Study on models for mean diameter of aerosol particle for analysis of radionuclide behaviour inside containment

Ann. Nucl. EnergyVol.23, No. 13, pp. 107%1090, 1996

Pergamon

Copyright © 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0306-4549/96$15.00+ 0.00

0306-4549(95)00119-0

STUDY ON MODELS FOR MEAN DIAMETER OF AEROSOL PARTICLE FOR ANALYSIS OF RADIONUCLIDE BEHAVIOUR INSIDE CONTAINMENT

J. S. BAEK, J. Y. HUH, N. H. LEE, J. H. JEONG, J. H. CHOI

Korea Atomic Energy Research Institute 150 Dukjin-dong, Yusong-gu Taejon, South Korea 305-353

(Received 21 October 1995)

Abstract

When high enthalpy liquid is discharged into a containment, thermal fragmentation is a dominant mechanism for the dispersion of liquid into droplets. The current methodfor aerosol size estimation in the SMART code used for CANDU containment analysis, however, results in too small aerosol diameter because it considers only aerodynamic atomization with very fast discharging velocity. The smaller the aerosol diameter, the less is the effect of aerosol removal mechanisms. Therefore, the amount of aerosol released into environment for some of the containment isolation failure cases and the resulting dose values are very conservative. Among several models to predict the drop diameter for a high enthalpy liquid jet, an appropriate model (Koestel, Gido and Lamkin model) has been selected for aerosol size calculation and incorporated in SMART code. This updated SMART code has been assessed for WALE (Water Aerosol Leakage Experiments)for the code verification. The calculated aerosol amount released into environment is still significantly higher than the experimental value but much lower compared to those predicted by non-updated (original) SMART code. Some of CANDU DBAs (Design Basis Accidents) have been analyzed by using updated and original SMART codes. The comparison of the results shows that the amount of each radionuclide isotope released into outer atmosphere is significantly reduced with the updated SMART code. Copyright © 1996 Elsevier Science Ltd

1. INTRODUCTION The LOCA (Loss Of Coolant Accident) with containment impairment case is one of the CANDU DBAs. rhe predicted amount of radionuclide released into environment by SMART computer code [Quraishi (1991)] seems too high for most LOCA with the containment impairment cases. This is mainly due to the very small liquid droplet generation by SMART when the high enthalpy primary system coolant is discharged into the :ontainment. If the airborne liquid droplets are very small, the removal mechanism becomes less effective. So, there is more chance for the smaller aerosol particles to remain inside containment without being removed and to be released into environment with activity. This will affect the public dose calculation directly. 1079

1080

J.S. Baek et

al.

The SMART code considers only aerodynamic atomization with very fast discharging velocity in calculation of mean diameter of the aerosol droplet. However, when high enthalpy liquid is discharged into a containment, thermal fragmentation is a dominant mechanism rather than aerodynamic atomization for the dispersion of liquid into droplets. Several models to predict the droplet diameter caused by thermal fragmentation have been examined. Among them, Koestel, Gido and Lamkin model has been selected and implemented in the SMART code. This updated SMART code has been verified by simulating the WALE [Fluke (1990)] and tested for the total loss of the containment isolation cases for a large LOCA with loss of Class IV power, feeder stagnation break and end fitting failure which are CANDU DBAs. In addition, diffusive deposition and turbulent coagulation models are incorporated in SMART code to give more effective aerosol deposition and coagulation and, therefore, to reduce the aerosol release to environment. 2. I N V E S T I G A T I O N OF C U R R E N T MODELS F O R D R O P L E T SIZE P R E D I C T I O N There are several papers which present models to predict the aerosol droplet size for jet flashing [Brown (1962), Eran (1977), Koestel (1980)]. Also, there is a paper which presents the correlation to predict diameter of droplet for bulk flashing [Anderson (1984)]. In this section, the three models available for jet flashing are investigated. 2.1 Brown and York Model

An experiment was conducted and a model to predict the droplet size was presented by Brown and York 1962 [Brown (1962)]. Their equation is as follows: Dmean = 1840 - 5.18T Nwe where,

