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17.P.14 The growth of hygroscopic aerosol in humid atmosphere
J. Aerosol Sci., VoI. 25, Suppl. 1, pp. $273-$274, 1994 Copyright(~1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021-8502194 $7.00 + 0.00
Pergamon
17 P 14 THE GROWTH OF HYGROSCOPIC AEROSOL IN HUMID ATMOSPHERE
F. SABATHIER*, A. ZOULALIAN+, T. ALBIOL* and Victor LAYLY*
*Institute for Protection and Nuclear Safety IPSNICEA - 13108 ST PAUL LEZ DURANCE - FRANCE
+ ESSTIB - Universit6 de Nancy, BP 239 54506 VANDOEUVRE LES NANCY CEDEX FRANCE
INTRODUCTION
In some severe reactor accidents in a pressurized water reactor (PWR), fission products could be released from the reactor coolant system to the containment and form hygroscopic aerosol such as cesium hydroxide;-, cesium carbonate and cesium iodide. These hygroscopic particles absorb water in humid atmosphere and if enough water is available (for high relative humidity) the particles grow larger. This phenomenon will increase their settling rate and contribute to decrease the release of radioactive compound from the containment. Experimental results for the growth of hygroscopic aerosol particles are required to assess this sedimentation rate of fission products and consequently the source term in release estimations to the environment. The overall objectives of the Piteas programme were to study aerosol behaviour under conditions which were as close as possible to those predicted to occur within the containment of a pressudsed water reactor dudng a severe accident. This experimental programme was performed with realistic temperatures (120 ° C) and pressures (several bar) under humid atmosphere in order to provide a database for validating IPSN containment aerosol computer codes. PITEAS FACILITY The Piteas facility compdses a vessel in which aerosols are injected by using an acoustic generator to produce cesium iodide (Csl) soluble aerosols between 1 and 5 pm diameter. The Piteas containment is a double stainless steel vessel with a total volume of 2.9 m 3, 1.2 m internal diameter and 3 m high. Organic thermofluid circulates to maintain uniform wall temperature, usual operating pressure ranges from 2 to 5 bar. MEASUREMENTS AND EXPERIMENTAL PROCEDURE
Gas and wall temperatures are measured by T-type thermocouples. The relative humidity measurement is obtained by a sensor which is designed for continous dew point measurements in gas at high temperature and pressure. Aerosol mass sampling are performed by using fiberglass filters. The aerosol size distibution of wet aerosols in Piteas vessel conditions is obtained with an optical particle size analyser and the dry particle size distdbution is measured by cascade impactors. One day before the test, the Piteas vessel containing 50 litres of demineralized water is heated up to 120 ° C. When the pressure is stabilized around 3.3 bar, the vessel is vented to the ambient pressure, then closed to reach a new stabilized pressure of about 2 bar in the vessel, the water is removed and the Csl aerosol injection is initiated for a duration of one hour in order to obtain a total pressure in the containment of 4 bar. At this pressure, injection is stopped and the study of aerosol settling can start.
$273
$274
F. SABATHIERet al.
Five experiments were performed under the same conditions except the relative humidity parameter (Table 1). The second experiment was a repeat of the first test to assess the reproductibility of a Piteas experiment. Vessel Pressure* (barI 4.0 4.0 4.0 4.0 4.1 the end of Csl injection.
Experiments
PCON 01 PCON 02 PCON 03 PCON 04 PCON 05 Conditions at
Temperature* (°C) 119 120 121 122 121
Duration of Csl Injection /minutes I 60 36 58 60 55
Relative Humidity* 95 95 = 100 90 52
TABLE 1 : PCON Test Matrix EXPERIMENTAL RESULTS AND DISCUSSION
Approximatively, for each experiment, at the end of Csl injection the suspended mass concentration was close to 0.7 g/m 3 which means that a mass of 2 g of soluble aerosol was present in the containment. The cascade impactors results obtained at the beginning of settling for each experiment show that the aerodynamic mass median diameter of dried aerosol is just above 3 pm with a corresponding geometric standard deviation of 1.8 The suspended mass concentration versus time for each experiment presents an exponential decrease during the first hour following the end of injection. This can be explained if one assumes that only stirred gravitational deposition takes place in the vessel. From experimental curves, the settling velocity and wet particle diameter can be deduced from Stokes law. The results are summarized in table 2 in which the particle diameter dp has been calculated by using a vessel height of 2.65 m and a particle density( of 1 g.cm "3 for all the tests expect for the 52 % humidity experiment where the Csl density (4.5 g.cm "°) has been chosen. Relative humidity 52 90 95 100
d~ (l~m) '2.2 5.4 8.1 10.6
TABLE 2 : Stokes Mean Diameter of Wet Particles
As humidity increases, the growth of hygroscopic Csl particles increases their settling rate. If one assumes that the dry particles have a Stokes mean diameter close to 2 pm, at 100 % relative humidity, the droplets have reached a size five times their initial value. Using a rate of aerosol growth in isothermal conditions for particle to reach their equilibrium diameter, the initial diameter fo dry aerosol has been calculated and the comparison with those obtained by cascade impactor is good. The Piteas programme clearly shows that hygroscopicity plays an important role in sedimentation and consequently the release of radioactive cesium (source term) can be reduced at high humidity conditions.
