1. UFOTRI: Accident assessment model for tritium
W. Raskob
Presented by D Galeriu, with mandate from author

2. Tritium model UFOTRI

3. Content Introduction Modelling Applications Basic approaches Foodchain
OBT formation
Applications
Tests of rice model
Dose assessments

4. Modes of application of UFOTRI
Deterministic (defined set of variables, e.g. predefined and constant weather)
Probabilistic (assessment of all possible weather for a certain period)
Near and/or far range
Plant species considered in UFOTRI
grass (fodder)
leafy vegetables (continuously harvested)
wheat
potatoes
rice

5. Endpoints of UFOTRI Concentration Organ doses Countermeasures
Air concentration and deposition
Time dependent concentration in air, soil and foodstuffs for selected points
time dependent dose values for selected points
Organ doses
Short term effective doses
long term effective doses
Countermeasures
Food restriction, areas and duration
Coupling with COSYMA for further evaluation

6. Submodel short term (hourly time step)
Transport and dispersion in the atmosphere
primary plume by Gaussian trajectory model
secondary plume by an area source model
Exchange atmosphere - plant - soil
deposition and reemission is expressed via resistance functions dependent on the prevailing meteorological conditions (for both tritium and water vapour)
Exchange atmosphere - soil
deposition of tritium to soil and evaporation of tritium and water is modelled via resistance functions
Transport in soil
water and tritium movement depends on the matrix forces
Cycling through the foodchains

7. Submodel long term
Compartment model for calculating the longer term behaviour of tritium in the foodchains
transfer rates are means, valid for the vegetation period and derived from equilibrium conditions
HTO and OBT are treated separately

8. Resistance approaches
Atmospheric source
Aerodynamic, Ra
Boundary, Rb
Stomatal, Rs
Cuticular, Rct
Ground, Rg
for various surfaces
Total Surface, Rc
Aerodynamic resistance Ra depends on turbulence and wind speed
Boundary layer resistance Rb depends on turbulence wind speed and surface properties
Total surface resistance Rc can be split up into canopy and ground related resistance
Canopy resistance depends on surface properties, temperature, photosynthetic active radiation, humidity, water content in soil

9. Stomata resistance- Jarvis approach
r st,min = minimum stomata resistance
Ip = incoming photosynthetic active radiation
c = efficiency constant
fl = weighting function for humidity
ft = weighting function for temperature
fw = weighting function for soil water content
Canopy resistance
Canopy resistance is the modified stomata resistance integrated
over the leaf area index L

10. Actual evapotranspiration (Penman-Monteith)
Ea = the actual evapotranspiration
l = the latent heat of evaporation in J kg-1
d = the gradient of the vapour pressure curve at ambient temperature in J m-3 K-1
Ia = the incoming solar radiation in W m-2
rAV = the sum of the atmospheric and the boundary resistance
cp = the specific heat of air at constant pressure in J kg-1 K-1
es = the actual saturation vapour pressure of air in N m-2
ea =the actual vapour pressure of air in N m-2
g = the psychrometer constant in J m-3 K-1
rx = the resistance of the surface
Transpiration of plants (modification of Ia and rx = rc)

11. Tritium concentration in plant
Basic assumptions:
100% saturation inside the stomata
equilibrium conditions prevail
with

12. Water movement in soil Simplified version of Darcey’s law is applied
k = hydraulic conductivity
S = suction tension
V = Darcy velocity
Z = vertical distance
Simplified equation used in UFOTRI
Gradient between two layers
Equilibrium conditions with complete exchange of water between two adiiacent layers
Soil resistance modeled using an effective diffusion
and depth of dry soil layer

13. HT deposition velocity
with the effective diffusivity
D0 = the diffusion coefficient of HT in air (0.634 E-04 m2 s-1)
zref = the reference depth in m (r = 23 mm)
QS = the maximum water content
QW = the water content at wilting point
tort = the soil torture factor
HTO deposition velocity
The deposition velocity is the reciprocal of the sum of the three resistance:
aerodynamic resistance
boundary layer resistance
soil resistance

14. Tritium food chain model
Aim is to develop a model which calculates the cycling of tritium in both forms (tissue free water tritium, TWT and organically bound tritium, OBT) through the foodchain, based on plant physiological knowledge
Processes to be considered:
phenological stages of crop development
sowing, emergence, anthesis, harvest
growth of crop based on photosynthesised organic matter
photosynthesis rate
respiration rate
Plant species considered in UFOTRI
grass (fodder)
leafy vegetables (continuously harvested)
wheat
Potatoes
rice

