- Email: [email protected]
Study on the hydrogenation of Zircaloy-4
Journal of Nuclear Materials 427 (2012) 121–125
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Study on the hydrogenation of Zircaloy-4 Ivaldete da Silva Dupim a, João M.L. Moreira a,⇑, Selma Luiza Silva b, Cecilia Chaves Guedes e Silva b, Oswaldo Nunes Jr. b, Ricardo Gonçalves Gomide b a b
Centro de Engenharia e Ciências Sociais Aplicada, CECS, Universidade Federal do ABC, Rua Santa Adélia, 166 Bairro Bangu, 09210-170 Santo André, SP, Brazil Centro Tecnológico da Marinha em São Paulo, Av. Professor Lineu Prestes, 2468, 05508-000 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 19 May 2011 Accepted 25 April 2012 Available online 3 May 2012
a b s t r a c t In this article we investigate producing Zirconium powder from discarded Zircaloy-4 material through the hydride–dehydride method. We restrict our study to the first part of the method, namely the hydrogenation process. Differential thermal analyses of the hydrogenation process of the Zircaloy-4 show that no hydrogen absorption occurs at temperatures below 573 K and hydrogen gas pressure of 25 kPa. When the system temperature is raised to around 770 K, with the same gas pressure, the protecting oxide layer of the specimens can be overcome and they are quickly hydrogenated. The bulk of the reaction occurs in about 5 min with the precipitation of Zirconium hydrides in the Zr-d and Zr-e phases. Once the temperature passes 573 K, the incubation time to initiate the reaction is short (about 5 min). Tests in a tube furnace system with larger samples, hydrogen pressure varying from 30 to 180 kPa, and temperature from 700 to 833.15 K, show that the specimens are fully hydrogenated and can be easily pulverized. The results indicate that the hydrogenation of the Zircaloy-4 chips can be successfully undertaken at temperatures around 770 K and hydrogen gas pressure as low as 30 kPa. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Zirconium alloys such as Zircaloy-4 are used in pressurized water nuclear reactors for manufacturing fuel rod cladding tubes and the overall structure of the nuclear fuel elements. A few hundreds of kg of Zircaloy-4 is usually discarded as scrap during the fabrication of a complete fuel load of a typical 1300 MW pressurized water reactor. There is an incentive today to recycle this type of material because of its inherent value to the nuclear industry, an expensive high purity material with very low neutron absorption cross-section, and to reduce consumption of raw materials [1]. A possible use for this discarded material is in advanced fuels for power reactors such as those based on dispersion of fissile material in a zirconium metal matrix or as an inert matrix for long term storage of uranium–transuranic oxide nuclear waste [2]. To manufacture such dispersion materials it is necessary a powder metallurgy work in which the fissile material is dispersed in the zirconium metal matrix. Zirconium is a ductile material, difficult to be crushed in ambient conditions, and its powder is usually obtained through the atomization and crushing processes, the latter being facilitated using cryogenic milling or hydride–dehydride methods [3,4]. Such discarded Zircaloy-4 materials have usually undergone strong stresses during the fuel element manufacture process, present rough surfaces, sharp edges, lattice defects and ⇑ Corresponding author. Tel.: +55 11 4996 0115; fax: +55 11 4996 3166. E-mail address: [email protected] (J.M.L. Moreira). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.04.032
relatively large specific surface areas. Materials presenting such conditions are usually more susceptible to the hydrogenation and dehydrogenation processes [4–6]. Hydrogenation is a reversible reaction, which is proportional to the H2 partial pressure in the environment surrounding the material. Under vacuum conditions, a dehydrogenation process takes place and H atoms are released from the material [5,6]. Most studies on zirconium hydrogenation are related to the Zircaloy cladding resistance against embrittlement during long term operation in power reactors at temperatures around 670 K, and during severe accidents at temperatures above 1270 K [6–15]. In these studies the hydrogen absorption reaction occurs simultaneously with oxidation due to the presence of steam in order to reproduce actual reactor operation conditions. Studies about Zircaloy-4 hydriding usually emphasize the initial stage when the H atoms overcome the oxide layer that protects the material. They indicate that the presence of an oxide layer and steam strongly affects the hydrogenation rate, and that Zircaloy4 presents good resistance against hydriding [8–11]. Meyer et al. [8] showed that the kinetics mechanism of Zircaloy hydriding can be divided into 3 distinct phases: slow, massive, and finally slow again. Kim et al. [9] studied the Zirconium hydriding in different physical forms and in two different pressures. They identified that there is an incubation time to initiate the hydriding process due to the oxide layer on the surface of the material. In a subsequent work [10] they noted that the hydriding reaction is slow when there is competition with oxidation, but with the lack of
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oxygen the hydriding process can be developed in minutes. For samples without oxide protection layers they obtained massive rates of hydriding at 670 K which dropped about three times at 623 K. Hong et al. [11], using thermogravimetric analysis, confirmed these results showing the importance of oxidation layers in the outcome of Zr hydriding processes. They studied the Zircaloy hydriding at 588 K and 673 K and observed that oxidized specimens exhibit lower hydriding rates and longer incubation times (42 h) than specimens without any oxidation layers. The hydrogen diffusion and solubility in Zr alloy lattices have also been studied [12–14]. Steinbruck [15] obtained the hydrogenation Sieverts constant for Zircaloy-4 at temperatures above 1273 K using thermogravimetric analysis, and noted that the hydriding process requires elevated temperatures to dissolve the oxide layer present on the metal surface. In summary, the literature emphasizes the importance of overcoming the oxide layer covering the Zircaloy-4 specimens in order to hydrogenate this material. In this article we restrict our investigation to the first part of the hydride–dehydride method for Zircaloy-4 pulverization, namely the hydrogenation process. The objectives of the research are determining appropriate conditions to overcome the protective oxide layer and initiate the hydrogenation reaction, and obtaining values for the process variables (temperature and H2 pressure) to successfully undertake the reaction. The approach taken in this research was to perform differential temperature analyses of the hydrogenation reaction and to hydrogenate larger samples in a tube furnace system. 