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Experimental results on the breakaway oxidation of the E110 cladding alloy under high-temperature isothermal conditions
Progress in Nuclear Energy 93 (2016) 89e95
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Experimental results on the breakaway oxidation of the E110 cladding alloy under high-temperature isothermal conditions bet Perez-Fero *, Tama s Novotny, Anna Pinte r-Csorda s, Miha ly Kunsta r, Zolta n Ho zer, Erzse rta Horva th, Lajos Matus Ma Hungarian Academy of Sciences Centre for Energy Research, H-1525, Budapest, P.O. Box 49, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2015 Received in revised form 11 July 2016 Accepted 8 August 2016
In order to evaluate the effect of zirconium breakaway oxidation on the behaviour of nuclear fuel in LOCA (Loss-of-coolant accident) conditions, high temperature (800e1200 C) oxidation tests with E110 type cladding were performed in steam atmosphere. The onset time of breakaway oxidation was detected using an online method based on hydrogen release. The experimental results showed that the breakaway oxidation starts not earlier than 5 min after the start of the oxidation that is longer than the duration of dry phase in a design basis accident LOCA. The experiments give preliminary indication that the breakaway oxidation does not play role in design basis accident LOCA conditions with the E110 alloy. This is to be confirmed by transient oxidation tests. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Zirconium alloy Cladding High temperature oxidation Breakaway phenomenon
1. Introduction Zirconium alloys are used in nuclear reactors as fuel cladding material due to their low neutron absorption cross section, good corrosion resistance and mechanical properties under operating conditions. However, significant oxidation may occur on the surface of the fuel cladding under loss-of-coolant accident (LOCA) conditions. Generally, a protective oxide film develops, decelerating further oxidation and hydrogen uptake, but the tetragonal to monoclinic phase transformation of the oxide can lead to the spalling of the oxide layer. This is the so called breakaway phenomenon, which is characterised by sudden increase of oxidation rate (Schanz and Leistikow, 1995; Qin et al., 2007; Billone et al., 2008; Baek and Jeong, 2008). Although the phenomenon is known for quite a long time, its mechanism is not entirely understood. A lot of factors have impact on the breakaway oxidation, for example: oxidation conditions, chemical composition of the alloy, impurities, surface roughness, etc. All of these factors should be taken into account to get explanations for this phenomenon. Zirconium claddings could lose their ductility because of the oxidation and the accompanying hydrogen absorption. The embrittlement of the cladding takes place faster in case of a spalling
* Corresponding author. ). E-mail address: [email protected] (E. Perez-Fero http://dx.doi.org/10.1016/j.pnucene.2016.08.006 0149-1970/© 2016 Elsevier Ltd. All rights reserved.
oxide layer, since the hydrogen uptake is enhanced under such conditions. The embrittlement of the material can give rise to the failure of the fuel rods under thermal and mechanical loads. For this reason, it is important to study the circumstances that may affect the mechanical properties of the fuel cladding. Breakaway oxidation characterises the Russian type E110 alloy in a wider range of temperature compared to Western alloys (e.g Zircaloy-4, M5® and Zirlo(™)). However, the available data on breakaway oxidation behaviour of this cladding alloy is scarce (Yegorova et al., 2005; Steinbrück et al., 2010, 2011; Nikulin et al., 2011; Yan et al., 2009; OECD, 2009). In recent years, several experiments have been carried out at the Hungarian Academy of Sciences Centre for Energy Research to investigate the oxidation, hydrogen uptake and mechanical performance of the E110 alloy zer et al., 2008; under accident conditions (Matus et al., 2001; Ho et al., 2010). Perez-Fero In this study, high temperature oxidation experiments were performed with E110 fuel cladding specimens to get an overview on its breakaway oxidation behaviour. We focused on the influence of the temperature on the breakaway phenomenon. The tests were carried out in steam environment in the temperature range of 800e1200 C. However, further experiments are needed to investigate the effects of other factors on the oxidation.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
90 Table 1 Chemical composition of the E110 alloy (ppm). Nb
Mg
Al
Si
Cr
Mn
Fe
Ni
Cu
Hf
F
Cl
Zr
10,000
0.5
0.5
1.0
10
0.1
45
15
0.5
100
30
3
Balance
was monitored online by a computerized data acquisition system. At the end of oxidation the sample was withdrawn to the cold part of the equipment. The extent of the oxidation was calculated on the basis of the measured mass gain.
