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Size distribution of aerosols released from heated LWR fuel rods
~ Pergamon
PU: 80021-8502(96)00306-0
J. AfflJsol Sci•• Vol. 27. Suppl. I. pp. 5467-5468. 1996 Copyrighl il:1 1996 Ebc:vier SciC1lCC Ltd Pnnled ,n Gmal Bril&in. An "Shu reoerved
0021-8502/96 SIS.OO + 0.00
SIZE DISTRIBUTION OF AEROSOLS RELEASED FROM HEATED LWR FUEL RODS
A. C:itrovszky, J. Freeska*, P. Janl, t: Matus*, A. Nagy RESEARCH INSTlTUI'E FOR SOLIDSTATE PHYSICS H-1525.BUDAPEST, P.O.BOX49, HUNGARY -RESEARCH INSTlTlJI'EFOR ATOMIC ENERGY H-1525,BUDAPEST, P. O. BOX 49, HUNGARY
INfRODUcnON
Should a nuclear accident occur, damaged high temperature fuel rods release aerosols, whose fonnation, size distribution and concentration are important factors when analysing the source term conditions of the accident. In this articlewe describe our investigations for modelling these phenomena. Theconcentration and sizedistribution of aerosols released by a simulated accident wereinvestigated by a laserparticle counter, by scanning electron microscopy and by chemical analysis of aerosols collected on sampling impactor targets. The consistency of the results obtained by different measurements demonstrates the convenience of the particular methods applied. The fuel rod samples contained some of the main chemical species offissionproducts formed up to 40 MWd/kgUbum-up level. EXPERIMENTAL The experimental setup consists of a special high temperature furnace with a graphite crucible containing the fuel rod samples. The area above the crucible was connected to a I m long quartz pipe. The sampling inlet of the airborne particle counterwas connected to the end of the pipe. The length of the sampling tube was 0.4 m. Changeable impactors within the pipe gave the possibility for sampling aerosols on target surfaces. The temperature was increased stepwise and was simultaneously monitored by a pyrometer and a thermocouple. The fuel rod samples were placed in the graphite crucible and heated up to 1600DC in an Ar flow. Aerosol release during the heating was measured in the Ar stream at the inlet of the pipe. Size distribution and concentration of the aerosol particles were measured continuously in automatic regime by an APC-Q3-2 laser light scattering single particle counter [1] and by rapidly changeable impactor targets. The duration of the measurement cycle of the particle counter was 20s, the time interval between measurements was 0.5s. The device presented results in 5 size ranges from 0.3 to 10 urn and in 1 sizerangehigherthan 10 urn. After stepwise temperature changing the aerosol concentration at first increased quickly and after a maximum decreased to about 1/3 of the highest value. The probability density function was estimated by computer evaluation of the data. Since the lower size limit of this device is 0.3 um, the size distribution below this limit was calculated by extrapolation down to 0 by usingthe probability density function. The size distribution obtained from the particle counter was in good agreement with the distribution obtained from electron microscope analysis of the aerosols collected on the sampling impactor targets. The heating and cooling of the samples during the measurements wereperformed in a similar way. The concentration of aerosols during the heating period changed by more than 5 orders of magnitude. Herewe can utilize the benefits of the APC-03-2 particle counter, viz. the possibility of measuring the concentration in an extremely wide range (up to 106 particles/litre) with high sampling efficiency without the considerable contamination of the testingchamber, having a double wall nozzle with aerodynamic focusing of the aerosol to be tested by purged air stream [2]. The typical temperature dependence of total aerosol concentration and concentration for various particle sizes was published elsewhere [3]. Ion-ehromatographic measurements of aerosols collected on sampling impactor targets showed that Cs and I dominated the release in the temperature region 900 1200 °C. The aerosol concentration maxima around 1000 DC correspond to these elements. The main aims of the articlewas to analyse the sizedistribution of the particles for various periods of the heating and cooling cycles. The data for size distribution measurements were sorted into 5 temperature regions (the groups of similar size distribution) during the heating period and one range during the cooling period. Within these regions we calculated the average values of the number of particles and concentration, those of the mass of the particles. of the fraction mass, and of the release velocity in number of particles/min and in g/min. When calculating the mass of the particles we supposed an average density of 1.5g/cm' andthe average diameter ofthe particles. 5467
S468
Abstracts of the 1996 European Aerosol Conference
