SRNS-F3200-2012-00019 Comparison of Parameters for Modeling Tritium Dispersion

1. SRNS-F3200-2012-00019 Comparison of Parameters for Modeling Tritium Dispersion Eric Hope Accident Analyst Nuclear & Criticality Safety Engineering Savannah River Nuclear Solutions, LLC April 18, 2012 2012 EFCOG Safety Analysis Workshop Santa Fe, New Mexico

2. 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.

3. Objective: Characterize the impact of MACCS2 input parameters on the result for a unit release of Tritium

4. Objective: This comparison is intended to look at responses of parameters, not as justification for any particular value

5. 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

6. Methodology • Baseline facility has large tritium inventory, small amounts of other species • Some activation products • No significant quantities of alpha emitting material

7. 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

8. 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

9. 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)

10. 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

11. Methodology Establish baseline parameters and results

12. 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

13. Methodology Vary parameters to determine impact on results

14. 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

15. 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

16. 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)

17. 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

18. Sensitivity to Surface Roughness Adjustment • Vertical turbulence increases dispersion • Adjustment to Sigma Z based on power law • 3 cm for prairie grass experiments

19. 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

20. 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

21. 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

22. 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.)

23. 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

24. 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

25. 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

26. Conclusion • The comparisons applicable only to facility where tritium is dominant release • Acute accident releases - Ground level only • Offsite receptor @ 11.5 km

27. 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

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

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

30. 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

31. References • 2010, EFCOG SAWG Presentation by Kevin Brotherton, “Significant Sensitivities in Developing Radiological Dose Consequences When Using MACCS2” • 2007, Paper by O’Kula & Thoman, WSRC-STI-2007-00027, “Modeling Atmospheric Releases of Tritium from Nuclear Installations” • 2004, MACCS2 Computer Code Application Guidance for Documented Safety Analysis, DOE-EH-4.2.1.4-MACCS2-Code Guidance (MACCS2 FGR) • 1998, Code Manual for MACCS2: Volume 1, User’s Guide, NUREG/CR-6613 • 2001, ICRP Database of Dose Coefficients & ICRP 72

32. Questions