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Oxidation of molten zirconium droplets in water
Nuclear Engineering and Design 354 (2019) 110225
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Oxidation of molten zirconium droplets in water ⁎
T
Louis Manickam, Qiang Guo, Andrei Komlev , Weimin Ma, Sevostian Bechta Kungliga Tekniska Högskolan (KTH), AlbaNova University Centre, Nuclear Power Safety Division, Roslagstullsbacken 21, SE-106 91 Stockholm, Sweden
A R T I C LE I N FO
A B S T R A C T
Keywords: Reactor safety Severe accident Fuel coolant interaction Zirconium oxidation Hydrogen production
Zirconium, which is used as the cladding material of nuclear fuel rods in LWRs, can react with steam in the case of a core meltdown accident. This results in the release of hydrogen which poses a significant risk of hydrogen explosion. Oxidation of Zr occurs either during the core degradation when the steam flows over the hot fuel rod surfaces or during an FCI when molten corium falls into a water pool (e.g. in the lower head). An experimental study was performed at the MISTEE-OX facility at KTH to observe and quantify the oxidation of molten zirconium droplets in a water pool. During the experimental runs, single droplets of molten zirconium were discharged into a subcooled water pool and the dynamic events were recorded using a high-speed camera. The bubble dynamics indicate a process of cyclic oxidation and hydrogen release from the rear periphery of a droplet while it is quenched in the water. The debris (solidified state of the droplet) after each run was collected for compositional and microstructural analysis via SEM/EDS. The obtained data were employed to estimate the oxidation fractions of the droplets and the results revealed several interesting insights into the oxidation phenomenon of the Zr melt. The water subcooling was observed to have a significant influence on the oxidation: the degree of oxidation decreased with increase in the water subcooling. Furthermore, the degree of oxidation was also influenced by the depth into the debris, forming compounds whose oxygen content decreases from the outer surface towards the core of the debris. Therefore, the qualitative and quantitative results presented in this paper are important in the context of developing a phenomenological understanding of the oxidation behaviour of zirconium melt during the FCI as well as to improve and validate the currently available models implemented in the state-of-art steam explosion codes.
1. Introduction A low thermal neutron capture cross-section and ideal thermalmechanical properties justify the use of zirconium alloy (Zircaloy) as a major component of nuclear fuel assemblies in LWRs. However, zirconium is a chemically reactive metal, which, when exposed to steam at high temperatures, increases the risks related to the release of excessive heat and hydrogen. Recommendations based on the lessons (Baba, 2013) learnt from the TMI-2 and Fukushima Daiichi accidents have laid specific emphasis on the development of ATF and reducing risks due to hydrogen release from an excessively heated core. However, while the development of ATF may be the final fix to completely eliminate the
release of hydrogen, it is equally important to develop sufficient understanding of the Zircaloy oxidation with steam under the aggressive conditions during a severe accident scenario, as the operating nuclear reactors and new installations of LWRs are expected to use the traditional fuel for a long time before the ATF is ready for deployment. While the oxidation of Zircaloy surface by steam has been extensively investigated, the behaviour of molten corium during the FCI has received little focus, which motivates the present study; i.e. to understand and characterise the zirconium oxidation phenomenon that occurs during the FCIs. FCI is a classical term used for a process where high-temperature molten materials come into contact with a cold volatile liquid such as
Abbreviations: Zr, Zirconium; LWR, Light Water Reactor; FCI, Fuel Coolant Interaction; KTH, Kungliga Tekniska Högskolan or Royal Institute of Technology, Sweden; MISTEE, Micro Interactions in Steam Explosion Energetics (facility); MISTEE-OX, Modified MISTEE facility to perform zirconium OXidation experiments; SEM/EDS, Scanning Electron Microscopy/Energy Dispersive Spectroscopy; TMI-2, The Three Mile Island Unit 2; ATF, Accident Tolerant Fuel; ANL, Argonne National Laboratory; TROI, Test for Real cOrium Interaction (facility); KROTOS, FCI-Experimental facility CEA-Cadarache-France; OECD, The Organisation for Economic Cooperation and Development; SERENA, Steam Explosion Resolution for Nuclear Application; HSUB, High water SUBcooling; MSUB, Medium water SUBcooling; STP, Standard Temperature and Pressure; SS, Stainless Steel ⁎ Corresponding author. E-mail addresses: [email protected] (L. Manickam), [email protected] (Q. Guo), [email protected] (A. Komlev), [email protected] (W. Ma), [email protected] (S. Bechta). https://doi.org/10.1016/j.nucengdes.2019.110225 Received 11 June 2019; Received in revised form 21 July 2019; Accepted 22 July 2019 Available online 01 August 2019 0029-5493/ © 2019 Elsevier B.V. All rights reserved.
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with added zirconium content (UO2-ZrO2-Zr) recorded the least energetics in the whole series of experiments conducted in the framework of the OECD SERENA project (Leskovar et al., 2016). Understandably, one may not assume the low energetics to be related to chemical augmentation considering the several integral effects associated with a phenomenon such as the FCI. These results clearly indicated the need to improve our understanding of zirconium metal oxidation behaviour during the FCI. Furthermore, a recent review on the status of steam explosion understanding and modelling (Meignen et al., 2014c) has also indicated the importance of performing small-scale experiments with liquid metal melts at very high temperatures to investigate the oxidation rate and integral tests to characterise the impact of global void generation. Thus, the scarcity of experimental data has stalled the development and improvement of oxidation models implemented for steam explosion analysis. Hence, the present study is a first step towards advancing our current state of knowledge on melt oxidation behaviour during the FCI. The MISTEE facility (Hansson et al., 2013; Zambaux et al., 2018) that has been originally used for steam explosion studies in the scale of single droplets was modified to study the oxidation of pure Zr droplets. Experiments at the scale of a single melt droplet level reduce the uncertainties related to integral mechanisms and are expected to provide conservative estimates. The data obtained from the current experiments provide preliminary insights into the dynamic mechanisms related to the zirconium melt oxidation during the FCI.
water. FCI can result in explosive steam generation that induces a strong dynamic loading on the surrounding structures threatening the overall integrity of the containment. The melt parameters, such as composition, superheat and water subcooling, pool depth, melt flow rate, and system pressure are considered to have a significant effect on the magnitude of the destructive mechanical energy released during an explosive FCI. Despite the significant efforts through FCI research, the occurrence of a steam explosion is still considered a stochastic event as the promoting/inhibiting effects of certain parameters of the melt and coolant on the explosive energy release remain unexplained. A review of the FCI experiments available elsewhere (Corradini, et al., 1988; Spencer et al., 1994; Cho et al., 1998; Magallon and Hohmann, 1997; Park et al., 2005; Hong et al., 2013; Meigner et al., 2014a,b) reveal interesting insights into some of the limiting as well as promoting mechanisms of steam explosions. Based on these insights, it is now widely accepted that the melt solidification mechanism and the relative void fraction in the dispersed region (pre-mixture) are the major factors that can affect the strength of a steam explosion (Meignen et al., 2014c). Molten Zr reacts exothermically with steam producing zirconium oxide and hydrogen which is released as a non-condensable gas (Eq. (1)).
