United Nations Scientific Committee on the Effects of Atomic Radiation

ATTACHMENT F-1

METHODOLOGY FOR ESTIMATING DOSES TO
NON-HUMAN BIOTAUNSCEAR 2013 Report, Annex A, Levels and effects of radiation exposure dueto the nuclear accident after the 2011 great east-Japan earthquake and tsunami,
Appendix F (Assessment of doses and effects for non-human biota)

Notes
The designations employed and the presentation of material in this publication do not imply theexpression of any opinion whatsoever on the part of the Secretariat of the United Nations
concerning the legal status of any country, territory, city or area, or of its authorities, orconcerning the delimitation of its frontiers or boundaries.
©

United Nations, August 2014. All rights reserved, worldwide.

This publication has not been formally edited.

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ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTAATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

3Contents
NOTES ......................................................................................................................................... 1
I. INTRODUCTION .................................................................................................................. 3
II. TRANSFER ........................................................................................................................... 4
A. Equilibrium approach to estimate activity concentrations in biota .................................. 5
B. Dynamic approach to estimate activity concentrations in terrestrial flora andfauna ................................................................................................................................. 7
C. Dynamic approach to estimate activity concentrations in aquatic organisms ................ 12
III. DOSES TO BIOTA .............................................................................................................. 14
REFERENCES ........................................................................................................................... 16

I.

INTRODUCTION

1.
In view of the great variety of plants and animals that could be exposed to radiation at
any given site of concern, it has been argued [Pentreath, 1999] that an assessment could be
reduced to manageable proportions by adopting reference exposure and dose models. This has
been the approach advocated by the Committee [UNSCEAR, 2008], consistent with theInternational Commission on Radiological Protection [ICRP, 2008]. The organisms selected in
the UNSCEAR 2008 Report form the basis for the exposure estimates made in the current
assessment (table 1).
Table 1. List of organisms selected by the Committee for assessing exposures [UNSCEAR,
2008]
Earthworm/soil invertebrate

Rat/burrowing mammal

Bee/above ground invertebrate

Wildgrass/grasses, herbs and crops

Pine tree/tree

Deer/herbivorous mammal

Duck/bird

Frog/amphibian

Brown seaweed/macroalgae

Trout/pelagic fish

Flatfish/benthic fish

Crab/crustacean

2.
The methodology that was used for both terrestrial and aquatic ecosystems was basedaround the ERICA Integrated Approach [Larsson, 2008] and consisted of the following steps:(a) select key species and radionuclides for analysis; (b) conduct the assessment preferablyusing actual radionuclide concentrations in the biota, measured over time and space from thefirst day of the accident; alternatively, (c) conduct an equilibrium assessment using
concentration ratios (CRs) to infer radionuclide concentrations in biota from measured
concentrations in media (soil and water), where appropriate; or (d) use kinetic models to
calculate “dynamic” concentrations in biota based on measured concentrations in media (i.e.
air, soil, freshwater, seawater, sediment); (e) enter the results into the ERICA Tool (dose4

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

calculation module) [Brown et al., 2008], to project dose rate as a function of time from which
integrated cumulative doses can be calculated. Once the dose rates were estimated, risk to nonhuman biota was assessed either deterministically or probabilistically.
3.
The ERICA Integrated Approach was designed to provide guidance on impacts ofradioactive material on the environment to ensure that decisions on environmental issues give
appropriate weight to the exposure, effects and risks from ionizing radiation exposure.
Emphasis was placed on protecting the structure and function of ecosystems fromradionuclides [Larsson, 2008], and supporting software (the ERICA Tool) was developed toserve this purpose [Brown et al., 2008]. The part of the approach that deals with the exposure
assessment (figure I) encapsulates the quantification of risk to organisms in the environment
through the application of transfer and dosimetric models and, for screening purposes, thecomparison of predicted dose rates with appropriately derived benchmarks.
Figure I. The ERICA Tool assessment

Activity concentrations in reference media
CR

DCCs

Activity concentrations
in reference organism

Occupancy
factors

DCC
Internal dose rate

External dose rates

Total absorbed dose rate

- FREDERICA database- Natural background
CR = concentration ratio, DCC = dose conversion coefficient (relating activity concentrations to dose rates); media is soil, water, sediment or
air depending upon the ecosystem, available data and radionuclide under consideration. The FREDERICA (radiation effects) database isdescribed in Copplestone et al. [2008].

