ML26027A172
| ML26027A172 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 10/13/2025 |
| From: | John Clements, Hay S Energy Solutions, Omaha Public Power District |
| To: | Division of Decommissioning, Uranium Recovery and Waste Programs |
| Shared Package | |
| ML26027A165 | List: |
| References | |
| EPID L-2025-LLN-0012 FC-24-006, Rev 1 | |
| Download: ML26027A172 (0) | |
Text
10/20/25
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 2 of 7 1.0 PURPOSE The primary purpose of this document is to evaluate the sensitivity of the Ludlum Model 44-20, 3.0 x 3.0 NaI detector when used to scan surfaces at the Fort Calhoun Station site locations where Co-60, Cs-137, and Eu-152 are potentially present as radioactive contamination. The scan Minimum Detectable Concentration (MDC) was determined as a function of background, scan speed, and radionuclide mixture.
2.0 TECHNICAL APPROACH The technical approach for establishing the MDC for gamma-emitting radionuclides utilizes the methodology and approach in MARSSIM Section 6.7.2.1 (NRC, 2000) and NUREG-1507 (Minimum Detectable Concentrations with Typical Radiation Survey Instruments for Various Contaminants and Field Conditions Revision 1 [NRC, 2020]) method for determining the scan MDC for gamma-emitting radionuclides. The Scan MDC is calculated using the following equation:
=
.x
.x The Scan MDC was calculated for a Ludlum Model 44-20 3 x 3 NaI detector with the endcap offset at 2, 3, and 5 inches from the surface.
2.1 Minimum Detectable Count Rate The Minimum Detectable Count Rate (MDCR) is determined based on the number of background counts during the observation interval and the acceptable decision error rates selected using the Data Quality Objectives process. The MDCR is calculated using the following equation:
=
x x (60/ )
Where:
MDCR =
minimum detectable count rate in counts per minute (cpm) d
=
index of sensitivity (Table 6-1, NUREG-1507 Rev. 1) bi
=
background counts in the observation interval i
=
observation interval (in seconds)
The index of sensitivity (d) is selected based on the willingness to accept the consequences of decision errors. The primary decision made during scanning is identifying the presence of residual radioactivity. The index of sensitivity is the difference between the probability of a false positive decision (deciding residual radioactivity is present when no residual radioactivity is present) and a true positive decision (deciding residual radioactivity is present when residual
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 3 of 7 radioactivity is present). The value of d selected for use at For Calhoun Station is 1.38, 95% true positive combined with 60% false positive.
The observation interval is determined based on the scan speed and the assumed size of the area of residual radioactivity. The area of residual radioactivity was modeled as a cylinder with a diameter of 0.5 meters, so a scan speed of 0.5 meters per second (m/s) would result in an observation interval of 1 second. A scan speed of 0.25 m/s would result in an observation interval of 2 seconds. Scan MDC values were calculated for both 1 and 2 second observation intervals.
The number of background counts during the observation interval is determined by the background count rate. Background count rates can vary significantly based on a combination of factors. Scan MDC values were calculated for a range of background count rates between 15,000 cpm and 100,000 cpm.
2.2 Surveyor Efficiency Human factors can influence surveyor performance during scan surveys. The factors that affect an individuals performance include, but are not limited to, survey technique, experience, the cost of false positive and false negative decision errors, and the a priori expectation of the likelihood that contamination will be identified. A surveyor efficiency of 0.5 was selected as an appropriate value for estimating field performance based on guidance in NUREG-1507.
If surveys are performed using gamma detection instrumentation coupled to a GPS unit and the data is subsequently post-processed using GIS software, the surveyor efficiency can be increased to 0.75.
2.3 Instrument Efficiency The instrument efficiency is defined here as the count rate to exposure rate ratio (CPMR) in units of counts per minute per microR per hour (cpm/
). NUREG-1507 Rev. 1 Table 6-5 lists 2,3
-20 3" x 3" NaI scintillation detector from exposure to a 0.662-MeV gamma from Cs-137 and a value of 1,165 from exposure to gamma rays emitted by Co-60.
