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High-temperature Oxidation Behavior of Zircaloy-4 and Zirlo in Steam Ambient
J. Mater. Sci. Technol., 2010, 26(9), 827-832.
High-temperature Oxidation Behavior of Zircaloy-4 and Zirlo in Steam Ambient Hyung Hoon Kim1) , Jun Ho Kim1) , Jin Young Moon1) , Ho Seong Lee1)† , Jeong Joo Kim1) and Young Suck Chai2) 1) School of Materials Science and Engineering, Kyungpook National University, 1370 Sangyeok-dong, Buk-gu, Daegu, 702-701, Republic of Korea 2) School of Mechanical Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si, Gyeongsangbuk-do, 712-749, Republic of Korea [Manuscript received April 14, 2009, in revised form April 6, 2010]
The oxidation characteristics for Zircaloy-4 and Zirlo in the temperature range of 700–1200◦ C under steam supply condition were investigated by using a modified thermo-gravimetric analyzer. The specimens were oxidized for 3600 s at each temperature and then quenched in a furnace. The oxidation rate constants were measured from the weight gains to evaluate the oxidation behavior in Zircaloy-4 and Zirlo. The weight gain rates of Zirlo were lower than those of Zircaloy-4, leading to the low rate constants. The different oxidation behaviors between both cladding materials were considered to be due to the difference in their chemical compositions. KEY WORDS: Zircaloy-4; Zirlo; Oxidation
1. Introduction Zirconium-based alloys have been used for a fuel cladding in nuclear power plants because they have good corrosion resistance and superior mechanical properties[1–6] . Fuel cladding prevents a leakage of radioactive materials into the coolant and acts as a security barrier. Therefore, the structural integrity of a fuel cladding under normal and abnormal operation conditions, such as loss of coolant accident (LOCA) or reactivity initiated accident (RIA), in nuclear power plants should be considered to guarantee safety[7,8] . During LOCA conditions, fuel cladding experiences high temperature deformation and oxidation with increasing temperature and then thermal shock induced by the reflooding of an emergency coolant into the reactor core[9] . In this circumstance, the equivalent cladding reacted (ECR) must not exceed the 17% criterion and the peak cladding temperature (PCT) should be below 1200◦ C[10] . Zircaloy-4 † Corresponding author. Prof.; E-mail address: [email protected] (H.S. Lee).
(Zr-1.5Sn-0.2Fe-0.1Cr) alloy has been generally used as a fuel cladding. However, at high burn-up, problems such as oxidation, hydriding, and oxide spallation have been reported[11,12] and thus the development of an improved fuel cladding is required. During last few decades, Zirlo (Zr-1.0Nb1.0Sn-0.1Fe)[1] , M5 (Zr-1.0Nb)[2] , E635 (Zr-1.2Sn1Nb-0.4Fe)[3] , MDA (Zr-0.5Nb-0.8Sn-0.2Fe-0.1Cr)[4] , NDA (Zr-0.1Nb-1.0Sn-0.27Fe-0.16Cr-0.01Ni)[5] and HANA (Zr-(0.4-1.5)Nb-(0-0.8)Sn-Fe-Cr)[6] have been developed. Among them, some have been used in nuclear power plants. Zr-based alloys for the fuel cladding have been developed by adding Nb to improve the corrosion resistance and mechanical properties[1–6] . The solubility limit of Nb in Zr is about 0.6 wt pct. Upon adding Nb in Zr above the solubility limit, β-Zr and β-Nb phases are formed depending on the heat-treatment temperature[13] . Multi-component Zr-based alloys containing Nb precipitate in a complex form. To date, some studies on high-temperature oxidation behaviors of Zr-based alloys have been carried out[8,13] . It was
H. Kim et al.: J. Mater. Sci. Technol., 2010, 26(9), 827–832
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reported that the oxidation behavior of Zircaloy-4 in the temperature ranges of 700–1200◦ C generally complied with a parabolic rate law, but did not obey the parabolic rate in the temperature ranges of 800– 1050◦ C, which was attributed to the phase transformation of the base metal and its oxide[14] . Under the steam oxidation tests of HANA-4 and Zircaloy-4, the oxidation resistance of HANA-4 was superior to that of Zircaloy-4, which resulted from the higher Nb and lower Sn content in HANA-4 cladding[9] . However, comparative studies on high-temperature oxidation behaviors of Zircaloy-4 and Zirlo have not been performed in detail. In this study, the oxidation kinetics of Zircaloy-4 and Zirlo alloys were investigated and compared in detail in the temperature ranges of 700–1200◦ C. In addition, the microstructural properties of both alloys were also characterized after steam oxidation. 2. Experimental The chemical compositions of Zircaloy-4 and Zirlo used in this research are shown in Table 1. They were Table 1 Chemical composition of Zircaloy-4 and Zirlo (wt pct) Zircaloy-4 Zirlo
Nb — 1.8
Sn 1.6 1.2
Cr 0.1 —
Fe 0.2 0.1
Zr Bal. Bal.
