ML26027A167
| ML26027A167 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 02/09/2021 |
| From: | Zoller 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-21-001, Rev 0 | |
| Download: ML26027A167 (0) | |
Text
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 1 of 10 SRRS - 2B.134, DTC - RPA907 FC-21-001 Revision 0 Use of ISOCS for FSS of Basements at FCS Prepared By:
Date Reviewed By:
Date Approved By:
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 2 of 10 SRRS - 2B.134, DTC - RPA907 Table of Contents 1
ISSUE STATEMENT......................................................................................................................... 4 2
BACKGROUND................................................................................................................................. 4 3
METHODOLOGY............................................................................................................................. 4 3.1 APPLICATION OF ISOCS IN FINAL STATUS SURVEY..................................................................... 4 3.2 USE OF ISOCS GEOMETRY SOFTWARE........................................................................................ 6 3.3 ISOCS GEOMETRIES FOR FINAL STATUS SURVEYS...................................................................... 7 3.3.1 Auxiliary Building..................................................................................................................... 7 3.3.2 Intake Structure......................................................................................................................... 8 3.3.3 Turbine Building....................................................................................................................... 8 3.3.4 Other Structures to Remain....................................................................................................... 8 3.3.5 Conclusion................................................................................................................................ 8 3.4 SENSITIVITY TO NON-UNIFORM AREAL CONTAMINATION.......................................................... 8 3.5 MINIMUM DETECTABLE CS-137 CONCENTRATION OF THE CIRCULAR PLANE MODEL.............. 10 4
ASSUMPTIONS................................................................................................................................ 10 5
RESULTS.......................................................................................................................................... 10 6
CALCULATIONS............................................................................................................................ 10 7
REFERENCES.................................................................................................................................. 10 List of Tables TABLE 1 - NON-UNIFORM CONTAMINATION EFFICIENCY RESULTS........................................................................... 9 List of Figures FIGURE 1 DEGREE COLLIMATOR FIELD OF VIEW............................................................................................... 6 FIGURE 2 - ISOCS GEOMETRY COMPOSER CIRCULAR PLANE TEMPLATE............................................................... 7
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 3 of 10 SRRS - 2B.134, DTC - RPA907 Acronyms and Initialisms CPP Calculations and Position Paper DCGL Derived Concentration Guideline Level FCS Fort Calhoun Station FOV Field of View FSS Final Status Survey ISOCS In-Situ Object Counting System LACBWR LaCrosse Boiling Water Reactor MARSSIM Multi-Agency Radiation Survey and Site Investigation Manual MDC Minimum Detectable Concentration ROC Radionuclides of Concern TSD Technical Support Document
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 4 of 10 SRRS - 2B.134, DTC - RPA907 1
Issue Statement The decommissioning of the Fort Calhoun Station (FCS) has been initiated. As part of the decommissioning, structures with basements will remain onsite at an elevation from 1001 and below. These structures to remain will be subject to a Final Status Survey (FSS). The use of the Canberra In-Situ Object Counting System (ISOCS) gamma spectroscopy system has been utilized successfully in past decommissioning sites, including the LaCrosse Boiling Water Reactor (LACBWR) and the Zion Nuclear Power Station. The utilization of the ISOCS for FSS of structures to remain at FCS needs to be evaluated based upon experiences from the past uses.
2
Background
The primary purpose of this Calculations and Position Paper (CPP) is to describe the efficiency calibration methods and use of in-situ gamma spectroscopy to perform the FSS of the FCS end-state sub-structures. This CPP also provides a general description of the in-situ gamma spectroscopy system application during FSS including nominal minimum detectable concentration (MDC) values. The calculations and efficiencies presented are based upon the LACBWR Technical Support Document (TSD) and field experiences. This is due to Radionuclides of Concern (ROC) for gamma emitters and structure material (concrete) of the structures to remain being identical between the two sites.
The objective of the FSS is to demonstrate that the residual radioactivity in each end state sub-structure at the time of license termination is below the Derived Concentration Guideline Levels (DCGL) for the ROC using a Sum of Fractions approach. The FSS will be performed with the ISOCS gamma spectroscopy system.
