ML25154A362

Audit Report Attachment - FCS LTP Rev 2

ML25154A362
Person / Time
Site:	Fort Calhoun
Issue date:	06/18/2025
From:	Reactor Decommissioning Branch
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ML25154A358	List: ML25154A362
References
EPID L-2024-LLA-0095
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FCS LTP Revision 2 Audit Summary of Topics Page 1 of 11 Attachment 1.

Derived Concentration Guideline Levels (DCGLs)

No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 1-a Clarify the inconsistencies between Table 2 of FC-24-004, rev. 1, and Tables 5-11, 6-23, and 6-24 of the revised License Termination Plan (LTP).

Additionally, identify which sets of derived concentration guideline level (DCGLs) the licensee plans to use as the compliance DCGLs for components of the Auxiliary Building basement, including grouted basement structures (i.e.,

trenches) and basement floors/walls.

Fort Calhoun Station (FCS) noted the inconsistences between the different tables of DCGLs in the different documents. FCS provided amended versions of Table 6-23 and Table 6-24, which identifies the proposed compliance DCGLs for the Auxiliary Building basement and grouted trenches. In response to this audit topic, FCS also notes that tables 5-7, 5-8, and 5-11 as well as the corresponding text have been revised.

This response addresses the inconsistencies noted. To support NRCs review of the FCS LTP Rev 2 License Amendment Request (LAR), FCS should submit the amended versions of Tables 6-23 and 6-24 formally to NRC.

1-b Clarify the role of the Base Case DCGLs, IC

[insignificant contributor] Dose Corrected Base Case DCGLs, and Operational DCGLs listed in Table 2 of FC-24-004, rev. 1, for trenches in the Auxiliary Building floor and provide the basis for why operational DCGLs are considered for the trenches but not for any of the other DCGLs being proposed in the revised LTP.

NRC staff determined the differences in these DCGLs do not impact conclusions being made regarding the use of DCGL values as it applies to this License Amendment Request (LAR) because the operational DCGLs are not applicable at this phase of decommissioning. No further information is needed.

FCS LTP Revision 2 Audit Summary of Topics Page 2 of 11 Attachment No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 1-c Clarify the process used for calculating compliance DCGL values and, if necessary, update the table(s) of values. This includes providing additional details explaining why the "Trench DCGL" values in Table 6-24 of the revised LTP and the "Industrial Use Concrete Excavation DCGL" values in Table 6-23 of the revised LTP are the same. In addition, discuss why the Wall/Floor DCGL values provided in Table 6-24 of the revised LTP are not the same as the Industrial Use Concrete Excavation DCGL values in Table 6-23 of the revised LTP.

FCS explained the DCGL calculations and clarified some of the issues and concerns expressed by NRC staff related to scenarios, equations, and calculations. This included addressing how the equations were used to calculate the scenario-specific DCGLs and, when applicable, the bases for specific assumptions (e.g., exposures to material associated with drilling spoils and excavation cannot occur in the same scenario). FCS also eliminated the adjustment factors used to account for doses associated with the excavation of the upper walls as it was found unnecessary and impacted the Sum of Fractions (SOF) calculation.

Updated Tables 6-23 and 6-24 were provided as a result of addressing this topic.

As noted in the response to 1-a, FCS should formally submit the amended versions of Tables 6-23 and 6-24 to the NRC.

1-d Clarify why the Trench DCGL values in Table 6-24 of the revised LTP and Industrial Use Concrete Excavation DCGL values in Table 6-23 of the revised LTP are the same. In addition, please discuss why the Wall/Floor DCGL values provided in Table 6-24 of the revised LTP are not the same as the Industrial Use Concrete Excavation DCGL values in Table 6-23 of the revised LTP.

The response to this topic is incorporated into the response to 1-c above.

FCS LTP Revision 2 Audit Summary of Topics Page 3 of 11 Attachment No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 1-e Clarify which DCGL values apply to the Resident Farmer (RF) excavation scenarios given that the new approach being proposed in Section 6.13.1 only considers excavated material associated with the top 3 meters (m) below ground surface (bgs) and uses an Industrial Use (IU) scenario to evaluate material more than 3 m bgs.

FCS provided written documentation that explains the role of the RF and the IU scenarios when developing DCGLs associated with material located greater than 3 m bgs. FCS proposes to use the smaller of the two DCGLs from the proposed RF excavation scenarios (i.e., the DCGLec). In addition, although doses are associated with exposures occurring to individuals located on the surface, the RF excavation scenario is limited to the excavation of walls located in the top 3 m bgs while the IU scenario considers exposure to all walls and floors excavated material associated with the Auxiliary Building, regardless of the depth it was excavated from.

