Radon Measurements at the Femp

Environmental radon monitoring activities at the DOE Femald Environmental Management Project (FEMP) have been conducted extensively since the early 1980s. Monitoring has been conducted at ambient concentration levels (< 1 pCVL Rn-222), inside buildings, and at significantly elevated levels (hundreds of thousands pCiL Rn222) within the K-65 silos that store concentrated radium bearing wastes. The purpose of this paper/presentation is to present and discuss some of the difficulties encountered/solutions (e.g. reliability, detection limits, affects of envirorunental factors, data transfer, etc.) that have been discovered while taking measurements using both alpha track-etch passive integrating detectors and alpha scintillation real-time detectors. A short summary and conclusion section is provided following each topic presented. INTRODUCTION The Femald Environmental Management Project (FEMP), formerly the Feed Materials Production Center (FMPC) a uranium metals processing facility, is today the focus of extensive environmental restoration activities. Owned by the U.S. Department of Energy (DOE), (lie site and surrounding areas are closely monitored for contamination and after data evaluation remedial techniques arc developed accordingly. In the late 1940s. after evaluating several locations, the government selected a 425 heckre (1,050-acre) site about 27 km (17 miles) northwest of downtown Cincinnati, Ohio, as the site for a new production facility. Ground was broken on May 1951, and the first uranium derby was produced at the site's Pilot Plant in October 1951. The major portion of construction was completed by 1954. For many years much of the uranium processed was slightly enriched up to 2% uranium-235. The majority of the uranium processed in recent years was depleted in the uranium-235 isotope less than the natural 0.71%. Two reinforced concrete storage silos were constructed in 1951 and 1952. Each silo is approximately 24.4 m (80 ft.) in diameter and 8.3m (27 ft.) high, yielding a volume of 3540 m3 (125,000 ft3 or one million gallons). The silos were constructed to provide storage for process residues from (he refining of high grade pitchblende ores from South Africa. These residues, known as K-65 residues, were received between 1952 and 1958. Presently, the combined inventory of waste material in the two K-65 silos is estimated at 5523 m3 (195,000 ft3). In 1991, a bentonite sealant was placed over the residues to reduce the amount of radon emitted into the headspace. Production was suspended in July 1989. In October 1990, the DOE transferred management responsibility for the site from its Defense Programs organization to the Office of Environmental Restoration and Waste Management. In February 1991, DOE announced its intention to formally end production and submitted a closure plan to Congress, becoming effective in June 1991. Subsequently, in August 1991, the site was renamed the Femald Environmental Management Project in accordance with the new mission. The 1993 International Radon Conference ENVIRONMENTAL RADON AIR MONITORING AT THE FEMP The FEMP has monitored radon levels routinely since the early 1980s. It is believed that the principal source of radon emissions from the FEMP is currently the K-65 silos due to their radon-emitting ore residues. Radon escapes through tiny cracks, and access ports, on top of the K-65 silos. To ensure emissions are monitored as efficiently as possible, radon concentration measurements are taken in (lie air at several locations: immediately adjacent to the silos, points at the FEMP facility fenceline, and in each silo's lieadspace. Waste pits 1, 2, 3, and 4 have been monitored and are considered minor sources of radon. Waste Pit 5 is a potential source of radon emissions when not covered with water. Waste Pit 6 is not considered a source of radon since very few radium-bearing materials are contained within it. It is DOE'S objective to conduct activities at its facilities such that radiation exposures to members of the public are maintained As Low As Reasonably Achievable (ALARA). Therefore, DOE facilities monitor all releases applicable to site activities. Since the FEMP stores radium-bearing materials onsite, radon concentrations in the atmosphere above facility surfaces or openings are regulated by DOE Order 5400.5. When added to background levels, these concentrations must not exceed the following limits: 100 pCi i at any given point An annual average concentration of 30 pCiL over the facility site, An annual average concentration of 3 pCik at or above any location outside the facility site, or Flux rates greater than 20 @/m2 per second from the storage of radon producing wastes. NESHAP subpart Q also has a flux-rate