wsc radioecology research group
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WSC Radioecology Research Group. A new methodology for the assessment of radiation doses to biota under non-equilibrium conditions. J. Vives i Batlle, R.C. Wilson, S.J. Watts, S.R. Jones, P. McDonald and S. Vives-Lynch. EC PROTECT Workpackage 2 Workshop, Vienna, 27 - 29 June 2007. - PowerPoint PPT PresentationTRANSCRIPT
WSC Radioecology Research WSC Radioecology Research GroupGroup
A new methodology for the A new methodology for the assessment of radiation assessment of radiation
doses to biota under non-doses to biota under non-equilibrium conditionsequilibrium conditions
J. Vives i Batlle, R.C. Wilson, S.J. Watts, S.R. Jones,
P. McDonald and S. Vives-Lynch
EC PROTECT Workpackage 2 Workshop, Vienna, 27 - 29 June 2007
IntroductioIntroductionn
Interest in recent years regarding protection of non-human biota
Different approaches: Environment Agency R&D 128 FASSET/ERICA RESRAD - Biota, Eden, EPIC-DOSES3D, etc.
All have one common theme:
Assume equilibrium within the system they are modelling
Current work builds on previous work but takes it to the next stage:
Non-equilibrium conditions
ObjectivesObjectives
Model the retention behaviour observed for many organisms and radionuclides.
Express model rate constants as a function of known parameters from the
literature.
Ensure the model automatically reduces to the old CF-based approach in the
non-dynamic case.
Incorporate dosimetry compatible with FASSET and EA R&D 128
methodologies.
Encode the model in a simple spreadsheet which assesses for lists of
radionuclides and biota over time.
Model DesignModel Design
Environment
(seawater)
Slow phaseFast phase Organism
Fast Uptake
Slow Uptake
Fast Release
Slow Release
Rad
ioac
tive
de
cay
Rad
ioactive d
ecay
Multi-phasic Multi-phasic releaserelease
Some organisms have fast followed by slow release, represented by two biological half-lives
Typical biphasic retention curve, representing the depuration of 131I from L. littorea (Wilson et al., 2005).
Model optionsModel options Three cases are possible:
No biological half-lives known use instant equilibration with a CF (current method). One biological half-life known use simple dynamic 2-compartment model. Two biological half-lives known use fully dynamic 3-compartment model.
Flow diagramFlow diagramBiokineticdatabase
Water activity
Dosimetry database
Calculate initial conditions of the system
2 TB1/2 known?
Slope transition known
No
Yes
% retention known?
Calculate 2 rate constants from TB1/2s
At least 1 TB1/2 known?
No
No
Apply npn-dynamic model using CF
No
Yes
Calculate remaining rate constants for basic model
(2 components)
Yes
Calculate remaining rate constants for advanced model
(3 components)
Run a loop for series of regular time steps
Yes
Refresh initial conditions of new time step using solution
form previous step
Refresh initial conditions of new time step using solution
form previous step
Write results into the spreadsheet
Basic equationsBasic equations
33313232321313
22232123231212
11131213132121
)0();()()()()(
)0();()()()()(
)0();()()()()(
qtqtqkktqktqkdt
tdq
qtqtqkktqktqkdt
tdq
qtqtqkktqktqkdt
tdq
Basic equationsBasic equations General solution:
Involves Laplace transformation, algebraic manipulation and some substitutions (, , d’s and ƒ’s are functions of the rate constants).
3,2,1,
)()()(
22
iefdq
efdqf
tq tiiitiiiii
Model parameterisationModel parameterisation Initial conditions:
0;Aq 32W1 qqV
Approximation 1 (organism is a faster
accumulator than the medium):
Approximation (organism holds less activity than the medium):
k21 << k12 and k31 << k13
q1 >> q2 or q3
ConsequencesConsequences Biphasic release:
Simple formulae for all the model constants:
3
132
12
2lnk;
2lnk
TT
13
31
12
21
1t k
k
k
k
)(
)(lim tq
tq
V
mCF B
)(
;2ln
k ;2ln2ln
k;2ln
k
31
313
1
3221
212
unknownxk
Tx
TCF
V
m
TT
Calculation of "x"Calculation of "x" If we know the % retained at time
(f100):
If we know when the release curve closes in to slope of the final phase (factor f ):
213
31
100
221
k2ln
;100
11
2lnk 12
1213
TV
CFmk
ef
eeTV
CFm kkk
)(
13
12
12
132131
1
)(
13
121221
1312
1312
k
k
1
1k
;k
k
1
11kk
kk
kk
effk
kk
effV
CFm
Sensitivity analysisSensitivity analysis
Basis for the dosimetryBasis for the dosimetry Same as EA R&D 128 and FASSET
(aquatic)
i
extitotal
waterisurface
watersedimenti
surfacesedimentexternal
iitotal
orgiernal
water
solidentse
drysolidentse
DCCCf
fCf
fH
DCCCH
CfCfC
,
int,int
dimdim
22
)1(
i
= summation over all nuclides
Corg , Cwater and Csediment = nuclide concs.
