Estimation of radioactive leakages into the Pacific Ocean due to Fukushima nuclear accident

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  • ORIGINAL ARTICLE

    Estimation of radioactive leakages into the Pacific Oceandue to Fukushima nuclear accident

    R. N. Nair Faby Sunny Manish Chopra

    L. K. Sharma V. D. Puranik A. K. Ghosh

    Received: 22 October 2012 / Accepted: 15 April 2013

    Springer-Verlag Berlin Heidelberg 2013

    Abstract High concentrations of several radionuclides

    were reported in the sea near the Fukushima Daiichi

    Nuclear Power Station (FDNPS) in Japan due to the

    nuclear accident that occurred on 11 March 2011. The

    main source of these concentrations was leakage of highly

    radioactive liquid effluent from a pit in the turbine building

    near the intake canal of Unit-2 of FDNPS through a crack

    in the concrete wall. In the immediate vicinity of the plant,

    seawater concentrations reached 68 MBq m-3 for 134Cs

    and 137Cs, and exceeded 100 MBq m-3 for 131I in early

    April 2011. These concentrations began to fall as of 11

    April 2011 and, at the end of April, had reached a value

    close to 0.1 MBq m-3 for 137Cs. Following the nuclear

    accident, the Tokyo Electric Power Company (TEPCO)

    had initiated intense monitoring of the environment

    including the Pacific Ocean. Seawater samples were col-

    lected and the concentrations of few radionuclides were

    measured on a wide spatial and temporal scale. In this

    study, the measured concentrations of different radionuc-

    lides near the south discharge canal of the FDNPS were

    used to estimate their leakages into the Pacific Ocean. The

    method is based on estimating the release rates that

    reproduce the concentration of radionuclides in seawater at

    a chosen location using a two-dimensional advectiondis-

    persion model in an iterative manner. The radioactive

    leakages were estimated as 5.68 PBq for 131I, 2.24 PBq for134Cs and 2.25 PBq for 137Cs. Leakages were also esti-

    mated for 99mTc, 136Cs, 140Ba and 140La and they range

    between 0.02 PBq (99mTc) and 0.53 PBq (140Ba). It was

    estimated that about 11.28 PBq of radioactivity in total was

    leaked into the Pacific Ocean from the damaged FDNPS.

    Out of this, 131I constitutes 50.3 %; 134Cs 20 %; 137Cs

    20 %; 140Ba 4.6 %; 136Cs 2.6 %; 140La 2.3 % and 99mTc

    0.2 % of the total radioactive leakage. Such quantitative

    estimates of radioactive leakages are essential prerequisites

    for short-term and local-scale as well as long-term and

    large-scale radiological impact assessment of the nuclear

    accident.

    Keywords Fukushima Daiichi Nuclear Power Station Nuclear accident Pacific Ocean Advectiondispersionmodel Radionuclide concentration Radioactive leakage Radioactive release Source reconstruction

    Introduction

    The Tohoku earthquake and tsunami of 11 March 2011

    caused extensive damage to the Fukushima Daiichi nuclear

    Electronic supplementary material The online version of thisarticle (doi:10.1007/s12665-013-2501-1) contains supplementarymaterial, which is available to authorized users.

    R. N. Nair (&) F. Sunny M. Chopra L. K. Sharma V. D. PuranikEnvironmental Assessment Division, Bhabha Atomic Research

    Centre, Mumbai 400 085, India

    e-mail: rnnair@barc.gov.in

    F. Sunny

    e-mail: fabys@barc.gov.in

    M. Chopra

    e-mail: mchopra@barc.gov.in

    L. K. Sharma

    e-mail: lksharma.physics@gmail.com

    V. D. Puranik

    e-mail: vdpuranik@gmail.com

    A. K. Ghosh

    Health, Safety and Environment Group, Bhabha Atomic

    Research Centre, Mumbai 400 085, India

    e-mail: ashok.ghosh@gmail.com

    123

    Environ Earth Sci

    DOI 10.1007/s12665-013-2501-1

  • power station (FDNPS) in Japan. This severe nuclear

    accident had resulted in large amounts of radioactive fall-

    out over land and the sea that peaked in the middle of

    March 2011 (Chino et al. 2011; Morino et al. 2011;

    Yasunari et al. 2011; NISA 2011; TEPCO 2011). In addi-

    tion to radioactive fallout over the sea, water used to cool

    the nuclear reactor cores leaked from the reactor buildings

    to the Pacific Ocean with large amounts of radioactivity of

    a spectrum of radionuclides (IAEA 2011; IRSN 2011;

    MEXT 2011; Tsumune et al. 2012). Other sources of

    marine radioactive contamination were the voluntary dis-

    charge of low contaminated water from the reactor build-

    ings to increase the on-site storage capacity for highly

    contaminated water and transport of radioactive material

    by rainwater runoffs of contaminated soils (Bailly du Bois

    et al. 2012). A leak of highly contaminated water to sea

    from a pit adjacent to Unit-2 of FDNPS through a crack on

    the wall was confirmed on 2 April 2011, which was sealed

    off on 6 April 2011 (TEPCO 2011). On 4 April 2011, a

    planned discharge to sea commenced from FDNPS of

    11,500 tonnes of low-level radioactive water that had been

    stored on site awaiting treatment to create storage capacity

    for the highly radioactive water that had collected in var-

    ious parts of the site. Nearly 70,000 tonnes of heavily

    contaminated water need to be moved from turbine build-

    ings and trenches so that workers could gain access

    (Wakeford 2011).

    The concentrations of 137Cs in seawater off the eastern

    Japan coast prior to the nuclear accident were between 1

    and 4 Bq m-3 (Nakanishi et al. 2011). Measurements of

    radionuclides in seawater revealed significantly high levels

    of concentrations towards the end of March 2011. Seawater

    concentrations reached 68 MBq m-3 for 134Cs and 137Cs

    and exceeded 100 MBq m-3 for 131I in early April 2011

    (Bailly du Bois et al. 2012). These concentrations began to

    fall as of 11 April 2011 and, at the end of April, had

    reached a value close to 0.1 MBq m-3 for 137Cs (IRSN

    2011). The concentrations measured in seawater before 30

    March 2011 were primarily due to radioactive fallout from

    the atmosphere and they varied from 2 to 27 kBq m-3 for137Cs and 3 to 57 kBq m-3 for 131I (IRSN 2011). The

    radionuclide with short half-lives like 131I ceased to be

    detectable after a few months and should not have any

    large-scale and long-term impact. Others, like 137Cs will

    persist in the marine environment for several years. Their

    persistence in the water column is dependent on the

    respective affinity of the radionuclides for the particles in

    suspension in surface waters which are likely to settle and

    carry the radionuclides to the sea bed. During the first

    month of leakage, the ratio of 134Cs/137Cs uniformly

    showed a value of unity. This makes the tracking of

    Fukushima-derived radionuclides in the ocean quite

    straightforward, since the only source of 134Cs, considering

    its short half-life, in the North Pacific Ocean at this time

    would be the FDNPS accident (Buesseler et al. 2012).