2. Materials and methods The Indústrias Nucleares do Brasil (INB) provided the Zircaloy-4 specimens used in this work. The material was obtained from scrap of the manufacture process of fuel cladding tubes for pressurized water reactors. Fig. 1 shows the general condition of this material which appears as long and winding ribbons of approximate rectangle cross-section and few millimeters thickness. The material was characterized with a Fluorescence X-ray EDS (HS EDX-800 Shimadzu), and Table 1 presents its composition. To study the hydrogenation process of the Zircaloy-4 we considered the differential thermal analysis (DTA) method because it allows identifying the start and end of the hydrogen absorption process in the samples. We used small samples of about 35 mg of Zircaloy-4 in these tests. The hydrogen content was inferred from the mass variation of the samples, considered as mainly due to hydrogen absorption, and from X-ray diffraction characterization. The masses were measured with an analytical scale with ±0.01 mg accuracy. The hydride samples were crushed and characterized with X-ray diffraction performed on a Shimadzu diffractometer (XRD 6000) with a 1°/min speed and Cu Ka radiation (k = 1.5406 Å). The particles size and morphology were characterized with a scanning electron microscopy. In the DTA system, the H2 pressure was 25 kPa, and the temperature profile was the following: start at 291.15 K, temperature increase rate of 20 K/min, a constant temperature during a time period (plateau), and temperature decrease at the same rate. The
Fig. 1. Zircaloy-4 chips provided by INB.
Table 1 INB Zircaloy-4 composition. Chemical composition (% weight)
Experimental
Zr
Sn
Fe
Cr
Fe + Cr
98.21
1.46
0.27
0.053
0.323
plateau temperatures considered were 573.15 K, 843.15 K and 943.15 K. Table 2 presents the conditions used in the DTA measurements. The DTA signal was obtained from the temperature difference between the sample and the reference crucibles. The signal presented a non-zero baseline, proportional to the temperature profile, because of small differences in the thermal capacities of both crucibles and in the heat transfer conditions to each one. Following a procedure suggested by Vizcaino et al. [12,13], a correction term proportional to the temperature profile was determined such that at the plateau level, where no reaction could be observed, its value was exactly equal to the DTA signal. This correction term was then subtracted from the original DTA signal. The material was sectioned in small chips of about 2 mm length to form the DTA samples. We obtained their masses from the weight difference between the empty and Zircaloy-4 full crucibles, and the mass gain, from the weight difference between the samples after and before hydrogenation. The other group of tests considered larger samples, of about 3500 mg, and different H2 pressure conditions during the hydrogenation process. These tests were performed in a tube furnace system with a heated cavity able to reach 923 K, and gas pressure levels up to 300 kPa. The H2 pressure varied from 30 to 180 kPa, above the one adopted in the DTA analysis, because the sample masses submitted to hydrogenation were about 100 times higher. The experimental procedure regarding the sample preparation and characterization were similar to those performed for the DTA samples. Briefly, the experimental procedure for performing the hydrogenation was as follows: (a) cleaning and sectioning the specimens, (b) positioning in a tungsten crucible, (c) weighting the sample, (d) imposing vacuum and purging with Argon gas for 2 min, (e) programming the test temperature profile, (f) performing the hydrogenation test with acquisition of temperature and DTA signals along the time, (g) cooling the specimens down to 298.15 K holding the H2 pressure constant and (h) crushing, weighting and characterizing the hydride specimens. The procedures in item (h) were performed in ambient conditions about 3 h after the end of the hydrogenation process. Thus the specimens may experience some oxidation in this period.
3. Results 3.1. The differential thermal analyses Table 2 shows the specimen masses before the hydrogenation, and their mass variations after the hydrogenation process. The T1 test, with plateau temperature of 573.15 K, no hydrogenation was observed even with the specimen staying under H2 pressure during 180 min. The T2 and T3 tests presented hydrogenation, and Fig. 2 shows the T2 test DTA signal and the temperature profile the sample underwent. Fig. 3 presents details of the DTA signal between the 18th and 28th min which shows exothermic effects characterized by the ‘‘knees’’ in the DTA signal curve. The signal variation in the beginning of the test is considered due to thermal accommodation of the DTA system [12,13]. At room temperature, the specimens were crushed and characterized. In Fig. 4 we show
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I. da Silva Dupim et al. / Journal of Nuclear Materials 427 (2012) 121–125 Table 2 Conditions during the Zircaloy-4 hydrogenation tests in the DTA system, mass before hydrogenation and mass variation after hydrogenation. Sample T1 T2 T3 a
Heating rate (K/mina) 20 20 20
Plateau duration and temperature a
180 min and 573.15 K 120 min and 843.15 K 180 min and 943.15 K
H2 pressure (kPa)
Sample mass (mg)
Mass variation (mg)
25 25 25
32.78 ± 0.014 40.70 ± 0.014 47.03 ± 0.014
– 1.30 ± 0.014 1.39 ± 0.014
‘‘min’’ means minute.
