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VAPORISATION RATES OF CsOH AND CsI IN CONDITIONS SIMULATING A SEVERE NUCLEAR ACCIDENT
PII: S0021-8502(00)00027-6
J. Aerosol Sci. Vol. 31, No. 9, pp. 1029}1043, 2000 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/00/$ - see front matter
VAPORISATION RATES OF CsOH AND CsI IN CONDITIONS SIMULATING A SEVERE NUCLEAR ACCIDENT Ari Auvinen,H Kari E. J. Lehtinen,H Juan Enriquez,H Jorma K. JokiniemiHR and Riitta ZilliacusS HVTT Energy, Aerosol Technology, P.O. Box 1401, 02044 VTT, Finland SVTT Chemical Technology, P.O. Box 1404, 02044 VTT, Finland (First received 8 December 1998; and in ,nal form 24 November 1999) Abstract*The vaporisation rates of volatile "ssion product compounds are critical parameters for modelling aerosol formation following a severe nuclear accident. The vaporisation of CsOH and CsI was studied in a pure steam atmosphere at ambient pressure (85}89 kPa) by increasing the temperature of the #ow furnace up to 10003C. For this purpose, samples were doped with a small amount of radioactive tracer. The vaporisation rate was then determined from the decrease in sample activity with time, using a germanium gamma detector placed outside the furnace. Calculated vaporisation rates obtained by solving complete velocity, temperature and vapour concentration pro"les surrounding the sample with FLUENT CFD-software, were in reasonable agreement with the data. A simple engineering calculation agrees almost perfectly with the FLUENT results, if a constant value, Sh+8, for the Sherwood number is used. 2000 Elsevier Science Ltd. All rights reserved
I N T RO DU CT I O N
Traditionally, it has been thought that in the worst case of a severe reactor accident the reactor core melts and volatile "ssion products are released rapidly through the primary coolant system to the containment building (U.S. Nuclear Regulatory Commission, 1975; Wright, 1994). In the containment, the "ssion products are mainly attached to aerosol particles, which in the humid atmosphere will grow and eventually settle onto the #oor and water pools (U.S. Nuclear Regulatory Commission, 1975, 1990; Jokiniemi, 1993). In such an accident the worst scenario is the highly unlikely early containment failure, where the integrity of the reactor building is lost early after the accident initiation before the "ssion products have had time to settle. Possible failure of the containment building at a later stage of an accident was thought not to pose such a dramatic threat (U.S. Nuclear Regulatory Commission, 1975, 1990). It is estimated, however, that as much as 80% of the "ssion products released from the reactor core may be deposited to the surfaces of the primary coolant circuit (Wright, 1994). Due to the decay heat of the radionuclides, heat transfer from the reactor core or chemical changes in the environment the deposited radionuclides may vaporise at a later stage of an accident and emerge from the reactor coolant system days or even weeks after the accident initiation (Donahue et al., 1986; U.S. Nuclear Regulatory Commission, 1990; Alonso et al., 1993; Lindholm et al., 1993; Sugimoto et al., 1994; Wright, 1994; European Commission, 1995). This phenomenon is called revaporisation and in case of a late containment failure it could lead to a major release of volatile "ssion products like cesium, iodine and tellurium to the environment. Only recently has the importance of revaporisation been recognised and now it is considered to be a signi"cant uncertainty in determining the source term from a severe reactor accident (U.S. Nuclear Regulatory Commission, 1990; Devell and Johansson, 1994; European Commission, 1995). The work described in this paper is part of a project to provide a detailed understanding of revaporisation phenomena. Our contribution has been to construct two #ow furnace facilities for "ssion product vaporisation rate measurements: one for use with synthetic
R Author to whom correspondence should be addressed. 1029
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radiolabelled samples at VTT and the other for analyses of the active Phebus-FPT-1 (Krischer and Rubinstein, 1992) samples at the CEC Joint Research Centre, Karlsruhe. In this paper the results of the experiments with radiolabelled samples are presented. The vaporisation rates of CsOH and CsI have been measured in temperature and #ow conditions relevant to severe accidents applying on-line gamma spectroscopy. The aim is to use these relatively simple cases as a "rst step in testing existing computer models for revaporisation. In order to resolve the complex #uid dynamics of the system, it was modelled with the FLUENT CFD-software (Fluent Inc., 1996). Thus the complete velocity, temperature and vapour concentration pro"les were obtained. By combining the experimental results with a detailed #uid dynamics simulation simple engineering correlations for "ssion product revaporisation can be obtained. Those correlations are especially useful for severe accident computer codes. The revaporisation of "ssion products from the surfaces of the primary circuit is already considered as a source of aerosols in RAFT (Im et al., 1985), VICTORIA (Heames et al., 1992) and MELCOR (Summers et al., 1993) codes, even though the database on the subject is very limited and partly also inconsistent. E XPE RI M EN T AL
Facility description A #ow furnace facility has been constructed to measure revaporisation rates of synthetic "ssion product samples. This was the "rst study, where the vaporisation of CsI and CsOH was measured in conditions comparable to the primary circuit. The similarities included steam atmosphere, AISI 304 stainless steel as surface material and the furnace tube diameter, which was close to the steam generator tubes. The boundary conditions for the #ow rates and temperature were taken from an earlier study (Lindholm et al., 1993). As a di!erence to the actual primary circuit conditions, the pressure inside the system was for safety reasons slightly under atmospheric. An additional bene"t of the #ow furnace facility was that very well-de"ned thermal}hydraulic conditions could be achieved for the measurements. A schematical picture of the experimental facility is presented in Fig. 1. The sample was put on a pre-oxidised AISI-304 stainless-steel sample plate (Fig. 2), which was placed into the #ow furnace over the second heating element, 100}150 mm from the outlet of the furnace. A thermocouple controlling the temperature of the furnace was in contact with the sample plate. In addition, the temperatures of both heating elements were measured. During the experiment the temperature of the furnace was increased from ambient up to 10003C. The vaporisation rate, which coincided with the decreasing activity
Fig. 1. The experimental facility to measure "ssion product revaporisation rates.
Vaporisation rates of CsOH and CsI
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Fig. 2. Sample plate used in the radio tracer studies.
of the radio-labelled sample, was measured outside the furnace with a germanium gamma detector. Each data collection lasted for about 70 s, thus de"ning the time resolution. A personal computer combined with a multichannel analyser and MicroSAMPO software was applied to analyse the data (Nikkinen and Aarnio, 1995). Since no standard was applied in the experiment, the "rst measurement was used as a calibration of the equipment. Samples were vaporised in a pure steam atmosphere at 85}89 kPa pressure. The mass #ow rate of steam was kept constant during each experiment. The #ow velocity inside the furnace was therefore dependent on temperature. Downstream of the furnace the gas #ow was cooled with ambient air in a porous tube diluter. The dilution ratio of the mass #ow rate was close to . During the cooling process "ssion product vapours formed aerosol particles, which were collected into a particle "lter. With such quench design the contamination of the facility could e!ectively be decreased. Between each experiment the processing tube, the "lter and the sample plate were changed. The "lter holder was used again after a decontamination. The use of gamma spectroscopy o!ered several advantages in the experiments. First of all, the measurements did not in#uence the studied phenomena. The vaporisation could be observed in real time, which enabled us to di!erentiate phenomena like melting and resuspension from vaporisation. Since there was no need for sampling, the measurements were representative and there was no dead volume. The selected method also allowed us to measure the vaporisation in a very wide range of conditions. After the vaporisation experiments, the mass balance of deposited material could be accurately solved by scanning the components with a gamma detector. Gamma spectroscopy will also be applied, when the revaporisation of Phebus-FPT-1 samples is measured at the CEC Joint Research Centre, Karlsruhe. In those experiments the selected method is very useful, because the mass of the "ssion product compounds is small and the activity of the samples is very high. Another important advantage, when compared for example to standard TGA, is the possibility to measure the vaporisation rate of di!erent "ssion products simultaneously. Results from CsOH vaporisation experiments Altogether three successful experiments were conducted in order to study the behaviour of CsOH. The experimental matrix is presented in Table 1. In all cases, samples were placed into the sample plate as 50 wt% CsOH}water solution. After the excess water was dried with an infrared heater, the samples were inserted inside the furnace into a nitrogen #ow. The geometry of the furnace and the sample plates remained identical throughout the experiments. Also maximum temperature, 10003C, as well as the steam #ow rate, 190 g/h, were constant. However, there were also a number of important di!erences between the experiments. The samples were vaporised in a #ow furnace by increasing the temperature with a constant ramp rate. In the "rst experiment the heating ramp was 103C/min, where as in later experiments, it was decreased to 23C/min. As a result, time and temperature
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Table 1. Experimental matrix for the radio tracer studies Experiment
Sample
1 2 3 4
CsOH CsOH CsOH CsI (94%) CsOH (6%) CsI CsI CsI
5 6 7
Mass (mg) 25 804 400 855 917 1517 1497
Temperature (3C) 500}1000 300}1000 400}1000 400}1000
Ramp Ramp Ramp Ramp
Flow rate (g/h) 103C/min 23C/min 23C/min 23C/min
