Modeling Atmospheric Releases of Molecular Tritium

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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

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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 gt 5 Years
• Most Significant Oxidation Occurs at the
  Atmosphere-Soil Interface

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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
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Effective Ground Plane Deposition Rate
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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
Where   CSW,dep annual mean concentration
of HTO in soil water deposition of HT.  
3.15?104 is 3.15?107 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.
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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

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Airborne Concentration Due to Re-Emission

• Defined for two heights above soil surface
• grVeg 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

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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.
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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.
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Dose Comparison Scenario

• ?/Q 1?10-6 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) 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