Assessing
potential exposures topeople in the postclosure period of awaste disposal facility
I. General aspects

Gerhard Proehl

Problem• Radioactive waste is generated world-wide during the application ofnuclear techniques− Energy production− Science and industry• Radioactive waste has to be disposed safely to ensure long-term isolationfrom the biosphere• International Safety Standards require a comprehensive safetyassessment of facilities for disposal of radioactive waste− A key element of the safety assessment is the assessment of potential
exposures to people after closure of the disposal facility− International Safety Standard recommend dose criteria for the exposures ofpeople

Problem (cont.)• The safety assessment covers time frames of several thousand
years up to a million years− Depending on the country and type of radioactive waste to be disposed• Disposal facilities have a complex system of safety features− Releases of radionuclides from waste repositories to the biosphere in the postclosure period may occur -̶ if at all ̶- only in the very far future• Compliance with the radiological criteria has to be demonstrated
today− Assessments of potential exposures of people possibly living close to thedisposal site have to be carried out− The consideration of long-time frames requires the consideration of changes inthe environment

Relevant radionuclides for disposal of radioactive
waste
Waste category Radionuclide
Low-level waste

Cs-137
Sr-90

Half-life (y)Remark30.1
28.5

Intermediate and Cl-36
high level waste Ni-59
Se-79
Zr-93
Nb-94
Tc-99

301 000 Potential high environmental mobility
75 00065 000 Potential high environmental mobility
1 530 000
20 300213 000 Potential high environmental mobility

Pd-107
Sn-126
I-129
Cs-1356 500 000
100 000
15 700 000 Potential high environmental mobility
2 300 000

U-238
- Th-230- Ra-226
Am-243- Pu-239- Pa-231
Np-2374 500 000 000
77 000 Daughter nuclide of U-238
1 600 Daughter nuclide of U-238
7 380
24 100 Daughter nuclide of Am-243
32 700 Daughter nuclide of Am-243
2 140 000 Potential high environmental mobility• Disposal of low-level waste− E.g. waste generated during the decontamination
activities after nuclear accidents− Disposal in near-surface facilities− Required isolation time: some 100 years

Time frames insafety assessments
fordisposal facilities• Disposal of intermediate level waste− Waste generated in nuclear facilities− Disposal in geological disposal at intermediate
depth (around 100 m).
− Required isolation time: Some 1000 years• Disposal of high-level waste− Spent fuel from nuclear power plants− Waste from reprocessing of fuel elements− Disposal in deep geological formations at depths
at around 500 m.
− Required isolation time: 10000 years or more
(up to 1 million y),depending on the national regulations

Biosphere in the post-closure period of a disposal
facility

BIOSPHERERiver
GEOSPHERE

Isolation
DISPOSALEXCAVATEDDAMAGED ZONE

Transport
limitation
Seals

WASTE PACKAGES

Confinement

MatrixOverpackWell

Radionuclides enter the biosphere
From the disposal area to thenear-surface aquifer•••Radionuclides are released
from the disposal
Radionuclides migrate throughthe overlying rock
Radionuclide contaminate thenear-surface aquifer

From the near-surface aquifer
to the biosphere••

Withdrawal of water via a wellIn case of high ground-waterlevel, radionuclides may directly
contaminate soils

Pathways for withdrawal of water from a well

Pathways for rising ground water

Plant model irrigation

Plant model for rising groundwaterFrom present to future conditions
Assessment for present
conditions
Environmental change• Climate• Lifestyle• Technology• Agriculture/soil

Assessment for future
conditions

Processes causing
radionuclide transfer in theenvironment

Important processes and features• Irrigation− Water demand of crops in different climates• Processes involved in the behaviour of radionuclides inplants− Interception of radionuclides in irrigation water by crops− Weathering and loss from plants− Systemic transport of radionuclides in plants− Uptake of radionuclides by crops via the rots from soil
− Migration in soil

• Processes involved in the transfer of radionuclides to animal
products− Feed and water intake− Metabolism in the animal− Transfer to meat milk and eggs

• Processing and culinary preparation• Intake of food
− Demand for nutrients− Plant and animal food products

Contamination routes for plant products
A Short-term
1 Direct deposition onto edible

parts of plants
2 Deposition onto leaves-> transport to the edible parts

B Long-term

3 Deposition on soil and uptake
through the roots
4 Resuspension of dust and redeposition on leaves and fruits

Interception for Cs
1,0000,900

Interception factor

Interception ofradionuclides
dissolved inirrigation
water

Fraction of activity retained by crops(interception) …
− … decreases with amount of irrigationwater− … increases with the development ofcrops− … highest during the peak season