Dwell 500, 600, 650, 700 Dwell 500, 600, 650, 700 Dwell 500, 600, 650, 700, 750
190 190 190 190 190 1100 72
resolutions were better in the later experiments. After the "rst experiment, also the activity of the samples was increased, which lead to a better statistical accuracy. The mass of the sample was 25 mg in the "rst, 803.5 mg in the second and 400 mg in the third experiment. The most signi"cant change however, was to alter the detector collimation after the second experiment. A signal originating from deposited cesium, lead to an overestimation of the sample mass and underestimation of the vaporisation rate in the "rst two experiments. This error signal could be almost eliminated with an improved detector collimation. The behaviour of the CsOH samples was found out to be complex. When the measurements were done with the improved collimation, it was found out, that the mass of the sample decreased at a constant rate although the temperature of the furnace was increased. It is thus very likely, that besides vaporisation the sample was #owing out of the sample plate. Because of this phenomenon, vaporisation could not be estimated in the third experiment. In the "rst two experiments the movement of the sample was also probable. This is why the sample surface area is not known. Otherwise the results from those experiments were less a!ected by the movement, since the collimation of the gamma detector was not so stringent. The movement of deposited liquid CsOH was observed previously in the LWR Aerosol Containment Experiment (LACE) program, when the aerosol transport within primary circuit pipe work was studied (Dickinson et al., 1987). It can thus be concluded, that the behaviour of CsOH would probably be similar in the primary circuit during a severe accident. If CsOH is condensed to the cooler structures of the circuit, it would most probably #ow on the surfaces and accumulate into the bends of the system. This phenomenon is especially likely to take place in a vertically placed steam generator. Despite the sample movement and the e!ect of deposition, at least a qualitative vaporisation rate of CsOH could be obtained from the "rst two experiments. The results agreed very well with each other, although the size of the sample and the heating rate di!ered greatly between the experiments. The vaporisation rates measured in the "rst two experiments are presented in Figs. 3 and 4. In experiment 1 the vaporisation started approximately at 5003C. The vaporisation rate increased for 28 min, until at 6203C it reached its maximum value of 0.66 mg/min. It is assumed that after this point the 25 mg sample was completely vaporised. In experiment 2 the vaporisation started at 5203C and increased rapidly for 120 min. At 6203C the vaporisation rate was approximately 1.0 mg/min. The maximum value for the vaporisation rate was 4.3 mg/min. It was reached at the temperature of 7603C and it stayed constant approximately 30 min up to 8243C. It is assumed that the sample was almost completely vaporised after that point. A higher temperature was reached in the second experiment, because the sample was much larger. Results from CsI vaporisation experiments Four experiments were conducted to study CsI revaporisation. The geometry of the sample plate and its placement were similar in all CsI experiments to that used in the last CsOH experiment. Unlike the previous experiments, the steam #ow rate was varied, in order to study its e!ect on vaporisation.
Vaporisation rates of CsOH and CsI
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Fig. 3. CsOH vaporisation rate in experiment 1.
Fig. 4. CsOH vaporisation rate in experiment 2.
It was much easier to measure the vaporisation of CsI samples when compared to CsOH samples. The deposition into the processing tube did not a!ect the results, because similar detector collimation was applied as in experiment 3. The samples did not #ow out of the sample plate either. Only with the highest #ow rate, there was another phenomenon that had to be taken into account. A small amount of sample material resuspended, when the steam #ow was initiated. Resuspension stopped after the #ow was stabilised. The resuspension was veri"ed after the experiment, when a small amount of unmelted CsI powder could be found downstream the sample plate from the cooler surfaces of the experimental set-up.
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Fig. 5. Sample mass and temperature in experiment 4.
Experiment 4 was performed with a constant heating ramp, like the previous CsOH experiments. The estimated sample mass is presented as a function of time in Fig. 5. The following formula was "tted with non-linear optimisation to the experimental values: m(t)"m(t!*t)!e 2>!A (t)*t, (1) where the temperature is in 3C, the mass of the sample in mg, surface area of the sample, A (t), in mm and time in s. The estimated values for parameters B and C were !10076 and 6.8409, respectively. The temperature range of the "tting was from 22 to 7603C. The temperature of the sample was calculated applying linear interpolation between measured points. It should be noted that the exponential part of equation (1) represents the vaporisation rate of CsI per square millimetre of the sample as a function of temperature. In experiments 5, 6 and 7, the temperature was increased stepwise. It was assumed, that during a temperature plateau the vaporisation rate would remain constant. Thus at each temperature plateau, measured values for the sample mass should form a straight line as a function of time. The slope of each line would then correspond to the vaporisation rate at that temperature. Finally, the results were divided by the estimated sample area in order to obtain the vaporisation rate per unit area of the sample. Since the surface area changed between the beginning and the end of each temperature plateau, the value representing it was an average of these two extremes. The variation of the surface area was included into the con"dence interval of the area-corrected vaporisation rate. Con"dence intervals were not calculated at 500 and at 6003C though, because below the melting point of CsI, the surface area of the powdery sample could only be roughly estimated. The vaporisation rates and their con"dence intervals are summarised in Table 2. The values are presented at each temperature and each #ow rate with and without surface area correction. Other "gures presented in the table are the number of measurement points used in the calculations and the variation of the estimated surface area. Vaporisation rates from the sixth experiment are calculated with the exponential part of equation (1). Several conclusions can be made from the tabulated results. At 5003C the null hypothesis for vaporisation could not be rejected at 5% level of signi"cance with low and medium steam #ow rates. With the highest #ow rate the vaporisation was signi"cant already at that temperature. The estimated value was 1.5 g/min per square meter of sample. The vaporisation rate increased exponentially as a function of temperature. It was signi"cant with all #ow rates already at 6003C. At 500 and 6003C the vaporisation appeared to be faster with the low #ow rate than with the medium #ow. The reason for this is not entirely clear. It is
Vaporisation rates of CsOH and CsI
1035
Table 2. Vaporisation rate of CsI Temperature (3C)
Mass #ow rate (g/min)
Number of measurements
Vaporisation rate (mg/min)
Vaporisation rate per 1 m (g/min)
Surface area (mm)
500 600 650 700 750
1.2 1.2 1.2 1.2 1.2
64 86 80 82 84
0.5$0.5 1.3$0.3 1.5$0.3 2.3$0.3 5.5$0.3
1.5H 4.0H 4.9$1.2 7.9$1.2 20.5$2.0
333}334R 329}333R 304}309 294}301 254}279
500 600 650 700
3.2 3.2 3.2 3.2
78 84 86 40
0.1$0.2 0.6$0.2 3.1$0.2 10.3$0.4
0.4H 2.1H 11.9$1.0 50.3$7.1
290}291R 286}289R 256}271 181}227
500 600 650 700
3.2 3.2 3.2 3.2
Exp. Exp. Exp. Exp.
0.03 0.8 2.7 8.1
0.1 2.9 10.4 31.5
283R 277R 263 256
500 600 650 700
18.4 18.4 18.4 18.4
0.5$0.3 2.3$0.3 4.1$0.3 9.7$0.3
1.5H 7.4H 14.7$1.5 44.5$6.2
325}326R 311}319R 273}290 192}243
82 84 86 56
4 4 4 4
HCon"dence intervals are not presented, because the area of the unmelted sample is not known. RThe area is estimated by assuming that the sample surface is smooth. In reality the area of the porous sample probably greater.
possible that the di!erence in the sample area was in this case larger than estimated. Faster vaporisation in experiment 9 could also be a result from the increased sample size. Vaporisation at temperatures above the melting point was notably slower with the lowest #ow rate than with other #ows. At the low #ow rate it increased from 5.0 g/min per square meter at 6503C up to 21 g/min at 7503C. The di!erence between the medium and the high #ow rate decreased, when the temperature was increased. At 6503C the vaporisation rate was 12 g/min per square meter of sample with the medium #ow and 15 g/min with the high #ow rate. At 7003C the vaporisation rate was approximately 47 g/min per square meter of the sample with both #ows. With the medium and the high #ow rate the samples also vaporised completely at 7003C. The performed CFD-calculations clearly showed the e!ect of the sample plate geometry to the #ow "eld. Increasing the steam #ow from 190 to 1100 g/h did not signi"cantly increase the #ow rate above the sample surface. In both cases, the #ow above the surface would be very slow, when compared to the #ow at the upper part of the tube. The #ow "eld above the sample with medium steam #ow rate is presented in Fig. 6. With the highest steam #ow rate, a recirculation zone at the sample plate entrance would grow, which may even decrease the vaporisation rate. The results from the CsI experiments were reproducible. CsI vaporisation rates calculated from experiments 4 and 5, which were done with the same #ow rate, agreed well with each other. The surface-areacorrected vaporisation rates are graphically presented in Fig. 7. Post test measurements Deposition of radioactive material to the surfaces of the test facility a!ected the "rst two revaporisation experiments. Signal coming from outside the sample plate lead to an overestimation of the sample mass and thus underestimation of the vaporisation rate. Two sets of measurements were conducted after each revaporisation experiment, in order to quantify the e!ect of deposition. Activity measurements during the disassembly of the system could verify, how much of the signal originated from the removed components at the end of an experiment. The mass balance and the deposition mechanisms could be deduced from the scanning of individual components.