    A 134Cs/137Cs activity ratio of unity is considerably

    higher than what it was 25 years ago when a ratio of 0.54

    was reported in Chernobyl fallout (Aarkrog 1988). In the

    oceans, the behaviour of cesium is conservative and

    about one per cent of it is attached to marine particles.

    However, this small fraction at such a high concentration

    levels as observed near the FDNPS will result in high

    concentrations in sediments and biota in seawater and

    will continue to remain so for at least 30100 years due

    to the long half-life of 137Cs (Bowen et al. 1980;

    Buesseler et al. 1991). Considerable attention was given

    to 131I leakages due to its relatively high activities and

    tendency to accumulate in the human thyroid if ingested

    via land-based food supply or if bio-concentrated by

    seaweeds and consumed as part of the Japanese diet.

    Coastal water concentrations of different radionuclides

    were decreased by a factor of 1,000 in the month fol-

    lowing the peak release. This is a consequence of ocean

    mixing and reduction in the radioactive leakages into the

    sea. According to Buesseler et al. (2012) direct radioac-

    tive releases into the Pacific Ocean were dominated by

    leakages from the FDNPS Unit 2 during April 16, 2011.

    The radioactive leakages from the turbine building of the

    FDNPS are considered as direct releases and radioactive

    fallout from atmospheric emissions as indirect releases

    into the Pacific Ocean in the present study.

    The releases of different radionuclides into the Pacific

    Ocean due to leakage of highly radioactive effluent from

    the FDNPS are estimated in this study using a simple and

    quick methodology (source reconstruction). The method-

    ology utilizes the concentrations at a chosen location

    measured at different times. Such quantitative estimates

    of radionuclide releases into the environment are neces-

    sary for assessing the short-term and local-scale as well as

    long-term and large-scale radiological impact of the

    nuclear accident. Few studies on the estimation of

    radioactive leakages into the Pacific Ocean following the

    nuclear accident at the FDNPS are reported in the liter-

    ature. IRSN (2011), based on the concentrations measured

    in the water pooled in the turbine hall of Unit-2, esti-

    mated that about 2.3 PBq of 137Cs could have been leaked

    into the sea. They also estimated that about 3.3 PBq of131I could have been leaked into the sea. According to

    Bailly du Bois et al. (2012) about 22 PBq of 137Cs was

    leaked into the Pacific Ocean in total at the end of the

    major leakage on 8 April 2011. Tsumune et al. (2012)

    used a regional ocean model to simulate 137Cs concen-

    trations due to leakages into the sea off Fukushima and

    found that a total amount of (3.5 0.5) PBq of 137Cs was

    leaked into the sea from 26 March 2011 to the end of

    May 2011. The simulated temporal change in 137Cs

    Environ Earth Sci

    123

  • concentrations near Fukushima agreed well with obser-

    vations in this case. Buesseler et al. (2012) based on a

    simple trapezoidal integration of the nuclide vs. depth

    profiles of concentrations estimated the inventory of 137Cs

    in the sea as 2 PBq following the nuclear accident at the

    FDNPS. TEPCO (2011) analyzed the accumulated waters

    in the turbine buildings of Unit-2; which leaked into the

    sea; and evaluated the concentrations of 131I (5.4 TBq

    m-3), 134Cs (1.8 TBq m-3) and 137Cs (1.8 TBq m-3).

    The diameter of the crack on the wall was reported as

    3 cm; height of the outflow as 75 cm and the flying

    distance of the outflow jet as 65 cm (TEPCO 2011).

    Using jet flow dynamics, TEPCO (2011) had calculated

    the total outflow volume for 5 days as 520 m3. The

    product of this volume and the concentrations of the ra-

    dionuclides in the pit yielded their leakages on an

    empirical basis. The estimated leakages were 2.81 PBq

    for 131I, 0.94 PBq for 134Cs and 0.94 PBq for 137Cs.

    Later, TEPCO (2012) revised the radioactive leakage

    estimates using an oceanic circulation and dispersion

    model. The model was used to estimate the radioactive

    leakages that reproduced the measured concentrations of

    radionuclides in the sea. The estimated leakages were

    11 PBq for 131I, 3.5 PBq for 134Cs and 3.6 PBq for 137Cs

    (TEPCO 2012).

    Most of these studies reported the leakage of 137Cs into

    the Pacific Ocean from FDNPS following the accident.

    Few studies reported the leakages of 131I and 137Cs. The

    present study estimates the radioactive leakages of 7 ra-

    dionuclides such as 131I, 134Cs, 136Cs, 137Cs, 99mTc, 140Ba

    and 140La based on their measured concentrations using

    an iteration technique of a two-dimensional advection

    dispersion model. Out of these radionuclides, 136Cs,99mTc, 140Ba and 140La are very short-lived ones. Their

    radiological impact on the environment is trivial. How-

    ever, knowledge on the estimates of their leakages during

    a nuclear accident is useful for accounting for all the

    radioactive releases from the damaged nuclear power

    plants as well as for understanding the features of such

    accidents.

    Methodology

    Advectiondispersion model

    Realistic and credible environmental models that simu-

    late the transfer and accumulation of radionuclides in

    specific media are essential for predicting doses and risks

    to human health and environmental quality from past,

    present and future contamination (Whicker et al. 1999).

    Many studies are available on modelling of both natural

    and anthropogenic radioactivity transport in the

    environment, dose and risk assessment (Yadigaroglu and

    Munera 1987; Thiessen et al. 1999; Bobba et al. 2000;

    IAEA 2001; Monte et al. 2009; Aksoy and Guney 2010;

    Gonzalez-Fernandez et al. 2012; Yadav et al. 2012).

    Environmental models can be classified into forward

    models and inverse models depending on their purpose.