Zr + 2H2 O(g) → ZrO2 + 2H2 (g) + 6.47 MJ/kg Zr at 1900 °C
(1)
The production of hydrogen during the late core degradation phase which results from the oxidation of the corium melt active reducers (i.e. U, Zr) is yet to be explained, and requires experimental investigation to consistently describe the related phenomenon. Generally, the melt oxidation during the FCI can be considered as a major source for the void produced in the pre-mixture due to the release of hydrogen. Moreover, it is also expected that the change in the metallic melt composition due to increase in the oxygen content can affect the physical-chemical behaviour of the melt during the crystallisation process. Further, the generation of the non-condensable gas (hydrogen) that envelops the melt droplet provides greater stability against direct meltwater contact. It is widely known that the non-condensable gases in the vapour film suppress the condensation of the vapour, which contributes to the occurrence of a steam explosion. The effect of non-condensable gases surrounding a melt droplet on vapour explosion has been studied experimentally and numerically by Leskovar et al. (2016). As discussed above, although melt oxidation has a limiting effect on steam explosion, it also has some adverse effects including the build-up of an explosive atmosphere through excess hydrogen production, increased melt thermal energy content, and delayed solidification through heat release; these are important mechanisms that strengthen the steam explosion (Meigner et al., 2014a,b; Leskovar et al., 2016). Apart from such logical propositions, a mechanistic understanding of the melt oxidation mechanism and its influence during the FCI is rather abstruse. The most comprehensive and systematic experimental work to date on zirconium oxidation during the FCI was performed at the ANL (Cho et al., 1998): Approximately 1 kg of melts of Zr-ZrO2 or Zr-SS systems were poured into the subcooled water to assess the extent of chemical augmentation during the FCI. The major experimental parameters used were water subcooling and the zirconium content in the melt. The results showed that the oxidation rate is independent of the zirconium content added to the melt mixture and increased at lower water subcooling. For explosive cases, the oxidation rate was estimated up to 99% and the energy of the explosion was observed to increase with the zirconium content indicating the vital role of chemical augmentation on steam explosion energetics. The interpretations of the results on chemical augmentation by TROI (Kim, et al., 2008) and KROTOS (NEA., 2014) tests performed with added zirconium metal to prototypic materials are not straight forward, considering the stochastic integral effects of a steam explosion. Generally, contradicting results were obtained when the experiments
2. MISTEE-OX experiments 2.1. Facility description To study prototypic oxidation behaviour during the FCI, we performed experiments with zirconium (99.8% purity). The MISTEE facility was modified to perform the oxidation experiments. The original MISTEE facility and its modifications are described in Manickam (2018) in more detail. The main components of the modified facility—called the MISTEEOX (Fig. 1)—include a furnace, water pool, and a high-speed visualisation system; the furnace is an induction heating system (20 kW, 50–250 kHz) capable of attaining temperatures of over 2000 °C. Ceramic tubes of alumina and zirconium oxide are arranged concentrically such that they envelop the crucible as radiation shields. The space between the crucible and radiation shields is packed with MgO or ZrO2 powder to help sustain the position of the crucible and tubes as well as to minimise heat loss. The crucible is made of graphite and has a 6 mm nozzle at the centre of its bottom. A melt droplet is suspended inside the crucible by a constant purge of argon through its nozzle. The temperature of the crucible is measured by a C-type thermocouple mounted from a hole on the crucible wall. A three-way valve isolates the furnace from the water pool. A high-speed camera (Redlake HG50LE) with an acquisition rate up to 5000 fps is used to record the interaction between the melt and water. The melt droplet is captured in a debris catcher suspended inside the water pool, which is a rectangular plexiglass tank (500 mm × 75 mm × 75 mm) arranged below the furnace.
2.2. Experimental conditions The conditions for the series of tests conducted at the MISTEE-OX droplet facility are summarised in Table 1. Approximately 2 g of zirconium was melted in the furnace and on attaining the desired melt temperature, the melt droplet was allowed to free fall through the opened nozzle into the water pool by arresting the gas purge in the crucible.
2
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Fig. 1. Schematic of MISTEE-OX facility.
3. Experimental results and discussion The forthcoming sections present a detailed account of the procedures and results obtained from the MISTEE-OX experiments. Within the scope of current work, the influence of water subcooling on melt oxidation is studied. An evaluation of the amount of released hydrogen based on the visual analysis of bubble formation is presented in Section 3.1. The recorded videos are analysed to attain insights into the phenomenon and data on hydrogen release during the FCI. Meanwhile, Section 3.2 characterises the microstructure and oxidation patterns of the quenched Zr samples. The possible mechanism of the molten Zr droplet oxidation in water pool based on the Zr-O phase diagram is proposed and discussed in Section 3.3. Section 3.4 is devoted to the uncertainty analysis based on the assumptions made for the visual analysis of bubble formation and oxygen measurement by EDS.
Fig. 2. Time sequence raw snapshots of the interaction between a single droplet of zirconium melt and water at ΔTsub ≈ 85 K (HSUB run 3).
3.1. Visual analysis of bubble formation
pattern related to zirconium oxidation. Initially, the melt is oxidised by the surrounding flow of steam causing instant release of heat and hydrogen (inferred by the melt glow at t = 90, 235, and 365 ms in Fig. 2). The growth of the bubble filled with non-condensable gas reduces the steam supply to the drop surface and thereby, the melt oxidation / hydrogen generation (inferred by the decrease in melt glow at t = 159 and 329 ms in Fig. 2). On reaching the threshold size for the current drop velocity in water, the gas bubble detaches from the rear of the droplet, thereby allowing contact between fresh steam and the drop surface. Thus, it logically follows that the temperature of the melt
3.1.1. Cyclic oxidation of molten Zr droplets in water Single droplets of molten Zr metal quenched in water depict a pattern of repetitive bubble growth and detachment along the rear periphery of the zirconium melt droplet (Fig. 2). The trail of bubbles left behind in the droplet trajectory does not seem to condense even under the HSUB, indicating that the bubbles are mostly filled with non-condensable gas (hydrogen). A preliminary observation of the recorded images suggests a number of “cyclic oxidation processes” of heat/hydrogen generation and subsequent bubble detachments from the drop during its free fall in water. The images clearly indicate a very specific Table 1 Experimental conditions of the zirconium melt/water interaction test series. Test ID
Initial mass of Zr melt, g
Temperature of Zr melt in the crucible, °C
Superheat of melt (ΔTsup), K
Temperature of water pool, °C
Subcooling of water, (ΔTsub), K
HSUB run 1 HSUB run 2 HSUB run 3 MSUB run 1 MSUB run 2
2.0 ± 0.1
2005 ± 20
150 ± 20
15 ± 1
85 ± 1
55 ± 1
45 ± 1
3
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melt, resulting in darker images when the light intensity reduces from the melt droplet because of the heat loss. Furthermore, digital image analysis is employed to extract the bubble parameters from the acquired sequence of raw images. As a general scheme, a series of procedures including background subtraction, noise reduction, edge detection, and morphological opening was performed to obtain the projected area of the bubbles (Fig. 5). Based on the above-mentioned assumptions, the kinetics of the hydrogen released during the interaction are estimated for the cases suitable for analysis (Fig. 6). The time scale in Fig. 6 corresponds to the time of the bubble detachment from the droplet. Furthermore, based on the bubble analysis, the O/Zr atomic ratio is estimated to be approximately 0.27 and 0.07 under medium and high subcooling conditions, respectively (the O/Zr atomic ratio of 2 corresponds to fully oxidised zirconium, i.e. the formation of ZrO2) (Fig. 7). A significant increase in the oxidation under the medium subcooling conditions can be clearly observed.
Fig. 3. Time sequence raw snapshots of the interaction between a single droplet of zirconium melt and water at ΔTsub ≈ 45 K (MSUB run 2).
3.2. SEM/EDS analysis
dynamically increases as a result of the heat released by the oxidation reaction while also being simultaneously quenched. The specific observations indicate that the oxidation process does not stop as soon as the bubble is filled with hydrogen, restricting the access of water vapour to the droplet; rather, the hydrogen bubble detaches, thereby providing a periodic access of fresh steam (Manickam, 2018). Similarly, time sequence snapshots of the progress in the interaction between water and zirconium melt under MSUB conditions are shown in Fig. 3. The long trail of non-condensable gas bubbles in the trajectory of the droplets is observed to be similar to that under the HSUB conditions. A typical velocity profile of the melt droplets during quenching under high and medium water subcooling is shown in Fig. 4. The significant difference in the velocity of the droplets observed under the different water subcooling conditions indicates that, under the HSUB conditions, the melt droplet reaches the catcher arrangement much more rapidly than under MSUB conditions.
3.2.1. Quenched Zr droplets under the high water subcooling SEM images depicting the microstructure of the solidified zirconium melt droplet under high water subcooling (HSUB runs 1–3) conditions are presented in Fig. 8. The scanning electron microscope (Hitachi S3700N) combined with the energy-dispersive X-ray spectroscope (Bruker’s XFlash Detector 4010) was used to perform electron-excited X-ray microanalysis of the solidified droplet. The analysis revealed stratified layers with varying oxygen content. A visual analysis of the presented microstructures allows us to observe the oxygen-enriched (lighter areas) and metal-enriched (darker areas) areas. The outer layer (k-area: crust region / melt–vapour interface) is clearly observed as the most oxygen-rich region (see, for example, label a2 in Fig. 8). Further, in the bulk towards the centre of the droplet, the concentration of oxygen is observed to decrease. Fig. 9 depicts the radial stratification of the O/Zr atomic ratio through the depth of the droplet for all the cases studied under the high subcooling conditions. We observe that the entire region of the droplet is neither fully oxidised nor un-oxidised: three or four distinct concentric layers with varied concentrations of oxygen can be identified in the droplet. The O/Zr atomic ratio values were obtained by the scan of a relatively larger area. For example, for the layers k, l, and m (in case of HSUB run 1), the scanning areas correspond to the thickness of the respective layers, and the length of the scanning area was in the range of 0.5–1.5 mm for different layers. The layer boundaries were determined visually, based on the change in the microstructural patterns exhibited within each layer of the droplet (see areas k, l, m, and n under labels a1, b1, and c1 in Fig. 8). The EDS technique employed in the study cannot detect the differences in the O/Zr atomic ratio over the respective layer and hence, it can be assumed that the O/ Zr atomic ratio is evenly distributed in the specific layer. The O/Zr atomic ratio at the outer surface estimated by EDS is in the approximate range of 0.8 and 0.9 (Fig. 9). The outer surface with the highest concentration of oxygen is observed to have a constant thickness in an approximate range of 200–300 µm, and is constantly distributed through the entire surface area of the droplet. The innermost layer of the droplet is almost evenly oxidised where the O/Zr atomic ratio is approximately 0.6. However, it is observed in the case of HSUB run 1 that the innermost layer shows a significantly reduced oxygen concentration as the O/Zr atomic ratio is approximately 0.2. The SEM image for case HSUB run 1, shows a well-formed crack at the outer surface (see label a1 in Fig. 8) which probably could have functioned as a mechanical barrier to oxygen migration towards the inner surfaces.
