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Supporting Documents for Response to Request for Additional Information Related to Partial Site Release and Recent Site Survey Activities, TSD-21-001, Rev. 2, Ludlum 44-10 Six-Detector Array
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Technical Support Document TSD 21-001 Calibration and Discrete Radioactive Particle Detection Sensitivity and Performance Assessment for a Ludlum 44-10 Six-Detector Array Revision 2 PREPARED BY / DATE:

Robert Yetter III LT/FSS QA/Documentation Specialist REVIEWED BY / DATE:

Robert Yetter Director Radiological Site Closure APPROVED BY / DATE:

Sarah Roberts Vice President Radiological Programs sroberts Digitally signed by Robert F Yetter III DN: C=US, O=EnergySolutions, CN=Robert F Yetter III, E=rfyetteriii@energysolutions.com Reason: I am the author of this document Location: your signing location here Date: 2021-12-06 13:19:10 Foxit PhantomPDF Version: 9.7.0 Robert F Yetter III Digitally signed by Sarah Roberts DN: OU=VP Rad Programs, O=ES, CN=Sarah Roberts, E=sroberts@energysolutions.com Reason: I am the author of this document Location: your signing location here Date: 2021-12-07 12:32:56 Foxit PhantomPDF Version: 9.7.2 Sarah Roberts

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 Revision 2, Summary of Changes Modified the following three parameters from deterministic to probabilistic: detector height, velocity and background. These parameter distributions are based on actual field conditions. Based on these modifications, parameter distributions are updated. Also added a section representing a posteriori data evaluation.

Table of Contents Introduction..................................................................................................................................... 1 Methods........................................................................................................................................... 1 2.1 Detector Calibration........................................................................................................... 1 2.2 Detector Array Setup.......................................................................................................... 2 Probabilistic a priori MDA Assessment............................................................................................ 3 System Performance Assessment and results................................................................................. 8 4.1 Performance Assessment................................................................................................... 8 4.2 A Priori Probabilistic Ebar and MDA Simulations.............................................................. 12 4.3 A posteriori System Performance Evaluation................................................................... 15 4.3.1 Data Assessment for Follow-up Investigations.................................................... 15 4.3.2 Field Testing for DRP Towed-Array Scanning, Manual Scanning, and Fixed Measurements..................................................................................................... 18 4.3.2.1 Towed Array Scanning Tests................................................................... 19 4.3.2.2 Empirical Evaluation of Handheld Instruments...................................... 22 Conclusions.................................................................................................................................... 23 Appendix A. NIST Source certificates........................................................................................................ A-1 Appendix B. Detector Calibration.............................................................................................................. B-1 Appendix C. Post Survey Data Statistical Analysis..................................................................................... C-5 Appendix D. : Summary of Ebar and MDA Simulations............................................................................ D-1 Appendix E. : Investigation areas, Statistical evaluation........................................................................... E-1

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 1

An Environmental Restoration Group (ERG) Model 104G GPS Array System was calibrated by Radiation Safety & Control Services, Inc. to support a final status survey of land areas at Zion Nuclear Generating Station in Zion, Illinois. The survey will focus on the detection of discrete radioactive particles (DRPs) rather than uniformly contaminated soil.

The 104G system incorporates a Ludlum 4612 12-Channel Counter and up to 12 Ludlum 44-10 2X2 NaI detectors. For this survey, six (6) 44-10 detectors will be used on a vehicle mounted array (with 2 additional detectors as spares) with the detectors spaced at approximately 12 inches. The two radionuclides of concern (ROC) for this calibration are Cs-137 and Co-60 as these are the principal gamma emitting radionuclides identified in DRPs at the site. The 8 detectors were optimized and balanced, and efficiencies vs. source-to-detector distance were recorded for each radionuclide using NIST-traceable point sources.

This TSD also includes an evaluation of the system performance in two aspects as described below.

The first is an a priori analysis involving two parts. One is a probabilistic analysis of the minimum detectable activity (MDA) to DRPs with consideration of the random variables that impact the relative positions of potential DRPs to the detectors during scanning operations. The other is an during preparations for scanning prior to conducting the survey.

The second is an a posteriori performance assessment that includes 3 parts. The first is an assessment of the DRP detection sensitivity using the post-processing identification of potential locations for follow-up investigations. The second is a test involving the placement of particles found at the site during survey activities where these particles were placed on the ground surface and scanned using the detector array and the third is where DRPs were placed 3 inches below the surface and surveyed using hand-scanning followed by monitoring using a portable gamma spectrometer.

