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Effect of niobium additions on initial hydriding kinetics of uranium
Journal of Nuclear Materials 449 (2014) 49–53
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of niobium additions on initial hydriding kinetics of uranium Ruiwen Li ⇑, Xiaolin Wang China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, Sichuan, China
a r t i c l e
i n f o
Article history: Received 18 August 2013 Accepted 28 February 2014 Available online 11 March 2014
a b s t r a c t To study the behavior of hydrogen corrosion at the surface of U, U–2.5 wt%Nb alloy and U–5.7 wt%Nb, a gas–solid reaction system with an in situ microscope was designed. The nucleation and growth of the hydride of the alloy were continuously observed and recorded by a computer. The different characteristics of the hydrides on U metal and U–2.5 wt%Nb showed that the later alloy is more susceptible to hydrogen corrosion than the former. The growth rate of hydride of U–2.5 wt%Nb, calculated by measuring the perimeter of the hydride spots recorded by the in situ microscope, exhibited a reaction temperature dependency in the range of 40–160 °C, for pressure of 0.8 105 Pa. An Arrhenius plot for growth rate versus temperature yielded activation energy of 24.34 kJ/mol for the hydriding of U–2.5 wt%Nb alloy. The maximum hydriding rate was obtained at 125 °C, whose thermodynamics reason was discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Uranium and its alloys have been widely used in the field of nuclear engineering: nuclear power, armor-piercing ammunition, and nuclear weapon. However, due to its reactivity to environment atmosphere, especially hydrogen, the surface corrosion on U have been seriously concerned from the beginning of its use. Hydrogen attack to uranium usually results in such problems as hydride burning, strength weakening, and hydrogen corrosion. Sometimes, the amount of hydrogen gas is not so much because it mainly comes from decomposition of macromolecule materials or water in air. So, the initiation of hydrogen corrosion of U in the environment containing a small quantity of H2 has been generally studied. Gouder, Fu and Wang give the valence band and core level spectroscopy of UH3 [1,2]. Valence band spectra showed that UH3 is metallic, and 5f electrons are itinerant. U 4f core level spectra of UH3 showed a main peak at slightly higher binding energy than U metal. The works by Gouder and Ao confirmed the formation of b-UH3 at low temperature after U hydriding [1,3]. Ab initio calculations were made to investigate thermodynamics, hydrogen saturation and phase transformation of uranium–hydrogen system [4–7]. The result showed that volume-expanded due to the formation of UH3 phase is the primary kinetic barrier to hydride formation. Many works have been focused on the initial kinetics of hydriding of U [8–16], for example, Mintz and Glascott developed a uranium hydride formation models in which the oxide overlayer acts a barrier to hydrogen diffusion. However, the reaction kinetics
⇑ Corresponding author. Tel.: +86 816 3626742; fax: +86 816 3625900. E-mail address: [email protected] (R. Li). http://dx.doi.org/10.1016/j.jnucmat.2014.02.036 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
of U–H2 has been shown to vary widely from study to study due to its dependence on a variety of factors. These factors that are apparently difficult to replicate or control include the surface characteristics of the uranium metal and the presence of gaseous impurity. Previous references showed that adding niobium to uranium can improve the anti-oxidation of U, and strengthen the mechanics properties. Yet, there is little information concerning the hydrogen corrosion of U–Nb alloy. In this work, the influence of alloying of uranium on the initial kinetics of hydriding was focused.
2. Experimental and materials A reaction cell with a quartz window was designed. Facing the vertically window, an in situ microscope model HIROXKH-7700 was positioned, which can record the morphology change of hydride formation on sample. The experimental device is given in Fig. 1. A tube furnace surrounded the reaction cell so that the sample was heated to equivalent temperatures. Thermocouple was mounted in the center of the reaction vessel to monitor the temperature of specimens. Temperature was monitored and maintained constant to within ±1 °C using a temperature programmed controller. The pressure was continuously measured with a computer-based data acquisition system to determine the subtle change. High pure hydrogen (99.999%) was used for the reaction studies, which was obtained from a LaNi5Hx bed. U, U–2.5 wt%Nb and U–5.7 wt%Nb alloy used here were made by casting, annealing, and then cut to pieces with Ø10 3 mm. The final structure of U–2.5 wt%Nb alloy is a double phase composition of (a + c1–2) with pearlite banded structure. By the annealing process of U–2.5 wt%Nb in this work, the percentage of a and c1–2 phase
50
R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
Fig. 1. The schematic figure of the experimental device.