Table 6-24 provides the lowest DCGL values from the three scenarios applicable for non-grouted walls/floors, RF in situ/drilling spoils, IU excavation, and IU in situ/drilling spoils (no excavation). The RF in situ/drilling spoils values are proposed to be applied for compliance for walls/floors.

Compliance DCGLs for the trenches listed in Table 6-24 are from the IU excavation scenario, which considers the excavation of all walls/floors and foundations of the Auxiliary Building, including the trenches. The other applicable scenarios considered are RF with grouted trenches and IU with grouted trenches.

The information provided in this response addresses the discussion topic.

NRC staff have no additional questions at this time.

FCS LTP Revision 2 Audit Summary of Topics Page 4 of 11 Attachment 2.

Use of DCGLs from year with highest relative dose vs. Peak year No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 2-a Clarify the basis for using an alternate approach of using DCGLs based on the year with the highest relative dose instead of using the largest DCGL for a specific radionuclide, regardless of the year in which the dose occurs; this approach differs from the approach previously approved for considering DCGLs outlined in the previously approved LTP.

FCS calculated a new DCGL for the UI scenario based on the year of Np-237 relative peak dose (year 341), rather than from the dose from each radionuclide at its year of peak dose (LTP, Rev 2, Section 6.13)

This approach takes into account that the Np-237 peak year provides the lowest DCGL of the radionuclide mixture used for the IU scenario but does not combine the highest relative doses for all of radionuclides that realistically would not all occur in the same year.

However, the licensees approach used the radionuclide mixture fractions from year zero when calculating doses at year 341. Given that radioactive decay and ingrowth occur over time, the licensee should justify why it is acceptable to apply mixture fractions established at year zero for all years beyond year zero.

2-b Justify the uncertainty associated with the changes to radionuclide concentration ratios given that the largest DCGL values are not being considered for all radionuclides.

FCS addressed the uncertainty associated with this approach by noting that their use of the 75th percentile of the Cs-137 fractions for selecting the mixture provides reasonable overall conservatism and is consistent with the use of the 75th percentile to address uncertainty in the parameter selection process for dose modeling and DCGL calculations. FCS also noted that the effect of decay and ingrowth on the dose in future years is fully accounted for in the RESRAD model and is therefore not a consideration when calculating relative dose for all years using equation 6-15. This response was adequate to address NRCs concerns.

2-c Clarify how the sum of fractions calculation would be performed given the different methods would be used for determining different DCGL values.

NRC staff confirmed that the proposed approach is consistent with NRC guidance provided in NUREG-1757, Vol. 2, Rev. 2.

2-d Clarify how the licensee will confirm that the final radionuclide concentrations will meet the regulatory requirements during the final status survey process.

NRC staff confirmed that the proposed approach is consistent with NRC guidance provided in NUREG-1757, Vol. 2, Rev. 2.

FCS LTP Revision 2 Audit Summary of Topics Page 5 of 11 Attachment 3.

New Areas to be Grouted No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 3-a Clarify what other areas of the Auxiliary Building basement have plans to be grouted and use DCGLs developed for the East Trenches.

FCS provided Aux Bldg Grouted Areas Draft C.docx which lists the areas other than the Auxiliary Building 989' East Trenches that are to be grouted. These areas include the 971' elevation Rooms 9, 10 &

11, Trench 31, Trench 32, 989' floor areas, and 971' sumps and floor.

The NRC staff now understands the licensees plans for grouting in the other Auxiliary Building basement areas.

3-b Describe how the licensee plans to develop radionuclides of concern (ROCs) for other basement areas to be grouted and if they will use mixture fractions from LTP Table 5-5 or determine new mixture fractions on a case-by-case basis using characterization, radiological assessment (RA), or remedial action support survey (RASS) data.

The 989' elevations, other than the trenches, will be using the original LTP mixture fractions and that these will be in the release records of each survey unit (SU). Based on this response, the statement in Section 5.2.5.1, that Table 5-5 provides the final radionuclide mixture fractions for the initial suite of radionuclides that apply to grouted basement structures, does not apply and these mixture fractions will be developed on an area specific basis. The staff finds this clarification acceptable because representative mixture fractions for all areas included in this LTP revision were either supplied in the LTP or will be included in the release records of each SU.

3-c If mixture fractions in Table 5-5 will apply to all basement structures to be grouted, clarify how the radionuclide mixtures fractions in Table 5-5, which were developed using samples from the Auxiliary Building East Trench, are representative of other basement areas to be grouted given the statement in revised LTP Section 6.13.

Resolved based on the response provided by FCS to 3-b that separate mix fractions were developed for separate areas.

FCS LTP Revision 2 Audit Summary of Topics Page 6 of 11 Attachment 4.