2.3. Method and measurement
2. Experimental procedures 2.1. Material The test series were carried out with E110 fuel cladding tubes produced by electrolytic method (currently used at Paks Nuclear Power Plant). The chemical composition of the alloy was determined by mass-spectrometry. The result of this analysis is shown in Table 1. 8 mm long specimens were cut from the original E110 tubes with an average outer diameter of 9.13 mm. The specimens were cleaned by total immersion in organic solvent and air-dried. 2.2. Equipment The zirconium specimens were oxidised in steam e argon mixture with 12 v/v% high purity argon gas (99.999 v/v%) under isothermal conditions. A high temperature tube furnace with a quartz tube was used for the oxidation experiments. The internal diameter of the quartz tube was 19 mm. The experimental apparatus (Fig. 1) consisted of a steam generator, a horizontally arranged three-zone resistance furnace with temperature control system, condenser and thermal conductivity detector (TCD). External heaters were applied in order to avoid condensation in the quartz tube. The steam was condensed in the condenser with an ice water mixture. The steam flow rate was 2.0e4.0 mg/cm2/s. It was determined by measuring the mass of the condensed water collected during the test period, divided by the oxidation time and the crosssectional area of the quartz tube. Before sample insertion, the test apparatus was sufficiently flushed with steam e argon mixture. After stabilization of temperature and steam flow, the quartz boat with the sample was pushed into the heated zone of the furnace. The hydrogen concentration of the outlet carrier gas was measured by a thermal conductivity detector during steam oxidation. The detector signal
An online method based on the hydrogen release during steam oxidation was used to study the breakaway phenomenon. The zirconium-steam reaction produces zirconium dioxide and hydrogen. The metal partly absorbs the formed hydrogen and the rest of it goes into the gas phase with the carrier gas (argon). This gas gets to the thermal conductivity detector through the condenser. Since hydrogen has a much higher thermal conductivity than argon, the formed hydrogen causes a significant change in thermal conductivity compared to carrier gas alone. The detector signal is directly proportional to the hydrogen concentration. Thermal conductivity detector was calibrated with high purity hydrogen (99.999 v/v%). The hydrogen release during high temperature zirconium oxidation is closely related to the properties of the oxide layer. At the beginning of the oxidation a dense and protective (tetragonal) oxide layer develops on the surface of the zirconium (Billone et al., 2008). Further oxidation is limited by this compact oxide layer. When the breakaway oxidation takes place, the steam penetrates into the cracks of the oxide and rapidly reaches the fresh metal surface. It results in faster oxidation kinetics and enhanced hydrogen formation. Consequently, the change of the hydrogen concentration in the carrier gas indicates the appearance of the oxide cracks. If the breakaway oxidation does not take place the hydrogen production smoothly decreases. The flowchart of the online method for the investigation of the breakaway oxidation is shown in Fig. 2. Fig. 3 represents the TCD signal during the steam oxidation of E110 alloy at 1000 C as a function of the oxidation time. Most important steps of the process are marked in Fig. 3. After insertion of the sample into the furnace (Step 1.), hydrogen formation starts due to the zirconium-steam reaction (Step 2.). A part of the hydrogen is absorbed by the zirconium sample; the remaining hydrogen goes into the gas phase with the argon. The protective oxide layer starts to develop, thus the oxidation rate and
Fig. 1. Schematic of the experimental apparatus used for the oxidation.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
91
Fig. 2. The flowchart of the online method for the investigation of the breakaway oxidation.
Fig. 3. Hydrogen release during oxidation of E110 sample at 1000 C for 2700 s.
hydrogen formation decrease (Step 3.). Cracks formed in the oxide (Step 4.) offer free path for the steam to reach the fresh metal surface. It results in higher oxidation rate and intensive hydrogen formation (Step 5.). Due to a new oxide layer formation on the fresh metal, the reaction rate decreases again (Step 6.) until the formation of new cracks (Step 7.). The next step (Step 8.) indicates the sample withdrawal, resulting in cessation of hydrogen formation (Step 9.).
3. Experimental results 3.1. Oxidation tests followed by online monitoring The hydrogen release during steam oxidation of E110 fuel cladding specimens was examined in the temperature range of 800e1200 C, with 50 C steps. The tests between 900 C and 1200 C were terminated after 2700 s. The oxidation at 800 C was carried out for longer time because of the lower reaction rate.
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et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
Fig. 4. Breakaway oxidation map of E110 alloy in the temperature range of 850e1050 C.
