and those that follow) should not have more than 0.50 of additional other component substances, preferably not more than 0.30 of additional other component substances, such as nickel, chromium, carbon, silicon, oxygen and the like, and with the remainder Zr . These provide alloys as building materials for the radiant environment which successfully work in an environment with lithium-containing water.

A preferred composition has a weight percent range for the alloy of 0.60-2.0 weight percent Nb, which includes weight percent for Fe and Sn: when Sn is 0.70, then Fe is 0.05 to 0.50; and when Sn is 0.85, then Fe is 0.05 to 0.50; where Sn extends between 0.70 and 0.85% by weight and where the weight percentage for Fe plus Sn is higher than 0.75. Because tin is beneficial for strength and creep strength, materials for the strength and creep limited applications will possess the higher tin levels (i.e., greater than 0.6 weight percent, within the specified ranges). The most preferred compositions of those described above will contain 0.80-1.20 Nb, with no more than 0.30 of further other component substances, and with the remainder Zr.

Autoclave corrosion results in both high water and vapor temperatures and in lithium-containing water exhibits lower corrosion weight increases (i.e., thinner oxide thickness) than prior art ZIRLO materials. These results indicate better performance inside a reactor than prior art ZIRLO materials. When these compositions, beta-forged, beta-heat treated and rapidly cooled, hot-worked in the temperature range of the alpha phase, and subsequently cold-worked several times with intermediate annealing in the alpha-temperature range, they contain Zr-Nb-Fe and / or beta-Nb precipitates. The goal is to produce a microstructure with a uniform distribution of small precipitates in the zirconium matrix.

One of the manufacturing sequences of the material for this invention, which is shown in fi g. 3, the steps include: (1) mixing the dry ingredients, (2) vacuum melting the ingredients, (3) forging the melt to a desired shape, (4) beta heat treatment 'followed by rapid cooling, (5) hot working, (5 ') an optional beta heat treatment followed by rapid cooling, (6) fl the number of steps of cold working and intermediate recrystallization annealing in the temperature range of the alpha phase at a temperature between about 500 ° C and 650 ° C, and (7) a final annealing in the form of a relaxation annealing or a recrystallization anneal at a temperature between about 450 ° C and 625 ° C.

The invention will now be illustrated by the following non-limiting examples: EXAMPLE Table 1 summarizes the experimental alloys made from zirconium sponges plus added by the added alloy additives in 150-pound blocks which later became strips. The ISO pound blocks were large enough to allow the material to be hot worked and cold worked in much the same manner as commercially produced material. The blocks are beta-forged, beta-heat treated and cooled rapidly, hot-rolled in the temperature range of the alpha phase, and then cold-rolled several times with intermediate alpha-solderings to final size.

This processing was compatible with the production possibilities and was also suitable for precipitation of small particles by processing in the temperature range for alpha.

The purpose of the processing was to produce a microstructure containing a uniform distribution of small precipitates of beta-Nb and / or Zr-Nb-Fe particles in the zirconium matrix. > l <* Ik All twelve alloys contain niobium exceeding the solubility limit of approximately 0,60% by weight. All alloys were tested in pure water at 360 ° C (680 ° F), pure steam at 427 ° C (800 ° F), and 360 ° C (680 ° F) water containing 70 ppm Li in the form of LiOH.

The corrosion rates (mg / dmz, day) for each alloy in the different environments are tabulated in Table 2. Relative corrosion rates are additionally provided in Table 2 to make it easier to compare the relative performance of the alloys. The aim was to identify compositions that had low thermal corrosion rates (that is, low velocities in clean water and pure steam), as well as resistance to accelerated corrosion in lithium-containing water. Both of these are believed to be important for good 526 648 Table 1 Alloy- Nb Fe Sn Fe + Sn designation (wt%) (wt%) (wt%) (wt%) 1 0.91 0.11 0.94 1.05 2 0.92 0.09 0.84 0.93 3 1.09 0.37 0.73 1.10 4 1.00 0.10 0.75 0.85 0.94 0.40 0, 40 0.80 6 1.42 0.30 0.48 0.78 7 * 1.33 0.42 1.32 1.74 8 * 0.95 0.11 1.27 1.38 9 * 1.98 0.21 0.27 0.48 * 0.93 0.11 0.43 0.54 11 ** 1.00 0.03 0.00 0.03 12 ** 2.60 0.05 0.00 0 , 05 Marginal Zr-Nb-Fe-Sn compositions Comparative example corrosion performance in nuclear reactor environments. 526 648 8 Table 2 Alloy 360 ° C Water 427 ° C Steam 360 ° C Water denomination containing 70 ppm Li Speed Relative Speed Relative Speed Relative (mg / dm2 / d) Speed (mg / dmz / d) Speed (mg / dmz / d) speed 1 0.38 1.00 2.75 1.00 0.59 1.00 2 0.36 0.96 2.51 0.91 0.56 0.95 3 0.37 0.97 2 .33 0.85 0.48 0.81 4 0.30 0.79 1.96 0.71 0.47 0.79 0.31 0.81 ', 1.86 0.68 0.68 0.64 6 0.31 0.83 2.13 0.78 0.43 0.73 7 * 0.47 1.24 3.43 1.25 0.58 0.98 s * 0.43 1.14 3.37 1.22 0.65 1.11 9 * 0.25 0.65 1.48 0.54 16.1 27.4 * 0.35 0.93 2.12 0.77 34.5 58.4 11 * * 0.20 0.53 1.06 0.39 83.0 141 12 ** 0.21 0.56 1.30 0.47 71.0 120 * Marginal Zr-Nb-Fe-Sn compositions Ik * Comparative Example In a diagram of tin (FIG. A) (in weight percent) versus iron (in weight percent), one area, enclosed by the solid lines, is generally described where outstanding corrosion performance is shown; this is the general area. The area with reduced tin content, is shown as the area enclosed by the dashed lines within the solid lines in fl g. A. Area 20 defines an area where, in general, the corrosion resistance of pure water and steam decreases with increasing tin content of the alloy. Area 30 defines an area where the alloy exhibits poor corrosion resistance in lithium-containing water. It is essential for this invention to be outside area 30.