Use of extracted basement and trench material as fill No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 4

Clarify the process the licensee will use to incorporate the trench DCGLs and the walls and floors DCGLs into the total compliance calculation for the site (Equations 5-6 and 5-7 of the revised LTP).

FCS noted that in Section 5.2.6.1.5 it states that Grouted basement structures are considered as basement surfaces and will be included as such for the summed compliance dose and that the highest sum of fraction value from a basement or grouted basement structure will be used in Equation 5-7. In other words, grouted basement structures are not considered to be an additional media component that needs to be accounted for in the total compliance calculation for the site.

5.

Area Factors No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 5-a Clarify if a trench is a survey unit.

FCS clarified that a trench is its own survey unit.

5-b Further justify how the use of the simple calculation based solely on the sizes of the survey unit and the radiologically elevated area can be applied without consideration for the specific radionuclides, specific exposure scenarios, specific exposure pathways, and specific modeling parameters being considered when evaluating whether a specific survey unit satisfies the regulatory requirements in 10 CFR 20.1402., or, if the simple calculation above cannot be justified, provide a table(s) of the area factors that will be considered when assessing elevated areas of basement walls, floor, and trenches (as was done for soil in Tables 5-23 and 5-24 of the revised LTP).

FCS provided additional details on how the simple calculation, which is based solely on the size of a survey unit without consideration for the specific radionuclides, exposure scenarios and applicable modeling parameters, can be applied to both soils associated with the resident farmer scenario and concrete/grout associated with an industrial use scenario. FCS also noted that the simple calculation was included in the previously approved LTP and that it is the standard area factors equation used to determine the activity in an elevated area that equates to the total activity in the survey unity. This response was adequate and addressed the NRC staffs concerns.

FCS LTP Revision 2 Audit Summary of Topics Page 7 of 11 Attachment 6.

Release Rate from Grout and Cement No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 6

Provide support for release rates (i.e., cumulative release) from grout and cement so that staff can make a determination that dose criteria specified in 10 CFR 20.1402 is not underestimated. The support should consider: (i) the 989-ft elevation trenches are in the zone of a fluctuating water table, (ii) some amount of advective flow could occur in fractures and contact planes of different materials, (iii) downward migration both by diffusion and advection, and (iv) selections of literature inputs that do not lead to underestimates of dose. Alternatively, provide justification that the uncertainty in the input values is encompassed in the release rate.

The licensee provided an 8-page assessment covering the physical processes affecting migration rates in concrete or grout mentioned in items (i), (ii) and (iii). To address the scale effects in the grouted trenches associated with the first three items, FCS described influx rates of groundwater into the trenches prior to and after trench treatments and provided a bounding estimate of dose using mixture fractions from characterization analyses and exposure due to drinking well water. For item (iv), covering selection of inputs for diffusion coefficient values for different elements, the licensee indicated that the values used for FCS were taken from literature review applied to Zion for application to concrete or grout media at FCS and explained why the values differed from those used at Zion.

The licensees response to Q6 addresses the discussion topic and should be submitted formally to NRC for staff to cite in the SER to support that a less likely scenario approach does not exceed 25 mrem/yr.

FCS LTP Revision 2 Audit Summary of Topics Page 8 of 11 Attachment 7.

Description of the Release from Pipes and Continuing Characterization No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 7

A description of the unusual occurrence involving the spread of contamination in the trenches is needed for staff to determine if the continuing characterization or final status survey plan will be adequate to meet requirements in 10 CFR 20.1501. Specifically, confirm that subsurface soil sampling below the trenches will be included in either the continuing characterization or final status survey plan for soils below basement foundations, or provide justification or support that ensures a release did not occur to the natural environment below the trenches either prior to or after excavation and remediation.

FCS stated that they will collect samples of sediments below slabs of grouted trench areas of the Auxiliary Building and Containment Building. Previously, no changes were made to continuing characterization in the LTP for Revision 2, which indicated continuing characterization would include sediment below slabs with a caveat that sampling may not occur due to groundwater influx and may not have considered reaching locations below slabs where trenches have been grouted.

FCS evaluated alternative technologies and indicated that they may use diagonal boring, drilling/boring through the slab, and/or hydraulic diverter technologies to reach and enable sampling of sediments below slab areas of the trenches after backfilling has occurred. Locations will be identified by a combination of physical measurements and geographic information systems to ensure samples are taken from the correct locations. The hydraulic diverters may be secured to the floor prior to backfilling and would control groundwater in-leakage while collecting soil samples. The diverters would additionally facilitate the collection of groundwater samples directly below the structures as opposed to monitoring wells in the groundwater monitoring network.

If the data collected during the continuing characterization is outside the bounds of the LTP as compared to the initial evaluation, this should be communicated to the NRC and a possible LTP revision may be needed. NRC has no further questions on this topic.

FCS LTP Revision 2 Audit Summary of Topics Page 9 of 11 Attachment 8.

