Modelling the environmental dispersion of radionuclides Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 28 April 2010

Lecture plan

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Dispersion models available in the ERICA Tool

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Other types of dispersion models that are available

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Key parameters that drive dispersion models for radioactivity in the environment

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Applicability to different scenarios/circumstances (e.g.

release directly to a protected site/end of pipe concentrations (e.g. mixing zones))

www.ceh.ac.uk/PROTECT

What reasons to use models?

 Often the receptor is not at a point of emission but is linked via an environmental pathway (dilution)   Need to predict media concentrations when (adequate) data are not available To conduct authorisation-based assessments for the protection and conservation of species listed under the EC Birds and Habitats Directives www.ceh.ac.uk/PROTECT

Part I: Dispersion modelling in ERICA

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IAEA SRS publication 19

  Designed to minimise under-prediction (conservative generic assessment) A default discharge period of 30 y is assumed (estimates doses for the 30 th year of discharge) www.ceh.ac.uk/PROTECT

Atmospheric dispersion

   Gaussian plume model version depending on the relationship between building height, HB & cross-sectional area of the building influencing flow, AB Assumes a predominant wind direction and neutral stability class Key inputs: discharge rate Q & location of source / receptor points (H, HB, AB and x) www.ceh.ac.uk/PROTECT

Atmospheric dispersion

(a) a) H > 2.5H

B b) H  2.5H

B (no building effects) & x > 2.5A

B ½ (airflow in the wake zone) c) H  2.5H

B & x  2.5A

B ½ (airflow in the cavity zone). Two cases:   source / receptor at same building surface not at same surface Not generally applicable at x > 20 km www.ceh.ac.uk/PROTECT (b) (c)

Basic dispersion equation

A Gaussian plume model for an elevated release is as follows:   

Q

2    10

z y

exp    

y

2 2  2

y

 

z



H S

2 

z

2  2    where C = the air concentration (Bq/m 3 ) or its time integral Bq.s/m 3 Q = release rate (Bq/s) or total amount released (Bq) u 10 = wind speed at 10 m above the ground (m/s)  z = standard deviation of the vertical Gaussian distribution (m)  y = standard deviation of the horizontal Gaussian distribution (m) H S = effective release height (m) x, y, z = rectilinear coordinates of the receptors

Importance of Release Height

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Key parameters

    Wind speed and direction  10 minute average from 10 m wind vane & anemometer Release height Precipitation  10 minute total rainfall (mm) from tipping bucket Stability or degree of turbulence (horizontal and vertical diffusion)  Manual estimate from nomogram using time of day, amount of cloud cover and global radiation level  Atmospheric boundary layer (time-dependent)   Convective and or mechanical turbulence Limits the vertical transport of pollutants www.ceh.ac.uk/PROTECT

R91 aerial dispersion model

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Based on the recommendations of the Working Group on Atmospheric Dispersion (NRPB-R91, -R122, -R123, -R124)

 Gaussian plume model  Meteorological conditions specified by:  Wind speed   Wind direction Pasquill-Gifford stability classification  Implemented in PC CREAM www.ceh.ac.uk/PROTECT

R91 - model limitations

 Model assumes constant meteorological and topographical conditions along plume trajectory  Prediction accuracy < 100 m and > 30 km limited  Source depletion unrealistic (deposition modelling & transfer factors are uncertain)  Developed for neutral conditions  Does not include  Buildings   Complex terrain e.g. hills and valleys Coastal effects www.ceh.ac.uk/PROTECT

Surface water dispersion

   Freshwater     Small lake (< 400 km 2 ) Large lake (≥400 km 2 ) Estuarine River Marine   Coastal Estuarine No model for open ocean waters www.ceh.ac.uk/PROTECT

Surface water dispersion

   Based on analytic solution of the advection diffusion equation describing transport in surface water for uniform flow conditions at steady state Processes included:  Flow downstream as transport (advection)     Mixing processes (turbulent dispersion) Concentration in sediment / suspended particles estimated from ERICA K d at receptor (equilibrium) Transportation in the direction of flow No loss to sediment between source and receptor In all cases water dispersion are assumed critical flow conditions, by taking the lowest in 30 years, the rate of current flow www.ceh.ac.uk/PROTECT

Rivers and coastal waters

L z

= distance to achieve full vertical mixing

  The river model assumes that both river discharge of radionuclides such as water harvesting is done in some of the banks, not in the midstream The estuary model is considered an average speed of the current representative of the behaviour of the tides.