The weighted CPMR for Eu-152 was calculated as described in NUREG-1507 Section 6.2.5. The exposure-rate-to-concentration ratio (ERC) for Eu-152 wa calculated using Microshield. The Ludlum 44-20 count rate verus exposure rate values for various gamma energies were taken from NUREG-1507 Table 6-3.
The weighted CPMR for each gamma energy was calculated using Equation 6.16 from NUREG-1507, Rev. 1:
=
x
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 4 of 7 The total weighted CPMR for Eu-152 is 2,550 cpm/ R/hr and is the sum of the energy-specific weighted CPMR values. The calculation of the total weighted CPMR for EU-152 is provided in Attachment 2.
2.4 Source Efficiency The source efficiency is defined as the exposure-rate-to-concentration ratio rate at some distance from a source with a well-defined geometry (i.e., areal extent and depth). The ERC values for Co-60 and Cs-137 independently, as well as for a mixture containing 97.8% Cs-137/2.03 Co-60/0.126% Eu-152, were generated from MicroShield. The mixture fraction comes from Table 5-3 of the License Termination Plan (LTP). To estimate the exposure rate of a hypothetical elevated area of contamination, MicroShield Version 8.03 was used to model a cylindrical volume of soil with a thickness of 6.0 inches, and a modeled area of 0.2 m2 (cylinder diameter of 0.5 m).
A separate series of MicroShield analyses were completed for Co-60 and Cs-137 independently, as well as for mixtures containing 97.8% Cs-137/2.03 Co-60/
0.126% Eu-152. The total soil concentration used for each analysis was set to a value of 1 pCi/g total activity concentration within the layer of soil. A soil density of 1.6 g/cm3 was used to represent the source material.
The dose point was modelled to the detector end cap. A dose point was established at 2, 3, 4, 5, and 6 inches above the source surface. Modeled exposure rates (in mR/hr, with buildup) were generated by MicroShield for the major gamma energies, which were then expressed as an ERC as µR/h per pCi/g.
2.5 Scan MDC Evaluation To calculate the Scan MDC, Equation 6.11 from NUREG-1507, Rev. 1 was used as follows.
Scan MDC =
x x
Where:
MDCR = Minimum Detectable (net) Count Rate for an ideal observer (cpm) p = Surveyor Efficiency (0.5, unitless)
CPMR = Count-rate-to-exposure-rate ratio (2,300 cpm/
for Cs-137, 1,165 cpm/
for Co-60, 2,550 cpm/ R/hr for Eu-152)
ERC = Exposure-rate-to-concentration ratio (µR/h per pCi/g)
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 5 of 7 The ERC values were generated using MicroShield, as discussed in Section 2.4.
The results of the MicroShield modeling are provided in Attachment 1.
The Scan MDC calculations for 100% Co-60, 100% Cs-137, and the LTP Table 5-3 mixture of radionuclides are provided in Table 1 for a detector background count rate of 25,000 cpm, and a scan speed of 0.25 m/s (2 sec observation interval). Scan MDCs for additional background count rates and scan speeds are provided in Attachment 2.
It should be noted that these calculations assume that a collimator is not used on the detector. The use of a collimator would alter the detector field of view and a separate calculation was performed and is provided in Attachment 5.
Table 1 Scan MDCS (pCi/g) at Various Detector Heights and Radionuclide Mixtures Detector Distance Inches (cm)
Nuclide Fraction Cs-137 97.8%
Co-60 2.03%
Eu-152 0.125%
Cs-137 100%
Co-60 100%
Surveyor Efficiency 0.50 2.0 (5.08) 2.39 2.32 1.12 3.0 (7.02) 2.89 2.80 1.35 5.0 (12.7) 4.05 3.93 1.91 Surveyor Efficiency 0.75 2.0 (5.08) 1.95 1.90 0.91 3.0 (7.02) 2.35 2.28 1.10 5.0 (12.7) 3.31 3.21 1.56 Background = 25,000 cpm Observation Interval = 2 seconds (scan rate 0.25 m/s)
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 6 of 7 Table 2 provides Action Levels in cpm for a range of background count rates. The Action Levels are based on a detector distance of 2 inches, a surveyor efficiency of0.75, and a scan speed of 0.25 m/s (2-second observation interval).