Fig. 6. The #ow "eld above the sample with medium steam #ow rate.
1036 A. Auvinen et al.
Vaporisation rates of CsOH and CsI
1037
Fig. 7. CsI vaporisation rate per square meter of the sample.
Table 3. Sources of detected activity at the end of revaporisation experiments presented as percentage of the initial sample activity Source of activity
Exp. 1 (%)
Processing tube Sample plate Thermocouple inlet Diluter Filter Background Total
8.0 1.5 4.4
14
Exp. 2 (%)
Exp. 3 (%)
Exp. 4 (%)
Exp. 5 (%)
Exp. 6 (%)
Exp. 7 (%)
15.0 1.5 1.1 0.7 0.4 0.3
2.3 1.9 * * * *
0.2 0.3 * * 0.1 1.2
2.3 4.4 * 0.1 * *
* 0.4 * * 0.1 *
8.7H * * * * *
19
4.2
1.8
6.8
0.5
8.7
HIn experiment 9 the sample plate and the processing tube were measured together.
Disassembly measurements The test facility was disassembled always on the day following the revaporisation experiment. Before the disassembly, activity of the apparatus was measured. The gamma detector was positioned at the same place as during the revaporisation experiment. The result of the "rst measurement corresponded thus to the activity detected at the end of an on-line experiment. The activity measurement was repeated, after each component of the facility was removed. Results from the disassembly measurements are presented as percentage of the initial sample activity in Table 3. As can be seen, a signi"cant amount of deposited material could be detected at the end of the "rst two revaporisation experiments with CsOH. Only a small fraction of this, approximately 1.5%, was coming from the sample plate. Majority of the signal originated from the processing tube. Also the thermocouple inlet between the processing tube and the diluter was, especially in the "rst experiment, a signi"cant source of pulses. In the second experiment the inlet was heated, which decreased the deposition in that component. After the second experiment, the thermocouple inlet was changed from downstream of the processing tube to upstream. Therefore, in the later measurements, there were no pulses coming from that component. Cesium accumulated in the "lter did not
1038
A. Auvinen et al.
in#uence the vaporisation measurements, since it was shielded from the detector with a lead wall. It is also evident from Table 3 that the improved detector collimation was working as intended. In experiments 3}7 the signal originating outside the sample plate did not have a signi"cant in#uence on the vaporisation measurements. Mass balance measurements The mass balance measurements in the "rst two experiments were relatively simple. The components of the apparatus were placed one by one in front of the gamma detector and their activity was measured for 30 min. The distance between the detector and the components was 500 mm. A rather long distance was required in order to decrease the geometric error resulting from di!erent dimensions of each component. The results were added up and scaled to 100%, in order to determine the relative deposition on each part of the system. They are presented in Table 4. Since no standards were applied and the samples were not measured before the experiment, results were not quantitative. Nevertheless, the results from these measurements were consistent with later experiments, which were conducted more accurately. From experiment 3 on the components were gamma scanned for the mass balance calculations. They were attached to a sample holder, so that their centre would be 250 mm from the surface of the detector. Two lead shields, both with a 20 mm gap, were placed between the detector and the measured object. Before each new measurement the sample holder was shifted 20 mm along the #ow axis of the measured component. In experiment 3 the results were scaled to 100%, because the sample was not measured before it was placed into the furnace. From experiments 4}7, the initial activity of the sample was applied to de"ne the 100% level. By scanning the components it was possible to remove the error related to their varied geometry. Also, the deposition pro"le of cesium in the "lter and in the processing tube, could be resolved. The validity of the measurements could be checked by comparing the results obtained with 604 and 795 keV gamma energies, since the gamma rays have a signi"cantly di!erent absorption behaviour. If the same value for deposited mass was estimated with both energies, the results were assumed to be correct. Several conclusions can be made from the mass balance measurements. As can be seen from Table 4, cesium was mostly collected into the particle "lter. Downstream of the "lter, the activity remained always below the detection limit. Also the deposition pro"le in the cylindrical "lter were, as expected, very even. The diluter was thus functioning as intended and reducing the deposition during the quenching process. Results estimated using the two gamma energies were in excellent agreement with each others. Furthermore, the total mass balance in the experiments was very close to 100%. It is thus believed, that the results from the mass balance measurements are very reliable. The only exception to this was experiment 4. When the issue was investigated, it was found that an incorrect energy calibration "le was applied at the day the "lter was measured. The most important deposition mechanism was condensation of CsOH and CsI vapour into the processing tube. The amount of condensed material was very much dependent on the applied steam #ow rate. With the lowest #ow rate a far greater fraction of the sample was condensed than with higher #ow rates. This was mainly due to a di!erent temperature gradient at the outlet of the processing tube. Also a longer residence time in the slow #ow probably increased the condensation. The condensation peak is clearly evident in Fig. 8, where the deposition pro"le of the processing tube from experiment 5 is presented. The last 3 cm of the processing tube were outside the furnace. In addition to this, obviously the diluter was cooling the tube outlet. When the #ow through the furnace was decreased, the cesiumiodide condensation peak moved into the furnace further away from the outlet of the processing tube. In the "rst two experiments a signi"cant CsOH condensation also took place in the thermocouple inlet, located between the processing tube and the diluter. After the second experiment, this component was moved to the upstream of the processing tube.
11 * 13 * 76
100A
Processing tube Sample plate TC inlet DiluterR Filter
Total
100A
19 2 6 1 72
Exp. 2 Q"3.2 g/min
12.8 1.8 * &3.2 82.2 100A
12.2 1.8 * &4.0 82.0 76.7
8.9 0.3 * ? 67.5S 85.6
8.6 0.2 * ? 76.8S
795 (keV)
604 (keV)
604 (keV)
795 (keV)
Exp. 4 Q"3.2 g/min
Exp. 3 Q"3.2 g/min
HThe sample plate was adhered into the processing tube during the experiment and was measured with the tube. RThe diluter could not be scanned in experiments 3}7 like the other components. SAn incorrect energy calibration "le was applied, when the "lter was measured. A Results were scaled to 100%, since the samples were not measured before they were placed into the furnace.
Exp. 1 Q"3.2 g/min
Component
98.3
19.7 4.1 * &4 70.5
604 (keV)
98.6
19.6 4.2 * &4 70.8
795 (keV)
Exp. 5 Q"3.2 g/min
Table 4. Deposition to the apparatus in the revaporisation experiments
95.7
2.5 0.2 * &7 86.0
604 (keV)
97.1
2.2 0.2 * &8 86.7
795 (keV)
Exp. 6 Q"18.4 g/min
97.2
47.2H ? * ? 50.1
604 (keV)
98.3
48.1H ? * ? 50.2
795 (keV)
Exp. 7 Q"1.2 g/min
Vaporisation rates of CsOH and CsI 1039
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Fig. 8. Processing tube mass pro"le in experiment 5.
As a secondary deposition mechanism, some cesium iodide had reacted from gas phase with the hot surface of the processing tube. This can be seen also in Fig. 8, as a uniform layer upstream of the condensation peak. The amount of cesium in this deposit was approximately 3.4 g/m. Reacted cesium did not vaporise below the temperature of 8003C and could thus not be in the form of CsI. When the temperature of the furnace was raised up to 10003C, the deposited layer had vanished. Approximately 2% of cesium reacted with the sample plate in the CsOH experiments. The fraction of the reacted material decreased to 0.2}0.3% in experiment 4, which was conducted with CsI in otherwise similar conditions. The amount of reacted CsI increased substantially in experiment 5, when the maximum temperature was decreased from 1000 to 8003C. Increasing the #ow rate in experiment 6 decreased the deposition in the sample plate. In experiment 7, the sample plate had adhered to the processing tube and could not be removed for the mass balance measurements. Therefore the deposition could not be quantitatively estimated. The results from the diluter measurements were not as accurate as from the other system components. The reason for this was a 40 mm thick permanent #ange connection at the inlet of the diluter, which e!ectively prevented the scanning of the component. Most of the deposition into the diluter took place at the inlet of the component. At the inlet, there was a 52 mm long steel tube before the porous tube began. Since the dilution #ow was at ambient temperature, the tube was signi"cantly cooler than the gas coming from the reactor. It is thus believed that the main deposition mechanism was thermophoresis, especially because the condensation of CsI to the wall mostly took place already in the processing tube. There was no deposition into the porous tube of the diluter. Although the diluter was used in every experiment, cesium activity measured from the porous tube remained below the detection limit at the end of the experimental period. Obviously, the #ow velocity of the dilution gas through the walls of the porous tube was greater than the deposition velocity of the particles. The 40 mm long outlet of the diluter was also protected from deposition by the dilution #ow. The #ow coming through the walls of the porous tube, directed most of the aerosol particles towards the middle of the #ow channel. Thus a layer of clean air was formed next to the walls. Only in experiment 6 a small amount of deposited cesium could be detected at the outlet of the diluter. In that experiment, there was the smallest deposition into the processing tube and also the highest vaporisation rate. Thus the concentration of particles must have been signi"cantly higher than in any other
Vaporisation rates of CsOH and CsI
1041
experiment. In experiment 6 the amount of CsI deposited to the outlet was 0.9% (8 mg) of the initial sample inventory. TH E OR Y
The local vaporisation rate (molecular #ux density) j of a "ssion product compound from the sample can be calculated from j"D n(r) at sample surface,
(2)
where D is the di!usion coe$cient of the compound in water vapour. To obtain j, it is "rst required to solve the vapour molecular concentration "eld n(r) from the equation of convective di!usion: v ) n" ) (D n).