A description of each of the performance evaluations is detailed within this TSD.

2.1 Detector Calibration The first step of the calibration process was determining the high-voltage settings to be used for each individual detector. A high-voltage (HV) plateau was performed for each detector using Cs-137 to determine the optimal operating voltage. The Ludlum 4612 Counter software HV Plateau utility was utilized to collect the data for this process. A 2/B) analysis was performed to optimize the efficiency of each detector using the guidance provided in Knoll1 detectors for similarity in response, high voltages other than the one with the highest S2/B were 1

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 2

considered and selected when deemed appropriate. These resulting HV settings were then programmed into the Ludlum 4612 Counter using the latest version of the Ludlum 4612 Counter software (V 2.3.3).

Because the isotopes of interest are Cs-137 and Co-60, the LLD for each detector was then set using a Co-57 source to discriminate gamma/X-ray energies below those of Co-57 (122 and 136 keV). This was accomplished by setting the LLD at a point where the observed count rate for Co-57 was approximately the same as the detector background. The upper-level discriminator was set to 3300 mV (maximum allowed by the Ludlum software) in order to ensure the high energy photons from Co-60 were included in the count rate.

The next step in the calibration process was to measure the detector response as a function of distance from a source of known radioactivity. Two NIST traceable sources were used; a Cs-137 button source (SN: 14290) and a Co-60 button source (SN: 2006-63-3). The decay corrected activity of each source was 0.7695 µCi for the Cs-137 source and 0.6056 µCi for the Co-60 source. The NIST traceable source certificates are included as Appendix A.

Each of the 44-10 detectors were centered on the source and the Ludlum 4612 software was set for 1 minute count times to obtain to obtain the total number of counts during the counting interval. The distance from the source to the end of the detector casing was varied from one inch up to a maximum casing. Therefore, this distance was added to the detector to source distance to compensate for the offset. A ten-minute background count was also performed for each detector. The net count rates are then determined by subtracting the background count rate from the gross count rate for each radionuclide and detector. Lastly, the detection efficiency, E(c/d), is determined by the ratio of the net count rate (cpm) to the source 4 pi activity emission rate (dpm). This calibration data is provided in Appendix B.

2.2 Detector Array Setup Six detectors are mounted to a support bar with center of detectors spaced at 12.4 inches. The height from detector center to ground is adjustable, depending on terrain uniformity. The setup is depicted in Figure 1.

Figure 1: Detector Array Setup

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 3

The five variables listed below will affect the minimum detectable activity (MDA) of the system. These include:

Background count rate (Rb): This variable uses all of the post-processed survey measurements

(~1.8 million) modeled as a lognormal distribution as provided in Appendix C.

Scan speed (V): Slower scan speed will enable lower MDA values, however minimum speed will be determined by the support vehicle restrictions. This analysis uses a pert probabilistic distribution for this parameter with minimum, mode, and maximum values of 0.25, 0.35, and 0.45 m/s respectively, as observed by the vehicle operator during the scan survey.

Ground-to-detector distance (detector center) for a hypothetical source location (Z). This was modeled as a uniform probabilistic parameter as estimated by the vehicle operator within the range of 0.25 to 5 inches. Note that the target distance for this parameter was 4 inches and was established/measured on level ground with the vehicle and system operators seating within the vehicle to account for loading of the vehicles suspension system.

Count time (scan interval): The system will apply a scan interval, t, of 1 second. However, we recognize that the beginning of the count interval could correspond to location of a hypothetical particle at any horizontal position within the distance traveled during the 1 second interval.

Therefore, we have created a probabilistic uniform distribution to define the count times and corresponding position as two time-intervals as further described in the mathematical derivation shown below.

DRP offset distance (h): This represents the relative position of a hypothetical particle in the direction of travel where the lowest efficiency (thus highest MDA) would be a particle located on the ground midway between two detectors and the highest efficiency (lowest MDA) is a particle in direct line with a detector. This analysis treats this parameter as a probabilistic value represented by a uniform distribution from the lowest (zero) to highest distance (6.2 in).

Table 1: Scan Parameter Definitions Parameter Value Units Definition Rb Lognormal Distribution, mean = 30.052, St. Dev. =

10.261 cps Background Count Rate from the scan survey results t

1 sec Scan Interval t1 Uniform Dist (0 to t) sec Approach time interval, probabilistic. (Time from count start to detector array centerline.)

t2 1-t1 sec Depart time interval, probabilistic (Time from detector array centerline to end count end-time.)

