Annals of Nuclear Energy 58 (2013) 95–101

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Annals of Nuclear Energy

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Simplified approach for reconstructing the atmospheric source termfor Fukushima Daiichi nuclear power plant accident using scantymeteorological data

0306-4549/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.anucene.2013.03.016

⇑ Corresponding author. Tel.: +91 22 25592077; fax: +91 22 25505151.E-mail address: rboza@barc.gov.in (R.B. Oza).

R.B. Oza a,⇑, S.P. Indumati a, V.D. Puranik a, D.N. Sharma b, A.K. Ghosh b

a Environmental Assessment Division, Bhabha Atomic Research Centre, Mumbai 400 085, Indiab Health, Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai 400 085, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 December 2012Received in revised form 1 March 2013Accepted 6 March 2013Available online 9 April 2013

Keywords:FukushimaGaussian puff modelAtmospheric dispersionSource termAir concentrationDeposited activity

The atmospheric source term for the Fukushima Daiichi nuclear power plant accident in March 2011 hasbeen estimated by a Gaussian puff based atmospheric dispersion model. The scanty meteorological dataavailable at irregular time intervals are utilized to demonstrate the utility of such data along with a sim-plified modeling approach to derive useful information. The source terms for I-131 and Cs-137 have beenestimated as a function of time from the observed values of activity concentration in the air and depos-ited activity on the ground. The model results suggest that during 12th March 2011–16th March 2011,9.29 � 1016 Bq of I-131 and 6.15 � 1015 Bq of Cs-137 might have got released to the environment.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The Great East Japan earthquake of magnitude 9 and subsequenttsunami on March 11, 2011 led to accidents at Fukushima DaiichiNuclear Power Plant (FDNPP) site. The accidental conditions atFDNPP released large amount of radioactive material to the environ-ment. The immediate concern of the local authority was to minimizethe impact of radioactive releases to the public residing in the nearbyareas by adopting suitable counter measures. The initial estimates ofthe release were made based on the information about the inventoryof radionuclides in the nuclear reactor and the behavior of radionuc-lides under core melt down condition (e.g. IRSN, 2012). Subse-quently, several attempts were made to reconstruct the sourceterm for radionuclides released to the environment by comparingatmospheric dispersion model results with the measured concen-tration levels, deposited activity levels and dose rate levels.

Chino et al. (2011) provided the source term estimation as afunction of time using SPEEDI and WSPEEDI-II atmospheric disper-sion models with meteorological data obtained using atmosphericdynamic models PHYSIC and MM5. The source terms provided byChino et al. (2011) were further refined by Katata et al. (2012)and Terada et al. (2012). Schoppner et al. (2012) provided thesource term estimation based on the data collected by Interna-

tional Monitoring System (IMS) stations installed by Comprehen-sive Nuclear-Test Ban Treaty Organization (CTBTO), by comparingit with the results of atmospheric dispersion model FLEXPART.The source term estimation is also carried out by the Tokyo ElectricPower Company (TEPCO, 2012), Nuclear and Industrial SafetyAgency (NISA, 2011) and Institute for Radiological Protection andNuclear Safety (IRSN, 2012), France.

The present work highlights the applicability of scant meteoro-logical data for getting rough estimation of the source term underaccidental conditions at nuclear power plant site using simplemodeling approach.

2. Model and methods

The model developed in the present study is based on the Gauss-ian puff approach, where release is simulated by sequence of puffscarrying the released activity depending upon the specified releaserate. The concentration at a given receptor location is estimated as:

Cðxg ;yg ;zg ;tÞ¼XNðtÞi¼1

QðiÞð2pÞ3=2rxryrz

exp �12

xg�xicðtÞrx

� �2 "

þyg�yicðtÞ

ry

� �2!#� exp �1

2zg�zic

rz

� �2 !"

þexp �12

zgþzic

rz

� �2 !

þexp �12

2zinv�zg�zic

rz

� �2 !#

ð1Þ

96 R.B. Oza et al. / Annals of Nuclear Energy 58 (2013) 95–101

Here, C(xg,yg,zg, t) is the instantaneous concentration in Bq m�3

at time t for the receptor location defined by xg, yg, and zg (all in m)in the x direction (West–East), y direction (South–North) and zdirection (vertical direction from the earth surface) respectively.N is the total number of puffs at given time t. Q(i) is the amountof activity carried by the ith puff in Bq, whereas xic, yic, and zic

(all in m) are the coordinates of the ith puff at time t in x, y andz directions respectively. It is to be noted that in our computations,zic remains invariant with respect to time. The rx, ry, and rz (all inm) are the puff dispersion parameters in the x, y and z directionsrespectively. As in most of the atmospheric dispersion modelswhich are based on Gaussian puff approach (e.g., RIMPUFF,Thykier-Nielsen et al., 2004), here also it is assumed that rx = ry.These dispersion parameters are taken from the well knownPasquill–Gifford dispersion curves where they are defined as afunction of distance and atmospheric stability categories (Hukkooet al., 1988). zinv is the height of inversion lid in meters.