4.
The derivation of dose-rate can be split into two components, these being transfer anddose estimation. The two components are considered in the following sections.

II.

TRANSFER

5.
In many cases, activity concentrations in plants and animals needed to be derived frommeasured or estimated activity concentrations of radionuclides in environmental media (soil,
water and/or sediments). Where possible, activity concentrations in plants and animals could be
used directly in subsequent dose-rate calculations but the expectation was that such data would
be quite limited compared to measured concentrations in environmental media. The approach

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

5taken to simulating transfer was twofold. The first approach was to use a simple model basedon concentration ratios where the assumption was that the activity concentrations in the bodyof a selected plant or animal were in equilibrium with the surrounding medium. Bearing in
mind that equilibrium approaches have limited applicability in situations where environmental
concentrations are changing rapidly with time [Coughtrey and Thorne, 1983; Brown et al.,2004], a second approach based upon kinetic models was also applied.
6.
In the intermediate phase after the Fukushima-Daiichi Nuclear Power Station (FDNPS)
accident (approximately the first two months), the fact that activity concentrations in various
compartments within the ecosystems were unlikely to have been in equilibrium provided
further support to the contention that it was more appropriate to model the activity
concentrations in selected biota using kinetic models. For example, in the context of the
FDNPS releases, Vives i Batlle [2011] considered that the activity concentrations in biota
predicted by equilibrium-based models may deviate significantly from actual activity
concentrations in the biota when activity concentrations were changing rapidly in time andspace. For example, the application of concentration ratios had a tendency to produce an overestimation in the initial phase when activity concentrations in media were increasing [Psaltaki
et al., 2012]. Hence, a dynamic modelling approach was likely more appropriate to provide anindication of changing dose rates in the early stages of the accident. However, in view of themore involved parameterization required, the dynamic modelling work was limited to selected
key species and a limited number of the radionuclides contributing most to dose.
7.
The application of equilibrium models was essentially the approach that had been
advocated by the Committee [UNSCEAR, 2008] for constant long-term releases of radioactive
material, for deriving activity concentrations in terrestrial and aquatic organisms (with the
exception of terrestrial flora where some consideration had also been given to deposition andinterception of radionuclides).
A.

Equilibrium approach to estimate activity concentrations in biota

8.
Concentration ratios are used to derive activity concentrations in the ERICA Tool
[Brown et al., 2008]. The approach was intended for assessing the impact of routine releases ofradionuclides to the environment. By way of example, for terrestrial biota the whole-bodyconcentration ratio, CRwo, is defined as in equation (1)A biotar
CR wo  b,soil
Ar(1)where:

A biota
is the activity concentration of radionuclide r in the whole organism of biotab,r“b” (Bq kg-1 fresh weight (f.w.));
A soilis the activity concentration of radionuclide r in soil (Bq kg-1 dry weight (d.w.))r
For aquatic organisms, activity concentrations in soil are replaced by those in water.
9.
The transfer database in the ERICA Tool based upon Hosseini et al. [2008], Beresford
et al. [2008] and supplemented by the more updated information provided in ICRP [2009] wasused to derive activity concentrations in non-human biota for the UNSCEAR evaluation.
Concentration ratios from ICRP [2009] are shown in tables 2–4.

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ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

Table 2. Concentration ratios for terrestrial ecosystem from ICRP [2009]
Element

Organism

Concentration ratio(Bq kg-1 f.w. per Bq kg-1 d.w.)