    Forward modelling is used to estimate the concentrations

    of pollutants in the environment based on flow and

    dispersion parameters when the source parameters are

    specified. Inverse modelling is used for parameter esti-

    mation including the source strength, location and time

    of origin from known concentrations observed at various

    locations and instants of time. However, forward mod-

    elling can also be used for parameter estimation provided

    the modelling scheme includes the possible uncertainties

    in the known parameters including those in the measured

    concentrations. Such a study has been carried for the

    source reconstruction in this work using a two-dimen-

    sional advectiondispersion model as shown below

    (IAEA 1985):

    oCot

    oox

    DxoCox

    ooy

    DyoCoy

    Ux oCox Uy

    oCoy

    kC Q1

    where C is the concentration of the radionuclide (Bq m-3),

    x is the axis along the flow direction (m), y is the lateral

    axis (m), Ux is the water flow velocity along the x-axis

    (m s-1), Uy is the water flow velocity along the y-axis

    (m s-1), Dx is the longitudinal hydrodynamic dispersion

    coefficient (m2 s-1), Dy is the lateral hydrodynamic dis-

    persion coefficient (m2 s-1), k is the radioactive decayconstant (s-1), Q is any source term (Bq m-3 s-1) and t is

    the time elapsed after release (s).

    Assuming a straight coastline, constant water flow

    velocity along the coast (Uy = 0), constant water depth and

    constant hydrodynamic dispersion coefficients, the solution

    of the two-dimensional advectiondispersion model for an

    instantaneous release of unit radioactivity (1 Bq) from a

    vertical line source at x = 0 and y = ys for a semi-infinite

    medium is given by (Schreiber 1978):

    Ci x; y; t X x; t Y y; t =h 2where x varies from -? to ?, y varies from 0 to ?, Ci isthe concentration of the radionuclide due to instantaneous

    release (Bq m-3) and h is the average mixing depth in the

    coastal sea (m). The definitions of X and Y are given below

    (Schreiber 1978):

    Xx; t 14pDxt

    p exp x Uxt2

    4Dxt

    ! kt

    " #3

    Environ Earth Sci

    123

  • Yy; t 14pDyt

    p exp y ys24Dyt

    !" # exp y ys

    2

    4Dyt

    !" #( )

    4where ys is the distance between discharge outfall and coast

    (m).

    The concentration of a radionuclide in seawater during

    the release period due to continuous release, Cd (Bq m-3),

    can be evaluated by integrating Eq. 2 with respect to time

    as given below:

    Cdx; y; t qZT

    0

    Cix; y; sds 5

    where q is the constant release (leakage) rate of the

    radionuclide for a specified release period (Bq s-1), T is the

    release period (s) and Ci is the concentration due to

    instantaneous release of unit radioactivity. Equation 5 can

    be used to estimate the concentrations of radionuclides due

    to continuous release during the release period.

    The concentration of a radionuclide in seawater during

    the post release period (after termination of the release)

    can be evaluated by the convolution integral given

    bedlow:

    Cpx; y; t qZT

    0

    Cix; y; t sds 6

    where Cp is the concentration during the post release period

    (Bq m-3) and t is the post release period (s) whose origin is

    at the end of the release period, T. The total time involved

    is (T ? t).

    The measured concentrations of radionuclides in sea-

    water on a spatial and temporal scale are the basic data to

    be used in the source reconstruction exercise. Equation 5

    was used to calculate the concentrations of different ra-

    dionuclides for a specified time within the release (leakage)

    period at a chosen location, where measured concentrations

    were available, assuming 1 Bq s-1 release of each radio-

    nuclide due to leakage. The resulting concentrations of

    each radionuclide at this distance were compared with the

    measured concentrations at the same distance and the ratio

    between them was used to scale up or scale down the

    release (leakage) rates of the radionuclides. These calcu-

    lations were repeated for many combinations of hydro-

    logical parameters within the range of their reported values

    pertaining to the study region for a radionuclide and the

    average release rate of the radionuclide due to leakage was

    derived. The total released quantity of a radionuclide due to

    leakage is equal to the product of the release rate and the

    period of release. The schematic of the methodology is

    given in the flow chart.

    Set release rate (q), Period of release (T), Number of parameter combinations (N)

    Select the advection-dispersion model (ADM)

    Select one set of hydrological parameters

    Collect the data on measured concentration for a specified time within the release period at a chosen location

    Simulate the ADM to calculate the concentration at the chosen location

    Estimate the ratio, R, between measured concentration and estimated concentration

    Scale up/down the assumed release rate, q, using the ratio, R (=qR)

    Are all the N sets of input data used?

    Yes

    No

    Estimate the average release rate and standard deviation. Find out the total release by multiplying the average release rate by the period of release

    Input data

    Large number of sampling points in the near shore and

    offshore regions of the Pacific Ocean were fixed by TEPCO

    as shown in Fig. 1. Seawater samples were collected at

    these sampling points regularly and analyses were carried

    Fig. 1 Sampling points in the Pacific Ocean near Japan (TEPCO2011). TEPCO sampling points are indicated by T and MEXT

    sampling points are indicated by M

    Environ Earth Sci

    123

  • out to evaluate the concentrations of several radionuclides

    (TEPCO 2011). Figure 1 shows that Sampling Point T1 is

    around the south discharge canal of FDNPS which is

    approximately 330 m from the Unit-4 canal. Sampling

    Point T2 is around the north discharge canal of FDNPS

    which is approximately 30 m from the discharge canal of

    Units-5 and 6. Sampling Point T3 is at the north discharge

    canal of Fukushima Daini Nuclear Power Station, which is

    approximately 10 km from the FDNPS in the downstream

    direction. Sampling Point T4 is along the Iwasawa Sea-

    coast in the downstream direction, which is about 16 km

    from the FDNPS or about 7 km from the Daini Nuclear

    Power Station at Fukushima. Sampling Point T5 is about

    15 km offshore from the FDNPS. Sampling Point T6 is

    about 15 km offshore from the Daini Nuclear Power Sta-

    tion at Fukushima. Sampling Point T7 is about 15 km

    offshore from the Iwasawa Beach. Sampling Point T8 is

    about 15 km offshore from Hirono Town; Sampling Point

    T9 is about 15 km offshore from Minami-Soma City and

    Sampling Point T10 is about 15 km offshore from Uke-

    dogawa River. The MEXT (Ministry of Education, Culture,

    Sports, Science and Technology) samplings points are

    indicated with M in this figure.

