Radionuclide dispersion modelling

Radiation Protection of the Environment (Environment Agency Course, July 2015)

Understand the purpose of dispersion models

Know the origin of the dispersion models used in ERICA

Be able to use some of the basic dispersion models provided in ERICA

By the end of the presentation and practical you should….

What happens if do not have media concentration?

Need method of predicting from release rates 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

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

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

Simple atmospheric dispersion model incorporating downwind transport (advection), mixing (turbulent diffusion) and effects of buildings

For continuous, long-term release (not accidents) Gaussian plume model (=normal distribution in vertical

and lateral axis) Not applicable >20 km from release in ERICA assume 20 km if >20 km

Assumes a predominant wind direction and neutral stability class (=doesn’t enhance or inhibit turbulence)

If you (really) want all the equations – see SRS-19

Atmospheric dispersion

Atmospheric dispersion

Atmospheric dispersion

Importance of Release Height

Effective stack height

Conditions for the plume

Conditions for the plume

Output Radionuclide

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

Freshwater Small lake

(< 400 km2) Large lake

(≥400 km2) Estuarine River

Marine Coastal Estuarine

No model for open ocean waters

Surface water dispersion

Processes included: Flow downstream as transport (advection) Mixing processes (turbulent dispersion) Concentration in sediment estimated from

ERICA Kd at receptor (equilibrium) No loss to sediment between source and

receptor Half-life drives difference between RN in

water

River flow conditions – 30 y low assumed

Processes and assumptions

Assumes a homogeneous concentration throughout the water body Expected life time of facility is required as input

Small lakes and reservoirs

>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

Large Lake

Estimates concentration along plume centre line

Estimates concentration along

shoreline

Some restrictions related to length discharge pipe and angle to shoreline receptor

Some restrictions related to short

receptor discharge point distances (mixing zone)

Lz = distance to achieve full vertical mixing (=7D)

Rivers (& Estuaries)

Estuaries model similar to rivers• Some tidal parameters used

Coastal waters

Dispersion along the coast • Shoreline or ‘in sea’

receptorsFor 10’s km (not >100 km)

Some restrictions related to: • short receptor discharge point

distances (mixing zone) • length discharge pipe and angle

to shoreline receptor

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

Summary limitations of IAEA SRS 19