measured by X-ray fluorescence (XRF, PW2400, Philips). For high-temperature oxidation tests, a tube-type fuel cladding was cut into 10 mm in length and then both sides were gradually polished up using SiC paper (Grit No. 2000). All samples were pickled in a solution of 5% HF, 45% HNO3 , 50% H2 O and then cleaned ultrasonically in an ethanol and acetone solution. The initial surface area and weight of the samples were measured to evaluate the variation of weight before and after steam oxidation. The apparatus for the high-temperature oxidation tests in a steam rich atmosphere was established by modifying the apparatus for thermogravimetric analysis (TGA). The variation of weight gains was continuously measured. The Ar gas flowed at a rate of 200 ml/min in order to minimize the initial oxidation during the heat-up to the desired temperature. The heat-up rate was set at 50◦ C/min. After the desired temperature was reached, the flow rate of Ar gas was set at 20 ml/min and the steam was supplied at a constant rate during the oxidation test. The amount of supplied steam was controlled by the dew point in order not to affect the weighing accuracy and to satisfy the conditions for a high-temperature oxidation. The dew point was maintained between 11 and 13◦ C. The oxidation tests were performed at 700–1200◦ C for 3600 s. The oxidized samples were cooled down to room temperature in a TGA. The weight gain was determined by an in-situ method within ±0.01 mg of accuracy.
After the steam oxidation tests, the microstructure of oxidized samples were investigated by optical microscopy. 3. Results and Discussion 3.1 Behavior of oxidation Figure 1 shows the continuous weight gains of the fuel claddings of Zircaloy-4 and Zirlo oxidized in steam atmosphere in the temperature ranges of 700–1200◦ C as a function of the oxidation time. In all temperature ranges, the weight gains at the early stage tend to increase according to the parabolic rate law[15] . This is considered to be due to the diffusion of the oxygen ions through the oxidation layer. The transition of the weight gains in the fuel cladding of Zirlo occurred at 1000◦ C after 1500 s. That is, a sudden increase of the oxidation rate was observed, which is called a breakaway oxidation[13] . But the rate transition was not observed in Zircaloy-4. This result is slightly different from the results reported by several other researchers[13,16] . This might be due to the different experimental methods and conditions. Even though it was not clear why the rate transition occurred at 1000◦ C, the transition of the weight gains could be considered to be attributed to the phase transformation in oxides. According to the Zr-O binary phase diagram, at this temperature, the ZrO2 oxide is transformed from a monoclinic to a tetragonal crystal structure[8] , resulting in the rate transition. This is in agreement with Baek s results on the change of the oxidation behaviors between 1000 and 1050◦ C[14] . The weight gains in Zircaloy-4 are larger than those in Zirlo except for 1000◦ C. The oxidation resistance of Zirlo would be superior to that of Zircaloy-4 in the temperature ranges except for 1000◦ C. This could be a result of the difference of the chemical composition in both materials. Zirlo contains 1.8% Nb and 1.2% Sn, compared with Zircaloy4 (0% Nb and 1.6% Sn). With the addition of Nb, β-Zr is stabilized and thus the oxidation resistance is improved. Sn contributes to the stabilization of α-Zr. Therefore, the superior oxidation resistance of Zirlo in comparison with Zircaloy-4 could be ascribed to the higher Nb and lower Sn content. Since the oxidation rate could be changed in terms of the oxidation temperature, the relationship between the weight gain and reaction time can be generalized as follows[15] : n Woxygen
absorbed
= Kn · t
where Kn is the oxidation rate constant (mg/dm2 )n /s). n and t are the oxidation rate exponent and reaction time (s), respectively. Figure 2 shows the oxidation rate exponent of Zircaloy-4 and Zirlo after steam oxidation. The oxidation rate exponents in both fuel claddings vary from 1.95 to 2.55. The oxidation reaction slightly deviates
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Fig. 1 Oxidation behaviors of the Zircaloy-4 and Zirlo at: (a) 700◦ C, (b) 800◦ C, (c) 900◦ C, (d) 1000◦ C, (e) 1100◦ C, (f) 1200◦ C
nent of Zircaloy-4 rapidly increases to 2.46. With the comparison of the rate exponents in Zircaloy-4 and Zirlo, the rate exponent of Zirlo is higher than that of Zircaloy-4 except for 1200◦ C. In the case of the oxidation reaction of the Zr alloys occurring in steam atmosphere at 1000–1100◦ C, the oxidation kinetics agreed with the parabolic rate law. Therefore, the oxidation rate constant during the oxidation can be expressed as follows[14] : Kn = Aexp (−Q/RT )
