Chemical reactivity and mobilization of beryllium exposed to steam

Fusion Engineering and Design 37 (1997) 543 – 552

Chemical reactivity and mobilization of beryllium exposed to steam K.A. McCarthy *, G.R. Smolik, R.A. Anderl, R.J. Pawelko, M.A. Oates, R.S. Wallace Idaho National Engineering and En6ironmental Laboratory, Lockheed Martin Idaho Technologies Inc., P.O. Box 1625, Idaho Falls, ID 83415 -3815, USA

Abstract Beryllium is used in many fusion reactor designs as either an armor for plasma facing surfaces, or as a neutron multiplier in the blanket. Beryllium used in a water-cooled design poses important safety issues related to the chemical reactivity of beryllium in steam and its toxicity. The Fusion Safety Program at the Idaho National Engineering and Environmental Laboratory has been investigating experimentally the chemical reactivity and mobilization of various forms of beryllium for the past 6 years. In this paper we present a summary of this work, including results from fully dense (irradiated and non-irradiated), plasma-sprayed, and 88% dense beryllium. Assembling this data helps us to assess where further testing is needed. Our data help guide designs such that accident temperatures stay below values necessary to ensure beryllium release limits and hydrogen generation limits are met. © 1997 Elsevier Science S.A. Keywords: Beryllium; Fusion Safety Program; Plasma; Hydrogen

1. Introduction Beryllium is present in the current International Thermonuclear Experimental Reactor (ITER) design as a plasma-facing armor (5 – 10 mm thick). In addition, the breeder blanket in ITER, and conceptual designs such as ARIES [1] incorporate beryllium as a neutron multiplier. The beryllium in fusion reactor designs can be of various forms, for example, initially fully dense (the properties of the beryllium can change under irradiation), initially porous (primarily blanket beryllium), and plasma-sprayed (with a variety of porosities). The * Corresponding author.

Fusion Safety Program (FSP) at the Idaho National Engineering and Enviromental Laboratory (INEEL) has studied experimentally each of these types of beryllium. When a reactor is watercooled, the possibility of water leaks into the vacuum vessel exists. Loss of Coolant Accident (LOCAs) are analyzed with codes such as MELCOR [2], which require information on chemical reactivity to adequately assess the consequences of potential accidents. The primary focus of our work has been on the chemical reactivity in steam and subsequent hydrogen production, however we have also completed scoping tests on mobilization in steam. The primary hazard in mobilized irradiated beryllium is not due to the induced radioac-

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tivity, but rather to the toxicity of beryllium if particulate is inhaled. Beryllium reacts exothermically with steam to produce hydrogen via the reaction: Be+ H2O “BeO + H2

(1)

If enough hydrogen is produced, given the right amount of air present, explosive conditions could result. The first tests performed at the INEEL focused on measuring reaction rates and mobilization of fully dense and 88% dense beryllium [3]. Our second test measured the reaction rate of plasmasprayed beryllium [4]. Our third test series focused on measuring the reaction rate of 88% dense beryllium, with a goal of identifying the temperature at which self-sustaining rections occur given the geomentry of our experiment [5]. Our fourth test series was primarily focused on measuring the reaction rate of irradiated beryllium, and included measurements of non-irradiated ‘control’ samples [6]. An important goal of this test series was to measure reaction rates in the 400 – 600°C temperature range, something we were not able to do in previous dense beryllium tests due to detection limit limitations. This paper summarizes the work done to date, and shows the progression of our experimental techniques. This information is important in determining the extent of our knowledge of the mobilization and chemical reactivity of beryllium exposed to steam, and planning future work.