If x on the same bank side and



Lz = 7D the radionuclide Condition for mixing is (y-y 0 )<<3.7x

concentration in water is assumed to be undiluted K d = Activity concentration on sediment (Bq kg -1 ) Activity concentration in seawater (Bq L -1 )

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Small lakes and reservoirs

Assumes a homogeneous concentration throughout the water body Expected life time of facility is required as input www.ceh.ac.uk/PROTECT

Limitations of IAEA SRS 19

 Simple environmental and dosimetric models as well as sets of necessary default data:  Simplest, linear compartment models  Simple screening approach (robust but conservative)  Short source-receptor distances  More complex / higher tier assessments:  Aerial model includes only one wind direction  Coastal dispersion model not intended for open waters e.g. oil/gas marine platform discharges  Surface water models assume geometry (e.g. river cross section) & flow characteristics (e.g. velocity, water depth) do not change significantly with distance / time  Assumes equilibrium e.g. water/sediment K d www.ceh.ac.uk/PROTECT

Part II: PC-Cream as a practical alternative for dispersion modelling

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Collective Dose Model - PC CREAM

    Consequences of Releases to the Environment Assessment Methodology A suite of models and data for performing radiological impact assessments of routine and continuous discharges Marine: Compartmental model for European waters (DORIS)  Seafood concentrations => Individual doses => Collective doses.

Aerial: Radial grid R-91 atmospheric dispersion model with (PLUME) with biokinetic transfer models (FARMLAND)  Ext. & internal irradiation => foodchain transfer (animal on pasture e.g. cow & plant uptake models) => dose www.ceh.ac.uk/PROTECT

Marine and aerial dispersion

Compartmental marine model Radial grid atmospheric model (continuous discharge) www.ceh.ac.uk/PROTECT

Dundalk

Irish Sea North West

Irish Sea West

Irish Sea North Irish Sea North East

Sellafield

Local compart.

Cumbrian Waters Irish Sea South East Liverpool And Morecambe Bays

Dublin

Marine and aerial dispersion

  Marine model (DORIS) => improvement   Has long-range geographical resolution - allows for offshore scenarios e.g. marine platform discharges Incorporates dynamic representation of water / sediment interaction Aerial model (PLUME) => no improvement     Still a gaussian dispersion model unsuitable for long distances (though it has been used in that way) Also assumes constant meteorological conditions Does not correct for plume filling the boundary layer Must use a better model e.g. Lagrangian particle dispersion - NAME www.ceh.ac.uk/PROTECT

Part III: Other alternative dispersion models

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www.ceh.ac.uk/PROTECT

Marine modelling

Geographically resolving marine models

   Allow for nonequilibrium situations e.g. acute release into protected site Advantages:   Resolves into a large geographical range Results more accurate (if properly calibrated) Disadvantages:    Data and CPU-hungry (small time step and grid sizes demand more computer resources) Run time dependent on grid size & time step Requires a more specialised type of user  Post-processing required for dose calculation (use as input to ERICA) www.ceh.ac.uk/PROTECT

Model characteristics

     Input requirements: Bathymetry, wind fields, tidal velocities, sediment distributions, source term Type of output: a grid map / table of activity concentration (resolution dependent on grid size) All use same advection/dispersion equations, differences are in grid size and time step Types of model:   Compartmental: Give average solutions in compartments connected by fluxes. Good for long-range dispersion in regional seas.

Finite differences: Equations discretised and solved over a rectangular mesh grid. Good for short-range dispersion in coastal areas Estuaries a special case: Deal with tides (rather than waves), density gradients, turbidity & c.

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Finite differences Compartmental www.ceh.ac.uk/PROTECT

Readily available models

   Long-range marine models (regional seas):   POSEIDON - N. Europe (similar to PC-CREAM model but redefines source term and some compartments - same sediment model based on MARINA) MEAD (in-house model available at WSC) Short-range marine models (coastal areas):   MIKE21 - Short time scales (DHI) - also for estuaries Delft 3D model, developed by DELFT   TELEMAC (LNH, France) - finite element model COASTOX (RODOS PV6 package) Estuarine models  DIVAST ( Dr Roger Proctor)  ECoS (PML, UK) - includes bio-uptake www.ceh.ac.uk/PROTECT

POSEIDON

     As seen previously (PC-CREAM section of the lecture) Area of interest divided into large area boxes and transfer at boundaries is dependent on the parameters in the adjacent boxes Contains sediment transport project (MARINA project) Simple, quick, easy to use radionuclide transport model  Continuous discharge   Time variable discharge Continuous leaching of an immersed solid material Post processing for annual dose to humans is intrinsic, hence only minor coding required for determination of dose to biota www.ceh.ac.uk/PROTECT

DHI MIKE21 model

      Two-dimensional depth averaged model for coastal waters Location defined on a grid - creates solution from previous time step Hydrodynamics solved using full time-dependent non-linear equations (continuity & conservation of momentum) Large, slow and complex when applied to an extensive region Suitable for short term (sub annual) assessments A post processor is required to determine biota concentrations and dose calculations www.ceh.ac.uk/PROTECT

