ML25345A131
| ML25345A131 | |
| Person / Time | |
|---|---|
| Site: | Oyster Creek |
| Issue date: | 12/11/2025 |
| From: | Holtec Decommissioning International, Radiation Safety & Control Services |
| To: | Office of Nuclear Material Safety and Safeguards, Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25345A092 | List:
|
| References | |
| HDI 25-039 24-060, Rev 1 | |
| Download: ML25345A131 (0) | |
Text
RSCS Technical Support Document No.24-060 Rev 1 In Situ Object Counting System (ISOCS) as Applied to Final Status Survey at Oyster Creek Nuclear Generating Station (OCNGS)
Prepared by: ______________________________________________
Martin Erickson, Sr. Radiological Engineer Reviewed by: __________________________________________________
Christopher Martel, PhD, CHP, Senior Director of Operations Approved by: _____________________________________________________
Chris Messier, Sr. Director of Engineering and Strategic Development Radiation Safety & Control Services, Inc 93 Ledge Road Seabrook, NH 03874 1-800-525-8339 www.radsafety.com April 2, 2025 RSCS Radiation Safety & Control Services
2 l P a g e Table of Contents 1
Introduction and Purpose.......................................................................................... 3 2
Equipment and Materials.......................................................................................... 3 3
Methods.................................................................................................................... 4 3.1 Defining a survey unit....................................................................................... 4 3.1.1 Class 1 Survey Units.................................................................................. 4 3.1.2 Class 2 Survey Units.................................................................................. 5 3.1.3 Class 3 Survey Units.................................................................................. 5 3.2 Concrete Surfaces............................................................................................ 5 3.3 Excavations....................................................................................................... 6 3.4 Efficiency Calibration for the ISOCS System.................................................... 6 3.5 Designing Survey Unit Scans............................................................................ 7 3.6 Documentation of Surveys................................................................................ 8 4
Considerations for Use of ISOCS for FSS Applications............................................ 9 4.1 General step-by-step procedure on using Genie to determine efficiency and MDC for radionuclides of concern.................................................................... 9 4.2 Examples of ISOCS use for Decommissioning Projects................................. 10 5
Conclusions............................................................................................................ 10 6
References............................................................................................................. 10 7 : Be5030V ISOCS Detector Characterization.................................... 12 Figures Figure 1 Mirion BEGe Broad Energy Detector................................................................ 3 Figure 2 ISOCS Orientation to a Wall, Floor or Soil Surface using the 90o Collimator.... 6 Figure 3 Geometry of a Scan.......................................................................................... 7 Figure 4 Plotted ISOCS Scan Locations......................................................................... 8 Figure 5: An Array of 2 m high ISOCS Sans for 100% Coverage................................... 8 Tables Table 1: Scan Survey Coverage Requirements.............................................................. 4 Table 2: OCNGS Class I DCGL Values for Soil from RESRAD*..................................... 5 Table 3: OCNGS Class I DCGL Values for Surfaces from RESRAD *............................ 6 Table 4: Table 4: ISOCS Soil MDAs as Compared to their DCGL Values at PG&E. 11
3 l P a g e 1 Introduction and Purpose The purpose of this report is to provide the basis for the use of the Canberra In-Situ Object Counting System (ISOCS) and Genie software [Reference 1] for surveying open surfaces such as land and walls, to demonstrate that release criteria have been met. Such surveys are often performed for site characterization as part of the Final Status Survey (FSS). The FSS is conducted in areas where contamination existed, remains, or has the potential to exist or remain. The use of the ISOCS unit in performing these surveys allows areas to be monitored for contamination in a more efficient and comprehensive manner.
Characterization surveys are designed and executed using the guidance provided in NUREG1575, Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM) [Reference 2] and NUREG-1757, Volume 2, Revision 1 [Reference 3].
2 Equipment and Materials The ISOCS BEGe detector, manufactured by Mirion Technologies, was procured for radiological measurements at the OCNGS. Other ISOCS detectors (e.g., reverse electrode coaxial detectors) may also be employed based on specific survey needs using the same methods described below.
Figure 1 Mirion BEGe Broad Energy Detector The detector system purchased includes the BEGe detector with the InSpector Multi-Channel Analyzer (MCA) unit connected to a laptop computer running the latest Genie data acquisition software. The detector may be used in an open configuration or inserted into a steel or lead shield collimator. The collimator can be either the 90o or 180o version.