Pergamon
17 P 14 THE GROWTH OF HYGROSCOPIC AEROSOL IN HUMID ATMOSPHERE
F. SABATHIER*, A. ZOULALIAN+, T. ALBIOL* and Victor LAYLY*
*Institute for Protection and Nuclear Safety IPSNICEA - 13108 ST PAUL LEZ DURANCE - FRANCE
+ ESSTIB - Universit6 de Nancy, BP 239 54506 VANDOEUVRE LES NANCY CEDEX FRANCE
INTRODUCTION
In some severe reactor accidents in a pressurized water reactor (PWR), fission products could be released from the reactor coolant system to the containment and form hygroscopic aerosol such as cesium hydroxide;-, cesium carbonate and cesium iodide. These hygroscopic particles absorb water in humid atmosphere and if enough water is available (for high relative humidity) the particles grow larger. This phenomenon will increase their settling rate and contribute to decrease the release of radioactive compound from the containment. Experimental results for the growth of hygroscopic aerosol particles are required to assess this sedimentation rate of fission products and consequently the source term in release estimations to the environment. The overall objectives of the Piteas programme were to study aerosol behaviour under conditions which were as close as possible to those predicted to occur within the containment of a pressudsed water reactor dudng a severe accident. This experimental programme was performed with realistic temperatures (120 ° C) and pressures (several bar) under humid atmosphere in order to provide a database for validating IPSN containment aerosol computer codes. PITEAS FACILITY The Piteas facility compdses a vessel in which aerosols are injected by using an acoustic generator to produce cesium iodide (Csl) soluble aerosols between 1 and 5 pm diameter. The Piteas containment is a double stainless steel vessel with a total volume of 2.9 m 3, 1.2 m internal diameter and 3 m high. Organic thermofluid circulates to maintain uniform wall temperature, usual operating pressure ranges from 2 to 5 bar. MEASUREMENTS AND EXPERIMENTAL PROCEDURE
Gas and wall temperatures are measured by T-type thermocouples. The relative humidity measurement is obtained by a sensor which is designed for continous dew point measurements in gas at high temperature and pressure. Aerosol mass sampling are performed by using fiberglass filters. The aerosol size distibution of wet aerosols in Piteas vessel conditions is obtained with an optical particle size analyser and the dry particle size distdbution is measured by cascade impactors. One day before the test, the Piteas vessel containing 50 litres of demineralized water is heated up to 120 ° C. When the pressure is stabilized around 3.3 bar, the vessel is vented to the ambient pressure, then closed to reach a new stabilized pressure of about 2 bar in the vessel, the water is removed and the Csl aerosol injection is initiated for a duration of one hour in order to obtain a total pressure in the containment of 4 bar. At this pressure, injection is stopped and the study of aerosol settling can start.
$273
$274
F. SABATHIERet al.
Five experiments were performed under the same conditions except the relative humidity parameter (Table 1). The second experiment was a repeat of the first test to assess the reproductibility of a Piteas experiment. Vessel Pressure* (barI 4.0 4.0 4.0 4.0 4.1 the end of Csl injection.
Experiments
PCON 01 PCON 02 PCON 03 PCON 04 PCON 05 Conditions at
Temperature* (°C) 119 120 121 122 121
Duration of Csl Injection /minutes I 60 36 58 60 55
Relative Humidity* 95 95 = 100 90 52
TABLE 1 : PCON Test Matrix EXPERIMENTAL RESULTS AND DISCUSSION
Approximatively, for each experiment, at the end of Csl injection the suspended mass concentration was close to 0.7 g/m 3 which means that a mass of 2 g of soluble aerosol was present in the containment. The cascade impactors results obtained at the beginning of settling for each experiment show that the aerodynamic mass median diameter of dried aerosol is just above 3 pm with a corresponding geometric standard deviation of 1.8 The suspended mass concentration versus time for each experiment presents an exponential decrease during the first hour following the end of injection. This can be explained if one assumes that only stirred gravitational deposition takes place in the vessel. From experimental curves, the settling velocity and wet particle diameter can be deduced from Stokes law. The results are summarized in table 2 in which the particle diameter dp has been calculated by using a vessel height of 2.65 m and a particle density( of 1 g.cm "3 for all the tests expect for the 52 % humidity experiment where the Csl density (4.5 g.cm "°) has been chosen. Relative humidity 52 90 95 100
d~ (l~m) '2.2 5.4 8.1 10.6
TABLE 2 : Stokes Mean Diameter of Wet Particles
As humidity increases, the growth of hygroscopic Csl particles increases their settling rate. If one assumes that the dry particles have a Stokes mean diameter close to 2 pm, at 100 % relative humidity, the droplets have reached a size five times their initial value. Using a rate of aerosol growth in isothermal conditions for particle to reach their equilibrium diameter, the initial diameter fo dry aerosol has been calculated and the comparison with those obtained by cascade impactor is good. The Piteas programme clearly shows that hygroscopicity plays an important role in sedimentation and consequently the release of radioactive cesium (source term) can be reduced at high humidity conditions.