Quenching Experiments with CrAl-coated Zircaloy Cladding in Reﬂooding Water Flows

: A quenching experiment is performed to investigate the heat transfer characteristics and cooling performance of CrAl-coated Zircaloy (Zr) cladding in a water ﬂow. The CrAl-coated Zr cladding is one of the accident tolerant fuels for light water reactors. The uncoated Zr cladding is also used in this quenching experiment for comparison. This experiment simulates reﬂood quenching of fuel rod during loss of coolant accident (LOCA) in nuclear power plant. The test conditions were determined to represent the peak cladding temperature, the coolant subcooling and the reﬂood velocity in the event of LOCA. The ﬂow visualization showed the ﬁlm boiling during early stage of reﬂood quenching and the transition to nucleate boiling. The ﬁlm layer decreases as the coolant subcooling increases and becomes wavy as the reﬂood velocity increases. The CrAl-coated Zr cladding showed more wavy and thinner ﬁlm than the uncoated Zr cladding. The rewetting temperature increases as the initial wall temperature and/or the coolant subcooling increases. The quench front velocity increases signiﬁcantly as the coolant subcooling increases. The reﬂood velocity has a negligible effect on rewetting temperature and quench front velocity.


Introduction
The zirconium-based cladding of fuel rods in nuclear reactors generates a large amount of hydrogen and heat due to the chemical reaction between zirconium and steam, particularly during any loss of the active cooling system. A hydrogen explosion occurred after the complete loss of the emergency core cooling system due to the station blackout at the Fukushima Daiichi nuclear power plant in 2011. Accident-tolerant fuels (ATFs) have been proposed to replace the zirconium-based cladding and uranium dioxide (UO 2 ) fuel in light water reactors (LWRs). Zinkle et al. [1] discussed the perspectives of ATFs for LWRs, which can enhance the safety margins for nuclear power systems. Kurata [2] identified the research and development metrics and the attributes of candidate ATF-concepts. The ATFs can be categorized into the modification of Zr cladding, the non-Zr cladding (FeCrAl alloy, silicon carbide composite) and the alternative fuel forms (fully ceramic microencapsulated fuel). Hence, the ATF claddings were developed for improving oxidation resistance and mechanical strength.
There are numerous studies of the ATF concepts focusing on fabrication technique and material performances [3][4][5][6][7][8][9][10][11][12]. However, there have been a limited number of studies on the thermal-hydraulic performance of the ATF cladding which is essential to evaluate the enhancement of safety margin under accident conditions. The CrAl-coated Zr cladding was proposed as the ATF candidate for near-term employment of ATFs in LWRs because of fabrication and neutron economics. Kim et al. [12] developed an arc-ion plating technique for CrAl coating and laser-beam scanning for oxide dispersion strengthened (ODS) treatment. The out-of-pile testing of the CrAl-coated Zr cladding showed the sufficient Figure 1 illustrates the schematic of the experimental apparatus for reflood quenching of cladding specimens. The quenching apparatus consists of test section, electrical heating element, coolant supply equipment, data acquisition system (DAS) and flow visualization system. The test section is a vertical tube specimen (uncoated and CrAl-coated Zr claddings) with quartz shroud. The heating element is made of tungsten rod and ceramic tube. The tungsten rod is heated by a direct current (DC) power supply with copper bars at the lower and upper ends. The ceramic tube is used for mounting thermocouples and isolating electrical current. In order to prevent equipment damage due to high temperature of cladding specimen, two cooling chambers are installed at the bottom and top sections of tube specimen. A constant-temperature bath circulates cooling water through the bottom and top chambers. The coolant supply system consists of a water tank, deionized (DI) water generator and pressure regulator. The pressure in the water tank is kept at constant by compressed air which is controlled by a pressure regulator. The water temperature is controlled by a heater inside water tank. The cooling water in the water tank is mixed using a recirculating pump to prevent thermal stratification. Thermal insulation and heating tape are used in the pipelines from the water tank to the test section in order to minimize the drop of coolant temperature due to heat losses. The cooling water is discharged into the drain tank through three outlets. The mass flow rate of coolant is measured using weight of discharged cooling water for constant duration. The data acquisition system is established using a Labview program to acquire the coolant temperature and the cladding temperature in real time. The K-type thermocouples (0.05 mm in diameter) are installed at the inlet chamber of test section and the cladding tube. A data acquisition device (9213 and 9221, National Instruments, Texas, USA) is used to process the thermocouple readings in every 0.2 s. The thermocouples (TCs) are calibrated using a calibrator (FLUKE, WA, USA) and the temperature measurement error is estimated to be ±3.7 °C A high-speed camera (FASTCAM SA3, Photron, CA, USA) is used to visualize the quenching at 500 frames per second (fps) in 512 × 1024 pixels. Since the recording time of high speed camera is short due to memory limit, a digital camera is also used to record whole process of reflood quenching at 30 fps. A Phoenix 