Fig. 4 and Fig. 5 show the TCD signals during oxidation tests of E110 at different temperatures versus the oxidation time. The amount of released hydrogen was influenced by the temperature. At the beginning of oxidation the TCD signal increased with increasing temperatures in all cases. The first cracks at each temperature are indicated in Fig. 4. Significant differences were found in breakaway oxidation times at different oxidation temperatures (Table 2). The breakaway oxidation of E110 occurred at longer oxidation times at 800 C (10700 s) and 850 C (1800 s). Contrarily, short breakaway times (330e510 s) were found in the temperature range between 900 C and 1050 C. The shortest oxidation time (330 s) was measured at 950 C. These oxide cracks were followed by further cracks. No breakaway oxidation behaviour was detected (the hydrogen production smoothly decreased without peaks
typical for breakaway) at 1100 C and above this temperature. A few tests were repeated at the same conditions to verify the results obtained from this online method. Fig. 6 represents the results of the replicated experiments at 1000 C. The curve characteristics were very similar for both experiments and nearly the same breakaway oxidation times were observed. 3.2. Additional examinations Additional oxidation tests were carried out for different times between 1000 C and 1200 C to produce samples for scanning electron microscope (SEM) analysis and metallographic examinations. The purpose of these tests was to get information on the microstructure of the oxide layers and to confirm the presence of oxide cracks that are characteristic of breakaway.
Fig. 5. Hydrogen release during oxidation of E110 samples at 1100 C, 1150 C and 1200 C.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero Table 2 The breakaway oxidation times at different oxidation temperatures. Test ID
Oxidation temperature ( C)
Breakaway time (s)
BE-20 BE-13 BE-12 BE-10 BE-06 BE-08 BE-09 BE-25 BE-26
800 850 900 950 1000 1050 1100 1150 1200
10,700 1800 500 330 410 510 e e e
Fig. 7 illustrates the extent of E110 oxidation (mass increase per surface area) as a function of the oxidation time at 1000 C. The post-test appearance of some E110 samples is also shown in this figure. A protective oxide layer formed on the sample which was oxidised for the shortest time. At 400 s the oxide layer started to peel off from the surface. The longer oxidation time resulted in the more cracked oxide layer. The SEM images of the E110 samples oxidised for 200 s and 400 s at 1000 C are shown in Fig. 8. The oxide layer was thin (<5 mm) and compact in case of the sample oxidised for 200 s. The first circumferential crack was observed at 400 s, which is in line with the result of the test carried out by online method. Fig. 9 shows the peeling oxide layer on the sample oxidised for 400 s. The optical micrographs of E110 cladding oxidised at 1100 C and 1200 C in steam are shown in Fig. 10. Thick, protective oxide layers were formed at both temperatures. No cracks were found in the oxide layers, indicating that breakaway oxidation did not occur. 4. Discussion Previous papers have shown that breakaway oxidation occurs in E110 cladding oxidised in steam at temperatures higher than 800 C (Yegorova et al., 2005; Steinbrück et al., 2011). They indicated pronounced breakaway oxidation between 900 C and 1000 C. Other studies focused on this temperature range and above (Baek and Jeong, 2008; Yan et al., 2009). The breakaway oxidation time is
93
usually defined on the basis of the weight-gain rate and the associated hydrogen uptake. In this study, the breakaway oxidation is followed by measuring the released hydrogen during steam oxidation. The applied method only measures the released hydrogen and cannot give information about the absorbed hydrogen. In (Grosse et al., 2009) it was reported that the hydrogen uptake of zirconium alloys during steam oxidation was connected with the phase transition in the oxide and it cannot be estimated from temperature and time data alone. According to the literature (Steinbrück et al., 2011), the tetragonal to monoclinic phase transition leads to the formation of circumferential cracks in the oxide layer. The formation of cracks causes spallation and the oxide loses its protective nature, thus the further oxidation is no longer limited by diffusion through the oxide layer. Due to the faster oxidation rate the hydrogen formation suddenly increases, which indicates the start of breakaway oxidation. Our results show that not only the hydrogen uptake but the hydrogen release is also influenced by the structure of the formed oxide layer. This statement is confirmed by scanning electron microscope analysis of oxidised samples before and after the breakaway. The SEM images of the sample beyond the breakaway time (400 s at 1000 C) revealed a major circumferential crack in the oxide layer which is characteristic of breakaway. This result is consistent with the study of M. Billone et al (Billone et al., 2008). who also reported early breakaway oxidation times at 1000 C. Our oxidation tests show similarly short breakaway times between 900 C and 1050 C. The shortest breakaway time for the Russian type E110 alloy is 330 s. Oxidation accompanied by oxide cracking does not take place at 1100 C and above. The metallographic examinations show thick and dense oxide layers without cracks, confirming the results from n-Moya et al., 2010) online method. It is generally admitted (Corvala that above 1050 C, the tetragonal oxide formed is dense and protective, even for thick oxide scale. The breakaway oxidation, which occurs in specific temperature ranges, has been identified by OECD experts (OECD, 2009) as a major problem for the cladding LOCA performance, since it is associated with a significant hydrogen pickup which degrades the
Fig. 6. Hydrogen release during replicated tests at 1000 C.