15. Photosynthesis

16. Photosynthesis rate (dependencies)
Plant properties
Plant development stage
Photosynthetic active radiation (PAR)
Leaf area index (LAI)
Leaf temperature (air temperature)
Opening of stomata
radiation
air humidity
air temperature
soil water content

17. Schematic growth curve for rice

18. Daily variation of growth curve

19. 1. Photosynthesis rate without any limitation, based on CO2 assimilation
Ppot = potential CO2 assimilation rate in g CO2 per m-2 h-1
Pm = maximum CO2 assimilation rate in CO2 per m-2 h-1
H = absorbed photosynthetically active radiation in W per m-2
e = initial light use efficiency in g CO2 per W m-2 h-1
R = respiration rate in g CO2 per m-2 h-1
2.Analytical solution of the basic equation with integration of the leaf area
over the canopy height
Iabs = radiation flux absorbed by the canopy
In0 = incoming photosynthetically
active radiation
L = leaf area in m-2/m-2, ranging from
zero to the total leaf area index
k = extinction coefficient (0.69)

20. 3. Temperature dependency of the maximum photosynthesis rate
DH1, DH2 = activation and denaturation energies for the electron transport, respectively, in cal
C = value for the formation of organic matter in mg CO2 per m-2 h-1
R = gas constant in cal/Kelvin per mol
DS = entropy change on denaturation of the electron transport system in cal/Kelvin per mol
T = air/leaf temperature in Kelvin
4. Respiration rate, expressed in CO2 equivalents
R = photorespiration + maintenance respiration
C1p Pc = photorespiration, dependent on the photosynthesis rate
C2m Wd = maintenance respiration, dependent on the plant weight
C1p C2m = constants

21. Dry matter production and OBT build-up
COA = conversion factor CO2 to dry matter
f(sr) = weighting function for the stomata opening
Radiation, temperature, humidity
soil water content
Tact = OBT build-up in Bq/h
Pact = organic matter build-up g/h
BM = basic metabolism
CTWT = mean TWT concentration in crop in Bq/g
fg = partitioning in the growing phase after anthesis
dis = distribution parameter (set to 2)

22. Contribution of pathways to the dose
Typical contribution (%) of the exposure pathways to the maximum dose at 1 km distance for a release of tritium as HTO or HT, calculated with the accidental tritium assessment model UFOTRI (local production and consumption)

23. Winter what Germany Ratio modelled / measured 14 experiments have been carried out in the years 1995 and 1996 The exposure of the wheat plants took place mostly in the linear growing phase after anthesis; Exposure times were distributed all over the day; this includes exposure in the morning, at mid-day, in the evening and during the night Samples were taken in most cases 1h, 3h, 12 h, 24 h, 7 d, 14 d and at harvest time. U-UFOTRI; P- PLANT OBT (trial model)

24. HTO reemission experiments from soil

26. RICE-Comparison of measured and calculated concentration ratios (Korea experiments)

27. Rice experiments performed in Japan
Potted rice plants were exposed with deuterium under daytime and night-time conditions
8 hours exposure
Half of the potted rice plants were flooded
Continuous measurements of air, leaf, stem and seed concentrations

28. Applications
Assessment calculations for the potential European fusion sites Cadarache (France), Studsvik (Sweden), one site in Italy and Greifswald (Germany)
Application in the ITER (International Thermonuclear Experimental Reactor) study to define the release limits for a generic site
Assessment calculation in the frame of the SEAFP (Safety and Environmental Aspects of Fusion Power) study

29. Deterministic release scenarios (ITER)
Three different release heights (ground level, stack and high speed exhaust from roof)
Two different dispersion parameter sets
Three different sets of weather conditions for accidental releases (‚average‘, ‚worst case‘ and ‚rain‘)
Four types of doses (plume dose, early dose, EDE without ingestion, EDE with ingestion)

30. Probabilistic calculations (Greifswald)
144 weather sequences describing the meteorological situation for one vegetation period
Ground level and stack releases (100 m)
Early (7 d) and chronic doses
Large initial areas of food interdiction when applying the EC-CFILs (activity concentration intervention levels in human foodstuffs) were estimated

31. Summary
UFOTRI considers most of the relevant transfer processes with dynamical approaches
UFOTRI is widely accepted in the frame of ITER
UFOTRI was applied for generic assessments and also site specific assessments in Europe
Future effort should be addressed to improve the modelling of formation and translocation of OBT as well as the reemission from soil
Tests for the generic rice model supports the approach to use plant physiological parameters within tritium models