Table 2 Action Levels (cpm) for a Range of Background Count Rates
=
Background===
Count Rate Scan MDC (pCi/g)
Investigation Level (cpm)
Background
Count Rate Scan MDC (pCi/g)
Investigation Level (cpm) 15,000 1.51 16,070 50,000 2.76 51.950 20,000 1.74 21,230 60,000 3.02 62,140 25,000 1.95 26,380 70,000 3.26 72,310 30,000 2.14 31,510 80,000 3.49 82,470 40,000 2.47 41,750 100,000 3.90 102,760 Radioactivity 97.8% Cs-137, 2.03% Co-60, 0.125% Eu-152 Detector endcap 2 inches above ground Surveyor efficiency 0.75 Observation Interval 2 seconds (0.25 m/s)
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 7 of 7
3.0 REFERENCES
3.1 Minimum Detectable Concentrations with Typical Radiation Survey for Instruments for Various Contaminants and Field Conditions NUREG-1507, Rev. 1, August 2020.
3.2 Fort Calhoun Station Decommissioning Project License Termination Plan Revision 1, 2023.
4.0 ATTACHMENTS 4.1 Attachment 1: MicroShield Reports for Various Nuclides and Distances 4.2 Attachment 2: Weighted Count Rate to Exposure Rate Ratio for Eu-152 4.3 Attachment 3: Scan MDC Calculations 4.4 Attachment 4: Evaluation of Scan MDCs used in the Phase 1 Survey Unit Release Records 4.5 Attachment 5: Scan MDC Calculations Using a Collimator Summary of Changes Revision Date Change Description 00 10/18/2024 New Record 01 10/13/2025 was added (Scan MDC Calculations Using a Collimator).
Minor typographical errors were corrected in the main document.
Text was added to refer to Attachment 5 - no revisions were made to the non-collimated Scan MDC results.
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 1 of ATTACHMENT 5 SCAN MDC CALCULATIONS USING A COLLIMATOR
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 2 of OVERVIEW The purpose of this attachment is to establish the sensitivity of the Ludlum Model 44-20, 3.0 x 3.0 NaI detector with a collimator installed and for use in scan surveys of FCS land areas. This evaluation assumes the use of Ludlum Model 7032-051 collimators -
these collimators are constructed from 99% lead and 1% antimony and have a wall thickness of 0.25 inches. The same general methodology to develop the Scan MDC, as described in Section 2.0 (Technical Approach) of this TSD, is used.
The scan MDC was calculated for a collimated 3 x 3 NaI detector that is offset from 2 to 12 inches above the land area surface - calculations were repeated in 1 inch increments. The offset distance of the detector causes the field of view (FOV) to increase as the distance from the land surface increases. The eleven FOVs were evaluated with respect to a potential hot spot that would be measured by the detector.
The approach utilized the following parameters:
Radionuclide mixture and source activity (i.e., energy and yield of gamma emissions),
Density of source media and the physical size of source (i.e., areal dimensions of the hypothetical land area source),
The source to detector probe geometry, Ambient background radiation in the area to be surveyed, Scan rate (observation interval),
Index of sensitivity, Efficiency of the surveyor.
DETECTOR FIELD OF VIEW (FOV) EVALUATION The FOV for a collimated Ludlum Model 44-20 detector was empirically determined using a Cs-137 source. The source was moved in a 180-degree arc around the collimated detector. The end cap of the detector/collimator was positioned at the midpoint of the arc, as shown in Figure 1.
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 3 of Figure 1 +/- Experimental Detector Setup for Field of View Determination The detector net count rate was determined at 28 equally spaced increments. Each increment represented approximately 6.7 degrees. The results of these measurements are provided in Table 1 and graphically depicted in Figure 2.