    Measured concentrations of different radionuclides such

    as 99mTc, 131I, 134Cs, 136Cs, 137Cs, 140Ba and 140La in

    seawater are available at Sampling Point T1 to Sampling

    Point T4 with a frequency of two times in a day since 20

    March 2011 onwards. However, Sampling Points T3 and

    T4 lie south of the FDNPS along the seacoast in the

    downstream direction. For example, Figs. 2 and 3 depict

    concentrations of 131I, 134Cs and 137Cs at Sampling Point

    T3 (10 km south of the FDNPS in the downstream direc-

    tion) and Sampling Point T4 (16 km south of the FDNPS in

    the downstream direction), respectively. Two peaks are

    visible for these radionuclides at both the sampling points

    on 29 March 2011 and 7 April 2011. The peak concen-

    trations of 131I vary between 3 and 3.8 MBq m-3 at both

    the sampling points. The concentrations of 134Cs and 137Cs

    are lower than those of 131I at least by a factor of 2.

    Advection will not have any effects on these sampling

    points at least during a period of few months as they lie at

    the downstream of FDNPS. However, the anti-cyclonic

    gyre in the Pacific Ocean may affect the concentration

    distribution at these sampling points after few months. The

    maximum possible effect of dispersion within a period of

    14 days will be seen only within a radius of 5 km from the

    discharge point [calculated based on the dispersion length

    which is equal to (Dxt)1/2 where Dx = 20 m

    2 s-1 and

    t = 14 days]. The magnitude of concentrations, their times

    of occurrence and the locations of these sampling points

    clearly indicate that these concentrations at T3 and T4 have

    not resulted from the radioactive leakage from the FDNPS.

    These data are included in Figs. 2 and 3 to show that there

    were sources other than the radioactive leakages into the

    Pacific Ocean (such as radioactive fallout from the

    atmosphere).

    It can be seen that the concentrations generated by the

    indirect releases are significantly lower than those due to

    the radioactive leakages. For example, the maximum

    concentration of 131I measured at Sampling Point T3 and

    Sampling Point T4 is about 3.8 MBq m-3, whereas it is

    about 120 MBq m-3 at Sampling Point T1 and Sampling

    Point T2. According to TEPCO (2011), the measured

    concentrations of 131I and 137Cs at 10 km offshore were 1.6

    and 0.32 MBq m-3, respectively, on 29 March 2011. The

    same at 16 km offshore were 1.3 and 0.23 MBq m-3,

    respectively. Such low concentrations in the offshore sea-

    waters are not caused by radioactive leakages from the

    FDNPS. They might have been caused by radioactive

    0.01

    0.1

    1

    10

    23-Mar 28-Mar 2-Apr 7-Apr 12-Apr 17-Apr 22-Apr 27-Apr 2-May

    Conc

    entra

    tion

    (MBq

    m-3 )

    I-131Cs-134Cs-137

    Fig. 2 Concentrations of 131I, 134Cs and 137Cs at Sampling Point T3[10 km south of FDNPS along the coast in downstream direction

    (TEPCO 2011)]

    0.01

    0.1

    1

    10

    23-Mar 28-Mar 2-Apr 7-Apr 12-Apr 17-Apr 22-Apr 27-Apr 2-May

    Conc

    entra

    tion

    (MBq

    m-3 )

    I-131Cs-134Cs-137

    Fig. 3 Concentrations of 131I, 134Cs and 137Cs at Sampling Point T4[16 km south of FDNPS along the coast in downstream direction

    (TEPCO 2011)]

    Environ Earth Sci

    123

  • fallout from the atmosphere. Stohl et al. (2012) and Morino

    et al. (2011) estimated that the accumulated atmospheric137Cs deposition to the ocean peaks at 50 to 200 kBq m-2.

    Hence, the measured concentrations of different radio-

    nuclides at Sampling Point T1 (330 m) are used for the

    estimation of radioactive leakages from the FDNPS. The

    concentrations of these radionuclides at Sampling Point T2

    (30 m) are used for the verification exercise. The concen-

    tration data at Sampling Point T1 and Sampling Point T2

    are presented in Figs. 4 and 5, respectively. TEPCO (2011)

    and IRSN (2011) also used the concentration data at

    Sampling Point T1 and Sampling Point T2 for estimating

    the leakages of different radionuclides into the Pacific

    Ocean.

    According to TEPCO (2011) the outflow of radioactive

    liquid effluent through a crack on the concrete wall of a pit

    in the turbine building near the intake canal of Unit-2 was

    observed at around 09:30 on 2 April 2011. The outflow was

    sealed at around 17:38 on 6 April 2011. TEPCO has no

    reasonable evidence to estimate when the outflow has

    started. However, some conclusions on the beginning of

    outflow can be drawn from the measured concentrations of

    different radionuclides at Sampling Point T1 and Sampling

    Point T2, which are available from 21 March 2011

    onwards. Table 1 presents the concentrations of 131I, 134Cs

    and 137Cs at Sampling Point T1 from 21 March 2011 to 29

    March 2011. The concentration of 131I on 21 March 2011

    was estimated as 5.07 MBq m-3. The concentrations of134Cs and 137Cs on this day were estimated as around

    1.5 MBq m-3. The concentrations of 131I, 134Cs and 137Cs

    on 23 March 2011 were measured as 5.9, 0.25 and

    0.25 MBq m-3, respectively. Table 1 indicates that the

    concentrations of these radionuclides at Sampling Point T1

    steadily increased from 24 March 2011 onwards. The

    concentrations of these radionuclides at Sampling Point T1

    started decreasing after 6 April 2011 (Fig. 4). The con-

    centrations at Sampling Point T2 also decreased after 6

    April 2011 (Fig. 5). Following any discharge into seawater,

    the concentration of a radionuclide at a sampling point

    normally increases from background value to a higher and

    constant value during the release period. The crack on the

    wall of the turbine building at the FDNPS might have

    occurred on 11 March 2011 due to the earthquake followed

    by the Tsunami. The sequence of events at the FDNPS,

    especially with respect to the cooling of the reactor cores,

    indicate that highly contaminated water might have started

    collecting in the turbine building from 12 March 2011

    onwards. Large quantities of contaminated water might

    have collected within a period of 1012 days. All these

    0.001

    0.01

    0.1

    1

    10

    100

    1000

    18-Mar 23-Mar 28-Mar 2-Apr 7-Apr 12-Apr 17-Apr 22-Apr 27-Apr 2-May 7-May

    Conc

    entra

    tion

    (MBq

    m-3 )

    Tc-99mI-131Cs-134Cs-136Cs-137Ba-140La-140

    Fig. 4 Concentrations of different radionuclides at Sampling PointT1 around 330 m south from the discharge canal of FDNPS Units 1-4