Cs

Pine tree/tree

7.5 × 10-2

I

Pine tree/tree

5.3 × 10-2

Te

Pine tree/tree

2.5 × 10-1

Cs

Wildgrass/grasses,herbs and crops

8.6 × 10-1

I

Wildgrass/grasses,herbs and crops

5.3 × 10-2Te

Wildgrass/grasses,herbs and crops

2.5 × 10-1

Cs

Bee/above ground invertebrate

4.7 × 10-3

I

Bee/above ground invertebrate

2.8 × 10-1

Te

Bee/above ground invertebrate

3.8 × 10-2

Cs

Earthworm/soil invertebrate

4.8 × 10-2

I

Earthworm/soil invertebrate

1.4 × 10-1

Te

Earthworm/soil invertebrate

3.8 × 10-2

Cs

Rat/burrowing mammal

2.2 × 10-1

I

Rat/burrowing mammal

4.0 × 10-1

Te

Rat/burrowing mammal

2.1 × 10-1

Cs

Deer/herbivorous mammal

1.6 × 100I

Deer/herbivorous mammal

4.0 × 10-1

Te

Deer/herbivorous mammal

2.1 × 10-1

Cs

Duck/bird

2.2 × 10-1

I

Duck/bird

4.0 × 10-1

Te

Duck/bird

2.1 × 10-1

Cs

Frog/amphibian

2.8 × 10-2

I

Frog/amphibian

4.0 × 10-1

Te

Frog/amphibian

2.1 × 10-1

Table 3. Concentration ratios for freshwater ecosystem from ICRP [2009]
Element

Organism

Concentration ratio(Bq kg-1 f.w. per Bq l-1)

Cs

Trout/pelagic fish

2.7 × 103

I

Trout/pelagic fish

6.2 × 101

Te

Trout/pelagic fish

2.8 × 102

Cs

Frog/amphibian

1.6 × 103I

Frog/amphibian

2.6 × 102Te

Frog/amphibian

2.8 × 102

Cs

Duck/bird

4.4 × 102I

Duck/bird

2.2 × 102

Te

Duck/bird

7.0 × 102ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

7

Table 4. Concentration ratios for marine ecosystem from ICRP [2009]
Element

Organism

Concentration ratio(Bq kg-1 f.w. per Bq l-1)

Cs

Flatfish/benthic fish

3.6 × 101

I

Flatfish/benthic fish

9.0 × 100

Te

Flatfish/benthic fish

1.0 × 103Cs

Crab/crustacean

1.4 × 101I

Crab/crustacean

3.0 × 100Te

Crab/crustacean

1.0 × 103

Cs

Brown seaweed/macroalgae

1.2 × 101

I

Brown seaweed/macroalgae

1.4 × 103

Te

Brown seaweed/macroalgae

1.0 × 104

B.

Dynamic approach to estimate activity concentrations in terrestrial
flora and fauna

10.
In view of the available data, it was most appropriate to split the modelling into flora(wild grass/grasses, herbs and crops and pine tree/tree) and fauna (deer/herbivorous mammal
and rat/burrowing mammal) on the basis of the classifications given in UNSCEAR [2008].
11.
Using a variant of the methodology given in UNSCEAR [2008], the activity
concentration in flora could be estimated from the total deposition density using an expression
accounting for interception by foliage, the direct deposition density onto soil, weathering lossesof radionuclides from vegetation and uptake from soil to plant.
12.
In the case of an acute deposition event, the radionuclide content on vegetation at timet, accumulated via direct deposition from the air, can be calculated (based on Brown et al.
[2003]) as:C air
flora ,rf flora  Dtot , rb flora [e(  (  flw ,r  r )t )](2)

where:
Cflora,r is the radionuclide activity concentration in flora from deposition (Bq kg-1 f.w.);
fflora is the interception fraction for a given flora (dimensionless);
Dtot,r is the total deposition density of radionuclide r (Bq m-2);
flw,r is the weathering constant for a given flora for radionuclide r (d-1);
r is the decay constant for radionuclide r(d-1);
b is standing biomass of the flora (kg m-2);
t is time (d).
For the same acute deposition, at time t, there is also a component of biota
13.
concentrations that arises from soil-to-plant transfer. In this case, an assumption was made thatfor this fraction of the concentration in the plant due to root uptake, equilibrium existed
between the activity concentration in the plant and the soil.