Typically for soil and concrete surfaces the 90o collimator is used. The Field of View (FOV) measured is defined by the distance between the collimator/detector from the surface being measured. The detector is mounted on a stationary frame, or a wheeled cart, or on a platform capable of being suspended by a crane. The method of deployment selected for the Oyster Creek site will be based on the environment in which it will be used. A battery for field operations can power all the equipment. Additionally, a wireless network may be installed, allowing the laptop running the system to be controlled remotely from another laptop, thus eliminating the need for a direct cable connection between the operator's station and the ISOCS unit.
4 l P a g e The greatest benefit of using this system for surveys in the field is the Geometry Composer [Reference 4] software which provides the user the ability to model complex counting geometries to generate custom efficiency calibrations, without the need for calibration sources. The Geometry Composer renders a model in 3-dimensional space and allows for entry of sample and geometry parameters such as size, density, and detector distance. The ISOCS software also includes the ISOCS Uncertainty Estimator which allows the user to analyze the sources of uncertainty within their geometry and use this information to properly set-up the measurements to increase accuracy of the results.
3 Methods 3.1 Defining a survey unit Surveys using the ISOCS are performed of areas defined as survey units. MARSSIM defines survey units according to their potential for elevated radioactivity, recognizing that not all areas have the same potential for contamination. Areas with a lower potential for contamination do not require the same level of survey coverage to achieve an acceptable level of confidence that the site meets the established released criteria. To operationalize this, survey units are divided into classes: Class 1, Class 2, and Class 3. The scan unit survey coverage requirements are presented in Table 1.
Table 1: Scan Survey Coverage Requirements Classification Class 1 Class 2 Class 3 Scan Coverage 100%
1 to 100%*
Judgmental (1 to 10%)
- For Class 2 Survey Units, the scan coverage will be proportional to the potential for detecting elevated activity or areas close to the release criterion, per MARSSIM Section 5.5.3. Historical information and individual measurement results collected during characterization will be used to correlate activity potential to scan coverage levels.
3.1.1 Class 1 Survey Units A Class 1 Survey Unit refers to a designated area within a contaminated site that is considered to have the highest potential for residual radioactive contamination, typically including areas with known prior spills, leaks, or disposal activities. Therefore, an area classified as a Class 1 survey unit is the highest priority area for detailed radiological investigation due to the likelihood of elevated contamination levels.
For an ISOCS unit being used for this survey, the primary assumption is that elevated radioactivity exists at the edge of the area being evaluated by a single measurement. It is critical to determine the largest size area that can be surveyed with the ISOCS that will meet the contamination level at the edge that is at or below the limit. The limit or guideline associated with this scan is defined as the Derived Concentration Guideline Level (DCGLW), where the W subscript signifies a value applied to a wide area scan. For standardization, a survey unit is typically defined as an area of one square meter.
5 l P a g e To determine the required scan Minimum Detectable Concentration (MDC), one must first determine the Elevated Measurement Concentration for a DCGL (DCGLEMC). Table 2 presents the DCGLW and the DCGLEMC values for Class I Areas for soil for the primary nuclides of concern at ONCGS, i.e., Cs-137 and Co-60.
Table 2: OCNGS Class I DCGL Values for Soil from RESRAD*
Nuclide DCGLw (pCi/g)
Area Factor (1m2)
DCGLEMC (pCi/g)
Cs-137 3.55E+01 9
3.20E+02 Co-60 7.94E+00 9
7.15E+01
- [Reference 5]
3.1.2 Class 2 Survey Units A Class 2 Survey Unit under MARSSIM refers to a designated area being surveyed for radioactive contamination where the expected level of residual radioactivity is below the derived concentration guideline level (DCGLW), meaning contamination is not anticipated to exceed acceptable limits but still requires a more thorough survey than a Class 3 unit with a lower probability of contamination. The investigation Level for Class 2 Survey units is presented in Table 2 as >DCGLw or >DCGLSCAN if DCGLSCAN is greater than DCGLW.
The detection limits are set such that DCGLW may be detected in a 1m2 area at the edge of the field of view.
3.1.3 Class 3 Survey Units A Class 3 Survey unit under MARSSIM is a designated area that has been preliminarily assessed as an area having a low probability of containing residual radioactivity, meaning it is considered to have a low risk of contamination and requires less intensive radiological surveys compared to Class 1 and Class 2 areas. The investigation level for Class 3 Survey units is provided in Table 2 as detectable greater than background. This requirement is met by investigating any scan that positively detects activity more than the site-assessed surface soil background.
