SRNS-F3200-2012-00019

Comparison of Parameters for Modeling Tritium Dispersion

Accident AnalystNuclear & Criticality Safety EngineeringSavannah River Nuclear Solutions, LLC

April 18, 2012

Eric Hope

Santa Fe, New Mexico

2012 EFCOG Safety Analysis Workshop

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Savannah River Nuclear Solutions

SRNS is the M&O contractor for DOE’s Savannah River Site in Aiken, S.C.

The primary initiatives for SRNS are national security, clean energy and environmental stewardship.

–We provide nuclear materials management to support national defense and U.S. nuclear nonproliferation efforts.

–We support the National Nuclear Security Administration by extracting tritium and delivering products to military and weapons design agencies.

–We develop and deploy environmental cleanup technologies.

–We conduct technology R&D on national energy independence initiatives.

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Objective:

Characterize the impact of MACCS2 input parameters on the result for a unit release of Tritium

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Objective:

This comparison is intended to look at responses

of parameters, not as justification for any

particular value

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Methodology

• Establish a baseline result• Vary one parameter at a time • Use a range that reflects current and anticipated requirements• Some parameters are essentially step functions

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Methodology

• Baseline facility has large tritium inventory, small amounts of other species

• Some activation products• No significant quantities of alpha emitting material

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Input Parameters

MACCS2 requires user input of parameters – some are:• Plume Depletion/Deposition Velocity• Sigma Z adjustment for surface roughness• Sigma Y adjustment for time basis/plume meander• Dispersion Coefficients• Resuspension Coefficient

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Baseline for MACCS2 Calculations

Conditions and parameters used in existing analysis:• Dry deposition with 0.5 cm/sec deposition velocity• Offsite receptor at 11.5 km from release• 95th dose quantile• Tadmor-Gur dispersion coefficients (far field)• Sigma Z adjusted for site specific surface roughness of 100

cm using 3 cm as basis• Sigma Y adjusted for plume meander (3 minute and 20 minute

releases) using 3 minute time basis

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Baseline for MACCS2 Calculations

Conditions and parameters used in existing analysis

• Source term assumes 100% instant conversion from elemental tritium to tritium oxide

• ICRP72 IDCF for elemental tritium – 1.8E-15 Sv/Bq• ICRP72 IDCF for tritium oxide - 1.8E-11 Sv/Bq• Oxide IDCF adjusted by 1.5x accounting for skin adsorption

( = 2.7E-11 Sv/Bq)

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Tritium in Several Forms

ICRP72 IDCF Values for various forms of Tritium• Organically Bound Tritium – 4.1E-11 Sv/Bq• Elemental Hydrogen – 1.8E-15 Sv/Bq• Tritiated Methane – 1.8E-13 Sv/Bq• Tritiated Water (Oxide) – 1.8E-11 Sv/Bq• Tritiated Water (1.5x Oxide) – 2.7E-11 Sv/Bq• Type F Tritium Compounds – 6.2E-12 Sv/Bq *• Type M Tritium Compounds – 4.5E-11 Sv/Bq *• Type S Tritium Compounds – 2.6E-10 Sv/Bq *

*1u AMAD - Influenced by particle size distribution

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Methodology

Establish baseline parameters and results

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Baseline for MACCS2 Calculations

Baseline result is 3.25E-10 Sv/Ci tritium oxide• Dry deposition 0.5 cm/sec deposition velocity• 20 minute release duration – 180 second time basis• 100 cm surface roughness – 3 cm basis• Tadmor-Gur far field dispersion coefficients• Ground level (0 meter) release• 11.5 km offsite receptor• 1.5x ICRP72 IDCF (2.7E-11 Sv/Bq)• 1E-04 m-1 Resuspension Coefficient• 95th Dose Quantile

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Methodology

Vary parameters to determine impact on results

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Sensitivity to Dry Deposition Velocity

• Plume depleted as it travels downwind• Larger effect as distance from source increases• Base case has 11.5 km offsite receptor

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Sensitivity to Dry Deposition Velocity

• MACCS2 Results for Sensitivity to various deposition velocity values

• All other baseline inputs held constant

• One Curie (3.7E+10 Bq) release

• 2001 met data only

Deposition Velocity (cm/sec)

MACCS2 Result (Sv/Ci)

% of Baseline

0.5 3.25E-10 100%

0.1 3.68E-10 113%

0.01 3.77E-10 116%

Deposition OFF

3.80E-10 117%

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Sensitivity to Time Basis Adjustment

• Plume meander increases dispersion• Adjustment to Sigma Y based on power law• 180 seconds for prairie grass experiments specified in

MACCS2 FGR• Time basis 600 seconds• Time basis equal to release duration (1200 seconds)

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Sensitivity to Time Basis Adjustment

• MACCS2 Results for Sensitivity to various Time Basis Adjustment values

• 20 minute release duration• All other baseline inputs

held constant• One Curie (3.7E+10 Bq)

release• 2001 met data only

Time Basis (sec)