300 device (Kromtek, Malaysia) is used to measure the contact angle on the surface of cladding specimens before and after the experiment. The measurement accuracy of contact angle is 0.1°. The surface roughness was also measured using a SJ-201 device (Mitutoyo, Kanagawa, Japan) with resolution of 0.02 μm. Table 1 lists the sensors and equipment used in this experiment. The coolant supply system consists of a water tank, deionized (DI) water generator and pressure regulator. The pressure in the water tank is kept at constant by compressed air which is controlled by a pressure regulator. The water temperature is controlled by a heater inside water tank. The cooling water in the water tank is mixed using a recirculating pump to prevent thermal stratification. Thermal insulation and heating tape are used in the pipelines from the water tank to the test section in order to minimize the drop of coolant temperature due to heat losses. The cooling water is discharged into the drain tank through three outlets. The mass flow rate of coolant is measured using weight of discharged cooling water for constant duration. The data acquisition system is established using a Labview program to acquire the coolant temperature and the cladding temperature in real time. The K-type thermocouples (0.05 mm in diameter) are installed at the inlet chamber of test section and the cladding tube. A data acquisition device (9213 and 9221, National Instruments, Texas, USA) is used to process the thermocouple readings in every 0.2 s. The thermocouples (TCs) are calibrated using a calibrator (FLUKE, WA, USA) and the temperature measurement error is estimated to be ±3.7 • C A high-speed camera (FASTCAM SA3, Photron, CA, USA) is used to visualize the quenching at 500 frames per second (fps) in 512 × 1024 pixels. Since the recording time of high speed camera is short due to memory limit, a digital camera is also used to record whole process of reflood quenching at 30 fps. A Phoenix 300 device (Kromtek, Malaysia) is used to measure the contact angle on the surface of cladding specimens before and after the experiment. The measurement accuracy of contact angle is 0.1 • . The surface roughness was also measured using a SJ-201 device (Mitutoyo, Kanagawa, Japan) with resolution of 0.02 µm. Table 1 lists the sensors and equipment used in this experiment.

Test Specimen
The test specimen shown in Figure 2 consists of a cladding tube, ceramic tube and tungsten rod. The ceramic tube is used to electrically isolate the cladding tube from the tungsten rod. The four TCs are placed in between ceramic tube and cladding tube. It is noted that four TCs (TC1-4) are installed in three axial locations with two TCs in the center of tube specimen. The axial distance between the TCs is 40 mm. The outer diameter, thickness and length of uncoated Zr cladding are 9.5, 0.57 and 400 mm, respectively. The diameter and length of tungsten rod is 4 and 500 mm.

Test Specimen
The test specimen shown in Figure 2 consists of a cladding tube, ceramic tube and tungsten rod. The ceramic tube is used to electrically isolate the cladding tube from the tungsten rod. The four TCs are placed in between ceramic tube and cladding tube. It is noted that four TCs (TC1-4) are installed in three axial locations with two TCs in the center of tube specimen. The axial distance between the TCs is 40 mm. The outer diameter, thickness and length of uncoated Zr cladding are 9.5, 0.57 and 400 mm, respectively. The diameter and length of tungsten rod is 4 and 500 mm. The CrAl-coated Zr cladding was fabricated by an arc-ion plating technique which is known as a high-energy deposition method providing excellent film density and economic efficiency [12]. The thickness of the CrAl coating layer is approximately 50 μm. The coated specimen is polished using #2400 mesh sandpaper. The mean surface roughness (Ra) is 0.19 μm and 1.3 μm for uncoated and CrAl-coated cladding specimens, respectively. Figure 3 shows the contact angle (θ for Zr and CrAl-coated Zr claddings before the quench experiment. The measured contact angle is 66 deg. and 57 deg. for Zr cladding and CrAl-coated Zr cladding, respectively. The CrAl-coated Zr cladding appears to be slightly more hydrophilic than the Zr cladding. The CrAl-coated Zr cladding was fabricated by an arc-ion plating technique which is known as a high-energy deposition method providing excellent film density and economic efficiency [12]. The thickness of the CrAl coating layer is approximately 50 µm. The coated specimen is polished using #2400 mesh sandpaper. The mean surface roughness (Ra) is 0.19 µm and 1.3 µm for uncoated and CrAl-coated cladding specimens, respectively. Figure 3 shows the contact angle (θ for Zr and CrAl-coated Zr claddings before the quench experiment. The measured contact angle is 66 deg. and 57 deg. for Zr cladding and CrAl-coated Zr cladding, respectively. The CrAl-coated Zr cladding appears to be slightly more hydrophilic than the Zr cladding.    Figure 4a is the Zr specimen after six times quenching tests at 600 °C. Figure 4b shows the SEM image of CrAl-coated Zr specimen after eight and six quenching tests at 600 °C and 800 °C, respectively. The Zr specimen shows an oxide layer of 3.93 μm thickness on the outer surface. It was also found that the oxide layer for Zr specimen increases as the quenching test is repeated. The CrAl-coated specimen in Figure 4b shows the coating layer of 50 μm on the outer surface of Zr cladding but no oxide layer. This indicates that the CrAl-coated Zr cladding is highly resistant to high temperature oxidation as designed.

