Presentation on theme: "Implications of Tritium Dose Conversion Factors in Deriving Regulatory Limits for Drinking Water and Effluent Compliance Ken Sejkora Entergy Nuclear Northeast."— Presentation transcript:
Implications of Tritium Dose Conversion Factors in Deriving Regulatory Limits for Drinking Water and Effluent Compliance Ken Sejkora Entergy Nuclear Northeast – Pilgrim Station Presented at the 16 th Annual RETS-REMP Workshop Mashantucket, CT / 26-28 June 2006
Reason for Concern Issue of tritium in groundwater has crossed jurisdictional boundaries… NRC, EPA, state radiation control programs are involved Various limits exist governing effluent compliance, safe drinking water standards, and dose/risk assessment Recognize the different limits, reasons for differences, and implications/applicability
EPA Drinking Water Standard …from the beginning Specified in 40 CFR 141.25 Refer to: www.epa.gov/safewater/rads/radfr.pdf 20,000 pCi/Liter, assumed to yield committed dose of 4 mrem/yr Based on drinking water consumption of 2 liters/day = 730 liters/yr 20,000 pCi/L * 730 L/yr = 1.46E+7 pCi/yr 4 mrem / 1.46E+7 pCi = 2.74E-7 mrem/pCi
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.
EPA Drinking Water Standard … where did it come from? Roots in ICRP-2, circa late 1950s “Second generation derivative” of NRC maximum permissible concentration (MPC) value 1 MPC = 500 mrem/yr total body Early MPC for tritium: 3E-3 uCi/mL (4 mrem / 500 mrem) * 3E-3 uCi/mL = 2.4E-5 uCi/mL = 24,000 pCi/L Rounded to 20,000 pCi/L… one significant figure “Gray-hair” phenomenon… derivation not documented, exists only in recollection of experts involved
Other ICRP-2 based Factors Direct activity-to-dose factors… not “back-calculated” from concentration limit Age-specific dose factors Regulatory Guide 1.109, Ingestion dose factors, Tables E-11 through E-14 Adult: 1.05E-7 mrem/pCi Child: 2.03E-7 mrem/pCi Teen: 1.06E-7 mrem/pCi Infant: 3.08E-7 mrem/pCi
Other ICRP-2 based Factors …continued LADTAP Factors… refinement of RG-1.109 factors Adult: 5.99E-8 mrem/pCiChild: 1.16E-7 mrem/pCi Teen: 6.04E-8mrem/pCiInfant: 1.76E-7 mrem/pCi 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
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 Ingestion DCF = 6.40E-8 mrem/pCi Table 2, Column 2 Concentration Limit - Public = 1E-3 uCi/mL, or 1,000,000 pCi/L assumed to yield committed dose of 50 mrem/yr
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, Adult: 7.09E-8 mrem/pCi 5-yr: 1.25E-7 mrem/pCi 15-yr: 7.07E-8 mrem/pCi 1-yr: 2.04E-7 mrem/pCi 10-yr: 9.06E-8 mrem/pCi Infant: 2.79E-7 mrem/pCi Standard among international community, but not widely adopted in USA, nor endorsed by NRC; however, adopted in EPA FGR-13
Which Limit to Use? RG-1.109/LADTAP for ODCM effluent release compliance, possible extension to REMP samples EPA Safe Drinking Water Standard adopted by most state programs… comes into play if offsite groundwater is involved ICRP-72 is not used (except for EPA FGR-13), despite being most modern and based on best available technology When dealing with the recent groundwater issue, one could argue we should use the most up-to-date methodology and science available. When dealing with the recent groundwater issue, one could argue we should use the most up-to-date methodology and science available.
Organically-bound tritium Existing limits (EPA, NRC) are based on tritium in the form of tritiated water High metabolism, rapid turnover/clearance from body Relatively low dose impact Organically-bound tritium, once incorporated into tissue, has much longer residence time in body More time = Higher dose impact Could result in lower limits than those based on conventional tritiated water metabolism Organically bound tritium is not of concern in drinking water, but could result in modifying concentration limits to account for higher dose/risk once in the body
Strontium-90 Exists at low levels in environment from weapons testing in 1950s through 1970s; often seen in REMP samples Has been detected at low levels in groundwater by some licensees There is a separate EPA drinking water standard for Sr-90 listed in 40CFR141 According to Federal Register, based on assumption that 8 pCi/L Sr-90 will yield 4 mrem/yr to bone marrow. Similar rationale to that used for H-3 at 20,000 pCi/L
Strontium-90 -- …error in drinking water standard? 1 MPC = 1500 mrem/yr critical organ (bone) MPC for Sr-90: 3E-7 uCi/mL (4 mrem / 1500 mrem) * 3E-7 uCi/mL = 8E-10 uCi/mL = 0.8 pCi/L, NOT 8 pCi/L 8 pCi/L * 730 L/yr = 5840 pCi/yr intake 5840 pCi/yr * 7.58E-3 mrem/pCi = 44 mrem/yr, NOT 4 mrem/yr EPA standard for Sr-90 appears to be 10X too high! Compliance with the EPA standard of 8 pCi/L would exceed the 10CFR50 Appendix I organ dose limit of 15 mrem/yr by about 3X
Summary Several limits exist for tritium in water EPA drinking water limit = 20,000 pCi/L NRC effluent limit = 1,000,000 pCi/L (1E-3 uCi/mL) Fractional limits imposed by states? Various limits are based on different assumptions, different bases EPA: 20,000 pCi/L = 4 mrem/yr, based on ICRP-2 NRC: 1,000,000 pCi/L = 50 mrem/yr, based on ICRP-30
Summary (continued) Other nuclides, forms may come into play Organically-bound tritium, Sr-90 Concern about non-conservative Sr-90 drinking water standard If one has to deal with multiple nuclides, it makes sense to use a single standard, and presumably the most modern one… ICRP-72