    (TEPCO 2011)

    0.001

    0.01

    0.1

    1

    10

    100

    1000

    18-Mar 23-Mar 28-Mar 2-Apr 7-Apr 12-Apr 17-Apr 22-Apr 27-Apr 2-May 7-May

    Conc

    entra

    tion

    (MBq

    m-3 )

    Tc-99mI-131Cs-134Cs-136Cs-137Ba-140La-140

    Fig. 5 Concentrations of different radionuclides at Sampling PointT2 around 30 m north from the discharge canal of FDNPS Units 56

    (TEPCO 2011)

    Table 1 Concentrations of radionuclides at Sampling Point T1(330 m) on different dates (TEPCO 2011)

    Date Time Concentration (MBq m-3)

    131I 134Cs 137Cs

    21 March 2011 14:30 5.07 1.49 1.48

    22 March 2011 06:30 1.19 0.15 0.15

    23 March 2011 08:50 5.9 0.25 0.25

    24 March 2011 10:25 4.2 0.45 0.44

    25 March 2011 08:30 50 7 7.2

    26 March 2011 14:30 74 12 12

    27 March 2011 08:30 11 1.9 1.9

    29 March 2011 08:20 100 24 24

    29 March 2011 1,355 130 31 32

    Environ Earth Sci

    123

  • observations point towards the inference that leakage of

    highly contaminated water from the turbine building might

    have started around 24 March 2011 and stopped around 6

    April 2011 leading to 14 days of release. The concentration

    levels at the discharge point were exceedingly high, with a

    peak 137Cs concentration of 68 MBq m-3 on 6 April 2011

    and the timing of peak release occured approximately one

    month after the earthquake (Buesseler et al. 2012). Tsu-

    mune et al. (2012) implied that the direct release was

    during 26 March to 6 April 2011 leading to a total release

    period of 12 days. Peaks of different radionuclides

    observed on 6 April 2011 followed by lower concentrations

    on further days indicate the possibility of stoppage of the

    direct release on 6 April. According to Bailly du Bois et al.

    (2012) the influence of leakages was particularly signifi-

    cant from 26 March 2011 to 8 April 2011 in the vicinity of

    the nuclear facilities and the drop in the concentrations

    measured after 10 April 2011 showed that there were far

    smaller leakages after this date.

    The FDNPS is located on the coast of the island of

    Honshu, more than 200 km north-east of Tokyo. The coast

    runs northsouth, facing the Pacific Ocean. The seabed

    shelves off gently to a depth of 200 m, 50 km from the

    coast and then drops suddenly to more than 5,000 m about

    100 km offshore (IRSN 2011). In the coastal zone, the

    currents are generated by the tides, wind and the general

    Pacific Ocean circulation. In the short-term, the tidal effect

    is predominant and it moves the masses of water with a

    rapid alternating motion along the coast towards the north

    and towards the south. The wind influences the circulation

    of the surface waters. The Kuroshio and Oyashio are the

    major ocean currents in the region, which are flowing

    towards northeast (Fig. 6). Ocean currents off Japan would

    lead to both southward transport of water along the coast

    via the Oyashio current, and northward-driven diversions

    due to surface wind shifts (Shimizu et al. 2001; Yasuda

    2003). The general large-scale circulation is the result of

    the interaction between the Kuroshio ocean current, which

    comes from the south and runs along the coast of Japan,

    and the Oyashio current, which comes from the north

    (IRSN 2011). The coastal waters in the vicinity of the

    FDNPS are situated in the zone where these two currents

    interact, creating variable gyratory currents. The net flow

    of seawater is towards the northeast direction as shown in

    Fig. 6. The velocities of these currents (Ux) vary from 0.4

    to 1.2 m s-1. The longitudinal dispersion coefficients (Dx)

    are reported to be varying between 10 and 20 m2 s-1. The

    lateral dispersion coefficients (Dy) vary between 1 and

    2 m2 s-1. These hydrological data of the Pacific Ocean

    reported by Yanagimoto and Taira (2003) and Stewart

    (2006) are used in the model. Ranges of these hydrological

    data are given in Table 2 with 5 values for each parameter.

    The maximum number of parameter combinations in this

    case is 125 (= 5 9 5 9 5). Hence, the number of param-

    eter combinations, N, is taken as 125. Model computations

    are carried out for 125 different combinations of Dx, Dy and

    Ux to generate meaningful statistics. For making such

    combinations, equal weightage has been given to all the

    parameters. An average seawater mixing depth of 5 m is

    used in the model (Inoue et al. 2012). Dispersion of the

    soluble radionuclides will mainly take place in the mixed

    layer. Therefore, two-dimensional advectiondispersion

    models are adequate to track the transport of radionuclides

    in this region.

    Fig. 6 Major ocean currents in the Pacific Ocean near Japan

    Table 2 Hydrological parameters used in the model

    Dx (m2 s-1) Dy (m

    2 s-1) Ux (m s-1)

    10.0 1.0 0.4

    12.0 1.2 0.6

    15.0 1.5 0.8

    18.0 1.8 1.0

    20.0 2.0 1.2

    Total combinations of parameters (N) = 5 9 5 9 5 = 125

    Table 3 Average leakage rate and total leakage of short-lived ra-dionuclides from the FDNPS estimated based on their measured

    concentrations at Sampling Point T1 (330 m) on 29 March 2011 at

    14:10 h

    Nuclide Half-life (y) Concentrationat SamplingPoint T1(MBq m-3)

    Averageleakage rate(TBqday-1)

    Total leakage(PBq)

    99mTc 6.872 9 10-4 0.16 1.23 0.26 0.02 0.004136Cs 3.605 9 10-2 2.8 21.2 4.62 0.29 0.06140Ba 3.494 9 10-2 5 37.8 8.24 0.53 0.12140La 4.597 9 10-3 2.5 19.0 4.12 0.27 0.06

    Environ Earth Sci

    123

  • Results and discussion

    Equation 5 is used to estimate the leakages of different

    radionuclides into the Pacific Ocean from the damaged

    FDNPS. The leakages of short-lived radionuclides such as99mTc, 136Cs, 140Ba and 140La are estimated using their

    measured concentrations on 29 March 2011 at 14:10 h at

    Sampling Point T1 (Table 3). Continuous and consistent

    measurements are not available for these radionuclides on

    other dates. The radioactive leakages of 131I, 134Cs and

    137Cs are estimated using their measured concentrations at

    Sampling Point T1 on several days as shown in Tables 4, 5,

    6, respectively.