The model uses virtual source logic to account for temporal var-iation of atmospheric stability category. In this, in case of change ofatmospheric stability category for a given puff, it estimates the vir-tual source distance assuming the current dispersion parameters inthe changed atmospheric condition, before considering the advec-tion of puffs in new meteorological condition. The dispersionparameters in changed atmospheric condition are then estimatedby adding the advection distance to the virtual source distance.The model considers reflection of the released material from theground surface (second term in the second square bracket of Eq.(1)) as well as from the mixing height (third term in the secondsquare bracket of Eq. (1)) and assumes that puff loses the verticaldistribution once 1.5 times rz becomes comparable to or morethan the mixing height.

The model was used with the meteorological data update inter-val of 1 h, with an advection time step of 1 min. The time inte-grated concentration and deposited activities are estimated foreach advection time step and added to estimate the total depositedactivity at the receptor. The instantaneous air concentration at thereceptor location is estimated only at the end of each hour. To esti-mate the deposited activity, a constant deposition velocity of0.02 ms�1 for I-131 and 0.004 ms�1 for Cs-137 was assumed. Thedepletion of activity in a puff due to dry deposition and radioactivedecay was estimated for each advection time step and the activitycarried by the puff was corrected accordingly.

For source term reconstruction, continuous release startingfrom 12th March 2011, 13:00 h to 17th March 2011, 00:00 h, fora total period of 108 h was assumed, with a release rate of3.7 � 1010 Bq h�1 for I-131 and Cs-137. The time period was judi-ciously chosen to cover the reported accidental conditions atUnit-1 (12th March 2011 at 15:30 h), Unit-3 (14th March 2011 at11:00 h), and Unit-4 as well as Unit-2 (15th March 2011 at06:00 h) of Fukushima Daiichi Nuclear Power Plant Site. It is tobe noted that in Unit-1, the explosion took place at around15:30 h on 12th March 2011, however, the release is consideredfrom 13:00 h to account for the releases that took place prior toexplosion. The domain considered for the computation is shownin Fig. 1, along with the receptors at which instantaneous air con-centration and deposited activities are estimated. The receptorsshown with the square symbols are finally used for the source termestimation whereas the receptors shown with cross symbols arenot considered either due to lack of measured data during the per-iod of interest or because they are not affected by the release dur-ing the period considered here. It is clear from the figure thatreceptors in the SW direction of the Fukushima plant site are usedfor the source term re-construction. The source term as a functionof time is reconstructed in two different ways: (1) based on themeasured concentration data and (2) based on the measureddeposited activity data. The methodology adopted in our study

for inversion algorithm is based on the methodology given by Chi-no et al. (2011). In this, the release rates of radionuclides (Bq h�1)are estimated by coupling environmental monitoring data withatmospheric dispersion model results for unit release rate(1 Bq h�1). The release rates are obtained by dividing measuredair concentration of I-131 and Cs-137 by the calculated ones atsampling points as follows:

Qi ¼ Mi � 3:7� 1010=Ci ð2Þ

where Qi is the release rate (Bq h�1) of radionuclide i when dis-charged into the atmosphere, Mi the measured air concentration(Bq m�3) of radionuclide i, and Ci is the estimated concentration(Bq m�3) of radionuclide i due to 3.7 � 1010 (Bq h�1) release rate.

Total release amounts are estimated by the time integration ofrelease rates as follows:

Si ¼X½Q ij � Tj� ð3Þ

where Si (Bq) is the total release amount of nuclide i, Qij the releaserate (Bq h�1) of nuclide i at time j, and Tj (h) is the duration of re-lease. The same methodology is also adopted for deposited activitydata.