T

(2-1)

= liquid jet temperature, °F

Nwe =

pgV2d/t~ = Weber number

pg =

density of gas phase surrounding jet

V

=

velocity of jet relative to gas medium

d

=

diameter of liquid jet

a

=

surface tension

If the liquid jet temperature is higher than 355.2°F (180°C), Equation (2-1) will give negative mean diameter. Therefore, this equation can not be used for high superheat liquid jet. 2.2 Sher and Elata Model

They obtained a model to predict the mean droplet size theoretically and performed an experiment to confirm their model [Eran (1977)]. Their model is expressed as follows: 2-2 Dmean

"=

\--~f ][C~iJ

where,

2 4 ~4--'~

surface tension, N/m pf =

density of liquid phase, kg/m 3

hfg= =

enthalpy difference between gas and liquid, J/kg

M =

molecular weight of gas, kg/mole

average value of the absolute pressure inside and outside nozzle, Pa

(2-2)

1081

Radionuclide behaviour inside containment Cp

specific heat at constant pressure of liquid, J/kg-K

T

absolute average temperature of liquid and saturation temperature at ambient pressure, K

R

universal gas constant ( = 8.314 J/mole-K)

~f =

thermal diffusivity of liquid, m2/s

O'g =

geometric standard deviation (=1.3 !)

V = AP =

dimensionless coefficient (plotted in Reference 5) pressure difference between inside and outside nozzle, Pa

Suppose that high enthalpy liquid with a temperature of 304 °C and pressure of 90.873 bar liquid is discharged into containment which is occupied by 100 °C saturated steam (very similar to the test condition of Fluke's experiment). The calculation has been conducted and the predicted mean diameter is too large to be applicable for this condition. There may be some misunderstanding in formulating the equation.

2.3 Koestel, Gido and Lamkin Model They developed a model to predict drop diameter for superheat liquid jet based on themodynamics and the phenomenon of free-surface generation as triggered by nucleation and calculated 16 I.tm of drop diameter for LOCA condition of PWR [Koestel (1980)]. They presented two correlations for drop size prediction. One is for heterogeneous nucleation and the other is for homogeneous nucleation. Here, both correlations are investigated and compared with experiment data.

2.3.1 Heterogeneous Nucleation Model The final equation in the Koestel, Gido and Lamkin's heterogeneous nucleation model [Koestel (1980)] is . cpBTf~[3 ~ \-~/1345'~ ~ l - - ~ f g j [~ IAorificeaf} ~---~-)

Dlnean

where,

Cp =

specific heat at constant pressure of liquid, J/kg-K

B

=

(Tf---T)/Tf = 0.07 [Koestel ( 1980)]

Tf

=

liquid temperature, K

T

=

absolute fragmentation temperature, K

hfg=

(2-3)

enthalpy difference between gas and liquid, J/kg

Doriflce = orifice diameter through which flow emerges, m o~f -- thermal diffusivity of liquid, m2/s =

C

=

maximum heat flux at the fragmentation temperature, W/m 2 coefficient determined from Brown and York correlation = 0.10-0.0030×Nwe forNwe< 12.5 = 0.058 - 0.0021 ×Nwe for Nwe > 12.5

The atomization rate is much slower than that of thermal fragmentation, therefore, thermal breakup is assumed to dominate and C =0.10 rrgs 1/2 in computing the void fraction at the point of dispersion [Koestel (1980)]. Nwe is Weber number described in Section 2.1. q is given in Figure B.4 of Koestel's paper (1980) and is attached as Figure 1.

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J.S. Baek et al.