Test Conditions and Procedure
This experiment is performed to evaluate the cooling performance of CrAl-coated Zr cladding during reflood quenching of LOCA in a pressurized water reactor (PWR). The experimental conditions depend on initial cladding temperature, coolant subcooling and reflood flowrate. The initial cladding temperatures (Tw) are 600 °C °C and 800 °C. The coolant temperature is changed from 50 °C to 95 °C. Since the saturation temperature of coolant (water) is 100 °C at 1 atm, the coolant subcooling (Tsub) ranges from 5 °C to 50 °C. The reflood flowrate of coolant is adjusted to vary the reflood velocity (Vc) from 0.02 m/s to 0.1 m/s, which simulates low and high reflood velocity during LOCA in PWR. Table 2 lists the summary of test conditions.   Figure 4a is the Zr specimen after six times quenching tests at 600 • C. Figure 4b shows the SEM image of CrAl-coated Zr specimen after eight and six quenching tests at 600 • C and 800 • C, respectively. The Zr specimen shows an oxide layer of 3.93 µm thickness on the outer surface. It was also found that the oxide layer for Zr specimen increases as the quenching test is repeated. The CrAl-coated specimen in Figure 4b shows the coating layer of 50 µm on the outer surface of Zr cladding but no oxide layer. This indicates that the CrAl-coated Zr cladding is highly resistant to high temperature oxidation as designed.    Figure 4a is the Zr specimen after six times quenching tests at 600 °C. Figure 4b shows the SEM image of CrAl-coated Zr specimen after eight and six quenching tests at 600 °C and 800 °C, respectively. The Zr specimen shows an oxide layer of 3.93 μm thickness on the outer surface. It was also found that the oxide layer for Zr specimen increases as the quenching test is repeated. The CrAl-coated specimen in Figure 4b shows the coating layer of 50 μm on the outer surface of Zr cladding but no oxide layer. This indicates that the CrAl-coated Zr cladding is highly resistant to high temperature oxidation as designed.

Test Conditions and Procedure
This experiment is performed to evaluate the cooling performance of CrAl-coated Zr cladding during reflood quenching of LOCA in a pressurized water reactor (PWR). The experimental conditions depend on initial cladding temperature, coolant subcooling and reflood flowrate. The initial cladding temperatures (Tw) are 600 °C °C and 800 °C. The coolant temperature is changed from 50 °C to 95 °C. Since the saturation temperature of coolant (water) is 100 °C at 1 atm, the coolant subcooling (Tsub) ranges from 5 °C to 50 °C. The reflood flowrate of coolant is adjusted to vary the reflood velocity (Vc) from 0.02 m/s to 0.1 m/s, which simulates low and high reflood velocity during LOCA in PWR. Table 2 lists the summary of test conditions.

Test Conditions and Procedure
This experiment is performed to evaluate the cooling performance of CrAl-coated Zr cladding during reflood quenching of LOCA in a pressurized water reactor (PWR). The experimental conditions depend on initial cladding temperature, coolant subcooling and reflood flowrate. The initial cladding temperatures (Tw) are 600 • C and 800 • C. The coolant temperature is changed from 50 • C to 95 • C. Since the saturation temperature of coolant (water) is 100 • C at 1 atm, the coolant subcooling (T sub ) ranges from 5 • C to 50 • C. The reflood flowrate of coolant is adjusted to vary the reflood velocity (Vc) from 0.02 m/s to 0.1 m/s, which simulates low and high reflood velocity during LOCA in PWR. Table 2 lists the summary of test conditions. The water tank is filled with DI water and the water temperature is maintained at the target temperature using an electrical heater. A water pump recirculates the DI water in water tank from bottom to top in order to prevent thermal stratification. The coolant pipeline from water tank to test section is thermally insulated and heated by ribbon heater in order to maintain the coolant temperature at target value. The electrical current of DC power supply for tungsten rod increased continually such that the cladding temperature reaches its target temperature. Figure 5 illustrates the heat-up of CrAl-coated Zr cladding. It took approximately 700 s for the cladding temperature to reach 800 • C. During the heat up of test specimen, the bottom and top cooling chambers are cooled down by circulating cold water from the water bath at constant temperature, i.e., 7 • C.  The water tank is filled with DI water and the water temperature is maintained at the target temperature using an electrical heater. A water pump recirculates the DI water in water tank from bottom to top in order to prevent thermal stratification. The coolant pipeline from water tank to test section is thermally insulated and heated by ribbon heater in order to maintain the coolant temperature at target value. The electrical current of DC power supply for tungsten rod increased continually such that the cladding temperature reaches its target temperature. Figure 5 illustrates the heat-up of CrAl-coated Zr cladding. It took approximately 700 s for the cladding temperature to reach 800 °C. During the heat up of test specimen, the bottom and top cooling chambers are cooled down by circulating cold water from the water bath at constant temperature, i.e., 7 °C. The water tank is pressurized with compressed air at constant pressure. The position of the gate valve in the coolant pipeline is adjusted to control the coolant flow rate at the target reflood velocity. Once the cladding specimen reaches its target temperature, the cooling water in compressed tank is injected into test section by opening the on/off valve in the pipeline. The data acquisition system acquires the coolant temperature at the inlet of test section as well as the cladding temperature in real time (0.2 sec) during quenching test.
