1. Modelling the environmental dispersion of radionuclides
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Modelling the environmental dispersion of radionuclides
Jordi Vives i Batlle Centre for Ecology and Hydrology, Lancaster, 1st – 3rd April 2014 1

2. What happens if do not have media concentrations?
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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 2

3. 1 - Dispersion modelling in ERICA
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
1 - Dispersion modelling in ERICA 3

4. SRS-19 is linked to ERICA help file
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Taken from IAEA SRS Publication 19
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
SRS-19 is linked to ERICA help file 4

5. Atmospheric dispersion
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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)
Key inputs: discharge rate Q & location of source / receptor points 5

6. Basic dispersion equation
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Importance of Release Height
Effective stack height 6

7. Conditions for the plume
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
(a)
(b)
(c)
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 7

8. Key parameters Wind speed and direction Release height Precipitation
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Key parameters
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 . 8

9. Output
Radionuclide activity concentrations in air (C,H,S & P) or soil (everything else)

10. Surface water dispersion
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Surface water dispersion
Freshwater
Small lake (< 400 km2)
Large lake (≥400 km2)
Estuarine
River
Marine
Coastal
No model for open ocean waters 10

11. Processes and assumptions
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Processes and assumptions
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. 11

12. Rivers and coastal waters
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Lz = 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.
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 maximum
Condition for mixing is x > 7D and (y-y0)<< 3.7x
concentration in sediment is assumed to be concentration in water x Kd
Kd = Activity concentration on sediment (Bq kg-1) xxxxxActivity concentration in seawater (Bq L-1) 12

13. Small lakes and reservoirs
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Small lakes and reservoirs
Assumes a homogeneous concentration throughout the water body
Expected life time of facility is required as input 13

14. Large Lake Surface area >400 km2
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Large Lake
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.’
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. 14

15. Limitations of IAEA SRS 19
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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
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 15

16. 2: PC CREAM as a practical alternative for dispersion modelling
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
2: PC CREAM as a practical alternative for dispersion modelling 16

17. Collective dose model PC CREAM
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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 17

18. Marine and aerial dispersion
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Marine and aerial dispersion
Radial grid - atmospheric model
Compartmental - marine model
(continuous discharge) 18

19. R91 aerial dispersion model
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
R91 aerial dispersion model
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 19

20. Degree of improvement of the models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Degree of improvement
of the models
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 20

21. 3. Alternative aerial models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
3. Alternative aerial models 21

22. New-generation plume models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
New-generation plume models
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 22

23. UK ADMS Modified Gaussian plume model
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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 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 23

24. 4. Alternative marine models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
4. Alternative marine models 24

25. Geographically-resolving marine models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Geographically-resolving marine models
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) 25

26. Model characteristics
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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, etc. 26

27. Model characteristics
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Model characteristics
Finite differences Compartmental 27

28. Some commonly available models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Some commonly 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 28

29. Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
DHI MIKE 21 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 29

30. Marine Environmental Advection Dispersion (MEAD)
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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
Has been combined with the ERICA methodology to make realistic assessments of impact on biota 30

31. More complex process models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
More complex process models
Extra modules for extra processes
More complex issues (eutrophication)
Wave interactions
Coastal morphology
Particle and slick tracking analysis
Sediment dynamics
ModelMaker biokinetic models
Dynamic interactions
with the sediments
Speciation
Dynamic uptake in biota 31

32. 5. Alternative river and estuary modelling
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
5. Alternative river and estuary modelling 32

33. River and estuary models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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) 33

34. Model characteristics
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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) 34

35. Common models Can be 1D, 2D or 3D models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
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) 35

36. Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Example - MIKE 11
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 36

37. Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Catchment modelling
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 37

38. Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Conclusions
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 38

39. Effect of using different models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Effect of using different models
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. 39

40. Effect of using different models (2)
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Effect of using different models (2)
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. 40

41. Summary of key points SRS19 model PC Cream Orther models Marine DORIS
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010
Summary of key points
SRS19 model
PC Cream
Orther models
Marine
DORIS
+ 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
+ very simple 1D model
+ 2D - 3D models
- only models riverbanks
+ Full representation of hydrodynamics
- Simple average flow conditions
+ Can deal with tides, concentration gradients
- 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 41

42. Links to alternative models
Radiological protection of the environment: CEH Lancaster 24th - 26th November 2010 42