0.80 0.79
-5
Pressure,10 Pa
0.78
U U-5.7Nb U-2.5Nb
0.77 0.76 0.75 0.74 0.73 0.72 0.71 0.70
0
200 400
600 800 1000 1200 1400 1600 1800 2000 2200 2400
Time,s Fig. 2. Double phase microstructure of U–2.5 wt%Nb alloy after being etched.
Fig. 4. Pressure change of U, U–2.5 wt%Nb and U–5.7 wt%Nb reaction with H2 at same temperature (125 °C).
was then evacuated to a vacuum of 103 Pa for 1 h. The sample was then heated up to a temperature of 170 °C under vacuum to outgas for 1.5 h, which was named preheating in many references. Then, the samples were corroded more quickly by high pure hydrogen at different temperatures, for pressure of 0.8 105 Pa. For a better comparison of U–2.5 wt%Nb with U and U5.7 wt%Nb, the pretreatment processes of hydriding of them were ensured in the same. 3. Results and discussion 3.1. The initiation of hydride formation
Fig. 3. The initial kinetics of U–2.5 wt%Nb reaction with H2: hydrogen pressure, reaction rate, and morphology change with time, T = 80 °C, P = 0.83 105 Pa.
are 50%, 50% respectively, seen in Fig. 2. The Nb content in a phase is about 1%, which is less than that in c1–2 phase(19.5%). U–5.7 wt%Nb is an a00 martensite and a-U is a orthorhombic crystal system. Before the sample was loaded in the cell, it was mechanically polished to a mirror-like surface. The reaction cell
To comprehensively reveal the initial kinetics of hydriding, such kinetics parameters as hydrogen pressure, hydrogen consumption rate, induction period, and hydride sites were determined integrally, seen in Fig. 3. Unlike hydriding of other metals, there is an induction period before hydride formation on U materials. After the induction period (during which there is no measurable pressure change and no hydride site), about a few minutes, some hydride nuclei occur on the surface. Hydrogen corrosion reactions occur at the limited region near the surface, which was proved by the cross sections detection by SEM and OM .At the same time, hydrogen pressure (left coordinate in Fig. 3) begin to drop slowly, and reaction rate (right coordinate in Fig. 3) increase non-linearly, as shown in Fig. 3. With the consumption of hydrogen, more and
R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
Fig. 5. The morphology of hydride: (a) on U and (b) on U–2.5 wt%Nb.
Fig. 6. In situ observation of hydride nucleus and growth on U–2.5wt% Nb alloy: (a) T = 100 °C, P = 0.8 105 Pa and (b) T = 125 °C, P = 0.8 105 Pa.
51
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R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
more hydride nuclei occurred, and the dimension of each nucleus increase at a stable rate. The whole reaction rate is determined by both the number of hydride nuclei and the growth rate of each hydride nucleus. A Raman spectrum was used to characterize the hydride sites on-line, which showed that the precipitates are UH3. But because UH3 is combustible in air, XRD is not made. 3.2. The difference between U, U–2.5wt%Nb and U–5.7wt%Nb For the hydriding kinetics of U, U–2.5 wt%Nb and U–5.7 wt%Nb, the main characteristics are primarily same, including the presence of induction period and hydride preference sites. But reaction rate, the time of induction period and the shape of hydride sites display considerable differences, seen in Figs. 4 and 5. Deduced from Fig. 4, Obviously, the sequence of induction time is U –5.7 wt%Nb > U > U–2.5 wt%Nb, which shows that U–2.5 wt%Nb alloy is more susceptible to hydrogen corrosion compare with U and U–5.7 wt%Nb. Moreover, once the reaction began, the hydriding rate of U–2.5 wt%Nb alloy is also higher than U, which is concluded from the slope of the curve of pressure versus time. As well as, the hydride morphology of U and U–2.5 wt%Nb alloy are different, seen in Fig. 5. The dimension of hydride spots on U display two types, namely, there are big nuclei, as well as many small nuclei (seen in inset in Fig. 5a) dispersed among the bigger. However, the dimension of hydride on U–2.5 wt%Nb alloy is more uniform relatively, seen in Fig. 5b. Density of nuclei on U–2.5 wt%Nb alloy is smaller than that of U, but the growth rate of each hydride spot on U– 2.5 wt%Nb alloy is greater than U. We did not observe that the hydride is exclusively restricted to the gamma or alpha phase.
