Presentation on theme: "Modeling Atmospheric Releases of Molecular Tritium 2005 RETS/REMP Workshop Jim Key Key Solutions, Inc. www.keysolutionsinc.com."— Presentation transcript:
Modeling Atmospheric Releases of Molecular Tritium 2005 RETS/REMP Workshop Jim Key Key Solutions, Inc.
Tritium Woes Keep It? –High Plant Inventories –Worker Exposure Problem –Increased Risk of Adverse Environmental Impact from Accidental Releases of High Concentrations TRITIUM
Tritium Woes Release It? –Via Liquid Effluents? Lowest Dose Impact High Political Impact for Some Sites –Via Gaseous Effluents? Higher Dose Impact Not ALARA
Dosimetric Impact of Liquid vs. Gaseous Releases of HTO Reg. Guide 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.)
A Different Idea Why Not Release to Atmosphere as HT? Significantly Lower Dose Impact Canadian Technology – Electrolytic Decomposition of HTO to HT and O 2 Canadians Release ~ 10 x More Tritium to Environment than U.S.
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
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 and NUREG 0133 Assume Tritium Released in the Form of Tritiated Water – HTO
Field Studies of Atmospheric HT 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
BIOMASS-3 IAEA Tritium Working Group Report “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
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.
BIOMASS-3 Examines 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
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
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
Oxidation of HT in Soil Described by “Deposition Velocity” - V d Typical Values: to m/sec Allows Determination of Ground Plane Concentration (activity/m 2 ) of HTO Resulting from Oxidation of HT
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
Effective Ground Plane Deposition
Effective Ground Plane Deposition Rate
Physical Transport Pathways 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.
Methodology Development Special Thanks to Ring Peterson at LLNL –NEWTRIT Model Described in HPS Journal, Feb Screening Model – Unrealistically Conservative –DCART Model (unpublished internal LLNL report, Sept. 2004). Incomplete Model But Rather a General Approach More Realistic Assumptions
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 and NUREG 0133 Approaches Easily Incorporated into Current ODCM Methodology
Soil Moisture Concentration Where: C SW,dep annual mean concentration of HTO in soil water deposition of HT 10 4 is 3.15 10 7 sec/yr m 3 /L. f r fraction of HTO retained in soil for plant root uptake (0.3). annual release rate of HT. Precipannual precipitation [m/yr].
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 m 3 /L (e.g. pCi/L of HTO in air to pCi/m 3 of HT in air) –Note must multiply by: to get pCi/m 3 HTO in air
Airborne Concentration Due to Re-Emission Defined for two heights above soil surface: –g r Veg 20 cm for vegetation uptake - typical value ~ 6 m 3 /L –g r Inh 1.5 m for inhalation exposure - typical value ~ 4 m 3 /L
Airborne Concentration Due to Re-Emission – Plant Exposure Where: C R-air concentration of HTO in air due to re-emission of HTO in soil. g r Veg concentration ratio of HTO in air to HT in air at height of vegetation (20 cm) [m 3 /L]. H A absolute atmospheric humidity [kg/m 3 ]. Water density of water [kg/L]
Concentration in Vegetation Where: 0.75fraction of vegetation what is water [L/Kg]. ratio of vapor pressure of HTO and H 2 O (1.1). H R relative humidity.
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. g r Inh concentration ratio of HTO in air to HT in air due to re-emission.
Dose Comparison Scenario /Q=1 sec/m 3 Q=1000 Ci/yr H A =8 gm/m 3 H R =70% Precipitation=30 inches/yr
HTO vs. HT Predicted Dose Dose (mrem) PathwayHTOHT Inhalation Vegetation Cow Milk Goat Milk Total
Liquid Release of HTO of Atmospheric 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