2.3.2 Homogeneous Nucleation Model The final equation in the Koestel, Gido and Lamkin's homogeneous nucleation model [Koestel (1980)] is Z

Dmean =

where,

I-

cpBTf

6

~

~ ~f

(2-4)

Cp =

specific heat at constant pressure of liquid, J/kg-K

B

(Tf--T)/Tf= 0.07 [Koestel (1980)]

=

liquid temperature, K

Tf = T

=

absolute fragmentation temperature, K enthalpy difference between gas and liquid, J/kg

hfg=

(xf = thermal diffusivity of liquid, m2/s Num = bubble population for homogeneous nucleation at Tf, number/m 3 C

=

coefficient determined from Brown and York correlation ( = 0.1)

Num is given in Figure B.3 of Koestel's paper (1980) and is attached as Figure 2. BOIUN

0FRAGMI~NTATION|

TI~MP~RATURE

SSo

(K)

E

~,oi

"~ ' ~1o~

°

to

I

i

i

i

,

,

4

BOtUING

(FRAGME[NTATION)

, G

,

,

,

•

TEMPERATUR

IO 3

i

('F)

i

,/~

i

t

t

i

I ~5o

T~'MPER.~TURF

Figure 1 Experimentally determined Maximum Heat Flux Values [Koestel (1980)]

I

,

,

•

i

,

600 t'F}

Figure 2 Experimental Homogeneous Nucleation Site Population Density [Koestei (1980)]

2.3.3 Drop Size Calculation Using Koestel, Gido and Lamkin Model Several mean diameters have been calculated by using Koestel, Gido and Lamkin model. The calculated diameter was compared to its experimental data. 2.3.3.1 Heterogeneous Model a. Brown and York's Experiment In this case, diameter of nozzle is 0.001 m, pressure is 3.7172 bar, temperature is 141 °C and Weber number is 11.3. The fragmentation temperature is given to be 385 K (112 °C) from the following relation: B = 0.07 = 141 + 2 7 3 - T 141 + 273 Cp, q, hfg and ctf are given from the steam table at 112 °C. The mean diameter has been calculated to be 27.8 lam using Equation (2-3). The experimental data gives the mean diameter as about 34.7 ~m. Therefore, the above equation underestimates the droplet diameter.

Radionuclide behaviour inside containment

1083

b. Fluke et al. Experiment The high enthalpy liquid with temperature of 304 °C and high pressure of 90.873 bar is discharged into containment occupied by 100 °C saturated steam. The diameter of nozzle seems to be 0.0037 m. In this case, the fragmentation temperature and mean diameter have been calculated to be 537 K (264 °C) and 19.6 p.m using Equation (2-3). The experimental data shows the mean diameter to be between 18 and 20 p.m. c. Large L O C A with Loss of Class IV Power Case of CANDU 6 Reactor For 100% ROH (Reactor Outlet Header) case, the break area is assumed to be 0.2594 m 2 and the initial liquid enthalpy seems to be 1407 kJ/kg. Therefore, the equivalent break diameter becomes 0.575 m and the discharge temperature is about 311 °C (584 K). The fragmentation temperature and mean diameter have been have been calculated as 534 K (270 °C) and 98.6 gm using Equation (2-3). The predicted diameter seems to be too large for this case. At high temperature above 576 K, homogeneous nucleation can become dominant [Koestel (1980)]. The mean diameter will be calculated again using homogeneous nucleation model in the following section. 2.3.3.2