8

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA Dtot ,r . (1  f flora )  f flora  (1  e   flw ,rt )  e rt 
C flora ,r  .CR flora ,r soil  d soil


soil

Where:(3)

soilis the dry soil density (kg m-3 d m.);dsoil is the depth of soil within which radionuclide r has become mixed (m);CRflora,r is the soil-to-plant concentration ratio for radionuclide r (dimensionless).

All other parameters are described above for equation (2).
Finally, growth dilution may play an important role in determining activity
14.
concentrations in grass because the period of deposition considered, i.e. early spring, normallycorresponds to substantial increases in vegetation biomass. A simple model was applied using
information for grass yield at various calendar dates [Müller and Pröhl, 1993], assuming thatthe change in biomass with time was linear (table 5).
Table 5. Assumed change in biomass with season
Date

15 March

15 May31 OctoberYield (kg m-2 f.w.)

0.05

1.5

1.5

15.
Application of this model also allowed for time-varying deposition rates to beconsidered. For this more complex situation, the problem could be solved numerically. There
was an assumption in this model that a representative interception fraction f for a given flora
type could be applied for the entire simulation period. Data compilations for agricultural
systems in relation to this parameter [IAEA, 2010] indicate that the interception fraction
depends on whether dry or wet deposition is occurring, the stage of development of the plantand plant type in question, the capacity of the canopy to retain water, elemental properties of
the radionuclide, and other factors such as amount and intensity of rainfall in the case of wet
deposition and particle sizes of the deposited material. The approach taken here was, therefore,
arguably simplistic but — in view of the numerous uncertainties involved — it at least
provided an indication of activity concentration levels following FDNPS releases and it
attempted to model the dynamics of interception and loss from flora in contrast to approaches
that consider soil-to-plant transfer only. In addition to the interception fraction, biomass (whichclearly relates to the stage of development of the plant) also required further consideration as
an important model parameter.
A map of vegetation coverage for Japan was available online [MOE, 2012]. Reference16.
to map 564017 in this series, an area in proximity to Fukushima-Daiichi, showed natural/seminatural ecosystems characterized by various plant communities including Korean hill/mountaincherry (Prunus vercunda), Japanese red pine (Pinus densiflora), red oak (Quercus serrata) andevergreen conifer plantations. Takahashi et al. [2002], provide information for a secondary
deciduous broad-leaved forest in Japan where the total above- and below-ground biomass wasgiven as 130 t ha-1 and the ratio of above- to below-ground biomass was 5.16. Clearly there
would be differences in biomass relating to plant community type, species and age of forest
stand but these data provide a reasonable indication for tree biomass for the subsequentcalculations.
Empirical information on grass biomass in Fukushima Prefecture were not found for the17.
dates of interest but generic information from published data could be used to provide
indicative values. Although Schino et al. [2003] studied grasslands in mountainous areas of