    The highest leakage among the short-lived radionuclides

    is observed for 140Ba (0.53 PBq) followed by 136Cs

    (0.29 PBq) and 140La (0.27 PBq). The lowest leakage is

    observed for 99mTc (0.02 PBq). These radionuclides are

    extremely short lived and hence will not sustain in seawater

    for more than few days. Their radiological impact on the

    environment is trivial. However, knowledge on the

    Table 4 Average leakage rateand total leakage of 131I from

    the FDNPS estimated based on

    measured concentrations at

    Sampling Point T1 (330 m)

    Half-life of131I = 2.203 9 10-2 y

    Date of

    sampling

    Time of

    sampling

    Leakage period

    (h)

    Concentration

    (MBq m-3)

    Leakage rate

    (PBq day-1)

    25 March 2011 08:30 46.1 50 0.38 0.08

    26 March 2011 08:20 69.9 30 0.23 0.05

    26 March 2011 14:20 76.1 74 0.56 0.12

    27 March 2011 08:30 94.1 11 0.08 0.02

    27 March 2011 13:50 99.4 11 0.08 0.02

    29 March 2011 08:20 142.0 10 0.75 0.16

    29 March 2011 13:55 148.0 130 0.98 0.22

    30 March 2011 08:20 166.0 32 0.24 0.06

    30 March 2011 13:55 172.0 180 1.38 0.33

    31 March 2011 08:40 190.0 74 0.57 0.14

    31 March 2011 14:00 196.0 87 0.67 0.16

    1 April 2011 08:20 214.0 71 0.54 0.12

    1 April 2011 14:00 220.0 38 0.29 0.06

    3 April 2011 08:40 262.0 29 0.21 0.04

    3 April 2011 13:50 268.0 25 0.18 0.04

    4 April 2011 09:20 287.0 11 0.08 0.02

    4 April 2011 14:20 292.0 41 0.30 0.06

    5 April 2011 08:55 311.0 16 0.12 0.03

    5 April 2011 14:10 316.0 11 0.08 0.02

    Average leakage rate 0.41 0.35

    Total leakage during 14 days (PBq) 5.68 4.89

    Table 5 Average leakage rateand total leakage of 134Cs from

    the FDNPS estimated based on

    measured concentrations at

    Sampling Point T1 (330 m)

    Half-life of 134Cs = 2.06 y

    Date of sampling Time of sampling Leakage period (h) Concentration (MBq m-3) Leakage rate

    (PBq day-1)

    26 March 2011 14:30 76.1 12.0 0.09 0.02

    29 March 2011 08:20 142.0 24.0 0.18 0.04

    29 March 2011 13:55 148.0 31.0 0.23 0.05

    30 March 2011 13:55 172.0 47.0 0.36 0.08

    31 March 2011 08:40 190.0 21.0 0.16 0.04

    31 March 2011 14:00 196.0 25.0 0.19 0.05

    1 April 2011 08:20 214.0 22.0 0.17 0.04

    1 April 2011 14:00 220.0 11.0 0.08 0.02

    3 April 2011 08:40 262.0 11.0 0.08 0.02

    3 April 2011 13:50 268.0 10.0 0.07 0.01

    4 April 2011 14:20 292.0 19.0 0.14 0.03

    Average leakage rate 0.16 0.09

    Total leakage during 14 days (PBq) 2.24 1.27

    Environ Earth Sci

    123

  • estimates of their leakages during a nuclear accident is

    useful for accounting for all the radioactive releases from

    the damaged plants. The highest leakage among 131I, 134Cs

    and 137Cs is observed for 131I (5.68 PBq) followed by 137Cs

    (2.25 PBq) and 134Cs (2.24 PBq). Among these radionuc-

    lides, concern of large-scale and long-term impacts exists

    mainly for 137Cs and to a certain extent for 134Cs.

    The concentrations of all the radionuclides at Sampling

    Point T2 (30 m) are found to be lower than those at

    Sampling Point T1 (330 m) for most of the sampling times.

    The highest concentration of 131I at 330 m is

    180 MBq m-3 (on 30 March 2011). The highest concen-

    tration of 131I at 30 m is 120 MBq m-3 (1 April 2011). On

    29 March 2011, the concentrations of 131I at 330 and 30 m

    are 130 and 51 MBq m-3, respectively. The concentrations

    of 134Cs and 137Cs are measured as 31 and 32 MBq m-3,

    respectively, at 330 m on this day. The concentrations of

    these radionuclides at 30 m are 12 MBq m-3 on this day.

    This indicates that Sampling Point T1 (330 m) lies most

    possibly in the plume centreline along the flow direction

    (y = 0) and Sampling Point T2 (30 m) probably lies away

    from the plume centre and flow direction. The estimated

    average leakages are used to reproduce the measured

    concentrations of different radionuclides at 30 m at dif-

    ferent distances along the y axis using Eq. 5. For a distance

    of y = 25 m, very good matching is observed between the

    measured and estimated concentrations of different radio-

    nuclides on 29 March 2011 as shown in Fig. 7. This good

    agreement between the measured and estimated concen-

    trations of different radionuclides provides certain confi-

    dence in the estimated radioactive leakages.

    Calculations show that about 11.28 PBq of radioactivity

    leaked into the Pacific Ocean from the damaged FDNPS.

    Out of this, 131I constitutes 50.3 %; 134Cs 20 %; 137Cs

    20 %; 140Ba 4.6 %; 136Cs 2.6 %; 140La 2.3 % and 99mTc

    0.2 % of the total radioactive leakage. This is apart from

    the radioactive fallout from the atmosphere into seawater,

    which is considered as a major portion of the atmospheric

    emissions between 12 March and 23 March 2011 (Bailly

    du Bois et al. 2012). Most of the radioactive emission in the

    atmosphere, in the case of Fukushima nuclear accident,

    was deposited into the Pacific Ocean. Most of the radio-

    active releases from the Chernobyl nuclear accident were

    airborne. About 20, 10, 13 and 6 % of reactor inventories

    for 131I, 134Cs, 137Cs and 140Ba were released into the

    atmosphere for the Chernobyl accident, respectively (Val-

    kovic 2000). It is, in general, observed that there is no

    linear relationship between the fission yields and airborne

    releases. For example, the fission yield of 131I is 2.88 %,

    whereas the fission yield of 137Cs is 6.14 %. However, the

    atmospheric emission of 131I is about 7 % more than that of137Cs in the case of Chernobyl nuclear accident (Valkovic

    2000). This is because the atmospheric emissions of ra-

    dionuclides depend on chemical properties of radionuc-

    lides: highly volatile 131I is preferentially emitted into the

    atmosphere.