Fig. 2 Oxidation rate exponent of each specimen with temperature
from the parabolic rate law. The rate exponents in both fuel claddings tend to increase with increasing temperature in the temperature ranges of 700–900◦ C. In Zirlo, the rate exponent gradually decreases to 1200◦ C. The oxidation behavior of Zircaloy-4 is similar to that of Zirlo. But at 1200◦ C, the rate expo-
where A is a constant (mg/dm2 )n /s). Q and R are the activation energy for the oxidation reaction and a universal gas constant (1.987 cal/mol·K), respectively. T is the oxidation temperature (K). Figure 3 shows the oxidation rate constants of Zircaloy-4 and Zirlo after steam oxidation. As the oxidation temperature increased, the rate constants for both claddings increased. It was found that the oxidation rate constant of Zircaloy-4 was higher than that of Zirlo. The Baker-Just relationship has been used as a safety criterion for the high-temperature
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Baker-Just relationship, which is shown in Fig. 3. It was noted that the absorbed oxygen content could be overestimated in this study. The slopes of both cladding materials are in agreement with that of the Baker-Just relationship. It indicated that the activation energy for the steam oxidation was the same in both claddings. 3.2 Microstructure
Fig. 3 Oxidation rate constant (Kn ) with temperature
oxidation for Zr-based alloys[14] . The oxidation rate constants for both alloys are higher than those of the
When the high-temperature oxidation occurs, the metal matrix has two distinct microstructures[16] . The oxygen-stabilized α-Zr phase is formed near the surface since the oxygen diffused from the oxide layer. Upon quenching from β region, the prior β-Zr phase is formed in the central region because of the suppression of the oxygen diffusion. Figures 4 and 5 show
Fig. 4 Microstructure of the Zircaloy-4 with oxidation temperature: (a) 700◦ C, (b) 800◦ C, (c) 900◦ C, (d) 1000◦ C, (e) 1100◦ C, (f) 1200◦ C
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Fig. 5 Microstructure of the Zirlo with oxidation temperature: (a) 700◦ C, (b) 800◦ C, (c) 900◦ C, (d) 1000◦ C, (e) 1100◦ C, (f) 1200◦ C
the microstructural evolution of the oxide layers and metal matrix after high temperature oxidation as a function of the oxidation temperature for Zircaloy-4 and Zirlo, respectively. For the Zircaloy-4 samples, only α-Zr phase was observed at 700 and 800◦ C. The grains were coarsened with increasing temperature up to 800◦ C. The oxygen-stabilized α-Zr phase and α+β Zr were observed at 900◦ C. With increasing temperature above 1000◦ C, the sample was divided into three regions: ZrO2 oxide layer, oxygen-stabilized αZr layer, and prior β-Zr layer, as shown in Fig. 4(d). As the oxidation temperature increased, the thickness of the oxide layer and the oxygen-stabilized αZr layer increased and that of prior β-Zr layer decreased. For the sample oxidized at 1200◦ C, it was
completely oxidized as a columnar-like structure and most of the prior β-Zr disappeared. On the other hand, the microstructure of Zirlo also revealed a threeregion structure similar to Zircaloy-4. However, for the Zirlo cladding materials, the suppression of the grain growth was observed by addition of Nb below 900◦ C. The thickness of the oxide layer abruptly increased at 1000◦ C. This abrupt change of the oxide thickness could be due to the phase transformation of the oxide and the rate transition of oxidation, as mentioned above. The border between the oxygenstabilized α-Zr and β-Zr phases was not clearly distinguished above 1000◦ C. This is due to the addition of Nb. The addition of Nb in Zr-based alloys contributed to the stabilization of the β-Zr, which pro-
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hibited the oxygen diffusion during the steam oxidation. The grain size of the oxygen-stabilized α-Zr and prior β-Zr phases in Zirlo was smaller than that of Zircaloy-4. 4. Conclusion The oxidation behaviors for Zircaloy-4 and Zirlo in the temperature ranges of 700–1200◦ C under steam supply condition by using a modified thermogravimetric analyzer were investigated. The results showed the oxidation resistance of Zirlo below 1200◦ C was superior to that of Zircaloy-4. This was confirmed by the higher rate exponent of Zirlo due to the difference of the chemical composition. It was believed that Nb contributed to the stabilization of β-Zr and so suppressed the oxidation of the metal matrix.
