ML26055A119
| ML26055A119 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 02/23/2026 |
| From: | Marlayna Vaaler Doell Reactor Decommissioning Branch |
| To: | Pearson B Omaha Public Power District |
| References | |
| EPID L-2025-LLN-0012 | |
| Download: ML26055A119 (0) | |
Text
From:
Marlayna Doell To:
PEARSON, BENJAMIN P Cc:
Jason Q. Spaide
Subject:
Request for Additional Information Re: Fort Calhoun - Phase 2 Final Status Survey Report Review - L-2025-LLN-0012 Date:
Monday, February 23, 2026 4:10:00 PM Attachments:
- Ben, By letter dated August 11, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession ML25224A124), as supplemented by letter dated December 4, 2025 (ML26027A205), Omaha Public Power District submitted the Final Status Survey Report for Phase 2 of the Final Status Survey Plan for the Fort Calhoun Station, Unit 1 (FCS) to the U.S. Nuclear Regulatory Commission (NRC) for review and approval associated with meeting the radiological release criteria described in the NRC-approved FCS License Termination Plan, Revision 2.
On February 14, 2025, the NRC staff sent the licensee DRAFT RAIs to ensure that the questions were understandable, the regulatory basis was clear, there is no proprietary information contained in the RAIs, and to determine if the information was previously docketed.The licensee determined that no clarification call was necessary to discuss the DRAFT RAIs. Based on a follow-up discussion with members of your staff, the NRC staff understands that the licensee intends to provide a response by March 31, 2026. The attached is the final version of the RAIs. These RAIs will be put in ADAMS as a publicly available document.
Thank you and please let me know if you have questions or need anything additional, Marlayna Marlayna Vaaler Doell
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Decommissioning Project Manager U.S. Nuclear Regulatory Commission NRR/DUWP/RDB Phone: 301.415.3178 E-mail: marlayna.doell@nrc.gov
REQUEST FOR ADDITIONAL INFORMATION PHASE 2 OF THE FINAL STATUS SURVEY REPORT OMAHA PUBLIC POWER DISTRICT FORT CALHOUN STATION, UNIT 1 DOCKET NO. 50-285 EPID: L-2025-LLN-0012 By letter dated August 11, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession ML25224A124), as supplement by letter dated December 4, 2025 (ML26027A205), Omaha Public Power District (OPPD, the licensee) submitted the Final Status Survey Report (FSSR) for Phase 2 of the Final Status Survey (FSS) Plan for the Fort Calhoun Station, Unit 1 (FCS) to the U.S. Nuclear Regulatory Commission (NRC) for review and approval associated with meeting the radiological release criteria described in the NRC-approved FCS License Termination Plan (LTP). To complete its review, the NRC staff requests the following additional information.
RAI-01-01: Background Radiation Basis:
Detailed information on the FSS measurements and sampling, including background radiation, is necessary to ensure the survey (in accordance with Section 20.1501, General, of Subpart F, Surveys and Monitoring, of Part 20, Standards for Protection Against Radiation, to Title 10 of the Code of Federal Regulations (10 CFR)) and LTP (in accordance with Paragraph (a)(9)(ii)(D) of Section 50.82, Termination of license, of 10 CFR) requirements are met. Guidance in NUREG-1757, Volume 2, Consolidated Decommissioning Guidance: Characterization, Survey, and Determination of Radiological Criteria - Final Report (ML22194A859), Section 4.4, Final Status Survey Design, discusses inclusion of background reference areas and materials to be used, along with a justification for their selection.
Issue:
Section 5.2.4, Reference Areas and Materials, of Revision 2 of the FCS LTP (ML25218A293),
hereinafter referred to as LTP Rev. 2 states that, if applied, media-specific backgrounds will be determined via measurements made in one or more reference areas and on various materials selected to represent the baseline radiological conditions for the site. For survey units with multiple materials, background data from reference areas may be subtracted from survey unit measurements (using paired observations) when the Sign test is applied.
