Acknowledgment and Disclaimer



Decontamination and Decommissioning
for the U.S. Nuclear Regulatory Commission



Project Description and Significance

The U.S. Nuclear Regulatory Commission (NRC) has the responsibility to assure safe and timely decommissioning of the nuclear facilities used in conjunction with NRC-licensed activities. This responsibility is partially fulfilled by providing guidance to licensees on how to plan for and prepare their sites for decontamination and decommissioning. In 1986, the NRC began a project to evaluate technical subjects related to residual radioactivity from decommissioning and license termination. This work includes radiation survey methods, economic costs for continued decontamination, and most important, the translation of residual radioactive contamination levels into annual doses. Since 1993, Sandia has supported NRC's decontamination and decommissioning efforts to provide a clear and consistent regulatory basis for determining the extent to which radioactive contamination must be removed or reduced in lands and structures for supporting defensible decisions to terminate a license and release the site for general use.




The NRC licenses thousands of nuclear facilities, and the number and complexity of these facilities that will require decommissioning are expected to increase. Before a site can be decommissioned and its license terminated, the licensee must assure that exposure to any residual radioactive contamination at the site will not pose an undue risk to human health and the environment. For sites that have little or no residual contamination, the decommissioning process will require no physical decontamination before the license is terminated. For other sites with more contamination, sometimes extensive and expensive physical decontamination will be required.


To address the range of sites that will require decommissioning, the NRC has adopted a generic screening methodology for assessing residual radioactive contamination. The generic methodology, developed by Pacific Northwest Laboratories, provides the technical basis, associated equations, and data tables for translating residual contamination levels to annual dose for decommissioned sites. The methodology is intended to be used as a screening tool for assessing potential doses from decommissioned sites based on a philosophy of moving from simple, prudently conservative calculations toward more realistic simulations, as necessary. This screening methodology is part of a process that includes three increasingly detailed methods for calculating screening model dose conversion factors (DCFs) for evaluating exposure to residual activity in soil and structures. Level 1 screening uses very simple assumptions and default parameters that are intended to be prudently, but not excessively, conservative. Level 2 screening allows the user to adjust certain parameters to more closely approximate conditions at the site being evaluated. All but the most contaminated sites should meet the regulatory dose limits with a Level 1 or Level 2 analysis. Level 3 modeling is very site specific, is required for sites with extensive contamination or complex facilities, and is beyond the scope of the screening methodology. For each screening level, four scenarios are analyzed, including building occupancy, building renovation, drinking water, and residential.


Sandia's Contribution

 During 1993, the NRC asked Sandia to use the screening methodology to calculate unit dose values to support the NRC's draft regulatory guide on release criteria for decommissioning. Sandia developed four separate FORTRAN computer codes for the building occupancy, building renovation, drinking water, and residential scenarios. However, these codes were developed for use by Sandia staff, and were not written with user friendliness, efficiency, or expandability in mind. In response to the NRC's need for a user-friendly software product for licensees, Sandia has developed DandD (Decontamination and Decommissioning), which incorporates the FORTRAN codes originally developed by Sandia within a graphical user interface (GUI). DandD is designed to run on Windows and has an extensive on-line user's manual that was developed using the Windows hypertext help system.


Sandia has also been conducting an analysis to define appropriate Level 1 default parameter values for the screening methodology. The default values are being defined such that the Level 2 analysis using site-specific data will produce doses that have a specified likelihood of being lower than the Level 1 doses. Sandia has developed and implemented a probabilistic approach for defining the appropriate default parameter values for a Level 1 analysis. SNL has also performed analyses with the DandD software to support the NRC's draft generic environmental impact statement (GEIS) for decommissioning and developed guidance for licensees in selecting defensible parameter values for Level 2 analyses.


Decommissioning of sites that are beyond the scope of the screening methodology falls under the purview of the NRC Site Decommissioning Management Plan (SDMP). Soils and structures at some of these sites contain more than routine levels and volumes of radiological contamination, and license termination and release of these sites for unrestricted use is more difficult to achieve. All of these sites require some degree of remedial action and a few of these licensed sites contain quantities of materials contaminated with low-level radioactivity to the extent that onsite stabilization and disposal, with some type of durable institutional control, may be the best way to ensure long-term protection of public health and the environment. These sites generally have buildings, former waste disposal areas, large piles of tailings, soil, and possibly ground water contaminated with low levels of uranium and thorium, or other radionuclides, and chemicals. Because remedial actions at SDMP sites involve an array of interrelated activities and assessments, an approach for guiding and supporting decommissioning at these sites is being investigated.


Sandia is supporting the development of a methodology for decommissioning SDMP sites. In previous work for the NRC, Sandia developed a performance assessment methodology (PAM) to evaluate postclosure performance of near-surface, low-level waste (LLW) disposal facilities (see "Risk Assessment Methodology for Disposal of Low-Level Radioactive Waste"). As part of its support for NRC's decontamination and decommissioning work, Sandia is evaluating the LLW PAM, including important exposure pathways, models, and codes, for application to SDMP sites.


Future Work

SNL will continue methodology and software development to implement enhancements that have been proposed for the DandD software. These include the following:
Future work for SDMP sites includes extending the LLW PAM for application to SDMP sites and developing a decision framework that integrates site characterization, dose assessments, and remedial actions and provides the technical bases for evaluating, defending, and treating the uncertainty of the data, assumptions, models, and codes used for assessment of SDMP sites.


For further information, contact:

Paul A. Davis
Sandia National Laboratories, MS-1345
Albuquerque, NM 87185-1345
Phone: (505) 848-0754
e-mail: padavis@sandia.gov

Submitted October 1996
Layout design by Wanda Mar.