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

1UFOTRIKarlsruhe Institute of Technology

UFOTRI:Accident assessment model for tritium

W. Raskob

Presented by D Galeriu, with mandate from author


Tritium model UFOTRI


Content

• Introduction

• Modelling– Basic approaches

– Foodchain

• OBT formation

• Applications– Tests of rice model

– Dose assessments


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


Endpoints of UFOTRI

• Concentration– 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


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


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


Resistance approaches

• 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

Atmospheric source

Aerodynamic, Ra

Boundary, Rb

Stomatal, Rs

Cuticular, Rct

Ground, Rg

for various surfacesT

otal

Su

rfac

e, R

c


Stomata resistance- Jarvis approach

r st,min = minimum stomata resistanceIp = incoming photosynthetic active radiationc = efficiency constant

fl = weighting function for humidity

ft = weighting function for temperature

fw = weighting function for soil water content

r rc

I f f fst stp l w t

,min 1

1

Canopy resistance rr

Lcst

Canopy resistance is the modified stomata resistance integrated over the leaf area index L


Actual evapotranspiration (Penman-Monteith)

Ea = the actual evapotranspiration = the latent heat of evaporation in J kg-1 = 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

= the psychrometer constant in J m-3 K-1

rx = the resistance of the surface

Transpiration of plants (modification of Ia and rx = rc)

E

I c e e r

r ra

a p s a av

x AV

( ) /

( / )1

I Ia ea LL

,. 1 0 398


Tritium concentration in plant

dCdt r

CG

1

Fr

i

G

C e kt

1

krG

F is the tritium flux to or from the canopyk is the time constant until equilibrium is the water content per unit area of leaf in g cm-2

is the concentration of tritium in air in pBq ml -1

C is the tritium concentration in tissue water in pBq g-1

rG is the total resistance in cm s-1

t is the time in s is the weight of water vapour in saturated air in g ml-1

is the H/T isotope ratio (set to 1.1)

Basic assumptions:• 100% saturation inside the stomata• equilibrium conditions prevail

with


Water movement in soilSimplified version of Darcey’s law is applied

k = hydraulic conductivity

S = suction tension

V = Darcy velocity

Z = vertical distance

dS

dZ

S S

D D

1 2

1 2 2/

V kdS

dZ

1

Simplified equation used in UFOTRIGradient between two layersEquilibrium conditions with complete exchange of water between two adiiacent layers

V kS S

D D1 2 1 21 2

1 2 21, , /

Soil resistance modeled using an effective diffusion and depth of dry soil layer


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)

S = the maximum water content

W = the water content at wilting point

tort = the soil torture factor

vD

z

D

zd HT

eff

ref d HT

eff

ref,

,

D DT

torteffa S W

0 7

2730

1 75

..

HTO deposition velocityv

r r rd HTOav bv soil

,

1

The deposition velocity is the reciprocal of the sum of the three resistance:aerodynamic resistanceboundary layer resistancesoil resistance


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


Photosynthesis


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


Schematic growth curve for rice


Daily variation of growth curve

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

1 6 11 16 21

Time of the day in hours

Ph

oto

syn

thes

is r

ate

(g/m

2 )

calculated


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

PP HP H

Rpotm

m

2.Analytical solution of the basic equation with integration of the leaf area over the canopy height

PPk

P kIP kI

Rcm m n

m abs

ln

0 Iabs = radiation flux absorbed by the canopyIn0 = incoming photosynthetically active radiationL = leaf area in m-2/m-2, ranging from zero to the total leaf area index

k = extinction coefficient (0.69)


3. Temperature dependency of the maximum photosynthesis rate

H1, H2 = activation and denaturation energies for the electron transport, respectively, in cal

C0 = value for the formation of organic matter in mg CO2 per m-2 h-1

R = gas constant in cal/Kelvin per mol

S = entropy change on denaturation of the electron transport system in cal/Kelvin per mol

T = air/leaf temperature in Kelvin

PC T

HRT

HRT

SR

m

0 158 10

1

09 1

2

. exp

exp exp

4. Respiration rate, expressed in CO2 equivalents

R C P C Wp c m d 1 2

R = photorespiration + maintenance respirationC1p Pc = photorespiration, dependent on the photosynthesis rateC2m Wd = maintenance respiration, dependent on the plant weightC1p C2m = constants


Dry matter production and OBT build-up

C = 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)

TWTgTWTactact CBMdisfCPT

P P COA f sract c ( )


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)

Inhalation (plume passage) Ingestion

HTO 20 80

HT < 0.1 > 90


Winter what Germany Ratio modelled / measured14 experiments have been carried out in the years 1995 and 1996The 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 nightSamples 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)

0.0

1.0

2.0

3.0

0 2 4 6 8 10 12 14

Experiment number

Rati

o m

od

ell

ed

/ m

es

ure

d

mo/me U

mo/me P


A: Sunrise Experiment

0

5

10

15

20

25

30

07:00 10:00 13:00 16:00 19:00

Time of Day

Re

emis

sio

n R

ate

[%h-1

] Reemission Rate(Experiment)

Reemission Rate(UFOTRI 2)

Reemission Rate(UFOTRI)

HTO reemission experiments from soil


B: Sunset Experiment

0

5

10

15

20

25

30

21:00 00:00 03:00 06:00 09:00

Time of Day

Re

emis

sio

n R

ate

[%h-1

] Reemission Rate(Experiment)

Reemission Rate(UFOTRI 2)

Reemission Rate(UFOTRI)


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

0.00

0.05

0.10

0.15

0.20

0.25

0.30

8.8 18.8 28.8 7.9 17.9 27.9 7.10

time of year

rati

o i

n %

of

TW

T a

ir /

OB

T s

eed

measured

calculated

harvest

heading


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

Measured ratio in%

Predicted (lineargrowing) in %

Predicted (earlygrowing) in %

Daytime 0.44 0.89 0.56Day-flooded 0.43 0.79 0.51Night-time 0.31 0.23 0.19Night-flooded 0.33 0.22 0.20


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


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)

Event category / limits Maximum dose

AccidentsHT: 50 gHTO: 5 g

Early dose EDE3.5E-04 Sv 1.0E-02 Sv2.7E-03 Sv 2.1E-02 Sv


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

release limits characteristic quantities of the dose distribution (Sv)scenario (Greifswald) max. value 95%-fractile 50%-fractile mean value

early EDE early EDE early EDE early EDE

80g HTO-elevated 5.5E-03 9.0E-02 1.1E-03 1.5E-02 5.0E-04 4.0E-03 5.8E-04 6.1E-03

5g HTO-ground 2.7E-03 1.0E-02 1.5E-03 6.5E-03 1.8E-04 1.8E-03 3.6E-04 2.5E-03

• Large initial areas of food interdiction when applying the EC-CFILs (activity concentration intervention levels in human foodstuffs) were estimated


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

Documents

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