FCSD-RA-LT-308, Radiation Surveys with Pipe Detectors, which was provided in response to the NRC request for supplemental information sent on November 21, 2025, notes that measurements may be adjusted for background radiation contribution through either the use of a reference area or by developing average background at the face of the pipe(s).
In Survey Unit (SU) 4202 the licensee used a Ludlum Model 44-159 Cesium Iodide detector to obtain static measurements of embedded piping to demonstrate compliance. The licensee states in the Survey Unit Release Record (SURR) for SU 4202 that background was subtracted from each measurement before the net gamma value (in counts per minute (cpm)) was converted to disintegrations per minute (dpm) using an efficiency factor. However, there is no specific discussion of (1) the chosen method for determining background in SU 4202, (2) a description of the reference area if applicable, and (3) the actual background results that were subtracted from each measurement shown in SURR Table 7-1, Survey Results.
Request:
Discuss the chosen method used for determining background radiation in SU 4202, describing and justifying the chosen reference areas if applicable, and include the results of this background determination.
RAI-01-02: Minimum Detectable Concentration (MDC)
Basis:
The regulations in 10 CFR 20.1501 require adequate surveys be performed to recognize and understand the sites radiological condition. Guidance in the Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM) (NUREG-1575, Revision 1) regarding survey instrument selection recommends an MDC at 10-50 percent of the Derived Concentration Guideline Limit (DCGL) for direct measurements. Additional guidance in NUREG-1757, Volume 2, Section 4.5.1, Areas of Review, in relation to the FSSR, states that the survey results for each survey unit should include measured sample concentrations in units that are comparable to the DCGLs and associated MDCs.
Issue:
LTP Rev. 2, Section 5.4.2.1, Instrument Selection, states that for direct measurements, MDCs less than 10 percent of the Operational DCGL (OpDCGL) are preferable, while up to 50 percent are acceptable. The NRC staff was unable to verify whether instrumentation in some survey units met this LTP criteria. The NRC staff is seeking further clarification regarding the following:
(I)
SU 4202 utilized a Ludlum Model 44-159 detector to obtain static measurements; however, the results did not include MDCs that can be used to demonstrate compliance.
Specifically, the results did not include the equation and parameter values used to calculate MDC.
(II)
SUs 3103, 4101, 4200, 4203, 4204, 4500, and 9012 utilized In Situ Object Counting System (ISOCS) to obtains static measurements for demonstrating compliance. ISOCS geometry files are included as attachments to the SURR for all survey units, but measurement MDCs are only included in the survey results for SU 4101 and SU 9012.
Request:
Provide MDC values directly comparable to the DCGLs for the survey units listed above, along with the equation and parameter values for SU 4202 as noted above, so that the NRC staff can verify the instrumentation used for FSS meets the applicable requirements and LTP criteria.
RAI-01-03: FSS of Basement Areas Basis:
Detailed information on the FSS measurements and sampling is necessary to ensure the survey (10 CFR 20.1501) and LTP (10 CFR 50.82(a)(9)(ii)(D)) requirements are met, ultimately demonstrating compliance with the unrestricted release criteria in 10 CFR 20.1402, Radiological criteria for unrestricted use. Specific guidance in MARSSIM discusses the use of a gross activity DCGL for surface activity.
Issue:
According to the FCS FSSR, a Ludlum Model 44-116 beta scintillation detector was used in SU 4501 and SU 9014 (basements) to obtain static measurements for demonstrating compliance, similar to what was done for SU 5700 and SU 9005 (above ground buildings).
For SU 5700 and SU 9005, OPPD calculated adjusted gross DCGLs due to the fact that radionuclide specific data is not acquired with static measurements. Gross measurements are compared to this adjusted gross DCGL in Table 7-3 of each SURR. However, this was not done in SU 4501 and SU 9014. No gross DCGLs were calculated and the results in Table 7-1 of each SURR are radionuclide specific. The FSSR does not explain how radionuclide specific results were obtained using the Ludlum Model 44-116 detector, or why a gross DCGL was not established for these survey units.
Request:
a.
Discuss how radionuclide specific results were obtained from static measurements using the Ludlum Model 44-116 detector for SU 4501 and SU 9014 and explain why this is appropriate given that beta measurement efficiency can vary widely with energy.
b.