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et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
Fig. 7. The measured mass increase per surface area for the steam oxidation of E110 alloy at different times at 1000 C.
Fig. 8. SEM images of the E110 samples oxidised for 200 s (left) and for 400 s (right) at 1000 C.
Fig. 9. Peeling oxide layer on the surface of the sample oxidised for 400 s at 1000 C.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
95
Fig. 10. Optical micrographs of E110 cladding oxidised for 3000 s at 1100 C (left) and for 1100 s at 1200 C (right).
mechanical properties of the cladding. Previous studies (Billone et al., 2010) showed et al., 2008; Yegorova et al., 2005; Perez-Fero that breakaway phenomenon in high temperature steam takes place with E110 alloy more easily than with other zirconium cladding alloys. The experiments described in this paper indicated that the breakaway oxidation does not play role in design basis accident LOCA conditions even with the E110 alloy, since the breakaway times are longer than the duration of dry phase in the LOCA event. In order to confirm the above, further investigation is needed. 5. Conclusions High temperature oxidation of E110 cladding specimens was carried out in steam environment between 800 C and 1200 C. These experiments were aimed at studying the breakaway phenomenon on the surface of zirconium using an online method. This technique is based on the online monitoring of hydrogen release during oxidation. Special attention was given to investigate the influence of the temperature on the breakaway oxidation. The breakaway oxidation times were determined. The breakaway effect was observed in the temperature range of 800e1050 C for E110. The test results indicated short breakaway times (330e510 s) at temperatures between 900 C and 1050 C. Longer breakaway times were found below 900 C. At higher temperatures (1100 Ce1200 C) the breakaway phenomenon did not occur. The results regarding breakaway oxidation were confirmed by different measuring methods. Taking into account that the dry phase in design basis LOCA typically lasts only for 2e4 min, it can be concluded that the breakaway oxidation will not start for E110 cladding during the accident. So the enhanced oxidation and hydrogen uptake associated with breakaway phenomenon will not danger the integrity of fuel cladding during the LOCA accident. Since, the oxidation tests were carried out under isothermal conditions, further transient oxidation tests are required to fully simulate LOCA events. Acknowledgments This work was funded by the Hungarian Atomic Energy Authority (HAEA), Research Contract No. OAH/NBI-ABA-15/09-M.
References Baek, Jong Hyuk, Jeong, Yong Hwan, 2008. Breakaway phenomenon of Zr-based alloys during a high-temperature oxidation. J. Nucl. Mater. 372, 152e159. Billone, M., Yan, Y., Burtseva, T., Daum, R., Scott, H., 2008. Cladding Embrittlement during Postulated Loss-of-coolant Accidents. NUREG/CR-6967, ANL-07/04. n-Moya, C., Desgranges, C., Toffolon-Masclet, C., Servant, C., Brachet, J.C., Corvala 2010. Numerical modeling of oxygen diffusion in the wall thickness of Low-Tin Zircaloy-4 fuel cladding tube during high temperature (1100e1250 C) steam oxidation. J. Nucl. Mater. 400, 196e204. Grosse, M., Lehmann, E., Steinbrück, M., Kühne, G., Stuckert, J., 2009. Influence of oxide layer morphology on hydrogen concentration in tin and niobium containing zirconium alloys after high temperature steam oxidation. J. Nucl. Mater. 385, 339e345. zer, Zolt } ri, Csaba, Matus, Lajos, Horva th, M Ho an, Gyo arta, 2008. Ductile-to-brittle transition of oxidised Zircaloy-4 and E110 claddings. J. Nucl. Mater. 373, 415e423. zer, Zolt szlo , 2001. Some experimental results on Matus, Lajos, Ho an, Vas aros, La hydrogen-cladding material interactions. HPR-356/27. In: Proceedings EHPG, Lillehammer. Nikulin, S.A., Rozhnov, A.B., Belov, V.A., Li, E.V., Glazkina, V.S., 2011. Influence of chemical composition of zirconium alloy E110 on embrittlement under LOCA condition e Part 1: oxidation kinetics and macro characteristics of structure and fracture. J. Nucl. Mater. 