In Fig. 1 the differential number of particles dN/dO is plottedfor each temperatureregion between 700 and 1600 ·C, where N is the number of particles and 0 is the diameter as a fimction of the size in
logarithmic scale. From these figures we can see that the size distribution has two ranges with different slope and their intersection is around 2 Jim. An explanation for the change of the slope can be the difference in the structure (and sometimes in origin) of smaller and larger particles [4,5]. The average approximation error of these curves is about 20 %. It is mentioned that the size distribution slightlychanged during the heating even as a function of the releaserate of aerosols and the temperature insidethe above mentioned regions. ieoo
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Fig. I. Thedifferential number ofparticlesas a function of the size for different temperature regions. CONCLUSION
Analysis of the size distribution of aerosols formed during the heating of LWR fuel rod samples in different temperature regions showed that their size distribution can be represented by two exponential functions with different slope having an intersection around 2 Jim. In each temperature region the same size distribution function is valid for low and high concentration of the aerosols. The average approximation error is about 20%. Chemical analysis provedthat the composition of aerosols in the 900 - 120Q·C temperature regionis dominated by Cs and L REFERENCES 1. A Czitrovszky, P. Jani, New design for a light scattering airborne particle counter and its application. (1993) Optical Engineering. vol. 32, No 10, pp. 2557- 2562. 2. A Czitrovszky, P. Jani, Application examples of APC-03-2 and APC-Q3-2A Airborne Particle Counters in a highly contaminated environment. Journal ofAerosol Science, vol. 26s, 1995. 3. A Czitrovszky, J. Frecska, L. Matus, P. Jani, Investigation of aerosol released from heatedLWRfuel rod and its properties, Proceedings of IV. International Aerosol Cont:, Los Angeles, pp. 784- 785, 1994. 4. O.K. Verma. A Sebestyen, JoA Julian, D.C.F. Muir, 0.5. Shaw,RMacDougalI, Particlesize distribution ofan aerosol and its sub-fractions, Ann. occup. Hyg., vol. 38, No. I, pp. 45-58,1994. 5. D.W. Dockery et al., An association between air pollution and mortality in six US cities. N. Engl. J. Med. , 1993, vol. 329, pp. 1753-1759.
PU: 80021-8502(96)00306-0
J. AfflJsol Sci•• Vol. 27. Suppl. I. pp. 5467-5468. 1996 Copyrighl il:1 1996 Ebc:vier SciC1lCC Ltd Pnnled ,n Gmal Bril&in. An "Shu reoerved
0021-8502/96 SIS.OO + 0.00
SIZE DISTRIBUTION OF AEROSOLS RELEASED FROM HEATED LWR FUEL RODS
A. C:itrovszky, J. Freeska*, P. Janl, t: Matus*, A. Nagy RESEARCH INSTlTUI'E FOR SOLIDSTATE PHYSICS H-1525.BUDAPEST, P.O.BOX49, HUNGARY -RESEARCH INSTlTlJI'EFOR ATOMIC ENERGY H-1525,BUDAPEST, P. O. BOX 49, HUNGARY
INfRODUcnON
Should a nuclear accident occur, damaged high temperature fuel rods release aerosols, whose fonnation, size distribution and concentration are important factors when analysing the source term conditions of the accident. In this articlewe describe our investigations for modelling these phenomena. Theconcentration and sizedistribution of aerosols released by a simulated accident wereinvestigated by a laserparticle counter, by scanning electron microscopy and by chemical analysis of aerosols collected on sampling impactor targets. The consistency of the results obtained by different measurements demonstrates the convenience of the particular methods applied. The fuel rod samples contained some of the main chemical species offissionproducts formed up to 40 MWd/kgUbum-up level. EXPERIMENTAL The experimental setup consists of a special high temperature furnace with a graphite crucible containing the fuel rod samples. The area above the crucible was connected to a I m long quartz pipe. The sampling inlet of the airborne particle counterwas connected to the end of the pipe. The length of the sampling tube was 0.4 m. Changeable impactors within the pipe gave the possibility for sampling aerosols on target surfaces. The temperature was increased stepwise and was simultaneously monitored by a pyrometer and a thermocouple. The fuel rod samples were placed in the graphite crucible and heated up to 1600DC in an Ar flow. Aerosol release during the heating was measured in the Ar stream at the inlet of the pipe. Size distribution and concentration of the aerosol particles were measured continuously in automatic regime by an APC-Q3-2 laser light scattering single particle counter [1] and by rapidly changeable impactor targets. The duration of the measurement cycle of the particle counter was 20s, the time interval between measurements was 0.5s. The device presented results in 5 size ranges from 0.3 to 10 urn and in 1 sizerangehigherthan 10 urn. After stepwise temperature changing the aerosol