Marine Environment Advection Dispersion

2 km grid    Applies advection - dispersion equations over an area and time Generates activity concentration predictions in water and sediment Has been combined with the ERICA methodology to make realistic assessments of impact on biota www.ceh.ac.uk/PROTECT

MEAD input data - water

Bathymetry for MEAD grid: resolution 2 km - 2 km Residual flow field (12 month MIKE21 simulation / averaged wind conditions) www.ceh.ac.uk/PROTECT

MEAD input data - sediment

Distribution of fine grained bed sediment www.ceh.ac.uk/PROTECT Distribution of suspended particles (modelled)

MEAD output - Cumbria coast

60 Co in winkles 137 Cs in cod / plaice 99 Tc in crab 241 Am in mussels Could be used to derive CFs for use in ERICA www.ceh.ac.uk/PROTECT

55.0

54.5

54.0

53.5

Cumbria fishing area

53.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-6.0

MEAD - long-range results

5.00

1.00

0.10

0.01

0.00

1000.00

100.00

50.00

10.00

55.0

54.5

1000.00

55.0

100.00

50.00

54.5

5000.00

1000.00

100.00

54.0

53.5

Cumbria fishing area 10.00

5.00

54.0

1.00

0.10

53.5

10.00

5.00

1.00

0.10

53.0

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

Predicted distribution of 137 Cs in seawater in 2000 0.01

0.00

53.0

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

Predicted distribution of 137 Cs in bed sediments in 2000 0.01

0.00

55.0

54.5

www.ceh.ac.uk/PROTECT

54.0

53.5

53.0

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

1.00

0.10

0.01

0.00

5000.00

1000.00

100.00

10.00

5.00

More complex process models

 Extra modules in MIKE21   More complex water quality issues e.g. eutrophication Wave interactions    Coastal morphology Particle and slick tracking analysis Sediment dynamics

Seawater_Pu_V_VI Discharge

Influx_Pu_III_IV Influx_Pu_V_VI Reduction Oxidation Influx_Pu_particulate

Seawater_Pu_III_IV

Desorption_susp Flushing_oxidised Adsorption_susp Flushing_reduced  ModelMaker biokinetic models  Dynamic interactions with sediment  Speciation  biota Dynamic uptake in www.ceh.ac.uk/PROTECT

Pelagic_fish Crustaceans Molluscs

Desorption_sed

Suspended_load Bottom_sediment

Flushing_colloidal

Colloidal Far_field Benthic_fish

Remobilisation Deposition Coagulation

Surface_sediment

Bioturbation Sedimentation

Middle_sediment

Burial Flushing_susp Adsorption_sed Adsorption_coll

River and estuary modelling

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River and estuary models

  Advantages:    Large geographical range Consider multiple dimensions of the problem (1 - 3D) Considers interconnected river networks  Results more accurate (if properly calibrated) Disadvantages - same as marine models:  Data hungry     Run time dependent on grid size & time step Requires a more specialised type of user CPU-hungry (as time step and grid size decreases it demands more computer resources) Post-processing required for dose calculation (use as input to ERICA) www.ceh.ac.uk/PROTECT

Model characteristics

 Input requirements: Bathymetry, rainfall and catchment data, sediment properties, network mapping, source term  Type of output: activity concentration in water and sediment, hydrodynamic data for river  All use same advection/dispersion equations as marine but differences in boundary conditions  Generally models solve equations to:  Give water depth and velocity over the model domain.

 Calculate dilution of a tracer (activity concentration) www.ceh.ac.uk/PROTECT

Common models

   Can be 1D, 2D or 3D models  1D river models: River represented by a line in downstream direction - widely used   2D models have some use where extra detail is required 3D models are rarely used unless very detailed process representation is needed Off-the-shelf models:  MIKE11 model developed by the DHI, Water and Environment (1D model)   VERSE (developed by WSC) MOIRA (Delft Hydraulics) Research models:  PRAIRIE (AEA Technology)  RIVTOX & LAKECO (RODOS PV6 package) www.ceh.ac.uk/PROTECT

www.ceh.ac.uk/PROTECT       MIKE11 - Industry standard code for river flow simulation River represented by a line in downstream direction River velocity is averaged over the area of flow Cross sections are used to give water depth predictions Can be steady flow (constant flow rate) or unsteady flow Use of cross sections can give an estimate of inundation extent but not flood plain velocity

www.ceh.ac.uk/PROTECT

Aerial modelling

New-generation Gaussian plume models

 Advanced models: ADMS, AERMOD   Gaussian in stable and neutral conditions Non-Gaussian (skewed) in unstable conditions  Continuous turbulence data rather than simplified stability categories to define boundary layer  Model includes the effects on dispersion from:   Buildings Complex terrain & coastal regions  ADMS a good choice www.ceh.ac.uk/PROTECT