in Bq kg–1 or Bq m–3
= density of sea water
fsolid = solids fraction of wet sediment (0.4).
int,itotalDCC and
extitotalDCC , = DCCs
in Gy h–1 per Bq kg-1
fsediment , fsurface and fwater = fractions of time in
different media
Model inputsModel inputs
Biokinetic Biokinetic ParametersParameters
Current data defaults from literature
User can edit with site-specific data
Model OutputsModel Outputs Reference organisms
Phytoplankton
Zooplankton
Macrophyte
Winkle
Benthic mollusc
Small benthic crustacean
Large benthic crustacean
Pelagic fish
Benthic fish
Nuclides 99Tc
125I, 129I & 131I
134Cs & 137Cs
238Pu, 239Pu & 241Pu
241Am
Weighted and un-weighted external and internal doses and activity concentrations
within biota produced
ValidationValidation 99Tc activity in lobsters: comparison with
model by Olsen and Vives i Batlle (2003)
129I activity in winkles: comparison with model by Vives i Batlle et al. (2006)
Results - Results - Long term Long term assessmentassessment
Pu benthic mollusc - TB1/2 = 474 days
Tc large benthic crustacean - TB1/2 = 56.8 & 114 days
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
1.00E+01
1950 1960 1970 1980 1990 2000
Time (year)
Tot
al w
eigh
ted
dose
rat
e (µ
Gy
h-1
)
Large benthic crustacean
Equilibrium model
Benthic mollusc
Dynamic model
Annual time steps
Results - Results - Short term Short term assessmentassessment
Tc in macrophytes - TB1/2 = 1.5 & 128 days
(a) Macrophyte
0
1
2
3
4
5
6
7
Date
Wei
ghte
d do
se ra
te (µ
Gy
h-1
)
Dynamic model
Equilibrium model
Daily time steps
Results - Results - Short term Short term assessmentassessment
Tc in winkles - TB1/2 = 142 days
Daily time steps(b) Winkle
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Date
Wei
gh
ted
do
se ra
te (
µG
y h-1
)
Dynamic model
Equilibrium model
Time-integrated dosesTime-integrated doses
differences between the integrated dose rates obtained from the two approaches increase with slowness of response of the organism to an input of radioactivity, due to the smoothing effect of the dynamic method.
Test Time period Organism Nuclide TB1/2 (d) % difference
Sellafield discharges 1952 - 2005 Winkle 137Cs 0.86 0.00
(short-term test) (50 y) Benthic fish 137Cs 64.7 0.00
Benthic Crust. 99Tc 56.8, 114 -1.48
Benthic mollusc 239Pu 474 8.63
Seawater at Drigg Jul 1997 – Jun 1999 Macrophyte 99Tc 1.5, 128 -7.1
(long-term test) (700 d) Winkle 99Tc 142 -16.6
25 Jan - 7 Mar 1998 Winkle 99Tc 1.5, 128 -37.1
(40 d) Macrophyte 99Tc 142 -78.3
ConclusionsConclusions
Successfully production of a dynamic model that makes
assessments to biota more realistic
Simple, user-friendly spreadsheet format similar to R&D 128
Model is rigorously tested and validated against CF and
dynamic research models
Can be edited with site-specific data
Expandable for extra nuclides and organisms
ReferencesReferences
Vives i Batlle, J., Wilson, R.C., Watts, S.J., Jones, S.R., McDonald, P. and Vives-Lynch, S. Dynamic model for the assessment of radiological exposure to marine biota. J. Environ. Radioactivity (submitted).
Vives i Batlle, J., Wilson, R. C., McDonald, P., and Parker, T. G. (2006) A biokinetic model for the uptake and release of radioiodine by the edible periwinkle Littorina littorea. In: P.P. Povinec, J.A. Sanchez-Cabeza (Eds): Radionuclides in the Environment, Volume 8. Elsevier, pp. 449 – 462.
Olsen, Y.S. and Vives i Batlle, J. (2003). A model for the bioaccumulation of 99Tc in lobsters (Homarus gammarus) from the West Cumbrian coast. J.
Environ. Radioactivity 67(3): 219-233.
AcknowledgementsAcknowledgements
The authors would like to thank the Nuclear Decommissioning Authority (NDA), UK, for funding this project.