418, 1e7. OECD, 2009. Nuclear Fuel Behaviour in Loss-of-coolant Accident (LOCA) Conditions, State-of-the-art Report. , Erzse bet, Gyo }ri, Csaba, Matus, Lajos, Vasa ros, L , Ho zer, Zolta n, Perez-Fero aszlo ter, Maro ti, La szlo , Horva th, Ma rta, Nagy, Imre, Pinte rWindberg, Pe s, 2010. Experimental database of E110 claddings Csord as, Anna, Novotny, Tama exposed to accident conditions. J. Nucl. Mater. 397, 48e54. Qin, W., Nam, C., Li, H.L., Szpunar, J.A., 2007. Effects of local stress on the stability of tetragonal phase in ZrO2 film. J. Alloys Compd. 437, 280e284. Schanz, G., Leistikow, S., October 04-06, 1995. Experimental results on hydrogen pick-up and storage potential of Zircaloy-4. In: International QUENCH Workshop, Karlsruhe. Steinbrück, M., Birchley, J., Boldyrev, A.V., Goryachev, A.V., Grosse, M., Haste, T.J., zer, Z., Kisselev, A.E., Nalivaev, V.I., Semishkin, V.P., Sepold, L., Stuckert, J., Ho r, N., Veshchunov, M.S., 2010. High-temperature oxidation and quench Ve behaviour of Zircaloy-4 and E110 cladding alloys. Prog. Nucl. Energy 52, 19e36. r, N., Grosse, M., 2011. Oxidation of advanced zirconium cladding Steinbrück, M., Ve alloys in steam at temperatures in the range of 600e1200 C. Oxid. Metals 76 (3e4), 215e232. Yan, Y., Burtseva, T.A., Billone, M.C., 2009. High-temperature steam-oxidation behavior of Zr-1Nb cladding alloy E110. J. Nucl. Mater. 393, 433e448. Yegorova, L., Lioutov, K., Jouravkova, N., Konobeev, A., Smirnov, V., Chesanov, V., Goryachev, A., 2005. Experimental Study of Embrittlement of Zr-1%Nb VVER Cladding under LOCA-relevant Conditions. NUREG/IA-0211/IRSN 2005-194/NSI RRC KI 3188.
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Experimental results on the breakaway oxidation of the E110 cladding alloy under high-temperature isothermal conditions bet Perez-Fero *, Tama s Novotny, Anna Pinte r-Csorda s, Miha ly Kunsta r, Zolta n Ho zer, Erzse rta Horva th, Lajos Matus Ma Hungarian Academy of Sciences Centre for Energy Research, H-1525, Budapest, P.O. Box 49, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2015 Received in revised form 11 July 2016 Accepted 8 August 2016
In order to evaluate the effect of zirconium breakaway oxidation on the behaviour of nuclear fuel in LOCA (Loss-of-coolant accident) conditions, high temperature (800e1200 C) oxidation tests with E110 type cladding were performed in steam atmosphere. The onset time of breakaway oxidation was detected using an online method based on hydrogen release. The experimental results showed that the breakaway oxidation starts not earlier than 5 min after the start of the oxidation that is longer than the duration of dry phase in a design basis accident LOCA. The experiments give preliminary indication that the breakaway oxidation does not play role in design basis accident LOCA conditions with the E110 alloy. This is to be confirmed by transient oxidation tests. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Zirconium alloy Cladding High temperature oxidation Breakaway phenomenon
1. Introduction Zirconium alloys are used in nuclear reactors as fuel cladding material due to their low neutron absorption cross section, good corrosion resistance and mechanical properties under operating conditions. However, significant oxidation may occur on the surface of the fuel cladding under loss-of-coolant accident (LOCA) conditions. Generally, a protective oxide film develops, decelerating further oxidation and hydrogen uptake, but the tetragonal to monoclinic phase transformation of the oxide can lead to the spalling of the oxide layer. This is the so called breakaway phenomenon, which is characterised by sudden increase of oxidation rate (Schanz and Leistikow, 1995; Qin et al., 2007; Billone et al., 2008; Baek and Jeong, 2008). Although the phenomenon is known for quite a long time, its mechanism is not entirely understood. A lot of factors have impact on the breakaway oxidation, for example: oxidation conditions, chemical composition of the alloy, impurities, surface roughness, etc. All of these factors should be taken into account to get explanations for this phenomenon. Zirconium claddings could lose their ductility because of the oxidation and the accompanying hydrogen absorption. The embrittlement of the cladding takes place faster in case of a spalling
* Corresponding author. ). E-mail address: [email protected] (E. Perez-Fero http://dx.doi.org/10.1016/j.pnucene.2016.08.006 0149-1970/© 2016 Elsevier Ltd. All rights reserved.