In the case of source term estimation based on measured con-centration data, the measured concentration for a particular dayand the peak instantaneous concentration estimated for the sameday are used to find out the source term. For the case of depositedactivity data, measured deposited activity data, which becameavailable after 19th March 2011, are used along with the estimatedtotal deposited activity at the end of simulation (i.e., on 17th March2011, 00:00 h) to reconstruct the source term. It is to be noted thatbased on UK Met Office data for Tokyo (and many other studies),the released activity during 16th March to 19th March 2011 wouldhave traveled towards Pacific Ocean and hence it justifies the com-parison of estimated deposited activity data on 17th March 2011with the measured deposited activity data on 19th March 2011, ex-cept for the weathering processes. The time dependency of sourceterm is ascertained by identifying the puffs which are contributingto the instantaneous air concentration as well as deposited activityat the receptor location at a given time.

3. Meteorological data

Few measured meteorological data were available for Fukushi-ma Daiichi NPP site at irregular time intervals from the official website of NISA. These data are presented in Table 1. Using wind speedand direction data given in this table, the wind components werederived and these were linearly interpolated to generate hourlyvalues of the wind speed and direction. The NISA web site didnot give meteorological data beyond 14th March 2011. Since therelease is also considered on 15th March 2011, the single pointmeteorological data given by U.K. Met Office for Tokyo city wereused for 15th March 2011 (wind speed of 4.0 m/s with wind direc-tion NE) and 16th March 2011 (wind speed of 4.9 m/s with winddirection WNW). Most of the studies reported so far indicate thaton 15th March 2011, predominantly the wind has blown fromNE to SW direction and on 16th March it has blown towards PacificOcean, which justifies the assumption of constant wind speed anddirection for these days up to some extent, however it may not berealistic. The atmospheric dispersion of pollutant also requiresinformation about the atmospheric stability category as well asmixing height at given point of time. These parameters are as-sumed to occur only as a function of time based on experience/lit-erature values and are presented in Table 2.

Fig. 1. Domain and receptor locations used for computations, approximate distance of the receptor from the source is given in the parenthesis in km.

Table 1Meteorological data collected from the web site of NISA.

Date DD/MM/YYYY Time Wind speed (ms�1) Direction

12/03/2011 13:12 1.8 SE12/03/2011 17:43 5.9 SSE12/03/2011 20:38 0.5 W13/03/2011 01:59 4.7 SW13/03/2011 12:20 2.6 NW13/03/2011 13:20 4.5 S13/03/2011 17:52 0.5 NNW13/03/2011 20:30 4.0 W14/03/2011 00:01 0.3 NNW14/03/2011 05:00 0.9 SSW14/03/2011 14:14 2.6 NNW14/03/2011 15:40 6.0 W

R.B. Oza et al. / Annals of Nuclear Energy 58 (2013) 95–101 97

4. Results and discussion

The calculated values of instantaneous air concentration anddeposited activity of I-131 at various receptor locations are shownin Fig. 2a–d as a function of time after start of release (The start ofrelease is considered at 13:00 h on 12th March 2011) for3.7 � 1010 Bq h�1 release rate. Fig. 2a and b give the calculated val-ues of instantaneous air concentration and deposited activityrespectively, for those receptor locations where instantaneous airconcentration values are used to reconstruct the source term.Whereas Fig. 2c and d give the same for those receptor locationsfor which deposited activity values are utilized for reconstructingthe source term. Similar results for Cs-137 are presented inFig. 3a–d in a similar fashion for 3.7 � 1010 Bq h�1 release rate. In

both the Figs. 2 and 3, legends for (a) and (b) are same and simi-larly legends for (c) and (d) are same. The results are presentedfor 60 h from the start of release to 90 h, because the receptorsconsidered for the source term reconstruction are getting affectedduring this period only. Though, there were receptors which got af-fected within first 24 h from the start of release (e.g., Minamisoma,Yamagata, Miyagi, and Onagawa), these are not shown due tounavailability of measured air activity concentration/depositedactivity data during the period of interest. For the same reason,the results of many other receptors are also not shown. The resultsof calculated values of deposited activity shown in Fig. 2b and d aswell as in Fig. 3b and d at 90 h after the start of simulation remainssame till the end of simulation (108 h after the start of simulation),as no fresh contribution is coming during this period for the recep-tors considered.

As can be seen from Fig. 2a and c as well as from Fig. 3a and c,different receptor locations experience peak concentration at dif-ferent points of time based on the meteorological conditions andrelative distances from the source. At the time of peak concentra-tion, the puffs which are contributing to the peak concentrationare identified and based on that, their time of release is ascer-tained. In this study, it was found that due to changing meteorolog-ical conditions, puffs released at different point of time cancontribute to the peak concentration level at receptor location.For example, the peak concentration estimated for Takasaki on15th March 2011 at 22 h (82 h after the start of release) has contri-bution from puffs released during 12th March 2011 at 13 h to 12thMarch 2011 at 15 h, in addition to having contribution from puffsreleased during 15th March 2011 at 04 h to 15th March 2011 at

Table 2Assumed occurrence of atmospheric stability category and mixing height as functionof time of the day.