Fig. 4. X-ray diffraction patterns for the Zircaloy-4 hydrided specimens T2 and T3 (for ZrHx, x is 1.6 for the d-phase and 2 for the e-phase; for ZrOx, x is 2).
Fig. 2. DTA signal and system temperature for the T2 test.
of the H2 pressure. Fig. 6 shows the final condition of the Zircaloy-4 specimens after hydrogenation. The hydrogenated specimens of all tests presented similar X-ray diffraction patterns, and we show in Fig. 7 the one from the E4 test. The absorption of hydrogen embrittled the specimens so that they could be manually crushed with a mortar. Fig. 8 shows the scanning electron microscopy image in which we observe the particles size and morphology. The powder had particles ranging from 2 to 150 lm. 4. Discussion of the results The results show the technical feasibility to perform the hydrogenation of the Zircaloy-4, which is the first part of the hydride– dehydride approach to obtain Zr powder. They provide information about the ranges of the temperature and H2 pressure in which the hydrogenation reaction can be carried out, the reaction kinetics, and about the characterization of the final hydrogenated material. 4.1. The differential thermal analyses
Fig. 3. Detail of the DTA signal and system temperature for the T2 test. The bulk of the hydrogenation reaction occurs in 6 min.
the X-ray diffraction patterns obtained at room temperature for the T2 and T3 specimens. 3.2. Large sample hydrogenation in the tube furnace system Table 3 presents the temperature profile, pressures, and the initial and final masses of the five tests performed in the tube furnace system. Since the hydrogenation reaction occurs at temperatures between 710 and 830 K (see Figs. 2 and 3), the temperature profile chosen for these tests covered that interval with a plateau at 833.15 K and duration of 180 min. Then the system was cooled down to ambient temperature. Fig. 5 presents the fractional mass gain obtained at the end of the hydrogenation process as a function
The T1 DTA test showed that at temperatures below 573 K, it is not possible to overcome the oxide protective layer covering the sample. No reaction occurred, even after 180 min of hydrogen exposition at this temperature. This result is in agreement with the literature that states that between 570 and 670 K, oxidized Zircaloy-4 specimens present low hydrogenation rate and the incubation time can take as long as 42 h [9–11]. On the other hand, the T2 and T3 DTA tests, reaching temperatures above 710 K, presented substantial hydrogenation at H2 pressure of 25 KPa. A large exothermic peak is seen between the18th and 30th min indicating that the reaction occurs during this period. The different plateau temperatures of T2 and T3 tests, 843.15 K and 943.15 K, respectively, does not affect the results. The two diffraction patterns shown in Fig. 4 are similar, the peaks show the predominant presence of ZrHx, where x is around 1.6 for the Zr-d-phase and around 2 for the Zr-e-phase [4]. The
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Table 3 Experimental conditions for the tube furnace hydrogenation tests with Zircaloy-4. Plateau duration and temperature for all tests were 180 min and 833, 15 °C. Sample
H2 pressure (KPa)
Temperature rates
Initial mass (mg)
Final mass (mg)
E1 E2 E3 E4 E5
30 60 100 130 180
10 K/min up to 573.15 K
3695.60 ± 0.014 2511.99 ± 0.014 3411.75 ± 0.014 3076.98 ± 0.014 2785.10 ± 0.014
3798.74 ± 0.014 2583.31 ± 0.014 3475.86 ± 0.014 3160.03 ± 0.014 2854.39 ± 0.014
2 K/min from 573.15 K to 833.15 K
Fig. 5. Mass gain obtained in tests E1 to E5 as a function of H2 pressure. The error is negligible (5 10 4%).
Fig. 6. General condition of the Zircaloy-4 material after hydrogenation.
Fig. 8. Scanning electron microscopy of the hydrogenated powder.
hydrogenation level of the material is high and that it became Zr hydride. In Fig. 4 it is also observed the presence of some ZrOx, with x being equal or below 2. This indicates that some oxygen uptake or some oxidation may have occurred and diffused into the material during the test or the waiting period before we performed the characterization measurements. The mass variation of the samples in Table 2 is therefore due to both hydrogen and oxygen additions to the original material. Figs. 2 and 3 show that the hydrogenation reaction with the discarded Zircaloy-4 chips started at about 710 K, with the exothermic solution of H in the metal lattice and the precipitation of Zr-Hx in different phases. The thin oxide layer must have started to dissolve when the sample temperature increased above 573 K, since below this temperature no reaction was observed in the T1 test. Above this temperature, the protective oxide layer starts to dissolve into the metal, and at about 710 K the hydrogen atoms from the dissociated H2 molecules start to diffuse into the metal. The bulk of the reaction occurs in about 5 min (Fig. 3). The incubation time to initiate the hydrogenation reaction in this material is short. Below 573 K no reaction occurs even for a hydrogenation period of 180 min (T1 test). Assuming that the relevant oxide dissolution reaction starts to occur only above this temperature, or about15 min after the test beginning, Figs. 2 and 3 show that some hydrogenation can be observed in the following 5th or 6th min. This result is in agreement with the literature for hydrogenation reactions with low oxygen concentration [9–11]. The quick reaction and the final high level of hydrogenation were not expected because of the difficulties reported in the literature for the hydrogen overcoming the oxide layer [8–15]. Despite the discarded Zircaloy-4 chips be oxidized, they are in such physical conditions that allow a quick hydrogenation process. 4.2. Large sample hydrogenation in the tube furnace system
Fig. 7. X-ray diffraction pattern of Test E4 after hydrogenation.