Homogeneous Model

a. Brown and York's Experiment The experiment conditions are described in Part a of Section 2.3.3.1. The mean diameter can not be calculated using homogeneous model because there is no homogeneous nucleation at low temperature. Therefore, the bubble population data is given for temperature above 520 K (247 °C) in Koestel's paper [Koestel (1980)]. Heterogeneous nucleation is dominant for water except at very high temperature [Collier (1991)]. b. Fluke et al. Experiment The experiment conditions are described in Part b of Section 2.3.3.1. The fragmentation temperature and mean diameter have been calculated as 537 K (264 °C) and 23.2 Ixm using Equation (2-4). The experimental data shows the number mean diameter to be between 18 and 20 gm. The homogeneous model overestimates the mean diameter slightly at this condition. c. Large L O C A with Loss of Class IV Power Case of CANDU 6 Reactor Discharge conditions for 100% ROH break case are described in Part c of Section 2.3.3.1. The fragmentation temperature and mean diameter have been calculated as 543 K (270 °C) and 20.5 gm using Equation (2-4). The homogeneous model predicts much smaller mean diameter compared to the heterogeneous model for this condition. 2.3.4 Discussion about Koestel, Gido and Lamkin Model As described above, there are two models of Koestel, Gido and Lamkin's correlation to predict the mean diameter of drop; heterogeneous and homogeneous models. It should be noted that the correct model has to be selected according to discharge condition. The heterogeneous model underestimates the mean drop diameter for low superheat liquid discharge. The homogeneous model can not be used for low superheat liquid discharge because the bubble generation is caused by heterogeneous nucleation and there is no bubble nucleation number data at low temperature. At high temperature above 576 K, however, the homogeneous nucleation plays the dominant role [Koestel (1980)]. 3. A D D I T I O N A L A E R O S O L REMOVAL AND C O A G U L A T I O N MECHANISMS The new method for the calculation of the mean diameter has been implemented in the SMART code. Three additional correlations related to aerosol removal and coagulation have also been incorporated. The three correlations are the diffusive deposition mechanism and the turbulent shear and inertial coagulation. These correlations are presented in the following sections.

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J . S . Baek et al.

3.1 Diffusive Deposition The SMART code considers two aerosol deposition mechanisms, namely gravitaional settling (or sedimentation) and diffusophoretic removal (Stefan flow). However, one of the aerosol deposition mechanism results from diffusion of aerosol in a concentration gradient, that is, from a higher to a lower concentration region. The diffusive deposition velocity is given by [Washington (1991)] oTCm Vdiff = 3~p.DpA

(3-1)

Boltzman constant (1.38 × 10-23 J/s-m2-K 4)

where, T

=

absolute temperature (K)

Ix

=

viscosity of air (N-s/m 2)

Dp =

diameter of particle (m)

A

diffusion boundary layer thickness (m) (assume ~-- 10-5 m)

=

Cm = l+¢tNkn =

Cunningham slip correction, which reduces the Stokes drag force to account for noncontinuum effects,

Nkn =

2~L/Dp =

~.

mean free path (m)

=

Knudson number

A+B exp(C/Nkn) A =

1.257

B =

0.4

C =

-1.1

The removal rate constant due to diffusive deposition, ~diff, is given by ~'diff -where

VdiffA V

(3-2)

A

=

settling area (m 2)

V

=

nodal volume (m 3)

3.2 Turbulent Coagulation The SMART code considers two coagulation mechanisms: coagulation due to Brownian motion of aerosol particles and coagulation due to different settling velocities in a gravitational field. Models for turbulent shear coagulation and turbulent inertial coagulation, which are very effective for the range 1 to 100 Ixm, are described as follows [Washington (1991)] : 3.2.1 "Ihrbulent Shear Coagulation The turbulent shear coagulation is represented by EQai r

3

(3-3)

Radionuclide behaviour inside containment where,

1085

coagulation kernel for turbulent shear coagulation £

=

turbulence dissipation rate (m2/s 3) [assume = 10-3 m2/s 3, Washington (1991)]

Pair =

air density (kg/m 3)

Ix =

viscosity of air (N-s/m 2)

D

aerosol particle diameter (m)

=

3.2.2 Turbulent Inertial Coagulation The turbulent inertial coagulation is given by _1

o.,4,(o, + where,

c

(3-4)

13i =

coagulation kernel for turbulent inertial coagulation

D

aerosol particle diameter (m)

=

turbulence dissipation rate (m2/s 3) [assume ~ 10 -3 m2/s 3, Washington (1991)] Pair

=

air density (kg/m 3)

V

=

settling velocity of aerosol particle (m/s)

C

=

1.5[min(Di, Dj)/( Di+Dj)] 2, collision efficiency

3.2.3 Combination of Turbulent Coagulation Kernels The total turbulent coagulation kernel,

~tur,

is given by (3-5)

