ML25013A318

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NRC Decommissioning Lessons Learned Meeting Slides- Guidance and Parameter Support
ML25013A318
Person / Time
Issue date: 01/15/2025
From: Cynthia Barr
NRC/NMSS/DDUWP/RTAB
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NRC Guidance Updates Decommissioning Lessons Learned Meeting January 15, 2025 Cynthia S. Barr, Senior Risk Analyst Division of Decommissioning, Uranium Recovery and Waste Programs Office of Nuclear Material Safety and Safeguards

Guidance Updates

  • Federal guidance updates
  • MARSSIM Rev. 2 (expected to be published in CY2025)
  • NRC guidance updates
  • Interim Staff Guidance (DUWP-ISG-02) (final issued September 2024)

Federal Radiological Survey Guidance MARSSIM Rev. 2

Image Credit: Oak Ridge Associated Universities.

Changes to MARSSIM Site-specific Scanning Surveys

Changes to MARSSIM Continuously Collected Data (CCD)

Image Credit: Oak Ridge Associated Universities.

Image Credit: PNNL-SA-157412, Pacific Northwest National Laboratory.

NRC Guidance Updates

  • Following issuance of NUREG-1757, Volume 2, Rev. 2, in July 2022, NRC developed additional guidance to supplement gaps
  • DUWP-ISG-02 was published in September 2024 and addresses surveys of open surfaces in the subsurface

Interim Staff Guidance DUWP-ISG-02 Radiological Surveys and Dose Modeling of the Subsurface to Support License Termination Extends NUREG-1575 Multi-Agency Radiation Survey and Site Investigation (MARSSIM) to the subsurface--

addressing surveys of open excavations, reactor basement substructures, and materials planned for reuse among other topics.

Image Credit (top right): Eric Darois, Subsurface Workshop Presentation, ML22136A164 Image Credit (bottom right): DUWP-ISG-02, Figure 2.9 Example (Iso-Pacific S3) Soil Sorting System.

Updates to NUREG/CR-7021 and NUREG-1507

  • NUREG/CR-7021 subsurface guidance is being updated and focuses on use of improved geostatistical tools and methods to provide remedial and final status survey decision support
  • PNNLs Visual Sample Plan is also being extended to three dimensions and updated to incorporate additional tools for complex subsurface problems
  • An update to NUREG-1507 update will support future MARSSIM updates on CCD surveys
  • Guidance updates will be incorporated into NUREG-1757, Volume 2, Revision 3 and MARSSIM, Revision 3 Image Credit: DUWP-ISG-02, Figure B.1, Variogram Surface from SADA.

Updated Guidance and Lessons Learned Related to Support for Risk-Significant Parameters Official Use Only 9

Updated Guidance on Selection of Deterministic Parameters

  • NRC sponsored development of probabilistic RESRAD codes and associated parameter distributions for use in probabilistic sensitivity analyses
  • ANLs Data Collection Handbook (updated in 2015)
  • NUREG/CR-7267 Default Parameter Values and Distribution in RESRAD-ONSITE V7.2, RESRAD-BUILD V3.5, and RESRAD-OFFSITE V4.0 Computer Codes published in May 2024 (supersedes NUREG/CR-6697)
  • However, some parameter distributions can vary several orders of magnitude and represent a large ranges of sites (e.g., distribution coefficients)
  • Therefore, for especially risk-significant parameters, additional support for deterministic parameter values may be needed (i.e., use of the 25th and 75th percentile of the parameter distribution may not be reasonably conservative) 10 Image Credit: Figure 2-2B, NUREG/CR-6708, Dissolved speciation of U(VI) as a function of total U(VI) concentration in an open atmosphere equilibrated with a partial pressure of CO2 of 10-3.5 atm. Total dissolved U(VI) = 10-6 M.

Updated Guidance on Selection of Deterministic Parameters (continued)

  • DUWP-ISG-02 provides examples of how literature values for distribution coefficients or Kds can vary substantially leading to large differences in results
  • While soil type may provide a first order approximation of the parameter value, other geochemical factors can be important (e.g., pH, Eh, complexing ions, solid minerals/coatings)
  • DUWP-ISG-02 provides a new Table 3.6 listing factors important to distribution coefficients for radionuclides of interest to reactor decommissioning
  • This information is expected to assist licensees in selection and support for deterministic parameter values 11

Lessons Learned 1: Selection of Kds based on soil type 12

  • Compilations of Kd values use soil texture categories which may differ from licensee data
  • Licensee borehole or geotechnical data on soil texture may not align with Kd soil texture
  • Additionally, licensees should select soil types that are representative of flow pathways from source areas to groundwater
  • In some cases, higher Kd s are more conservative (when surface dose pathways dominate) while in other cases lower Kd s are more conservative (when the groundwater pathway dominates)

Lessons Learned 2: Benefit and difficulty of obtaining site-specific Kds 13

  • Site-specific laboratory studies are not always needed, particularly if the radionuclide has low uncertainty regarding its high mobility or low mobility (i.e., reasonably conservative values can be justified). See Table 3.6 in DUWP-ISG-02.
  • Multiple lines of evidence to support the values selected may be needed commensurate with risk-significance
  • Site-specific Kds may be beneficial considering costs of justifying selected values or costs associated with clean-up to lower DCGLs DCGLs=Derived Concentration Guidelines Levels (or clean-up levels)

Types of Methods to Obtain Additional Support

  • Look-up tables with site-specific information on soil type and geochemical conditions
  • In situ batch method (i.e., matched pore water, solids field or laboratory analysis)
  • Field modeling method (e.g., migration rate observations)
  • Koc method (empirical equations using organic carbon percent)
  • Laboratory batch test, use representative subsurface materials and geochemical conditions
  • Laboratory flow through (or column) method
  • Geochemical modeling 14

Summary of Guidance on Support for Risk-Significant Parameters

  • Licensees can use probabilistic sensitivity analysis to identify risk-significant radionuclides, pathways, and parameters
  • Default parameter distributions in RESRAD-ONSITE and -BUILD can be used to perform probabilistic sensitivity analysis
  • If a parameter is found to be risk-significant, multiple lines of evidence may be needed to support selection of deterministic parameters
  • Representative site materials/conditions should be considered in selecting or estimating values

Back-up Slides Official Use Only 16

Support for Risk-Significant Parameters

  • 25th percentile values (if lower values of the parameter value resulted in higher doses) or
  • 75th percentile values (if higher values of the parameter value resulted in higher doses)
  • Changes were made in Revision 2 such that licensees may need to provide additional support for risk-significant parameters such as distribution coefficients or Kds (i.e., 25th or 75th percentile of the RESRAD parameter distribution are not automatically approved for use) 17

Factors Important to Kd for Reactor ROCs

  • Table 3.6 provides information on typical radionuclides of concern for reactors
  • The table lists important geochemical factors influencing Kd for those radionuclides
  • Qualitative information about the importance of sorption and degree of uncertainty is provided
  • Information on available look-up tables is also provided 18
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