Acknowledgement This work was supported by the Korea Science & Engineering Foundation through the BAERI Program (Grant No. M20508110003). REFERENCES [1 ] G.P. Sabol, G.R. Kilp, M.G. Balfour and E. Roberts: Development of a Cladding Alloy for High Burnup, in Proc. 8th Int. Symp. on Zirconium in the Nuclear Industry, ASTM STP, 1989, 1023, 227.
[2 ] J.P. Mardon, D. Charquet and J. Senevat: Influence of Composition and Fabrication Process on out-of-pile and in-pile Properties of M5 Alloy, in Zirconium in the Nuclear Industry: Twelfth Symposium, ASTM STP, 2000, 1354, 505. [3 ] V. Nikulina, P.P. Markelov and M. M. Peregud: J. Nucl. Mater., 1996, 238, 205. [4 ] M. Garde: ASTM STP, 1991, 1132, 566. [5 ] T. Isobe and Y. Matsuo: ASTM STP, 1991, 1132, 346. [6 ] Y.H. Jeong, S.Y. Park, M.H. Lee, B.K. Chio, J.H. Baek, J.Y. Park, J.H. Kim and H.G. Ki: J. Nucl. Sci. Technol., 2006, 43, 977. [7 ] F. Nagase, T. Otomo and H. Uetsuka: J. Nucl. Sci. Technol., 2003, 40, 213. [8 ] V.F. Urbanic: ASTM STP, 1977, 633, 168. [9 ] J.H. Baek and Y.H. Jeong: J. Nucl. Mater., 2007, 361, 30. [10] USNRC-SRP (Standard Review Plan), Sec. 4.2, NUREG-0800. [11] J.H. Kim, B.K. Choi, J.H. Baek and Y.H. Jeong: J. Nucl. Eng. Des., 2006, 236, 2386. [12] H.M. Chung and T.F. Kassner: J. Nucl. Mater., 1979, 84, 327. [13] J.H. Baek and Y.H. Jeong: J. Nucl. Mater., 2008, 372, 152. [14] J.H. Baek, K.B. Park and Y.H. Jeong: J. Nucl. Mater., 2004, 335, 443. [15] Y. Niu and F. Gesmundo: J. Mater. Sci. Technol., 2003, 19, 545. [16] H.G. Kim, J.H. Baek, S.D. Kim and Y.H. Jeong: J. Nucl. Mater., 2008, 372, 304.
High-temperature Oxidation Behavior of Zircaloy-4 and Zirlo in Steam Ambient Hyung Hoon Kim1) , Jun Ho Kim1) , Jin Young Moon1) , Ho Seong Lee1)† , Jeong Joo Kim1) and Young Suck Chai2) 1) School of Materials Science and Engineering, Kyungpook National University, 1370 Sangyeok-dong, Buk-gu, Daegu, 702-701, Republic of Korea 2) School of Mechanical Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si, Gyeongsangbuk-do, 712-749, Republic of Korea [Manuscript received April 14, 2009, in revised form April 6, 2010]
The oxidation characteristics for Zircaloy-4 and Zirlo in the temperature range of 700–1200◦ C under steam supply condition were investigated by using a modified thermo-gravimetric analyzer. The specimens were oxidized for 3600 s at each temperature and then quenched in a furnace. The oxidation rate constants were measured from the weight gains to evaluate the oxidation behavior in Zircaloy-4 and Zirlo. The weight gain rates of Zirlo were lower than those of Zircaloy-4, leading to the low rate constants. The different oxidation behaviors between both cladding materials were considered to be due to the difference in their chemical compositions. KEY WORDS: Zircaloy-4; Zirlo; Oxidation
1. Introduction Zirconium-based alloys have been used for a fuel cladding in nuclear power plants because they have good corrosion resistance and superior mechanical properties[1–6] . Fuel cladding prevents a leakage of radioactive materials into the coolant and acts as a security barrier. Therefore, the structural integrity of a fuel cladding under normal and abnormal operation conditions, such as loss of coolant accident (LOCA) or reactivity initiated accident (RIA), in nuclear power plants should be considered to guarantee safety[7,8] . During LOCA conditions, fuel cladding experiences high temperature deformation and oxidation with increasing temperature and then thermal shock induced by the reflooding of an emergency coolant into the reactor core[9] . In this circumstance, the equivalent cladding reacted (ECR) must not exceed the 17% criterion and the peak cladding temperature (PCT) should be below 1200◦ C[10] . Zircaloy-4 † Corresponding author. Prof.; E-mail address: [email protected] (H.S. Lee).