A high-speed camera and a digital camera are used to visualize two-phase flow pattern during quenching test. The high-speed camera captures the fine structure of the two- The water tank is pressurized with compressed air at constant pressure. The position of the gate valve in the coolant pipeline is adjusted to control the coolant flow rate at the target reflood velocity. Once the cladding specimen reaches its target temperature, the cooling water in compressed tank is injected into test section by opening the on/off valve in the pipeline. The data acquisition system acquires the coolant temperature at the inlet of test section as well as the cladding temperature in real time (0.2 s) during quenching test.
A high-speed camera and a digital camera are used to visualize two-phase flow pattern during quenching test. The high-speed camera captures the fine structure of the two-phase flow pattern near the quench front. The digital camera visualizes the quenching process in the whole region of the active test section. The DC power for heater rod is turned off when the cladding temperature is lower than approximately 200 • C. The quenching experiment is complete when the cladding specimen is cooled down to the coolant temperature.

Flow Visualization of Reflood Quenching
The digital camera records dynamic images of flow transition from bottom to top of test section during quenching experiments. The high-speed camera captures the flow images in the local region of test section. The videos taken using the digital and high-speed cameras are converted into still pictures. In the early stage of quenching, a vapor film develops on the outer surface of tube specimen due to the high cladding temperature. The vapor film is collapsed by a quench front that goes up from the bottom of the test section. The bubbly flow is observed to follow the film flow. Then, the reflood quenching ends up with single-phase convective flow as the quench front reaches the top of test section. Figure 6 shows the digital and high-speed camera pictures for the Zr specimen quenching at Tw = 800 • C, T sub = 5 • C and Vc = 0.05 m/s. The digital-camera image at 0 s shows a cloud of bubbles in lower region below the water level indicated by a white line. The digital-camera picture at 25 s shows the quench front (dotted line) in the bottom region and the collapse of the vapor film. It shows the bubbly flow and film flow upstream and downstream of the quench front, respectively. The quench front appears to reach the central and top regions of the test section at 38 s and 43 s, respectively. The high-speed camera pictures show details of the flow structure in the local region (approximately 40 mm in height) during a short period of time, i.e., 3 s. A thick and wavy film is formed on the hot surface of the cladding specimen before the quench front passes by. The bubbly flow follows the film flow upstream of the quench front as it moves upwards. phase flow pattern near the quench front. The digital camera visualizes the quenching process in the whole region of the active test section. The DC power for heater rod is turned off when the cladding temperature is lower than approximately 200 °C. The quenching experiment is complete when the cladding specimen is cooled down to the coolant temperature.

Flow Visualization of Reflood Quenching
The digital camera records dynamic images of flow transition from bottom to top of test section during quenching experiments. The high-speed camera captures the flow images in the local region of test section. The videos taken using the digital and high-speed cameras are converted into still pictures. In the early stage of quenching, a vapor film develops on the outer surface of tube specimen due to the high cladding temperature. The vapor film is collapsed by a quench front that goes up from the bottom of the test section. The bubbly flow is observed to follow the film flow. Then, the reflood quenching ends up with single-phase convective flow as the quench front reaches the top of test section. Figure 6 shows the digital and high-speed camera pictures for the Zr specimen quenching at Tw = 800 °C, Tsub = 5 °C and Vc = 0.05 m/s. The digital-camera image at 0 sec shows a cloud of bubbles in lower region below the water level indicated by a white line.    for Zr cladding appears to be symmetric in circumferential direction as shown in Figure 8. The flow pattern and the speed of quench front appear to be almost same for uncoated and CrAl-coated Zr claddings. However, Figure 9 shows an asymmetric quench front for the CrAl-coated Zr cladding. This might be caused by non-uniform coating in Figure 4 and an unsmooth surface of CrAl-coated Zr cladding. Lee et al. [18] provided SEM images of CrAl-coated Zr cladding that show an unsmooth surface after polishing. It can be noted that the quench front at high subcooling is much faster than that at low subcooling. The top-down quenching was also observed under certain test conditions because of rapid cooling in the top portion of cladding specimen.  