7.0 FIELD MEASUREMENT METHODS AND INSTRUMENTATION
Data for the BOMARC site will be generated from direct measurements of soil and volumetric samples and scanning surveys of surfaces. Direct measurements are samples collected and analyzed to determine the average activity in a survey unit. Scanning is part of both measurement methods that is done to identify areas of elevated activity that may not be detected by sampling measurement methods.
7.1 Direct Measurements
A High purity Germanium (HpGe) detector will be used for field analysis of volumetric samples. Proper calibration, traceable to the National Institute of Standards and Technology, will be done in accordance with approved procedures prior to operation of the system. The calibration source shall contain 241Am. Calibration shall be done for each specific geometry used in the field. Significant changes in the operating environment, physical hardware or software will require calibration as determined by the project Certified Health Physicist. Routine source checks with a source containing 241Am will be done to ensure proper operation of the system. These checks will be done at a minimum daily, prior to system use, logged, and available for trend analysis. If significant changes in the quality of the spectrum, the operating environment, physical hardware or software occur, additional source checks will be required by the project Certified Health Physicist.
A control chart may be used to track the system operation. Prior to performing field measurements an evaluation of the detector sensitivity shall be done. The minimum detectable concentration (MDC) by gamma spectroscopy for soil samples should be no greater than 1 pCi/g, 239/240Pu. This would translate to an MDC for 241Am of approximately 0.19 pCi/g.
The area is contaminated with Weapons Grade Plutonium (WGP) which is known to have a heterogeneous distribution within soil. This heterogeneity causes a loss of precision in individual sample activities and a high variability in counting statistics. Some samples may have the activity distributed throughout the soil and others may have approximately the same amount of activity in one particle of soil. Activity distributed in samples this way presents a problem in determining the concentration of the soil being remediated.
Sample counting for soil samples that are to represent the final status survey data will be done after the soil has been dried and sieved. The best sample counting methodology will be determined with the on-site gamma spectroscopy system to minimize the influence of heterogeneity within the sample on the precision of the sample results. This will include a determination of the optimum and minimum sample mass.
For example, if it is determined that a 100 gram sample in a petri dish geometry, counted twice, gives data that has a low coefficient of variation the resultant geometric mean would be used as a data point in the determination of the Fs for that survey unit. The geometric mean is calculated with:
7.2 Scanning Surveys
Building surface scanning surveys shall be capable of detecting 300 dpm/100cm2 with a least a 95% probability. Scanning rate will be adjusted to allow the observer to achieve a good scanning efficiency. Intermittent scaler counts may be used is suspected areas above 300 dpm/100 cm2 are found to determine more accurately the fixed contamination. The scan minimum detectable concentration (MDC) will be determined from the minimum detectable count rate (MDCR) by applying conversion factors for surface efficiency, surveyor efficiency, instrument efficiency and probe area using:
Where MDCR = minimum detectable count rate
,i = instrument efficiency
,s = surface efficiency
p = surveyor efficiency, assumed to be 0.5
The scanning sensitivity of the meters used to scan soil will be approximated from physical characteristics, manufacturers data, calibration information and confirmed on site. When surface scanning of land areas is done in support of the final status survey the area factor for the survey unit will be determined from the size of the area encompassed by the sampling pattern and Table 5.6 from Reference 2.1. For example, a surface area of 300 m2 has an area factor of 4.4. This area factor will be multiplied by 8 pCi/g (DCGLw ) to get an acceptable concentration (DCGLEMC) in the area encompassed by the sampling pattern. If the scanning sensitivity is less than this number no additional data points are needed to demonstrate compliance with the elevated measurement comparison criteria.
If the result of the DCGLw times the area factor is less than the scanning sensitivity (the scanning sensitivity is greater than the DCGLEMC) the number of data points is insufficient to demonstrate compliance with the elevated measurement comparison criteria. The scanning sensitivity will be divided by the DCGLw to determine an area factor represented by the instrumentation. This area factor will be used to determine the equivalent area from Table 5.6 from Reference 2.1. A triangular grid pattern will be set up that gives that equivalent area and this will provide a new, higher number of data points that should be performed in the survey unit.