presence of only Zr-d and Zr-e-phases, with H/Zr atom ratio higher than 1.6, and no peaks of pure Zr metal mean that the final
The hydrogenation tests shown in Fig. 5 with large specimens (E1–E5) indicate that increasing the hydrogen gas pressure from 30 to 180 kPa does not increase their final fractional gain of mass, as the Sievert’s law would predict [15]. Excluding the E3 test
I. da Silva Dupim et al. / Journal of Nuclear Materials 427 (2012) 121–125
(100 kPa), which shows a much lower value for the fractional gain of mass compared to the other tests, the overall trend is a slow decrease of this variable as a function of the H2 pressure. We note in Fig. 7 that the X-ray diffraction patterns of these tests are similar to the ones from the DTA analyses (Fig. 4). Thus these specimens are completely hydrogenated and also present some oxygen in their final state. For low hydrogen concentrations in a metal, Sievert’s law states that this concentration is proportional to the square root of the H2 partial pressure. Since the hydrogenated specimens were completely hydrided for all pressures, the Sievert’s law behavior is not observed, and Fig. 5 shows an almost flat fractional gain of mass as a function of pressure in the interval from 30 to 180 kPa. This result indicates that a hydrogen gas pressure of 30 kPa is sufficient to fully hydrogenate the material in the test conditions presented in Table 3. In summary, the Zircaloy-4 chips have specific properties that enable a quick hydrogenation process. We are able to fully hydrogenate specimens with about 3500 mg at temperatures around 770 K and hydrogen gas partial pressure as low as 30 kPa.
5. Conclusions It is important today to recycle materials because of their inherent value, and the necessity to reduce consumption of raw materials. Using differential thermal analysis, we investigated an alternative to recycle discarded Zircaloy-4 scrap obtained from the manufacture process of fuel clading tubes for pressurized water reactors, namely the production of Zr powder through the hydride–dehydride method. We studied the first part of the process, i.e., the hydrogenation process, and the results show that is possible to hydrogenate and pulverize the material. The differential thermal analyses show no hydrogenation at temperatures below 573 K and hydrogen gas pressure of 25 kPa. When the system temperature is raised to around 770 K, with the same gas pressure, the protecting oxide layer of the specimens can be overcome and they are quickly hydrogenated. The bulk of the reaction occurs in about 5 min with the precipitation of Zirconium hydrides in the Zr-d and Zr-e phases. Once the temperature passes 573 K, the incubation time to initiate the reaction is short (about 5 min). Tests in a tube furnace system, with hydrogen pressure varying from 30 to 180 kPa and temperature from 700 to 833.15 K, show that specimens of about 3500 mg are fully hydrogenated and can be easily pulverized. The results indicate that the hydrogenation of the Zircaloy-4 chips can be successfully
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undertaken at temperatures around 770 K and hydrogen gas pressure as low as 30 kPa. To obtain Zr powder as the final product, we must remove the absorbed H from the hydride material. The process of dehydrogenation is facilitated if the H content of the specimens is low, which can be obtained with lower hydrogen gas pressure during the hydrogenation process. For future work, using thermal gravimetry or simultaneous thermal analysis, we intend to study the hydrogenation process at H2 pressure levels below 30 kPa, and to study the dehydrogenation of the hydrided Zircaloy-4. In addition, we intend to quantify the content of hydrogen and oxygen in the final states of each process. Acknowledgements The first author wants to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçamento de Pessoal de Nível Superior (CAPES) for their financial support. Also the first author thanks José Marcelo Soares and Claudio Padovani for their important help during the measurements performed at the LABMAT/CTMSP. All authors thank and Centro Tecnológico da Marinha em São Paulo (CTMSP) for supporting this research, and also the very helpful comments of the reviewers. References [1] Nuclear power and sustainable development, International Atomic Energy Agency, 2006. [2] A.R. Totemeier, S.M. McDeavitt, J. Mater. Sci. 44 (2009) 5494–5500. [3] Powder production technology, In International Powder Metallurgy Directory, (accessed 20.10.2011). [4] S. Banerjee, D. Mukhopadhyay, Phase Transformation – Examples from Titanium and Zirconium alloys, Pergamon, Amsterdam, 2007 (p. 721). [5] A. Zuttel, Mater. Today 6 (2003) 24–33. [6] R.A. Oriani, The physical and metallurgical aspects of hydrogen in metals, in: ICCF4 Fourth International Conference on Cold Fusion, Lahaina, Maui: Elecric Power Research Institute, Palo Alto CA., 1993. (accessed 20.10.2011). [7] Y.S. Kim, J. Nucl. Mater. 378 (2008) 30–34. [8] G. Meyer, M. Kobrinski, J.P. Abriata, J.C. Bolcich, J. Nucl. Mater. 229 (1996) 48– 56. [9] Y.S. Kim, W. Wang, D.R. Olander, S.K. Yagnik, J. Nucl. Mater. 240 (1996) 27–31. [10] Y.S. Kim, W. Wang, D.R. Olander, S.K. Yagnik, J. Nucl. Mater. 246 (1997) 43–52. [11] H.S. Hong, L. Sihver, D.R. Olander, L. Hallstadius, J. Nucl. Mater. 336 (2005) 113–119. [12] P. Vizcaino, A.D. Banchik, J.P. Abriata, J. Nucl. Mater. 336 (2005) 54–64. [13] P. Vizcaino, A.D. Banchik, J.P. Abriata, J. Nucl. Mater. 304 (2002) 96–106. [14] K. Une, S. Ishimoto, J. Nucl. Sci. Tech. 41 (2004) 949–952. [15] M. Steinbruck, J. Nucl. Mater. 334 (2004) 58–64.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Study on the hydrogenation of Zircaloy-4 Ivaldete da Silva Dupim a, João M.L. Moreira a,⇑, Selma Luiza Silva b, Cecilia Chaves Guedes e Silva b, Oswaldo Nunes Jr. b, Ricardo Gonçalves Gomide b a b
