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Modeling Atmospheric Releases of Molecular Tritium
2005 RETS/REMP Workshop
Jim Key
Key Solutions, Inc.
www.keysolutionsinc.com
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Tritium Woes
• Keep It?– High Plant Inventories– Worker Exposure Problem– Increased Risk of Adverse
Environmental Impact from Accidental Releases of High Concentrations
TRITIUM
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Tritium Woes
• Release It?– Via Liquid Effluents?
• Lowest Dose Impact
• High Political Impact for Some Sites
– Via Gaseous Effluents?• Higher Dose Impact
• Not ALARA
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Dosimetric Impact of Liquid vs. Gaseous Releases of HTO
• Reg. Guide 1.109 and NUREG 0133 Models Indicate Significant Increase in HTO Dose for Atmospheric vs. Liquid Releases
• Exact Dose Increase is Site Specific but Typically 10 Times or Greater
• Significant Risk of Site Contamination (condensation on build surfaces, etc.)
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A Different Idea
• Why Not Release to Atmosphere as HT?
• Significantly Lower Dose Impact• Canadian Technology – Electrolytic
Decomposition of HTO to HT and O2
• Canadians Release ~ 10 x More Tritium to Environment than U.S.
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Dosimetric Impact of HT vs. HTO
• Radiotoxicity of HTO ~ 20,000-25,000 Times that of HT (ICRP-30)
• Only Significant Dose Impact Occurs Following Oxidation of HT to HTO and Subsequent Exposure
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Why Model Molecular Tritium?
• Need Ability to Predict Environmental Concentrations for Decision Making.
• If Tritium is Released Atmospherically as HT, then ODCM Must be Revised to Model Doses.
• Reg. Guide 1.109 and NUREG 0133 Assume Tritium Released in the Form of Tritiated Water – HTO
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Field Studies of AtmosphericHT Releases
• AECL – Chalk River Laboratory, Canada– 1986 – 18.5 Ci of HT Released– Pure HT Release
• Savannah River Site, USA– 1974 – 479,000 Ci of HT Released– 1975 – 182,000 Ci of HT Released– Estimated 99% HT, 1% HTO
• Short Term Releases
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BIOMASS-3
• IAEA Tritium Working Group Report - 2003 - “Modeling the Environmental Transport of Tritium in the Vicinity of Long Term Atmospheric and Sub-Surface Sources”
• Provides Comparison of Numerous Tritium Models Against Field Measurements
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BIOMASS-3
• Models Atmospheric Releases of Molecular Tritium (HT) as well as Tritiated Water (HTO)
• These are all screening models and as such result in very conservative estimates of Tritium exposure.
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BIOMASS-3Examines Models Used By:
• AECL – Canada• BEAK – Canada• ANDRA – France• CEA – France• FZK – Germany• ZSR – Germany
• JAERI – Japan • NIPNE – Romania• VNIIEF – Russia• SESAB – Sweden• LLNL – USA
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Oxidation of HT to HTO• Oxidation in
Atmosphere is Very Slow Process with Half Life of > 5 Years
• Most Significant Oxidation Occurs at the Atmosphere-Soil Interface
HT
HTO
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Oxidation of HT to HTO in Soil
• Result of Bacterial Action in Soil
• Oxidation Efficiency is Highly Dependant on Organic Content of Soil– Sterilized Clay Loam ~ 3.4%– Natural Clay Loam 100%
• Occurs Very Quickly ~ hours
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Oxidation of HT in Soil
• Described by “Deposition Velocity” - Vd
• Typical Values: 0.00003 to .00034 m/sec
• Allows Determination of Ground Plane Concentration (activity/m2) of HTO Resulting from Oxidation of HT
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Atmospheric Dispersion of HT
• HT Has Approximately 6% Density of Air• Might Seem that HT Would Quickly
Diffuse Out of Plume• Field Studies Have Shown that HT Remains
Entrapped in Plume in the Near Field• BIOMASS-3 Models All Model HT
Dispersion Using Standard Gaussian Plume Model
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Effective Ground Plane Deposition
2HTd
3 m1QDsecmVmsecQ
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Effective Ground Plane Deposition Rate
sec
CiQ
sec
mV
m
secQ
sec
m/CiDep
HTHTd3
2
HTO
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Physical TransportPathways Considered
• Soil Moisture– Deposition of HT onto ground plane with
subsequent oxidation to HTO.