(3)
Here, v is the #ow velocity "eld, which has to be calculated from Navier}Stokes equations. Usually, in engineering analysis, it is typical to approximate the gradient in equation (1) by a di!erence in discrete values in the following fashion (Incropera and DeWitt, 1996): n !n (x) j+Sh D , (4) d where d is the hydraulic diameter of the #ow passage, n is the vapour molecular concentration at the sample surface and n (x) the average concentration in the cross-section at x (the coordinate along the #ow axis). Sh is the Sherwood number, which in this case is used as a "tting parameter, to take into account the complex nature of the #ow around the sample plate. By using this approximation, the equation of convective di!usion, for the average concentration n (x) in our system of interest (see Fig. 2.) reduces to dn Sh DP (n !n ). " (5) dx Aud Here u is the average #ow velocity. P and A are the wetted perimeter and the cross sectional area of the #ow passage, respectively. Equation (5), with the initial condition n "0, has a straightforward solution, which by inserting into equation (4) gives for the total revaporating mass #ux J:
p QM Sh DS J" 1 1!exp . (6) R¹ Qd Here P is the saturation vapour pressure, Q the volumetric #ow rate of gas, R the gas 1 constant, S the area of sample, M the molar mass of vapour, ¹ the temperature, D the di!usion coe$cient and d the hydraulic diameter of tube. The detailed #ow, temperature and CsOH vapour concentration "elds were also calculated using the FLUENT CFD-software (Fluent Inc., 1996). The detailed results will not be presented here, but in short, the result was that the temperature is rather uniform and that the velocity "eld contains areas of recirculation before and after the front and back edges of the sample crucible. Knowing the vapour concentration "eld then directly gives us the evaporative #ux of CsOH from equation (2). The FLUENT calculation was done at 600, 650 and 7003C. As can be seen from the approximate equation (6), in the expression for the total mass #ux J, there are variables that depend on the temperature ¹ ( p , D and Q) in addition to its explicit dependence on ¹. The results are presented graphically in Fig. 4. Equation (6) was then "tted to the FLUENT results by adjusting the Sherwood number Sh. The "t is also presented in Fig. 4, as well as the experimentally obtained mass #ux J. A visual inspection shows that the correlation matches the three numerically calculated points (obtained using FLUENT) quite nicely, when the constant value Sh"8 is used. It can also be seen that the model results match the experimental results reasonably well, at
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A. Auvinen et al.
least below the temperature ¹"7003C. After this temperature it is possible that the evaporated vapour reacts with the tube walls and the detector also sees a signal from this reacted CsOH. CO NC LU SIO N S
An experimental facility was constructed for "ssion product vaporisation rate measurements to use with synthetic radiolabelled samples. This was the "rst study, where the vaporisation of CsI and CsOH was measured in conditions comparable to the primary circuit. The facility was proven to function well. Quantitative data on CsI vaporisation with three di!erent #ow rates are obtained in a temperature range from 500 to 7503C. Above this temperature the samples were almost completely vaporised. The behaviour of CsOH samples was found out to be more complex. In addition to vaporisation, the movement of liquid CsOH on the stainless-steel surface was evident. As a possible implication to this, cesium may well accumulate in a di!erent place than it is deposited in a primary circuit. Despite the sample movement, at least qualitative vaporisation rates for CsOH were obtained. The results from the "rst two experiments agreed well with each other. Quantitative data about the deposition mechanisms in the experimental line could be obtained from the post-test measurements. The most important mechanism was condensation on the processing tube. The amount of condensed material was very much dependent on the applied steam #ow rate. With the lowest #ow rate a far greater fraction of the sample was condensed than with higher #ow rates. This was due to a di!erent temperature gradient at the outlet of the processing tube. As a secondary deposition mechanism, cesium reacted from gas phase with the hot surface of the processing tube. The amount of cesium in this uniform layer was approximately 3.4 g m\. Reacted cesium did not vaporise below the temperature of 8003C. When the temperature of the furnace was increased up to 10003C, the deposited layer had vanished. The #ow, temperature and concentration "elds of the system were simulated numerically with the FLUENT CFD-software. The results from CsOH vaporisation experiments agreed well with FLUENT modelling in the temperature range from 500 to 7003C. The sample movement and the reaction with the processing tube in#uenced these results though, especially at high temperatures. A simple engineering calculation agrees almost perfectly with the FLUENT results, if a constant value for the Sherwood number is used. Such correlations are especially useful for computer codes like RAFT, VICTORIA and MELCOR that cannot use much computing e!ort for single phenomena. The CsI experiments are currently being modelled with FLUENT software. Another #ow furnace facility, based on this experimental set-up, has also been assembled. In that facility, revaporisation of "ssion products from samples obtained from Phebus FPT-1 experiment will be studied. The furnace is reduced in size to avoid excessive lead shielding weight and to make handling of the process tube in a glove box practicable. However, the experimental conditions like gas composition, gas #ow rates and temperature ramp rate remain as close as possible to those described in this paper. Acknowledgements*We would like to acknowledge the people at the VTT Aerosol Technology Group for advice and ideas regarding the experimental set-up. This work has been supported by the European Community via contract FI4S-CT96-0019.
REF ER E NCE S Alonso, A., Carretero, T., Ezquerra, F. and Hontanon, E. (1993) State of the art report on revaporisation of the RCS condensate under LWR accident conditions. CTN-56/93. Devell, L. and Johansson, K. (1994) Speci"c features of cesium chemistry and physics a!ecting reactor accident source term predictions. ISSN 1104-1374, ISRN SKI-R-94:29-SE, NEA/CSNI/R(94)28. Dickinson, D., Hilliard, R., Muhlestein, L., Mechan, D. and Carraro, G. (1987) Aerosol Behavior in LWR containment bypass piping*results of LACE test LA3. LACE TR-011, 1987.
Vaporisation rates of CsOH and CsI
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Donahue, M., Hazzan, M., Metcalf, J. and Warman, E. (1986) Source term evaluation for accident conditions*an analysis of retention/revaporisation in BWR mark-II power plant. IAEA-SM-281/23. European Commission (1995) Plant assessments, identi"cation of uncertainties in source term analysis. In Nuclear science and technology. EUR Report 16502 EN, ISSN 1018-5594, ISBN 92-827-5284-4, C9071/PWR/93/REP/ 001 ST(93)-P35 Issue C Rev. 1. Fluent Inc. (1996) Fluent user's guide, release 4.4. Heames, T. J. et al. (1992) VICTORIA: a mechanistic model of radionuclide behavior in the reactor coolant system under severe accident conditions. USNRC Report NUREG/CR-5545 Rev. 1. Im, K. H., Ahluwalia, R. K. and Chuang, C. F. (1985) RAFT: a computer model for formation and transport of "ssion product aerosols in LWR primary systems. Aerosol Sci. Technol. 4, 125. Incropera, F. P. and DeWitt, D. P. (1996) Fundamentals of Heat and Mass Transfer. Wiley, New York. Jokiniemi, J. (Ed). (1993) Physical and chemical characteristics of aerosols in the containment. NEA/CSNI/R(93)7. Krischer, W. and Rubinstein, M. C. (1992) The Phebus Fission product Project*Presentation of the Experimental Programme and Test Facility. Elsevier Applied Science, London. Lindholm, I., SjoK vall, H., Jokiniemi, J. and MaK kynen, J. (1993) A comparative study of "ssion product behaviour in TVO I nuclear power plant. VARA-15/93, CEC/RCA-ST(95)-P169. Nikkinen, M. and Aarnio, P. (1995) Sampo 90 V. 3.4-3.5 Manual. Helsinki University of Technology. Sugimoto, J., Kajimoto, M., Hashimoto, K. and Soda, K. (Eds). (1994) Short overview on the de"nition and signi"cance of the late phase "ssion product aerosol/vapor source. NEA/CSNI/R(94)30. Summers, R. M. et al. (1993) MELCOR 1.8.2: a computer code for nuclear reactor severe accident source term and risk assessment analyses. USNRC Report NUREG/CR-5531. U.S. Nuclear Regulatory Commission, (1975) Reactor safety study*an assessment of accident risks in U.S. commercial nuclear power plants. WASH-1400, NUREG-75/014. U.S. Nuclear Regulatory Commission, (1990) Severe accident risks: an assessment for "ve U.S. nuclear power plants. NUREG-1150. Wright, A. L. (Ed). (1994) Primary system "ssion product release and transport. NUREG/CR-6193, NEA/CSNI/R(94)2, ORNL/TM-12681.