3.1.2. Hydrogen release estimation To estimate the amount of hydrogen released from the exothermic chemical reaction, we studied the gaseous wake in the trail of the melt droplet. We estimated the volume of hydrogen generated during the interaction at approximately 50 mm below the water level based on the following simplified assumptions: (1) the steam in the bubbles detached from the melt droplet completely condense on reaching the surface layer of the water pool and (2) the bubble contains pure hydrogen at STP. However, it is instructive to note that the resolution of the highspeed camera, which is a function of the number of pixels in the sensor, is inversely proportional to the recording speed; hence, the recorded video is limited to approximately 5 s. Moreover, high-speed imaging usually suffers from low light collectiveness. This, combined with the necessity to neutralise the light emitted from the high temperature
0HOW GURSOHW YHORFLW\ PV
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.100
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3.2.2. Quenched Zr droplets under the medium water subcooling The SEM images depicting the microstructure of the solidified zirconium melt droplet under medium water subcooling (MSUB runs 1–2) conditions are presented in Fig. 10. We observe that the droplets exhibit irregular morphology compared to those under high subcooling
0.500
Fig. 4. Temporal velocity profile of the melt droplet in water under different water subcooling conditions. 4
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Fig. 5. Image processing for the estimation of the temporal distribution of hydrogen (left). Labels 1 and 2 depict typical individual hydrogen bubbles (right).
unlike under the high subcooling conditions, the formation of evenly distributed concentric layers is not observed over the entire surface of the droplet. The O/Zr atomic ratio at the outermost surface is in the range between 1.1 and 1.5 which is approximately 75% oxidation, unlike under the high subcooling conditions where it is approximately 45% oxidation. Moreover, we observe that the O/Zr atomic ratio on the innermost surface ranges between 0.85 and 1, which is approximately 50% oxidation under medium subcooling conditions whereas under the high subcooling conditions the O/Zr atomic ratio is 0.7 which indicates approximately 35% oxidation. A significant difference is also observed between the hydrogen bubble formation analysis and SEM/EDS analysis: the latter shows increased oxidation compared to the former. It is instructive to note that the results of the SEM/EDS analysis should be treated with caution considering the uncertainties involved in the estimation of a relatively lighter element such as oxygen. However, the obtained data can be used for a qualitative comparison of the oxygen concentration between different areas in a sample. Therefore, the use of other techniques/measurements is recommended for an accurate estimation of the O/Zr ratio.
Fig. 6. Hydrogen generation history during Zr melt and water interactions.
3.3. Zr droplet oxidation mechanism 3.3.1. Analysis of the phase transitions according to the Zr–O phase diagram As mentioned earlier, the oxidation (O/Zr atomic ratio) of Zr droplets in medium subcooled water is uniformly distributed over the entire volume (see Fig. 10). This indicates that oxygen mass transfer from the surface to the bulk volume is very fast, which is typical for mass transfer in the liquid state. This is probably because in the case of liquid droplets, the oxygen transfer is not limited to diffusion through the oxidised Zr layer unlike during Zr solid material oxidation where the formation of the ZrO2 layer can be clearly observed on the surface (see, for example Yang et al., 2014). The results of the EDS analysis show that the total oxygen concentration of the obtained droplets is about 40–60 at.%; and, according to the phase diagram (Fig. 11), region of the mentioned composition can be achieved by two ways:
Fig. 7. Evolution of oxygen content in the melt drop estimated from hydrogen generation data.
conditions where the droplets resemble a nearly spheroidal morphology. These observations indicate that the droplets could still be in the liquid state under medium subcooling conditions, allowing the melt to spread in the catcher unlike under the high subcooling conditions where the droplet could have already formed a crust, which limits its shape to that of a spheroid. The images depict a similar pattern of melt oxidation where the outermost layer shows the highest degree of oxygen concentration in the droplet. However, the thickness of this outer layer is not constant as was observed under the high subcooling conditions. The thickness of the layer ranges between 180 and 320 µm. Furthermore, similar to the high subcooling conditions, the oxygen content is stratified from the outer surface to the centre of the droplet (see labels a, b, and c for cases MSUB runs 1 and 2 in Fig. 10). However,
1) Below the solidus line, by increasing the oxygen content and undergoing the following phase transitions in the solid state: β-Zr (O) → α-Zr(O) → (α-Zr(O) + ZrO2) 2) Above the liquidus line, by increasing the oxygen content and quenching the liquid phase with the corresponding composition: Liquid → (α-Zr(O) + ZrO2) Because oxidation of solidified droplet is extremely unlikely and hence, we can safely assume that the oxidation occurred only in the liquid state for a few seconds as explained above. The exothermic oxidation reaction causes a simultaneous increase in the temperature of 5
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Fig. 8. SEM images showing microstructure of the solidified Zr droplet cross-sections for all cases studied under high subcooling conditions.
oxidation characterised by the average O/Zr atomic ratio in the quenched droplets is plotted as a function of the particle diameter, which corresponds to the mean particle diameter estimated from the discrete spherical fragments collected after the experiments in Baker (1962). The O/Zr atomic ratio was observed to decrease with increasing particle diameter. Although not a fair comparison, the experimental data obtained at the MISTEE-OX seem to be in good agreement with these results.
the melt as well as the oxygen concentration in the droplet before the actual rapid solidification. The observed oxygen concentration shows that the melt superheat could have potentially increased by approximately 200 K under high subcooling conditions. The oxygen concentration on the inner surface of the droplet near the centre is approximately in the range up to 40 at.%, indicating that the temperature at the centre of the droplet could have remained relatively lower that at the crust. Similarly, under medium subcooling conditions, the maximum oxygen content observed in the crust region is in the range of approximately 60 at.%, indicating an increase of approximately 400 K in the melt superheat. The oxygen concentration on the inner surface of the droplet near the centre is approximately 50 at.%, indicating a pattern similar to the high subcooling conditions. However, the aforementioned hypothesis is a rather simplified thought and the actual mechanisms could be far more complex, which require further research. It is important to note that for the HSUB samples, the whole oxidation process (oxygen transfer for almost 3 mm droplet radius) probably occurred during the fall of the droplet into the water pool, i.e. in ~0.5 s. To study this process, the kinetics of the droplet oxidation must be considered; however, this can complicate the experimental procedure in terms of a relatively precise temperature measurement, which is not a trivial part of high-temperature and high-speed processes. The O/Zr atomic ratio in the quenched drops that was estimated based on the hydrogen generation data obtained from the bubble analysis of the MISTEE-OX experiments was compared with the experimental data available in Baker, 1962 (Fig. 12). The final Zr
3.3.2. Impact of non-condensable gas formation on steam explosion It is important to note that the zirconium melt droplets do not seem to undergo spontaneous steam explosion even under high subcooling conditions, indicating the stability of the vapour and gas film surrounding the melt droplet. The integral experiments performed at the ANL did not involve spontaneously triggered steam explosions (Cho et al., 1998). In the current experiments with Zr melt, a cyclic process of hydrogen generation and release is observed, leaving behind a trail of bubbles in the droplet trajectory even under high subcooling conditions (Fig. 2). For a general comparison, Fig. 13 shows a droplet of molten alumina discharged into water under almost similar test conditions, as the melting point of alumina (2050 °C) is comparable to that of zirconium. We observe that the alumina droplets falling into highly subcooled water (ΔTsub: 80 K) do not show the formation of a large trail of bubbles indicating rapid condensation of the generated steam except during the initial phase of entering into the water pool where the entrapped non-condensable gases detach from the melt droplet periphery
Fig. 9. Oxygen concentration profile from the surface to the bulk of the quenched drops after performing tests under high subcooling conditions. 6
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Fig. 10. SEM images showing the microstructure of the solidified Zr droplet cross-sections for all cases studied under medium subcooling conditions.
or triggered.