MACCS2 Result (Sv/Ci)

% of Baseline

180 3.25E-10 100%

600 4.02E-10 124%

1200 4.68E-10 144%

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Sensitivity to Surface Roughness Adjustment

• Vertical turbulence increases dispersion• Adjustment to Sigma Z based on power law• 3 cm for prairie grass experiments

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Sensitivity to Surface Roughness Adjustment

• MACCS2 Results for Sensitivity to various surface roughness values

• 3 cm basis• All other baseline inputs

held constant• One Curie (3.7E+10 Bq)

release• 2001 met data only

Surface Roughness

(cm)

MACCS2 Result (Sv/Ci)

% of Baseline

100 3.25E-10 100%

80 3.36E-10 103%

30 3.81E-10 117%

3 5.33E-10 164%

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Sensitivity to Resuspension

• Depletion of the plume results in a ground concentration (Bq/m2)

• Wind and traffic suspend deposited activity back into atmosphere

• Resuspension Concentration Coefficient of 1E-04 is default value in MACCS2 FGR – Baseline Input

• EARLY module input• CHRONC module calculates environmental collective

population doses using 1E-05 after emergency response phase

• 2007 paper by O’Kula and Thoman

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Sensitivity to Resuspension

• MACCS2 Results for Sensitivity to various Resuspension Coefficients

• All other baseline inputs held constant

• One Curie (3.7E+10 Bq) release

• 2001 met data only• 0.5 cm/sec dry deposition

may influence magnitude

Re-suspension Coefficient

(m-1)

MACCS2 Result (Sv/Ci)

% of Baseline

1E-04 3.25E-10 100%

1E-05 3.14E-10 97%

1E-06 3.13E-10 96%

1E-10 3.13E-10 96%

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Sensitivity to Dispersion Coefficients

• Typically not practical to get site specific data(budget, schedule, technical hurdles, etc.)

• Choices of default dispersion coefficients

• Selection based on applicability to conditions (surface roughness, elevation, terrain, distance, time of release, etc.)

Sensitivity to Dispersion Coefficients

Default Sets of Dispersion Coefficients

•Tadmor-Gur widely used for ground releases•Briggs typically used for elevated releases•Pasquill-Gifford discussed in MACCS2 documentation (T-G is an implementation of P-G)•Julich developed in Germany for elevated release from very tall stack

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Sensitivity to Dispersion Coefficients

Software Quality Assurance Issues

•HSS 2009-05 error in MACSS2 v1.13.1 and v2.4•Error dependent on input form of dispersion coefficients•Other errors reported for “new” plume meander model•The results for the next slide generated using MACCS2 v2.5; all previous results based on “Toolbox” v1.13.1

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Sensitivity to Dispersion Coefficients

• MACCS2 Results for Sensitivity to various dispersion coefficients

• All other baseline inputs held constant - issues

– 0 meter ground release– Sigma Z surface roughness

adjustment to 100 cm from 3 cm basis

– Sigma Y time basis adjustment– Distance to receptor

Dispersion Coefficient

Set

MACCS2 Result (Sv/Ci)

% of Baseline

Tadmor-Gur (5 to 50 km)

3.25E-10 100%

Pasquill-Gifford

(E-K)

3.14E-10 97%

Briggs Open Country

3.53E-10 109%

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Conclusion

• The comparisons applicable only to facility where tritium is dominant release

• Acute accident releases - Ground level only• Offsite receptor @ 11.5 km

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Conclusion

• Tadmor-Gur dispersion coefficients appear to be a reasonable choice for ground level releases

• All three evaluated dispersion coefficients gave roughly comparable results for 11.5 km receptor

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Conclusion

• Changing the deposition velocity parameter from 0.5 to 0 cm/sec may increase the offsite dose to this receptor by about 17%

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Conclusion

• Changes to other parameters besides deposition velocity might have an even larger impact on offsite dose

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Conclusion

• Potential future technologies emphasizing tritium compounds may be impacted by filtered and unfiltered release parameters

• Unlike current source term assumption of 100% instant conversion to oxide

• Needed research publications into response of tritium compounds under thermal stress

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References

1. 2010, EFCOG SAWG Presentation by Kevin Brotherton, “Significant Sensitivities in Developing Radiological Dose Consequences When Using MACCS2”

2. 2007, Paper by O’Kula & Thoman, WSRC-STI-2007-00027, “Modeling Atmospheric Releases of Tritium from Nuclear Installations”

3. 2004, MACCS2 Computer Code Application Guidance for Documented Safety Analysis, DOE-EH-4.2.1.4-MACCS2-Code Guidance (MACCS2 FGR)

4. 1998, Code Manual for MACCS2: Volume 1, User’s Guide, NUREG/CR-6613

5. 2001, ICRP Database of Dose Coefficients & ICRP 72

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Questions