V Probabilistic mph Velocity (units conversion)

V Probabilistic, Pert Distribution (Min = 0.25, mode =0.35, max=0.45) m/sec Velocity V

Probabilistic in/sec Velocity (units conversion)

H 12.40 in Detector Spacing

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 4

Parameter Value Units Definition h

Uniform Dist (0 to H/2) in horizontal distance to hypothetical particle, probabilistic Z

Probabilistic, Pert Distribution (Min = 1.5, mode = 4, max = 6.25) in Height from Ground to Detector Centerline X1max Depends on t1 in Max Ground Travel Distance for t1 X2max Depends on t2 in Max Ground Travel Distance for t2 P1max Depends on V, t1, h, Z in max distance to hypothetical particle at start of count (t1)

P2max Depends on V, t2, h, Z in max distance to hypothetical particle at end of count (t2)

P1min = P2min Depends on H, Z in min distance (detector closest to particle)

The calibration data provided in Appendix B is summarized in Table 2 and Table 3 for Co-60 and Cs-137 respectively.

Table 2: Co-60 Detection Efficiencies Source to Detector Center Distance (Inches)

Detector 1

Detector 2

Detector 3

Detector 5

Detector 6

Detector 7

Detector 8

Detector 9

Average 1.25 1.73E-01 1.92E-01 1.50E-01 1.46E-01 1.75E-01 2.06E-01 1.54E-01 1.86E-01 1.73E-01 2.25 6.18E-02 6.74E-02 5.66E-02 5.25E-02 5.94E-02 7.00E-02 5.44E-02 6.25E-02 6.06E-02 3.75 2.36E-02 2.59E-02 2.10E-02 2.10E-02 2.27E-02 2.33E-02 2.20E-02 2.32E-02 2.28E-02 5.25 1.35E-02 1.45E-02 1.12E-02 1.15E-02 1.17E-02 1.20E-02 1.15E-02 1.23E-02 1.23E-02 7.25 8.20E-03 8.87E-03 6.29E-03 6.01E-03 6.33E-03 6.26E-03 6.03E-03 6.44E-03 6.80E-03 11.25 4.93E-03 5.18E-03 2.56E-03 2.76E-03 2.76E-03 2.59E-03 2.77E-03 2.90E-03 3.31E-03 13.25 4.13E-03 4.38E-03 1.90E-03 2.05E-03 2.03E-03 1.91E-03 2.13E-03 2.07E-03 2.57E-03 19.25 3.24E-03 3.32E-03 1.02E-03 1.10E-03 9.96E-04 9.29E-04 1.16E-03 1.12E-03 1.61E-03 Table 3: Cs-137 Detection Efficiencies Source to Detector Center Distance (Inches)

Detector 1

Detector 2

Detector 3

Detector 5

Detector 6

Detector 7

Detector 8

Detector 9

Average

.25 8.53E-02 9.57E-02 7.65E-02 7.64E-02 8.75E-02 1.02E-01 8.55E-02 9.78E-02 8.83E-02 2.25 2.92E-02 3.31E-02 2.83E-02 2.92E-02 3.04E-02 3.43E-02 3.09E-02 3.29E-02 3.10E-02 3.75 1.09E-02 1.18E-02 1.07E-02 1.07E-02 1.15E-02 1.23E-02 1.10E-02 1.24E-02 1.14E-02 5.25 5.56E-03 5.91E-03 5.60E-03 5.75E-03 6.07E-03 6.08E-03 5.82E-03 6.36E-03 5.89E-03 7.25 2.94E-03 2.89E-03 3.01E-03 3.08E-03 3.13E-03 3.31E-03 3.44E-03 3.37E-03 3.15E-03 11.25 1.22E-03 1.12E-03 1.29E-03 1.46E-03 1.41E-03 1.45E-03 1.47E-03 1.61E-03 1.38E-03 13.25 9.08E-04 7.37E-04 9.32E-04 9.70E-04 9.50E-04 1.04E-03 1.04E-03 1.11E-03 9.60E-04 19.25 4.20E-04 1.02E-04 3.74E-04 6.21E-04 6.27E-04 5.70E-04 5.84E-04 6.71E-04 4.96E-04 A plot of the average efficiency versus source to detector centerline distance is provided in Figure 2 and Figure 3 along with their respective trendline fits to power functions with excellent correlation coefficients.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 5

Figure 2: Average Co-60 Efficiency vs Distance (in)

Figure 3: Average Cs-137 Efficiency vs Distance (in)

A summary of the curve function fitting parameters using the equation of the form shown in Equation 1 total detection efficiency,

, for the total count time, t, are provided in Table 4.

