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Derivation of Dose-Based Detection Limits for Drinking Water and Effluent Compliance

Derivation of Dose-Based Detection Limits for Drinking Water and Effluent Compliance. Ken Sejkora Entergy Nuclear Northeast – Pilgrim Station Presented at the 17 th Annual RETS-REMP Workshop Philadelphia, PA / 25-27 June 2007. Reason for Concern. NRC vs. EPA … different limits due to:

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Derivation of Dose-Based Detection Limits for Drinking Water and Effluent Compliance

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  1. Derivation of Dose-Based Detection Limits for Drinking Water and Effluent Compliance Ken Sejkora Entergy Nuclear Northeast – Pilgrim Station Presented at the 17th Annual RETS-REMP Workshop Philadelphia, PA / 25-27 June 2007

  2. Reason for Concern • NRC vs. EPA … different limits due to: • Age-specific differences • Ingestion rates • Dose models and dose factors – ICRP-2 vs. ICRP-30 • Total body vs. Critical Organ • Various limits exist governing effluent compliance, safe drinking water standards, and dose/risk assessment

  3. Basis for Detection Limit • Based on equal dose consequence?? • Approach used by EPA for drinking water standard • Based on argument of “reasonable survey”?? • Loose basis of current ODCM LLD requirements • What’s reasonable… 1 hour analysis? 8 hour analysis? Radiochemical separation? • Dose is a common denominator representative of risk, and should be used as basis, but reasonableness should be considered in cases of extremely low LLDs.

  4. NRC Effluent Concentration Limits • Specified in Appendix B to Part 20--Annual Limits on Intake (ALIs) and Derived Air Concentrations (DACs) of Radionuclides for Occupational Exposure; Effluent Concentrations; Concentrations for Release to Sewerage • Derived from ICRP-30 dose models, adult age class • Committed dose of 100 mrem/yr for public • Preamble to Effluent Concentration Limit Tables “Consideration of non-stochastic limits has not been included in deriving the air and water effluent concentration limits because non-stochastic effects are presumed not to occur at the dose levels established for individual members of the public.”

  5. EPA Drinking Water Standard • Specified in 40 CFR 141.25 • Refer to: www.epa.gov/safewater/rads/radfr.pdf • Committed dose of 4 mrem/yr • Based on drinking water consumption of 2 liters/day = 730 liters/yr • H-3 = 20,000 pCi/L • Sr-90 = 8 pCi/L

  6. EPA Drinking Water Standard … where did it come from? • Roots in ICRP-2, circa late 1950s • Based on NRC maximum permissible concentration (MPC) value, where 1 MPC = 500 mrem/yr total body… “second generation derivative” • Step 1: Dose Conversion Factor = mrem/pCi • Step 2: Max perm. Intake = pCi to yield 4 mrem/yr • Step 3: Max perm. Conc. = pCi/L to yield intake of MPI, assuming ingestion rate of 730 L/yr

  7. EPA Drinking Water Standard …why it’s important • Standard limit adopted by state regulatory agencies… often takes precedence over ODCM limits if groundwater is involved • Used as basis for tables in standard ODCM guidance – NUREG-1301/1302 • Table 3.12-2, “Reporting Levels”… 20,000 pCi/L for drinking water; 30,000 pCi/L for non-DW • Table 4.12-1, “Lower Limit of Detection”… set at 1/10 of Reporting Level = 2,000 pCi/L for DW; 3,000 pCi/L for non-DW.

  8. Other ICRP-2 based Factors • Regulatory Guide 1.109, age-specific DCFs • LADTAP Factors… refinement of RG-1.109 factors • RG-1.109 and LADTAP dose conversion factors are the de facto values used in most plants’ ODCMs, effluent control programs, and effluent dose calculation software

  9. ICRP-30 based Factors • Based on ICRP-26 “standard man” approach, occupational exposure… circa early 1970s • Single age class = adult male • Basis for current values in 10CFR20 Appendix B, and EPA Federal Guidance Report 11

  10. ICRP-60/72 Factors • Based on ICRP-60 metabolic models… most modern, circa late 1990s • Non-occupational exposure, latest risk quality factors, multiple age classes • Standard among international community, but not widely adopted in USA, nor endorsed by NRC; however, adopted in EPA FGR-13

  11. Deriving dose-based LLD: BodyStep 1: Dose Factors

  12. Deriving dose-based LLD: BodyStep 2: Maximum Permissible Intake

  13. Deriving dose-based LLD: BodyStep 3: Drinking Water Concentration

  14. Deriving dose-based LLD: OrganStep 1: Dose Factors

  15. Deriving dose-based LLD: OrganStep 2: Maximum Permissible Intake

  16. Deriving dose-based LLD: OrganStep 3: Drinking Water Concentration

  17. Comparison of Derived Concentrations Total Body vs. Maximum Organ

  18. Comparison of Derived Concentrations Total Body vs. Maximum Organ

  19. EPA vs. NRC Approach • EPA drinking water approach only addresses intake and dose from drinking water ingestion • NRC RG-1.109 approach assumes multiple pathways: • Drinking water ingestion • Fish and shellfish ingestion • Shoreline sediment • Food crop ingestion from irrigated plants • Animal product ingestion… milk, meat • Far-field dilution… dose consequence based on concentrations at location of exposed receptor, instead of concentrations at point of discharge • How much dose are we missing?

  20. Deriving Total Pathway Dose Factors • Use Reg Guide 1.109 approach and pathways, assuming freshwater lake • Assume irrigation 50% of year • Assume concentration of 1 pCi/L in water, derive corresponding concentration in each pathway media type • Apply age-specific usage factors to each pathway media type • Determine age-specific dose from each pathway media type

  21. Combined Pathway DoseAdult Total Body

  22. Combined Pathway DoseTeen Total Body

  23. Combined Pathway DoseChild Total Body

  24. Combined Pathway DoseInfant Total Body

  25. Combined Pathway DoseICRP-30 Adult Total Body

  26. Combined Pathway LLD: BodyStep 1: Dose per Unit Concentration

  27. Combined Pathway LLD: BodyStep 2: Pathway Water Concentration

  28. Comparison of Derived Concentrations Drinking Water vs. Combined Pathway

  29. Comparison of Derived Concentrations Drinking Water vs. Combined Pathway

  30. Which LLDs to Use? • RG-1.109/LADTAP pathway approach includes exposure “missed” by drinking water standard • Maximum organ values are not applicable… 10CFR20 Appendix B basis for discounting non-stochastic effects at low doses involved • Age-specific dose factors and use factors • Beyond scope of ICRP-30… ICRP-2 or ICRP-72? • When dealing with deriving new concentration limits and LLDs, one could argue we should use the most up-to-date methodology and science available… ICRP-72

  31. Summary • Current EPA approach outlined in 40CFR141 only addresses dose from drinking water ingestion • Overlooks dose from other pathways, which can contribute significantly more dose than water ingestion alone • Considers single age class, overlooks age-specific variability in dose factors and consumption rates

  32. Summary (continued) • Use of organ-specific dose factors yields lower concentration limits • Based on NRC approach in Appendix B to 10CFR20, non-stochastic dose effects are not applicable at the low dose levels assumed to occur from environmental exposure • Concentration limits and LLDs should be derived from effective total body doses

  33. Summary (continued) • A consistent methodology should be used to derive water concentrations based on dose, but some LLDs may be difficult to achieve, and may necessitate a “reasonable survey” approach • Consideration should be given to deriving stochastic dose-based concentrations using the most modern standard… ICRP-72

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