U.S. Public Health Service, detailed to U.S. Environmental Protection Agency, Las Vegas, Nevada, United States of America

Transcription

1 MULTIAGENCY RADIATION SURVEY AND SITE INVESTIGATION MANUAL (MARSSIM) OVERVIEW AND USE FOR FINAL STATUS (VERIFICATION) SURVEYS AT SEVERAL SITES IN THE UNITED STATES C.PETULLO XA U.S. Public Health Service, detailed to U.S. Environmental Protection Agency, Las Vegas, Nevada, United States of America M. DOEHNERT U.S. Environmental Protection Agency, Office of Radiation and Indoor Air, Washington, DC; United States of America R. MECK, G. POWERS U.S. Nuclear Regulatory Commission, Rockville, Maryland, United States of America K. DUVALL U.S. Department of Energy, Washington, DC; United States of America C. GOGOLAK U.S. Department of Energy, Environmental Measurements Laboratory, New York, New York; United States of America D. ALBERTH U.S. Army, Aberdeen Proving Ground, Maryland; United States of America J. COLEMAN U.S. Air Force, Boiling Air Force Base, Washington, DC; United States of America K. MARTILLA U.S. Air Force, Brooks Air Force Base, San Antonio, Texas; United States of America D. FARRAND, L. FRAGOSO U.S. Navy, Norfolk, Virginia; United States of America Abstract This paper will provide an overview of the Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM). MARSSIM was written by the four United States agencies 148

2 most involved with radioactive site cleanups. MARSSM provides a single, nationally consistent, flexible, performance based approach for scientifically planning, conducting and evaluating cleanup demonstration measurements and making subsequent decisions. MARSSIM works with any cleanup goal, dose, risk, or radioactivity concentration. In addition, this paper will report on the use of the MARSSIM methodology for Final Status (Verification) Surveys at several types of sites in the United States. MARSSIM's use at these sites produced improved survey plans and caused fewer costly measurements to demonstrate that the sites had been remediated. Overview Objectives MARSSIM was written in order to provide a consensus technical guidance document oriented toward dose or risk based regulations for; planning and determining survey objectives and data quality needs; conducting sampling and measurements; assessing if the sampling objectives were met; making decisions based on assessment results. History MARSSIM began in January, 1994, and was developed through the efforts of the Multi-Agency Radiation Survey and Site Investigation Manual Workgroup. MARSSIM received intensive review by federal agencies, states, the public and a formal multi-agency technical peer review. Scope MARSSIM has limited scope: it focuses on surface soils (to about 15 cm) and building surfaces only. Groundwater, surface water and subsurface soils are NOT addressed, to name a few. MARSSIM applies to "real property" or real estate - what you can't take with you, such as exit signs, which stay on the wall, and not pictures which you can remove and take with you. Decision Framework MARSSIM provides a decision framework that is comprised of existing approaches, alternate (new) approaches and a MARSSIM approach that is self-contained and fully described in the MARSSIM. When used in conjunction with MARSSIM, the existing and alternate (new) approaches can still benefit from using many parts of MARSSIM. Demonstrating Compliance From a technical perspective, demonstrating compliance with a cleanup standard written in the form of a release criterion as dose or risk based regulation involves: 149

3 Terminology Translating the dose/risk level via modeling into activity concentrations or areas (e.g., pci/gm, pci/cm 2 ). This area is outside the scope of MARSSIM. Measuring the activity concentration via surveys and sampling. Deciding if the area under consideration meets the cleanup standard. Before one can begin to understand MARSSIM, a few key terms need to be defined: (a) Area Classification (Class 1, Class 2, and Class 3): Implies the type of Final Status Survey an Area is projected to require. Class 1 applies to areas with the highest potential for contamination and small areas of elevated activity. Class 2 applies to areas with low potential for delivering a dose above the release criterion and little or no potential for small areas of elevated activity. Class 3 applies to areas with little or no potential for delivering a dose above the release criterion or for small areas of elevated activity. (b) (c) (d) (e) Derived Concentration Guideline Level (DCGL): A derived radionuclide specific activity concentration within a Survey Unit corresponding to the release criterion. The DCGL is derived from activity/dose relationships through various exposure pathway scenarios (models). Release Criterion: The regulatory limit expressed in terms of dose or risk. Small Area of Elevated Activity: The maximum point estimate of contamination or "hot spot". Note: the term "hot spot" has been purposefully omitted from MARSSIM because the term often has different meanings based on operational or local program concerns. Survey Unit: A geographical area consisting of structures or land areas of specified size and shape at a remediated site where a separate decision will be made whether it meets or exceeds the cleanup standard. Process When survey results are used to support a decision, one needs to ensure that the data will support that decision with satisfactory confidence. Usually one will make a correct decision after evaluating the data. However, since uncertainty in the survey results is unavoidable, positive actions must be taken to manage the uncertainty in the survey results so that sound, defensible decisions may be made. These actions include proper survey planning to control known causes of uncertainty, proper application of quality (QC) procedures during implementation of the survey plan to detect and control significant sources of error, and careful analysis of uncertainty before the data are used to support decision making. These actions describe the flow of data throughout each type of survey, and are combined in the Data Life Cycle shown below. 150