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

9central Italy, the work provides an indication of variations in grass biomass that can arise fromseasonality and the presence of different species. The recorded range of grass biomass in thisaforementioned study was approximately 60 to almost 700 g m-2 providing a useful context for
selecting an appropriate biomass value for “wild grass/grasses, herbs and crops”.
The growing season for Japanese pampas grass (Miscanthus sinensis) for sites in
18.
northern Japan is the period May to October [Shoji et al., 2012]. If this observation isconsidered typical for other grass species, it seems likely that the timing of the main deposition
events following atmospheric radionuclide releases from the FDNPS, i.e. mid-March to early
April, preceded any substantial new growth. Nonetheless, by mid-April it was possible tocollect grass samples in areas affected by the Fukushima-Daiichi emissions, as evidenced by
the supplementary material published by Yasunari et al. [2011], so it seems likely that somecoverage of herbaceous vegetation would have been present at the time of atmospheric releases
leading to a degree of interception, albeit to a relatively low degree, of deposited activity. Thisappears to be borne out by evidence from the early phase of the accident where it was possible
to measure elevated concentrations of 131I and 137Cs in herbaceous “weeds” [NERH, 2011].
Yasunari et al. [2011] showed, using model simulations, that the highest deposition
19.
densities of radiocaesium downwind of FDNPS were clearly aligned with satellite-observed
precipitation. There were numerous rainfall events during the emission period [Kato et al.,2012] as exemplified by a daily accumulated rainfall in excess of 20 mm in areas to the northeast of the FDNPS that occurred on 21 March 2011 [Yasunari et al., 2011].
Taking all of the above points into consideration, the grass biomass was likely to have20.
been relatively low, as was the interception fraction. Best estimate values of 0.05 kg m-2 forbiomass and 0.05 (dimensionless) for the interception fraction of radiocaesium on grass wereselected.
The IAEA [2010] provides values for the mass interception fraction, fB (defined as the21.
interception fraction divided by biomass) of caesium deposited on grass ranging from 0.7 to
5.5, the values depending upon antecedent rainfall — the low value corresponding to a
relatively high rainfall event. Furthermore, an fB value of 1.1 was considered as typical for
caesium deposited on grass following the Chernobyl accident. The selected values forinterception fraction for caesium on grass and grass biomass for the model simulations
corresponded to an fB value of 1, a level which was reassuringly commensurate with the value
expressing deposition from the Chernobyl accident.
Limitations to the concentration ratios used arose from an incompatibility between
22.
empirical data based on long-term steady-state conditions with a period directly following an
accident. The CR values used (table 2) were based on empirical datasets from field
investigations collated to avoid including data pertaining to the period directly following
depositional events (e.g. global weapons test fallout and deposition following the Chernobyl
accident for some radionuclides such as Cs, Pu, Sr and Am) and, thus, should omit values
pertaining to deposition onto the surface of vegetation [Beresford et al., 2008]. These default
CR data are generally assumed to correspond to, and thus are applicable for, a soil depth of
10 cm. There was thus an inconsistency with the observed distributions of radionuclides shortly
following deposition. For example, Kato et al. [2012] reported that more than 86% of total
radiocaesium and 79% of total 131I were absorbed in the upper 2.0 cm in a soil profile from acultivated area that had relatively high deposition density, sampled at the end of April 2011.
This area was in proximity to the FDNPS site (less than 50 km distant, in a north-easterly
direction). Furthermore, bioavailability of radiocaesium has been observed to decrease withtime following its introduction to soils [Vidal et al., 1995] implying that CRs based upon long-

10

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

term post-depositional datasets might not appropriately reflect the transfer occurring in theearly phase depositional environment. Indeed this contention is evidenced by reviews ofpublished information on 137Cs in the soil–plant system shortly after the Chernobyl accident[Fesenko et al., 2009]. Finally, the soil type, as defined by various soil properties, strongly
influences transfer to plants [IAEA, 2010] and there will undoubtedly be differences in the soil
types upon which the default data were based and the soil types in Japan for which the transfer
parameters were applied.
Although some information exists on soil–to–grass transfer for the short term after23.
accidents [Fesenko et al., 2009] and differs substantially from equilibrium transfer factors,
these data are, by the author’s own admission, insufficient for adequate transfer estimation.
Furthermore, with the model constructed and parameterized in its current configuration, direct
deposition dominates the total activity concentration in vegetation in the initial weeks. Thismeans that although CR values are highly uncertain, their influence on the dose rates calculated
was relatively unimportant.
The parameters were assigned different default values as shown in table 6. Two24.
categories of flora — pine tree/tree and wild grass/grasses, herbs and crops — were considered.
Table 6. Parameters used to model activity concentrations in flora with time
Parameter

Value

Units and notes

References

soil

1300

kg m-3

Yasunari et al. [2011]

dsoil

0.05

m,
Based on soil sampling
information provided withdeposition data

MEXT

Pine tree/tree, Cs

0.7

Dimensionless,
IAEA present 50–90% deposition
for global fallout and 50–60 year
old pine stands

Bunzl and Schimmack [1989]
IAEA [2010]

Pine tree/tree, I

0.7

Wild grass/grasses,
herbs and crops, Cs

0.05

f varies from 0.84 (drydeposition) to 0.027 (wetdeposition, heavy rain)

IAEA [2010]

Wild grass/grasses,
herbs and crops, I

0.03

fB for Chernobyl deposits = 0.7(cf. 1.1. for Cs)

