Modelling the environmental dispersion of radionuclides Jordi Vives i Batlle Centre for Ecology and...
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Transcript of Modelling the environmental dispersion of radionuclides Jordi Vives i Batlle Centre for Ecology and...
Modelling the environmental dispersion of radionuclides
Jordi Vives i Batlle
Centre for Ecology and Hydrology, Lancaster, 1st – 3rd April 2014
www.radioecology-exchange.org
What happens if do not have media concentrations?
Need method of predicting from release rates over a dilution pathway between source and receptor
If have dispersion model can run and input predictions If not then ERICA has some screening level models
built-in to enable this in Tiers 1 and 2
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1 - Dispersion modelling in ERICA
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Designed to minimise under-prediction (conservative generic assessment): ‘Under no circumstances would doses be underestimated by more than a factor of ten.’
A default discharge period of 30 y is assumed (estimates doses for the 30th year of discharge)
Models - atmospheric, freshwater (lakes and rivers) and coastal water models available
Taken from IAEA SRS Publication 19
SRS-19 is linked to ERICA help file
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Gaussian plume model version depending on the relationship between building height & cross-sectional area of the building influencing flow
Assumes a predominant wind direction and neutral stability class (=doesn’t enhance or inhibit turbulence)
Atmospheric dispersion
Key inputs: discharge rate Q & location of source / receptor points
www.ceh.ac.uk/PROTECT
A Gaussian plume model for an elevated release is as follows:
C x y z
Q
u
y z H
z y y
S
z
, , exp
2 2 210
2
2
2
2
where C = the air concentration (Bq/m3) or its time integral Bq.s/m3
Q = release rate (Bq/s) or total amount released (Bq) u10 = 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) HS = effective release height (m) x, y, z = rectilinear coordinates of the receptors
Importance of Release Height
Basic dispersion equation
Effective stack height
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(a) (b)
(c)
Conditions for the plume
a) H > 2.5HB (building height): No building effects
b) H 2.5HB & x > 2.5AB½ (cross-sectional area of building): Airflow in the wake zone
c) H 2.5HB & x 2.5AB½: Airflow in the cavity zone. Two cases:
source / receptor at same building surface not at same surface
Not generally applicable at > 20 km from stack
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Wind speed and direction 10 minute average from 10 m wind vane & anemometer
Release height Precipitation
10 minute total rainfall (mm) 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
Key parameters
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Output Radionuclide
activity concentrations in air (C,H,S & P) or soil (everything else)
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Freshwater Small lake (<
400 km2) Large lake
(≥400 km2) Estuarine River
Marine Coastal Estuarine
No model for open ocean waters
Surface water dispersion
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Based on analytical 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 Kd at receptor (equilibrium) Transportation in the direction of flow No loss to sediment between source and receptor
In all cases water dispersion assumes critical flow conditions, by taking the lowest in 30 years, instead of the rate of current flow
The only difference between RNs in predicted water concentrations as material disperses is decay by their different radiological half-lives.
Processes and assumptions
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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.
Some restrictions related to short receptor discharge point distances (mixing zone) and length discharge pipe and angle to shoreline receptor
For 10’s of km maximumCondition for mixing is x > 7D and (y-y0)<< 3.7xconcentration in sediment is assumed to be concentration in water x Kd
Lz = distance to achieve full vertical mixing
• Kd = Activity concentration on sediment (Bq kg-1) xxxxxActivity concentration in seawater (Bq L-1)
Rivers and coastal waters
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Assumes a homogeneous concentration throughout the water body Expected life time of facility is required as input
Small lakes and reservoirs
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Surface area >400 km2
As a rough rule a lake can be considered to be large when the opposite side of the lake is not visible to a person standing on a 30 m high shore.’
Large Lake
Some restrictions related to length discharge pipe and angle to shoreline receptor, short receptor discharge point distances (mixing zone)
Estimates concentration along shoreline and along plume centre line.
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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 Equilibrium between liquid and solid phases - Kd
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) which do not change significantly with distance / time
End of pipe mixing zones require hydrodynamic models
Limitations of IAEA SRS 19
www.ceh.ac.uk/PROTECT
2: PC CREAM as a practical alternative for dispersion
modelling
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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
Collective dose model PC CREAM
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Compartmental - marine model
(continuous discharge)
Radial grid - atmospheric model
Sellafield
Dundalk
Dublin
Irish SeaNorth West
Irish SeaNorth
Irish SeaNorth East
Irish SeaSouth East
Liverpool And Morecambe Bays
CumbrianWaters
Irish SeaWest
Localcompart.
