Concentration of Tritium in Liquid Samples by Electrolysis
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17th Annual RETS – REMP
Workshop
Stan Morton, Ph.D.
June 27, 2007
Philadelphia, PA June 27, 2007
Tritium
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• Heavy Isotope of Hydrogen
• 12.3 year half-life
• Beta Decays to Stable He-3
• Low-energy Beta Particle– 18.6 keV beta-max– 5.7 keV beta-avg
• Hydrogen and Tritium interchangeable
Tritium Unit (TU)
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TU ≡ 0.118 Bq/L ≡ 3.19 pCi/L
Or
1 Bq/L ≡ 8.47 TU
1TU ≡ Concentration of 10-18
Sources of Tritium
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• Cosmogenic• Nuclear Weapons – Atmospheric detonation
• By-Product Nuclear Power Reactors
– Boron
– Lithium
• Fission Process
Cosmogenic
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• Spallation – Cosmic-rays interact with atomic nitrogen:
14N (n, 3H) → 12C
• Reaction altitude from 11 to 16 km
• Adds to Precipitation Level
Nuclear Weapons - Atmospheric
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• Testing 1940s through 1970s
• Precipitation Levels peaked in 1963
• Additional 52 X 1018 Bq to Global Inventory
• Estimated remnant of 100 – 400 pCi/L in precipitation
Precipitation Concentrations
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Tritium Production from Boron
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• Boron-10 (19.9%) high thermal neutron cross section – 3835 barns
• Control rods for BWRs and PWRs
• Chemical shim and reactivity control in PWRs
10B(n,2α) → 3H10B(n,α) → 7Li(n,nα) → 3H
Tritium Production from Lithium
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• Most acceptable hydroxide for pH control in some PWR primary coolant
• 6Li (7.5%) 940 barns
• Principle reactions:7Li(n,nα) → 3H8Li(n,α) → 3H
Tritium from Fission
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• Lesser extent from fission
• Fission yield for 235U is ~0.01%
Tritium Inventory
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• Estimated normal releases 0.02 x 1018 Bq/year• Estimated off-normal releases 0.001 x 1018
Bq/year• Steady-state buildup 0.4 x 1018 Bq globally• Legacy release 0.4 x 1018 Bq/year• Steady-state buildup 7.4 x 1018 Bq globally
Tritium Global Inventory
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• Atmospheric detonations 50s & 60s:
185 to 240 X 1018 Bq
• Legacy today:
52 X 1018 Bq
• Combined natural and anthropogenic global inventory of approximately:
53 X 1018 Bq or ~ 10 Bq/L
Regulatory Limits
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• Weak beta and rapid elimination produce a minor constituent in dose evaluation
• EPA 1976 Drinking Water Standard is 4 mrem/yr = 20,000 pCi/L
• 1991 4 mrem/yr = 60,900; retained 1976 limit.
Dose Considerations
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• Human retention studies provide three-component half-lives– 6 to 12 days – turnover of pool of body water– 10 to 34 days – involved in carbon-tritium
chemistry– 130 to 550 days – organic molecules with
slow turnover rates
Perception vs. Risk
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• Braidwood – 1600 pCi/L ≡ 0.3 mrem/yr; Exelon Corporation bought the farm
• Decatur Daily headline: …TVA, NRC are ‘flippant’ over tritium leaks
• Morris Daily Herald: Dresden leak levels 25 times the allowable drinking water limits
• Arizona Republic: Palo Verde’s tritium leak may impact the groundwater.
• Harford Courant: Haddam a few gallons a day of tritium contaminated water breaches 6-foot thick concrete wall.
Concentration of Tritium by Electrolysis
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• 2006, GEL Labs recognized need
• Quantitative below 150 – 200 pCi/L
• Reliable method
• Well defined turn-around-times
• Quality driven
• Customer service
Concentration of Tritium by Electrolysis
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• Exploits slight differences in physical properties between hydrogen and tritium
• Molecule of water containing hydrogen more likely to dissociate by electrolysis than molecule of water containing tritium.
Overview
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• Aliquant of 500 mL sample screened for ‘high’ levels of tritium and extraneous emitters
• Shipped to Richland Service Center (RSC) in Richland, Washington
• Enrichment process
• Returned in closed vial for LSC
Ambient and Environmental Considerations
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• All commercial analytical labs evaporate hundreds of liters of tritium laden water
• May produce elevated levels of ambient tritium
• Environmental concentrations vary by region– Eastern and Southern states highest levels– Mid-west and Western States lowest levels
Richland Service Center
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Electrolysis Instrumentation
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Electrolysis Cold-Water Bath
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RSC Tritium Laboratory
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Procedure
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• 300 mL distilled
• 250 mL concentrated by electrolysis for maximum sensitivity
• Batch size– 12 samples– 2 background– LCS containing NIST traceable tritium
standard
Procedure (cont’d)
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• Cold water bath (~ 5°C)
• Constant current until 25 mL remaining
• Reduce current until 12 – 15 ml
• Volume reduction and enrichment requires 12 – 14 days
• Vacuum distillation to remove NaOH
Procedure (cont’d)
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• Volume determined by weight
X = Vi / Vf
• Enrichment determined from LCSs
Y = Cf / Ci
• Transferred to LSC vial
• Returned to Charleston for beta counting
MARLAP Approach
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• Multi-Agency Radiological Laboratory Analytical Protocols (MARLAP) Manual
• Nationally consistent approach to producing analytical data
• Performance-based approach for selecting analytical protocol
• Project specific criteria
MARLAP Method Validation
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Validation Level
Applications Sample Type1
Acceptance Criteria2
Levels3 (Concentrations)
Replicates No. of Analyses
A Without
Additional Validation
Existing Validated Method
____ Method
Previously Validated B thru E
____ ____ ____
B
Same or similar Matrix
Internal PT
Measured Value Within ±2.8μMR or ±2.8φMR Known Value
3 3 9
C Similar Matrix/
New Application
Internal or
external PT
Measured Value Within ±2.9μMR or ±2.9φMR Known Value
3 3 15
D
Newly Developed or
Adapted Method
Internal or
external PT
Measured Value Within ±3.0μMR or ±3.0φMR Known Value
3 3 21
E
Newly Developed or
Adapted Method
MVRM* Samples
Measured Value Within ±3.0μMR or ±3.0φMR Known Value
3 3 21
Newly Developed Methods
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• Newly developed use Level D or E
• Increased number of replicates
• Best estimate of precision and bias
• Unique matrix
Determination of Uncertainty
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• Method validation determines method uncertainty
• Sample uncertainty menu includes– Method uncertainty– Liquid-scintillation counting statistics– Background subtraction– NIST standards, decay time, half-life, etc.
Uncertainty Considerations
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• Uncertainty increases as activity approaches detection limit
• Because of such effects, as analyte concentrations drop, the relative uncertainty associated with the result tends to increase, first to a substantial fraction of the result and finally to the point where the (symmetric) uncertainty interval includes zero. This region is typically associated with the practical limit of detection for a given method.
Questions & Contact Information
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Contact Information:
Stan Morton, Manager, Radiobioassay Programs 303.349.8345 [email protected]
Bob Wills, Manager, Nuclear Programs 843.556.8171 [email protected]