(Zr-1.5Sn-0.2Fe-0.1Cr) alloy has been generally used as a fuel cladding. However, at high burn-up, problems such as oxidation, hydriding, and oxide spallation have been reported[11,12] and thus the development of an improved fuel cladding is required. During last few decades, Zirlo (Zr-1.0Nb1.0Sn-0.1Fe)[1] , M5 (Zr-1.0Nb)[2] , E635 (Zr-1.2Sn1Nb-0.4Fe)[3] , MDA (Zr-0.5Nb-0.8Sn-0.2Fe-0.1Cr)[4] , NDA (Zr-0.1Nb-1.0Sn-0.27Fe-0.16Cr-0.01Ni)[5] and HANA (Zr-(0.4-1.5)Nb-(0-0.8)Sn-Fe-Cr)[6] have been developed. Among them, some have been used in nuclear power plants. Zr-based alloys for the fuel cladding have been developed by adding Nb to improve the corrosion resistance and mechanical properties[1–6] . The solubility limit of Nb in Zr is about 0.6 wt pct. Upon adding Nb in Zr above the solubility limit, β-Zr and β-Nb phases are formed depending on the heat-treatment temperature[13] . Multi-component Zr-based alloys containing Nb precipitate in a complex form. To date, some studies on high-temperature oxidation behaviors of Zr-based alloys have been carried out[8,13] . It was
H. Kim et al.: J. Mater. Sci. Technol., 2010, 26(9), 827–832
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reported that the oxidation behavior of Zircaloy-4 in the temperature ranges of 700–1200◦ C generally complied with a parabolic rate law, but did not obey the parabolic rate in the temperature ranges of 800– 1050◦ C, which was attributed to the phase transformation of the base metal and its oxide[14] . Under the steam oxidation tests of HANA-4 and Zircaloy-4, the oxidation resistance of HANA-4 was superior to that of Zircaloy-4, which resulted from the higher Nb and lower Sn content in HANA-4 cladding[9] . However, comparative studies on high-temperature oxidation behaviors of Zircaloy-4 and Zirlo have not been performed in detail. In this study, the oxidation kinetics of Zircaloy-4 and Zirlo alloys were investigated and compared in detail in the temperature ranges of 700–1200◦ C. In addition, the microstructural properties of both alloys were also characterized after steam oxidation. 2. Experimental The chemical compositions of Zircaloy-4 and Zirlo used in this research are shown in Table 1. They were Table 1 Chemical composition of Zircaloy-4 and Zirlo (wt pct) Zircaloy-4 Zirlo
Nb — 1.8
Sn 1.6 1.2
Cr 0.1 —
Fe 0.2 0.1
Zr Bal. Bal.
measured by X-ray fluorescence (XRF, PW2400, Philips). For high-temperature oxidation tests, a tube-type fuel cladding was cut into 10 mm in length and then both sides were gradually polished up using SiC paper (Grit No. 2000). All samples were pickled in a solution of 5% HF, 45% HNO3 , 50% H2 O and then cleaned ultrasonically in an ethanol and acetone solution. The initial surface area and weight of the samples were measured to evaluate the variation of weight before and after steam oxidation. The apparatus for the high-temperature oxidation tests in a steam rich atmosphere was established by modifying the apparatus for thermogravimetric analysis (TGA). The variation of weight gains was continuously measured. The Ar gas flowed at a rate of 200 ml/min in order to minimize the initial oxidation during the heat-up to the desired temperature. The heat-up rate was set at 50◦ C/min. After the desired temperature was reached, the flow rate of Ar gas was set at 20 ml/min and the steam was supplied at a constant rate during the oxidation test. The amount of supplied steam was controlled by the dew point in order not to affect the weighing accuracy and to satisfy the conditions for a high-temperature oxidation. The dew point was maintained between 11 and 13◦ C. The oxidation tests were performed at 700–1200◦ C for 3600 s. The oxidized samples were cooled down to room temperature in a TGA. The weight gain was determined by an in-situ method within ±0.01 mg of accuracy.