2. Description of experiments

2.1. Sample preparation The fully dense beryllium samples used in Series 1 were disc-shaped, prepared from round bar, approximately 25.4 mm in diameter and 2.54 mm thick. Chemical etching was used to remove surface deformation layers from machining. The 88% dense samples used in Series 1 and 3 were prepared from billets produced from Brush-Wellman structural powder, SP-200-F, with nominal particle size of 10 to 12 mm. The 44 mm diameter billets were prepared by isostatically cold pressing

and then sintered for 4 h at 1100°C under vacuum to provide an approximately 88% dense material. The porous samples had a diameter of 25.4 mm and various thicknesses (2.54, 7.4 and 12.7 mm) to check for geometry effects. The plasma-sprayed samples used in Series 2 were prepared by Battelle Columbus. Our measurements of the density by immersion methods indicated densities were 85–93% of theoretical density. These samples were prepared prior to 1992 and do not represent the current capabilities of producing higher quality, more dense deposits. The samples used in Series 4 were produced from the same grade of powder as the fully dense and 88% dense samples. These samples were cylindrical in shape with a 0.76 cm diameter and three different lengths, 0.635, 2.032 and 3.051 cm. No chemical etching was used to remove surface deformation layers from machining. Irradiation of a portion of these samples took place in the Experimental Breeder Reactor II (EBR-II), a fission reactor with a fast neutron environment [7]. Nominal fast neutron fluences ( \ 0.11 MeV neutrons) were estimated to range from 5.2× 1022 to 6.7× 1022 n cm − 2. The nominal irradiation temperature was 400°C.

2.2. Experimental method The first three test series [3–5] were done in our Volatilization of Activation Product Oxides Reactor (VAPOR) facility. In these experiments, gas produced during exposure of the heated sample to steam was collected and measured volumetrically (in the last year, we added a gas chromatograph to VAPOR, enabling us to measure hydrogen directly, however these experiments were done before this instrument was available). A new facility was developed for the irradiated beryllium tests. The Steam-Reactivity Measurement System (SRMS) has an on-line mass spectrometer to measure gas production. Schematics of the VAPOR and SRMS Facilities are shown in Figs. 1 and 2. A detailed description of these two facilities can be found in Ref. [8]. In the first and second test series, the sample was heated inductively. This type of heating produced an environment prototypic to a fusion acci-

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dent where the sample, representing the plasmafacing material, was at a higher temperature than the steam flowing by the sample. Temperature control with this heating method is somewhat difficult, particularly when exothermic reactions take place, increasing the temperature of the sample. A thermocouple embedded in the bulk of the sample was used to monitor the temperature of the sample, and power was increased or decreased to maintain the desired temperature. Additionally, environments with different sample and gas temperatures are more difficult to model than the tube furnace environments where temperatures are more uniform. Thus our third set of beryllium experiments consisted of tests with inductive heating and tests in tube furnaces with radiative heating to compare results from the two heating methods. In both cases, temperature was monitored through a thermocouple embedded in the sample. The tests with resistive tube furnace heating provide more uniform temperatures, and generally, more conservative results since mobilization and hydrogen production tend to be slightly higher in these tests. This method was used in Series 4.

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In both facilities, the beryllium sample was heated to the desired test temperature under an argon gas purge. Then, steam at atmospheric pressure (0.85 atm. at the Idaho National Engineering and Environmental Laboratory) flowed through the system. Steam input was 0.167 g s − 1 (1100 cm min − 1) for Series 1–3 and 0.033 g s − 1 (220 cm min − 1) for Series 4. Measurement of hydrogen production by more than one method is important to increase confidence in the data. In addition to either volumetric hydrogen measurement or measurement by a mass spectrometer, we often measure weight change of the sample and calculate hydrogen production based on this measurement and the reaction in Eq. (1). Each method has advantages and disadvantages. The ability to detect a gas leak is very important because air leaks can develop in a system. Volumetric measurement methods cannot detect air leaks, however the mass spectrometer can. A detailed description of methods can be found in Ref. [9]. During the tests, mobilized species deposited on the various components. Glassware components were acid cleaned following each test. The solutions were processed and analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) to determine mobilization rates. A summary of heating methods, data obtained, and gas measurement methods for the four test series is shown in Table 1. 3. Beryllium mobilization

Fig. 1. Schematic of Volatilization of Activation Product Oxides Reactor (VAPOR).