Fig. 7. Growth rate of hydride: perimeter dependence of growth time.
3.3. Determination of growth rate of hydride The process of hydride sites growth was recorded by the optical microscope described above (Section 1) at a rate of one picture per 3 s. The morphology of hydride of U–2.5 wt%Nb in different stages was displayed in Fig. 6. Then, 3–4 hydride spots which characterized typically the hydride growth were chosen to measure their perimeters. This measurement was made automatically by the instrument to exclude artificial effects, which can improve the measuring precision. Consequently, the diameter of each hydride was calculated based on the assumption that each hydride spot is round in shape. In this way, the dimensions of hydride spots changed with time were determined, seen in Fig. 7. By fitting these data, the slope (growth rate) of the line was calculated. So, the growth rates of hydride spots were calculated under different reaction condition. As shown in Fig. 7, every hydride spot grew at an approximately same rate under a reaction condition. However, due to chemical inhomogeneity, the growth rates under one reaction condition presented little difference. Thus, a mean rate was adopted on 3–4 hydride spots. 3.4. Influence of temperature on rate of hydride growth on U–2.5wt%Nb Fig. 8 shows the hydride growth rates on U–2.5 wt%Nb, which were calculated according to the way described in last section, as a function of reaction temperature of 40 °C, 60 °C, 80 °C, 100 °C, 125 °C, 140 °C, 160 °C, for hydrogen pressures of 0.8 105 Pa. To study the effect of temperature on hydriding kinetics, we have strictly ensured the hydrogen pressures are same. But in practice, artificial errors are unavoidable. The plot for ln v versus 1/T is divided evidently into two stages, i.e., high temperature and low temperature. At low temperature, an Arrhenius function has been fitted to the form described by Eq. (1):
v ¼ 12332:6 exp
24336:5 RT
ð1Þ
Fig. 8. Growth rate dependence of reactive temperature.
where v is the reaction rate, T is the reaction temperature, R is gas constant. For a zero order reaction of this hydriding reaction, the reaction rate constant (k) is equal to the reaction rate value (v). For U–2.5 wt%Nb, Eq. (1) yields an activation energy of 24.34 kJ/mol. We have also done many experiments on U reaction kinetics with H2, and got an activation energy of 60 kJ/mol which was accordant with Ref. [9]. Obviously, activation energy of U–2.5 wt%Nb was less than that of U. This was also consistent with the result of induction time in Section 3.2. Activation energy is associated with the sensitivity of the formation of hydride, consequently, it can be concluded again that U–2.5 wt%Nb alloy is more susceptible to hydrogen corrosion compare with pure U. At high temperature stage, a maximum rate reaches at 125 °C, and then, the rate decreases sharply with increasing of temperature. There are only two data (>125 °C) in Fig. 8, but the trend is obvious. In fact, when we increase temperature any more, the reaction rate is too slow to detect. This phenomenon can be explained by Eq. (2), which describes the bidirectional reactions like as U with H2 system in the whole temperature range:
v¼
dcA 1 ¼k 1 Kc dt
ð2Þ
where k is rate constant, Kc is equilibrium constant. For an exothermic reaction of this system, with an increasing of reaction temperature, Kc decrease, but k increase. So, there is a maximum reaction rate at a temperature, 125 °C for U–2.5 wt%Nb alloy with H2 system.
R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
4. Conclusions The hydriding of U, U–2.5 wt%Nb and U–5.7 wt%Nb were studied by an in situ microscope, and there are obvious differences between them in hydride morphology, rate and reaction activation energy. The reaction temperature dependency of growth rate of the hydride has been determined, in the range 40–160 °C, for pressure of 0.8 105 Pa. An Arrhenius plot for v versus T yields activation energy of 24.34 kJ/mol for U–2.5 wt%Nb alloy, and 60 kJ/mol for U. A maximum velocity of hydride of U–2.5 wt%Nb exists at the temperature of 125 °C, and thermodynamics explain for that was discussed. The results showed that U–2.5 wt%Nb alloy is more susceptible to hydrogen corrosion comparing with U and U–5.7 wt%Nb. Acknowledgements This work was partially supported by the Development Fund of China Academy of Engineering Physics. (Contract number: 2011B0301056).