oxide layer, since the hydrogen uptake is enhanced under such conditions. The embrittlement of the material can give rise to the failure of the fuel rods under thermal and mechanical loads. For this reason, it is important to study the circumstances that may affect the mechanical properties of the fuel cladding. Breakaway oxidation characterises the Russian type E110 alloy in a wider range of temperature compared to Western alloys (e.g Zircaloy-4, M5® and Zirlo(™)). However, the available data on breakaway oxidation behaviour of this cladding alloy is scarce (Yegorova et al., 2005; Steinbrück et al., 2010, 2011; Nikulin et al., 2011; Yan et al., 2009; OECD, 2009). In recent years, several experiments have been carried out at the Hungarian Academy of Sciences Centre for Energy Research to investigate the oxidation, hydrogen uptake and mechanical performance of the E110 alloy zer et al., 2008; under accident conditions (Matus et al., 2001; Ho et al., 2010). Perez-Fero In this study, high temperature oxidation experiments were performed with E110 fuel cladding specimens to get an overview on its breakaway oxidation behaviour. We focused on the influence of the temperature on the breakaway phenomenon. The tests were carried out in steam environment in the temperature range of 800e1200 C. However, further experiments are needed to investigate the effects of other factors on the oxidation.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
90 Table 1 Chemical composition of the E110 alloy (ppm). Nb
Mg
Al
Si
Cr
Mn
Fe
Ni
Cu
Hf
F
Cl
Zr
10,000
0.5
0.5
1.0
10
0.1
45
15
0.5
100
30
3
Balance
was monitored online by a computerized data acquisition system. At the end of oxidation the sample was withdrawn to the cold part of the equipment. The extent of the oxidation was calculated on the basis of the measured mass gain.
2.3. Method and measurement
2. Experimental procedures 2.1. Material The test series were carried out with E110 fuel cladding tubes produced by electrolytic method (currently used at Paks Nuclear Power Plant). The chemical composition of the alloy was determined by mass-spectrometry. The result of this analysis is shown in Table 1. 8 mm long specimens were cut from the original E110 tubes with an average outer diameter of 9.13 mm. The specimens were cleaned by total immersion in organic solvent and air-dried. 2.2. Equipment The zirconium specimens were oxidised in steam e argon mixture with 12 v/v% high purity argon gas (99.999 v/v%) under isothermal conditions. A high temperature tube furnace with a quartz tube was used for the oxidation experiments. The internal diameter of the quartz tube was 19 mm. The experimental apparatus (Fig. 1) consisted of a steam generator, a horizontally arranged three-zone resistance furnace with temperature control system, condenser and thermal conductivity detector (TCD). External heaters were applied in order to avoid condensation in the quartz tube. The steam was condensed in the condenser with an ice water mixture. The steam flow rate was 2.0e4.0 mg/cm2/s. It was determined by measuring the mass of the condensed water collected during the test period, divided by the oxidation time and the crosssectional area of the quartz tube. Before sample insertion, the test apparatus was sufficiently flushed with steam e argon mixture. After stabilization of temperature and steam flow, the quartz boat with the sample was pushed into the heated zone of the furnace. The hydrogen concentration of the outlet carrier gas was measured by a thermal conductivity detector during steam oxidation. The detector signal
An online method based on the hydrogen release during steam oxidation was used to study the breakaway phenomenon. The zirconium-steam reaction produces zirconium dioxide and hydrogen. The metal partly absorbs the formed hydrogen and the rest of it goes into the gas phase with the carrier gas (argon). This gas gets to the thermal conductivity detector through the condenser. Since hydrogen has a much higher thermal conductivity than argon, the formed hydrogen causes a significant change in thermal conductivity compared to carrier gas alone. The detector signal is directly proportional to the hydrogen concentration. Thermal conductivity detector was calibrated with high purity hydrogen (99.999 v/v%). The hydrogen release during high temperature zirconium oxidation is closely related to the properties of the oxide layer. At the beginning of the oxidation a dense and protective (tetragonal) oxide layer develops on the surface of the zirconium (Billone et al., 2008). Further oxidation is limited by this compact oxide layer. When the breakaway oxidation takes place, the steam penetrates into the cracks of the oxide and rapidly reaches the fresh metal surface. It results in faster oxidation kinetics and enhanced hydrogen formation. Consequently, the change of the hydrogen concentration in the carrier gas indicates the appearance of the oxide cracks. If the breakaway oxidation does not take place the hydrogen production smoothly decreases. The flowchart of the online method for the investigation of the breakaway oxidation is shown in Fig. 2. Fig. 3 represents the TCD signal during the steam oxidation of E110 alloy at 1000 C as a function of the oxidation time. Most important steps of the process are marked in Fig. 3. After insertion of the sample into the furnace (Step 1.), hydrogen formation starts due to the zirconium-steam reaction (Step 2.). A part of the hydrogen is absorbed by the zirconium sample; the remaining hydrogen goes into the gas phase with the argon. The protective oxide layer starts to develop, thus the oxidation rate and
Fig. 1. Schematic of the experimental apparatus used for the oxidation.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
91
Fig. 2. The flowchart of the online method for the investigation of the breakaway oxidation.
