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2017 Water Reactor Fuel Performance MeetingSeptember 10 (Sun) ~ 14 (Thu), 2017

Ramada Plaza Jeju • Jeju Island, Korea

VISUAL OBSERVATION OF BALLOONING AND BURST PHENOMENA OF VVER FUEL CLADDINGS

Richárd Nagy 1, Márton Király2, Tamás Szepesi3

1 Hungarian Academy of Sciences Centre For Energy Research (MTA EK), Fuel and Reactor Materials DepartmentBudapest, Hungary, 1525 Budapest 114., P.O. Box 49. [email protected]

2 Hungarian Academy of Sciences Centre For Energy Research (MTA EK), Fuel and Reactor Materials DepartmentBudapest, Hungary, 1525 Budapest 114., P.O. Box 49. [email protected]

3 Hungarian Academy of Sciences Wigner Research Center for Physics (MTA Wigner), Department of Plasma PhysicsBudapest, Hungary, [email protected]

ABSTRACT: Keeping the integrity of fuel elements is crucial in the life of a nuclear power plant. One of the mostresearched phenomenon regarding the LOCA is the ballooning and burst of fuel claddings. Our goal was to visually study theballooning and the crack propagation during the high pressure burst of the different (traditional and sponge-based material)

E110 fuel claddings using regular and high-speed cameras.

E110 samples were inspected under high internal argon pressures at high temperatures. For the purpose of visualinspection, a special telescope was designed and mounted onto a tube furnace in order to record the high temperaturephenomena during ballooning and burst events. We performed 10 burst measurements at around 800 °C with various

pressure increase rates. The appearance of non-uniform deformations were visible over 80% of the burst pressure. Theregular HD (high spatial resolution digital) camera observations revealed the kinetics of diameter change during ballooning.

The crack propagation during the burst was recorded using the high-speed camera.

We found that every sample has bent by approximately 5° relative to its axis. Bending happened during ballooning and wasnot caused by the rocket effect of the high pressure gas escaping during the burst. Every sample opened up on the convex side

of the bend. Prior to the burst, a hot spot appeared with approximately 100 °C higher temperature at the same locationwhere the crack propagation would initiate and it was also visible at the crack tip. The crack propagation took 0.2 ms

according to our observations. Significant thinning is observed in the wall thickness of the tubes. Axial grooves have formedon the surface of the sponge-based E110 samples under tension.

KEYWORDS: E110, ballooning, burst, cladding, visual observation

I. INTRODUCTION

Previously, several experiments have been conducted to measure burst pressures at temperatures between 600 – 1100 °C.Some of them were single rod tests, others were bundle (integral) tests: Hungarian measurements of Russian type claddings[1], burst and ballooning measurements [2,3] CODEX experiments [4], German QUENCH tests [5] and also Russian singlerod and bundle tests [6] showed a general view of the behavior of E110. These experiments revealed the burst pressuresunder certain circumstances, the final shape of the ballooned rod and the bending of the rods.

We aimed to reproduce our previous data, and to study the ballooning and burst phenomena in detail and to visually studythe ballooning and the crack propagation during the high pressure burst of the different (traditional and sponge-basedmaterial) E110 fuel claddings using regular and high-speed cameras. Our results give us new experimental data for fuelperformance code calculations in FRAPTRAN, FUROM, TRANSURANUS, etc. The experimental setup consisted of four

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parts: the optical system, the pressure system, the control unit and the furnace control. We controlled the increment rate of theinternal pressure of the sample.

II. OPTICAL SYSTEM

We have constructed two identical optical-infrared telescopes, operable at 1000 °C, to be able to visually observe thecladding samples during the tests. To avoid fatigue or stress caused by different thermal expansion, a telescopic dual tubesystem was used to adjust the effective focal length. Some other issues had to be considered during construction. At the pointof burst a high pressure gas front hits the front lens, therefore thick lenses were designed, and the first lens also had to resistinternal stresses caused by the 10 °C/mm temperature gradient.