We have measured mobilization from fully dense, and 88% dense beryllium [3,5]. Larger pieces of beryllium oxide that resulted from spalling during testing, cool-down, or handling were sometimes visually apparent in the test chambers. Such pieces of oxide were tapped out and not included in the chemical analyses. The mobilization measurements in the third test series were made only during tests where samples were heated radiatively. It was difficult to remove samples from the horizontal test chambers of the tube furnace without dislodging additional oxide. Mobilization products for these tests were collected only from chambers located downstream.

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Fig. 2. Schematic of Steam-Reactivity Measurement System (SRMS).

The samples of porous beryllium swelled as the reaction took place due to the volumetric expansion from the formation of BeO around the sintered beryllium powder particles. This reduced thermocouple contact with specimens. This sometimes resulted in temperature measurments below actual temperatures which resulted in experiment operators increasing power and causing additional over-heating. Finally, the porous beryllium samples would sometimes undergo a self-staining reaction and increase to temperatures of around 1230°C. This behaviour occured for targeted temperatures above 675°C. Thus there are no mobilization data for the porous beryllium between 675 and 1230°C. The mobilization data from Series 1 and 3 plotted in Fig. 3 show that there is significant scatter. Although there were differences in the tests methods, the scatter is likely from mobilization occurring by spalled oxide particles. Oxide spalling is a somewhat irregular process, and for beryllium, is further complicated by the quite different oxide characteristics we observed at vari-

ous temperatures. A shiny tenacious dark gray oxide developed on dense beryllium samples exposed at and below 600°C. Exposures near 700°C produced a loose, granular oxide quite prone to being dislodged. At temperatures near 830°C a crusty outer layer developed over a porous inner oxidized zone. Samples tested near 1025°C developed a very hard white, sinitered-like appearing oxide. The character of the oxide formed near 700°C suggests, or supports, the observance of higher mobilization at this temperature compared to rates measured near 600 and 830°C. The mobilization we measured at higher temperatures likely includes some volatilization from a hydroxide species produced by the reaction: BeO(s)+ H2O(g)“ Be(OH)2(g)

(2)

This process was presented as the mechanism for volatilization by [10,11]. Stuart and Price [10] measured a release rate of 0.75 mg s − 1 from a 3-cm deep BeO pebble bed at 1205°C. This compares fairly well with the rate of 1.68 mg s − 1 from our 1230°C test. Although the effective surface

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Table 1 A summary of heating methods, data obtained, and hydrogen measurement methods Test series and Be Form

Mob. Data

H2 Data

Inductive heating

Radiative heating

Weight change

Vol. H2 measurement

Mass spec. H2 measurement

Series 1-100% dense Series 1-88% dense Series 2-plasma-sprayed Series 3-88% dense Series 4-initially fully dense, irradiated Series 4-fully dense control samples

Yes Yes No Yes No

Yes Yes Yes Yes Yes

Yes Yes Yes Yes No

No No No Yes Yes

Yes Yes No Yes Yes

Yes Yes Yes Yes No

No No No No Yes

No

Yes

No

Yes

Yes

No

Yes

area of the bed could be quite large, our sample also turned into a voluminous oxide with substantially increased area relative to initial surface area. We used a steam flow rate 30 times higher than that used by Stuart and Price. The agreement between the two studies may, therefore, just be coincidental considering differences in effective surface areas and flow rates. Conway et al. [11], measured a 0.054 mg s − 1 release rate of beryllium from a sintered BeO tube having an exposed area of 14.9 cm2 at 1230°C. This is two orders of magnitude lower than our measurement at 1230°C. The lower rate could be due to flow rate, which Conway et al. [11] did not report, or to the environment which was air with 0.8 wt.% water vapour. In summary, it seems likely that the mobilization process during our high temperature

tests could involve both volatilization and removal of the relatively light BeO particles (density of 3.01 g cm − 3) due to our flow rate of 0.18 m s − 1 (1100 cm min − 1). We fit a curve to the data assuming the form [12] G= At B exp(− C/RT)

(3)

where G is the mass flux (g/m2 − h) averaged over the test duration, A is a constant, t is time (h), B is the time exponent, C is the activation energy (J mol − 1), R is the gas constant (8.314 J mol − 1-K), and T is temperature (K). A time exponent (B) near 0 would indicate that the mass flux did not change with time. This would suggest some constant rate process such as an unprotective oxide layer or gas phase diffusion. A time exponent near − 1 would indicate that the mass flux decreased rapidly with increased time and the mass mobilization could be dominated by initial short-term exposure of relatively unoxidized surfaces. The curve fit to the data results in G= 12 487t − 0.23 exp(− 99 289/RT) 2

Fig. 3. Mass of beryllium mobilized in steam, per m2 exposed area, per h at temperature. Curve fit assuming 1 h exposure time shown.