4 (a) (b) (c) (d) Planning Phase: Plan for data collection using the Data Quality Objectives (DQO) process and develop a quality assurance project plan. Implementation Phase: Collect data using documented measurement techniques and associated Quality Assurance and Quality Control activities. Assessment Phase: Evaluate the collected data against the survey objectives using data verification, data validation and Data Quality Assessment. Decision-Making Phase: Make the decision keeping in mind some decisions are quantitative, while others are qualitative based on best available evidence and professional judgement. Radiation Survey and Site Investigation Process (RSSI) - Six Steps The RSSI process is a series of six steps used to collect information to demonstrate compliance in order to release a site. The six steps are: Site Identification, Historical Site Assessment (HSA), Scoping Survey, Characterization Survey, Remedial Action Support Survey and Final Status Survey (FSS). MARSSIM focuses on the Final Status (Verification) Survey - the last step in the process. The success, design and conduct of the FSS is based on the prior surveys. Each and every action and survey should have clearly defined objectives - why is the survey being performed and what questions is it trying to answer. Use of MARSSIM at Several Sites in the United States Use of MARSSIM at a site contaminated with thorium A final status survey was conducted by the Environmental Survey and Site Assessment Program of the Oak Ridge Institute for Science and Education. The objective of the final status survey was to implement MARSSIM methodology in Class 1 and Class 2 land area survey units at a site contaminated with thorium. Survey activities conducted included document reviews, surface scans and surface soil sampling and analysis. Both survey units passed the Wilcoxon Rank Sum (WRS) statistical test, but the Class 1 survey unit did not pass the Elevated Measurement Comparison (EMC). The WRS test was used to evaluate the Th-232 concentrations in the Class 1 and Class 2 survey units because the contaminant of concern (Th-232) was present in background. Additionally, while the Class 2 survey unit did pass the release criterion, it was evident that the background reference area did not sufficiently represent the Class 2 area - further emphasizing the need for proper reference area selection. Use of MARSSIM at a Light Water Reactor (LWR) One of the first field tests of MARSSIM was performed at a LWR site. The areas selected for the survey contained low-levels of residual radioactivity from site operations that would be classified as Class 1 or Class 2. To be an effective test, the level of "residual activity" was selected to be near the cleanup level, since an area with either no residual activity, or a heavily contaminated area would be easy to identify using almost any reasonable procedure. 151

5 The field test was conducted on both interior and exterior survey units. The radionuclides of concern were 60 Co and 137 Cs. The interior Class 1 survey unit was 61 m 2 in size. A GM probe and sealer unit were used for the surface measurements. The following activity and concentration release criteria were used, based on a conservative, surface activity screening level approach: 60 Co 1260 dpm/100 cm 2 and 137 Cs 1120 dpm/100 cm 2 Since the measurements were not radionuclide specific, a reference area was found in a nearby building constructed of similar material. Background in the reference area was estimated to be about 2240 dpm/100 cm 2 with a standard deviation of 400 dpm/100 cm 2. Results: Samples Average Std. Deviation Median Reference Area Survey Unit Wilcoxon Rank Sum Test (WRS): The sum of the ranks of the adjusted reference area measurements, 2432, is greater than the critical value Thus, the null hypothesis that the Survey Unit as a whole uniformly exceeds the release criterion is rejected. Elevated Measurement Comparison (EMC) with DCGL=30: One area of 1.5 m 2 (Area Factor of 8) exceeds 240 GM counts; An area of 6 m 2 (Area Factor less than 3) averages about 230; A 24 m 2 area (Area Factor of 1.2) averages about 130. The Survey Unit Fails based on the results of the EMC. Use of MARSSIM on a Depleted Uranium Survey Unit A final status survey was also conducted on a 500 m 2 survey unit at a facility that had been contaminated with depleted U (90% 238 U, 8% 234 U 1% 235 U) and subsequently remediated. The DCGL was determined to be 1.03 pci/g for 238 U. Natural background for 238 U in the vicinity was estimated to be about 1 pci/g with a standard deviation of approximately 30%. During the DQO process it was determined that there was no radium contamination at the site. Thus the 226 Ra concentration in each sample could be used as a surrogate to determine the net residual 238 U activity in each sample. This eliminated the need for sampling a background reference area and reduced the total number of samples required for the survey. In-situ gamma spectrometry was used at 24 locations in the survey unit to determine that the survey unit met the release criterion. Conclusion As part of the process of preparing a site for unrestricted release, it must be demonstrated that residual radioactivity in facilities and environmental media has been reduced to acceptable levels. Typically, compliance with radiological criteria for unrestricted release was demonstrated by 152

6 conducting final status surveys of the site or facility and reporting the survey results to the cognizant regulatory agency for evaluation. Previous federal guidance for such surveys had limitations that become apparent as residual radioactivity approaches background levels. Among these are the prescription of fixed sampling densities and the assumption that the data are normally (or lognormally) distributed. MARSSIM provides a nonparametric statistical technique that does not require the data to have a specific distribution, can be applied to data containing non-detects, and specifies sampling densities that depend on the specific DQOs of the user. This methodology, after being successfully applied to several types of sites, generally results in final status survey plans that are flexible and accurate, while being more efficient than previous methods. 153