The telescopes consisted of a flat-corrected condenser objective lens (with 3 lenses, 27 mm in diameter) and an ocular lens(40 mm in diameter) made of high purity quartz (Heraeus). The lenses were held in place by quartz spacer rings at a certainrelative distance from each other. We were able to use this telescope with several independent cameras and optical systems,as the telescope was separately focusable using the ocular, so the lenses of all cameras could adjust to it.

Before the experiment, we set up the telescopes to see the whole length of the sample. The telescope was approximately110 mm away from the sample and this determined a maximum of 50° angle of view and we could achieve a 0.1 mm/pixelscale was realized in the “full HD” (64 bit colored, 24 megapixels spatial resolution, non-binned CMOS sensor) video. Thecameras used were Nikon 5200 DSLRs with 18-200 mm zoom lenses. The proper length of the samples was tested to fit tothe observable area. We found that 85 mm cladding sample length (10 mm of which is inside the pipe connection) isappropriate to keep a high spatial resolution on the video, although longer samples could also be used with smaller spatialresolution. A welded sample can be seen on Fig. 1. From left to right, the lid of the furnace, the pressurizing pipe, somethermocouple and the optical reference grid is visible. The right tip of the pipe is the plugged sample. A heat resistant steelsquare grid was installed into the furnace to calibrate the spatial resolution of the videos and to reveal the optical errors of theoptical system. With it we were able to measure the diameter change of the tubes after the experiment.

Fig. 1: An end-welded 85 mm long sample with the connection to the pressurizing pipes, the interchangeable furnace toppiece, the thermocouples and the spatial reference grid.

Two stubs were mounted to the inside of the furnace. These stubs held the two telescopes opposite to each other. Becauseof their metal tube construction, both slots represented a colder spot in the furnace, allowing us to distinguish the profile ofthe sample from the background.

III. FURNACE AND PRESSURE SYSTEM SETUP

A three-zone steel tube furnace was used for holding the sample temperature at 800 °C. The two outer zones wereregulated by an embedded PID controller wired together with the central PID controller which regulated the central zone.Both controllers were guided by K-type thermocouples located inside the furnace. Heating it up to 800 °C took about 3 hours,the temperature stability was ±3 °C, with a 0.6 h periodical cycling caused by the large thermal inertia of the system.

During the tests high pressure argon gas was fed into the cladding samples by a regulated needle valve, in order tomaintain a certain rate of internal pressure increase. Gas pressure increment had to compensate for the volume growth, the

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thermal expansion, and also for some possible leakage of the pipes and sealing rings. The pressure was measured by twoSuco 0720 type, linear mechanical–inductive transducers. This type of transducer could measure pressure between 0.5 MPaand 25 MPa, and the output was direct current between 4 mA and 20 mA. It was connected to a shunt resistor and directvoltage was measured. To calibrate the pressure sensors we used a valve to regulate the argon flow into a buffer tank. A MAI-250 type Bourdon-tube manometer was used to measure the pressure of this buffer.

Simple moving average and difference quotient was calculated from the measured pressure to operate the needle valve. Inorder to avoid the uneven pressurizing of the samples, a small buffer tank was installed parallel to the sample connecting viaa 0.7 mm inner diameter capillary. The stepper motor regulating the needle valve was driven in single step mode (200steps/rotation), therefore the needle valve movement was smooth enough to let in only small amounts of argon gas. For dataacquisition and control we used a Measurement Computing USB-2408 multifunction measurement device. The coils of thestepper motor were individually driven by a power amplifier circuit connected to the digital output of the DAQ unit. Thepressure increase rate control for the stepper motor and the data logging program was developed in LabView 2014.