J. Aerosol Sci. Vol. 31, No. 9, pp. 1029}1043, 2000 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-8502/00/$ - see front matter
VAPORISATION RATES OF CsOH AND CsI IN CONDITIONS SIMULATING A SEVERE NUCLEAR ACCIDENT Ari Auvinen,H Kari E. J. Lehtinen,H Juan Enriquez,H Jorma K. JokiniemiHR and Riitta ZilliacusS HVTT Energy, Aerosol Technology, P.O. Box 1401, 02044 VTT, Finland SVTT Chemical Technology, P.O. Box 1404, 02044 VTT, Finland (First received 8 December 1998; and in ,nal form 24 November 1999) Abstract*The vaporisation rates of volatile "ssion product compounds are critical parameters for modelling aerosol formation following a severe nuclear accident. The vaporisation of CsOH and CsI was studied in a pure steam atmosphere at ambient pressure (85}89 kPa) by increasing the temperature of the #ow furnace up to 10003C. For this purpose, samples were doped with a small amount of radioactive tracer. The vaporisation rate was then determined from the decrease in sample activity with time, using a germanium gamma detector placed outside the furnace. Calculated vaporisation rates obtained by solving complete velocity, temperature and vapour concentration pro"les surrounding the sample with FLUENT CFD-software, were in reasonable agreement with the data. A simple engineering calculation agrees almost perfectly with the FLUENT results, if a constant value, Sh+8, for the Sherwood number is used. 2000 Elsevier Science Ltd. All rights reserved
I N T RO DU CT I O N
Traditionally, it has been thought that in the worst case of a severe reactor accident the reactor core melts and volatile "ssion products are released rapidly through the primary coolant system to the containment building (U.S. Nuclear Regulatory Commission, 1975; Wright, 1994). In the containment, the "ssion products are mainly attached to aerosol particles, which in the humid atmosphere will grow and eventually settle onto the #oor and water pools (U.S. Nuclear Regulatory Commission, 1975, 1990; Jokiniemi, 1993). In such an accident the worst scenario is the highly unlikely early containment failure, where the integrity of the reactor building is lost early after the accident initiation before the "ssion products have had time to settle. Possible failure of the containment building at a later stage of an accident was thought not to pose such a dramatic threat (U.S. Nuclear Regulatory Commission, 1975, 1990). It is estimated, however, that as much as 80% of the "ssion products released from the reactor core may be deposited to the surfaces of the primary coolant circuit (Wright, 1994). Due to the decay heat of the radionuclides, heat transfer from the reactor core or chemical changes in the environment the deposited radionuclides may vaporise at a later stage of an accident and emerge from the reactor coolant system days or even weeks after the accident initiation (Donahue et al., 1986; U.S. Nuclear Regulatory Commission, 1990; Alonso et al., 1993; Lindholm et al., 1993; Sugimoto et al., 1994; Wright, 1994; European Commission, 1995). This phenomenon is called revaporisation and in case of a late containment failure it could lead to a major release of volatile "ssion products like cesium, iodine and tellurium to the environment. Only recently has the importance of revaporisation been recognised and now it is considered to be a signi"cant uncertainty in determining the source term from a severe reactor accident (U.S. Nuclear Regulatory Commission, 1990; Devell and Johansson, 1994; European Commission, 1995). The work described in this paper is part of a project to provide a detailed understanding of revaporisation phenomena. Our contribution has been to construct two #ow furnace facilities for "ssion product vaporisation rate measurements: one for use with synthetic
R Author to whom correspondence should be addressed. 1029
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radiolabelled samples at VTT and the other for analyses of the active Phebus-FPT-1 (Krischer and Rubinstein, 1992) samples at the CEC Joint Research Centre, Karlsruhe. In this paper the results of the experiments with radiolabelled samples are presented. The vaporisation rates of CsOH and CsI have been measured in temperature and #ow conditions relevant to severe accidents applying on-line gamma spectroscopy. The aim is to use these relatively simple cases as a "rst step in testing existing computer models for revaporisation. In order to resolve the complex #uid dynamics of the system, it was modelled with the FLUENT CFD-software (Fluent Inc., 1996). Thus the complete velocity, temperature and vapour concentration pro"les were obtained. By combining the experimental results with a detailed #uid dynamics simulation simple engineering correlations for "ssion product revaporisation can be obtained. Those correlations are especially useful for severe accident computer codes. The revaporisation of "ssion products from the surfaces of the primary circuit is already considered as a source of aerosols in RAFT (Im et al., 1985), VICTORIA (Heames et al., 1992) and MELCOR (Summers et al., 1993) codes, even though the database on the subject is very limited and partly also inconsistent. E XPE RI M EN T AL
Facility description A #ow furnace facility has been constructed to measure revaporisation rates of synthetic "ssion product samples. This was the "rst study, where the vaporisation of CsI and CsOH was measured in conditions comparable to the primary circuit. The similarities included steam atmosphere, AISI 304 stainless steel as surface material and the furnace tube diameter, which was close to the steam generator tubes. The boundary conditions for the #ow rates and temperature were taken from an earlier study (Lindholm et al., 1993). As a di!erence to the actual primary circuit conditions, the pressure inside the system was for safety reasons slightly under atmospheric. An additional bene"t of the #ow furnace facility was that very well-de"ned thermal}hydraulic conditions could be achieved for the measurements. A schematical picture of the experimental facility is presented in Fig. 1. The sample was put on a pre-oxidised AISI-304 stainless-steel sample plate (Fig. 2), which was placed into the #ow furnace over the second heating element, 100}150 mm from the outlet of the furnace. A thermocouple controlling the temperature of the furnace was in contact with the sample plate. In addition, the temperatures of both heating elements were measured. During the experiment the temperature of the furnace was increased from ambient up to 10003C. The vaporisation rate, which coincided with the decreasing activity
Fig. 1. The experimental facility to measure "ssion product revaporisation rates.
Vaporisation rates of CsOH and CsI
1031
Fig. 2. Sample plate used in the radio tracer studies.
of the radio-labelled sample, was measured outside the furnace with a germanium gamma detector. Each data collection lasted for about 70 s, thus de"ning the time resolution. A personal computer combined with a multichannel analyser and MicroSAMPO software was applied to analyse the data (Nikkinen and Aarnio, 1995). Since no standard was applied in the experiment, the "rst measurement was used as a calibration of the equipment. Samples were vaporised in a pure steam atmosphere at 85}89 kPa pressure. The mass #ow rate of steam was kept constant during each experiment. The #ow velocity inside the furnace was therefore dependent on temperature. Downstream of the furnace the gas #ow was cooled with ambient air in a porous tube diluter. The dilution ratio of the mass #ow rate was close to . During the cooling process "ssion product vapours formed aerosol particles, which were collected into a particle "lter. With such quench design the contamination of the facility could e!ectively be decreased. Between each experiment the processing tube, the "lter and the sample plate were changed. The "lter holder was used again after a decontamination. The use of gamma spectroscopy o!ered several advantages in the experiments. First of all, the measurements did not in#uence the studied phenomena. The vaporisation could be observed in real time, which enabled us to di!erentiate phenomena like melting and resuspension from vaporisation. Since there was no need for sampling, the measurements were representative and there was no dead volume. The selected method also allowed us to measure the vaporisation in a very wide range of conditions. After the vaporisation experiments, the mass balance of deposited material could be accurately solved by scanning the components with a gamma detector. Gamma spectroscopy will also be applied, when the revaporisation of Phebus-FPT-1 samples is measured at the CEC Joint Research Centre, Karlsruhe. In those experiments the selected method is very useful, because the mass of the "ssion product compounds is small and the activity of the samples is very high. Another important advantage, when compared for example to standard TGA, is the possibility to measure the vaporisation rate of di!erent "ssion products simultaneously. Results from CsOH vaporisation experiments Altogether three successful experiments were conducted in order to study the behaviour of CsOH. The experimental matrix is presented in Table 1. In all cases, samples were placed into the sample plate as 50 wt% CsOH}water solution. After the excess water was dried with an infrared heater, the samples were inserted inside the furnace into a nitrogen #ow. The geometry of the furnace and the sample plates remained identical throughout the experiments. Also maximum temperature, 10003C, as well as the steam #ow rate, 190 g/h, were constant. However, there were also a number of important di!erences between the experiments. The samples were vaporised in a #ow furnace by increasing the temperature with a constant ramp rate. In the "rst experiment the heating ramp was 103C/min, where as in later experiments, it was decreased to 23C/min. As a result, time and temperature