Centro de Engenharia e Ciências Sociais Aplicada, CECS, Universidade Federal do ABC, Rua Santa Adélia, 166 Bairro Bangu, 09210-170 Santo André, SP, Brazil Centro Tecnológico da Marinha em São Paulo, Av. Professor Lineu Prestes, 2468, 05508-000 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 19 May 2011 Accepted 25 April 2012 Available online 3 May 2012
a b s t r a c t In this article we investigate producing Zirconium powder from discarded Zircaloy-4 material through the hydride–dehydride method. We restrict our study to the first part of the method, namely the hydrogenation process. Differential thermal analyses of the hydrogenation process of the Zircaloy-4 show that no hydrogen absorption occurs at temperatures below 573 K and hydrogen gas pressure of 25 kPa. When the system temperature is raised to around 770 K, with the same gas pressure, the protecting oxide layer of the specimens can be overcome and they are quickly hydrogenated. The bulk of the reaction occurs in about 5 min with the precipitation of Zirconium hydrides in the Zr-d and Zr-e phases. Once the temperature passes 573 K, the incubation time to initiate the reaction is short (about 5 min). Tests in a tube furnace system with larger samples, hydrogen pressure varying from 30 to 180 kPa, and temperature from 700 to 833.15 K, show that the specimens are fully hydrogenated and can be easily pulverized. The results indicate that the hydrogenation of the Zircaloy-4 chips can be successfully undertaken at temperatures around 770 K and hydrogen gas pressure as low as 30 kPa. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Zirconium alloys such as Zircaloy-4 are used in pressurized water nuclear reactors for manufacturing fuel rod cladding tubes and the overall structure of the nuclear fuel elements. A few hundreds of kg of Zircaloy-4 is usually discarded as scrap during the fabrication of a complete fuel load of a typical 1300 MW pressurized water reactor. There is an incentive today to recycle this type of material because of its inherent value to the nuclear industry, an expensive high purity material with very low neutron absorption cross-section, and to reduce consumption of raw materials [1]. A possible use for this discarded material is in advanced fuels for power reactors such as those based on dispersion of fissile material in a zirconium metal matrix or as an inert matrix for long term storage of uranium–transuranic oxide nuclear waste [2]. To manufacture such dispersion materials it is necessary a powder metallurgy work in which the fissile material is dispersed in the zirconium metal matrix. Zirconium is a ductile material, difficult to be crushed in ambient conditions, and its powder is usually obtained through the atomization and crushing processes, the latter being facilitated using cryogenic milling or hydride–dehydride methods [3,4]. Such discarded Zircaloy-4 materials have usually undergone strong stresses during the fuel element manufacture process, present rough surfaces, sharp edges, lattice defects and ⇑ Corresponding author. Tel.: +55 11 4996 0115; fax: +55 11 4996 3166. E-mail address: [email protected] (J.M.L. Moreira). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.04.032
relatively large specific surface areas. Materials presenting such conditions are usually more susceptible to the hydrogenation and dehydrogenation processes [4–6]. Hydrogenation is a reversible reaction, which is proportional to the H2 partial pressure in the environment surrounding the material. Under vacuum conditions, a dehydrogenation process takes place and H atoms are released from the material [5,6]. Most studies on zirconium hydrogenation are related to the Zircaloy cladding resistance against embrittlement during long term operation in power reactors at temperatures around 670 K, and during severe accidents at temperatures above 1270 K [6–15]. In these studies the hydrogen absorption reaction occurs simultaneously with oxidation due to the presence of steam in order to reproduce actual reactor operation conditions. Studies about Zircaloy-4 hydriding usually emphasize the initial stage when the H atoms overcome the oxide layer that protects the material. They indicate that the presence of an oxide layer and steam strongly affects the hydrogenation rate, and that Zircaloy4 presents good resistance against hydriding [8–11]. Meyer et al. [8] showed that the kinetics mechanism of Zircaloy hydriding can be divided into 3 distinct phases: slow, massive, and finally slow again. Kim et al. [9] studied the Zirconium hydriding in different physical forms and in two different pressures. They identified that there is an incubation time to initiate the hydriding process due to the oxide layer on the surface of the material. In a subsequent work [10] they noted that the hydriding reaction is slow when there is competition with oxidation, but with the lack of