Table 4: Curve Fit Parameters for the Average Detector Efficiency Isotope a

m Co-60 0.2382

-1.745 Cs-137 0.1406

-1.914 The ERG scanning system records the count rate measurements from each detector for a fixed time interval (1 second) at the vehicle speed during the survey. As such, during the measurement interval, y = 0.2382x-1.745 R² = 0.9997 0

0.05 0.1 0.15 0.2 0

5 10 15 20 25 Average Eff, Co-60 (c/d) y = 0.1406x-1.914 R² = 0.9993 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0

5 10 15 20 25 Average Cs-137 Eff, c/d Equation 1

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 6

the detection efficiency from a point source will change. To accommodate these changes during each measurement interval, the mean efficiency during the interval is needed which then can be applied to a probabilistic Monte Carlo model as discussed later. In this model, the measurement time, t (1 sec), is divided into two sub-intervals t1 and t2, representing the approach and departure time intervals for encountering a particle on the ground. For these time intervals the maximum and minimum distances Pmax and Pmin can be calculated for each of these time intervals. Therefore, the mean detection efficiency

, for the total count time, t, can be determined from the efficiency for intervals t1 and t2 which are dependent on P1max, P2max, P1min and P2min as shown in Equation 2 and further derived in Equation 3 through Equation 5.

Solving the denominator integrals is shown in Equation 4 Solving the numerator integrals is shown in Equation 5.

Finally, substituting the limits of integration is shown in Equation 6.

Equation 3 Equation 4 Equation 5 Equation 6 Equation 2

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 7

Determination of the Pimin and Pimax (for i=1 or 2) parameter values are calculated using simple triangle solutions in terms of the parameters in Table 1 and are shown in Equation 7 through Equation 9.

The system a priori minimum detectable activity (MDA) is calculated from the same construct as used within MARSSIM for scanning as shown in Equation 10.

Where: Si is the minimum detectable number of net source counts and where:

is the statistic that governs the false positive proportion (5%) and true positive i is the scan interval in seconds, Equation 7 Equation 8 Equation 9

=

Equation 10

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 8

p is the surveyor efficiency, (set to 1.0 for the scan array), and is the mean efficiency in interval We recognize that the value of the surveyor efficiency, p, applied in this analysis (1.0) deviates from the values typically used for manual scanning which relies on the surveyor pausing once an increase in the audible signal is identified. In the case of this scan array, the system does not rely on such a human intervention thereby justifying the selected value, which is consistent with the approach presented by ERG, the system manufacturer2.

Substituting the above values and converting to the variable definitions used in Table 1 yields the expression represented by Equation 11.

Since the efficiency calculation shown in Equation 6 uses probabilistic parameter distributions, the analysis requires a Monte-Carlo tool to calculate simulation distributions, particularly, the mean efficiency and MDA distributions. This is performed using ModelRiskTM, distributed by Vose Software as a commercially available ExcelTM plug-in software tool. This tool can simulate over 100 distributions within an ExcelTM workbook and has a variety of output reports and data displays. The analysis executed 10,000 simulations and provide the mean efficiency (Ebar) and MDA distributions for Co-60 and Cs-137 as summarized in Appendix D.

4.1 Performance Assessment An aluminum detector rack was fabricated and each of 6 model 44-10 2-inch diameter detectors were mounted to the rack along with the GPS antenna centered on the rack between the detectors. Following detector calibration, the Model 104G was assembled, the software was set-up, and the rack attached to a vehicle as shown Figure 4 and operated in the RSCS parking area with NIST-traceable radioactive sources placed in known locations. These sources are listed in Table 5.

2 Alecksen, T. and Whicker, R. 2016. Scan MDCs for GPS-Based Gamma Radiation Surveys. Operational Radiation Safety, Health Physics 111 (Supplement 2): S123-S132.