The high-speed images also show clearly the quench front (dot line) moving upwards. The quench front for Zr cladding appears to be symmetric in circumferential direction as shown in Figure 8. The flow pattern and the speed of quench front appear to be almost same for uncoated and CrAl-coated Zr claddings. However, Figure 9 shows an asymmetric quench front for the CrAl-coated Zr cladding. This might be caused by non-uniform coating in Figure 4 and an unsmooth surface of CrAl-coated Zr cladding. Lee et al. [18] provided SEM images of CrAl-coated Zr cladding that show an unsmooth surface after polishing. It can be noted that the quench front at high subcooling is much faster than that at low subcooling. The top-down quenching was also observed under certain test conditions because of rapid cooling in the top portion of cladding specimen. for Zr cladding appears to be symmetric in circumferential direction as shown in Figure 8. The flow pattern and the speed of quench front appear to be almost same for uncoated and CrAl-coated Zr claddings. However, Figure 9 shows an asymmetric quench front for the CrAl-coated Zr cladding. This might be caused by non-uniform coating in Figure 4 and an unsmooth surface of CrAl-coated Zr cladding. Lee et al. [18] provided SEM images of CrAl-coated Zr cladding that show an unsmooth surface after polishing. It can be noted that the quench front at high subcooling is much faster than that at low subcooling. The top-down quenching was also observed under certain test conditions because of rapid cooling in the top portion of cladding specimen.        Figure 11 shows the quenching curve of the CrAl-coated Zr at Tw = 600 • C, T sub = 5 • C and Vc = 0.05 m/s. It shows a gradual decrease of the cladding temperature due to film boiling in the early stages of quenching. The transition to nucleate boiling can be also seen in the quenching curve by the rapid drop in cladding temperature in between 10 s and 20 s. Figure 11b clearly shows the sequence of transition boiling from bottom to top of the cladding specimen. The cladding temperature at the bottom (TC1) and top (TC4) sections show the transition at 10 s and 16 s, respectively. It is also noted that the cladding temperature at the middle section (TC2 and TC3) indicates transition boiling at the same time, e.g., 14 s. The rewetting temperatures (RWTs) appear to be approximately 490 • C and 525 • C at the middle and bottom sections, respectively. The RWT is defined as the transition temperature from film boiling to nucleate boiling in this paper as indicated in Figure 11b.

Parametric Analysis of Quenching Curve
Energies 2021, 14, x FOR PEER REVIEW 10 of 17 Figure 11 shows the quenching curve of the CrAl-coated Zr at Tw = 600 °C, Tsub = 5 °C and Vc = 0.05 m/s. It shows a gradual decrease of the cladding temperature due to film boiling in the early stages of quenching. The transition to nucleate boiling can be also seen in the quenching curve by the rapid drop in cladding temperature in between 10 s and 20 s. Figure 11b clearly shows the sequence of transition boiling from bottom to top of the cladding specimen. The cladding temperature at the bottom (TC1) and top (TC4) sections show the transition at 10 s and 16 s, respectively. It is also noted that the cladding temperature at the middle section (TC2 and TC3) indicates transition boiling at the same time, e.g., 14 s. The rewetting temperatures (RWTs) appear to be approximately 490 °C and 525 °C at the middle and bottom sections, respectively. The RWT is defined as the transition temperature from film boiling to nucleate boiling in this paper as indicated in Figure 11b.  Figure 12 shows the effect of initial cladding temperature on the quenching curve for the CrAl-coated Zr cladding. The test specimen was heated to reach the initial target temperatures, i.e., 600 °C and 800 °C. It is noted that the cladding temperature in the top location (TC4) is slightly lower than at others (TC1 and TC2). The coolant subcooling was approximately 5 °C with the reflood velocity of 0.05 m/s. The quenching curves show a longer period of film boiling and higher RWTs as the cladding temperature increases. It is noted that the initial cladding temperature at the top section (TC4) is approximately 10% lower than those at the bottom and middle sections (TC1 & TC2). This is due to nonuniform heating of the cladding tube in the axial direction and higher heat loss to the top region. The RWT values in the middle (TC2) are approximately 494 °C and 655 °C for the cladding temperatures of 600 °C and 800 °C, respectively. The cooling rate of CrAl-coated Zr cladding in film boiling is estimated at 9.7 °C/s and 7.6 °C/s for Tw = 600 °C and Tw = 800 °C, respectively. Hence, the RWT increases but the cooling rate in film boiling regime decreases as the cladding temperature increases.  Figure 12 shows the effect of initial cladding temperature on the quenching curve for the CrAl-coated Zr cladding. The test specimen was heated to reach the initial target temperatures, i.e., 600 • C and 800 • C. It is noted that the cladding temperature in the top location (TC4) is slightly lower than at others (TC1 and TC2). The coolant subcooling was approximately 5 • C with the reflood velocity of 0.05 m/s. The quenching curves show a longer period of film boiling and higher RWTs as the cladding temperature increases. It is noted that the initial cladding temperature at the top section (TC4) is approximately 10% lower than those at the bottom and middle sections (TC1 & TC2). This is due to nonuniform heating of the cladding tube in the axial direction and higher heat loss to the top region. The RWT values in the middle (TC2) are approximately 494 • C and 655 • C for the cladding temperatures of 600 • C and 800 • C, respectively. The cooling rate of CrAl-coated Zr cladding in film boiling is estimated at 9.7 • C/s and 7.6 • C/s for Tw = 600 • C and Tw = 800 • C, respectively. Hence, the RWT increases but the cooling rate in film boiling regime decreases as the cladding temperature increases.  