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oxygen the hydriding process can be developed in minutes. For samples without oxide protection layers they obtained massive rates of hydriding at 670 K which dropped about three times at 623 K. Hong et al. [11], using thermogravimetric analysis, confirmed these results showing the importance of oxidation layers in the outcome of Zr hydriding processes. They studied the Zircaloy hydriding at 588 K and 673 K and observed that oxidized specimens exhibit lower hydriding rates and longer incubation times (42 h) than specimens without any oxidation layers. The hydrogen diffusion and solubility in Zr alloy lattices have also been studied [12–14]. Steinbruck [15] obtained the hydrogenation Sieverts constant for Zircaloy-4 at temperatures above 1273 K using thermogravimetric analysis, and noted that the hydriding process requires elevated temperatures to dissolve the oxide layer present on the metal surface. In summary, the literature emphasizes the importance of overcoming the oxide layer covering the Zircaloy-4 specimens in order to hydrogenate this material. In this article we restrict our investigation to the first part of the hydride–dehydride method for Zircaloy-4 pulverization, namely the hydrogenation process. The objectives of the research are determining appropriate conditions to overcome the protective oxide layer and initiate the hydrogenation reaction, and obtaining values for the process variables (temperature and H2 pressure) to successfully undertake the reaction. The approach taken in this research was to perform differential temperature analyses of the hydrogenation reaction and to hydrogenate larger samples in a tube furnace system. 2. Materials and methods The Indústrias Nucleares do Brasil (INB) provided the Zircaloy-4 specimens used in this work. The material was obtained from scrap of the manufacture process of fuel cladding tubes for pressurized water reactors. Fig. 1 shows the general condition of this material which appears as long and winding ribbons of approximate rectangle cross-section and few millimeters thickness. The material was characterized with a Fluorescence X-ray EDS (HS EDX-800 Shimadzu), and Table 1 presents its composition. To study the hydrogenation process of the Zircaloy-4 we considered the differential thermal analysis (DTA) method because it allows identifying the start and end of the hydrogen absorption process in the samples. We used small samples of about 35 mg of Zircaloy-4 in these tests. The hydrogen content was inferred from the mass variation of the samples, considered as mainly due to hydrogen absorption, and from X-ray diffraction characterization. The masses were measured with an analytical scale with ±0.01 mg accuracy. The hydride samples were crushed and characterized with X-ray diffraction performed on a Shimadzu diffractometer (XRD 6000) with a 1°/min speed and Cu Ka radiation (k = 1.5406 Å). The particles size and morphology were characterized with a scanning electron microscopy. In the DTA system, the H2 pressure was 25 kPa, and the temperature profile was the following: start at 291.15 K, temperature increase rate of 20 K/min, a constant temperature during a time period (plateau), and temperature decrease at the same rate. The
Fig. 1. Zircaloy-4 chips provided by INB.
Table 1 INB Zircaloy-4 composition. Chemical composition (% weight)
Experimental
Zr
Sn
Fe
Cr
Fe + Cr
98.21
1.46
0.27
0.053
0.323
plateau temperatures considered were 573.15 K, 843.15 K and 943.15 K. Table 2 presents the conditions used in the DTA measurements. The DTA signal was obtained from the temperature difference between the sample and the reference crucibles. The signal presented a non-zero baseline, proportional to the temperature profile, because of small differences in the thermal capacities of both crucibles and in the heat transfer conditions to each one. Following a procedure suggested by Vizcaino et al. [12,13], a correction term proportional to the temperature profile was determined such that at the plateau level, where no reaction could be observed, its value was exactly equal to the DTA signal. This correction term was then subtracted from the original DTA signal. The material was sectioned in small chips of about 2 mm length to form the DTA samples. We obtained their masses from the weight difference between the empty and Zircaloy-4 full crucibles, and the mass gain, from the weight difference between the samples after and before hydrogenation. The other group of tests considered larger samples, of about 3500 mg, and different H2 pressure conditions during the hydrogenation process. These tests were performed in a tube furnace system with a heated cavity able to reach 923 K, and gas pressure levels up to 300 kPa. The H2 pressure varied from 30 to 180 kPa, above the one adopted in the DTA analysis, because the sample masses submitted to hydrogenation were about 100 times higher. The experimental procedure regarding the sample preparation and characterization were similar to those performed for the DTA samples. Briefly, the experimental procedure for performing the hydrogenation was as follows: (a) cleaning and sectioning the specimens, (b) positioning in a tungsten crucible, (c) weighting the sample, (d) imposing vacuum and purging with Argon gas for 2 min, (e) programming the test temperature profile, (f) performing the hydrogenation test with acquisition of temperature and DTA signals along the time, (g) cooling the specimens down to 298.15 K holding the H2 pressure constant and (h) crushing, weighting and characterizing the hydride specimens. The procedures in item (h) were performed in ambient conditions about 3 h after the end of the hydrogenation process. Thus the specimens may experience some oxidation in this period.