53
The authors would like to thank G. Li for the hydriding experiment help, X. Lai and J. Yang for their technical support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Gouder, R. Eloirdi, F. Wastin, Phys. Rev. B. 70 (2004) 235108-1. X. Wang, X. Fu, Z. Zhao, Surf. Rev. Lett. 10 (2003) 325. B. Ao, X. Wang, Y. Wei, Y. Zhang, Appl. Cryst. 40 (2007) 796. D.T. Christopher, L. Turab, R.S. Lillard, Acta Mater. 58 (2010) 1045. D.T. Christopher, R.S. Lillard, Acta Mater. 57 (2009) 4707. K. Balasubramanian, W.J. Siekhaus, W. Mc, J. Chem. Phys. 119 (2003) 5889. J.L. Nie, H.Y. Xiao, X.T. Zu, F. Gao, J. Phys.: Condens. Matter 20 (2008) 1. J. Bloch, M.H. Mintz, The Kinetics of Hydride Formation in Uranium, Israel IAEC Report, 2001. J. Glascott, Sci. Tech. J. AWE. 6 (2003) 16. J. Bloch, M.H. Mintz, J. Alloys Compd. 241 (1996) 224. J. Bloch, M.H. Mintz, J. Less Common Met. 81 (1981) 301. J. Bloch, J. Less Common Met. 103 (1984) 163. S.G. Bazley, J.R. Petherbridge, J. Glascott, Solid State Ionics 211 (2012) 1. R.W. Li, X.L. Wang, G. Li, At. Energy Sci. Technol. 43 (2009) 683. R.W. Li, X.L. Wang, G. Li, Rare Met. Mater. Eng. 39 (2010) 1423. N. Shamir, Appl. Surf. Sci. 253 (2007) 5957.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of niobium additions on initial hydriding kinetics of uranium Ruiwen Li ⇑, Xiaolin Wang China Academy of Engineering Physics, P.O. Box 919-71, Mianyang 621900, Sichuan, China
a r t i c l e
i n f o
Article history: Received 18 August 2013 Accepted 28 February 2014 Available online 11 March 2014
a b s t r a c t To study the behavior of hydrogen corrosion at the surface of U, U–2.5 wt%Nb alloy and U–5.7 wt%Nb, a gas–solid reaction system with an in situ microscope was designed. The nucleation and growth of the hydride of the alloy were continuously observed and recorded by a computer. The different characteristics of the hydrides on U metal and U–2.5 wt%Nb showed that the later alloy is more susceptible to hydrogen corrosion than the former. The growth rate of hydride of U–2.5 wt%Nb, calculated by measuring the perimeter of the hydride spots recorded by the in situ microscope, exhibited a reaction temperature dependency in the range of 40–160 °C, for pressure of 0.8 105 Pa. An Arrhenius plot for growth rate versus temperature yielded activation energy of 24.34 kJ/mol for the hydriding of U–2.5 wt%Nb alloy. The maximum hydriding rate was obtained at 125 °C, whose thermodynamics reason was discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Uranium and its alloys have been widely used in the field of nuclear engineering: nuclear power, armor-piercing ammunition, and nuclear weapon. However, due to its reactivity to environment atmosphere, especially hydrogen, the surface corrosion on U have been seriously concerned from the beginning of its use. Hydrogen attack to uranium usually results in such problems as hydride burning, strength weakening, and hydrogen corrosion. Sometimes, the amount of hydrogen gas is not so much because it mainly comes from decomposition of macromolecule materials or water in air. So, the initiation of hydrogen corrosion of U in the environment containing a small quantity of H2 has been generally studied. Gouder, Fu and Wang give the valence band and core level spectroscopy of UH3 [1,2]. Valence band spectra showed that UH3 is metallic, and 5f electrons are itinerant. U 4f core level spectra of UH3 showed a main peak at slightly higher binding energy than U metal. The works by Gouder and Ao confirmed the formation of b-UH3 at low temperature after U hydriding [1,3]. Ab initio calculations were made to investigate thermodynamics, hydrogen saturation and phase transformation of uranium–hydrogen system [4–7]. The result showed that volume-expanded due to the formation of UH3 phase is the primary kinetic barrier to hydride formation. Many works have been focused on the initial kinetics of hydriding of U [8–16], for example, Mintz and Glascott developed a uranium hydride formation models in which the oxide overlayer acts a barrier to hydrogen diffusion. However, the reaction kinetics
⇑ Corresponding author. Tel.: +86 816 3626742; fax: +86 816 3625900. E-mail address: [email protected] (R. Li). http://dx.doi.org/10.1016/j.jnucmat.2014.02.036 0022-3115/Ó 2014 Elsevier B.V. All rights reserved.