In order to determine a reasonable FOV, the net count rates (net cpm) were evaluated for the arc locations to each side of the detector center point. The furthest arc locations exhibiting a count rate within 20% of the maximum (center point) value were considered the limits of the FOV, ensuring that a symmetrical distance was maintained on both sides. The arc degrees between these two points (126.7 and 46.7) is 80 degrees, which represents the angle ³a + b' shown in Figure 3. It is assumed that this FOV angle will remain constant over the range of detector heights to be evaluated, and the FOV diameters can be calculated for varying detector heights via the geometric (tangent) calculations shown in Figure 3. The resulting diameters are shown in Table 2 below - these were then used as the dimensions determine Scan MDCs at varying detector heights.
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 4 of Table 1 - Data from Field of View Determination Degrees Location CPM Net CPM Relative Response 0.0 1
12900 3638 0.603 6.7 2
13200 3938 0.652 13.3 3
13300 4038 0.669 20.0 4
13500 4238 0.702 26.7 5
13600 4338 0.718 33.3 6
13700 4438 0.735 40.0 7
14300 5038 0.834 46.7 8
14300 5038 0.834 53.3 9
14700 5438 0.901 60.0 10 15100 5838 0.967 66.7 11 15200 5938 0.983 73.3 12 15400 6138 1.017 80.0 13 15300 6038 1.000 86.7 14 15300 6038 1.000 93.3 15 15100 5838 0.967 100.0 16 15100 5838 0.967 106.7 17 14900 5638 0.934 113.3 18 14800 5538 0.917 120.0 19 14400 5138 0.851 126.7 20 14200 4938 0.818 133.3 21 13900 4638 0.768 140.0 22 13700 4438 0.735 146.7 23 13500 4238 0.702 153.3 24 13300 4038 0.669 160.0 25 13100 3838 0.636 166.7 26 12900 3638 0.603 173.3 27 12400 3138 0.520 180.0 28 12300 3038 0.503 Figure 2 - Field of View Detection Profile 3000 3500 4000 4500 5000 5500 6000 6500 0
1 2
3 4
5 6
7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Net Count Rate (cpm)
Source Position Plane of Detector vs. Net CPM
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 5 of Figure 3 - Field of View Calculations Table 2 - Field of View Dimensions Detector Height (in.)
FOV Radius (in.)
FOV Diameter (in.)
2 1.68 3.36 3
2.52 5.03 4
3.36 6.71 5
4.20 8.39 6
5.03 10.07 7
5.87 11.75 8
6.71 13.43 9
7.55 15.10 10 8.39 16.78 11 9.23 18.46 12 10.07 20.14
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 6 of SCAN MDC EVALUATION The Scan MDC was determined following the methodology described in Section 2.0 above. For the MDCR calculation, was set to 1.38 in accordance with Table 6-1 of NUREG-1507, Rev. 1. This value assumes that surveys will be performed to accept a true positive proportion of 0.95 and a false positive proportion of 0.60. The observation interval was determined for each of the FOV sizes using the estimated average scan speed of 0.25 m/s and the diameter of each calculated FOV. The background counts in the observation interval (bi) were determined using the background value of 25900 cpm and each calculated observation interval. This background count rate was used as it was the maximum background value observed in scoping surveys performed within FCS land areas expected to undergo collimated NaI surveys. Consistent with the non-collimated MicroShield evaluations, the total soil concentration used for each analysis was set to a value of 1 pCi/g total activity concentration within the layer of soil, and a soil density of 1.6 g/cm3 was used to represent the source material.
To complete the Scan MDC calculation, the surveyor efficiency (p) was set to 0.50 per MARSSIM recommendations. The CPMR values described in Section 2.3 above (2,300 cpm/
for Cs-137, 1,165 cpm/
for Co-60, 2,550 cpm/ R/hr for Eu-152) were used, and ERC values were generated using MicroShield, as discussed in further detail below. A table summarizing the Scan MDC input parameters is provided in Table 3 below.