4. M O D I F I C A T I O N OF SMART CODE The SMART code predicts the radionuclide behaviour inside containment and release of activity into the environment using the thermohydraulic transients calculated by other containment thermohydraulic computer code. The mean diameter and distribution of aerosol drops is also calculated by SMART code. It has been modified to incorporate the new mean diameter calculation mechanism and by adding the diffusive deposition model and the turbulent shear and inertial coagulation models. 4.1 Incorporation of the Model for Mean Diameter of Aerosol Particles Koestel, Gido and Lamkin model (heterogeneous and homogeneous models) have been implemented. As discussed in Section 2.3.3, the drop diameter is calculated using both models. One subroutine of SMART code, has been modified to calculate mean diameter of aerosol particles and additional three subroutines have been created to calculate the thermal properties, maximum heat flux and bubble number which are used in Koestel, Gido and Lamkin model. When drop mean diameter is calculated using Koestel, Gido and Lamkin model, the discharge enthalpy is assumed as a saturated liquid water enthalpy even though it may be two phase. This is a conservative assumption in a view of aerosol generation.

1086

J.S. Back et

al.

Also, a drop diameter is calculated using aerodynamic atomization model expressed by ; Weo

(4-1)

Daerodynami c -- Q g V 2

We = Weber number ( = 12)

where,

= surface tension, N/m pg = density of gas phase surrounding jet, kg/m 3 V

= velocity of jet relative to gas medium, m/s

V is calculated using the mass flow rate, break area and specific volume provided in the discharge input data from the thermalhydraulic calculation of primary heat transport system (this method is different from original one of SMART code). That is, V ~ m Gv

(4--2)

ABreak where,

G

=

mass flow rate, kg/s

v

=

specific volume, m3/kg

ABreak =

break area, m 2

The predicted drop diameters by the above three correlations (homogeneous and heterogeneous thermal fragmentation models and aerodynamic atomization model) are compared and the smallest diameter is selected as the final mean diameter to distribute aerosol using log-normal disrtibution shape with 1.4 standard deviation.

4.2 Incorporation of the Models for Diffusive Deposition and Turbulent Coagulation In order to consider these mechanisms, the models described in Section 3 have been incorporated in SMART code by modifying two subroutines. The effects of these mechanisms have been examined separately. 4.3 Simple Validation of Updated S M A R T Code The original SMART code had been verified already [Quraishi (1992)]. The updated SMART code have been assessed by simulating the WALE [Fluke (1990)]. Detailed description about WALE is omitted here. Figure 3 and 4 show the carry-over fractions for typical cases of which discharge pressures are 10 MPa and 5 MPa. The carry-over is defined as the leakage of water aerosol transported with the steam flow. In the legend, 'original', 'coag+depo', diameter' and 'all' represent the results predicted by original model, by considering only additional coagulations and diffusive deposition, by considering only new model of mean droplet diameter and by considering all of them, respectively. If it can be said that the equilibrium values of predicted carry--over fractions are reached at the end of simulation, the carry--over fraction is much reduced by the updated code rather than the original code but still very high compared to the experiment values, about the maximum of 3% carry-over fraction [Fluke (1990)]. And it can be also seen that the effects of the additional diffusive deposition and the turbulent coagulations are very small compared to the effect of the mean diameter of the aerosol particles.

Radionuclide behaviour inside containment 50

50

4O-

40

1087

"~ 30tl.

t~ 2~

c?

¢. ~ " ~ - . ~

_~_

2o-

. original coag+depo--- olameler all_ _ . __

original coag+depo--~ dtameter~ all~ . _ _

560

560

iobo

~sbo 20bo 25'00 Time (s) Figure 3 Carry-Over Fractions of Water Aerosol for 10MPa and 311"C Discharge

lobo

t5~

2obo

25'00

3000

Time (s)