(i.e. at t = 79 ms in Fig. 13). The non-condensable gases have a significant effect on steam explosion triggering as well as the overall explosion yield since their pressure within the vapour film reduces the condensation of vapour, which essentially dictates the occurrence of vapour explosion (Corradini et al., 1988; Akiyoshi et al., 1990). The release of non-condensable gas due to the Zr droplet oxidation reaction under water shows that the triggering of the steam explosion can be suppressed even under high subcooling conditions while the alumina droplets can be relatively easily triggered under similar conditions. Furthermore, the cyclic process of bubble release also indicates that the Zr droplet is left with a thinner film during bubble detachment, promoting the effective participation of the Zr melt droplet in steam explosion. Considering a pre-mixture with many droplets of discrete sizes undergoing varied cycles of bubble growth and departure, it is possible that the active mass that could effectively participate in steam explosion is significantly reduced. However, it would be useful to investigate, in future works, the threshold bubble volume required to suppress the triggering of steam explosion to identify the trigger strength required to quantify the time scales under which the bubbles cannot be compressed
3.4. Analysis of uncertainties A comparison of the degree of zirconium oxidation observed by the visual bubbles (Section 3.1) and EDS analyses (Section 3.2) revealed significant discrepancies. According to the EDS analysis, the increase in the concentration of oxygen (i.e. O/Zr ratio) in the quenched Zr droplets (Fig. 9) is observed to correspond to the values of the HSUB and MSUB conditions for the bubbles analysis (Fig. 7). The bubble volume analysis method includes following uncertainties of the O/Zr ratio determination: - The released gas is an “ideal” gas and fully non-condensable (in the time-point of analysis). - The temperature of the gas is equal to the temperature of water and its pressure equals the normal atmospheric pressure. - The size (depth) of the gas bubble in a direction perpendicular to the image equals the width of the bubble (horizontal size).
Fig. 11. Zirconium–Oxygen phase diagram (Massalski, 1990). 7
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Fig. 12. O/Zr atomic ratio as a function of the mean particle diameter.
experimental work (typically by performing experiments involving a wide range of conditions including melt temperature and droplet diameter) with improved measurements (i.e. temperature measurement during melt droplet quenching in water and use of techniques other than EDS for oxygen measurement, for example wavelength dispersive X-ray analysis (WDS) or carbo-thermal reduction) and by implementing the use of metal mixtures (e.g. Fe-Zr).
- There are errors in the digital image analysis (more detailed described in paragraph 3.1). Based on the above-mentioned uncertainties of the described method, the estimated error of oxygen concentration (O/Zr ratio) is approximately 20%. It is widely known that the EDS methods can involve considerable errors in the analysis of relatively lighter elements such as oxygen (Gondstein, 2003). Moreover, only one cross-section for each droplet was analysed, which leads to an increase in the error of the statistic results; however, this is the basis of the assumption of equivalent oxygen distribution in the quenched droplet. As this raises doubts regarding the use of the quantitative values of the O/Zr ratio, the presented EDS data is considered somewhat more reliable for the qualitative characterisation of the samples, especially for a comparative analysis of the O/Zr ratio between the HSUB and MSUB conditions. Considering issues such as the complexity of the phenomenon, scarcity of experimental data, and importance of such experimental data for the model development, it is vital to follow up this
4. Conclusions and future prospects In the pursuit of understanding the FCIs during a postulated severe accident of LWRs, the oxidation behaviour of molten zirconium droplets falling into a water pool was experimentally studied at the MISTEE-OX facility. The complex bubble and droplet dynamics due to the chemical reaction as well as rapid heat and mass transfer were recorded using a high-speed camera, and the resulting debris was analysed by SEM/EDS. The experiments revealed several interesting insights into the oxidation phenomenon of molten zirconium droplets falling in a water pool. The major conclusions from this study can be
Fig. 13. Time sequence raw snapshots of the interaction between alumina melt and water (ΔTsup ≈ 100 K, ΔTsub ≈ 15 K). 8
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(NKS), and Swiss Federal Nuclear Safety Inspectorate (ENSI) under the APRI-MSWI program. The authors are also grateful to Per Sköld at the KTH-NPS Laboratory for his technical support with the experimental setup.
summarised as follows:
• A pattern of “cyclic oxidation” is observed in the high-speed images. • • • •
The images reveal melt glow, indicating an increase in the temperature or, in other words, an oxidation reaction whenever the bubble surrounding the melt droplet detaches, and subsequently, a fresh flow of steam is available. Spontaneously triggered steam explosions were not observed in the present study, indicating that the oxidation (and hydrogen generation) may suppress the potential for a spontaneous steam explosion. Water subcooling has a substantial effect on the oxidation process. The O/Zr atomic ratios for the quenched Zr droplets obtained based on the bubble volume analysis are approximately 0.07 and 0.27 under high and medium subcooling conditions, respectively. The SEM/EDS analysis of a solidified droplet shows the formation of concentric layers having constant thickness. Concentration of oxygen decreases from the outer layer towards the centre of the droplet. The SEM/EDS analysis of a solidified droplet with a hollow core or central pore shows that the oxygen concentration on the surface of the central pore is similar to that on the crust. However, the thickness of the surface of the central pore with high oxygen concentration is substantially lesser (36 µm) than the outer crust (284 µm).
References Akiyoshi, R., Nishio, S., Tanasawa, I., 1990. A study on the effect of non-condensible gas in the vapor film on vapor explosion. Int. J. Heat Mass Transf. 33, 603–609. Baba, M., 2013. Fukushima accident: what happened? Radiat. Meas. 55, 17–21. Baker L., Just L., 1962. Studies of metal-water interactions at high temperature-III. ANL report No: 6548. Argonne national laboratory, Illinois. Cho, D.H., Armstrong, D.R., Gunther, W.H., 1998. Experiments on Interactions between Zirconium-Containing Melt and Water. NUREG/CR-5372, ANL, Argonne, IL, USA. Corradini, M.L., Kim, B.J., Oh, M.D., 1988. Vapor explosions in light water-reactors – a review of theory and modelling. Prog. Nucl. Energy 22, 1–117. Goldstein, J.I., et al., 2003. Scanning Electron Microscopy and X-Ray Microanalysis, third ed. Springer US 689 p. Hansson, R.C., Dinh, T.N., Manickam, L., 2013. A study of the effect of binary oxide materials in a single droplet vapor explosion. Nucl. Eng. Des. 264, 168–175. Hong, S.W., Piluso, P., Leskovar, M., 2013. Status of OECD-SERENA project for the resolution of ex-vessel steam explosion risks. J. Energy Power Eng. 7, 423–431. Kim, J., Min, B., Park, I., Kim, H., Hong, S., 2008. Steam explosion experiments using partially oxidized corium. J. Mech. Sci. Technol. 22. Leskovar, M., Centrih, V., Ursic, M., 2016. Analysis of oxidation influence on steam explosion energetics. Ann. Nucl. Energy 90, 441–449. Magallon, D., Hohmann, H., 1997. Experimental investigation of 150-kg-scale corium melt jet quenching in water. Nucl. Eng. Des. 177, 321–337. Manickam, L., 2018. An experimental study on melt fragmentation, oxidation and steam explosion during fuel coolant interaction (Doctoral thesis). KTH Royal Institute of Technology, Stockholm, Sweden. Massalski, T.B., 1990. Binary Alloy Phase Diagram. ASM International, Materials Park. Meignen, R., Picchi, S., Lamome, J., Raverdy, B., Escobar, S.C., Nicaise, G., 2014a. The challenge of modeling fuel–coolant interaction: Part I – Premixing. Nucl. Eng. Des. 280, 511–527. Meignen, R., Raverdy, B., Picchi, S., Lamome, J., 2014b. The challenge of modeling fuel–coolant interaction: Part II – steam explosion. Nucl. Eng. Des. 280, 528–541. Meignen, R., Raverdy, B., Buck, M., Pohlner, P., Kudinov, P., Ma, W., Brayer, C., Piluso, P., Hong, S., Leskovar, M., Ursic, M., Albrecht, G., Lindholm, I., Ivanov, I., 2014c. Status of steam explosion understanding and modelling. Ann. Nucl. Energy 74, 125–133. NEA, 2014. OECD research program on fuel-coolant interactions-Steam explosion resolution for nuclear applications SERENA –Phase 2 final report, NEA/CNSI/R. Park, H.S., Hannson, R.C., Sehgal, B.R., 2005. Fine fragmentation of molten droplet in highly subcooled water due to vapor explosion observed by X-ray radiography. Exp. Therm Fluid Sci. 29 (3), 351–361. Spencer, B.W., Wang, K., Blomquist, C.A., McUmber, L.M., Schneider, J.P., 1994. Fragmentation and quench behavior of corium melt streams in water. NUREG/CR6133, ANL, Argonne, IL, USA. Yang, F., Zhao, X., Xiao, P., 2014. In situ measurement of stresses and phase compositions of the zirconia scale during oxidation of zirconium by Raman spectroscopy. Oxid. Met. 81, 331–343. Zambaux, J.A., Manickam, L., Meignen, R., Ma, W., Bechta, S., Picchi, S., 2018. Study on thermal fragmentation characteristics of a superheated alumina droplet. Ann. Nucl. Energy 119, 352–361.