Equation 11

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 9

Figure 4: Photograph of the Assembly Rack with NaI Detectors

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 10 Table 5: Sources Used for Performance Test Isotope Co-60 Cs-137 Cs-137 Activity (uCi) 0.9708 0.2536 0.919 Assay Date 1-Jul-13 1-Jul-13 1-Jul-13 Decay to Date 4-Aug-21 4-Aug-21 4-Aug-21 Decayed Activity (uCi) 0.33 0.21 0.76 SN R-171C R-171F R-171H The system was driven at the slow speed possible (between approximately 0.5 and 1 mph) and three passes were made in the area while collecting GPS and detector count rate data using the Model 104G software. At the conclusion of the test, the data was transferred to the ARC-GIS software platform for post-processing which allows for user-input of any combination of filters to be applied as shown in Figure 5.

As shown in Figure 5, the sources were placed in three locations and the post-processing correctly identified the sources in the proper locations. In this case, post-processed data was simply color-scaled based on the population and range of the data and does not necessarily represent the post-processing that will apply to the site survey data. The data also depicts some elevated readings in the north and east portions of the surveyed area showing light-green regions. These are due to elevated background readings from recently applied asphalt over a portion of the area. This shows a relative uniform region that may require some level of investigation during the post measurement phase of the survey.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 11 Figure 5: Results of Post-Processed Test Data

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 12 Figure 6 shows the same set of test data superimposed over a satellite photo of the test area. This image used the smoothed data from the ERG system and was created to only illustrate the ability to overlay a data set onto a satellite image rather than to compare the output against Figure 5.

Figure 6 Test Data Superimposed on a Satellite Photograph 4.2 A Priori Probabilistic Ebar and MDA Simulations As discussed in Section 3, MDA simulations were performed for 10,000 trials while stochastically varying Rb, h, V, Z and t1 using the distributions from Table 1. The results provide distributions of the mean detection efficiencies (Ebar) and MDAs as shown in Figure 7 through Figure 10 and summarized Table 6 for the 5th, 50th, and 95th percentile of the MDAs.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 13 Figure 7: Co-60 Mean Efficiency Distribution (Ebar)

Figure 8: Co-60 MDA Distribution

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 14 Figure 9: Cs-137 Mean Efficiency Distribution (Ebar)

Figure 10: Cs-137 MDA Distribution Table 6: Summary of a priori Simulation Results Parameter Percentile Co-60 Cs-137 Ebar 5th

.00455

.00185 Ebar 50th

.0074

.0031 Ebar 95th

.0137

.00624 MDA 5th

.0329

.073 MDA 50th

.0634

.149 MDA 95th

.111

.273

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 15 Overall, the system performance can be characterized by the MDA distribution markers of the 5th and 95th percentiles. This generally shows that for Co-60 these MDAs vary from 0.03 to.11 uCi and from 0.07 to 0.27 uCi for Cs-137.

to derive potential particle sizes and hypothetical internal and external doses.

4.3 A posteriori System Performance Evaluation 4.3.1 Data Assessment for Follow-up Investigations The measurement post processing methods include a detailed, multi-step process using Arc-GIS software capable of managing very large data sets correlated with GPS positional data. As noted earlier, the total number survey measurements were nearly 1.8 million with each tagged to a GPS location. This section presents the initial assessment of the measurement data that will allow for additional estimates of the MDAs through this assessment process rather than using the derived MDA construct from Section

3. The initial data analysis within Arc-GIS showed large areas of elevated measurements that appeared to represent areas where naturally occurring radioactive material (NORM) was present and this was confirmed with portable gamma spectrometer measurements. This is depicted in Figure 11 for the central portion of the site where the color-coded scale is a z-score scale that ranges from data at or below the site mean count rate up to greater than 7 standard deviations (z-scores). Since the site data shows a wide range of values likely due to NORM variability, this initial approach for the data analysis was believed to potentially mask discrete elevated measurements that could be a result of DRPs. To overcome this potential constraint,

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 16 Figure 11: Arc-GIS Representation of Center-Site Data Showing Measurement Results Converted to z-scores (multiples of St.

Dev.) Using All Site Data The fishnet analysis is an Arc-GIS feature that allows the site to be subdivided into connected blocks with dimensioned defined by the user. For the measurements conducted here, the dimensions of each fishnet block was 20 X 20 feet with each block containing ~300 to 1200 individual measurements. The Arc-GIS software then allows individual measurements to be specifically flagged using the data distribution within each fishnet block. Figure 12 shows the fishnet grid with 3090 blocks covering the surveyed site areas.