Figure 13 shows the effect of reflood velocity on the quenching performance of CrAlcoated Zr cladding. The reflood velocity (Vc) was changed to 0.02, 0.05 and 0.1 m/s. The initial cladding temperature is 800 °C and the coolant subcooling is 7-15 °C for low subcooling and 50 °C for high subcooling. The low reflood velocity (Vc = 0.02 m/s) appears to result in faster cooling under low subcooling conditions in Figure 13a. This is because the coolant subcooling for low reflooding is higher than those for high reflooding cases. It should be noted that the coolant temperature is slightly different from the target value under the low subcooling conditions. Figure 13b compares the quenching curves at high subcooling, i.e., Tsub = 50 °C. It shows almost same quenching curves for the three reflooding cases. Therefore, the reflood velocity seems to have a minimal effect on quenching characteristics such as RWT and cooling rate in film boiling.  Figure 13 shows the effect of reflood velocity on the quenching performance of CrAlcoated Zr cladding. The reflood velocity (Vc) was changed to 0.02, 0.05 and 0.1 m/s. The initial cladding temperature is 800 • C and the coolant subcooling is 7-15 • C for low subcooling and 50 • C for high subcooling. The low reflood velocity (Vc = 0.02 m/s) appears to result in faster cooling under low subcooling conditions in Figure 13a. This is because the coolant subcooling for low reflooding is higher than those for high reflooding cases. It should be noted that the coolant temperature is slightly different from the target value under the low subcooling conditions. Figure 13b compares the quenching curves at high subcooling, i.e., T sub = 50 • C. It shows almost same quenching curves for the three reflooding cases. Therefore, the reflood velocity seems to have a minimal effect on quenching characteristics such as RWT and cooling rate in film boiling. Figure 14 shows the effect of coolant subcooling for CrAl-coated Zr cladding. The initial cladding temperature is 600 • C and 800 • C with the constant reflood velocity of 0.05 m/s. The coolant subcooling ranges from 5 • C to 50 • C. The duration of film boiling decreases significantly as the coolant subcooling increases. The vapor film is very thin and disappears shortly after the coolant reflooding under high subcooling conditions, e.g., T sub > 30 • C. The duration of film boiling is 10 s for the subcooling of 10 • C at Tw = 600 • C as shown in in Figure 14a. Figure 14b indicates the duration of film boiling for 14 s and 7 s for the subcooling at 10 • C and 20 • C, respectively. The RWT appears to increase significantly when the coolant subcooling is higher than 20 • C. Hence, the fast cooling is possible for the CrAl-coated Zr cladding if the coolant subcooling is higher than 20 • C.  Figure 14 shows the effect of coolant subcooling for CrAl-coated Zr cladding. The initial cladding temperature is 600 °C and 800 °C with the constant reflood velocity of 0.05 m/s. The coolant subcooling ranges from 5 °C to 50 °C. The duration of film boiling decreases significantly as the coolant subcooling increases. The vapor film is very thin and disappears shortly after the coolant reflooding under high subcooling conditions, e.g., Tsub > 30 °C. The duration of film boiling is 10 s for the subcooling of 10 °C at Tw = 600 °C as shown in in Figure 14a. Figure 14b indicates the duration of film boiling for 14 s and 7 s for the subcooling at 10 °C and 20 °C, respectively. The RWT appears to increase significantly when the coolant subcooling is higher than 20 °C. Hence, the fast cooling is possible for the CrAl-coated Zr cladding if the coolant subcooling is higher than 20 °C.  Figure 15 compares the quenching curves of uncoated and CrAl-coated Zr specimens for initial cladding temperatures of 600 °C and 800 °C. In the case of high subcooling, e.g., Tsub = 50 °C, the cladding temperature decreases rapidly as soon as the coolant reflooding   Figure 14 shows the effect of coolant subcooling for CrAl-coated Zr cladding. The initial cladding temperature is 600 °C and 800 °C with the constant reflood velocity of 0.05 m/s. The coolant subcooling ranges from 5 °C to 50 °C. The duration of film boiling decreases significantly as the coolant subcooling increases. The vapor film is very thin and disappears shortly after the coolant reflooding under high subcooling conditions, e.g., Tsub > 30 °C. The duration of film boiling is 10 s for the subcooling of 10 °C at Tw = 600 °C as shown in in Figure 14a. Figure 14b indicates the duration of film boiling for 14 s and 7 s for the subcooling at 10 °C and 20 °C, respectively. The RWT appears to increase significantly when the coolant subcooling is higher than 20 °C. Hence, the fast cooling is possible for the CrAl-coated Zr cladding if the coolant subcooling is higher than 20 °C. CrAl-coated Zr  Figure 15 compares the quenching curves of uncoated and CrAl-coated Zr specimens for initial cladding temperatures of 600 °C and 800 °C. In the case of high subcooling, e.g., Tsub = 50 °C, the cladding temperature decreases rapidly as soon as the coolant reflooding  Figure 15 compares the quenching curves of uncoated and CrAl-coated Zr specimens for initial cladding temperatures of 600 • C and 800 • C. In the case of high subcooling, e.g., T sub = 50 • C, the cladding temperature decreases rapidly as soon as the coolant reflooding begins. This is because the thin film quickly collapses due to the quench