Modeling Atmospheric Releases of Molecular Tritium 2005 RETS/REMP Workshop Jim Key Key Solutions, Inc. www.keysolutionsinc.com.
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Modeling Atmospheric Releases of Molecular Tritium 2005 RETS/REMP Workshop Jim Key Key Solutions, Inc. www.keysolutionsinc.com 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 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.) 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. 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 1.109 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 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 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” - Vd • Typical Values: 0.00003 to .00034 m/sec • Allows Determination of Ground Plane Concentration (activity/m2) 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 Q sec m V 3 HT d m sec D Q1 m 2 Effective Ground Plane Deposition Rate Ci / m 2 DepHTO sec sec Ci HT m Q 3 Vd Q HT m sec sec 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. 2002. • 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 1.109 and NUREG 0133 Approaches • Easily Incorporated into Current ODCM Methodology Soil Moisture Concentration CSM ,dep Where: CSW,dep 3.15104 fr Q HT Precip Q Q 3.1510 f 4 Vd Precip HT r annual mean concentration of HTO in soil water deposition of HT. is 3.15107 sec/yr 10-3 m3/L. fraction of HTO retained in soil for plant root uptake (0.3). annual release rate of HT. annual 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 m3/L (e.g. pCi/L of HTO in air to pCi/m3 of HT in air) – Note must multiply by: AbsoluteHumidity kg m3 WaterDensitykg L to get pCi/m3 HTO in air Airborne Concentration Due to Re-Emission Defined for two heights above soil surface: – grVeg – grInh 20 cm for vegetation uptake - typical value ~ 6 m3/L 1.5 m for inhalation exposure - typical value ~ 4 m3/L Airborne Concentration Due to Re-Emission – Plant Exposure CRair Where: CR-air grVeg HA Water Veg Q QHT g r HA Water concentration of HTO in air due to re-emission of HTO in soil. concentration ratio of HTO in air to HT in air at height of vegetation (20 cm) [m3/L]. absolute atmospheric humidity [kg/m3]. density of water [kg/L] Concentration in Vegetation C Veg Q Q HT HR g HA Veg r Where: 0.75 0.75 f r Vd 3.1510 1 H R P r ecip 4 fraction of vegetation what is water [L/Kg]. ratio of vapor pressure of HTO and H2O (1.1). HR relative humidity. Airborne Concentration Due to ReEmission – Inhalation Exposure C Inhal R air HA Inh Q Q HT g r Water Where: CInhal R air 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. Dose Comparison Scenario /Q = Q HA HR Precipitation = = = = 110-6 sec/m3 1000 Ci/yr 8 gm/m3 70% 30 inches/yr 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 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