concentration at first increased quickly and after a maximum decreased to about 1/3 of the highest value. The probability density function was estimated by computer evaluation of the data. Since the lower size limit of this device is 0.3 um, the size distribution below this limit was calculated by extrapolation down to 0 by usingthe probability density function. The size distribution obtained from the particle counter was in good agreement with the distribution obtained from electron microscope analysis of the aerosols collected on the sampling impactor targets. The heating and cooling of the samples during the measurements wereperformed in a similar way. The concentration of aerosols during the heating period changed by more than 5 orders of magnitude. Herewe can utilize the benefits of the APC-03-2 particle counter, viz. the possibility of measuring the concentration in an extremely wide range (up to 106 particles/litre) with high sampling efficiency without the considerable contamination of the testingchamber, having a double wall nozzle with aerodynamic focusing of the aerosol to be tested by purged air stream [2]. The typical temperature dependence of total aerosol concentration and concentration for various particle sizes was published elsewhere [3]. Ion-ehromatographic measurements of aerosols collected on sampling impactor targets showed that Cs and I dominated the release in the temperature region 900 1200 °C. The aerosol concentration maxima around 1000 DC correspond to these elements. The main aims of the articlewas to analyse the sizedistribution of the particles for various periods of the heating and cooling cycles. The data for size distribution measurements were sorted into 5 temperature regions (the groups of similar size distribution) during the heating period and one range during the cooling period. Within these regions we calculated the average values of the number of particles and concentration, those of the mass of the particles. of the fraction mass, and of the release velocity in number of particles/min and in g/min. When calculating the mass of the particles we supposed an average density of 1.5g/cm' andthe average diameter ofthe particles. 5467
S468
Abstracts of the 1996 European Aerosol Conference
In Fig. 1 the differential number of particles dN/dO is plottedfor each temperatureregion between 700 and 1600 ·C, where N is the number of particles and 0 is the diameter as a fimction of the size in
logarithmic scale. From these figures we can see that the size distribution has two ranges with different slope and their intersection is around 2 Jim. An explanation for the change of the slope can be the difference in the structure (and sometimes in origin) of smaller and larger particles [4,5]. The average approximation error of these curves is about 20 %. It is mentioned that the size distribution slightlychanged during the heating even as a function of the releaserate of aerosols and the temperature insidethe above mentioned regions. ieoo
A
100
01
'-1\'
.00
10000
D
'000
10
0 .00
"'". z
-.
<.
2.00
-.
' .00 100 100 dlome"!" Cum)
1000
000
100
.&00
100
dtO""etf"
100
Cum)
1580-500'C
o
'00'
100 1000
(uml
COohn9 petlod
"~:~,o~r:~·o '"
,- ~ ~ ~
600
d.,.......
"
1200
10000
'000
F
'00 10 10
01
1
o.oot--:'!::--':"T."--:'!::'""-:'!~=~::'\.· 0.00
Fig. I. Thedifferential number ofparticlesas a function of the size for different temperature regions. CONCLUSION
Analysis of the size distribution of aerosols formed during the heating of LWR fuel rod samples in different temperature regions showed that their size distribution can be represented by two exponential functions with different slope having an intersection around 2 Jim. In each temperature region the same size distribution function is valid for low and high concentration of the aerosols. The average approximation error is about 20%. Chemical analysis provedthat the composition of aerosols in the 900 - 120Q·C temperature regionis dominated by Cs and L REFERENCES 1. A Czitrovszky, P. Jani, New design for a light scattering airborne particle counter and its application. (1993) Optical Engineering. vol. 32, No 10, pp. 2557- 2562. 2. A Czitrovszky, P. Jani, Application examples of APC-03-2 and APC-Q3-2A Airborne Particle Counters in a highly contaminated environment. Journal ofAerosol Science, vol. 26s, 1995. 3. A Czitrovszky, J. Frecska, L. Matus, P. Jani, Investigation of aerosol released from heatedLWRfuel rod and its properties, Proceedings of IV. International Aerosol Cont:, Los Angeles, pp. 784- 785, 1994. 4. O.K. Verma. A Sebestyen, JoA Julian, D.C.F. Muir, 0.5. Shaw,RMacDougalI, Particlesize distribution ofan aerosol and its sub-fractions, Ann. occup. Hyg., vol. 38, No. I, pp. 45-58,1994. 5. D.W. Dockery et al., An association between air pollution and mortality in six US cities. N. Engl. J. Med. , 1993, vol. 329, pp. 1753-1759.