The initial data analysis flagged all data that exceeded the mean plus 7 standard deviations within each grid. Once this data subset was identified and filtered within Arc-GIS, locations corresponding to discrete and isolated measurements were flagged for follow-up investigations. Figure 13 shows the results of applying the fishnet criteria to all site data and only showing values in excess of 4 sigma. For each flagged measurement location, the mean, maximum and standard deviation was calculated for all data withing a 20 foot diameter circle was calculated. Appendix E provides the summary statistics for each of the 44 initial investigation areas and includes the net count rate above each areas average count rate.

From this, a summary of the analysis is shown in Table 7. This shows the count rates associated with the minimum, maximum, 50th percentiles of all 44 locations and the corresponding activities for Cs-137 and Co-60 when applying the 50th percentile mean efficiency (Ebar) from section 4.2. This shows that this investigation (>= 7 sigma) would have identified a DRP on the ground surface of activities corresponding to inferred DRP activities of 0.262 and 0.115 uCi for Cs-137 and Co-60 respectively.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 17 Table 7: Summary of Statistics for Initial 44 Investigations Inferred Activity of Flagged Result, uCi Parameter Count Rate, cps Cs-137 Co-60 Minimum 17.7 0.149 0.066 Maximum 72.2 0.610 0.267 Average 33.1 0.280 0.123 50th 31.1 0.262 0.115 50th Ebar, c/d N/A 0.0032 0.0073 Figure 12: Site Survey Areas with Fishnet Blocks

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 18 Figure 13: Site Survey Areas with Fishnet Criteria Applied As this DRP investigation progresses, the method provided here for identifying areas for further investigations can be expanded to include lower z-scores. However, as this criterion is lowered, a substantial increase in the number of locations exceeding the criteria may be realized. Therefore, a fraction can be selected where locations are either selected randomly or based on location and site history. This additional analysis will be presented in the final report for the DRP survey.

4.3.2 Field Testing for DRP Towed-Array Scanning, Manual Scanning, and Fixed Measurements A series of tests was performed to evaluate the detection capabilities for the various instruments used in the DRP Identification and Remediation Plan. The items evaluated were discovered during the recent FSS walkover surveys in SUs 12203A/B/C, 12212 and 122213. The items consisted of 2 pieces of elevated activity rubble and 5 particles. Details of the items are included in Table 8.

Table 8: Summary of DRPs Identified During FSS ID CR Number Survey Unit Description Depth found at Elevated Activity Rubble 1 ES-ZION-CR-2021-0049 12203B concrete fragment (1 cm x 1.5 cm) 6" Particle 1 ES-ZION-CR-2021-0052 12203A DRP 1"

Particle 2 ES-ZION-CR-2021-0058 12203C DRP 1"

Particle 3 ES-ZION-CR-2021-0060 12213 DRP 4"

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 19 ID CR Number Survey Unit Description Depth found at Elevated Activity Rubble 2 ES-ZION-CR-2021-0064 12112 concrete fragment

(~2.54 cm in diameter) 6" Particle 4 ES-ZION-CR-2021-0066 12113 DRP surface Particle 5 ES-ZION-CR-2021-0069 12112 DRP 6"

The items were surveyed with a Ludlum Model 2350-1 with a 44-10 scintillation probe, a Ludlum Model 3 with 44-9 G-M probe and a Ludlum Model 19 or Thermo RO-20 dose rate meter. A Bicron Fieldspec Digital Gamma Spectroscopy Unit was used for isotopic identification. Survey data for each item is included in Table 9.

Table 9: Summary of DRP Radiological Profiles ID On Contact 2350/44-10 Gamma count Rate (CPM) count rate (CPM)

Contact Dose Rate Isotopes Identified with Fieldspec Elevated Activity Rubble 1

28,000 15000 70 uR/hr Cs-137, Ag-110m Particle 1 35,000 28000 50 uR/hr Co-60, Zn-65 Particle 2 21,000 4200 30 uR/hr Co-60 Particle 3 53,000 7000 36 uR/hr Co-60, Fe-59 and Mn-54 Elevated Activity Rubble 2

550,000 17000 1.0 mr/hr OW 0.6 mr/hr CW Eu-152, Co-60 Particle 4 13,000 6000 14 uR/hr Co-60, Zn-65 Particle 5 44,000 24000 60 uR/hr Co-60 4.3.2.1 Towed Array Scanning Tests On October 6, 2021, an evaluation was performed of the ability of the Gamma Survey Array to detect actual DRPs and elevated activity rubble debris. Detector 5 (SN 391744) was selected for the static evaluation as it was the detector with the lowest calibration efficiency. The Gamma Survey Array was positioned, and a static background count rate recorded for detector 5. The target items were placed directly onto the face of detector 5. The count rate was allowed to stabilize, and the result recorded.