3.2 Concrete Surfaces As mentioned in the introduction of this TSD, the ISOCS unit may be used to perform assays of concrete surfaces (e.g., building surfaces). To perform these surveys, the ISOCS unit is positioned perpendicular to the surface using a 90o collimator, as shown in Figure 2. Using the Geometry Composer within the ISOCS software a model is developed as appropriate for the survey plan data quality objectives (DQOs). The investigation levels for Cs-137 and Co-60 for surface surveys are included in Table 3.
6 l P a g e Table 3: OCNGS Class I DCGL Values for Surfaces from RESRAD
- Nuclide DCGLw (dpm/100cm2)
Area Factor (1m2)
DCGLEMC (dpm/100cm2)
Cs-137 4.70E+04 12 5.64E+05 Co-60 1.35E+04 11 1.49E+05
- [Reference 6]
3.3 Excavations Survey units may include excavations, which are of particular concern from a safety perspective. ISOCS may be used to perform scans in these areas and has the benefit of limiting the number of people and time in the excavation. The ISOCS modeling is developed in accordance with the DQOs of the survey plan. The detector must be positioned perpendicular to the surface being scanned using a 90o collimator. Depending on the depth and extent of the excavation, the use of an ISOCS system can present detector positioning challenges.
3.4 Efficiency Calibration for the ISOCS System The central feature of portable ISOCS technology is to support in-situ gamma spectroscopy by applying mathematically derived efficiency calibrations. Due to the nature of the environment and surfaces being evaluated (assayed), input parameters for the ISOCS efficiency calibrations will be reviewed case-by-case to ensure the applicability of the resultant efficiency. Material densities and other parameters applied to efficiency calibrations will be documented. In practice, a single efficiency calibration file may be applied to most measurements. The geometry most generally employed will be a circular plane assuming uniformly distributed activity. Efficiency calibrations will address a depth of 15cm for soil and a depth of up to 5cm for concrete surfaces to account for activity embedded in cracks, etc. Other geometries (e.g., exponential circular plane, rectangular plane, etc.) will be applied as warranted by the physical attributes of the area or surface being evaluated. Efficiency calibrations are developed by radiological engineers who have received training with respect to the ISOCS software. Efficiency calibrations will be Figure 2 ISOCS Orientation to a Wall, Floor or Soil Surface using the 90o Collimator
7 l P a g e documented following OCNGS procedures. Attenuation by standing water will need to be modeled into the assay and carefully verified so as not to understate the depth.
Experience has shown that it becomes difficult to meet detection limits with more than two inches of water in a scan assay. If there is evidence of standing water (i.e., a water sheen is visible on the soil media), then an appropriately thick layer of water attenuator will be added to the model.
3.5 Designing Survey Unit Scans Surveys to address the Class 1, 2, or 3 criteria discussed above will be designed using software specific for this purpose such as Visual Sample Plan (VSP). VSP supports the development of a defensible sampling plan based on statistical sampling theory and the statistical analysis of sample results to support confident decision-making. This software helps answer the questions of how many samples are needed, where the samples should be taken, and others. Surveys designed in VSP typically use a triangular grid pattern to identify measurement locations. Our application assumes that the 90-degree collimators are installed and that the detector face is orientated downward and lifted to the desired height, h, above the horizontal plane. The detector's field of view is described by a circle of radius r (i.e., the base of a right circular cone, r = h).
The detector response at point P with an energy E is a function of the concentration of the radionuclide (Ca), in Ci/m2, the calibration factor for the detector for energy (E), the height of the detector above the surface (h), in meters, and the radius of the area (r), in m2, and is described as:
() = ()[
(2 + 2) 2
] 1 The spacing between scan areas is determined by the scan area radius and the number of scan areas needed to cover 100% of the survey unit as shown in Figure 4. A random r
h P
Ca Figure 3 Geometry of a Scan Right Circular Cone Venex--,-.
Axis--
8 l P a g e starting point within the survey unit is selected, and the statistically determined number of desired samples (N) is used to establish a triangular grid with a random start point of appropriate grid spacing. This input may be determined by using the familiar triangular grid equation from the MARSSIM, Chapter 5.