The efficiency calibrations presented in this CPP are based on the initial characterization data and are expected to be applicable to the conditions at the time of the FSS. However, further characterization will be performed to validate and adjust, if necessary, the geometries. Final Status Survey measurements with the ISOCS can proceed before continuing characterization is completed using efficiency curves based on the initial characterization data. The release record will include a discussion of the basis for the efficiency curve selected and a review of applicable continuing characterization data completed after the ISOCS FSS measurements were performed.
The efficiency curve used for the final analysis of the ISOCS FSS data will be based upon the analysis of all the characterization data.
The efficiency calibrations are intended to be conservative. The efficiency calibrations assume a uniform areal distribution of residual radioactivity. The effect of non-uniform areal contamination is also evaluated to ensure that a conservative estimate of the ROC concentration over the ISOCS Field of View (FOV) will be calculated.
3 Methodology 3.1 Application of ISOCS in Final Status Survey The DCGLs developed for the License Termination Plan in each sub-structure is the metric that will be used to demonstrate compliance with the 25 mrem/yr dose criterion in 10 CFR 20.1402.
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 5 of 10 SRRS - 2B.134, DTC - RPA907 For survey planning purposes and the assessments provided in this CPP, the assumed configuration of the ISOCS system during FSS is a three-meter distance from the surface to be measured with a 90-degree, 50 mm lead collimator. This configuration results in a 28.3 m2 FOV.
This configuration may be changed as necessary depending on actual conditions encountered during FSS.
Consistent with the graded approach recommended in the Multi-Agency Radiation Survey and Site Investigation Manual (Reference 1), the number of ISOCS measurements in each sub-structure is a function of the potential for residual radioactivity to be present. Class 1 structures will have ISOCS measurements performed over 100% of the surface.
The source term geometry (i.e., concentration depth profile and areal distribution of the residual radioactivity in structures) is required to generate efficiency curves (i.e., efficiency as a function of energy) for the ISOCS gamma spectroscopy measurements. The concrete cores obtained during characterization, as described in FC-020-12, Fort Calhoun Station Decommissioning Project Radiological Characterization Report (Reference 2), provide information regarding the distribution of activity with depth for each structure. The areal distribution is assumed to be uniform for the efficiency calibration. The effect of non-uniform areal contamination is evaluated separately in Section 3.4.
Demolition of structures at FCS will include the removal of the above grade structure to an elevation of 1001. The sub-structures that will remain at license termination include:
Auxiliary Building (971 and 989 foot elevations)
Containment Building Turbine Building Circulation Tunnels Intake Structure Neutralization Basin The characterization data is documented and analyzed in FC-020-012.
Characterization of the structural surfaces of basements that will remain at the end-state consisted primarily of the acquisition and radiological analysis of concrete samples from the foundation walls and basement floors of the Auxiliary, Intake, and Turbine Buildings. The locations where the core samples were taken are presented in FC-020-012.
The locations selected for the concrete sampling were biased toward locations where physical or observed radiological measurements indicated the presence of fixed and/or volumetric contamination of the concrete media. When possible, locations were determined based upon elevated observed contact dose rates or count rates. In addition, visual observations of floor and wall surfaces were used to identify potential locations of surface contamination, such as discoloration or standing water. The goal was to identify, to the extent possible, the locations that exhibited the highest potential of representing the worst case bounding radiological condition for concrete in each survey unit. This judgmental sampling approach also ensured
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 6 of 10 SRRS - 2B.134, DTC - RPA907 there was sufficient source term in the concrete samples to achieve the sensitivities required to determine the radionuclide fractions of gamma emitters.
3.2 Use of ISOCS Geometry Software The ISOCS Geometry Composer Software contains two relevant geometry templates for the FSS that allow the physical parameters of the measurement to be entered to determine the efficiency curves: the Exponential Circular Plane and Circular Plane templates. Based upon the data presented in FC-020-012, only the Circular Plane template should be needed to perform measurements in end state sub-structures.
