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 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

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-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

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

2HTd

3 m1QDsecmVmsecQ

Effective Ground Plane Deposition Rate

sec

CiQ

sec

mV

m

secQ

sec

m/CiDep

HTHTd3

2

HTO

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.

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

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

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

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

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

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

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

Dose Comparison Scenario

/Q = 110-6 sec/m3

Q = 1000 Ci/yr

HA = 8 gm/m3

HR = 70%

Precipitation = 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 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