IAEA, 2010

Pine tree

11

kg m-2

Takahashi et al. [2002]

Wild grass/grasses,
herbs and crops

0.05–1.5kg m-2

Table 5

Pine tree/tree, Cs

7.6 × 10-3d-1; IAEA [2010] considers a
range of 7.6 × 10-3 to 2.8 × 10-2

IAEA [2010]

Pine tree/tree, I

7.6 × 10-3

Wild grass/grasses,
herbs and crops, Cs
Wild grass/grasses,
herbs and crops, I

5.0 × 10-2

d-1, Table VIII, p.37 [IAEA,
1996]

IAEA [1996]

f

f

b

flw,rflw,rDependencies: flora,
radionuclide

IAEA [2010]

7.0 × 10-2

25.
Examples of kinetic models for terrestrial environments have been published in the
open literature and one of these, the FASTer model, was selected for further application

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

11[Beresford et al., 2010; Brown et al., 2003]. For herbivorous mammals, the data used for input
could be those specifying the activity concentrations in grass as expressed above. Details wererequired regarding biokinetic parameters for various representative animals/fauna as described
below.



dC r , a i  n  x i .AE r ,i . FMI .C r ,i  C r , a . r , a
M
dt
i 1

(4)

Where :
xi is the fraction of the diet associated with dietary component i;
AEr,i is the assimilation efficiency (dimensionless) for radionuclide r within dietarycomponent i;
FMI/M is the ingestion rate per unit mass of animal (kg f.w. d-1 per kg f.w.);
Cr,i is the activity concentration of radionuclide r in dietary component i (Bq kg-1 f.w.);
Cr,a is the activity concentration of radionuclide r in the whole-body of the animal(Bq kg-1 f.w.);
r,a is the effective loss rate of radionuclide r from animal (d-1) incorporating both
excretion rate and physical decay of the radionuclide.
This model was applied to estimate the transfer to deer/herbivorous mammals and26.
rat/burrowing mammals. Fresh matter ingestion rates (FMI) were derived using allometric
relationships of the form given in equation (5) as shown in table 7. The animal masses thatwere used in these derivations were extracted from ICRP [2008].
FMI = a.Mb

(5)

Where:
a is the multiplication constant in the allometric relationship for fresh matter intake by
the animal (kg d-1);
b is the exponent in the allometric relationship for fresh matter intake by the animal(relative units);
M is the mass of the animal (kg).

Table 7. Estimated fresh matter ingestion rates, FMI, for the various animals selected for study
Organism

FMI (kg/d)Comments and references

Deer/herbivorous mammal

6.3 × 100

Mass = 245 kg [ICRP, 2008]
FMI for herbivores (kg d-1) = 0.1995M0.628 from Nagy [2001]

Rat/burrowing mammal

8.4 × 10-2

Mass = 0.314 kg [ICRP, 2008]
FMI for Rodentia (kg d-1) = 0.2296M0.864 from Nagy [2001]

27.
Similarly, r,a the effective loss rate of radionuclide, r, from animal, a, can be derived
using allometric relationships (table 8) and the animal masses specified above (table 7)

12

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

Table 8. Allometric equations used to derive effective loss rates (d-1) for studied animals fromthe mass of animals (kg) [Brown et al.,2003]
Element

Allometric equations

Caesium

r ,a ln 218.36 M 0.24
ln 2
r , a 
16.7 M 0.13

Iodine28.

The various parameters required in the model runs are thus specified in table 9.

Table 9. Parameters used in dynamic model runs
Parameter

Dependencies: fauna,
flora, chemical element

Value

Units

Notes (references)xi

Grass (deer)
Tree (deer)
Deer, Cs
Deer, I

dimensionlessdimensionlessdimensionlessdimensionlessdimensionless
kg f.w. d-1 per kg(FMI/M)

d-1
d-1

Table 8; Mass = 245 kg
Table 8; Mass = 245 kg

FMI/M

Deer

0.5
0.5
1
1
1
2.6 × 10-2

r,aDeer, Cs
Deer, I

1.01 × 10-2
2.03 × 10-2

AE

C.