The Circular Plane template allows the radioactivity to be distributed in any manner in up to ten layers of sources/absorbers (i.e., Items 2 through 11 in Figure 2). Item 1.1, wall thickness, is set to zero for end-state sub-structure floors and walls. Item 1.2 is the circular plane diameter, which will typically be set to 6.0 meters when the source to detector distance is set to 3.0 meters (Item 14.1).
A generic Circular Plane geometry is shown in Figure 1.
Figure 1 Degree Collimator Field of View Figure 2 shows the Circular Plane template input parameters.
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 7 of 10 SRRS - 2B.134, DTC - RPA907 Figure 2 - ISOCS Geometry Composer Circular Plane Template 3.3 ISOCS Geometries for Final Status Surveys 3.3.1 Auxiliary Building The 971 and 989 elevations of the Auxiliary Building were subjected to 47 sampling locations.
Analytical results indicate the majority of the contamination is present in the top 1-2 inches of concrete, specifically the top 0.5 inches. There were locations where Cs-137 was detectable at depths greater than the top 1-2 inches, with certain locations having detectable activity at up to 6 inches. It is anticipated that remediation will be performed in the Auxiliary Building to reach the
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 8 of 10 SRRS - 2B.134, DTC - RPA907 DCGL for release. Continuing characterization will be required to confirm depths of contamination at the time of FSS as well as the mixture of hard-to-detect radionuclides.
3.3.2 Intake Structure Six sample locations were selected in the Intake Building for characterization. No ROC were detected in the first 0.5-inch samples.
3.3.3 Turbine Building Sixteen sample locations were selected in the Turbine Building for characterization. No ROC were detected in the first 0.5-inch samples utilizing the on-site gamma spectroscopy system.
Results of one sample sent offsite for analysis indicated Cs-137 at a nominal concentration of 0.354 pCi/g.
3.3.4 Other Structures to Remain No samples were collected from the Circulation Tunnels or the Neutralization Basin during characterization. Contamination present, if any, would be limited to the top 0.5 inches, based upon the use of the structures.
3.3.5 Conclusion The entire source term in the Containment Building basement will be removed. All concrete will be removed to expose the steel liner. The only residual radioactivity that should remain in the Reactor Building basement is contaminated concrete dust remaining from the demolition process.
Therefore, a simple Circular Plane geometry with a single 1/2-inch layer of contamination could be used for most end state sub-structure Final Status Surveys. As stated previously, continuing characterization will be required to confirm depths of contamination at the time of FSS as well as the mixture of hard to detect radionuclides. Final characterization data will be needed to verify these assumptions.
3.4 Sensitivity to Non-Uniform Areal Contamination The development and use of the Circular Plane geometry discussed above assumes that the residual radioactivity is uniformly distributed over the ISOCS FOV. This section evaluates the effect of non-uniform areal contamination on the calculation of total activity using ISOCS. This section replicates the evaluations performed in the Zion TSD 14-022, Use of In-Situ Gamma Spectroscopy for Final Status Survey of End State Structures (Reference 3).
To determine the effect of non-uniform contamination, two sets of three Circular Plane geometry templates (all with a contamination depth of 1.27 cm) were developed. The first set assumed that the contaminated area was limited to a 1 m2 circular area, as opposed to the full 28.3 m2 FOV.
The location of the 1 m2 areas were defined by three offset distances from the detector centerline, which were 0 cm, 122 cm, and 244 cm. The 244 cm offset places the 1m2 area at the edge of the 28.3 m2 FOV (i.e., the furthest distance from the detector). The second set of non-uniform
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 9 of 10 SRRS - 2B.134, DTC - RPA907 distributions evaluated assumed that the contaminated area was a 0.5 m2 circular area with offset distances from the centerline of 0 cm, 130 cm, and 260 cm.
Efficiency files were generated for each configuration. The relevant excerpts from the geometry composer model report and results reports are summarized in Table 1.
Inspection of Table 1 data shows that the Cs-137 efficiencies for single 1.0 m2 areas of contamination exceed the uniform Circular Plane efficiency by a factor of 2.43 and 1.59 at offset distances of 0 cm and 122 cm, respectively. At a distance of 187 cm, the efficiencies of the uniform and 1 m2 area are equal. At offset distances greater than 187 cm, the 1 m2 area efficiencies are less than the uniform. In summary, a 1 m2 area with a center that is offset less than 187 cm would result in overestimating the total activity when using the uniform Circular Plane efficiency, and 1 m2 spots with offsets greater than 187 cm would result in an underestimate. The same general analysis applies to the 0.5 m2 area spots. The 0.5 m2 spot efficiency equals the uniform Circular Plane efficiency at the same offset distance of 187 cm.