Figure 4 Carry-Over Fractions of Water Aerosol for 5MPa and 263.9°C Discharge

As a result of the calculation for the verification of the updated SMART code, it can be said that even though the carry-over fraction has been much reduced with updated SMART code, there is still large conservatism for it compared to the experiment data. 5. C A L C U L A T I O N S F O R S O M E O F C O N T A I N M E N T I S O L A T I O N F A I L U R E CASES By using the updated SMART code, the following three cases have been assessed; the total loss of the containment isolation cases for a large LOCA with loss of Class IV power, feeder stagnation break and end fitting failure of typical CANDU 6 reactor. The containment has been modelled using 14 nodes and 37 links. The description about the nodalization and calculation method are not given here but J. S. Back's report explains them in detail. The results have been compared with the results predicted by the original version. First the droplet size has been compared for a large LOCA with loss of Class IV power. Figure 5 shows the droplet diameters using the three aerosol generation models (heterogeneous and homogeneous thermal fragmentation and aerodynamic atomization). During the initial period, the calculated diameter using the Koestel, Gido and Lamkin's heterogeneous and homogeneous correlations seems to be very similar to the diameters calculated in Sections 2.3.1 and 2.3.2. From this figure, it can be seen that homogeneous model gives the smallest diameter until around 30 seconds and after that time aerodynamic atomization model predicts the smallest one. In this figure, the sudden increase of the mean diameter to I0 m stands for that the diameter can not be calculated by using this specific model. 100000013

lOOOOtX~ Heterogeneous Homogeneous - - ~ Aerodynamic - -

I000000~ looooo

orig,oal--

.o

loooo-

r,

ioooo tooo

too~ -

e~

2b 4b 6'0 go 160 ii0 I~o l~o i~o 200 Time (seconds)

Figure 5 Comparison of the Drop Diameters of Several Models

|

//~--

loo IO

io~ I

Modified

1oooooo-

-

0

I/

II

o

,

50

16o

,

15o

2o0

160 260 360 460 560 660 760 860 960 loo0 Time (seconds)

Figure 6 Comparison of the Mean Diameters for Original and Modified Models

Throughout the simulation period, the smallest diameter is selected as the mean diameter of the serosol particles. Figure 6 shows the mean diameters for a large LOCA with loss of Class IV power for time periods of 910 seconds and 200 seconds using both the modified and original versions. As expected, the mean diameter predicted by the modified version is larger compared to the original one.

1088

J.S. Baek et al.

The resulting amounts of activity released into the outer atmosphere have been compared. First, the effect of the new mean diameter model is examined and then the effect of the additional diffusive deposition and turbulent coagulations is investigated. Figures 7 to 9 show the 1-131 activity concentrations in the accident node into which the high enthaipy coolant and radionuclides are discharged for three cases. In the legend,'modified' and 'original' stand for the results predicted by modified version only for new diameter model and original version, respectively. As expected, the new model for the mean droplet diameter gives lower concentrations rather than the original model for all cases. Because of larger diameters calculated by the new model, the aerosol removal mechanisms become very effective and it results in lower concentration of activity. 1.0 . 0.g

o~

original modified ~

1.0

¢.

0.6

o3!

0.4

0.2-

0.2

t

original modified ~

"z

26O

46O 660 Time (s)

8~0

0.1-

k,,,.. 6

1000

0

Figure 7 1-131 Concentration in Accident Node for LBLOCA + Loss of Class IV Power with Total Loss of Isolation Case

0.15, l 0.12

0.3 0.2 0.1

260

0.06-0"09"

.4'0

6b

46O 600 Time (s)

8b

16o

860

1000

Figure 8 1-131 Concentration in Accident Node for Feeder Stagnation Break with Total Loss of Isolation Case

60 original modified - - ~

50-

0.15o=

2'o

!

.

0.034

~

<

. ~~

/ 30-

/

//

/

/

/ I

/

original_stack original_inlet- - - modified_stack~ modified_inlet~ . ______----

I

2 oi ~--~=---

26O

= ....