of U–H2 has been shown to vary widely from study to study due to its dependence on a variety of factors. These factors that are apparently difficult to replicate or control include the surface characteristics of the uranium metal and the presence of gaseous impurity. Previous references showed that adding niobium to uranium can improve the anti-oxidation of U, and strengthen the mechanics properties. Yet, there is little information concerning the hydrogen corrosion of U–Nb alloy. In this work, the influence of alloying of uranium on the initial kinetics of hydriding was focused.
2. Experimental and materials A reaction cell with a quartz window was designed. Facing the vertically window, an in situ microscope model HIROXKH-7700 was positioned, which can record the morphology change of hydride formation on sample. The experimental device is given in Fig. 1. A tube furnace surrounded the reaction cell so that the sample was heated to equivalent temperatures. Thermocouple was mounted in the center of the reaction vessel to monitor the temperature of specimens. Temperature was monitored and maintained constant to within ±1 °C using a temperature programmed controller. The pressure was continuously measured with a computer-based data acquisition system to determine the subtle change. High pure hydrogen (99.999%) was used for the reaction studies, which was obtained from a LaNi5Hx bed. U, U–2.5 wt%Nb and U–5.7 wt%Nb alloy used here were made by casting, annealing, and then cut to pieces with Ø10 3 mm. The final structure of U–2.5 wt%Nb alloy is a double phase composition of (a + c1–2) with pearlite banded structure. By the annealing process of U–2.5 wt%Nb in this work, the percentage of a and c1–2 phase
50
R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
Fig. 1. The schematic figure of the experimental device.
0.80 0.79
-5
Pressure,10 Pa
0.78
U U-5.7Nb U-2.5Nb
0.77 0.76 0.75 0.74 0.73 0.72 0.71 0.70
0
200 400
600 800 1000 1200 1400 1600 1800 2000 2200 2400
Time,s Fig. 2. Double phase microstructure of U–2.5 wt%Nb alloy after being etched.
Fig. 4. Pressure change of U, U–2.5 wt%Nb and U–5.7 wt%Nb reaction with H2 at same temperature (125 °C).
was then evacuated to a vacuum of 103 Pa for 1 h. The sample was then heated up to a temperature of 170 °C under vacuum to outgas for 1.5 h, which was named preheating in many references. Then, the samples were corroded more quickly by high pure hydrogen at different temperatures, for pressure of 0.8 105 Pa. For a better comparison of U–2.5 wt%Nb with U and U5.7 wt%Nb, the pretreatment processes of hydriding of them were ensured in the same. 3. Results and discussion 3.1. The initiation of hydride formation
Fig. 3. The initial kinetics of U–2.5 wt%Nb reaction with H2: hydrogen pressure, reaction rate, and morphology change with time, T = 80 °C, P = 0.83 105 Pa.
are 50%, 50% respectively, seen in Fig. 2. The Nb content in a phase is about 1%, which is less than that in c1–2 phase(19.5%). U–5.7 wt%Nb is an a00 martensite and a-U is a orthorhombic crystal system. Before the sample was loaded in the cell, it was mechanically polished to a mirror-like surface. The reaction cell
To comprehensively reveal the initial kinetics of hydriding, such kinetics parameters as hydrogen pressure, hydrogen consumption rate, induction period, and hydride sites were determined integrally, seen in Fig. 3. Unlike hydriding of other metals, there is an induction period before hydride formation on U materials. After the induction period (during which there is no measurable pressure change and no hydride site), about a few minutes, some hydride nuclei occur on the surface. Hydrogen corrosion reactions occur at the limited region near the surface, which was proved by the cross sections detection by SEM and OM .At the same time, hydrogen pressure (left coordinate in Fig. 3) begin to drop slowly, and reaction rate (right coordinate in Fig. 3) increase non-linearly, as shown in Fig. 3. With the consumption of hydrogen, more and
R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
Fig. 5. The morphology of hydride: (a) on U and (b) on U–2.5 wt%Nb.