7.3 Radiological Surveys of Ground Surface Areas
Soils at the site will be radiologically surveyed using a FIDLER detector with a scaler/rate meter and the detector system tied to a global positioning system (gps). The FIDLER is capable of detecting low background levels for the low energy Am-241 x-rays. The positional and radiological data will be recorded at a frequency of once every two seconds. Four different types of gamma surveys of soils will be performed during the course of this project:
· A walkover gamma survey of the perimeter and/or portions of the site will be conducted prior to the start of remedial activities;
· Walkover surveys of each "pit" during excavation activities;
· Routine surveys of roads, parking lots, load-out areas, decon pads, and soil stockpile areas during remedial activities to ensure that radioactive materials are not being spilled or dispersed around the site as a result of the remedial activities; and
· Final status survey of the disturbed areas following remedial activities, but prior to placement of clean topsoil.
A correlation investigation will be conducted to determine the comparability of field survey scanning measurements to actual radionuclide concentrations in soil. The purpose of the investigation is to support the excavation activities with real time delineation surveying.
As part of the investigation, a review of the historical surveying and analytical results collected during the Characterization Study and other previous investigations will be conducted to determine if a correlation factor can be calculated between radiological survey results and laboratory analytical results.
7.3.1 Walkover Gamma Survey of Site Prior to Remedial Activities
Prior to the start of remediation activities, a walkover gamma survey will be performed for the perimeter of the excavation area. The initial perimeter of the excavation will be based on the results of the Characterization Report. The surveying of the perimeter will be based on a 5-meter grid system. Others areas including roads, load-out area, parking lot, trailer area, and soil stockpile area will also be surveyed. These areas will be surveyed in order to document the lack of contaminant migration during remediation. Survey results will be plotted for visual representation of the radiological conditions.
Permanent reference points will be established around the Site which are outside of the areas to be disturbed and which can serve as benchmarks for walkover radiological surveys, physical engineering surveys, or other activities requiring benchmarks. Walkover survey grids will be designed using the reference points as cornices to each land parcel. The survey will be performed by operators walking straight parallel lines over a parcel area at a constant velocity while "swinging" the detector close to the ground surface. Guide markers will be established to assure that the operators walk straight lines and achieve complete area coverage. Once a parcel survey is completed, the data will be checked for completeness and accuracy, and plotted showing contours of the gross gamma readings. If the results appear to be reasonable and complete, the survey team will move on to the next parcel until the designated area to be surveyed is completed.
7.3.2 Walkover Gamma Surveys of Excavation Pits
A radiological survey team will survey the excavated area after the removal of each 2-foot lift. The survey team will walk straight lines while swinging the gamma radiation detector as close to the ground as possible. The areas where gross gamma readings exceed the threshold value will be clearly marked with pin flags. When the grid survey is completed, the team will return to the flagged areas and refine the outline of the area containing >8 pCi/g soils. Once the perimeter of each area is better defined, spray paint, lath, or flagging will be used to delineate the area boundary. This type of radiological survey will be conducted only in areas where excavation equipment is not operating.
7.3.3 Radiation Surveys Of Roads, Parking Lots, Load-out Areas, and Soil Stockpile Areas During Remedial Activities
Access control and erosion control procedures will be implemented to mitigate the possibility of radiologically impacted material exiting the control areas. Although these procedures will be implemented and enforced, a potential exists for materials to exit the control areas due to circumstances such as spillage or flooding. To capture and reduce the potential for tracking of contamination outside the controlled areas, the roads, parking lots, and load-out areas will be surveyed regularly. Routine surveys of these areas and good "housekeeping practices" will help reduce the potential for dispersion of the radionuclides.
Roads, load-out areas, the parking lot, and clean soil storage area will be surveyed periodically. If the readings reach the threshold level of 2 times the background reading for the area being surveyed, further investigation and sampling will be conducted to determine the cause of the increase. Where appropriate, a more sensitive meter will be used with integrating counts. For this type of meter a one-minute integration may show an investigation level that is at 50% above background as an action level.