Explain why an adjusted gross DCGL was not calculated in SU 4501 and SU 9014 for direct comparison to gross measurements by converting from cpm to dpm using an established detector efficiency for the radionuclide mix, similar to what was done in SU 5700 and SU 9005.
RAI-01-04: Instrument Efficiencies Basis:
Detailed information on the FSS measurements and sampling is necessary to ensure the survey (10 CFR 20.1501) and LTP (10 CFR 50.82(a)(9)(ii)(D)) requirements are met, ultimately demonstrating compliance with unrestricted release criteria in 10 CFR 20.1402.
Issue:
As noted in RAI-01-03, in several survey units, a Ludlum Model 44-116 detector was used to obtain gross beta measurements for comparison to adjusted gross DCGLs. Detector efficiencies are listed in Table 7-2 of the SURRs for SU 5700 and SU 9005 and are used to convert the measured cpm values to dpm for eventual comparison to the DCGL.
The executive summary of the FCS FSSR indicates that instrument efficiencies were derived from surface emission rate of the radioactive source(s) used during calibration. Footnote (b) to Table 5-32 in LTP Rev. 2 notes that Cesium-137 (Cs-137) was typically employed as the calibration source. However, the radionuclide mixture present in these survey units when using the Ludlum Model 44-116 detector includes Cobalt-60, which has a lower weighted beta energy compared to Cs-137. Consequently, if Cs-137 was used to establish detector efficiency, the calculated results (dpm) could be underestimated as opposed to establishing some weighted efficiency or conservatively determining efficiency with a lower beta energy radionuclide.
Request:
Clarify the radioactive source(s) used during calibration and explain why they were adequate for determining detector efficiency when using the Ludlum Model 44-116 detector to obtain gross measurements for comparison to an adjusted gross DCGL.
RAI-01-05: Surrogate Ratio Verification Basis:
To validate the adequacy of the methods used to demonstrate, with reasonable assurance, compliance with the unrestricted release criteria (10 CFR 20.1402), the site must have provisions for determining contributions from all radionuclides of concern (ROCs), including hard to detect (HTD) radionuclides, in the final dose calculations. Furthermore, detailed information on the FSS measurements and sampling is necessary to ensure the survey (10 CFR 20.1501) and LTP (10 CFR 50.82(a)(9)(ii)(D)) requirements are met.
Issue:
Several items were identified regarding verification of surrogate ratios that the NRC staff is seeking clarification on:
(I)
SU 4200 - The SURR for SU 4200 states that 3 concrete cores were sent offsite for HTD analysis and to See Attachment 2 for data concerning the concrete cores. is not included and the SURR does not contain any discussion of the offsite HTD results, so it is not clear if there were any positively detected ROCs in the concrete core samples that could be used to verify surrogate ratios.
(II)
SU 4202 - There is no discussion in the SURR for SU 4202 regarding whether any samples were sent for HTD analysis. LTP Rev. 2, Section 5.2.6.8, Demonstrating Compliance with Dose Criterion, states that if material remains after cleanout of buried pipe and embedded pipe, at least one volumetric sample will be collected during FSS to confirm the HTD ratio.
(III)
SU 9012 - Section 5 of the SURR for SU 9012 states that 8 concrete disks from 3 core locations were collected, then 1 was sent for HTD analysis. This is confirmed in SURR Table 7-4, which includes a summary of the offsite HTD results. LTP Rev. 2, Section 5.2.6.8 states "the number of samples collected and analyzed for HTD ROCs will be 10 percent of the number of ISOCS measurements or 10 percent of volumetric concrete samples." There were 21 systematic ISOCS measurements in SU 9012, which should equate to at least 2 HTD samples. In addition, SURR Table 7-4 contains an error and does not include any activity values for the sample sent for HTD analysis.
Request:
a.
For item (I), clarify if any ROCs were positively identified in the samples sent for offsite HTD analysis in SU 4200. If so, clarify if surrogate ratios were verified and include laboratory results as applicable.
b.