VI. EXPERIMENTAL

During the experiment, the pressure increase rate was tightly controlled. The pressure increase rate was tested between 2kPa/s and 0.5 MPa/s with 99.9% accuracy in linearity, with 200 ms period sampling. This 200 ms was the time constant ofthe control program, so all measurements, data loggings, calculations and control actions happened with this frequency.During the current set of measurements 0.05 MPa/s was used to in nearly all tests. The control program counted the numberof the steps given to the stepper motor, so at the end of the test the needle valve could automatically be closed by reversingthe motor direction. A trigger pulse was also created by the software to stop the high-speed camera recording after burst hadoccurred.

While heating up the furnace, the samples were tested with 5 MPa argon pressure at room temperature to look for gas leaksat the connections and the welds. The furnace and the telescopes were heated up with a mock-up sample inside the furnace inorder to avoid unwanted oxide layer formation on the external surface of the samples. This was necessary as the furnace wasnot airtight. The furnace was therefore constantly washed with argon to purge out air and to protect it from oxidizing.

After reaching isothermal state we set up the optics and the cameras and manually focused them on the mock-up. Finally,we swapped the mock-up to the real sample and closed the furnace again. In most cases the tests started with a small initialpressure to monitor the gas loss before the test. After an intensive argon flush and a new isothermal state, controlled amountof high pressure argon was charged into the sample. We logged the temperature at the highest and the lowest tip of thesample. The sample was positioned at the center of the furnace, where we logged central temperature, which also controlledthe furnace. We logged the pressure of the argon gas within the sample, as it is seen in Fig. 2, and the pressurization rate wasconfirmed to be very linear. After the burst we quickly swapped the sample and the mock-up sample again to avoid anyfurther oxidation.

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Fig. 2. The pressure history of a (11_17/06) measurement.

V. RESULTS OF THE PRELIMINARY TESTS

In this current series of experiments we performed 4 preliminary measurements and 6 high-speed camera recordings. Allmeasurements were done at around 800 °C, though some inaccuracy or intentional deviation from this was noticeable in theexperiment settings. The burst pressure was determined for each sample, as shown in Table 1. The burst pressure on the giventemperature and pressurization rate was known from previous experiments to be around 8.4 MPa [6]. Our measured datawere - for the most part – slightly above this data.

Table 1. Measured data of the preliminary experiments. (E110 sp. means the new E110 claddings manufactured on spongebase material.)

Samplenumber

Samplelength (mm)

Pressurizationrate (MPa/s)

Temperature(ºC)

Burst pressure(MPa)

Samplematerial

1. 150 0.048 795 7.69 E110 sp.

2. 65 0.043 755 9.11 E110 sp.

3. 70 0.069 805 9.41 E110 sp.

4. 85 0.039 820 8.99 E110 sp.

We measured the diameter based on the images and determined the diameter increment in time. The diametrical change of thesamples relative to the axial position and the internal pressure can be seen in Fig. 3. The evolution of the sample diameter at agiven axial position (60 mm) versus the internal pressure and the measurement time (with constant pressurization rate) is seen

in Fig. 4. We found that 10% diameter increase happened at about 80% of the final burst pressure. During the tests every

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sample has bent by approximately 5° relative to its axis before the burst.

Fig. 3. The diametrical change of the sample relative to the axial position and the internal pressure.

Fig. 4. The evolution of the sample diameter at a given axial position (60 mm) versus the internal pressure and themeasurement time (with constant pressurization rate).

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Every burst was preceded by a local heat-up. This hot spot appeared not earlier than 1-1.5 s prior to the cracking and burst(Fig. 5). The temperature rise of these spots were about 100 °C higher than their surroundings based on comparing the colorof the hot spot and the color of the rest of the sample. This local heat-up may be caused by the sudden mechanical work ofthe high pressure argon gas ballooning and rupturing the tube.