Table 1 - Non-uniform Contamination Efficiency Results Energy (KeV)
Uniform Efficiency 1 m2 0 cm Offset 1 m2 122 cm Offset 1 m2 244 cm Offset 0.5 m2 0 cm Offset 0.5 m2 130 cm Offset 0.5 m2 260 cm Offset 661.6 2.24E-06 5.43E-06 3.57E-06 1.39E-06 5.58E-06 3.42E-06 1.20E-06 Ratio to Uniform 1.00 2.43 1.59 0.62 2.49 1.53 0.54 Area-Weighted Average 1.2 1.1 The area weighting was performed by calculating the circular areas inside and outside of the offset distance where the uniform efficiencies and the spot efficiencies are equal (i.e., 187 cm).
The ratios for offset distances less than the equal distance were averaged and applied to the inside circular area. The single ratio with an offset greater than the equal distance was applied to the circular area outside of the equal distance.
Calculating an area-weighted average 1 m2 spot efficiency from the ratio data in Table 1 shows that if several 1 m2 spots were randomly distributed in the ISOCS measurement FOV, the effective efficiency would be a factor of 1.2 times higher than the uniform Circular Plane efficiency. This would result in an overestimate of total activity if the ISOCS spectrums were analyzed with the uniform Circular Plane efficiency. The area-weighted average 0.5 m2 spot efficiency of randomly distributed 0.5 m2 spots is a factor of 1.1 times higher than the uniform Circular Plane efficiency and would also result in an overestimate of total activity if the ISOCS spectrums were analyzed with the uniform Circular Plane efficiency.
The analyses above assume that there is no contamination present in the FOV other than a single hot spot at the distances shown. Any increase in the number and areal extent of contamination beyond the single 1 m2 and 0.5 m2 areas assumed in Table 1, up to full uniform contamination over the entire 28 m2 FOV, would result in a convergence between non-uniform and uniform efficiencies.
Use of ISOCS for FSS of Basements at FCS FC-21-001 Revision 0 Page 10 of 10 SRRS - 2B.134, DTC - RPA907 The distribution and location of non-uniform elevated areas are expected to be randomly located with equal probability of being at any distance from the detector centerline. In addition, the sub-structures are expected to contain very low levels of residual radioactivity at license termination.
Therefore, based upon the evaluations presented in this section, no efficiency adjustment for potential non-uniform areal contamination is considered necessary.
3.5 Minimum Detectable Cs-137 Concentration of the Circular Plane Model Typical MDCs for Cs-137 at the LACBWR project were in the range of 2 - 4E+4 pCi/m2 for the Waste Gas Tank Vault FSS, and 1 - 2E+4 pCi/m2 for the Reactor Building FSS. These MDCs are well below the typical values for basement DCGLs. The MDCs for both Zion and LACBWR were not of a concern for FSS purposes.
4 Assumptions None 5
Results The ISOCS gamma spectroscopy hardware and software can be used to perform the FSS of the FCS end-state sub-structures.
Past use of a 600-second spectrum of concrete, assuming uniform contamination at a depth of 1.27 cm in the concrete, results in MDCs below the typical DCGLs for basement structures.
No adjustment to the ISOCS efficiency calibrations are necessary to account for isolated spots of contamination.
Additional concrete samples should be collected after end state sub-structure remediation or in end state sub-structures that have not been previously evaluated, to provide validation to the final FSS ISOCS geometries.
6 Calculations None 7
References
- 1. NUREG-1575, Multi-Agency Radiation Survey and Site Investigation Manual
- 2. FC-020-12, Fort Calhoun Station Decommissioning Project Radiological Characterization Report
- 3. Zion TSD 14-022, Use of In-Situ Gamma Spectroscopy for Final Status Survey of End State Structures