3. Results 3.1. The differential thermal analyses Table 2 shows the specimen masses before the hydrogenation, and their mass variations after the hydrogenation process. The T1 test, with plateau temperature of 573.15 K, no hydrogenation was observed even with the specimen staying under H2 pressure during 180 min. The T2 and T3 tests presented hydrogenation, and Fig. 2 shows the T2 test DTA signal and the temperature profile the sample underwent. Fig. 3 presents details of the DTA signal between the 18th and 28th min which shows exothermic effects characterized by the ‘‘knees’’ in the DTA signal curve. The signal variation in the beginning of the test is considered due to thermal accommodation of the DTA system [12,13]. At room temperature, the specimens were crushed and characterized. In Fig. 4 we show
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I. da Silva Dupim et al. / Journal of Nuclear Materials 427 (2012) 121–125 Table 2 Conditions during the Zircaloy-4 hydrogenation tests in the DTA system, mass before hydrogenation and mass variation after hydrogenation. Sample T1 T2 T3 a
Heating rate (K/mina) 20 20 20
Plateau duration and temperature a
180 min and 573.15 K 120 min and 843.15 K 180 min and 943.15 K
H2 pressure (kPa)
Sample mass (mg)
Mass variation (mg)
25 25 25
32.78 ± 0.014 40.70 ± 0.014 47.03 ± 0.014
– 1.30 ± 0.014 1.39 ± 0.014
‘‘min’’ means minute.
Fig. 4. X-ray diffraction patterns for the Zircaloy-4 hydrided specimens T2 and T3 (for ZrHx, x is 1.6 for the d-phase and 2 for the e-phase; for ZrOx, x is 2).
Fig. 2. DTA signal and system temperature for the T2 test.
of the H2 pressure. Fig. 6 shows the final condition of the Zircaloy-4 specimens after hydrogenation. The hydrogenated specimens of all tests presented similar X-ray diffraction patterns, and we show in Fig. 7 the one from the E4 test. The absorption of hydrogen embrittled the specimens so that they could be manually crushed with a mortar. Fig. 8 shows the scanning electron microscopy image in which we observe the particles size and morphology. The powder had particles ranging from 2 to 150 lm. 4. Discussion of the results The results show the technical feasibility to perform the hydrogenation of the Zircaloy-4, which is the first part of the hydride– dehydride approach to obtain Zr powder. They provide information about the ranges of the temperature and H2 pressure in which the hydrogenation reaction can be carried out, the reaction kinetics, and about the characterization of the final hydrogenated material. 4.1. The differential thermal analyses
Fig. 3. Detail of the DTA signal and system temperature for the T2 test. The bulk of the hydrogenation reaction occurs in 6 min.
the X-ray diffraction patterns obtained at room temperature for the T2 and T3 specimens. 3.2. Large sample hydrogenation in the tube furnace system Table 3 presents the temperature profile, pressures, and the initial and final masses of the five tests performed in the tube furnace system. Since the hydrogenation reaction occurs at temperatures between 710 and 830 K (see Figs. 2 and 3), the temperature profile chosen for these tests covered that interval with a plateau at 833.15 K and duration of 180 min. Then the system was cooled down to ambient temperature. Fig. 5 presents the fractional mass gain obtained at the end of the hydrogenation process as a function
The T1 DTA test showed that at temperatures below 573 K, it is not possible to overcome the oxide protective layer covering the sample. No reaction occurred, even after 180 min of hydrogen exposition at this temperature. This result is in agreement with the literature that states that between 570 and 670 K, oxidized Zircaloy-4 specimens present low hydrogenation rate and the incubation time can take as long as 42 h [9–11]. On the other hand, the T2 and T3 DTA tests, reaching temperatures above 710 K, presented substantial hydrogenation at H2 pressure of 25 KPa. A large exothermic peak is seen between the18th and 30th min indicating that the reaction occurs during this period. The different plateau temperatures of T2 and T3 tests, 843.15 K and 943.15 K, respectively, does not affect the results. The two diffraction patterns shown in Fig. 4 are similar, the peaks show the predominant presence of ZrHx, where x is around 1.6 for the Zr-d-phase and around 2 for the Zr-e-phase [4]. The
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Table 3 Experimental conditions for the tube furnace hydrogenation tests with Zircaloy-4. Plateau duration and temperature for all tests were 180 min and 833, 15 °C. Sample
H2 pressure (KPa)
Temperature rates
Initial mass (mg)
Final mass (mg)
E1 E2 E3 E4 E5
30 60 100 130 180
10 K/min up to 573.15 K
3695.60 ± 0.014 2511.99 ± 0.014 3411.75 ± 0.014 3076.98 ± 0.014 2785.10 ± 0.014
3798.74 ± 0.014 2583.31 ± 0.014 3475.86 ± 0.014 3160.03 ± 0.014 2854.39 ± 0.014
2 K/min from 573.15 K to 833.15 K
Fig. 5. Mass gain obtained in tests E1 to E5 as a function of H2 pressure. The error is negligible (5 10 4%).
Fig. 6. General condition of the Zircaloy-4 material after hydrogenation.
