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

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 95 th 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 95 th 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 Deposition Velocity (cm/sec) MACCS2 Result (Sv/Ci) % of Baseline E-10100% E-10113% E-10116% Deposition OFF 3.80E-10117%

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 Time Basis (sec) MACCS2 Result (Sv/Ci) % of Baseline E-10100% E-10124% E-10144%

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 Surface Roughness (cm) MACCS2 Result (Sv/Ci) % of Baseline E-10100% E-10103% E-10117% 35.33E-10164%

20 Sensitivity to Resuspension Depletion of the plume results in a ground concentration (Bq/m 2 ) 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 Re- suspension Coefficient (m -1 ) MACCS2 Result (Sv/Ci) % of Baseline 1E E-10100% 1E E-1097% 1E E-1096% 1E E-1096%

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.)

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 23

Sensitivity to Dispersion Coefficients Software Quality Assurance Issues HSS error in MACSS2 v 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” v

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 Dispersion Coefficient Set MACCS2 Result (Sv/Ci) % of Baseline Tadmor-Gur (5 to 50 km) 3.25E-10100% Pasquill- Gifford (E-K) 3.14E-1097% Briggs Open Country 3.53E-10109%

26 Conclusion The comparisons applicable only to facility where tritium is dominant release Acute accident releases - Ground level only Offsite 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 , EFCOG SAWG Presentation by Kevin Brotherton, “Significant Sensitivities in Developing Radiological Dose Consequences When Using MACCS2” , Paper by O’Kula & Thoman, WSRC-STI , “Modeling Atmospheric Releases of Tritium from Nuclear Installations” , MACCS2 Computer Code Application Guidance for Documented Safety Analysis, DOE-EH MACCS2-Code Guidance (MACCS2 FGR) , Code Manual for MACCS2: Volume 1, User’s Guide, NUREG/CR , ICRP Database of Dose Coefficients & ICRP 72

32 Questions