Scan MDC Inputs No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 8

Clarify which set of typical detector height and scan rate inputs described above (revised LTP Section 5.4.1.1.2, revised LTP Section 5.4.2.4.4, and revised LTP Table 5-29) FCS will utilize during surveys. In addition, provide the scan minimum detectable concentration (MDC) values using these inputs so that NRC staff can verify and ensure that methods to be employed will be consistent with the stated MDCs.

FCS clarified that the typical detector height utilized for land surveys is 2 with a scan speed of 0.25 m/sec. For concrete surfaces, the typical detector height is 6 with a scan speed of 0.25 m/sec. Typical scan MDCs for land surveys are on the order of 2.09 pCi/g for backgrounds of 30,000 cpm with the 3 x 3 NaI detector. Typical scan MDCs for concrete surfaces are on the order of 2.33E+07 pCi/m2 with background count rates of 18,000 cpm. FCS noted that due to the high scan MDCs associated with concrete surfaces, this type of survey can only be used in survey units where the grouted structures DCGLs are being applied. The NRC finds this acceptable as it provides clarification on survey methods and scan MDC inputs.

NRC staff were able to reproduce the typical scan MDC given by the licensee during the audit clarification for concrete surfaces.

9.

Scan MDC No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 9

Confirm that a Ludlum 44-10 detector will be used on auxiliary building basement upper walls. In addition, explain how the scan MDC will meet LTP requirements as outlined in revised LTP Sections 5.4.2.1 and 5.5.2.1 or explain what processes will be used to ensure sufficient scan sensitivity to detect small areas of elevated activity.

The licensee clarified that limits on using the Ludlum 44-10 detector can be inferred from the LTP. Any areas where the scan MDC for the Ludlum 44-10 would not meet the LTP requirements, an In Situ Object Counting System (ISOCS) would be used.

FCS LTP Revision 2 Audit Summary of Topics Page 10 of 11 Attachment 10.

Insignificant Contributors Verification No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 10 Clarify if RASS samples were used to verify insignificant contributor (IC) dose in the Auxiliary Building East Trench. If verification was done, provide data and discussion for determining IC dose in the Auxiliary Building East Trenches using RASS samples similar to Section 6.8, RASS Surrogate Ratios, of FC-24-004. If verification was not done, explain why.

The RASS samples were not sent out by FCS in this case because characterization surveys were done so close in time to RASS they felt it was not necessary. IC dose was calculated using Auxiliary Building East Trench mixture fractions, derived from characterization data, and the applicable DCGLs. NRC has no further questions on this topic.

11.

Investigations & Remediation No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 11 Confirm whether the investigation levels and remediation will be based on either (1) Base Case DCGLs as stated in FC-24-004, or (2) DCGLEMC

[elevated measurement comparison] as stated in revised LTP Table 5-30 and revised LTP Section 5.5.5.2.

The licensee confirmed that the investigation levels and remediation are based on the DCGLEMC.

FCS LTP Revision 2 Audit Summary of Topics Page 11 of 11 Attachment 12.

Relative Dose Fractions No.

Discussion Topic NRC Staff Observations & Proposed Closure Path 12-a Explain how FCS plans to calculate DCGLs for walls/floors and trench areas other than the Auxiliary Building East Trenches using mixture fractions specific to those areas or explain how the current selected DCGLs associated with the Auxiliary Building are representative for all areas to be grouted, even those with different radionuclide mixture fractions.

FCS noted that the DCGLs provided are applicable for all auxiliary building basement areas to be grouted and that mixture fraction are only used to determine the year with the largest relative dose for the Basement Fill Model excavation scenario, which is the basis for selecting the applicable IU concrete excavation DCGL.

FCS also noted that the relative dose is attributed to year zero for all three mixture fractions; thus the IU concrete excavation DCGLs for all three areas are from year zero. Specific details regarding the use of this approach are addressed in the response to 2-b.

12-b Clarify how FCS plans to submit DCGLs for grouted areas other than the Auxiliary Basement East Trenches, such as 971 elevation grouted areas, for approval prior to implementing final status survey activities.

FCS noted that there are three mixture fractions associated with the Auxiliary Building basement. The NRC has evaluated calculations associated with the mixture fractions for the trenches and walls/floors; the 971 elevation values have not been evaluated by NRC staff at this point but will be provided in the next LTP revision. According to FCS, all three mixtures result in a relative dose attributed to year zero. Therefore, using the proposed approach, IU excavation DCGLs are the concrete DCGLs associated with year zero.

12-c Explain how FCS plans to incorporate specific mixture fractions for each year that the DCGL values are determined over the 1000-year evaluation period.

FCS noted that the mixture fractions provided, which are associated with year zero, apply to all years.