Fig. 6. In situ observation of hydride nucleus and growth on U–2.5wt% Nb alloy: (a) T = 100 °C, P = 0.8 105 Pa and (b) T = 125 °C, P = 0.8 105 Pa.
51
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R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
more hydride nuclei occurred, and the dimension of each nucleus increase at a stable rate. The whole reaction rate is determined by both the number of hydride nuclei and the growth rate of each hydride nucleus. A Raman spectrum was used to characterize the hydride sites on-line, which showed that the precipitates are UH3. But because UH3 is combustible in air, XRD is not made. 3.2. The difference between U, U–2.5wt%Nb and U–5.7wt%Nb For the hydriding kinetics of U, U–2.5 wt%Nb and U–5.7 wt%Nb, the main characteristics are primarily same, including the presence of induction period and hydride preference sites. But reaction rate, the time of induction period and the shape of hydride sites display considerable differences, seen in Figs. 4 and 5. Deduced from Fig. 4, Obviously, the sequence of induction time is U –5.7 wt%Nb > U > U–2.5 wt%Nb, which shows that U–2.5 wt%Nb alloy is more susceptible to hydrogen corrosion compare with U and U–5.7 wt%Nb. Moreover, once the reaction began, the hydriding rate of U–2.5 wt%Nb alloy is also higher than U, which is concluded from the slope of the curve of pressure versus time. As well as, the hydride morphology of U and U–2.5 wt%Nb alloy are different, seen in Fig. 5. The dimension of hydride spots on U display two types, namely, there are big nuclei, as well as many small nuclei (seen in inset in Fig. 5a) dispersed among the bigger. However, the dimension of hydride on U–2.5 wt%Nb alloy is more uniform relatively, seen in Fig. 5b. Density of nuclei on U–2.5 wt%Nb alloy is smaller than that of U, but the growth rate of each hydride spot on U– 2.5 wt%Nb alloy is greater than U. We did not observe that the hydride is exclusively restricted to the gamma or alpha phase.
Fig. 7. Growth rate of hydride: perimeter dependence of growth time.
3.3. Determination of growth rate of hydride The process of hydride sites growth was recorded by the optical microscope described above (Section 1) at a rate of one picture per 3 s. The morphology of hydride of U–2.5 wt%Nb in different stages was displayed in Fig. 6. Then, 3–4 hydride spots which characterized typically the hydride growth were chosen to measure their perimeters. This measurement was made automatically by the instrument to exclude artificial effects, which can improve the measuring precision. Consequently, the diameter of each hydride was calculated based on the assumption that each hydride spot is round in shape. In this way, the dimensions of hydride spots changed with time were determined, seen in Fig. 7. By fitting these data, the slope (growth rate) of the line was calculated. So, the growth rates of hydride spots were calculated under different reaction condition. As shown in Fig. 7, every hydride spot grew at an approximately same rate under a reaction condition. However, due to chemical inhomogeneity, the growth rates under one reaction condition presented little difference. Thus, a mean rate was adopted on 3–4 hydride spots. 3.4. Influence of temperature on rate of hydride growth on U–2.5wt%Nb Fig. 8 shows the hydride growth rates on U–2.5 wt%Nb, which were calculated according to the way described in last section, as a function of reaction temperature of 40 °C, 60 °C, 80 °C, 100 °C, 125 °C, 140 °C, 160 °C, for hydrogen pressures of 0.8 105 Pa. To study the effect of temperature on hydriding kinetics, we have strictly ensured the hydrogen pressures are same. But in practice, artificial errors are unavoidable. The plot for ln v versus 1/T is divided evidently into two stages, i.e., high temperature and low temperature. At low temperature, an Arrhenius function has been fitted to the form described by Eq. (1):
v ¼ 12332:6 exp
24336:5 RT
ð1Þ
Fig. 8. Growth rate dependence of reactive temperature.