front as shown in Figures 8 and 9. The CrAl-coated Zr cladding shows a cooling performance similar to the Zr cladding for high subcooling conditions. For the coolant subcooling of 10 • C and initial temperature of 600 • C seen in Figure 15a, the CrAl-coated cladding shows slightly slower cooling in film boiling regime than the Zr cladding. This seems to be the effect of the higher initial temperature of CrAl-coated Zr cladding. It is noted that the initial cladding temperature depends largely on the experimental conditions such as ambient temperature and heat loss. However, the CrAl-coated Zr cladding shows much faster cooling at T sub = 5 • C and Tw = 800 • C in Figure 15b. The transition to nucleate boiling occurs at 20 s and 40 s for the CrAl-coated Zr and the Zr, respectively. The faster cooling at high temperature and low subcooling for CrAl-coated Zr cladding attributes to smaller contact angle and higher surface roughness. temperature depends largely on the experimental conditions such as ambient temperature and heat loss. However, the CrAl-coated Zr cladding shows much faster cooling at Tsub = 5 °C and Tw = 800 °C in Figure 15b. The transition to nucleate boiling occurs at 20 s and 40 s for the CrAl-coated Zr and the Zr, respectively. The faster cooling at high temperature and low subcooling for CrAl-coated Zr cladding attributes to smaller contact angle and higher surface roughness.  Figure 16 compares the measurements of rewetting temperature with the previous correlations. The RWT measured in this experiment is obtained from the quenching curve of the cladding temperature at the bottom (TC1). The measurement error for RWT is estimated at ±25 °C and ±40 °C for the Zr and the CrAl-coated Zr, respectively. This error is estimated from the variation of cladding temperature during the time interval of data acquisition (±0.2 sec) during transition boiling. The RWT increases as the coolant subcooling and/or initial cladding temperature increases. There is no significant difference in RWT between the Zr cladding and the CrAl-coated Zr cladding.  Figure 16 compares the measurements of rewetting temperature with the previous correlations. The RWT measured in this experiment is obtained from the quenching curve of the cladding temperature at the bottom (TC1). The measurement error for RWT is estimated at ±25 • C and ±40 • C for the Zr and the CrAl-coated Zr, respectively. This error is estimated from the variation of cladding temperature during the time interval of data acquisition (±0.2 s) during transition boiling. The RWT increases as the coolant subcooling and/or initial cladding temperature increases. There is no significant difference in RWT between the Zr cladding and the CrAl-coated Zr cladding. Tw 600 o C 800 o C CrAl-coated Zr Zr Kim & Lee [19] Drucker & Dhir [20] Rewetting temperature, RWT ( o C)

Quenching Performance of CrAl-Coated Zr Cladding
ΔT sub ( o C) The RWT correlations used in this study are those of Kim and Lee [19], and Drucker and Dhir [20]. Kim and Lee [19] proposed the prediction correlation of apparent rewetting temperature ( ) in Equation (1)   The RWT correlations used in this study are those of Kim and Lee [19], and Drucker and Dhir [20]. Kim and Lee [19] proposed the prediction correlation of apparent rewetting temperature (T AR ) in Equation (1) under the bottom flooding conditions of vertical circular tubes: where, T w , T sat , and T f are the wall temperature, saturation temperature, and coolant temperature, respectively. c p.w , k w , and ρ w are the specific heat, thermal conductivity, and density of test tube, respectively. δ, G, and z are the tube wall thickness, liquid mass flux, and axial distance from the inlet, respectively. Drucker and Dhir [20] conducted quenching experiments using a rod bundle with four Zircaloy test specimens. They developed the correlation of quenching temperature (T Q ) considering the effects of flooding velocity and coolant subcooling given in Equation (2): where, U indicates the flooding velocity and ∆T sub is the coolant subcooling. The RWT correlations in Figure 16 show the increase of RWT (T AR or T Q ) as the coolant subcooling increases. The Kim and Lee correlation also shows the increase of RWT as the initial specimen temperature increases. The measured RWTs in this experiment are somewhat higher than the correlation predictions. This is because the TCs appear to be in incomplete contact with the cladding tube caused by thermal expansion and shrinkage during quenching test. However, the dependence of RWT on initial specimen temperature and coolant subcooling is consistent with the correlations. Figure 17 compares the speed of quench front depending on coolant subcooling for uncoated and CrAl-coated Zr claddings. The initial cladding temperatures are 600 • C and 800 • C with the reflood velocity of 0.05 m/s. The speed of quench front (V QF ) is estimated by accounting for traveling time from the bottom TC (TC1) to the top TC (TC4) as follows: where, ∆z is the distance between TC1 and TC4, i.e., 0.08 m. ∆t is the difference in rewetting time between TC1 and TC4. The measurement error in the quench front velocity is 4% and 20% at low subcooling and high subcooling, respectively. The speed of the quench front can also be estimated by analyzing the progress of the quench front in high-speed camera images. The speed of the quench front increases significantly as the coolant subcooling increases. However, it decreases as the initial specimen temperature increases.