The expanding argon cooled the samples down to approximately 550 °C, in 200 ms. We observed that the oxide layerchipped off from the surface of Sample 1. It is clearly observable that breaking structure of the oxide oriented parallel to theaxis of the cladding tube. Also, axial grooves have formed on the surface of the sponge-based E110 samples under tensionwhich may have initiated this oxide spalling (Fig. 6).

We could measure that the adiabatic gas expansion of argon cooled the cladding sample down by about 300 °C that frozethe aperture of the gap in position.

Fig. 5. The bending of sample 3 and the hot spot appearing before the burst. The figure on the left shows the sample at 5 MPainternal pressure, the right picture was taken 1/30 s before the burst (snapshots taken from the 30 frames/s HD video).

Fig. 6. a.) Black spots and stripes are seen as longitudinally oriented damages in the oxide layer on the surface of theSample 1, and b.) an optical microscopic image of grooves on Sample 4 at around the tip of the crack.

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VI. RESULT OF THE HIGH-SPEED CAMERA MEASUREMENTS

After the first set of preliminary experiments we performed a second set, and the measured data are shown in Table2. Using a Photron SA-5 high-speed camera (courtesy of MTA Wigner) we recorded bursts with 2000, 25 000 and 100 000frames per second (fps). These revealed the crack propagation and the opening of the gap. The 2000 fps video images areseen in Fig. 7, and similarly, the series of 25 000 fps video images are seen in figure Fig. 8. To compare these series, in thefirst one we couldn’t see any details about the burst and the crack propagation, because the phenomenon is much quicker than0.5 ms. As it is seen in the Fig. 8., the opening lasts no longer than 0.2 ms.

Table 2. Measured data of the high-speed camera experiments.

Samplenumber

Samplelength (mm)

Pressurizationrate (MPa/s)

Temperature(ºC)

Burst pressure(MPa)

Samplematerial

5. 85 0.048 770 9.15 E110 sp.

6. 85 0.038 796 7.69 E110 sp.

7. 85 0.049 850 8.06 E110

8. 85 0.048 760 9.96 E110 sp.

9. 85 0.046 847 7.42 E110

10. 85 0.037 758 10.26 E110 sp.

t0 = 0ms t1 = 0,5 ms t2 = 1 ms t3 = 1,5 ms t4 = 2 ms

Fig. 7. The result of our first recording that shows the temporal bend after the burst (t=0).

In Fig. 7. the shape of the fuel cladding tube is observable during and right after the burst. At t2 = 1 ms, a slight bend to theleft is seen, that disappear by t4 = 2 ms. This little bend was caused by the blast of the argon gas. However, this rocket effectdid not bend the cladding tube permanently, and that means that much longer time is needed to bend the rod permanently, asthe elastic deformation needs time to move the dislocations. The permanent deformation requires longer time than amillisecond.

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t0 = 0.00 ms t0 + 0,04 ms t0 + 0,08 ms t0 + 0,12 ms t0 + 0,16 ms

Fig. 8. The sequence of the crack opening.

In the final test, the high-speed camera was set to 100 000 frames per second. This speed is so rapid that we couldn’t stopthe recording within human reaction time to avoid memory allocation overflow, therefore a trigger signal generator wasadded to our LabView virtual instrumentation. The trigger stopped recording within 10 ms, which represented about 1000extra images after the burst. These images revealed some chipped-off pieces of the cladding tube were flying. We couldn’t getnew information about the crack opening as the burst happened perpendicular to the camera and only the profile of theopening could be seen.

VII. Summary

The main goal of this research was to visually observe the high temperature burst of traditional and sponge based E110fuel claddings. We built a high temperature optical telescope to record the cladding rod at around 800 °C. We also constructeda regulated high pressure argon pressurizing system that was able to create a linear increment of internal pressure of argonwithin the zirconium sample with 99.9% accuracy.

The burst pressures were in good agreement with earlier experiments performed with E110 claddings. However, thepresent tests provided detailed information on the ballooning process and burst of the cladding tube. The tests did not showany significant difference between the traditional and sponge-based alloys.