Dynamic approach to estimate activity concentrations in aquaticorganisms

29.
A relatively simple biokinetic model based on first-order exchange kinetics between themedium and the organism can be defined for the aquatic ecosystem using only the biologicalhalf-life of elimination and the CR. The simplest approach, which was used here, was toassume a two-compartment first-order kinetic model with constant rates of uptake andexcretion from water (ku and kel), as shown in figure II.
Figure II. Simple two-compartment biokinetic model

Seawater(Aw)
Uptake (ku)Elimination (kel)Organism(Ao)30.
The kinetic rates ku and kel can be simply written in terms of the biological half-life ofelimination after uptake from water (TB1/2), the length of time required for all combined
physiological processes to cause the loss of half of the bioaccumulated radionuclide from anorganism. Hence:ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

kel 

ln 2
ln 2 mku 
CF
TB1/ 2 and
TB1 / 2 V

13

(6)

31.
The above explanation is somewhat simplified because radionuclide turnover by marineorganisms can be multiphasic, i.e. with an initial fast release followed by a slower, longer term
release, with a certain point in time marking the transition from one phase to another. There aremany instances of this, such as the time-dependence of technetium and plutonium in European
lobsters [Olsen and Vives i Batlle, 2003; Swift, 1992]; 241Am and 237Pu in mussels [Guary andFowler, 1981] or the typical multiphasic release curve representing the depuration of 131I from
Littorina littorea [Wilson et al., 2007; Wilson et al., 2005].
32.
An adequate compromise is a three-compartment model which makes the working
simplification that marine organisms absorb radionuclides mainly from the surrounding
seawater, allowing one to relate their radioactivity concentration to their environment. Such amodel includes direct exchange with seawater of specific activity Aw through both a fastcompartment of activity Af and a slow compartment of activity As:

dAw
 k fel A f  k els As  (k fu  k su ) Aw
dt
dA f
dA
 k fu Aw  k fel A f ; s  k su Aw  k els As
dt
dt

(7)

33.
In this way, the model incorporates biphasic release, being sufficiently simple to besolved analytically without recourse to numerical computation. The model (figure III) requires
relatively few parameters, rendering the approach practical for use in radiological assessments[Vives i Batlle et al., 2008].
Figure III. Basis of a dynamic model for the transfer of radionuclides to marine biota[Vives i Batlle et al., 2008]

Seawater(A2)sf

Uptake (ku )Uptake (ku )fDirect excretion (kel )sDelayed excretion (kel )Fast phase(A2)

Slow phase(A3)

Radioactive decay ()34.
There is the more advanced ECOMOD radioecological approach, which describes the
dynamic processes of radionuclide migration in aquatic biota as radioactive tracers of stableanalogous elements involved in growth and metabolism of organisms [Kryshev and Ryabov,
2000; Kryshev and Sazykina, 1986; Sazykina, 2000]. The ECOMOD modelling approach was

14

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

developed to calculate non-equilibrium, non-linear processes in aquatic ecosystems, combining
radioecological and ecological equations.
35.
This approach considers that, for a growing organism of total mass M(t), theradionuclide activity concentration y (Bq kg-1, fresh weight) in a given tissue changes
according to the following equation [Kryshev and Ryabov, 2000]:B 1 dM  Q1A  1 dMB dy     a r  a r  X (t )y  A 
M M dt  Q0  M dt
Mdtwhere:(8) is the radioactive decay constant (s-1);
a is a proportionality coefficient between the rate of biological loss of a radionuclide

from a target tissue and the general metabolic rate of the organism;
Br is the metabolic rate (kg s-1);
X(t) is the radionuclide activity concentration in food (Bq kg-1, fresh weight) or in water(Bq L-1) when bioassimilation occurs directly from water at time t;
A
Q1A , Q0 are the stable element concentrations in a given tissue and in food or water(mg kg-1), respectively.