Next, the DRPs were placed on the ground at the mid-point between detectors 5 and 6. The count rates were allowed to stabilize and the results for detectors 5 and 6 recorded. The results of the evaluation for each target item are presented in the table below along with the inferred activities of the particles containing Co-60 using the contact (1.25 in.) detection efficiency for detector 5.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 20 Table 10: Evaluation of Site-Identified DRP Characteristics ID Description On Contact w/ Det. 5 (CPM)

Inferred

Activity, uCi On ground @ mid-point of Det. 5 & Det. 6 BKG Static Background 2692 N/A N/A N/A Elevated Activity Rubble 1 ES-ZION-CR-2021-0049 18782 N/A N/A N/A Elevated Activity Rubble 2 ES-ZION-CR-2021-0064 370361 N/A N/A N/A Particle 1 ES-ZION-CR-2021-0052 30121 0.085 Det. 5 = 3849 CPM Det. 6 = 3491 CPM Particle 2 ES-ZION-CR-2021-0058 17112 0.044 Det. 5 = 3295 CPM Det. 6 = 3147 CPM Particle 3 ES-ZION-CR-2021-0060 41625 0.120 Det. 5 = 4729 CPM Det. 6 = 3888 CPM Particle 4 ES-ZION-CR-2021-0066 10469 0.024 Det. 5 = 3241 CPM Det. 6 = 2789 CPM Particle 5 ES-ZION-CR-2021-0069 37178 0.106 Det. 5 = 4612 CPM Det. 6 = 3980 CPM Elevated Activity Rubble 2, Particle 3 and Particle 5 were selected for the next phase of the evaluation.

The selected target items were then taken into a low background area in SU 10203C. The items were then placed one at a time onto the ground and the array was driven over the target item. The data was saved to an individual file for each target item. The Arc-GIS data file identification numbers for each target item are provided in Table 12.

Table 11: Arc-GIS Data File Numbers for Array Test ID Arc-GIS File #

Elevated Activity Rubble 2 100621-030707-2 Particle 3 100621-031355-2 Particle 5 100621-032717-2 Figure 14 and Figure 15 show the results of the data interpretation using Arc-GIS for two of the three DRPs. These show that the system clearly identifies the presence of these particles during a scan array survey with these DRPs at the ground surface. The third particle (#5) was not identified. The likely cause of the missed detection was the position of the particle relative to the start of the 1 second acquisition,

t. For example, if t2 from Table 1 were at or near zero, the detection efficiency would be at a low value potentially causing a non-detection. This likely cause is also supported since the detection of particle #3 was clear despite the close inferred activity between #3 and #5. Also, the detection of particle #3 with an inferred activity of 0.12 uCi is close to the theoretical predictions of both the probabilistic model and the post survey data assessment demonstration.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 21 Figure 14: Image of Scan Results for the 500k cpm DRP Figure 15: Image of Scan Results of 53kcpm DRP Particle Location Particle Location

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 22 4.3.2.2 Empirical Evaluation of Handheld Instruments On November 13, 2021, a series of tests were performed using the handheld instrumentation used to perform the surveys in the DRP Identification and Remediation Plan. The items selected for this evaluation were:

1. Elevated activity rubble 1,

2. Particle 2,

3. Particle 3 and,

4. Elevated activity rubble 2.

The four targets were taken to SU 10204C. An average background was performed at the assessment site in in a low background area within the survey unit. Each target item was evaluated separately. The deep. A scan survey over the buried item was performed using a Ludlum 2350-1 with 44-10 probe and the results documented. A gamma spectroscopy assessment using a Canberra Inspector 1000 was performed and documented. The item was then placed on the soil surface and a gamma spectroscopy assessment re-performed and documented. The results are presented in Table 12.