Atmospheric stabilitycategory (% ofoccurrence assumed)

Time of theoccurrenceassumed (h)

Mixing height (m) assumedbased on atmosphericstability category

A (8.3) 14, 15 1400B (12.5) 12, 13, 16 1000C (8.3) 11, 17 900D (25) 8, 9, 10, 18, 19, 20 800E (20.8) 7, 21, 22, 23, 24 400F (25) 1, 2, 3, 4, 5, 6 100

98 R.B. Oza et al. / Annals of Nuclear Energy 58 (2013) 95–101

10 h. However, it is assumed that contribution of puffs released on12th March is expected to be negligible as compared to the contri-bution of puffs released on 15th March, mainly because of theduration of release that is affecting the concentration and alsodue to the ages of the puffs. Accordingly the most probable releaseperiod associated with peak simulated concentration is derived inTables 3 and 4.

The estimated deposited activity at various receptor locationsas a function of time after start of release is shown in Fig. 2b andd for I-131 and in Fig. 3b and d for Cs-137. As can be seen, the

Fig. 2. (a) and (b) Estimated values of instantaneous air concentration and deposited actconcentration values are used for source term reconstruction and (c) and (d) are estimatefor I-131 at those receptor locations where deposited activity values are used for source tsame.

deposited activity at various receptor location starts increasing asa function of time and attains constant value for those receptorswhich experience only single peak for instantaneous concentration(e.g., see the instantaneous concentration values for Takasaki inFig. 2a and deposited activity in Fig. 2b). The receptors having mul-tiple peaks in instantaneous concentration (e.g., Tokyo in Fig. 2a),can give rise to step like structures in deposited activity (e.g., Tokyoin Fig. 2b).

The source term reconstructed using the measured and esti-mated concentration values are presented in Table 3 and thesource term reconstructed using measured and estimated depos-ited activity values are presented in Table 4. In general it is foundthat the source terms estimated using deposited activity data arelower than that estimated using concentration data. This couldbe due to the large uncertainty associated with the depositionvelocities. The reported deposition velocity for I-131 varies from0.001 to 0.02 ms�1, and for Cs-137 it varies from 0.0004 to0.005 ms�1(Brandt, 1998) and is expected to have diurnal as wellas seasonal variation. In our study, a constant deposition velocityof 0.02 ms�1 for I-131 and 0.004 ms�1 for Cs-137 is used. More-over, wet deposition is not considered in the present modeling ap-proach, which may additionally increase the estimated source termbased on deposited activity data. As can be seen from Tables 3 and

ivity data respectively for I-131 at those receptor locations where instantaneous aird values of instantaneous air concentration and deposited activity data respectivelyerm reconstruction. Legends for (a) and (b) are same and similarly for (c) and (d) are

Fig. 3. (a) and (b) Estimated values of instantaneous air concentration and deposited activity data respectively for Cs-137 at those receptor locations where instantaneous airconcentration values are used for source term reconstruction and (c) and (d) are estimated values of instantaneous air concentration and deposited activity data respectivelyfor Cs-137 at those receptor locations where deposited activity values are used for source term reconstruction. Legends for (a) and (b) are same and similarly for (c) and (d)are same.

R.B. Oza et al. / Annals of Nuclear Energy 58 (2013) 95–101 99

4, the source term estimation varies by orders of magnitude,depending upon the method used and also depending upon thereceptor location. In further discussion, the highest release rateestimated (which is based on the measured and estimated concen-tration levels) is considered as the estimated release rate.

Tables 3 and 4 show that none of the receptors that are used forsource term reconstruction got predominantly affected by the re-lease that took place on 12th March 2011, and hence, release ratefor this date could not be ascertained. However, the peak instanta-neous concentration value estimated for Chiba on 15th March2011 is predominantly due to the release that took place between13th March 2011 at 11 h to 14th March 2011 at 15 h. On 14thMarch 2011, explosion took place in Unit-3 at about 11:00 h, how-ever, since Chiba is at 228 km from the source, the source termestimated based on the measured and estimated concentration atChiba on 15th March 2011 could be attributed to the release periodof 13th March. Since the release rate could not be ascertained for12th March, it is assumed that this release rate prevailed fromthe start of release (i.e., 12th March 2011, 13:00 h) to the time ofexplosion in Unit-3 (i.e., 14th March 2011, 11:00 h), for a total per-iod of 47 h. The release rate estimated for this period is2.12 � 1013 Bq h�1 for I-131 and 1.65 � 1011 Bq h�1 for Cs-137.