(4)

The fit, as shown by an r value of 0.67, indicates significant scatter in the data. The relatively small value of the time exponent implies that the mass flux was not a strong function of exposure time. These data indicate a need for further mobilization tests. The FSP plans to measure mobilization of beryllium during 1997 and 1998. We expect these tests to help clarify the mobilization mechanisms and the scatter within our data.

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on the difference between the chemical reactivity of irradiated beryllium and non-irradiated beryllium. Fig. 4 shows data from the four test series and indicates the hydrogen measurement method used for each data point shown. We discuss the results of the various tests in the following paragraphs.

4.1. Porous beryllium

Fig. 4. Reaction rate data from dense beryllium tests. The data are labeled INELww-xx-y,z; ww refers to the test year, xx refers to the hydrogen measurement method WG, hydrogen based on weight gain; GMS, hydrogen measured by mass spectrometer; GV, hydrogen measured volumetrically), y, z refers to the shape and type of sample (D,D, fully dense disc-shaped beryllium; C,D, fully dense cylindrically-shaped beryllium; I, irradiated cylindrically-shaped beryllium; P, porous beryllium; PS, plasma-sprayed beryllium).

4. Chemical reactivity Because of the importance of the chemical reactivity issue for beryllium, we have tested beryllium chemical reactivity in all four test series. Our first test series showed 88% dense beryllium to be extremely reactive compared to dense beryllium, up to two orders of magnitude at temperatures below 600°C [3]. Inductively heated samples developed self-sustaining reactions due to the exothermic heat of reaction at temperatures between 600 and 700°C. These tests did not completely explain the mechanisms causing this behaviour. The onset of thermal instability appeared to have a temperature dependent incubation period and some dependence upon specimen geometry, however our third test series indicated this was not the case [5]. Our second test series showed plasma-spray beryllium to be even more reactive than the porous beryllium, up to three to five times higher [4]. Our most recent test series [6] helped us to further understand earlier tests, in addition to providing information

The porous beryllium was tested for the blanket program (lower-density beryllium was used in a blanket design to provide increased thermal resistivity). The third test series showed that the onset of thermal instability for the porous beryllium was between 650 and 675°C. This temperature was common for both inductively and radiatively heated samples. However it is important to note that this temperature is geometry-dependent, since the heat transfer characteristics will determine how efficiently heat is conducted from the sample. The reaction rates of porous beryllium depended both on the surface area and sample mass. As the reaction proceeded, the samples developed greater porosity. This enhanced the access of steam within the specimen resulted in greater relative proportion of the reaction occurring within the specimen. Reaction rates thus increased relative to initial rates based on external surface area rates based on external surface area. The slope for the reaction rates of porous beryllium on the reciprocal temperature plot was not exactly parallel to that for dense beryllium as suggested after the first test series [3]. The lower slope for the porous beryllium likely means a different mechanism controlling the reaction. The hydrogen produced may have started limiting the ingress of the steam. This may be one reason we did not see a geometry effect in the third test series.The following equation is a curve fit to the porous beryllium data, H2(l m − 2 · s)= 20 022 exp(−10 873/T)

(5)

2

where T is in °K. The r for this fit was 0.92. We plan to test porous beryllium again in 1998.