Fig. 3. Hydrogen release during oxidation of E110 sample at 1000 C for 2700 s.
hydrogen formation decrease (Step 3.). Cracks formed in the oxide (Step 4.) offer free path for the steam to reach the fresh metal surface. It results in higher oxidation rate and intensive hydrogen formation (Step 5.). Due to a new oxide layer formation on the fresh metal, the reaction rate decreases again (Step 6.) until the formation of new cracks (Step 7.). The next step (Step 8.) indicates the sample withdrawal, resulting in cessation of hydrogen formation (Step 9.).
3. Experimental results 3.1. Oxidation tests followed by online monitoring The hydrogen release during steam oxidation of E110 fuel cladding specimens was examined in the temperature range of 800e1200 C, with 50 C steps. The tests between 900 C and 1200 C were terminated after 2700 s. The oxidation at 800 C was carried out for longer time because of the lower reaction rate.
92
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
Fig. 4. Breakaway oxidation map of E110 alloy in the temperature range of 850e1050 C.
Fig. 4 and Fig. 5 show the TCD signals during oxidation tests of E110 at different temperatures versus the oxidation time. The amount of released hydrogen was influenced by the temperature. At the beginning of oxidation the TCD signal increased with increasing temperatures in all cases. The first cracks at each temperature are indicated in Fig. 4. Significant differences were found in breakaway oxidation times at different oxidation temperatures (Table 2). The breakaway oxidation of E110 occurred at longer oxidation times at 800 C (10700 s) and 850 C (1800 s). Contrarily, short breakaway times (330e510 s) were found in the temperature range between 900 C and 1050 C. The shortest oxidation time (330 s) was measured at 950 C. These oxide cracks were followed by further cracks. No breakaway oxidation behaviour was detected (the hydrogen production smoothly decreased without peaks
typical for breakaway) at 1100 C and above this temperature. A few tests were repeated at the same conditions to verify the results obtained from this online method. Fig. 6 represents the results of the replicated experiments at 1000 C. The curve characteristics were very similar for both experiments and nearly the same breakaway oxidation times were observed. 3.2. Additional examinations Additional oxidation tests were carried out for different times between 1000 C and 1200 C to produce samples for scanning electron microscope (SEM) analysis and metallographic examinations. The purpose of these tests was to get information on the microstructure of the oxide layers and to confirm the presence of oxide cracks that are characteristic of breakaway.
Fig. 5. Hydrogen release during oxidation of E110 samples at 1100 C, 1150 C and 1200 C.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero Table 2 The breakaway oxidation times at different oxidation temperatures. Test ID
Oxidation temperature ( C)
Breakaway time (s)
BE-20 BE-13 BE-12 BE-10 BE-06 BE-08 BE-09 BE-25 BE-26
800 850 900 950 1000 1050 1100 1150 1200
10,700 1800 500 330 410 510 e e e
Fig. 7 illustrates the extent of E110 oxidation (mass increase per surface area) as a function of the oxidation time at 1000 C. The post-test appearance of some E110 samples is also shown in this figure. A protective oxide layer formed on the sample which was oxidised for the shortest time. At 400 s the oxide layer started to peel off from the surface. The longer oxidation time resulted in the more cracked oxide layer. The SEM images of the E110 samples oxidised for 200 s and 400 s at 1000 C are shown in Fig. 8. The oxide layer was thin (<5 mm) and compact in case of the sample oxidised for 200 s. The first circumferential crack was observed at 400 s, which is in line with the result of the test carried out by online method. Fig. 9 shows the peeling oxide layer on the sample oxidised for 400 s. The optical micrographs of E110 cladding oxidised at 1100 C and 1200 C in steam are shown in Fig. 10. Thick, protective oxide layers were formed at both temperatures. No cracks were found in the oxide layers, indicating that breakaway oxidation did not occur. 4. Discussion Previous papers have shown that breakaway oxidation occurs in E110 cladding oxidised in steam at temperatures higher than 800 C (Yegorova et al., 2005; Steinbrück et al., 2011). They indicated pronounced breakaway oxidation between 900 C and 1000 C. Other studies focused on this temperature range and above (Baek and Jeong, 2008; Yan et al., 2009). The breakaway oxidation time is