For item (II), clarify if any material remained after cleanout of embedded pipe and if samples were sent for HTD analysis in SU 4202.
c.
For item (III), clarify the number of samples sent for HTD analysis in SU 9012. If less than 10 percent (2 samples) were sent for HTD analysis, explain why the criteria in LTP Rev. 2, Section 5.2.6.8 were not met. In addition, include activity values for the sample sent for HTD analysis in SURR Table 7-4.
RAI-01-06: Relative Shift Basis:
The regulations in 10 CFR 20.1501 require adequate surveys be performed to recognize and understand the sites radiological condition. MARSSIM Section 5.5.2.3, Contaminant Not Present in Background Determining Numbers of Data Points for Statistical Tests, provides guidance for determining the number of measurements (N) required for the statistical test to demonstrate compliance with the Sign Test. The number depends on, among other parameters, the relative shift.
Issue:
The SURR for SU 4202 does not discuss what data was used to establish the relative shift and no statistical quantities for any remedial action support survey or characterization data are included. The NRC staff is unable to verify whether the appropriate number of random samples were collected to satisfy the survey requirements.
Request:
Clarify what data was used to establish the relative shift for SU 4202 and include summary statistical quantities (i.e., Table 3-1 in other SURRs), if applicable, so that the NRC staff can verify the appropriate number of random measurements were collected.
RAI-01-07: Investigations Basis:
The regulations in 10 CFR 20.1501 require adequate surveys be performed to recognize and understand the sites radiological condition. NUREG-1757, Volume 2, Section 4.5, Final Status Survey Report, states that a discussion of anomalous data, including any areas of elevated direct radiation detected during scanning that exceeded the Investigation Level, or any measurement locations in excess of the DCGLW, should be provided.
Issue:
LTP Rev. 2, Section 5.5.5.2, Remediation, Reclassification, and Resurvey, states that in Class 3 survey units, such as SU 5700, if an individual measurement exceeds 50 percent of the OpDCGL, then the survey unit, or portion of the unit, will be investigated. If the investigation confirms residual radioactivity in excess of 50 percent of the OpDCGL, then the survey unit, or the impacted portion of the survey unit, will be reclassified to a Class 1 or a Class 2 survey unit and the survey will be re-designed and re-performed as a Class 1 or Class 2 survey.
SURR Table 7-3 for SU 5700 summarizes scan results for 14 random, 16 judgmental, and 2 quality control (QC) static measurements. The results for 1 random sample and 3 judgmental samples exceed the action level of 1,118 dpm per square centimeter (50 percent of the adjusted gross OpDCGL). Furthermore, SURR Table 16-1, Survey Unit 5700 Scan Data, for SU 5700 appears to indicate investigations were conducted that identified Potassium-40 as present.
However, Section 9 of the SURR for SU 5700 states that no investigations were performed during the performance or analyses of the survey.
Request:
Clarify if investigations were performed in SU 5700, and if so, discuss the results of those investigations.
RAI-01-08: Quality Control Assessment Basis:
The regulations in 10 CFR 50.82(a)(9)(ii)(D) require detailed plans for the final radiation survey be included in the LTP. The regulations in 10 CFR 20.1501 require adequate surveys be performed to recognize and understand the sites radiological condition. Guidance in NUREG-1757 discusses submittal and NRC review of the licensees QA/QC program.
Issue:
LTP Rev. 2, Section 5.6.2.1, QC Replicate Measurements, states that replicate measurements will be performed on 5 percent of the static and scan locations in each applicable survey unit at locations chosen at random.
The SURR for SU 4200 states that 14 random and 15 judgmental ISOCS measurements were taken. In addition, Section 1 of the SURR states that 28 static measurements were collected and included as judgmental measurements for the FSS of the basement floor/wall survey unit, resulting in 57 total measurements. However, the SURR for SU 4200 indicates that only 2 (3.5 percent) QC replicate measurements were collected.
Request:
Clarify if additional QC replicate measurements were taken in SU 4200 to fulfill the LTP Rev. 2 criteria of 5 percent, or explain why additional replicate measurements did not need to be taken.