4.2. Plasma-sprayed beryllium Because plasma-spraying is one of the probable first wall repair methods, we tested plama-sprayed

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beryllium samples. As shown in Fig. 4, the plasma-sprayed samples were even more reactive than the porous samples. We believe this is due to the rougher surfaces and finer particles formed during spraying compared to those resulting from pressing and sintering. Because the samples were sprayed in air, some air was probably drawn into the beryllium stream during spraying. If oxygen pickup and inclusions were present in the samples, the density was even lower than that measured by immersion methods. The sprayed deposits could actually contain more closed porosity than indicated. After this test series, Los Alamos began spraying beryllium, and has been able to produce samples with densities in the range of 92 –96% (R. Castro, personal communication). Because of the higher density, reaction rates should be lower than the Battelle samples. We plan to test some of these samples in 1997. Because the test temperature range was so limited, we did not attempt a curve fit to the data. There is a significant scatter in the data, however it is consistenely higher than the porous beryllium data.

4.3. Fully dense beryllium 4.3.1. Non-irradiated samples Tests with dense beryllium are our most extensive to date. As shown in Fig. 4, our Series 1 hydrogen based on weight gain are consistently lower than the volumetric measurement at 600 and 700°C (there were no series 1 tests below 600°C). The detection limit of the volumetric measurement method in VAPOR for the dense sample surface area and a 3-h test is  0.7 – 1.4 × 10 − 3 1 m − 2-s. Thus the volumetric measurement at 600°C is set to the detection limit, and is therefore too high and is not included in the curve fit. Volumetric measurement of gas does not provide information on the gas composition, thus argon from the pre-steam purge may have contributed significantly to the volumetric measurement at low temperatures. However, the hydrogen calculation based on weight gain could be low due to possible lost spalled oxide. Thus we include hydrogen based on volumetric measurements and weight gain calculations in the curve fits.

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There are two dintinct, fairly linear, regions on the plot in Fig. 4. The transition between the regions is located near 900°C. There is likely some change in mechanism around this temperature. At 600°C and below, the data indicate that the reactivity behaviour was quite different for the cylindrical samples relative to the dic-shaped samples. In most cases at 600°C, the total quantity of hydrogen generated by the cylindrical samples was greater than that generated by the disc samples, for comparable steam exposure times. This could indicate that at the lower temperatures, the beryllium cylinders were more reactive or the developing oxide layer was less protective because of the damaged surface on the cylinders (which were not chemically etched after machining) [13]. An important accomplishment in Series 4 was extending the dense beryllium chemical reactivity database down to 450°C. As accident analyses become more sophisticated and designers work towards designs that decrease accident temperatures, the 400–600°C temperature range becomes very important.

4.3.2. Irradiated beryllium Because of the high reaction rate shown by the porous beryllium, there was concern that irradiated beryllium could have significantly higher reaction rates than non-irradiated beryllium. We performed three types of experiments: (1) annealing studies to measure the swelling and gas release behaviour for irradiated beryllium; (2) chemical reactivity experiments for unirradiated beryllium (control samples) when exposed to steam, and (3) chemical reactivity experiments for irradiated beryllium samples exposed to steam under the same conditions as the control samples [6]. Measurements of the mass, density, and physical dimensions (length and diameter) were made both before and after the anneal tests. The density of the unirradiated beryllium was 1.854 g cm − 3 (fully dense) and the density of the irradiated beryllium was 1.825 g cm − 3, prior to anneal tests, indicating 1.6% swelling during the EBR-II irradiation. Key results of the immersion density measurements are summarized in Fig. 5 that shows volumetric swelling for each specimen as a function of anneal temperature. Annealing at tempera-

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tures from 450 to 600°C resulted in little change in the swelling relative to that observed for the as-received irradiated material. However, annealing to temperatures above 600°C resulted in swelling, relative to unirradiated material, that ranged from 14% at 700°C to 56% at 1200°C. Our gas analysis measurements indicated that tritum and helium gas release behaviour during annealing tests was a complex function of both temperature and time at temperature. No measurable tritium and helium were observed from the specimens annealed to temperatures at or below 600°C. For tests above 800°C, bursts of helium and tritium occured concurrently indicating that the tritium and helium reside predominantly in common bubbles in the irradiated material and are subsequently released concurrently during the thermal anneal. Fig. 6 shows the gas release rates during the 1000°C anneal. The irradiated beryllium samples were tested at furnace temperatures of 450, 500, 600, and 700°C. Based on the measured hydrogen generation rates, the chemical reactivity behaviour was similar in magnitude for both the irradiated and control samples tested in steam at and below 600°C. However, the irradiated sample tested at a furnace

Fig. 5. Swelling of irradiated beryllium as a function of annealing temperature. Incubation times are listed for each point.