A. Auvinen et al.
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Table 1. Experimental matrix for the radio tracer studies Experiment
Sample
1 2 3 4
CsOH CsOH CsOH CsI (94%) CsOH (6%) CsI CsI CsI
5 6 7
Mass (mg) 25 804 400 855 917 1517 1497
Temperature (3C) 500}1000 300}1000 400}1000 400}1000
Ramp Ramp Ramp Ramp
Flow rate (g/h) 103C/min 23C/min 23C/min 23C/min
Dwell 500, 600, 650, 700 Dwell 500, 600, 650, 700 Dwell 500, 600, 650, 700, 750
190 190 190 190 190 1100 72
resolutions were better in the later experiments. After the "rst experiment, also the activity of the samples was increased, which lead to a better statistical accuracy. The mass of the sample was 25 mg in the "rst, 803.5 mg in the second and 400 mg in the third experiment. The most signi"cant change however, was to alter the detector collimation after the second experiment. A signal originating from deposited cesium, lead to an overestimation of the sample mass and underestimation of the vaporisation rate in the "rst two experiments. This error signal could be almost eliminated with an improved detector collimation. The behaviour of the CsOH samples was found out to be complex. When the measurements were done with the improved collimation, it was found out, that the mass of the sample decreased at a constant rate although the temperature of the furnace was increased. It is thus very likely, that besides vaporisation the sample was #owing out of the sample plate. Because of this phenomenon, vaporisation could not be estimated in the third experiment. In the "rst two experiments the movement of the sample was also probable. This is why the sample surface area is not known. Otherwise the results from those experiments were less a!ected by the movement, since the collimation of the gamma detector was not so stringent. The movement of deposited liquid CsOH was observed previously in the LWR Aerosol Containment Experiment (LACE) program, when the aerosol transport within primary circuit pipe work was studied (Dickinson et al., 1987). It can thus be concluded, that the behaviour of CsOH would probably be similar in the primary circuit during a severe accident. If CsOH is condensed to the cooler structures of the circuit, it would most probably #ow on the surfaces and accumulate into the bends of the system. This phenomenon is especially likely to take place in a vertically placed steam generator. Despite the sample movement and the e!ect of deposition, at least a qualitative vaporisation rate of CsOH could be obtained from the "rst two experiments. The results agreed very well with each other, although the size of the sample and the heating rate di!ered greatly between the experiments. The vaporisation rates measured in the "rst two experiments are presented in Figs. 3 and 4. In experiment 1 the vaporisation started approximately at 5003C. The vaporisation rate increased for 28 min, until at 6203C it reached its maximum value of 0.66 mg/min. It is assumed that after this point the 25 mg sample was completely vaporised. In experiment 2 the vaporisation started at 5203C and increased rapidly for 120 min. At 6203C the vaporisation rate was approximately 1.0 mg/min. The maximum value for the vaporisation rate was 4.3 mg/min. It was reached at the temperature of 7603C and it stayed constant approximately 30 min up to 8243C. It is assumed that the sample was almost completely vaporised after that point. A higher temperature was reached in the second experiment, because the sample was much larger. Results from CsI vaporisation experiments Four experiments were conducted to study CsI revaporisation. The geometry of the sample plate and its placement were similar in all CsI experiments to that used in the last CsOH experiment. Unlike the previous experiments, the steam #ow rate was varied, in order to study its e!ect on vaporisation.
Vaporisation rates of CsOH and CsI
1033
Fig. 3. CsOH vaporisation rate in experiment 1.
Fig. 4. CsOH vaporisation rate in experiment 2.
It was much easier to measure the vaporisation of CsI samples when compared to CsOH samples. The deposition into the processing tube did not a!ect the results, because similar detector collimation was applied as in experiment 3. The samples did not #ow out of the sample plate either. Only with the highest #ow rate, there was another phenomenon that had to be taken into account. A small amount of sample material resuspended, when the steam #ow was initiated. Resuspension stopped after the #ow was stabilised. The resuspension was veri"ed after the experiment, when a small amount of unmelted CsI powder could be found downstream the sample plate from the cooler surfaces of the experimental set-up.
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Fig. 5. Sample mass and temperature in experiment 4.
Experiment 4 was performed with a constant heating ramp, like the previous CsOH experiments. The estimated sample mass is presented as a function of time in Fig. 5. The following formula was "tted with non-linear optimisation to the experimental values: m(t)"m(t!*t)!e 2>!A (t)*t, (1) where the temperature is in 3C, the mass of the sample in mg, surface area of the sample, A (t), in mm and time in s. The estimated values for parameters B and C were !10076 and 6.8409, respectively. The temperature range of the "tting was from 22 to 7603C. The temperature of the sample was calculated applying linear interpolation between measured points. It should be noted that the exponential part of equation (1) represents the vaporisation rate of CsI per square millimetre of the sample as a function of temperature. In experiments 5, 6 and 7, the temperature was increased stepwise. It was assumed, that during a temperature plateau the vaporisation rate would remain constant. Thus at each temperature plateau, measured values for the sample mass should form a straight line as a function of time. The slope of each line would then correspond to the vaporisation rate at that temperature. Finally, the results were divided by the estimated sample area in order to obtain the vaporisation rate per unit area of the sample. Since the surface area changed between the beginning and the end of each temperature plateau, the value representing it was an average of these two extremes. The variation of the surface area was included into the con"dence interval of the area-corrected vaporisation rate. Con"dence intervals were not calculated at 500 and at 6003C though, because below the melting point of CsI, the surface area of the powdery sample could only be roughly estimated. The vaporisation rates and their con"dence intervals are summarised in Table 2. The values are presented at each temperature and each #ow rate with and without surface area correction. Other "gures presented in the table are the number of measurement points used in the calculations and the variation of the estimated surface area. Vaporisation rates from the sixth experiment are calculated with the exponential part of equation (1). Several conclusions can be made from the tabulated results. At 5003C the null hypothesis for vaporisation could not be rejected at 5% level of signi"cance with low and medium steam #ow rates. With the highest #ow rate the vaporisation was signi"cant already at that temperature. The estimated value was 1.5 g/min per square meter of sample. The vaporisation rate increased exponentially as a function of temperature. It was signi"cant with all #ow rates already at 6003C. At 500 and 6003C the vaporisation appeared to be faster with the low #ow rate than with the medium #ow. The reason for this is not entirely clear. It is
Vaporisation rates of CsOH and CsI
1035
Table 2. Vaporisation rate of CsI Temperature (3C)
Mass #ow rate (g/min)
Number of measurements
Vaporisation rate (mg/min)
Vaporisation rate per 1 m (g/min)
Surface area (mm)
500 600 650 700 750
1.2 1.2 1.2 1.2 1.2
64 86 80 82 84
0.5$0.5 1.3$0.3 1.5$0.3 2.3$0.3 5.5$0.3
1.5H 4.0H 4.9$1.2 7.9$1.2 20.5$2.0
333}334R 329}333R 304}309 294}301 254}279
500 600 650 700
3.2 3.2 3.2 3.2
78 84 86 40
0.1$0.2 0.6$0.2 3.1$0.2 10.3$0.4
0.4H 2.1H 11.9$1.0 50.3$7.1
290}291R 286}289R 256}271 181}227
500 600 650 700
3.2 3.2 3.2 3.2
Exp. Exp. Exp. Exp.
0.03 0.8 2.7 8.1
0.1 2.9 10.4 31.5
283R 277R 263 256
500 600 650 700
18.4 18.4 18.4 18.4
0.5$0.3 2.3$0.3 4.1$0.3 9.7$0.3
1.5H 7.4H 14.7$1.5 44.5$6.2
325}326R 311}319R 273}290 192}243
82 84 86 56
4 4 4 4
HCon"dence intervals are not presented, because the area of the unmelted sample is not known. RThe area is estimated by assuming that the sample surface is smooth. In reality the area of the porous sample probably greater.
possible that the di!erence in the sample area was in this case larger than estimated. Faster vaporisation in experiment 9 could also be a result from the increased sample size. Vaporisation at temperatures above the melting point was notably slower with the lowest #ow rate than with other #ows. At the low #ow rate it increased from 5.0 g/min per square meter at 6503C up to 21 g/min at 7503C. The di!erence between the medium and the high #ow rate decreased, when the temperature was increased. At 6503C the vaporisation rate was 12 g/min per square meter of sample with the medium #ow and 15 g/min with the high #ow rate. At 7003C the vaporisation rate was approximately 47 g/min per square meter of the sample with both #ows. With the medium and the high #ow rate the samples also vaporised completely at 7003C. The performed CFD-calculations clearly showed the e!ect of the sample plate geometry to the #ow "eld. Increasing the steam #ow from 190 to 1100 g/h did not signi"cantly increase the #ow rate above the sample surface. In both cases, the #ow above the surface would be very slow, when compared to the #ow at the upper part of the tube. The #ow "eld above the sample with medium steam #ow rate is presented in Fig. 6. With the highest steam #ow rate, a recirculation zone at the sample plate entrance would grow, which may even decrease the vaporisation rate. The results from the CsI experiments were reproducible. CsI vaporisation rates calculated from experiments 4 and 5, which were done with the same #ow rate, agreed well with each other. The surface-areacorrected vaporisation rates are graphically presented in Fig. 7. Post test measurements Deposition of radioactive material to the surfaces of the test facility a!ected the "rst two revaporisation experiments. Signal coming from outside the sample plate lead to an overestimation of the sample mass and thus underestimation of the vaporisation rate. Two sets of measurements were conducted after each revaporisation experiment, in order to quantify the e!ect of deposition. Activity measurements during the disassembly of the system could verify, how much of the signal originated from the removed components at the end of an experiment. The mass balance and the deposition mechanisms could be deduced from the scanning of individual components.