Marine and aerial dispersion
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Gaussian plume model
Meteorological conditions specified by: Wind speed, Wind
direction, Pasquill-Gifford stability classification
Implemented in PC CREAM and CROM Model assumes constant meteorological and topographical
conditions along plume trajectory Prediction accuracy < 100 m and > 20 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
R91 aerial dispersion model
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Marine model (DORIS) => improvement Has long-range geographical resolution Incorporates dynamic representation of water /
sediment interaction Aerial model (PLUME) => no improvement
Still a gaussian dispersion model unsuitable for long distances > 20 km
Also assumes constant meteorological conditions Does not correct for plume filling the boundary layer
Degree of improvement of the models
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3. Alternative aerial models
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Include deviations from idealised Gaussian plume model Include turbulence data rather than simplified stability
categories to define boundary layer Include particulate vs gases and chemical interactions Model includes the effects on dispersion from:
Complex buildings Complex terrain & coastal regions
Advanced models: ADMS, AERMOD Gaussian in stable and neutral conditions Non-Gaussian (skewed) in unstable conditions
New-generation plume models
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Modified Gaussian plume model Gaussian in stable
and neutral conditions Skewed non-Gaussian
in unstable conditions Boundary layer based on
turbulence parameters
UK ADMS
Model includes: Meteorological pre-processor, 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
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4. Alternative marine models
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Allow for non-equilibrium 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 specialist users
Post-processing required for dose calculation (use as input to ERICA)
Geographically-resolving marine models
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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, etc.
Model characteristics
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Finite differences Compartmental
Model characteristics
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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
Some commonly available models
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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
DHI MIKE 21 model
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Has been combined with the ERICA methodology to make realistic assessments of impact on biota
Marine Environmental Advection Dispersion (MEAD)
Runs on a 2-km 2-dimensional grid Input: bathymetry, wind field,
sediment distribution maps Applies advection - dispersion
equations over an area and time Generates long-range radioactivity
predictions in water and sediment
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Extra modules for extra processes More complex issues
(eutrophication) Wave interactions Coastal morphology Particle and slick tracking analysis Sediment dynamics
D isso lv e d
Su sp e n d e dSe d im e n t
A d so rp tio n
D e so rp tio nR e su sp e n sio n
D e p o s itio n
A d v e c tio n
D if fu s io n
D if fu s io n
D e so rp tio n
A d so rp tio n
D e p o site dSe d im e n t
A d v e c tio n
Discharge
ModelMaker biokinetic models
Dynamic interactions with the sediments
SpeciationDynamic uptake in
biota
Seawater_Pu_III_IVSeawater_Pu_V_VI
Suspended_load
Reduction
Oxidation
Desorption_sed Adsorption_sed
Adsorption_coll
Adsorption_susp
DepositionRemobilisation Coagulation
Desorption_susp
SedimentationBioturbation
Flushing_oxidisedFlushing_reduced
Flushing_colloidal
Flushing_susp
Burial
Discharge
Influx_Pu_III_IV
Influx_Pu_V_VI
Influx_Pu_particulate
Surface_sediment
Bottom_sedimentMiddle_sediment
Far_field
Colloidal
Pelagic_fish
Crustaceans
Molluscs
Benthic_fish
More complex process models
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5. Alternative river and estuary modelling
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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)
River and estuary models
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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)
Model characteristics
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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)
Common models
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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
Example - MIKE 11
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Convert rainfall over the catchment to river flow out the catchment
Represent the processes illustrated, however in two possible ways: empirical relationship from rainfall to runoff
(cannot be used to simulate changing conditions) Complex physically based models where all
processes are explicitly represented Example: DHI MIKE-SHE, HP1 (HYDRUS +
PHREEQC) SVAT modelling
Catchment modelling
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ERICA uses the IAEA SRS 19 dispersion models to work out a simple, conservative source - receptor interaction
SRS 19 has some shortcomings PC-CREAM can be used as an alternative to the SRS-19
marine model 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 – must match complexity to need
Conclusions
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Uncertainty associated with the application of aquatic SRS models: Models generally conservative. From factor of 2 to 10 difference with respect to a dynamic
model. Uncertainty associated with the application of a
Gaussian plume model for continuous releases: About a factor of 4 or 10 for a flat and complex terrain
respectively. At distances < 2.5 times the square root of the frontal area
of the building, the model provides conservative results. For distances of about 2.5 the above, the model tends to
underpredict for wind speeds above 5-m s-1.