As given in Table 3, the release rate estimated for 14th March2011 is based on the measured and estimated concentration at

Tokyo for 15th March 2011. This works out to be 8.77 � 1014 -Bq h�1 for I-131 and 2.93 � 1013 Bq h�1 for Cs-137. It is assumedthat this release rate (probably due to the accidental condition atUnit-3) prevailed from the time of explosion at Unit-3 (i.e., 14thMarch 2011, 11:00 h) to the explosion at Unit-4 and 2 (i.e., 15thMarch 2011, 06:00 h) for a period of 19 h.

The release for 15th March 2011 is ascertained based on themeasured and estimated concentration value at Takasaki, whichpredominantly got affected by the release that took place on15th March 2011. This shows release rate of I-131 as1.79 � 1015 Bq h�1 and that for Cs-137 as 1.33 � 1014 Bq h�1.Here, it is assumed that this release prevailed from 15th March2011, 06:00 h, till the end of simulation on 16th March 2011,for total period of 42 h. In fact from 16th March onwards, thewind direction was towards sea, and hence source term for16th March could not be ascertained. In view of this, the sourceterm for 15th March 2011, is assumed to prevail on 16th March2011 also.

The report published by IRSN suggests that the main releaseperiod ended on 17th March 2011, hence the time period consid-ered in the present study seems to be good enough to approxi-mately estimate the total release.

It is believed that release rate would have varied significantlyduring the period for which we have estimated a constant release

Table 3Probable release rate and associated release period for I-131 and Cs-137, based on measured and estimated air concentration data.

Receptor location [Ref. formeasured concentration]approximate distance from thesource (km)

Date on whichmeasurement wascarried out DD/MM/YYYY

Measured I-131& [Cs-137]concentration(Bq m�3)

Peak estimatedconcentration of I-131, [Cs-137] (Bqm�3) for3.7 � 1010 Bq h�1 release rate

Estimated release rate ofI-131 & [Cs-137] (Bq h�1)

Most Probable release periodassociated with peaksimulated concentration DD/MM/YYYY/HH

Takasaki [Stoehlker et al.,2011] 215 km

15/03/2011 15 [5] 3.094 � 10�4 [1.390 � 10�3] 1.79 � 1015 [1.33 � 1014] 15/03/2011/04–15/03/2011/10

Tokyo [IRSN, 2012] 227 km 15/03/2011 240 [60] 1.013 � 10�2 [7.574 � 10�2] 8.77 � 1014 [2.93 � 1013] 14/03/2011/10–14/03/2011/21

16/03/2011 25 [5] 2.449 � 10�2 [3.156 � 10�1] 3.78 � 1013 [5.86 � 1011] 15/03/2011/01–15/03/2011/10

Chiba [Amano et al., 2011]228 km

15/03/2011 33 [0.87] 5.755 � 10�2 [1.945 � 10�1] 2.12 � 1013 [1.65 � 1011] 13/03/2011/11–14/03/2011/15

16/03/2011 7.4 [0.12] 1.804 � 10�2 [2.684 � 10�1] 1.52 � 1013 [1.65 � 1010] 15/03/2011/01–15/03/2011/10

Table 5Estimate of total released quantity of I-131 over a period of 108 h.

Starting time (DD/MM/YYYY/HH)

Ending time (DD/MM/YYYY/HH)

No. ofhours

Estimated release rate of I-131(Bq h�1)

Total release of I-131 (Bq) during no. ofhours

12/03/2011/13 14/03/2011/11 47 2.12 � 1013 9.96 � 1014

14/03/2011/11 15/03/2011/06 19 8.77 � 1014 1.67 � 1016

15/03/2011/06 17/03/2011/00 42 1.79 � 1015 7.52 � 1016

Total estimated release of I-131 in the present study during 108 h(Bq) 9.29 � 1016

Estimate given by NISA, February 2012, (Bq) 1.50 � 1017

Estimate given by TEPCO, May 2012, (Bq) 5.00 � 1017

Estimate given by IRSN, March 2012, (Bq) 1.97 � 1017

Table 4Probable release rate and associated release period for I-131 and Cs-137, based on measured and estimated ground deposited activity data.