UK ADMS

   Modified Gaussian plume model  Gaussian in stable and neutral conditions  Skewed non-Gaussian in unstable conditions Boundary layer based on turbulence parameters Model includes:     Meteorological preprocessor, buildings, complex terrain Wet deposition, gravitational settling and dry deposition Short term fluctuations in concentration Chemical reactions     Radioactive decay and gamma-dose Condensed plume visibility & plume rise vs. distance Jets and directional releases Short to annual timescales www.ceh.ac.uk/PROTECT

ADMS input Parameters

   Meteorological data (site specific & Met Office)  Wind speed, wind direction, date, time, latitude, boundary layer height, cloud cover Boundary Layer Height   Height at which surface effects influence dispersion ADMS calculates boundary layer properties for different heights based on meteorology Monin-Obukhov Length   Measure of height at which mechanical turbulence is more significant than convection or stratification ADMS calculates M-O length based on meteorology and ground roughness www.ceh.ac.uk/PROTECT

Types of output

NOx Concentration (ug/m3)

400 Stack with building 300 Concentration plot 200 100 0 -100 -200 -300 -400 0 100 200 300 400 500 Metres 600 700 800 900 1000 5 4 3 2 9 8 7 6 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 www.ceh.ac.uk/PROTECT

Terrestrial (biosphere) modelling

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Catchment modelling

  Convert rainfall over the catchment to river flow out the catchment Represent the processes illustrated, however in two possible ways:  Simple “black box” type model such as empirical relationship from rainfall to runoff (cannot be used to simulate changing conditions)  Complex physically based models where all processes are explicitly represented www.ceh.ac.uk/PROTECT

Example Model - MIKE SHE

    Integrated groundwater surface water solution Advanced rainfall runoff model with extensive process representation Intense parameter demand One of the more widely used models  A good choice when the close linkage of surface water and ground water is important to the study Graham, D.N. and M. B. Butts (2005) Flexible, integrated watershed modelling with MIKE SHE. In Watershed Models, Eds. V.P. Singh & D.K. Frevert Pages 245-272, CRC Press. ISBN: 0849336090. www.ceh.ac.uk/PROTECT

Conclusions

     ERICA uses the IAEA SRS 19 dispersion models to work out a simple, conservative source receptor interaction SRS 19 have some shortcomings PC-CREAM can be used as an alternative suite of dispersion models There are further off-the-shelf models performing radiological impact assessments of routine and continuous discharges ranging from simple to complex Key criteria of simplicity of use and number of parameters need to be considered www.ceh.ac.uk/PROTECT

Links to alternative models

Model

ADMS 4 AERMOD DELFT 3D DIVAST EcoS 3 HEC-RAS IAEA SRS 19 ISIS MEAD MIKE11 MIKE21 MIKE3 MIKE-SHE

Organisation

CERC EPA DELFT Hydraulics Cardiff University PML HEC (USACE) IAEA Halcrow WSC DHI DHI DHI DHI

Link

http://www.cerc.co.uk/environmental-software/ADMS-model.html

http://www.epa.gov/scram001/dispersion_prefrec.htm#aermod (Freeware) http://delftsoftware.wldelft.nl/index.php?option=com_content&task=blog category&id=13&Itemid=34 http://hrc.engineering.cf.ac.uk/ http://www.pml.ac.uk/ http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html

(Freeware) www-pub.iaea.org/MTCD/publications/PDF/Pub1103_scr.pdf

http://www.halcrow.com/isis/isisfree.asp

(Freeware) http://www.halcrow.com/isis/default.asp

(Professional edition) http://www.westlakes.org

(in-house model) http://www.dhigroup.com/Software/WaterResources/MIKE11.aspx

http://www.dhigroup.com/Software/Marine/MIKE21.aspx

http://www.dhigroup.com/Software/Marine/MIKE3.aspx

http://www.dhigroup.com/Software/WaterResources/MIKESHE.aspx

MOIRA-PLUS PC CREAM 08 POSEIDON PRAIRIE R91 RODOS PV6 EU MOIRA programme HPA CEPN AEA Technology NRPB EU RODOS http://user.tninet.se/~fde729o/MOIRA/Software.htm

(Freeware) http://www.hpa.org.uk/web/HPAweb&HPAwebStandard/HPAweb_C/11 95733792183 http://www.cepn.asso.fr/en1/logiciels.html

http://www.aeat.co.uk/ http://www.admlc.org.uk/NRPB-R91.htm

http://www.rodos.fzk.de/rodos.html

(Freeware, password protected) (COASTOX, RIVTOX & LAKECO) programme TELEMAC 2 & 3D VERSE SOGREAH WSC www.ceh.ac.uk/PROTECT http://www.telemacsystem.com/index.php?option=com_jdownloads&Itemid=31&task= viewcategory&catid=3&lang=en (Freeware) http://www.westlakes.org

(in-house model)