    Table 6 Average leakage rateand total leakage of 137Cs from

    the FDNPS estimated based on

    measured concentrations at

    Sampling Point T1 (330 m)

    Half-life of 137Cs = 30.2 y

    Date of sampling Time of sampling Leakage period (h) Concentration (MBq m-3) Leakage rate

    (PBq day-1)

    26 March 2011 14:30 76.1 12.0 0.09 0.02

    29 March 2011 08:20 142.0 24.0 0.18 0.04

    29 March 2011 13:55 148.0 32.0 0.24 0.05

    30 March 2011 13:55 172.0 47.0 0.36 0.08

    31 March 2011 08:40 190.0 21.0 0.16 0.04

    31 March 2011 14:00 196.0 25.0 0.19 0.05

    1 April 2011 08:20 214.0 22.0 0.17 0.04

    1 April 2011 14:00 220.0 11.0 0.08 0.02

    3 April 2011 08:40 262.0 11.0 0.08 0.02

    3 April 2011 13:50 268.0 10.0 0.07 0.01

    4 April 2011 14:20 292.0 19.0 0.14 0.03

    Average leakage rate 0.16 0.09

    Total leakage during 14 days (PBq) 2.25 1.28

    0.01

    0.1

    1

    10

    100

    Tc-99m I-131 Cs-134 Cs-136 Cs-137 Ba-140 La-140

    Conc

    entra

    tion

    (MBq

    m-3 )

    MeasuredEstimated

    Fig. 7 Comparison of measured and estimated concentrations ofradionuclides at Sampling Point T2 (30 m) on 29 March 2011 at

    14:10

    Environ Earth Sci

    123

  • Many times 137Cs fluxes give a good estimate of other

    radionuclides released due to nuclear accidents. Tables 5

    and 6 show that the 134Cs/137Cs activity ratio in the leakage

    is about 1. Such a ratio indicates recent nuclear accidents as

    this ratio decreases with time due to the short half-life of134Cs (Aarkrog 1988). The 134Cs/137Cs activity ratio in the

    measured concentrations at 30 m and 330 m was about 1 as

    shown in Fig. 8. In the present study, the 131I/134Cs activity

    ratio in radioactive leakage was about 2.5. Figure 8 also

    depicts the 131I/134Cs ratio in the measured concentrations

    at 30 m (Sampling Point T2) and 330 m (Sampling Point

    T1). At 30 m, 131I/134Cs ratio increased from 1.5 to 8.6 and

    then fell to 0.32. At 330 m, 131I/134Cs ratio increased from

    3.4 to 24 and fell to 0.18. The activity ratio of 131I/134Cs at

    30 m on 24 March 2011 was about 8.6 and reduced to 3.8

    on 31 March 2011 after about 8 days indicating the

    radioactive decay of 131I. The activity ratio of 131I/134Cs at

    330 m on 24 March 2011 was about 7.9 and reduced to 3.9

    after 8 days (neglecting the high ratios on 25 and 26 March

    2011). The 131I/137Cs ratio is a useful tool for determining

    whether 137Cs concentrations in the ocean result from

    radioactive leakage into the ocean or radioactive fallout

    from the atmosphere (Tsumune et al. 2012). If this ratio in

    the seawater follows the radioactive decay of 131I with

    time, then the source is derived from direct releases such as

    radioactive leakages; otherwise, the source is derived from

    indirect releases such as radioactive fallout from the

    atmosphere. This is because of the preferential deposition

    of 131I and 134Cs (or 137Cs) due to their particle sizes during

    emission. The fact that the 131I/134Cs ratios at both these

    distances before 24 March do not follow the radioactive

    decay of 131I indicates that radioactive leakages into the

    Pacific Ocean had approximately commenced from 24

    March 2011. Bailly du Bois et al. (2012) showed that a

    major portion of the atmospheric emissions occurred

    between 12 March and 23 March 2011. The 131I/134Cs

    activity ratios in soil were found to be varying between 50

    and 61 (Tagami et al. 2011). According to Chino et al.

    (2011), the 131I/134Cs activity ratios in the atmosphere

    varied larger than those in soil.

    Figure 9 presents the concentrations of 131I, 134Cs and137Cs at Sampling Point T1 (330 m) during the post release

    period, i.e. from 6 April 2011 onwards. The concentration

    data are fitted into exponential equations. The correlation

    coefficient of the fitting is about 0.94. The overall depletion

    rates of 131I, 134Cs and 137Cs in seawater work out to be

    0.01, 0.0069 and 0.007 h-1, respectively. The radioactive

    decay constants of 131I, 134Cs and 137Cs are 3.592 9 10-3,

    3.84 9 10-5 and 2.62 9 10-6 h-1, respectively. If the

    radioactive decay rates are removed from the overall

    depletion rates of these nuclides in seawater, their envi-

    ronmental depletion rates work out to be 6.41 9 10-3,

    6.86 9 10-3 and 6.99 9 10-3 h-1, respectively. These

    depletion rates are almost constant, indicating the influence

    of advection and dispersion over any other nuclide-

    dependent removal such as sorption by suspended sediment

    particles. The average environmental depletion rate of

    these radionuclides in seawater works out to be about

    6.75 9 10-3 h-1, which corresponds to an environmental

    half-life of 148.15 h (6.2 days) for the leakages of these

    radionuclides. Bailly du Bois et al. (2012) estimated the

    environmental half-life of 137Cs in seawater as 7 days for

    its leakage.