After the steam oxidation tests, the microstructure of oxidized samples were investigated by optical microscopy. 3. Results and Discussion 3.1 Behavior of oxidation Figure 1 shows the continuous weight gains of the fuel claddings of Zircaloy-4 and Zirlo oxidized in steam atmosphere in the temperature ranges of 700–1200◦ C as a function of the oxidation time. In all temperature ranges, the weight gains at the early stage tend to increase according to the parabolic rate law[15] . This is considered to be due to the diffusion of the oxygen ions through the oxidation layer. The transition of the weight gains in the fuel cladding of Zirlo occurred at 1000◦ C after 1500 s. That is, a sudden increase of the oxidation rate was observed, which is called a breakaway oxidation[13] . But the rate transition was not observed in Zircaloy-4. This result is slightly different from the results reported by several other researchers[13,16] . This might be due to the different experimental methods and conditions. Even though it was not clear why the rate transition occurred at 1000◦ C, the transition of the weight gains could be considered to be attributed to the phase transformation in oxides. According to the Zr-O binary phase diagram, at this temperature, the ZrO2 oxide is transformed from a monoclinic to a tetragonal crystal structure[8] , resulting in the rate transition. This is in agreement with Baek s results on the change of the oxidation behaviors between 1000 and 1050◦ C[14] . The weight gains in Zircaloy-4 are larger than those in Zirlo except for 1000◦ C. The oxidation resistance of Zirlo would be superior to that of Zircaloy-4 in the temperature ranges except for 1000◦ C. This could be a result of the difference of the chemical composition in both materials. Zirlo contains 1.8% Nb and 1.2% Sn, compared with Zircaloy4 (0% Nb and 1.6% Sn). With the addition of Nb, β-Zr is stabilized and thus the oxidation resistance is improved. Sn contributes to the stabilization of α-Zr. Therefore, the superior oxidation resistance of Zirlo in comparison with Zircaloy-4 could be ascribed to the higher Nb and lower Sn content. Since the oxidation rate could be changed in terms of the oxidation temperature, the relationship between the weight gain and reaction time can be generalized as follows[15] : n Woxygen
absorbed
= Kn · t
where Kn is the oxidation rate constant (mg/dm2 )n /s). n and t are the oxidation rate exponent and reaction time (s), respectively. Figure 2 shows the oxidation rate exponent of Zircaloy-4 and Zirlo after steam oxidation. The oxidation rate exponents in both fuel claddings vary from 1.95 to 2.55. The oxidation reaction slightly deviates
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Fig. 1 Oxidation behaviors of the Zircaloy-4 and Zirlo at: (a) 700◦ C, (b) 800◦ C, (c) 900◦ C, (d) 1000◦ C, (e) 1100◦ C, (f) 1200◦ C
nent of Zircaloy-4 rapidly increases to 2.46. With the comparison of the rate exponents in Zircaloy-4 and Zirlo, the rate exponent of Zirlo is higher than that of Zircaloy-4 except for 1200◦ C. In the case of the oxidation reaction of the Zr alloys occurring in steam atmosphere at 1000–1100◦ C, the oxidation kinetics agreed with the parabolic rate law. Therefore, the oxidation rate constant during the oxidation can be expressed as follows[14] : Kn = Aexp (−Q/RT )
Fig. 2 Oxidation rate exponent of each specimen with temperature
from the parabolic rate law. The rate exponents in both fuel claddings tend to increase with increasing temperature in the temperature ranges of 700–900◦ C. In Zirlo, the rate exponent gradually decreases to 1200◦ C. The oxidation behavior of Zircaloy-4 is similar to that of Zirlo. But at 1200◦ C, the rate expo-
where A is a constant (mg/dm2 )n /s). Q and R are the activation energy for the oxidation reaction and a universal gas constant (1.987 cal/mol·K), respectively. T is the oxidation temperature (K). Figure 3 shows the oxidation rate constants of Zircaloy-4 and Zirlo after steam oxidation. As the oxidation temperature increased, the rate constants for both claddings increased. It was found that the oxidation rate constant of Zircaloy-4 was higher than that of Zirlo. The Baker-Just relationship has been used as a safety criterion for the high-temperature