36.
In the simplest case where X(t) = X = const; y(0) = 0, assuming that the mass of theorganism is constant and neglecting radioactive decay, the solution represents the classicalform of the radionuclide bioaccumulation curve:

For the depuration case, in which X(t) = 0, y(0) = Y0 , the solution gives the simple monophasic
depuration curve:Ba r ty(t)  y(0)e M

III. DOSES TO BIOTA
37.
Once activity concentrations in environmental media and biota had been collated andcalculated, dose rates were derived through application of the ERICA Tool. The basic
underlying equations (equations 9 and 10) use activity concentration data in order to deriveabsorbed dose rates due to internal (Dint) and external (Dext) exposure (in units of Gy h-1). The
total absorbed dose rate is the sum of these components, through the application of doseconversion coefficients (DCCs).
 b   C b * DCC b
D
int
i
int,i

(9)

i

Where:
C ib is the average concentration of radionuclide i in the reference organism b (Bq kg-1
fresh weight);

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

15b
DCC int,
i is the radionuclide-specific dose conversion coefficient (DCC) for internal

exposure defined as the ratio between the average activity concentration of radionuclide
i in the organism j and the dose rate to the organism b (µGy h-1 per Bq kg-1 fresh
weight).
 b   v  Cref * DCCb
D
ext
z
zi
ext ,ziz(10)iWhere:
vz is the occupancy factor, i.e. fraction of the time that the organism b spends at a
specified position z in its habitat;
Cziref is the average concentration of radionuclide i in the reference media at a given
location z (Bq kg-1 fresh weight or dry weight (soil or sediment) or Bq L-1 (water));
DCC jext,zi is the dose conversion coefficient for external exposure defined as the ratio
between the average activity concentration of radionuclide i in the reference media
corresponding to the location z and the dose rate to organism b (µGy h-1 per Bq kg-1
fresh weight or Bq L-1).
38.
The DCCs used correspond to those reported in ICRP [2009]. Occupancy factors fororganisms were selected such that they might characterize simplified yet realistic exposure
geometry (table 10).
Table 10. Source target exposure geometry assumed for selected organisms
Organism
Earthworm/soil invertebrate

Exposure geometry assumption
In soil, volumetric source

Wildgrass/grasses, herbs and crops

On soil, volumetric source

Duck/bird

1.On soil, volumetric source
2. At water–air interface, aquatic source

Trout/pelagic fish

In water column, aquatic source

Rat/burrowing mammal

In soil, volumetric source

Pine tree/tree

On soil, volumetric source

Frog/amphibian

1.On soil, volumetric source
2. In water column, aquatic source

Flatfish/benthic fish

At water–sediment interface, aquatic source

Bee/above ground invertebrate

On soil, volumetric source

Deer/herbivorous mammal

On soil, volumetric source

Brown seaweed/macroalgae

At water–sediment interface, aquatic source

Crab/crustacean

At water–sediment interface, aquatic source

39.
Weighted total dose rates (in µGy h-1) are derived through the application of weighting
factors (dimensionless) for alpha, low beta and high beta–gamma radiation (equations 11 and
12).

DCC int  wf low  DCC int,low  wf    DCC int,   wf   DCC int,

(11)

DCC ext  wf low  DCC ext ,low  wf    DCC ext , (12)Where:
wf = weighting factors for various components of radiation (low β, β + γ and α);
DCC = dose conversion coefficients in µGyh-1 per Bq L-1 or Bq kg-1.

16

ATTACHMENT F-1: METHODOLOGY FOR ESTIMATING DOSES TO NON-HUMAN BIOTA

40.
Default radiation weighting factors of 10 for alpha radiation, 1 for low energy beta and
1 for (high energy) beta and gamma radiation were applied in this assessment in line with those
applied in UNSCEAR [2008].
41.
The dosimetric calculation underpinning the derivation of DCCs is dealt with in detailelsewhere [Ulanovsky and Pröhl, 2006; Ulanovsky et al., 2008]. Radioactive progeny were
included in the DCCs for their parent if their half-lives were shorter than 10 days. DCCs forinternal exposure were derived assuming a homogeneous distribution of the radionuclide in theorganism; the error introduced by this assumption was, in view of the assessment goals,
considered to be of minor significance [Gómez-Ros et al., 2008].

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