Fig. 1 is a graph showing the effect of Sn on the relative corrosion rate of the alloys in both 360 ° C (680 ° F) water (shown as triangles) and 427 ° C (800 ° F) steam (shown as points). A decreasing corrosion rate with decreasing Sn content is obvious. Favorable thermal corrosion resistance in 360 ° C water and 427 ° C steam is observed for all alloys except alloys 7 and 8, shown as the group of \ 526 648 points 40. Alloys 7 and 8 are the only alloys with a Sn content higher than 1, 0% by weight.

A clear separation between good and poor corrosion resistance in lithium-containing water is shown in Fig. 2, which is a graph of relative corrosion rate against Fe plus Sn content. Since the change in corrosion behavior is abrupt, a limit for Fe and Sn of about 0 was identified. 75% by weight; that is, Fe plus Sn must be higher than 0.75% by weight to achieve accelerated corrosion resistance due to lithium. Alloys 9 to 12, shown as points 50, were the only alloys that exhibited accelerated corrosion in lithium-containing water. In addition, alloys 9 to 12 were the only alloys with Fe plus Sn values lower than 0.75% by weight as tabulated in Table 1_.

Based on experimental results, the following compositions are identified to achieve good thermal corrosion resistance, as well as resistance to accelerated corrosion in lithium water: Fe plus Sn higher than 0.75% by weight (ensures resistance to accelerated corrosion in lithium water); Sn lower is or equal to 1.0% by weight (gives good thermal corrosion resistance with the recognition that lower tin is better); Fe between 0.05 wt% and 0.50 wt% (this limitation is based on the range of Fe included in the group of alloys; also, zirconium sponges typically contain hundreds of ppm of Fe as an impurity; the lower limit identifies iron as present at levels higher than those of a pollutant); Nb between 0.6% by weight and 2.0% by weight (niobium must exceed the solubility limit; the lowest Nb in the group of alloys was 0.9% by weight, therefore, a preferred minimum limit for Nb is 0.80% by weight. %; maximum Nb can be determined by neutron cross section, and a preferred upper limit is 1.2 Weight-Wo).

It is to be understood that the present invention may take other forms without departing from the spirit of essential attributes thereof, and consequently reference should be made to both the appended claims and the foregoing specification for indicating the scope of the invention.

Claims (1)

1. 0 15 20 25 30 526 648 10 PATENT CLAIMS Zirconium alloy with a low tin content mainly consisting of, in weight percent: 0.60-2.00 Nb; and with the ratio of the Sn to Fe content defined by the area enclosed by a graph of tin to iron with the following coordinates: when Sn is 0.65, then Fe is 0.10 to 0.50; when Sn is 0.70, then Fe is 0.05 to 0.50; when Sn is 0.85, then Fe is 0.05 to 0.50; where Sn is in the range between 0.65 and 0.90% by weight and where the weight percentage for Fe plus Sn is higher than 0.75, with not more than 0.50 of additional other component substances and with the remainder Zr. An alloy according to claim 1, wherein the Sn content, in weight percent, is between 0.25 and 0.85, wherein the upper limit of 0.85 ensures good thermal corrosion resistance and the lower limit, which is dependent on the Fe content, provides corrosion resistance in lithium-containing water. Alloy according to Claim 1 or 2, in which the Fe content, in% by weight, is between 0.05 and 0.5, in which the Fe content depends on the Sn content. Alloy according to claim 1, where Sn is 0.70, then Fe is 0.05 to 0.50; and where Sn is 0.85, then Fe is 0.05 to 0.50; where Sn extends between 0.70 and 0.85% by weight and where the weight percentage for Fe plus Sn is higher than 0.75. Alloy according to claims 1-4, containing 0.80-1.20 Nb, with not more than 0.30 of further other component substances, and with the residue Zr. Alloy according to any one of the preceding claims, with not more than 0.30 of additional component components. Alloy according to one of the preceding claims, which is resistant to corrosion in pure water and steam and in lithium-containing water. Zirconium alloy had a tin content according to any one of the preceding claims, wherein Sn ranges between 0.65% by weight and 0.85% by weight.