Receptor location [Ref. formeasured deposited activity]approximate distance from thesource (km)

Date ofmeasurement

Measureddeposited activityof I-131 & [Cs-137]in Bq m�2

Estimated deposited activityof I-131 & [Cs-137] in Bq m�2

for 3.7 � 1010 Bq h�1 releaserate

Estimated release rate ofI-131 & [Cs-137] Bq h�1

Most probable release periodassociated with peaksimulated concentration DD/MM/YYYY/HH

Ibaraki [MEXT] 131 km 19/03/2011 880 [86] 78.00 [42.88] 4.17 � 1011 [7.42 � 1010] 14/03/2011/14–14/03/2011/19 and 15/03/2011/09–15/03/2011/15

Tochigi [MEXT] 142 km 19/03/2011 1300 [62] 39.50 [25.28] 1.22 � 1012 [9.07 � 1010] 14/03/2011/21–15/03/2011/00and 15/03/2011/08–15/03/2011/15

Gumma [MEXT] 209 km 19/03/2011 230 [84] 2.632 � 10�2 [3.439 � 10�2] 3.23 � 1014 [9.04 � 1013] 15/03/2011/05–15/03/2011/10Saitama [MEXT] 211 km 19/03/2011 64 [ND]a 7.73 3.06 � 1011 14/03/2011/13–14/03/2011/

21 and 15/03/2011/02–15/03/2011/11

Kanagawa [MEXT] 273 km 19/03/2011 40 [ND] 5.49 2.69 � 1011 13/03/2011/20–14/03/2011/20and 14/03/2011/19–15/03/2011/07

Yamanshi [MEXT] 293 km 19/03/2011 175 [ND] 3.235 � 10�2 2.00 � 1014 14/03/2011/19–15/03/2011/06

a ND: Not Detected.

Table 6Estimate of total released quantity of Cs-137 over a period of 108 h.

Starting time DD/MM/YYYY/HH

Ending time DD/MM/YYYY/HH

No. ofhours

Estimated release rate of Cs-137(Bq h�1)

Total release of Cs-137 (Bq) during no. ofhours

12/03/2011/13 14/03/2011/11 47 1.65 � 1011 7.75 � 1012

14/03/2011/11 15/03/2011/06 19 2.93 � 1013 5.57 � 1014

15/03/2011/06 17/03/2011/00 42 1.33 � 1014 5.59 � 1015

Total estimated release of Cs-137 in the present study during 108 h(Bq) 6.15 � 1015

Estimate given by NISA, February 2012, (Bq) 8.20 � 1015

Estimate given by TEPCO, May 2012, (Bq) 1.0 0 � 1016

Estimate given by IRSN, March 2012, (Bq) 2.10 � 1016

100 R.B. Oza et al. / Annals of Nuclear Energy 58 (2013) 95–101

R.B. Oza et al. / Annals of Nuclear Energy 58 (2013) 95–101 101

rate. However, if the highest source term estimated for the speci-fied time interval as discussed above is considered, then the totalestimated release of I-131 over a period of 108 h works out to be9.29 � 1016 Bq and that of Cs-137 works out to be 6.15 � 1015 Bqfor the same period. The details of this, along with the estimationof NISA, TEPCO and IRSN are presented in Table 5 for I-131 andin Table 6 for Cs-137. As can be seen from these tables, the presentestimation is in reasonable agreement with the same carried outby NISA, TEPCO and IRSN, even though the meteorological dataused in the present study are scanty and modeling approach usedis relatively simple.

5. Summary and conclusions

The study shows that even scanty meteorological data alongwith a simplified modeling approach, may give reasonable sourceterm estimation under accidental conditions at nuclear power plant.It was found that the source term estimated based on the concentra-tion data and deposited activity data varies by orders of magnitude.As mentioned earlier, the source term estimation based on concen-tration data will certainly have an advantage compared to the samebased on deposited activity data, due to partial omission of highlyuncertain parameters, such as deposition velocity, and washout.The estimated total release of I-131 works out to be 9.29 �1016 Bq and that of Cs-137 works out to be 6.15 � 1015 Bq during12th March 2011, 13:00 h to 17th March 2011, 00:00 h.

Acknowledgement

Authors would like to thank Dr. R.N. Nair, Head, EnvironmentalModeling Section, Environmental Assessment Division, BARC, foruseful discussion during the course of this study.

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