requirement, but will not be applicable until on-going remedial actions have been conducted and the final remedial action to abate the radon emission problem has taken place. These actions are enacted to comply with (he requirements of the Federal Facility Compliance AgreementFederal Facility Agreement (FFCNFFA). Therefore, all actions related to the control and abatement of radon-222 at the FEMP are performed in cooperation with USEPA. RADON MONITORING METHODOLOGIES To determine radon concentrations in the environment, (lie FEMP uses two monitoring systems for (lie alpha particles produced as radon gas decays: alpha track-etch detectors and real-time alpha scintillation detectors. The alpha track-etch detector is consists of a filtered cup surrounding a plastic chip, the molecules of which are damaged when contacted by alpha particles. The filter acts as a diffusion barrier, preventing airborne alpha particles and radon daughters from entering. Therefore, only radon atoms are free to diffuse into the cup. When alpha particles from radon and its daughters interact with the plastic, ionimtion damage produces latent tracks. These (racks can be visualized by chemical treatment or electrochemical etching. The number of etches or tracks in the material is equal to the number of alpha particles that have reached the plastic. This number can then be related to the average concentration of radon in (lie cup. Real-time alpha scintillation monitors record ambient environmental radon concentrations on an hourly basis. They detect alpha particles by using a scintillation cell. They operate in the passive mode, allowing air to diffuse into the detector through a foam barrier. Radon present in (he diffused air will decay along with its alpha-emitting daughters, and the alpha particles strike the sensitive ZnS scintillator which lines the interior of the cell. Each interaction produces light pulses, which are amplified by a photomultiplier tube and then are counted. It takes about a half-hour to achieve the same radon gas equilibrium inside the detector as is present in the surrounding air. RADON MONITORING RESULTS The following accounts describe some of the difficulties encountered in various radon monitoring activities and resolutions to the problems encountered while using alpha track-etch and real-time scintillation detectors in die sampling program. The 1993 International Radon Conference Aloha Tea ck-etch D e t e m Environmental data has been collected for several locations: at the site boundary, background locations, area residences, and at locations adjacent to the silos and in the predominant wind direction relative to them. Together with limited indoor building monitoring, a total of 65 locations are monitored. The detectors are changed quarterly and provide long-term integrated radon measurements. The typical monitoring setup involves the use of three or four cups depending upon (lie location. In the past there have been occasions where substantial differences in radon concentration values have been recorded for cups at the same location. To determine the precision and reliability of the alpha track-etch cups, an experiment using replicate measurements was designed to assess the variability of the measurements at a particular outdoor exposure level. For this study, the northeast comer of the K-65 silo fenceline was chosen to provide the highest onsite concentrations (typically > 1 pCilL), the area near the meteorological tower to represent typical onsite concentrations, and an offsite location which would be typical of background radon levels with no influence from sources at the FEMP. The measurement period was approximately 3 months (from December 1992 until mid-March 1993). Twenty cups were used at both the K-65 silo location and the meteorological tower, and 21 cups were used at the offsite location. The cups at each location were placed only inches from each other to ensure they were measuring the same ambient radon concentrations. The following equation was used to determine the sample number at each location: n = number of detectors at each measurement location 0 = 0.2 (This value of standard deviation was adopted based on the evaluation of previous routine environmental radon measurement data collected). e = 10% (This value of expected outlier frequency was also based on (lie evaluation of previous routine environmental radon measurement data collected). z = 1.645 (This value was selected from the normal curve to obtain a 95% confidence interval for the measurements). Analysis of the data yielded the following results (based on net count data): Table 1. Outlier Data LOCATION DATA BELOW THE OUTLIERS MINIMUM SENSITIVITY OF THE DETECTOR NE Comer of Silo Area 0% Met. Tower Background 20 % (upper extreme)