This paper presented the results of the first series of experiments performed in the MISTEE-OX facility on Zr oxidation. The current research program complements the pioneering studies performed at the ANL: while the ANL experiments are integral in nature (using 1 kg of melt and mixtures of Zr and SS or ZrO2), the MISTEE-OX experiments are performed at the single droplet level providing scope for in-depth characterisation. While the experiments produced significant insights into the Zr oxidation phenomenon in a water pool, it is important to consider the uncertainties involved in the measurement of oxygen concentration using the SEM/EDS analysis. Hence, the results must be verified by other microscopic techniques, and improvements in the experimental measurements (typically hydrogen) are foreseen in future works. Acknowledgements This study was made possible by the support from SAFEST EU Project of FP7 [Grant Agreement No. 604771] and Swedish Nuclear Radiation Protection Authority (SSM), Swedish Power Companies, European Commission (SARNET-2), Nordic Nuclear Safety Program
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Oxidation of molten zirconium droplets in water ⁎
T
Louis Manickam, Qiang Guo, Andrei Komlev , Weimin Ma, Sevostian Bechta Kungliga Tekniska Högskolan (KTH), AlbaNova University Centre, Nuclear Power Safety Division, Roslagstullsbacken 21, SE-106 91 Stockholm, Sweden
A R T I C LE I N FO
A B S T R A C T
Keywords: Reactor safety Severe accident Fuel coolant interaction Zirconium oxidation Hydrogen production
Zirconium, which is used as the cladding material of nuclear fuel rods in LWRs, can react with steam in the case of a core meltdown accident. This results in the release of hydrogen which poses a significant risk of hydrogen explosion. Oxidation of Zr occurs either during the core degradation when the steam flows over the hot fuel rod surfaces or during an FCI when molten corium falls into a water pool (e.g. in the lower head). An experimental study was performed at the MISTEE-OX facility at KTH to observe and quantify the oxidation of molten zirconium droplets in a water pool. During the experimental runs, single droplets of molten zirconium were discharged into a subcooled water pool and the dynamic events were recorded using a high-speed camera. The bubble dynamics indicate a process of cyclic oxidation and hydrogen release from the rear periphery of a droplet while it is quenched in the water. The debris (solidified state of the droplet) after each run was collected for compositional and microstructural analysis via SEM/EDS. The obtained data were employed to estimate the oxidation fractions of the droplets and the results revealed several interesting insights into the oxidation phenomenon of the Zr melt. The water subcooling was observed to have a significant influence on the oxidation: the degree of oxidation decreased with increase in the water subcooling. Furthermore, the degree of oxidation was also influenced by the depth into the debris, forming compounds whose oxygen content decreases from the outer surface towards the core of the debris. Therefore, the qualitative and quantitative results presented in this paper are important in the context of developing a phenomenological understanding of the oxidation behaviour of zirconium melt during the FCI as well as to improve and validate the currently available models implemented in the state-of-art steam explosion codes.
1. Introduction A low thermal neutron capture cross-section and ideal thermalmechanical properties justify the use of zirconium alloy (Zircaloy) as a major component of nuclear fuel assemblies in LWRs. However, zirconium is a chemically reactive metal, which, when exposed to steam at high temperatures, increases the risks related to the release of excessive heat and hydrogen. Recommendations based on the lessons (Baba, 2013) learnt from the TMI-2 and Fukushima Daiichi accidents have laid specific emphasis on the development of ATF and reducing risks due to hydrogen release from an excessively heated core. However, while the development of ATF may be the final fix to completely eliminate the
release of hydrogen, it is equally important to develop sufficient understanding of the Zircaloy oxidation with steam under the aggressive conditions during a severe accident scenario, as the operating nuclear reactors and new installations of LWRs are expected to use the traditional fuel for a long time before the ATF is ready for deployment. While the oxidation of Zircaloy surface by steam has been extensively investigated, the behaviour of molten corium during the FCI has received little focus, which motivates the present study; i.e. to understand and characterise the zirconium oxidation phenomenon that occurs during the FCIs. FCI is a classical term used for a process where high-temperature molten materials come into contact with a cold volatile liquid such as
Abbreviations: Zr, Zirconium; LWR, Light Water Reactor; FCI, Fuel Coolant Interaction; KTH, Kungliga Tekniska Högskolan or Royal Institute of Technology, Sweden; MISTEE, Micro Interactions in Steam Explosion Energetics (facility); MISTEE-OX, Modified MISTEE facility to perform zirconium OXidation experiments; SEM/EDS, Scanning Electron Microscopy/Energy Dispersive Spectroscopy; TMI-2, The Three Mile Island Unit 2; ATF, Accident Tolerant Fuel; ANL, Argonne National Laboratory; TROI, Test for Real cOrium Interaction (facility); KROTOS, FCI-Experimental facility CEA-Cadarache-France; OECD, The Organisation for Economic Cooperation and Development; SERENA, Steam Explosion Resolution for Nuclear Application; HSUB, High water SUBcooling; MSUB, Medium water SUBcooling; STP, Standard Temperature and Pressure; SS, Stainless Steel ⁎ Corresponding author. E-mail addresses: [email protected] (L. Manickam), [email protected] (Q. Guo), [email protected] (A. Komlev), [email protected] (W. Ma), [email protected] (S. Bechta). https://doi.org/10.1016/j.nucengdes.2019.110225 Received 11 June 2019; Received in revised form 21 July 2019; Accepted 22 July 2019 Available online 01 August 2019 0029-5493/ © 2019 Elsevier B.V. All rights reserved.
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with added zirconium content (UO2-ZrO2-Zr) recorded the least energetics in the whole series of experiments conducted in the framework of the OECD SERENA project (Leskovar et al., 2016). Understandably, one may not assume the low energetics to be related to chemical augmentation considering the several integral effects associated with a phenomenon such as the FCI. These results clearly indicated the need to improve our understanding of zirconium metal oxidation behaviour during the FCI. Furthermore, a recent review on the status of steam explosion understanding and modelling (Meignen et al., 2014c) has also indicated the importance of performing small-scale experiments with liquid metal melts at very high temperatures to investigate the oxidation rate and integral tests to characterise the impact of global void generation. Thus, the scarcity of experimental data has stalled the development and improvement of oxidation models implemented for steam explosion analysis. Hence, the present study is a first step towards advancing our current state of knowledge on melt oxidation behaviour during the FCI. The MISTEE facility (Hansson et al., 2013; Zambaux et al., 2018) that has been originally used for steam explosion studies in the scale of single droplets was modified to study the oxidation of pure Zr droplets. Experiments at the scale of a single melt droplet level reduce the uncertainties related to integral mechanisms and are expected to provide conservative estimates. The data obtained from the current experiments provide preliminary insights into the dynamic mechanisms related to the zirconium melt oxidation during the FCI.
water. FCI can result in explosive steam generation that induces a strong dynamic loading on the surrounding structures threatening the overall integrity of the containment. The melt parameters, such as composition, superheat and water subcooling, pool depth, melt flow rate, and system pressure are considered to have a significant effect on the magnitude of the destructive mechanical energy released during an explosive FCI. Despite the significant efforts through FCI research, the occurrence of a steam explosion is still considered a stochastic event as the promoting/inhibiting effects of certain parameters of the melt and coolant on the explosive energy release remain unexplained. A review of the FCI experiments available elsewhere (Corradini, et al., 1988; Spencer et al., 1994; Cho et al., 1998; Magallon and Hohmann, 1997; Park et al., 2005; Hong et al., 2013; Meigner et al., 2014a,b) reveal interesting insights into some of the limiting as well as promoting mechanisms of steam explosions. Based on these insights, it is now widely accepted that the melt solidification mechanism and the relative void fraction in the dispersed region (pre-mixture) are the major factors that can affect the strength of a steam explosion (Meignen et al., 2014c). Molten Zr reacts exothermically with steam producing zirconium oxide and hydrogen which is released as a non-condensable gas (Eq. (1)).