Fig. 8. Scanning electron microscopy of the hydrogenated powder.
hydrogenation level of the material is high and that it became Zr hydride. In Fig. 4 it is also observed the presence of some ZrOx, with x being equal or below 2. This indicates that some oxygen uptake or some oxidation may have occurred and diffused into the material during the test or the waiting period before we performed the characterization measurements. The mass variation of the samples in Table 2 is therefore due to both hydrogen and oxygen additions to the original material. Figs. 2 and 3 show that the hydrogenation reaction with the discarded Zircaloy-4 chips started at about 710 K, with the exothermic solution of H in the metal lattice and the precipitation of Zr-Hx in different phases. The thin oxide layer must have started to dissolve when the sample temperature increased above 573 K, since below this temperature no reaction was observed in the T1 test. Above this temperature, the protective oxide layer starts to dissolve into the metal, and at about 710 K the hydrogen atoms from the dissociated H2 molecules start to diffuse into the metal. The bulk of the reaction occurs in about 5 min (Fig. 3). The incubation time to initiate the hydrogenation reaction in this material is short. Below 573 K no reaction occurs even for a hydrogenation period of 180 min (T1 test). Assuming that the relevant oxide dissolution reaction starts to occur only above this temperature, or about15 min after the test beginning, Figs. 2 and 3 show that some hydrogenation can be observed in the following 5th or 6th min. This result is in agreement with the literature for hydrogenation reactions with low oxygen concentration [9–11]. The quick reaction and the final high level of hydrogenation were not expected because of the difficulties reported in the literature for the hydrogen overcoming the oxide layer [8–15]. Despite the discarded Zircaloy-4 chips be oxidized, they are in such physical conditions that allow a quick hydrogenation process. 4.2. Large sample hydrogenation in the tube furnace system
Fig. 7. X-ray diffraction pattern of Test E4 after hydrogenation.
presence of only Zr-d and Zr-e-phases, with H/Zr atom ratio higher than 1.6, and no peaks of pure Zr metal mean that the final
The hydrogenation tests shown in Fig. 5 with large specimens (E1–E5) indicate that increasing the hydrogen gas pressure from 30 to 180 kPa does not increase their final fractional gain of mass, as the Sievert’s law would predict [15]. Excluding the E3 test
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(100 kPa), which shows a much lower value for the fractional gain of mass compared to the other tests, the overall trend is a slow decrease of this variable as a function of the H2 pressure. We note in Fig. 7 that the X-ray diffraction patterns of these tests are similar to the ones from the DTA analyses (Fig. 4). Thus these specimens are completely hydrogenated and also present some oxygen in their final state. For low hydrogen concentrations in a metal, Sievert’s law states that this concentration is proportional to the square root of the H2 partial pressure. Since the hydrogenated specimens were completely hydrided for all pressures, the Sievert’s law behavior is not observed, and Fig. 5 shows an almost flat fractional gain of mass as a function of pressure in the interval from 30 to 180 kPa. This result indicates that a hydrogen gas pressure of 30 kPa is sufficient to fully hydrogenate the material in the test conditions presented in Table 3. In summary, the Zircaloy-4 chips have specific properties that enable a quick hydrogenation process. We are able to fully hydrogenate specimens with about 3500 mg at temperatures around 770 K and hydrogen gas partial pressure as low as 30 kPa.
5. Conclusions It is important today to recycle materials because of their inherent value, and the necessity to reduce consumption of raw materials. Using differential thermal analysis, we investigated an alternative to recycle discarded Zircaloy-4 scrap obtained from the manufacture process of fuel clading tubes for pressurized water reactors, namely the production of Zr powder through the hydride–dehydride method. We studied the first part of the process, i.e., the hydrogenation process, and the results show that is possible to hydrogenate and pulverize the material. The differential thermal analyses show no hydrogenation at temperatures below 573 K and hydrogen gas pressure of 25 kPa. When the system temperature is raised to around 770 K, with the same gas pressure, the protecting oxide layer of the specimens can be overcome and they are quickly hydrogenated. The bulk of the reaction occurs in about 5 min with the precipitation of Zirconium hydrides in the Zr-d and Zr-e phases. Once the temperature passes 573 K, the incubation time to initiate the reaction is short (about 5 min). Tests in a tube furnace system, with hydrogen pressure varying from 30 to 180 kPa and temperature from 700 to 833.15 K, show that specimens of about 3500 mg are fully hydrogenated and can be easily pulverized. The results indicate that the hydrogenation of the Zircaloy-4 chips can be successfully
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undertaken at temperatures around 770 K and hydrogen gas pressure as low as 30 kPa. To obtain Zr powder as the final product, we must remove the absorbed H from the hydride material. The process of dehydrogenation is facilitated if the H content of the specimens is low, which can be obtained with lower hydrogen gas pressure during the hydrogenation process. For future work, using thermal gravimetry or simultaneous thermal analysis, we intend to study the hydrogenation process at H2 pressure levels below 30 kPa, and to study the dehydrogenation of the hydrided Zircaloy-4. In addition, we intend to quantify the content of hydrogen and oxygen in the final states of each process. Acknowledgements The first author wants to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçamento de Pessoal de Nível Superior (CAPES) for their financial support. Also the first author thanks José Marcelo Soares and Claudio Padovani for their important help during the measurements performed at the LABMAT/CTMSP. All authors thank and Centro Tecnológico da Marinha em São Paulo (CTMSP) for supporting this research, and also the very helpful comments of the reviewers. References [1] Nuclear power and sustainable development, International Atomic Energy Agency, 2006. [2] A.R. Totemeier, S.M. McDeavitt, J. Mater. Sci. 44 (2009) 5494–5500. [3] Powder production technology, In International Powder Metallurgy Directory,