Table 3 Scan MDC Input Parameters Input Name Value Background Rate (cpm) cpm 25900 cpm Average Scan Rate (m/s) m/s 0.25 m/s Index of Sensitivity (from NUREG-1507 Table 6-1) d' 1.38 Surveyor Efficiency p
0.5 Cs-137 sensitivity (from NUREG-1507 Table 6-3)
CPMR 2300 cpm/µR/h Co-60 sensitivity (from NUREG-1507 Table 6-3)
CPMR 1165 cpm/µR/h Eu-152 sensitivity (from FC-24-006, R0)
CPMR 2550 cpm/µR/h The ERC was calculated separately for 100% Co-60, 100% Cs-137, and 100% Eu-152 these results are provided in Table 4. In order to account for the anticipated mixture at FCS (97.8% Cs-137, 2.03% Co-60, 0.125% Eu-152) and the different CPMR values for each of the three radionuclides in the calculation of the Mixture Scan MDC, the 100%
ERC for each radionuclide was multiplied times the respective radionuclide s mixture fraction and CPMR, as shown in the following equation:
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 7 of Mixture Scan MDC =
x x
x Where:
i
= Radionuclide (Cs-137, Co-60, or Eu-152),
= Minimum Detectable Count Rate, p
= Surveyor Efficiency, ERCi,100%
= ERC calculated for a 100% mixture of each radionuclide, i, CPMRi
= CPMR for each radionuclide, i, and Fractioni
= Mixture Fraction for each radionuclide, i, (i.e., 97.8% Cs-137, 2.03% Co-60, 0.125% Eu-152).
A Mixture ERC was also determined as follows:
Mixture ERC =
x The 100% ERC values are shown in Table 4 below, followed by a summary of the Mixture ERC and Scan MDC results in Table 5. The associated MicroShield reports are provided at the end of this Attachment.
Table 4 - ERCs at Various Detector Heights for 100% Cs-137, Co-60, and Eu-152 Source Height (Thickness) - (in.)
Detector Height (in.)
Hot spot Diameter (in.)
(
(
/pCi/g) 100% Eu-152 ERC (
/pCi/g) 6 2
3.36 2.959E-01 1.296E+00 5.963E-01 6
3 5.03 2.454E-01 1.071E+00 4.942E-01 6
4 6.71 2.059E-01 8.963E-01 4.145E-01 6
5 8.39 1.745E-01 7.581E-01 3.510E-01 6
6 10.07 1.492E-01 6.474E-01 3.001E-01 6
7 11.75 1.286E-01 5.578E-01 2.587E-01 6
8 13.43 1.118E-01 4.847E-01 2.249E-01 6
9 15.10 9.794E-02 4.245E-01 1.970E-01 6
10 16.78 8.641E-02 3.745E-01 1.738E-01 6
11 18.46 7.663E-02 3.321E-01 1.541E-01 6
12 20.14 6.800E-02 2.945E-01 1.367E-01
Ludlum 44-20 Detector Sensitivity FC-24-006 Page 8 of Table 5 - Mixture Scan MDCs and ERCs at Various Detector Heights (97.8% Cs-137, 2.03% Co-60, 0.125% Eu-152)
Source Height (Thickness) - (in.)
Detector Height (in.)
Hot spot Diameter (in.)
Mixture ERC
(
/pCi/g)
Mixture Scan MDC (pCi/g) 6 2
3.36 3.1645E-01 5.97 6
3 5.03 2.6237E-01 5.88 6
4 6.71 2.2009E-01 6.07 6
5 8.39 1.8649E-01 6.40 6
6 10.07 1.5944E-01 6.84 6
7 11.75 1.3742E-01 7.34 6
8 13.43 1.1946E-01 7.90 6
9 15.10 1.0465E-01 8.50 6
10 16.78 9.2330E-02 9.14 6
11 18.46 8.1880E-02 9.83 6
12 20.14 7.2655E-02 10.61
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