In this series of measurements we found that the fuel rods increased their diameter in two ways. We observed a globalincrease in diameter at the beginning, then we saw a local bulge formation prior to the burst. We used full HD cameras torecord the increase in cladding diameter. We measured the diameter based on the video images and determined the diameterchange in time. We found that 10% of diameter increase happened at about 80% of the final burst pressure.

We also found a hot spot on the surface of the cladding samples at the burst location 1-1.5 second prior to the crackopening with approximately 100 °C higher temperature at the same location where the crack propagation would initiate. Thislocal heat-up may be caused by the sudden mechanical work of the high pressure argon gas ballooning and rupturing the tube.

We found that the bending of the cladding is not caused by the rocket effect of the expanding argon streaming out of thecrack after the burst. Bending is simultaneously happening with the bulge formation. The rocket effect caused a bend but itwas reversible. We could measure that the adiabatic gas expansion of argon could cool the cladding sample down by about300 °C that froze the aperture of the gap in position.

We received a high-speed camera on behalf of the MTA Wigner Research Centre for Physics. The maximal resolution wecould reach was 100 000 fps. This recording revealed that the crack propagation lasted about 0.2 ms. We observed severalpieces of cladding that have chipped off the sample during the burst.

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ACKNOWLEDGMENTS

This research was supported by Paks Nuclear Power Plant, Hungary. We are thankful to our colleagues at MTA EK forproviding useful information and technical expertise on order to achieve the set research goals.

We’d also like to thank the Hungarian Academy of Sciences Wigner Research Center for Physics for the high-speedcamera and Tamás Szepesi for his assistance with the high-speed camera operations.

The present work was supported by the National Research, Development and Innovation Fund of Hungary (contractnumber: NVKP_16-1-2016-0014).

REFERENCES

1. HÓZER, ZOLTÁN, PEREZ-FERÓ, ERZSÉBET, NOVOTNY, TAMÁS, NAGY, IMRE, HORVÁTH, MÁRTA,PINTÉR-CSORDÁS, ANNA, VIMI, ANDRÁS, KUNSTÁR, MIHÁLY, AND KEMÉNY, TAMÁS, ExperimentalComparison of the Behavior of E110 and E110G Claddings at High Temperature, Zirconium in the Nuclear Industry:17th International Symposium, STP 1543, Robert Comstock and Pierre Barberis, Eds., pp. 932–951,doi:10.1520/STP154320120165, ASTM International, West Conshohocken, PA (2014).

2. HÓZER, Z., GYŐRI, C., HORVÁTH, M., NAGY, I., MARÓTI, L., MATUS, L., WINDBERG, P., and FRECSKA, J.,Ballooning Experiments With VVER Cladding, Nucl. Technol., Vol. 152, (2005), pp. 273–285.

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4. Z. HÓZER: Summary of Core Degradation Experiments CODEX, Proceedings of EUROSAFE, Berlin November 2002,seminar 2 paper 2.

5. STEINBRÜCK, M., BIRCHLEY, J., BOLDYREV, A. V., GORYACHEV, A. V., GROSSE, M., HASTE, T. J.,HÓZER,Z., KISSELEV, A. E., NALIVAEV, V. I., SEMISHKIN, V. P., SEPOLD, L., STUCKERT, J., VÉR, N.,ANDVESHCHUNOV, M. S., High-Temperature Oxidation and Quench Behaviour of Zircaloy-4 and E110 Cladding Alloys,Prog. Nucl. Energy, Vol. 52, 2010, pp. 19–36.

6. L. YEGOROVA, Data Base on the Behavior of High Burnup Fuel Rods with Zr-1%Nb Cladding and UO2 Fuel (VVERType) under Reactivity Accident Conditions: Test and Calculation Results (NUREG/IA-0156, Volume 3, IPSN 99/08-3,NSI RRC KI-2179).

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