• Airborne Concentration from Soil Re-Emission– Emission of HTO (oxidized HT) into air from
soil moisture.
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Methodology Development
• Special Thanks to Ring Peterson at LLNL– NEWTRIT Model Described in HPS Journal,
Feb. 2002.• Screening Model – Unrealistically Conservative
– DCART Model (unpublished internal LLNL report, Sept. 2004).
• Incomplete Model But Rather a General Approach
• More Realistic Assumptions
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Methodology Development
• Methodology Presented Here Makes Use of DCART Strategy for Predicting Environmental Concentrations of HTO Due to Atmospheric Releases of HT
• Methodology Designed to be Compatible with Reg Guide 1.109 and NUREG 0133 Approaches
• Easily Incorporated into Current ODCM Methodology
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Soil Moisture Concentration
Precip
1015.3 4,
dHTrdepSM
VQQfC
Where: CSW,dep annual mean concentration of HTO in soil water
deposition of HT.
3.15104 is 3.15107 sec/yr 10-3 m3/L.
fr fraction of HTO retained in soil for plant root uptake (0.3).
annual release rate of HT.
Precip annual precipitation [m/yr].
HTQ
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Airborne Concentration Due to Re-Emission
• Described in terms of HTO in air to HT in air based on field measurements.
• Specified in units of m3/L (e.g. pCi/L of HTO in air to pCi/m3 of HT in air)– Note must multiply by:
to get pCi/m3 HTO in air
LkgDensityWater
mkgHumidity Absolute 3
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Airborne Concentration Due to Re-Emission
Defined for two heights above soil surface:– gr
Veg 20 cm for vegetation uptake
- typical value ~ 6 m3/L
– grInh 1.5 m for inhalation exposure
- typical value ~ 4 m3/L
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Airborne Concentration Due to Re-Emission – Plant Exposure
Where: CR-air concentration of HTO in air due to re-emission of HTO
in soil.
grVeg concentration ratio of HTO in air to HT in air at height
of vegetation (20 cm) [m3/L].
HA absolute atmospheric humidity [kg/m3].
Water density of water [kg/L]
WaterAVegrHTairR HgQQC
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Concentration in Vegetation
Where: 0.75 fraction of vegetation what is water [L/Kg].
ratio of vapor pressure of HTO and H2O (1.1).
HR relative humidity.
ecipPr
VfH11015.3
H
gH
75.0QQC
drR
4
A
VegrR
HTVeg
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Airborne Concentration Due to Re-Emission – Inhalation Exposure
Where: airborne concentration of HTO in air at 1.5 m due to
re-emission from soil.
grInh concentration ratio of HTO in air to HT in air due to
re-emission.
Water
AInhrHT
InhalairR
HgQQC
InhalairRC
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Dose Comparison Scenario
/Q = 110-6 sec/m3
Q = 1000 Ci/yr
HA = 8 gm/m3
HR = 70%
Precipitation = 30 inches/yr
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HTO vs. HT Predicted Dose
Dose (mrem)
Pathway HTO HT
Inhalation 0.036 0.001
Vegetation 0.157 0.012
Cow Milk 0.050 0.004
Goat Milk 0.136 0.010
Total 0.328 0.023
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Liquid Release of HTO ofAtmospheric Release of HT?
• Both Appear to Have the Same Dose Impact
• Exact Comparison Requires Site Specific Analysis
• Obviously Is Not Cost Beneficial If Liquid Discharge is an Option
• Possible Option Where Liquid Releases Are Not Viable