0,800

Leafarea
index

0,700

1
1,50,600

2
2,50,500

340,40050,30070,2000,100
0,000
0,1110Rainfall (mm)100Interception depends on the chemical form (Hoffman
Mean of 5 plant species, 8.6 mm rain

21,510,5cations

I-131

S-35

Ce-141

Sr-85

Cr-51

Be-70Cd-109

Mass interception fraction (m²/kg)

et al., 1995)

anions(mass interception fraction = interception fraction normalized to the biomass)

Translocation

Active transport of elements in plants− Defines the amount of activity
transported from leaves to edible parts
Depends on− Element• Mobile elements (xylem + phloem)• Immobile elements (only phloem)− Stage of development• Pronounced seasonality• Foliar uptake may exceed root uptake byorders of magnitude

Translocation factors for wheat and barley

Left: Total activity in grain [Bq/m2] per Total activity deposited on the plant [Bq/m2]
Right: Activity concentration in grain [Bq/kg] per Total activity deposited on the plant [Bq/m2]
Voigt, G, Proehl, G, and Mueller, H. Experiments on the seasonality of the cesium translocation in cereals, potatoes and vegetables. 1991.
Web. doi:10.1007/BF01210514

Loss ofradionuclides fromplants due toweathering• Post-deposition activity loss from plants− Rainfall, fog, foliar abrasion− Including the decrease of activity
concentration due to increase in biomass(growth dilution)• Influencing factors− Time after deposition• Loss rate declines with time afterdeposition− Age of plants• Higher for young plants− Rainfall, fog

Cs-137 and I-131 activity invegetables after single
deposition event

Radionuclide
uptakefrom soil

Long-term source of plantcontamination, depending on• Soil characteristics− Sorption capacity (Sand, loam and clay
content, Organic matter)− pH value− Redox potential (esp. iodine, plutonium)− Concentration of antagonists• Cs vs K, Sr vs Ca• Use of fertilizer• Chemical form of the deposit− Soluble vs inert particles• Time since the contamination− Progressing sorption, fixation andincorporation processes

Quantificationof the uptakeof radionuclides
from soil

Transfer factor soil-plant TF

TF= Activity(plant) / Activity(soil)[Bq/kg fresh per Bq/kg dry]Typical values
fortransfer factors
soil-plant• Strontium:−0.1 - 1• Caesium−Well managed soils: 0.001 – 0.1−Organic, acid soils: 0.1-10• Technetium:− 0.1 – 10
• Iodine:− 0.001 - 1• Plutonium, americium :−0.00001 – 0.001• Pronounced variability, also on the samesite

Resuspensionof soil• Defines the flux of radionuclides
from soil to atmosphere• Depends on− Soil texture and humidity− Vegetation cover− Wind speed• Areas particularly affected byresuspension− Arid regions− In temperate climates,
resuspension during storms
may cause a relevant activity
loss from soil• Erosion by waterErosion:
Degradation of soil
due toremoval of soilmaterial
by wind and water− Kinetic energy of rain destroys soil
aggregates− Soil will be transported downhill• Factors increasing water erosion− Precipitation and contribution ofheavy rain showers− Slope− High fraction of sand and silt
− Low content of clay and organic
matter− Poor vegetation• Relevance− Up to 200 t/(ha*a ) (=> 1 cm soil)

Erosion by water• Rain splash affects the soil surface• Creeks in a sloping fieldTransfer toanimalproducts• Use of contaminatedfeedstuffs• Transfer to meat, milk,eggs

Simple model for the time-dependence of theactivity in animal products
Quantification• Transfer factor (steady stateconditions)• Biological half-lives

TF feed-(meat/milk/eggs) [d/kg]== Activity(meat/milk/eggs) [Bq/kg] / Daily activity(animal) [Bq/d]For more
information:

Compilation ofparameters forenvironmentaltransferSU • Long time frames have to be considered
100 years for low level waste
M −− Several
Several 1000 years for intermediate level waste− 10000 and more for high level waste
M • Radionuclides
enter the biosphereof water from a wellA −− Abstraction
Rising groundwater in case of high water tables• Safety assessment has to include all relevant pathways
R − Intake of food and drinking water− Inhalation of resuspended soil
− External exposure
Y • Safetyassessment has to consider all processes leading to an• Licensing of facilities for disposal of radioactive waste requires asafety assessment− The assessment of radiation doses to people in the post-closure period
is a key element of the safety assessment

exposure of people− Irrigation− Uptake of radionuclides from soil
− Radionuclide sorption and migration in soil
− Transfer to animal products− Erosion and resuspension