Dose consequence of environmental water lld values and implications to derivation of revised values
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Dose Consequence of Environmental Water LLD Values and Implications to Derivation of Revised Values. Ken Sejkora Entergy Nuclear Northeast – Pilgrim Station Presented at the 19 th Annual RETS-REMP Workshop South Bend, IN / 22-24 June 2009. REMP LLDs - General.

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Ken Sejkora Entergy Nuclear Northeast – Pilgrim Station

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Dose Consequence of Environmental Water LLD Values and Implications to Derivation of Revised Values

Ken Sejkora

Entergy Nuclear Northeast – Pilgrim Station

Presented at the 19th Annual RETS-REMP Workshop

South Bend, IN / 22-24 June 2009


REMP LLDs - General

  • Specified in Table 4.12-1 of NUREG-1302/1302

  • Derived in late 1960s to early 1970s… limited documentation of pedigree, “gray-hair” phenomenon

  • Loosely based on dose consequence and assumption of “reasonable survey”

  • Nuclide list based on major nuclides anticipated to be seen in radwaste source term

  • Values are likely outdated when compared to “modern” standards


REMP LLDs - Dose

  • Original dose factors based on ICRP-2, circa 1950s

  • Dose coefficients have undergone several revisions… ICRP-26/30, ICRP-68/72, newer. Newer factors used throughout international community, limited endorsement by EPA in Federal Guidance Report series.

  • Concept of ‘risk’ based dose consequence based on effective dose equivalent… basis of effluent control limits in 10CFR20 Appendix B, Table 2


REMP LLDs – Reasonable Survey

  • Based on “state-of-the-art” in 1960s

  • What is a “reasonable survey”?

    • Length of counting time – 1hr? Over-night?

    • Analyze as-is, or process? Separation required?

  • Underlying assumptions not documented

  • Function of sample volume, sample geometry, detector efficiency, interfering nuclides (including natural activity)

  • Significant improvements in detector efficiencies and nuclide identification algorithms since 1960s

    • Caveat – just because we can “see” tritium down to 150 pCi/L doesn’t mean we have to set the LLD at 150 pCi/L!


REMP LLDs – Critical Nuclides

  • Origin of current list unknown… GALE source term?

  • How does list compare to what has been seen in 30+ years of power reactor operation?

    • Assuming 100 reactors operating for an average of 20 years, over 2000 “reactor-years” of REMP and effluent data are available for analysis; these are REMP LLDs – emphasis should be on historical REMP sample results

  • Is current list based on activity levels anticipated, or dose impact anticipated?

  • Should some nuclides be removed, and others added?

  • Largely gamma emitters… what about hard-to-detects?


Reason For Concern - 1

  • Current list of nuclides and LLD values is nearly 40 years old, poorly documented

  • Current list may not reflect modern standards or past operating experience

  • Current list was proposed for inclusion in DG-4013, the new revision to Regulatory Guide 4.1

    • Do we want to incorporate LLD values from 40 years ago, with undocumented pedigree, into new standards?


Reason For Concern - 2

  • What rationale is to be used to derive required LLDs for nuclides and/or exposure pathways not in current list?

  • Need to have consistent approach for all licensees to apply if posed with deriving their own LLDs for a specific nuclide or pathway

  • LLDs chosen should result in a similar dose/risk consequence -- Dose-based LLDs!


Method of Approach

  • Evaluate dose impact of current LLD requirements

    • Age-specific dose coefficients; ICRP-2 and ICRP-72

    • Age-specific media usage factors

  • Derive revised LLD values based on a normalized dose impact of 1 mrem/yr

    • I am NOT suggesting or endorsing 1 mrem/yr as the limiting dose for establishing LLDs!