Fig. 6. Tritium release rate and relative helium signal during 1000°C anneal of irradiated beryllium sample.

temperature of 700°C reacted much differently than the control sample. Hydrogen generation rates and tritium and helium mobilization measurments are shown in Fig. 7. The tritium and helium releases and hydrogen generation of the sample prior to introduction of steam at 178 min were very similar to the sample annealed to 700°C. (The gap in the data in Fig. 7 was due to a temporary isolation of the sample reaction chamber with only pure argon carrier gas flowing through the ion chamber). Following steam introduction into the system, tritium and helium mobilization rates and hydrogen generation rates increased very rapidly, in a concurrent manner, to values that were orders of

Fig. 7. Hydrogen, tritium and helium signals during 700°C irradiated beryllium test in steam.

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magnitude higher than those observed for the control sample that was tested at 700°C. During this experiment we observed an increase in the light emission from the sample furnace interior, indicating that the specimen temperature had risen to temperatures possibly in excess of 1000°C. We interpret the gas generation behaviour for the irradiated specimen tested at 700°C as follows. During the heatup to 700°C and the conditioning at 700°C prior to initiation of steam exposure, we observed tritium release behaviour that indicated swelling development and evolution of a network of interconnected porosity that intersects the surface. Based on the anneal experiments, the swelling was about 14%. Consequently, when the sample was exposed to steam, the increased surface area associated with the porosity network resulted in a steam-beryllium reaction that accelerated rapidly as the exothermic heat of reaction caused the specimen temperature to rise, probably to 1000°C or higher (shown by an error bar in Fig. 4). This intense reaction caused liberation of much of the entrapped tritium and helium and further increased the reactivity until the sample was nearly consumed. Further experiments are planned to provide a better understanding of chemical reactivity behaviour for irradiated beryllium exposed to steam. This work includes measurements of density changes and effective surface areas of irradiated specimens that show swelling from annealing, measurements of chemical reactivity at temperatures below 700°C for the annealed, irradiated samples with characterized surfaceconnected porosity and a test with a thermocoupled sample to provide the temperature history of the sample during exposure to steam. Modeling to understand the influence of the higher neutron fluence relative to ITER is also planned.

4.3.3. Cur6e fit to the dense beryllium data Because there are two distinct, fairly linear, regions on the plot in Fig. 4, we fit the data at 900°C and above separate from the data at 900°C and below. We use all Series 1 data with the exception of the volumetric measurement at

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600°C, which was below detection limits. All irradiated beryllium points were included in the fits with the exception of the test furnace temperature of 700°C. For the temperature range of 900°C and above, the equation fitting the data is H2(l m − 2 · s)= 88 242 exp(−13 387/T)

(6)

where T is in °K. The r 2 for this fit was 0.82. For the temperature range of 900°C and below, the equation fitting the data is H2(l m − 2 · s)= 7.103× 1010 exp(− 28 935/T)

(7)

The r 2 for this fit was 0.86. Further testing is planned for 1997–1998 to provide a better understanding of our results, and aid in recommendation of curve fits to be used in safety analyses.

5. Discussion We have presented a summary of our mobilization and chemical reactivity data for various types of beryllium. Scoping mobilization data will be supplemented by additional tests during 1997. Results of plasma-spray beryllium tests indicate that the quality of the plasma spraying must be improved over the samples tested. Tests in 1997 with higher-density samples produced by Los Alamos National Laboratory should result in lower reaction rates. Porous beryllium also had unacceptably high reaction rates. The chemical reactivity of irradiated beryllium samples was similar in magnitude to unirradiated fully dense samples at 600°C and below. Further experiments are planned in 1997–1998 to provide a better understanding of the chemical reactivity behavior of irradiated beryllium.

Acknowledgements This work was supported by the US Department of Energy, Office of Fusion Energy, DOE Idaho Operations Office, Contract DE-AC0776ID01570.

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