Table 12: Summary of Field Measurements of DRPs ID On Contact 2350/44-10 Gamma Count rate (CPM)

Fieldspec Isotopes Identified Average

Background

(CPM)

Buried scan result 2350/44

-10 (CPM)

Inspector3

" Depth Isotopes Identified Inspector Surface Isotopes Identified Elevated Activity Rubble 1 28,000 Cs-137, Ag-110m 3849 6880 K-40, Cs-137 K-40, Cs-137 Particle 2 21,000 Co-60 3849 6050 K-40, Co-60 K-40, Co-60 Particle 3 53,000 Co-60, Fe-59 and Mn-54 3849 10600 K-40, Co-60 K-40, Co-60 Elevated Activity Rubble 2 550,000 Eu-152, Co-60 3849 78200 Eu-152, Co-60 Eu-152, Co-60 Table 11 shows that each of the methods used for manual scanning could detect all particles used in the test to a depth of 3 inches in soil. These particle-inferred activities range from approximately 0.05 uCi (Co-60) to 0.14 uCi (Cs-137) using a slow scanning measurement technique. Also, the field-gamma spectrometer identified the key applicable plant-related gamma-emitting radionuclides in all cases.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 23 The Model 104G GPS Array System was calibrated, assembled, and tested for system performance characteristics. The pre-survey tests involved placement of 3 small (point) NIST-Traceable radioactive sources in an outdoor parking lot with the array connected to a passenger vehicle and the vehicle operated several times over the sources. During the test, the system operator observed the live time display of the computer output and the data was later downloaded to a GIS system for further post processing. The three sources used in the test were successfully located through both the local monitoring as well as post-processing.

Based on the detailed detector calibrations and the detector configurations attached to the detector array mounted to the UTV, a theoretical probabilistic detection efficiency and minimum detectable activity (MDA) model was derived. This model used the following probabilistic input parameters:

Vehicle velocity, Detector height to ground surface, Detector approach and departure intervals from a hypothetical particle, Horizontal particle position, and Background count rate.

The analysis efficiency and MDA model used the ModelRisk software where specific distributions were applied to the probabilistic parameters for 10,000 simulations. From this analysis, the theoretical 50th percentile MDA ranges from 0.063 to 0.149 uCi for Co-60 and Cs-137 respectively using the MDA expression for scanning from MARSSIM and by applying the mean detection efficiency as determined using the probabilistic model.

Two additional estimates of the system performance are provided. The first applies the post processing criteria to the array site survey data that includes approximately 1.8 million measurements. Using a seven-sigma-criteria applied to a fishnet method, the sensitivity for particles is estimated to be 0.15 and 0.26 uCi for Co-60 and Cs-137 respectively. The second method involved empirical testing using particles collected from the site during prior final status survey activities. This test clearly shows that the system is able to detect DRPs of 0.12 and 0.23 uCi respectively.

A summary of the detection sensitivities for each of the methods is summarized in Table 13. This shows that the detection sensitivities compare well between the post processing method and the empirical data and that the theoretical estimates appear to under-report the MDAs by approximately a factor of 1.7.

Table 13: Summary of Array DRP Sensitivities Methodology Co-60 Sensitivity, uCi Cs-137 Sensitivity, uCi Probabilistic Model, 50th Percentile 0.063 0.15 Post-Process data Assessment, 7 sigma 0.115 0.26 Array Drive-Over 0.12 0.231

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 24 Lastly, the sensitivity of the hand-scanning used for the DRP survey are shown to be reasonably sensitive and lower than the scan array. This is principally because of closer distances and slower speeds to suspect DRPs. Also, the use of a portable gamma spectrometer is shown to be capable of identifying plant-related radionuclides within a background containing naturally occurring radionuclides.

We believe the methodologies and detection sensitivities described here are consistent with Revision 1 to NUREG-1507 (particularly the post processing discussions in Chapter 6) where a posteriori s

PP) are discussed. In our case, we have demonstrated, through empirical studies also discussed in NUREG 1507, align well with theoretical considerations of detection and process sensitivities.

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 Page A-1 APPENDIX A.

Cs-137 S/N 14290

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 Page A-2 APPENDIX A (CONTINUED)

Co-60 S/N 2006-63-2

Zion Station Restoration Project Technical Support Document TSD 21-001 Revision 2 Page B-1 APPENDIX B.

Detector 1 Ludlum 44-10 SN: 150852 HV = 900 Volts and LLD set @ 500 10 Min BKGD counts = 28,972 Rb = 2,897.2 cpm Cs-137 Source (SN 14290) = 1,707,500 dpm Co-60 Source (SN 2006-63-3) = 1,341,016 dpm LLD Check: Co-57 Count = 2,810 Source to Detector Center Distance (Inches)