As shown in Figures 4 and 5, once the triangular grid is constructed, it is necessary to verify that 100% of the area is covered by plotting the scan measurement locations and the field of view for each measurement in a drafting program such as AutoCAD. Additional scan assays on the periphery of the survey unit may be readily added by plotting additional measurement locations using the sample points along the triangular grid. The manually added scan assay locations are shown in red.
Figure 4 Plotted ISOCS Scan Locations Figure 5: An Array of 2 m high ISOCS Sans for 100% Coverage 3.6 Documentation of Surveys Personnel specifically trained to operate the system perform data collection activities.
Data collection activities address environmental conditions that may impact soil moisture content. Logs are maintained to provide a mechanism to annotate such conditions and ensure that efficiency calibration files address the in-situ condition(s). In extreme cases (e.g., standing water, etc.), specific conditions are addressed to ensure that analysis results reflect the conditions. As previously discussed with respect to water, when unique environmental conditions exist that may impact analysis results, conservative compensatory factors can be applied to the analysis of the data.
2m
9 l P a g e 4 Considerations for Use of ISOCS for FSS Applications The use of the in-situ techniques will be limited to characterized HPGe detectors using appropriate geometries and will be used in conjunction with the Mirion Genie software suite. All operations will be conducted in accordance with applicable site procedures.
Additionally, the following conditions must be satisfied: The geometries must be reviewed by a Subject Matter Expert (SME) to ensure they are correctly developed, and accurate or conservative approximations of the media are being measured.
Caution must be used when applying geometries for ISOCS scanning. Careful verification that the environmental conditions and geometric arrangement are appropriate to the detector geometry is critical to ensuring the accuracy of the results.
Field conditions may also significantly influence the practical applicability of the ISOCS as a field instrument. Experience has shown that the impact of attenuation from standing water may be particularly problematic in achieving the required detection sensitivity.
Consequently, it is recommended that standing water be avoided to the extent practical and sufficient counting times are planned for where it is impractical to eliminate.
4.1 General step-by-step procedure on using Genie to determine efficiency and MDC for radionuclides of concern In situ surveys for soil or floor is used as an example. The technique described is based on a circular or rectangular plane used in the ISOCS software system. For this example, we will select the rectangular plane. The general steps to perform the actual measurement is as follows.
- 1. Attach the appropriate shield and collimator to the detector.
- 2. Position the detector vertically on its stand, looking perpendicular to the floor or soil.
- 3. Perform a count
- 4. Measure and record the distance between the detector and the surface, the size of the area being assayed (if a large open area, use a large value such as 20 meters)
- 5. Obtain samples and measure the thickness, composition and density and relative concentration of each radioactivity layer.
To obtain the efficiency of the detector for specific radionuclides,
- 1. Launch the ISOCS calibration routine
- 2. Select the detector and collimator used.
- 3. Select the rectangular plane template and enter all of the data recorded.
- 4. Run the calibration using the analysis of the sample spectrum.
- 5. Obtain the efficiency and the MDC for all radionuclides of interest.
- 6. Compare the MDC to the DCGLW.
- 7. Using the geometry composer select a location at the edge of the area radius and determine the MDC for the radionuclides of concern.
- 8. Compare the MDC to the MDCEMC to ensure it is below.
10 l P a g e 4.2 Examples of ISOCS use for Decommissioning Projects The detector and method described in this document has been successfully used in other decommissioning projects. For example, the ISOCS unit was used to survey soils at the Pacific Gas & Electric (PG&E) site for decommissioning.8 For this project, a range of MDAs (pCi/gm) for Cs-137 and Co-60 in soils were determined from RESRAD scenarios. Geometry composer was also used to determine the efficiency for a 1m2 area at the edge of the field of view at 2 meters height using the 90o collimator. The geometry composer result generated an efficiency curve for gamma ray energies from 10 keV to 2.5 MeV. Sample results were used to validate the model. The MDAs calculated for a 10-minute count interval compared to the DCGLW and DCGLEMC are presented in Table 4.
Table 4: ISOCS Derived Soil MDAs for Cs-137 and Co-60 as Compared to their DCGL Values at PG&E*
The data in Table 4 shows that the MDA for the radionuclides of concern both in the field of view or at the edge of the field of view are below the DCGLW and DCGLEMC.
Therefore, the use of ISOCS was demonstrated to accurately measure the concentration of radionuclides Cs-137 and Co-60 to a depth of 15 cm over an area of radius 2 meters and, therefore, can be used for decision purposes.