Stepanek et al. [21] also found the same dependence of quench front velocity on coolant subcooling and initial temperature from the reflood quenching of stainless steel (SS) rod. The quench front velocity of this experiment agrees well with the previous experimental data of Seshadri and Shirvan [13], and Stepanek et al. [21], as shown in Figure 17. The CrAl-coated Zr appears to show slightly higher quench front velocity than the Zr at Tw = 800 • C. However, the quench front velocity of CrAl-coated Zr is lower than the Zr one at Tw = 600 • C. Hence, it can be judged that the quench front velocity of CrAl-coated Zr is comparable to the Zr result.

= ∆
where, ∆z is the distance between TC1 and TC4, i.e., 0.08 m. ∆t is the difference in rewetting time between TC1 and TC4. The measurement error in the quench front velocity is 4% and 20% at low subcooling and high subcooling, respectively. The speed of the quench front can also be estimated by analyzing the progress of the quench front in highspeed camera images. Tw 600 o C 800 o C CrAl-coated Zr Zr Zr [13] Cr-coated [13] SS [21] Speed of quench front, V QF (m/s) ΔT sub ( o C) The speed of the quench front increases significantly as the coolant subcooling increases. However, it decreases as the initial specimen temperature increases.
Stepanek et al. [21] also found the same dependence of quench front velocity on coolant subcooling and initial temperature from the reflood quenching of stainless steel (SS) rod. The quench front velocity of this experiment agrees well with the previous experimental data of Seshadri and Shirvan [13], and Stepanek et al. [21], as shown in Figure 17. The CrAl-coated Zr appears to show slightly higher quench front velocity than the Zr at Tw = 800 °C. However, the quench front velocity of CrAl-coated Zr is lower than the Zr one at Tw = 600 °C. Hence, it can be judged that the quench front velocity of CrAl-coated Zr is comparable to the Zr result.

Conclusions
A reflood quenching experiment was conducted to evaluate the cooling performance of accident-tolerant cladding, i.e., CrAl-coated Zr tube against the commercial Zr tube. The experimental conditions are an initial tube temperature of 600 • C and 800 • C, the coolant subcooling from 5 • C to 50 • C and the reflood velocity from 0.02 m/s to 0.1 m/s. The flow pattern and quenching curve are compared for CrAl-coated Zr and Zr tube specimens. The contact angle, rewetting temperature and quench front velocity are also compared to evaluate the quenching performance of CrAl-coated Zr cladding. The findings of this quenching experiment can be summarized as below: (1) The CrAl-coated Zr cladding shows the flow pattern and heat transfer mode similar to the uncoated Zr cladding during reflood quenching. The vapor film for the CrAlcoated Zr specimen shows thinner and more turbulent wave than the Zr one at low subcooling. At high subcooling, the CrAl-coated Zr cladding shows asymmetric quench front that moves upwards quickly. (2) The quenching curves for the high subcooling condition do not show any noticeable difference between uncoated Zr and CrAl-coated Zr claddings. The CrAl-coated Zr specimen shows fast quenching with high RWT and high speed of quench front at high initial cladding temperature (Tw = 800 • C) and low subcooling (T sub ≤ 10 • C). (3) The RWT increases as the coolant subcooling and/or initial cladding temperature increases. The reflood velocity has a negligible effect on RWT. There is no significant difference in RWT between the Zr cladding and the CrAl-coated Zr cladding. (4) The quench front velocity of CrAl-coated Zr is slightly higher than the Zr one at low subcooling but becomes lower at high subcooling and low initial wall temperature. Hence, it can be concluded that the quench front velocity of CrAl-coated Zr is comparable to the Zr result. (5) The CrAl-coated Zry-4 cladding is quite promising for the ATF candidate which can result in acceptable quenching performance as well as suppression of hydrogen