    • “Normalized” factor allows easy scaling to any dose target deemed “acceptable”


Dose Impact – Current Water LLDs

  • Illustrate technique. Need to repeat for other exposure pathways and nuclides

    Dose = Concentration * Usage * Dose Factor

    mrem/yr pCi/L L/yr mrem/pCi

  • Usage factors for Adult, Teen, Child, Infant

    • Are Regulatory Guide 1.109 usage values valid or current?

  • Dose coefficients for Adult, Teen, Child, Infant

    • Reg Guide 1.109 (LADTAP) values – Outdated, but provide insight to what may have been used in derivation of original values

    • Avoid ICRP-30 factors… single age group (Adult), occupational

    • ICRP-72 factors… used 4 of 6 available age classes


Dose Consequence from Current LLD Requirement Concentrations


Derived Normalized LLDs – 1 mrem/yr Dose Consequence

  • Existing LLD concentration will yield corresponding dose consequence

    • Only accounts for drinking water ingestion. Other exposure pathways not included

    • If existing dose consequence is less than 1 mrem/yr, increase LLD proportionally

    • If existing dose consequence is greater than 1 mrem/yr, decrease LLD proportionally

  • How? -- Multiply arithmetic inverse of dose consequence by existing LLD concentration value


Derived Water Concentrations for 1 mrem/yr Dose Consequence


Limitations and Concerns -1

  • Only addresses drinking water pathway

    • Data from 2007 RETS-REMP Presentation indicate including other pathways may increase dose by 2x to 600x over drinking water alone… nuclide dependent

    • However, the above effect is offset if analysis of other pathways (fish, shellfish, crops) shows buildup is not occurring and contributing to increasing total dose from all pathways

  • Approach assumes water concentration is at or above LLD 100% of the time

    • In reality, concentration is likely much less than LLD, and dose consequence is much less than 1 mrem/yr


Limitations and Concerns -2

  • Calculated EDE dose from ICRP-72 dose factors is often much higher than “Total Body” dose calculated from ICRP-2 factors

    • Potential to grossly underestimate true “total body” dose used to demonstrate compliance with dose limits… NON-CONSERVATIVE

    • ICRP-72 factors are more modern and widely accepted and used by the international community


Where to from here… - 1

  • Encourage NRC to derive new dose-based LLD targets for inclusion in revision to Regulatory Guide 4.1

    • Based on use of ICRP-72 or newer dose coefficients

    • Provide guidance and methodology for deriving LLDs for nuclides of interest not addressed in table

    • Endorse application of stochastic, EDE-based dose/risk assumptions… eliminate need for non-stochastic, organ-specific dose calculations


Where to from here… - 2

  • Encourage revision of list of critical nuclides included in LLD table

    • Primary emphasis should be on those nuclides yielding majority of dose; secondary consideration to activity

    • Consider data gleaned from review of historical REMP and effluent data from over 2000 reactor-years of operation to determine most-important nuclides; 1˚ emphasis on REMP sample data, 2˚ consideration on effluent data… EPRI? Industry working group? Graduate student research project?


Where to from here… - 3

  • Encourage establishment of dose/risk based LLDs with risk commensurate with other regulatory programs

    • The same dose/risk value used to derive MARSSIM DCGLs should be considered for establishing dose-based LLDs; already accepted and endorsed by NRC, EPA, and DOE agreement

    • MARSSIM DCGL values may prove to be a viable alternative to dose-based LLDs


Summary - 1

  • Current REMP LLD values are likely outdated, poorly documented, and may be non-conservative in some cases

  • The current list of critical nuclides may not be representative of those observed through 30+ years of commercial power reactor operations

  • Existing/current REMP LLD values are not “robust” enough for inclusion in new revisions to NRC guidance documents


Summary - 2

  • Establishment of EDE-based LLDs would provide a consistent, uniform approach and risk-basis across various nuclides and pathways

  • Current ICRP-2 dose coefficients from Regulatory Guide 1.109 and LADTAP are likely underestimating total body dose; Reg Guide 1.109 dose coefficients need to be modernized


Questions?


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