Zr + 2H2 O(g) → ZrO2 + 2H2 (g) + 6.47 MJ/kg Zr at 1900 °C
(1)
The production of hydrogen during the late core degradation phase which results from the oxidation of the corium melt active reducers (i.e. U, Zr) is yet to be explained, and requires experimental investigation to consistently describe the related phenomenon. Generally, the melt oxidation during the FCI can be considered as a major source for the void produced in the pre-mixture due to the release of hydrogen. Moreover, it is also expected that the change in the metallic melt composition due to increase in the oxygen content can affect the physical-chemical behaviour of the melt during the crystallisation process. Further, the generation of the non-condensable gas (hydrogen) that envelops the melt droplet provides greater stability against direct meltwater contact. It is widely known that the non-condensable gases in the vapour film suppress the condensation of the vapour, which contributes to the occurrence of a steam explosion. The effect of non-condensable gases surrounding a melt droplet on vapour explosion has been studied experimentally and numerically by Leskovar et al. (2016). As discussed above, although melt oxidation has a limiting effect on steam explosion, it also has some adverse effects including the build-up of an explosive atmosphere through excess hydrogen production, increased melt thermal energy content, and delayed solidification through heat release; these are important mechanisms that strengthen the steam explosion (Meigner et al., 2014a,b; Leskovar et al., 2016). Apart from such logical propositions, a mechanistic understanding of the melt oxidation mechanism and its influence during the FCI is rather abstruse. The most comprehensive and systematic experimental work to date on zirconium oxidation during the FCI was performed at the ANL (Cho et al., 1998): Approximately 1 kg of melts of Zr-ZrO2 or Zr-SS systems were poured into the subcooled water to assess the extent of chemical augmentation during the FCI. The major experimental parameters used were water subcooling and the zirconium content in the melt. The results showed that the oxidation rate is independent of the zirconium content added to the melt mixture and increased at lower water subcooling. For explosive cases, the oxidation rate was estimated up to 99% and the energy of the explosion was observed to increase with the zirconium content indicating the vital role of chemical augmentation on steam explosion energetics. The interpretations of the results on chemical augmentation by TROI (Kim, et al., 2008) and KROTOS (NEA., 2014) tests performed with added zirconium metal to prototypic materials are not straight forward, considering the stochastic integral effects of a steam explosion. Generally, contradicting results were obtained when the experiments
2. MISTEE-OX experiments 2.1. Facility description To study prototypic oxidation behaviour during the FCI, we performed experiments with zirconium (99.8% purity). The MISTEE facility was modified to perform the oxidation experiments. The original MISTEE facility and its modifications are described in Manickam (2018) in more detail. The main components of the modified facility—called the MISTEEOX (Fig. 1)—include a furnace, water pool, and a high-speed visualisation system; the furnace is an induction heating system (20 kW, 50–250 kHz) capable of attaining temperatures of over 2000 °C. Ceramic tubes of alumina and zirconium oxide are arranged concentrically such that they envelop the crucible as radiation shields. The space between the crucible and radiation shields is packed with MgO or ZrO2 powder to help sustain the position of the crucible and tubes as well as to minimise heat loss. The crucible is made of graphite and has a 6 mm nozzle at the centre of its bottom. A melt droplet is suspended inside the crucible by a constant purge of argon through its nozzle. The temperature of the crucible is measured by a C-type thermocouple mounted from a hole on the crucible wall. A three-way valve isolates the furnace from the water pool. A high-speed camera (Redlake HG50LE) with an acquisition rate up to 5000 fps is used to record the interaction between the melt and water. The melt droplet is captured in a debris catcher suspended inside the water pool, which is a rectangular plexiglass tank (500 mm × 75 mm × 75 mm) arranged below the furnace.
2.2. Experimental conditions The conditions for the series of tests conducted at the MISTEE-OX droplet facility are summarised in Table 1. Approximately 2 g of zirconium was melted in the furnace and on attaining the desired melt temperature, the melt droplet was allowed to free fall through the opened nozzle into the water pool by arresting the gas purge in the crucible.
2
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Fig. 1. Schematic of MISTEE-OX facility.
3. Experimental results and discussion The forthcoming sections present a detailed account of the procedures and results obtained from the MISTEE-OX experiments. Within the scope of current work, the influence of water subcooling on melt oxidation is studied. An evaluation of the amount of released hydrogen based on the visual analysis of bubble formation is presented in Section 3.1. The recorded videos are analysed to attain insights into the phenomenon and data on hydrogen release during the FCI. Meanwhile, Section 3.2 characterises the microstructure and oxidation patterns of the quenched Zr samples. The possible mechanism of the molten Zr droplet oxidation in water pool based on the Zr-O phase diagram is proposed and discussed in Section 3.3. Section 3.4 is devoted to the uncertainty analysis based on the assumptions made for the visual analysis of bubble formation and oxygen measurement by EDS.
Fig. 2. Time sequence raw snapshots of the interaction between a single droplet of zirconium melt and water at ΔTsub ≈ 85 K (HSUB run 3).
3.1. Visual analysis of bubble formation
pattern related to zirconium oxidation. Initially, the melt is oxidised by the surrounding flow of steam causing instant release of heat and hydrogen (inferred by the melt glow at t = 90, 235, and 365 ms in Fig. 2). The growth of the bubble filled with non-condensable gas reduces the steam supply to the drop surface and thereby, the melt oxidation / hydrogen generation (inferred by the decrease in melt glow at t = 159 and 329 ms in Fig. 2). On reaching the threshold size for the current drop velocity in water, the gas bubble detaches from the rear of the droplet, thereby allowing contact between fresh steam and the drop surface. Thus, it logically follows that the temperature of the melt
3.1.1. Cyclic oxidation of molten Zr droplets in water Single droplets of molten Zr metal quenched in water depict a pattern of repetitive bubble growth and detachment along the rear periphery of the zirconium melt droplet (Fig. 2). The trail of bubbles left behind in the droplet trajectory does not seem to condense even under the HSUB, indicating that the bubbles are mostly filled with non-condensable gas (hydrogen). A preliminary observation of the recorded images suggests a number of “cyclic oxidation processes” of heat/hydrogen generation and subsequent bubble detachments from the drop during its free fall in water. The images clearly indicate a very specific Table 1 Experimental conditions of the zirconium melt/water interaction test series. Test ID
Initial mass of Zr melt, g
Temperature of Zr melt in the crucible, °C
Superheat of melt (ΔTsup), K
Temperature of water pool, °C
Subcooling of water, (ΔTsub), K
HSUB run 1 HSUB run 2 HSUB run 3 MSUB run 1 MSUB run 2
2.0 ± 0.1
2005 ± 20
150 ± 20
15 ± 1
85 ± 1
55 ± 1
45 ± 1
3
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melt, resulting in darker images when the light intensity reduces from the melt droplet because of the heat loss. Furthermore, digital image analysis is employed to extract the bubble parameters from the acquired sequence of raw images. As a general scheme, a series of procedures including background subtraction, noise reduction, edge detection, and morphological opening was performed to obtain the projected area of the bubbles (Fig. 5). Based on the above-mentioned assumptions, the kinetics of the hydrogen released during the interaction are estimated for the cases suitable for analysis (Fig. 6). The time scale in Fig. 6 corresponds to the time of the bubble detachment from the droplet. Furthermore, based on the bubble analysis, the O/Zr atomic ratio is estimated to be approximately 0.27 and 0.07 under medium and high subcooling conditions, respectively (the O/Zr atomic ratio of 2 corresponds to fully oxidised zirconium, i.e. the formation of ZrO2) (Fig. 7). A significant increase in the oxidation under the medium subcooling conditions can be clearly observed.
Fig. 3. Time sequence raw snapshots of the interaction between a single droplet of zirconium melt and water at ΔTsub ≈ 45 K (MSUB run 2).