Fig. 6. The #ow "eld above the sample with medium steam #ow rate.
1036 A. Auvinen et al.
Vaporisation rates of CsOH and CsI
1037
Fig. 7. CsI vaporisation rate per square meter of the sample.
Table 3. Sources of detected activity at the end of revaporisation experiments presented as percentage of the initial sample activity Source of activity
Exp. 1 (%)
Processing tube Sample plate Thermocouple inlet Diluter Filter Background Total
8.0 1.5 4.4
14
Exp. 2 (%)
Exp. 3 (%)
Exp. 4 (%)
Exp. 5 (%)
Exp. 6 (%)
Exp. 7 (%)
15.0 1.5 1.1 0.7 0.4 0.3
2.3 1.9 * * * *
0.2 0.3 * * 0.1 1.2
2.3 4.4 * 0.1 * *
* 0.4 * * 0.1 *
8.7H * * * * *
19
4.2
1.8
6.8
0.5
8.7
HIn experiment 9 the sample plate and the processing tube were measured together.
Disassembly measurements The test facility was disassembled always on the day following the revaporisation experiment. Before the disassembly, activity of the apparatus was measured. The gamma detector was positioned at the same place as during the revaporisation experiment. The result of the "rst measurement corresponded thus to the activity detected at the end of an on-line experiment. The activity measurement was repeated, after each component of the facility was removed. Results from the disassembly measurements are presented as percentage of the initial sample activity in Table 3. As can be seen, a signi"cant amount of deposited material could be detected at the end of the "rst two revaporisation experiments with CsOH. Only a small fraction of this, approximately 1.5%, was coming from the sample plate. Majority of the signal originated from the processing tube. Also the thermocouple inlet between the processing tube and the diluter was, especially in the "rst experiment, a signi"cant source of pulses. In the second experiment the inlet was heated, which decreased the deposition in that component. After the second experiment, the thermocouple inlet was changed from downstream of the processing tube to upstream. Therefore, in the later measurements, there were no pulses coming from that component. Cesium accumulated in the "lter did not
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A. Auvinen et al.
in#uence the vaporisation measurements, since it was shielded from the detector with a lead wall. It is also evident from Table 3 that the improved detector collimation was working as intended. In experiments 3}7 the signal originating outside the sample plate did not have a signi"cant in#uence on the vaporisation measurements. Mass balance measurements The mass balance measurements in the "rst two experiments were relatively simple. The components of the apparatus were placed one by one in front of the gamma detector and their activity was measured for 30 min. The distance between the detector and the components was 500 mm. A rather long distance was required in order to decrease the geometric error resulting from di!erent dimensions of each component. The results were added up and scaled to 100%, in order to determine the relative deposition on each part of the system. They are presented in Table 4. Since no standards were applied and the samples were not measured before the experiment, results were not quantitative. Nevertheless, the results from these measurements were consistent with later experiments, which were conducted more accurately. From experiment 3 on the components were gamma scanned for the mass balance calculations. They were attached to a sample holder, so that their centre would be 250 mm from the surface of the detector. Two lead shields, both with a 20 mm gap, were placed between the detector and the measured object. Before each new measurement the sample holder was shifted 20 mm along the #ow axis of the measured component. In experiment 3 the results were scaled to 100%, because the sample was not measured before it was placed into the furnace. From experiments 4}7, the initial activity of the sample was applied to de"ne the 100% level. By scanning the components it was possible to remove the error related to their varied geometry. Also, the deposition pro"le of cesium in the "lter and in the processing tube, could be resolved. The validity of the measurements could be checked by comparing the results obtained with 604 and 795 keV gamma energies, since the gamma rays have a signi"cantly di!erent absorption behaviour. If the same value for deposited mass was estimated with both energies, the results were assumed to be correct. Several conclusions can be made from the mass balance measurements. As can be seen from Table 4, cesium was mostly collected into the particle "lter. Downstream of the "lter, the activity remained always below the detection limit. Also the deposition pro"le in the cylindrical "lter were, as expected, very even. The diluter was thus functioning as intended and reducing the deposition during the quenching process. Results estimated using the two gamma energies were in excellent agreement with each others. Furthermore, the total mass balance in the experiments was very close to 100%. It is thus believed, that the results from the mass balance measurements are very reliable. The only exception to this was experiment 4. When the issue was investigated, it was found that an incorrect energy calibration "le was applied at the day the "lter was measured. The most important deposition mechanism was condensation of CsOH and CsI vapour into the processing tube. The amount of condensed material was very much dependent on the applied steam #ow rate. With the lowest #ow rate a far greater fraction of the sample was condensed than with higher #ow rates. This was mainly due to a di!erent temperature gradient at the outlet of the processing tube. Also a longer residence time in the slow #ow probably increased the condensation. The condensation peak is clearly evident in Fig. 8, where the deposition pro"le of the processing tube from experiment 5 is presented. The last 3 cm of the processing tube were outside the furnace. In addition to this, obviously the diluter was cooling the tube outlet. When the #ow through the furnace was decreased, the cesiumiodide condensation peak moved into the furnace further away from the outlet of the processing tube. In the "rst two experiments a signi"cant CsOH condensation also took place in the thermocouple inlet, located between the processing tube and the diluter. After the second experiment, this component was moved to the upstream of the processing tube.
11 * 13 * 76
100A
Processing tube Sample plate TC inlet DiluterR Filter
Total
100A
19 2 6 1 72
Exp. 2 Q"3.2 g/min
12.8 1.8 * &3.2 82.2 100A
12.2 1.8 * &4.0 82.0 76.7
8.9 0.3 * ? 67.5S 85.6
8.6 0.2 * ? 76.8S
795 (keV)
604 (keV)
604 (keV)
795 (keV)
Exp. 4 Q"3.2 g/min
Exp. 3 Q"3.2 g/min
HThe sample plate was adhered into the processing tube during the experiment and was measured with the tube. RThe diluter could not be scanned in experiments 3}7 like the other components. SAn incorrect energy calibration "le was applied, when the "lter was measured. A Results were scaled to 100%, since the samples were not measured before they were placed into the furnace.
Exp. 1 Q"3.2 g/min
Component
98.3
19.7 4.1 * &4 70.5
604 (keV)
98.6
19.6 4.2 * &4 70.8
795 (keV)
Exp. 5 Q"3.2 g/min
Table 4. Deposition to the apparatus in the revaporisation experiments
95.7
2.5 0.2 * &7 86.0
604 (keV)
97.1
2.2 0.2 * &8 86.7
795 (keV)
Exp. 6 Q"18.4 g/min
97.2
47.2H ? * ? 50.1
604 (keV)
98.3
48.1H ? * ? 50.2
795 (keV)
Exp. 7 Q"1.2 g/min
Vaporisation rates of CsOH and CsI 1039
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Fig. 8. Processing tube mass pro"le in experiment 5.
As a secondary deposition mechanism, some cesium iodide had reacted from gas phase with the hot surface of the processing tube. This can be seen also in Fig. 8, as a uniform layer upstream of the condensation peak. The amount of cesium in this deposit was approximately 3.4 g/m. Reacted cesium did not vaporise below the temperature of 8003C and could thus not be in the form of CsI. When the temperature of the furnace was raised up to 10003C, the deposited layer had vanished. Approximately 2% of cesium reacted with the sample plate in the CsOH experiments. The fraction of the reacted material decreased to 0.2}0.3% in experiment 4, which was conducted with CsI in otherwise similar conditions. The amount of reacted CsI increased substantially in experiment 5, when the maximum temperature was decreased from 1000 to 8003C. Increasing the #ow rate in experiment 6 decreased the deposition in the sample plate. In experiment 7, the sample plate had adhered to the processing tube and could not be removed for the mass balance measurements. Therefore the deposition could not be quantitatively estimated. The results from the diluter measurements were not as accurate as from the other system components. The reason for this was a 40 mm thick permanent #ange connection at the inlet of the diluter, which e!ectively prevented the scanning of the component. Most of the deposition into the diluter took place at the inlet of the component. At the inlet, there was a 52 mm long steel tube before the porous tube began. Since the dilution #ow was at ambient temperature, the tube was signi"cantly cooler than the gas coming from the reactor. It is thus believed that the main deposition mechanism was thermophoresis, especially because the condensation of CsI to the wall mostly took place already in the processing tube. There was no deposition into the porous tube of the diluter. Although the diluter was used in every experiment, cesium activity measured from the porous tube remained below the detection limit at the end of the experimental period. Obviously, the #ow velocity of the dilution gas through the walls of the porous tube was greater than the deposition velocity of the particles. The 40 mm long outlet of the diluter was also protected from deposition by the dilution #ow. The #ow coming through the walls of the porous tube, directed most of the aerosol particles towards the middle of the #ow channel. Thus a layer of clean air was formed next to the walls. Only in experiment 6 a small amount of deposited cesium could be detected at the outlet of the diluter. In that experiment, there was the smallest deposition into the processing tube and also the highest vaporisation rate. Thus the concentration of particles must have been signi"cantly higher than in any other
Vaporisation rates of CsOH and CsI
1041
experiment. In experiment 6 the amount of CsI deposited to the outlet was 0.9% (8 mg) of the initial sample inventory. TH E OR Y
The local vaporisation rate (molecular #ux density) j of a "ssion product compound from the sample can be calculated from j"D n(r) at sample surface,
(2)
where D is the di!usion coe$cient of the compound in water vapour. To obtain j, it is "rst required to solve the vapour molecular concentration "eld n(r) from the equation of convective di!usion: v ) n" ) (D n).