400Time -(s~600 --

86O

1000

Figure 9 1-131 Concentration in Accident Node for End Fitting Failure with Total Loss of Isolation Case

01 0

26O

400Time(s)600

86O

1000

Figure I0 Integrated I-I31 Activity Amount Released into Environment for LBLOCA + Loss of Class IV Power with Total Loss of Isolation Case

Figures 10 to 12 show the integrated activity amounts of 1-131 released into the environment for three cases. In the legend, 'stack' and 'inlet' stand for the 1-131 activity amount released through the ventilation stack and inlet, respectively and 'modified' represents only consideration of the new diameter model. As can be seen in these figures, the 1-131 activity amounts are much reduced with the modified version. Because of the lower concentrations in the accident nodes, less activity is transferred to other nodes and finally into outer atmosphere.

Radionuclide behaviour inside containment

1089

30

50 e~ / I-

T-

/ /

- -

20

- -

<

/

/ /

/ - -

modified_stack - modified_inlet - - - - -

/

2ff

original_stack original_inlet

/

/

15

/

/

/

/

originalstack original_inlet

/

-- --

modified s t a c k - - modified-inlet - - . - -

/ o

fl

10-

/

...7

o

2~0

4~0

660

S00

1000

2~0

Time (s)

Figure 11 Integrated 1-131 Activity Amount Released into Environment for Feeder Stagnation Break with Total Loss of Isolation Case

460 6~0 Time (s)

860

1000

Figure 12 Integrated 1-131 Activity Amount Released into Environment for End Fitting Failure with Total Loss of Isolation Case

Table 1 shows the comparison of amounts of I-131 activity released into the environment with considering only the mean diameter models. The activity values of 1-131 are integrated ones until the time at which the operator turns off the fan switch at 15 miniture after the certain signal of accident. During this period, most activity is released. As shown in this table, the new method gives much lower activity amount released into the environment due to the larger droplet diameter, especially for the feeder stagnation and end fitting failure cases.

Table I Comparison of lntegrated l-131 Activity Amount Released lnto Environment for the Modified and Original Versions of SMART for Large LOCA with Loss of Class IV Power, Feeder Stagnation Break and End Fitting Failure Large LOCA with Loss of Class IV Power (TBq)

Feeder Stagnation Break (TBq)

End Fitting Failure (TBq)

Stack

Inlet

Stack

Inlet

Stack

Inlet

Original

29,6

50.6

20.2

41.4

11.1

25.2

Modified

14.4

20.4

1.35

2.54

2.73

5.55

Note:

All cases have been considered for the total loss of containment isolation. And 'Modified' stands for the results predicted by considering only the new model for the mean diameter of aerosol particles.

Table 2 shows the effect of the diffusive deposition and turbulent coagulation. The comparison has been conducted for the large L O C A with loss of Class IV case. As described above and shown in this table, the effect on the amount of activity released into the environment is small compared to the effect of the diameter change.

Table 2 Effect of Diffusive Deposition and Turbulent Shear and Inertia Coagulations on Integrated 1-131 Activity Amount Released Into Environment for large LOCA with loss of Class IV case Diameter (TBq) Modified Note:

Total (TBq)

Stack

Inlet

Stack

Inlet

14.4

20.4

12.7

17.9

This case has been considered for the total loss of containment isolation. And 'Diameter' and 'Total' represent the results predicted by considering only the new model for the mean diameter of aerosol particles and the additional diffusive deposition and turbulent coagulations as well as the new diameter model, respectively.

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al.

6. CONCLUSION Several models to predict drop diameter have been investigated. Among them, Koestel, Gido and Lamkin's heterogeneous and homogeneous fragmentation models seem to predict the mean diameter of drop reasonably and, therefore, are incorporated into the SMART code. The calculation for the jet velocity used in the aerodynamic atomization model is modified to make it realistic. The minimum droplet diameter predicted by the three correlations is selected as the mean diameter. In additional, diffusive deposition and turbulent shear and inertial coagulation models have been implemented in the SMART code. The aerosol diameter change has very significant effect. The effect of diffusive deposition and turbulent coagulation, however, is negligible. The new method gives much higher mean diameters and much lower activity amount released into environment compared to the original method. Therefore, it can be said that as some conservatism has been removed from the released aerosol amount, some conservatism in public dose calculation can be removed.

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