where v is the reaction rate, T is the reaction temperature, R is gas constant. For a zero order reaction of this hydriding reaction, the reaction rate constant (k) is equal to the reaction rate value (v). For U–2.5 wt%Nb, Eq. (1) yields an activation energy of 24.34 kJ/mol. We have also done many experiments on U reaction kinetics with H2, and got an activation energy of 60 kJ/mol which was accordant with Ref. [9]. Obviously, activation energy of U–2.5 wt%Nb was less than that of U. This was also consistent with the result of induction time in Section 3.2. Activation energy is associated with the sensitivity of the formation of hydride, consequently, it can be concluded again that U–2.5 wt%Nb alloy is more susceptible to hydrogen corrosion compare with pure U. At high temperature stage, a maximum rate reaches at 125 °C, and then, the rate decreases sharply with increasing of temperature. There are only two data (>125 °C) in Fig. 8, but the trend is obvious. In fact, when we increase temperature any more, the reaction rate is too slow to detect. This phenomenon can be explained by Eq. (2), which describes the bidirectional reactions like as U with H2 system in the whole temperature range:
v¼
dcA 1 ¼k 1 Kc dt
ð2Þ
where k is rate constant, Kc is equilibrium constant. For an exothermic reaction of this system, with an increasing of reaction temperature, Kc decrease, but k increase. So, there is a maximum reaction rate at a temperature, 125 °C for U–2.5 wt%Nb alloy with H2 system.
R. Li, X. Wang / Journal of Nuclear Materials 449 (2014) 49–53
4. Conclusions The hydriding of U, U–2.5 wt%Nb and U–5.7 wt%Nb were studied by an in situ microscope, and there are obvious differences between them in hydride morphology, rate and reaction activation energy. The reaction temperature dependency of growth rate of the hydride has been determined, in the range 40–160 °C, for pressure of 0.8 105 Pa. An Arrhenius plot for v versus T yields activation energy of 24.34 kJ/mol for U–2.5 wt%Nb alloy, and 60 kJ/mol for U. A maximum velocity of hydride of U–2.5 wt%Nb exists at the temperature of 125 °C, and thermodynamics explain for that was discussed. The results showed that U–2.5 wt%Nb alloy is more susceptible to hydrogen corrosion comparing with U and U–5.7 wt%Nb. Acknowledgements This work was partially supported by the Development Fund of China Academy of Engineering Physics. (Contract number: 2011B0301056).
53
The authors would like to thank G. Li for the hydriding experiment help, X. Lai and J. Yang for their technical support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Gouder, R. Eloirdi, F. Wastin, Phys. Rev. B. 70 (2004) 235108-1. X. Wang, X. Fu, Z. Zhao, Surf. Rev. Lett. 10 (2003) 325. B. Ao, X. Wang, Y. Wei, Y. Zhang, Appl. Cryst. 40 (2007) 796. D.T. Christopher, L. Turab, R.S. Lillard, Acta Mater. 58 (2010) 1045. D.T. Christopher, R.S. Lillard, Acta Mater. 57 (2009) 4707. K. Balasubramanian, W.J. Siekhaus, W. Mc, J. Chem. Phys. 119 (2003) 5889. J.L. Nie, H.Y. Xiao, X.T. Zu, F. Gao, J. Phys.: Condens. Matter 20 (2008) 1. J. Bloch, M.H. Mintz, The Kinetics of Hydride Formation in Uranium, Israel IAEC Report, 2001. J. Glascott, Sci. Tech. J. AWE. 6 (2003) 16. J. Bloch, M.H. Mintz, J. Alloys Compd. 241 (1996) 224. J. Bloch, M.H. Mintz, J. Less Common Met. 81 (1981) 301. J. Bloch, J. Less Common Met. 103 (1984) 163. S.G. Bazley, J.R. Petherbridge, J. Glascott, Solid State Ionics 211 (2012) 1. R.W. Li, X.L. Wang, G. Li, At. Energy Sci. Technol. 43 (2009) 683. R.W. Li, X.L. Wang, G. Li, Rare Met. Mater. Eng. 39 (2010) 1423. N. Shamir, Appl. Surf. Sci. 253 (2007) 5957.