(3)
Here, v is the #ow velocity "eld, which has to be calculated from Navier}Stokes equations. Usually, in engineering analysis, it is typical to approximate the gradient in equation (1) by a di!erence in discrete values in the following fashion (Incropera and DeWitt, 1996): n !n (x) j+Sh D , (4) d where d is the hydraulic diameter of the #ow passage, n is the vapour molecular concentration at the sample surface and n (x) the average concentration in the cross-section at x (the coordinate along the #ow axis). Sh is the Sherwood number, which in this case is used as a "tting parameter, to take into account the complex nature of the #ow around the sample plate. By using this approximation, the equation of convective di!usion, for the average concentration n (x) in our system of interest (see Fig. 2.) reduces to dn Sh DP (n !n ). " (5) dx Aud Here u is the average #ow velocity. P and A are the wetted perimeter and the cross sectional area of the #ow passage, respectively. Equation (5), with the initial condition n "0, has a straightforward solution, which by inserting into equation (4) gives for the total revaporating mass #ux J:
p QM Sh DS J" 1 1!exp . (6) R¹ Qd Here P is the saturation vapour pressure, Q the volumetric #ow rate of gas, R the gas 1 constant, S the area of sample, M the molar mass of vapour, ¹ the temperature, D the di!usion coe$cient and d the hydraulic diameter of tube. The detailed #ow, temperature and CsOH vapour concentration "elds were also calculated using the FLUENT CFD-software (Fluent Inc., 1996). The detailed results will not be presented here, but in short, the result was that the temperature is rather uniform and that the velocity "eld contains areas of recirculation before and after the front and back edges of the sample crucible. Knowing the vapour concentration "eld then directly gives us the evaporative #ux of CsOH from equation (2). The FLUENT calculation was done at 600, 650 and 7003C. As can be seen from the approximate equation (6), in the expression for the total mass #ux J, there are variables that depend on the temperature ¹ ( p , D and Q) in addition to its explicit dependence on ¹. The results are presented graphically in Fig. 4. Equation (6) was then "tted to the FLUENT results by adjusting the Sherwood number Sh. The "t is also presented in Fig. 4, as well as the experimentally obtained mass #ux J. A visual inspection shows that the correlation matches the three numerically calculated points (obtained using FLUENT) quite nicely, when the constant value Sh"8 is used. It can also be seen that the model results match the experimental results reasonably well, at
1042
A. Auvinen et al.
least below the temperature ¹"7003C. After this temperature it is possible that the evaporated vapour reacts with the tube walls and the detector also sees a signal from this reacted CsOH. CO NC LU SIO N S
An experimental facility was constructed for "ssion product vaporisation rate measurements to use with synthetic radiolabelled samples. This was the "rst study, where the vaporisation of CsI and CsOH was measured in conditions comparable to the primary circuit. The facility was proven to function well. Quantitative data on CsI vaporisation with three di!erent #ow rates are obtained in a temperature range from 500 to 7503C. Above this temperature the samples were almost completely vaporised. The behaviour of CsOH samples was found out to be more complex. In addition to vaporisation, the movement of liquid CsOH on the stainless-steel surface was evident. As a possible implication to this, cesium may well accumulate in a di!erent place than it is deposited in a primary circuit. Despite the sample movement, at least qualitative vaporisation rates for CsOH were obtained. The results from the "rst two experiments agreed well with each other. Quantitative data about the deposition mechanisms in the experimental line could be obtained from the post-test measurements. The most important mechanism was condensation on the processing tube. The amount of condensed material was very much dependent on the applied steam #ow rate. With the lowest #ow rate a far greater fraction of the sample was condensed than with higher #ow rates. This was due to a di!erent temperature gradient at the outlet of the processing tube. As a secondary deposition mechanism, cesium reacted from gas phase with the hot surface of the processing tube. The amount of cesium in this uniform layer was approximately 3.4 g m\. Reacted cesium did not vaporise below the temperature of 8003C. When the temperature of the furnace was increased up to 10003C, the deposited layer had vanished. The #ow, temperature and concentration "elds of the system were simulated numerically with the FLUENT CFD-software. The results from CsOH vaporisation experiments agreed well with FLUENT modelling in the temperature range from 500 to 7003C. The sample movement and the reaction with the processing tube in#uenced these results though, especially at high temperatures. A simple engineering calculation agrees almost perfectly with the FLUENT results, if a constant value for the Sherwood number is used. Such correlations are especially useful for computer codes like RAFT, VICTORIA and MELCOR that cannot use much computing e!ort for single phenomena. The CsI experiments are currently being modelled with FLUENT software. Another #ow furnace facility, based on this experimental set-up, has also been assembled. In that facility, revaporisation of "ssion products from samples obtained from Phebus FPT-1 experiment will be studied. The furnace is reduced in size to avoid excessive lead shielding weight and to make handling of the process tube in a glove box practicable. However, the experimental conditions like gas composition, gas #ow rates and temperature ramp rate remain as close as possible to those described in this paper. Acknowledgements*We would like to acknowledge the people at the VTT Aerosol Technology Group for advice and ideas regarding the experimental set-up. This work has been supported by the European Community via contract FI4S-CT96-0019.
REF ER E NCE S Alonso, A., Carretero, T., Ezquerra, F. and Hontanon, E. (1993) State of the art report on revaporisation of the RCS condensate under LWR accident conditions. CTN-56/93. Devell, L. and Johansson, K. (1994) Speci"c features of cesium chemistry and physics a!ecting reactor accident source term predictions. ISSN 1104-1374, ISRN SKI-R-94:29-SE, NEA/CSNI/R(94)28. Dickinson, D., Hilliard, R., Muhlestein, L., Mechan, D. and Carraro, G. (1987) Aerosol Behavior in LWR containment bypass piping*results of LACE test LA3. LACE TR-011, 1987.
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Donahue, M., Hazzan, M., Metcalf, J. and Warman, E. (1986) Source term evaluation for accident conditions*an analysis of retention/revaporisation in BWR mark-II power plant. IAEA-SM-281/23. European Commission (1995) Plant assessments, identi"cation of uncertainties in source term analysis. In Nuclear science and technology. EUR Report 16502 EN, ISSN 1018-5594, ISBN 92-827-5284-4, C9071/PWR/93/REP/ 001 ST(93)-P35 Issue C Rev. 1. Fluent Inc. (1996) Fluent user's guide, release 4.4. Heames, T. J. et al. (1992) VICTORIA: a mechanistic model of radionuclide behavior in the reactor coolant system under severe accident conditions. USNRC Report NUREG/CR-5545 Rev. 1. Im, K. H., Ahluwalia, R. K. and Chuang, C. F. (1985) RAFT: a computer model for formation and transport of "ssion product aerosols in LWR primary systems. Aerosol Sci. Technol. 4, 125. Incropera, F. P. and DeWitt, D. P. (1996) Fundamentals of Heat and Mass Transfer. Wiley, New York. Jokiniemi, J. (Ed). (1993) Physical and chemical characteristics of aerosols in the containment. NEA/CSNI/R(93)7. Krischer, W. and Rubinstein, M. C. (1992) The Phebus Fission product Project*Presentation of the Experimental Programme and Test Facility. Elsevier Applied Science, London. Lindholm, I., SjoK vall, H., Jokiniemi, J. and MaK kynen, J. (1993) A comparative study of "ssion product behaviour in TVO I nuclear power plant. VARA-15/93, CEC/RCA-ST(95)-P169. Nikkinen, M. and Aarnio, P. (1995) Sampo 90 V. 3.4-3.5 Manual. Helsinki University of Technology. Sugimoto, J., Kajimoto, M., Hashimoto, K. and Soda, K. (Eds). (1994) Short overview on the de"nition and signi"cance of the late phase "ssion product aerosol/vapor source. NEA/CSNI/R(94)30. Summers, R. M. et al. (1993) MELCOR 1.8.2: a computer code for nuclear reactor severe accident source term and risk assessment analyses. USNRC Report NUREG/CR-5531. U.S. Nuclear Regulatory Commission, (1975) Reactor safety study*an assessment of accident risks in U.S. commercial nuclear power plants. WASH-1400, NUREG-75/014. U.S. Nuclear Regulatory Commission, (1990) Severe accident risks: an assessment for "ve U.S. nuclear power plants. NUREG-1150. Wright, A. L. (Ed). (1994) Primary system "ssion product release and transport. NUREG/CR-6193, NEA/CSNI/R(94)2, ORNL/TM-12681.