3.2. SEM/EDS analysis
dynamically increases as a result of the heat released by the oxidation reaction while also being simultaneously quenched. The specific observations indicate that the oxidation process does not stop as soon as the bubble is filled with hydrogen, restricting the access of water vapour to the droplet; rather, the hydrogen bubble detaches, thereby providing a periodic access of fresh steam (Manickam, 2018). Similarly, time sequence snapshots of the progress in the interaction between water and zirconium melt under MSUB conditions are shown in Fig. 3. The long trail of non-condensable gas bubbles in the trajectory of the droplets is observed to be similar to that under the HSUB conditions. A typical velocity profile of the melt droplets during quenching under high and medium water subcooling is shown in Fig. 4. The significant difference in the velocity of the droplets observed under the different water subcooling conditions indicates that, under the HSUB conditions, the melt droplet reaches the catcher arrangement much more rapidly than under MSUB conditions.
3.2.1. Quenched Zr droplets under the high water subcooling SEM images depicting the microstructure of the solidified zirconium melt droplet under high water subcooling (HSUB runs 1–3) conditions are presented in Fig. 8. The scanning electron microscope (Hitachi S3700N) combined with the energy-dispersive X-ray spectroscope (Bruker’s XFlash Detector 4010) was used to perform electron-excited X-ray microanalysis of the solidified droplet. The analysis revealed stratified layers with varying oxygen content. A visual analysis of the presented microstructures allows us to observe the oxygen-enriched (lighter areas) and metal-enriched (darker areas) areas. The outer layer (k-area: crust region / melt–vapour interface) is clearly observed as the most oxygen-rich region (see, for example, label a2 in Fig. 8). Further, in the bulk towards the centre of the droplet, the concentration of oxygen is observed to decrease. Fig. 9 depicts the radial stratification of the O/Zr atomic ratio through the depth of the droplet for all the cases studied under the high subcooling conditions. We observe that the entire region of the droplet is neither fully oxidised nor un-oxidised: three or four distinct concentric layers with varied concentrations of oxygen can be identified in the droplet. The O/Zr atomic ratio values were obtained by the scan of a relatively larger area. For example, for the layers k, l, and m (in case of HSUB run 1), the scanning areas correspond to the thickness of the respective layers, and the length of the scanning area was in the range of 0.5–1.5 mm for different layers. The layer boundaries were determined visually, based on the change in the microstructural patterns exhibited within each layer of the droplet (see areas k, l, m, and n under labels a1, b1, and c1 in Fig. 8). The EDS technique employed in the study cannot detect the differences in the O/Zr atomic ratio over the respective layer and hence, it can be assumed that the O/ Zr atomic ratio is evenly distributed in the specific layer. The O/Zr atomic ratio at the outer surface estimated by EDS is in the approximate range of 0.8 and 0.9 (Fig. 9). The outer surface with the highest concentration of oxygen is observed to have a constant thickness in an approximate range of 200–300 µm, and is constantly distributed through the entire surface area of the droplet. The innermost layer of the droplet is almost evenly oxidised where the O/Zr atomic ratio is approximately 0.6. However, it is observed in the case of HSUB run 1 that the innermost layer shows a significantly reduced oxygen concentration as the O/Zr atomic ratio is approximately 0.2. The SEM image for case HSUB run 1, shows a well-formed crack at the outer surface (see label a1 in Fig. 8) which probably could have functioned as a mechanical barrier to oxygen migration towards the inner surfaces.
3.1.2. Hydrogen release estimation To estimate the amount of hydrogen released from the exothermic chemical reaction, we studied the gaseous wake in the trail of the melt droplet. We estimated the volume of hydrogen generated during the interaction at approximately 50 mm below the water level based on the following simplified assumptions: (1) the steam in the bubbles detached from the melt droplet completely condense on reaching the surface layer of the water pool and (2) the bubble contains pure hydrogen at STP. However, it is instructive to note that the resolution of the highspeed camera, which is a function of the number of pixels in the sensor, is inversely proportional to the recording speed; hence, the recorded video is limited to approximately 5 s. Moreover, high-speed imaging usually suffers from low light collectiveness. This, combined with the necessity to neutralise the light emitted from the high temperature
0HOW GURSOHW YHORFLW\ PV
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.100
¨7VXE 45. ¨7VXS 150. ¨7VXE 5. ¨7VXS 150. 0.200
0.300 7LPH V
0.400
3.2.2. Quenched Zr droplets under the medium water subcooling The SEM images depicting the microstructure of the solidified zirconium melt droplet under medium water subcooling (MSUB runs 1–2) conditions are presented in Fig. 10. We observe that the droplets exhibit irregular morphology compared to those under high subcooling
0.500
Fig. 4. Temporal velocity profile of the melt droplet in water under different water subcooling conditions. 4
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Fig. 5. Image processing for the estimation of the temporal distribution of hydrogen (left). Labels 1 and 2 depict typical individual hydrogen bubbles (right).
unlike under the high subcooling conditions, the formation of evenly distributed concentric layers is not observed over the entire surface of the droplet. The O/Zr atomic ratio at the outermost surface is in the range between 1.1 and 1.5 which is approximately 75% oxidation, unlike under the high subcooling conditions where it is approximately 45% oxidation. Moreover, we observe that the O/Zr atomic ratio on the innermost surface ranges between 0.85 and 1, which is approximately 50% oxidation under medium subcooling conditions whereas under the high subcooling conditions the O/Zr atomic ratio is 0.7 which indicates approximately 35% oxidation. A significant difference is also observed between the hydrogen bubble formation analysis and SEM/EDS analysis: the latter shows increased oxidation compared to the former. It is instructive to note that the results of the SEM/EDS analysis should be treated with caution considering the uncertainties involved in the estimation of a relatively lighter element such as oxygen. However, the obtained data can be used for a qualitative comparison of the oxygen concentration between different areas in a sample. Therefore, the use of other techniques/measurements is recommended for an accurate estimation of the O/Zr ratio.
Fig. 6. Hydrogen generation history during Zr melt and water interactions.
3.3. Zr droplet oxidation mechanism 3.3.1. Analysis of the phase transitions according to the Zr–O phase diagram As mentioned earlier, the oxidation (O/Zr atomic ratio) of Zr droplets in medium subcooled water is uniformly distributed over the entire volume (see Fig. 10). This indicates that oxygen mass transfer from the surface to the bulk volume is very fast, which is typical for mass transfer in the liquid state. This is probably because in the case of liquid droplets, the oxygen transfer is not limited to diffusion through the oxidised Zr layer unlike during Zr solid material oxidation where the formation of the ZrO2 layer can be clearly observed on the surface (see, for example Yang et al., 2014). The results of the EDS analysis show that the total oxygen concentration of the obtained droplets is about 40–60 at.%; and, according to the phase diagram (Fig. 11), region of the mentioned composition can be achieved by two ways:
Fig. 7. Evolution of oxygen content in the melt drop estimated from hydrogen generation data.
conditions where the droplets resemble a nearly spheroidal morphology. These observations indicate that the droplets could still be in the liquid state under medium subcooling conditions, allowing the melt to spread in the catcher unlike under the high subcooling conditions where the droplet could have already formed a crust, which limits its shape to that of a spheroid. The images depict a similar pattern of melt oxidation where the outermost layer shows the highest degree of oxygen concentration in the droplet. However, the thickness of this outer layer is not constant as was observed under the high subcooling conditions. The thickness of the layer ranges between 180 and 320 µm. Furthermore, similar to the high subcooling conditions, the oxygen content is stratified from the outer surface to the centre of the droplet (see labels a, b, and c for cases MSUB runs 1 and 2 in Fig. 10). However,
1) Below the solidus line, by increasing the oxygen content and undergoing the following phase transitions in the solid state: β-Zr (O) → α-Zr(O) → (α-Zr(O) + ZrO2) 2) Above the liquidus line, by increasing the oxygen content and quenching the liquid phase with the corresponding composition: Liquid → (α-Zr(O) + ZrO2) Because oxidation of solidified droplet is extremely unlikely and hence, we can safely assume that the oxidation occurred only in the liquid state for a few seconds as explained above. The exothermic oxidation reaction causes a simultaneous increase in the temperature of 5
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Fig. 8. SEM images showing microstructure of the solidified Zr droplet cross-sections for all cases studied under high subcooling conditions.
oxidation characterised by the average O/Zr atomic ratio in the quenched droplets is plotted as a function of the particle diameter, which corresponds to the mean particle diameter estimated from the discrete spherical fragments collected after the experiments in Baker (1962). The O/Zr atomic ratio was observed to decrease with increasing particle diameter. Although not a fair comparison, the experimental data obtained at the MISTEE-OX seem to be in good agreement with these results.