830
H. Kim et al.: J. Mater. Sci. Technol., 2010, 26(9), 827–832
Baker-Just relationship, which is shown in Fig. 3. It was noted that the absorbed oxygen content could be overestimated in this study. The slopes of both cladding materials are in agreement with that of the Baker-Just relationship. It indicated that the activation energy for the steam oxidation was the same in both claddings. 3.2 Microstructure
Fig. 3 Oxidation rate constant (Kn ) with temperature
oxidation for Zr-based alloys[14] . The oxidation rate constants for both alloys are higher than those of the
When the high-temperature oxidation occurs, the metal matrix has two distinct microstructures[16] . The oxygen-stabilized α-Zr phase is formed near the surface since the oxygen diffused from the oxide layer. Upon quenching from β region, the prior β-Zr phase is formed in the central region because of the suppression of the oxygen diffusion. Figures 4 and 5 show
Fig. 4 Microstructure of the Zircaloy-4 with oxidation temperature: (a) 700◦ C, (b) 800◦ C, (c) 900◦ C, (d) 1000◦ C, (e) 1100◦ C, (f) 1200◦ C
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Fig. 5 Microstructure of the Zirlo with oxidation temperature: (a) 700◦ C, (b) 800◦ C, (c) 900◦ C, (d) 1000◦ C, (e) 1100◦ C, (f) 1200◦ C
the microstructural evolution of the oxide layers and metal matrix after high temperature oxidation as a function of the oxidation temperature for Zircaloy-4 and Zirlo, respectively. For the Zircaloy-4 samples, only α-Zr phase was observed at 700 and 800◦ C. The grains were coarsened with increasing temperature up to 800◦ C. The oxygen-stabilized α-Zr phase and α+β Zr were observed at 900◦ C. With increasing temperature above 1000◦ C, the sample was divided into three regions: ZrO2 oxide layer, oxygen-stabilized αZr layer, and prior β-Zr layer, as shown in Fig. 4(d). As the oxidation temperature increased, the thickness of the oxide layer and the oxygen-stabilized αZr layer increased and that of prior β-Zr layer decreased. For the sample oxidized at 1200◦ C, it was
completely oxidized as a columnar-like structure and most of the prior β-Zr disappeared. On the other hand, the microstructure of Zirlo also revealed a threeregion structure similar to Zircaloy-4. However, for the Zirlo cladding materials, the suppression of the grain growth was observed by addition of Nb below 900◦ C. The thickness of the oxide layer abruptly increased at 1000◦ C. This abrupt change of the oxide thickness could be due to the phase transformation of the oxide and the rate transition of oxidation, as mentioned above. The border between the oxygenstabilized α-Zr and β-Zr phases was not clearly distinguished above 1000◦ C. This is due to the addition of Nb. The addition of Nb in Zr-based alloys contributed to the stabilization of the β-Zr, which pro-
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hibited the oxygen diffusion during the steam oxidation. The grain size of the oxygen-stabilized α-Zr and prior β-Zr phases in Zirlo was smaller than that of Zircaloy-4. 4. Conclusion The oxidation behaviors for Zircaloy-4 and Zirlo in the temperature ranges of 700–1200◦ C under steam supply condition by using a modified thermogravimetric analyzer were investigated. The results showed the oxidation resistance of Zirlo below 1200◦ C was superior to that of Zircaloy-4. This was confirmed by the higher rate exponent of Zirlo due to the difference of the chemical composition. It was believed that Nb contributed to the stabilization of β-Zr and so suppressed the oxidation of the metal matrix.
Acknowledgement This work was supported by the Korea Science & Engineering Foundation through the BAERI Program (Grant No. M20508110003). REFERENCES [1 ] G.P. Sabol, G.R. Kilp, M.G. Balfour and E. Roberts: Development of a Cladding Alloy for High Burnup, in Proc. 8th Int. Symp. on Zirconium in the Nuclear Industry, ASTM STP, 1989, 1023, 227.
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