Effect of using different models
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For aerial, PC-Cream is no improvement to SRS 19 For marine, PC cream has a dynamic compartment
model Effect of using such a fully dynamic model:
In periods where concentrations in compartments increase, dynamic model estimates of transfer will be lower than for equilibrium model (‘build-up effect’)
In period where environmental concentrations decrease, dynamic model estimates higher than equilibrium model (‘memory effect’)
Diffcult to generalise, but differences could be up to a factor of 10.
Effect of using different models (2)
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Summary of key pointsSRS19 model PC Cream Orther modelsMarine DORIS Marine + point in coast + Large compartment box model + compartmental models for large areas
+ requires very few parameters + Dynamic transfer to water and sediments
+ Grid models for fine resolutions (small areas)
- no offshore dispersion - requires more parameters + Dynamic / time-variable discharges - very simple equilibrium model (Kd based)
- Does not work well at fine resolution
- parameter hungry (bathimetry, gridding, etc)
River, lake, reservoir N/A River, lake, reservoir + very simple 1D model + 2D - 3D models - only models riverbanks + Full representation of hydrodynamics
- Simple average flow conditions + Can deal with tides, concentration gradients
- very simple equilibrium model (Kd based) + Dynamic / time-variable discharges - Simple linear river + Complex river networks
- parameter hungry (bathymetry, gridding, etc.)
Aerial PLUME AERMOD, ADMS, etc. + limited range 100 m to 20 km - Same as SRS19 + Non Gaussian for unstable conditions + constant meteorology + Buildings and terrain + Gaussian plume, still conditions + Solute modelling + Complex meteorology
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Model Organisation LinkADMS 4 CERC http://www.cerc.co.uk/environmental-software/ADMS-model.htmlAERMOD EPA http://www.epa.gov/scram001/dispersion_prefrec.htm#aermod
(Freeware)DELFT 3D DELFT
Hydraulicshttp://delftsoftware.wldelft.nl/index.php?option=com_content&task=blogcategory&id=13&Itemid=34
DIVAST CardiffUniversity
http://hrc.engineering.cf.ac.uk/
EcoS 3 PML http://www.pml.ac.uk/
HEC-RAS HEC(USACE)
http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html (Freeware)
IAEA SRS 19 IAEA www-pub.iaea.org/MTCD/publications/PDF/Pub1103_scr.pdfISIS Halcrow http://www.halcrow.com/isis/isisfree.asp (Freeware)
http://www.halcrow.com/isis/default.asp (Professional edition)MEAD WSC http://www.westlakes.org (in-house model)MIKE11 DHI http://www.dhigroup.com/Software/WaterResources/MIKE11.aspxMIKE21 DHI http://www.dhigroup.com/Software/Marine/MIKE21.aspxMIKE3 DHI http://www.dhigroup.com/Software/Marine/MIKE3.aspxMIKE-SHE DHI http://www.dhigroup.com/Software/WaterResources/MIKESHE.aspxMOIRA-PLUS EU MOIRA
programmehttp://user.tninet.se/~fde729o/MOIRA/Software.htm (Freeware)
PC CREAM 08 HPA http://www.hpa.org.uk/web/HPAweb&HPAwebStandard/HPAweb_C/1195733792183
POSEIDON CEPN http://www.cepn.asso.fr/en1/logiciels.htmlPRAIRIE AEA
Technologyhttp://www.aeat.co.uk/
R91 NRPB http://www.admlc.org.uk/NRPB-R91.htmRODOS PV6(COASTOX,RIVTOX &LAKECO)
EU RODOSprogramme
http://www.rodos.fzk.de/rodos.html (Freeware, password protected)
TELEMAC 2 &3D
SOGREAH http://www.telemacsystem.com/index.php?option=com_jdownloads&Itemid=31&task=viewcategory&catid=3&lang=en (Freeware)
VERSE WSC http://www.westlakes.org (in-house model)
Links to alternative models