5 Conclusions This report describes the methodology for using the ISOCS system to determine gamma ray energy efficiencies which are used to calculate the MDA for radionuclides. The MDAs are then compared to the DCGLW and DCGLEMC as part of decommissioning efforts. This document also presented the application of this methodology at PG&E as part of their Final Status Survey efforts where they demonstrated that ISCOCS system was capable of detecting radionuclide concentrations at levels 20 times below the DCGLs.
6 References
- 1.
Genie 2000 Operations Manual V3.4, Canberra, 2017
- 2.
NUREG-1575, MARSSIM "Multi-Agency Radiation Site Survey and Investigation Manual," Rev 1, August 2000.
- 3.
NUREG-1757, Consolidated Decommissioning Guidance Volume 2, Revision 1, July 2022
- 4.
Genie 2000 Geometry Composer Manual V4.3, Canberra 2017 Nuclide DCGLW (pCi/gm)
DCGLEMC (pCi/gm)
MDA (pCi/gm)
In FOV Edge FOV Cs-137 7.9 110 0.3 3.13 Co-60 3.8 380 0.183 1.89
[Reference 7]
11 l P a g e
- 5.
OCNGS ENG-OCS-008 Site-specific Derived Concentration Guideline Levels for Soil-Industrial Use Scenario"
- 6.
OCNGS ENG-OCS-006 Site-specific Derived Concentration Guideline Levels for Buildings/Structures
- 7.
In Situ Object Counting System (ISOCS) as Applied to Scan Requirements in Support of Final Status Survey at HBPP, September 26, 2012
12 l P a g e 7 Attachment 1: Be5030V ISOCS Detector Characterization CANBERRA ff (2 :i DETECTOR SPECIFQ ATION AND PERFORMANCE DATA Specifications Doc. No.: DPF-009 Rev: H Date: 10/5/20 I 2 Detector Model BE5030V Detector Sena! Number _____
..;:l.;.40::..:5:..:S _ ___
Preamp I Ifie r Model _____
AE_G_IS_-_,P_A ____
Preamplifier Serial Number ____
22::.:0:..:9.::2.:.2*..:c0.:.
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Cryostat Model ____
U.:,H_;_V;_*.;.4*_:R.:.DC
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CryoCooler Model _
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CryoCooler Senal Number _____
...:0:.:.7.:.13=--- ---
Rela~ve Effic,ency _ _______ %
Resolution s 2.000 keV FWHM at 1332 keV
keV FWTM @ 1.33 MeV s 1.000 kcV at I 22 keV keV FWHM @ s. 9 keV Peak/Compton ________ : I Well Diameter ________ mm Well Depth ________ mm Endcap Sile 4.00
" Dia Endcap Length _ ___
4_.3_1 _ __ " Length Cryostat Oescnpllon Enelosure 5/N: 13000257 Physical Characteristics Dtameter 80.4 mm Area 5000 mm' LengthfTJuctmess 32.2 mm Well Diameter mm Distance from Window 8.0 mm Well Depth mm Window ThIci<ness 0.80 mm Active Volume cc Window Matenal Machined Aluminum Electrical Characteristics Depleij0<1 Voltage
(*)2500 Vdc D,g,tal Shaping Times 5.6
µs (Rise Time)
Reoommended Bias Voltage
(-)3000 Vdc 1.2
µs (Flat Top)
Test Point Voltage at Recommended Bias
(+)0.80 Vdc BLR Mode Auto Reset Interval at Recommended Bias sec Cryocoote, Set Point
-173
- c Capaotance at Reoommended Bias pf Measured Performance Isotope 57Co
- co "Fe 57Co *
,o*cd
' 0'Cd
'°'Cd Ratio Energy (keV) 122 1332 5.9 6.4 22 88 22:88 FWHM (keV) 0.798 1.674 FWTM (keV) l.526 3.059 Peak/Compton/Bl<gd 65.0:1 Efficiency%
45.5
- Substit 4Jtes for Hfe In some cases where "fe peaks i re not well separated Cool Down Time __
.....;;l=..
2 _ __ Hrs Temp Sensor (Cold) --- --- V PRTDl ___
- 1_6_7_
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- c LN2 Loss Rate L/D Temp Sensor (Wanm)
V PRTD2
- 173.0
- c Tested By: _________
Den n_
is_Ba_l_lester Date: _
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Approved By: ____ _____ Joh"'-n-'Costa--'-'-- ------
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800 Reseanll Parkway, Meriden, er USI\\ 06450
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