    A comparison of measured and estimated time history of131I concentrations at Sampling Point T2 is depicted in

    Fig. 10. The oscillations of measured concentrations as

    observed in Fig. 10 indicate that the release is not constant

    and there exists influence of local oscillatory forcing

    0.01

    0.1

    1

    10

    100

    18-Mar 23-Mar 28-Mar 2-Apr 7-Apr 12-Apr 17-Apr 22-Apr 27-Apr 2-May 7-May

    Rat

    io

    I-131/Cs-134 at 30 mCs-134/Cs-137 at 30 mI-131/Cs-134 at 330 mCs-134/Cs-137 at 330 m

    Fig. 8 Ratios of 131I/134Cs and 134Cs/137Cs in seawater at SamplingPoint T1 (330 m) and Sampling Point T2 (30 m)

    y = 2.9114e-0.0069x

    R2 = 0.8818

    y = 3.0555e-0.007x

    R2 = 0.8863

    y = 4.2503e-0.0105x

    R2 = 0.8937

    0.01

    0.1

    1

    10

    0 48 96 144 192 240 288 336 384 432 480 528 576 624

    Time (hr)

    Conc

    entra

    tion

    (MBq

    m-3 )

    Cs-134Cs-137I-131Expon. (Cs-134)Expon. (Cs-137)Expon. (I-131)

    Cs-134

    Cs-137

    I-131

    Fig. 9 Concentrations of 131I, 134Cs and 137Cs at Sampling Point T1(330 m) after termination of release where 0 h corresponds to 14:10 h

    on 6 April 2011

    Environ Earth Sci

    123

  • functions such as tides on concentrations as Sampling Point

    T2 is very close to the coast. Tides will cause temporal

    changes in the current field dominated by the Kuroshio

    Current. The present study is based on an analytical model

    which cannot account for non-constant release rate or the

    influence of oscillatory forcing functions such as tides.

    However, within these uncertainties, the study provides

    gross estimates of the leakages of different radionuclides

    into the Pacific Ocean. The increase of concentrations

    during the release period as well as the peak concentrations

    are well depicted by the model as shown in Fig. 10.

    A comparison is carried out between the presently

    estimated radioactive leakages of 131I, 134Cs and 137Cs into

    the Pacific Ocean from the damaged FDNPS and those

    reported by different agencies (Table 7). Only three esti-

    mates such as IRSN (2011); TEPCO (2011 and 2012) are

    available for 131I leakages so far. Since TEPCO revised the

    leakage estimates in 2012, their estimates reported in 2011

    may be discarded for all the radionuclides. The 131I leakage

    estimated in the present study is about two times lower than

    that of TEPCO (2012). It is about 1.7 times higher than that

    of IRSN (2011). The leakage of 134Cs estimated in the

    present study is about 1.6 times lower than that of TEPCO

    (2012). The leakage of 137Cs varies from 2 PBq (Buesseler

    et al. 2012) to 22 PBq (Bailly du Bois et al. 2012), and the

    highest reported leakage is about 11 times higher than the

    lowest reported value. According to Estournel et al. (2012),

    the source term estimated by Bailly du Bois et al. (2012),

    based on an analysis of observations, is much higher.

    Estournel et al. (2012) explained in detail how such a high

    leakage is obtained by Bailly du Bois et al. (2012). If the

    estimation of Bailly du Bois et al. (2012) is excluded, the

    highest leakage (4.3 PBq) is about 2.2 times higher than

    the lowest value (2 PBq). Variation by such a factor can be

    regarded as acceptable considering the non-constant leak-

    age rates, uncertainties in measurements and data and

    usage of different modelling methods. The leakages of137Cs estimated by IRSN (2011); Buesseler et al. (2012)

    and the present study match fairly well.

    Conclusions

    Leakage estimates of different radionuclides into the

    Pacific Ocean from a concrete pit containing highly

    radioactive effluent in the turbine building of the damaged

    FDNPS are derived using a two-dimensional advection

    dispersion model. It is estimated that about 11.28 PBq of

    0.001

    0.01

    0.1

    1

    10

    100

    1000

    0 100 200 300 400 500 600 700 800 900 1000Time (hr)

    Conc

    entra

    tion

    (MBq

    m-3 )

    MeasuredModelled

    Fig. 10 Comparison of measured and estimated concentrations of131I at Sampling Point T2 (30 m)

    Table 7 Leakages of 131I, 134Cs and 137Cs into the Pacific Ocean from the FDNPS estimated by different agencies

    Agency Estimated release

    (PBq)

    Remarks

    131I 134Cs 137Cs

    IRSN (2011) 3.3 2.3 Based on concentrations measured in the water pooled in the turbine hall of Unit-2. (Main leakage

    period: 25 March11 April 2011)

    Bailly du Bois

    et al. (2012)

    22 Based on measurements performed by TEPCO. (Main leakage period: 26 March8 April 2011)

    Tsumune et al.

    (2012)

    3.5 Using a regional ocean model. (Main leakage period: 26 March6 April 2011)

    Buesseler et al.

    (2012)

    2 Based on simple trapezoidal integration of the nuclide concentrations vs. depth profiles in the ocean.

    (Main leakage period: 1 April6 April 2011)

    Estournel et al.

    (2012)

    4.3 Based on numerical model simulation and comparison with observed concentrations in the vicinity

    of the two outlets of the nuclear power plants. (Main leakage period: 25 March8 April 2011)

    TEPCO (2011) 2.81 0.94 0.94 Based on discharge concentrations and leak jet dynamics (Main leakage period: 2 April6 April

    2011)

    TEPCO (2011) 11 3.5 3.6 Based on iteration of an oceanic circulation and dispersion model

    Present study 5.68 2.24 2.25 Based on iteration of two-dimensional advectiondispersion model. (Leakage period: 24 March6

    April 2011)

    Environ Earth Sci

    123

  • radioactivity was leaked into the Pacific Ocean from the

    damaged FDNPS. Out of this, 131I constitutes 50.3 %;134Cs 20 %; 137Cs 20 %; 140Ba 4.6 %; 136Cs 2.6 %; 140La

    2.3 % and 99mTc 0.2 % of the total leakage. The environ-

    mental half-life of 131I, 134Cs and 137Cs in seawater with

    respect to the leakage is estimated as 6.2 days. The esti-

    mated leakages of different radionuclides vary between

    0.02 PBq (99mTc) and 5.68 PBq (131I). The leakages of134Cs and 137Cs are estimated as 2.24 and 2.25 PBq,

    respectively. The study shows that advectiondispersion

    modelling coupled with measurements is highly useful to

    extract early information on the quantity and extent of

    radioactive releases into the environment during nuclear

    accidents.

    Acknowledgments Authors express sincere thanks to TEPCO,MEXT and NISA for making available large amounts of data per-

    taining to the FDNPS nuclear accident in the public domain ever since

    the occurrence of the accident. Thanks are also due to Dr. D. N.

    Sharma; Director; Health, Safety and Environment Group; Bhabha

    Atomic Research Centre for his keen interest in the study.

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    Estimation of radioactive leakages into the Pacific Ocean due to Fukushima nuclear accidentAbstractIntroductionMethodologyAdvection--dispersion modelInput data

    Results and discussionConclusionsAcknowledgmentsReferences

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