7.3.4 Final Gamma Radiation Survey of Site at the End of Remedial Activities
When remediation activities have been completed in an excavation area, a QC final gamma radiation walkover survey will be performed as a double-check to increase the confidence that the area meets the release criterion.
The QC gamma crosswalk procedures will result in a complete gamma survey of the unit which will be documented on a survey form. Crosswalk data will be documented by linking the rate meter/scaler with a GPS system. The resulting data printouts will depict a grid pattern across the unit, which will identify the path of the survey lines with the corresponding gamma readout, expressed in cpm.
Once the radiation survey indicates that it is highly probable that the area meets the release criterion, then the Final Status Survey can be performed for the area that has been surveyed. The survey equipment used for the final gamma radiation survey is described in Table 7.1.
Table 7.1
Radiological Screening Instruments
|
TASK |
FIELD INSTRUMENTS |
|
Radiation Grid Surveys(Site wide and Excavation areas) |
8-inch FIDLER detector with single channel scaler/rate meter combined with GPS system and data recording capabilities |
|
Radiation Surveys of Ancillary Areas(Roads, Parking Lots, Railway, Loadout Area, soil storage areas) |
8-inch FIDLER detector with single channel scaler/rate meter |
|
Scanning of Vehicles and Equipment for Free ReleaseScanning of Shelter Debris |
Hand-held gross alpha survey meter (for surface measurements)Hand-held gross alpha survey meter (for surface measurements)5-inch FIDLER detector with single channel scaler/rate meter |
|
Scanning shipping containers to comply with DOT regulations |
MicroR survey meter |
|
Perimeter Air Monitoring |
High volume air samplers and appropriate filters |
|
Wipe Samples (Removable Contamination) |
Protean Gas proportional Counter |
7.4 Radiological Surveys of Shelter and Debris
Radiological surveys of the shelter will conducted for total and removable alpha contamination using a protean gas flow proportional detector, and total alpha radiation using hand held meters. Because the shelter will be removed, a radiological survey of 100% of the shelter will be performed using a 1x1 meter grid system.
Any area not passing the release criteria will be decontaminated. Decontaminated areas will be re-surveyed as above to confirm compliance with the release criteria.
7.5 Radiological Surveys of Vehicles, Containers, Equipment, and Other Solid Surfaces
Equipment, intermodal containers, personnel, or vehicles that enter a radiologically controlled area must be monitored for surface contamination before release from the Site for unrestricted use. First, the item's surface will be surveyed using hand-held gross alpha instruments (Table 7.1) to determine total values of alpha radiation. Then, levels of removable surface alpha contamination will be quantified based on the collection and on-site analysis of surface wipe samples. Finally, these data will be compared against release criteria.
For loaded soil containers, each individual container will be surveyed using the equipment and procedures presented above. In general, the outside of the container will be visually inspected. Any loose dirt, mud, or other debris attached to the container will be removed. The surface of the car will then be surveyed with a hand-held beta/gamma survey instrument and a hand-held gross alpha survey instrument (Table 7.1) around all sides, top, and underneath of each car. The readings and locations where found will be recorded on the Survey form. The MicroR meter will then be held at a height equal to the vertical center of the container and a distance of one meter away. The surveyor will slowly walk around the container, keeping the instrument at a distance of one meter, and record the highest reading observed for all four sides on the Survey form.
Five surface wipe samples will be collected from each loaded container ready to leave the Site. These will be collected from surface locations that yielded the highest gross alpha and gross beta/gamma exposure readings.
Vehicles, equipment, personnel, and other items that have entered radiologically controlled areas will also be surveyed and cleared before leaving the Site. The equipment and procedures for performing a "Free Release" survey are contained in Table 7.1.
Besides using these procedures to test items leaving the Site, shipment containers coming onto the Site will also be radiologically surveyed in order to verify that they are clean before being loaded with BOMARC material. If the shipping containers are not "clean," then they will not be accepted and will be returned to the vender at the vender's expense.
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