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usually defined on the basis of the weight-gain rate and the associated hydrogen uptake. In this study, the breakaway oxidation is followed by measuring the released hydrogen during steam oxidation. The applied method only measures the released hydrogen and cannot give information about the absorbed hydrogen. In (Grosse et al., 2009) it was reported that the hydrogen uptake of zirconium alloys during steam oxidation was connected with the phase transition in the oxide and it cannot be estimated from temperature and time data alone. According to the literature (Steinbrück et al., 2011), the tetragonal to monoclinic phase transition leads to the formation of circumferential cracks in the oxide layer. The formation of cracks causes spallation and the oxide loses its protective nature, thus the further oxidation is no longer limited by diffusion through the oxide layer. Due to the faster oxidation rate the hydrogen formation suddenly increases, which indicates the start of breakaway oxidation. Our results show that not only the hydrogen uptake but the hydrogen release is also influenced by the structure of the formed oxide layer. This statement is confirmed by scanning electron microscope analysis of oxidised samples before and after the breakaway. The SEM images of the sample beyond the breakaway time (400 s at 1000 C) revealed a major circumferential crack in the oxide layer which is characteristic of breakaway. This result is consistent with the study of M. Billone et al (Billone et al., 2008). who also reported early breakaway oxidation times at 1000 C. Our oxidation tests show similarly short breakaway times between 900 C and 1050 C. The shortest breakaway time for the Russian type E110 alloy is 330 s. Oxidation accompanied by oxide cracking does not take place at 1100 C and above. The metallographic examinations show thick and dense oxide layers without cracks, confirming the results from n-Moya et al., 2010) online method. It is generally admitted (Corvala that above 1050 C, the tetragonal oxide formed is dense and protective, even for thick oxide scale. The breakaway oxidation, which occurs in specific temperature ranges, has been identified by OECD experts (OECD, 2009) as a major problem for the cladding LOCA performance, since it is associated with a significant hydrogen pickup which degrades the
Fig. 6. Hydrogen release during replicated tests at 1000 C.
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et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
Fig. 7. The measured mass increase per surface area for the steam oxidation of E110 alloy at different times at 1000 C.
Fig. 8. SEM images of the E110 samples oxidised for 200 s (left) and for 400 s (right) at 1000 C.
Fig. 9. Peeling oxide layer on the surface of the sample oxidised for 400 s at 1000 C.
et al. / Progress in Nuclear Energy 93 (2016) 89e95 E. Perez-Fero
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Fig. 10. Optical micrographs of E110 cladding oxidised for 3000 s at 1100 C (left) and for 1100 s at 1200 C (right).
mechanical properties of the cladding. Previous studies (Billone et al., 2010) showed et al., 2008; Yegorova et al., 2005; Perez-Fero that breakaway phenomenon in high temperature steam takes place with E110 alloy more easily than with other zirconium cladding alloys. The experiments described in this paper indicated that the breakaway oxidation does not play role in design basis accident LOCA conditions even with the E110 alloy, since the breakaway times are longer than the duration of dry phase in the LOCA event. In order to confirm the above, further investigation is needed. 5. Conclusions High temperature oxidation of E110 cladding specimens was carried out in steam environment between 800 C and 1200 C. These experiments were aimed at studying the breakaway phenomenon on the surface of zirconium using an online method. This technique is based on the online monitoring of hydrogen release during oxidation. Special attention was given to investigate the influence of the temperature on the breakaway oxidation. The breakaway oxidation times were determined. The breakaway effect was observed in the temperature range of 800e1050 C for E110. The test results indicated short breakaway times (330e510 s) at temperatures between 900 C and 1050 C. Longer breakaway times were found below 900 C. At higher temperatures (1100 Ce1200 C) the breakaway phenomenon did not occur. The results regarding breakaway oxidation were confirmed by different measuring methods. Taking into account that the dry phase in design basis LOCA typically lasts only for 2e4 min, it can be concluded that the breakaway oxidation will not start for E110 cladding during the accident. So the enhanced oxidation and hydrogen uptake associated with breakaway phenomenon will not danger the integrity of fuel cladding during the LOCA accident. Since, the oxidation tests were carried out under isothermal conditions, further transient oxidation tests are required to fully simulate LOCA events. Acknowledgments This work was funded by the Hungarian Atomic Energy Authority (HAEA), Research Contract No. OAH/NBI-ABA-15/09-M.
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