the melt as well as the oxygen concentration in the droplet before the actual rapid solidification. The observed oxygen concentration shows that the melt superheat could have potentially increased by approximately 200 K under high subcooling conditions. The oxygen concentration on the inner surface of the droplet near the centre is approximately in the range up to 40 at.%, indicating that the temperature at the centre of the droplet could have remained relatively lower that at the crust. Similarly, under medium subcooling conditions, the maximum oxygen content observed in the crust region is in the range of approximately 60 at.%, indicating an increase of approximately 400 K in the melt superheat. The oxygen concentration on the inner surface of the droplet near the centre is approximately 50 at.%, indicating a pattern similar to the high subcooling conditions. However, the aforementioned hypothesis is a rather simplified thought and the actual mechanisms could be far more complex, which require further research. It is important to note that for the HSUB samples, the whole oxidation process (oxygen transfer for almost 3 mm droplet radius) probably occurred during the fall of the droplet into the water pool, i.e. in ~0.5 s. To study this process, the kinetics of the droplet oxidation must be considered; however, this can complicate the experimental procedure in terms of a relatively precise temperature measurement, which is not a trivial part of high-temperature and high-speed processes. The O/Zr atomic ratio in the quenched drops that was estimated based on the hydrogen generation data obtained from the bubble analysis of the MISTEE-OX experiments was compared with the experimental data available in Baker, 1962 (Fig. 12). The final Zr
3.3.2. Impact of non-condensable gas formation on steam explosion It is important to note that the zirconium melt droplets do not seem to undergo spontaneous steam explosion even under high subcooling conditions, indicating the stability of the vapour and gas film surrounding the melt droplet. The integral experiments performed at the ANL did not involve spontaneously triggered steam explosions (Cho et al., 1998). In the current experiments with Zr melt, a cyclic process of hydrogen generation and release is observed, leaving behind a trail of bubbles in the droplet trajectory even under high subcooling conditions (Fig. 2). For a general comparison, Fig. 13 shows a droplet of molten alumina discharged into water under almost similar test conditions, as the melting point of alumina (2050 °C) is comparable to that of zirconium. We observe that the alumina droplets falling into highly subcooled water (ΔTsub: 80 K) do not show the formation of a large trail of bubbles indicating rapid condensation of the generated steam except during the initial phase of entering into the water pool where the entrapped non-condensable gases detach from the melt droplet periphery
Fig. 9. Oxygen concentration profile from the surface to the bulk of the quenched drops after performing tests under high subcooling conditions. 6
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Fig. 10. SEM images showing the microstructure of the solidified Zr droplet cross-sections for all cases studied under medium subcooling conditions.
or triggered.
(i.e. at t = 79 ms in Fig. 13). The non-condensable gases have a significant effect on steam explosion triggering as well as the overall explosion yield since their pressure within the vapour film reduces the condensation of vapour, which essentially dictates the occurrence of vapour explosion (Corradini et al., 1988; Akiyoshi et al., 1990). The release of non-condensable gas due to the Zr droplet oxidation reaction under water shows that the triggering of the steam explosion can be suppressed even under high subcooling conditions while the alumina droplets can be relatively easily triggered under similar conditions. Furthermore, the cyclic process of bubble release also indicates that the Zr droplet is left with a thinner film during bubble detachment, promoting the effective participation of the Zr melt droplet in steam explosion. Considering a pre-mixture with many droplets of discrete sizes undergoing varied cycles of bubble growth and departure, it is possible that the active mass that could effectively participate in steam explosion is significantly reduced. However, it would be useful to investigate, in future works, the threshold bubble volume required to suppress the triggering of steam explosion to identify the trigger strength required to quantify the time scales under which the bubbles cannot be compressed
3.4. Analysis of uncertainties A comparison of the degree of zirconium oxidation observed by the visual bubbles (Section 3.1) and EDS analyses (Section 3.2) revealed significant discrepancies. According to the EDS analysis, the increase in the concentration of oxygen (i.e. O/Zr ratio) in the quenched Zr droplets (Fig. 9) is observed to correspond to the values of the HSUB and MSUB conditions for the bubbles analysis (Fig. 7). The bubble volume analysis method includes following uncertainties of the O/Zr ratio determination: - The released gas is an “ideal” gas and fully non-condensable (in the time-point of analysis). - The temperature of the gas is equal to the temperature of water and its pressure equals the normal atmospheric pressure. - The size (depth) of the gas bubble in a direction perpendicular to the image equals the width of the bubble (horizontal size).
Fig. 11. Zirconium–Oxygen phase diagram (Massalski, 1990). 7
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Fig. 12. O/Zr atomic ratio as a function of the mean particle diameter.
experimental work (typically by performing experiments involving a wide range of conditions including melt temperature and droplet diameter) with improved measurements (i.e. temperature measurement during melt droplet quenching in water and use of techniques other than EDS for oxygen measurement, for example wavelength dispersive X-ray analysis (WDS) or carbo-thermal reduction) and by implementing the use of metal mixtures (e.g. Fe-Zr).
- There are errors in the digital image analysis (more detailed described in paragraph 3.1). Based on the above-mentioned uncertainties of the described method, the estimated error of oxygen concentration (O/Zr ratio) is approximately 20%. It is widely known that the EDS methods can involve considerable errors in the analysis of relatively lighter elements such as oxygen (Gondstein, 2003). Moreover, only one cross-section for each droplet was analysed, which leads to an increase in the error of the statistic results; however, this is the basis of the assumption of equivalent oxygen distribution in the quenched droplet. As this raises doubts regarding the use of the quantitative values of the O/Zr ratio, the presented EDS data is considered somewhat more reliable for the qualitative characterisation of the samples, especially for a comparative analysis of the O/Zr ratio between the HSUB and MSUB conditions. Considering issues such as the complexity of the phenomenon, scarcity of experimental data, and importance of such experimental data for the model development, it is vital to follow up this
4. Conclusions and future prospects In the pursuit of understanding the FCIs during a postulated severe accident of LWRs, the oxidation behaviour of molten zirconium droplets falling into a water pool was experimentally studied at the MISTEE-OX facility. The complex bubble and droplet dynamics due to the chemical reaction as well as rapid heat and mass transfer were recorded using a high-speed camera, and the resulting debris was analysed by SEM/EDS. The experiments revealed several interesting insights into the oxidation phenomenon of molten zirconium droplets falling in a water pool. The major conclusions from this study can be
Fig. 13. Time sequence raw snapshots of the interaction between alumina melt and water (ΔTsup ≈ 100 K, ΔTsub ≈ 15 K). 8
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(NKS), and Swiss Federal Nuclear Safety Inspectorate (ENSI) under the APRI-MSWI program. The authors are also grateful to Per Sköld at the KTH-NPS Laboratory for his technical support with the experimental setup.
summarised as follows:
• A pattern of “cyclic oxidation” is observed in the high-speed images. • • • •
The images reveal melt glow, indicating an increase in the temperature or, in other words, an oxidation reaction whenever the bubble surrounding the melt droplet detaches, and subsequently, a fresh flow of steam is available. Spontaneously triggered steam explosions were not observed in the present study, indicating that the oxidation (and hydrogen generation) may suppress the potential for a spontaneous steam explosion. Water subcooling has a substantial effect on the oxidation process. The O/Zr atomic ratios for the quenched Zr droplets obtained based on the bubble volume analysis are approximately 0.07 and 0.27 under high and medium subcooling conditions, respectively. The SEM/EDS analysis of a solidified droplet shows the formation of concentric layers having constant thickness. Concentration of oxygen decreases from the outer layer towards the centre of the droplet. The SEM/EDS analysis of a solidified droplet with a hollow core or central pore shows that the oxygen concentration on the surface of the central pore is similar to that on the crust. However, the thickness of the surface of the central pore with high oxygen concentration is substantially lesser (36 µm) than the outer crust (284 µm).
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This paper presented the results of the first series of experiments performed in the MISTEE-OX facility on Zr oxidation. The current research program complements the pioneering studies performed at the ANL: while the ANL experiments are integral in nature (using 1 kg of melt and mixtures of Zr and SS or ZrO2), the MISTEE-OX experiments are performed at the single droplet level providing scope for in-depth characterisation. While the experiments produced significant insights into the Zr oxidation phenomenon in a water pool, it is important to consider the uncertainties involved in the measurement of oxygen concentration using the SEM/EDS analysis. Hence, the results must be verified by other microscopic techniques, and improvements in the experimental measurements (typically hydrogen) are foreseen in future works. Acknowledgements This study was made possible by the support from SAFEST EU Project of FP7 [Grant Agreement No. 604771] and Swedish Nuclear Radiation Protection Authority (SSM), Swedish Power Companies, European Commission (SARNET-2), Nordic Nuclear Safety Program
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