ML25154A363
| ML25154A363 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 06/18/2025 |
| From: | Reactor Decommissioning Branch |
| To: | |
| Shared Package | |
| ML25154A358 | List: |
| References | |
| EPID L-2024-LLA-0095 | |
| Download: ML25154A363 (1) | |
Text
Enclosure 2 REQUEST FOR ADDITIONAL INFORMATION RELATED TO AMENDMENT NO. 303 TO APPROVE REVISION 2 OF THE LICENSE TERMINATION PLAN FOR OMAHA PUBLIC POWER DISTRICT FORT CALHOUN STATION, UNIT 1 FACILITY OPERATING LICENSE NO. DPR-40 DOCKET NO. 50-285 Regulations in 10 CFR 20.1402 contain the unrestricted release dose criterion for license termination. Clearly stated and technically supported derived concentration guideline levels (DCGLs) are needed to accurately assess the radiological conditions of the site relative to the dose criterion in 10 CFR 20.1402. NUREG-1757, Vol. 2, Rev. 2, and NUREG-1575, provide guidance on the calculation and use of DCGLs for demonstrating compliance with 10 CFR 20.1402.
Issue:
Clarifications are needed regarding the relative dose approach used for deriving industrial use (IU) scenario concrete excavation DCGLs for the Auxiliary Building (Aux Building) basement.
A common approach for modelling the dose from excavated concrete is to conservatively assume that excavated concrete takes on the properties of soil when brought to the ground surface rather than remaining as discrete chunks of concrete. In addition, the projected peak dose is calculated for the excavated concrete by summing the individual peak doses from each radionuclide in the concrete, even if it involves the use of DCGL values from different years.
FCS proposes to use this approach when considering drilling spoil and in situ DCGLs associated with the IU scenario but proposes an alternate approach for determining IU concrete excavation DCGLs.
FCS proposes to calculate the Aux Building basement IU concrete excavation DCGLs using the soil DCGLs calculated using RESRAD along with the surface area and thickness of the walls associated with the auxiliary building basement and the density of concrete. This calculation was performed using a spreadsheet. NRC staff modified the RESRAD analysis used to derive the IU soil excavation DCGLs for Area of the contaminated zone, Thickness of contaminated zone, Length parallel to aquifer flow, and Density of contaminated zone parameter values to match the inputs used in the spreadsheet calculation but were unable to confirm that the proposed concrete excavation DCGLs corresponded to a dose of 25 mrem/yr.
FCS also proposes to use a value identified as the relative dose fraction for each radionuclide in the known mixture of radionuclides associated with the Aux Building basement walls and floors and trenches to decide which IU concrete excavation DCGLs to use when considering compliance DCGLs. Section 3.3 of NUREG-1757, Vol. 2, Rev. 2., discusses the use of relative dose contributions when determining radionuclides of concern and insignificant contributors but the NRC considers use of a radionuclides relative dose fraction to be an alternative approach for deriving compliance DCGLs. As proposed, FCS is basing its selection of IU concrete excavation DCGLs on the summation of relative dose fractions calculated using mixture fractions based on samples collected at time zero, RESRAD-calculated IU concrete excavation DCGLs, and the equation provided below.
Relative Dose Fraction mrem yr
= Mixture fraction of specific radionuclide (unitless)
Industrial Use concrete excavation DCGL ( pCi m2 )
According to FCS, dividing the mixture fraction value for a specific radionuclide by the corresponding IU concrete excavation DCGL results in a radionuclide-specific relative dose contribution based on the fraction of the specific radionuclide that currently exists in the mixture.
However, the NRC staff determined that equation 6-9 does not result in a projected dose contribution but instead results in a quantity with the units m2/pCi.
Furthermore, the proposed approach considers only the initial mixture fractions for walls and floors and trenches. According to FCS, only the initial mixture fractions are needed when evaluating relative dose over time since decay and ingrowth associated with individual radionuclides are considered by RESRAD when calculating the IU concrete excavation DCGLs at time zero (i.e. license termination).
NRC staff note that the use of initial mixture fractions for the walls and floors of the Aux Building, where the initial mixture fraction for Np-237 is zero, does not appear to consider a dose contribution from Np-237 in any year. Specifically, dividing the initial mixture fraction of Np-237 (zero) by the IU concrete excavation DCGLs for Np-237 results in a Np-237 relative dose fraction value of zero for all years. This is inconsistent with the idea that even if no Np-237 exists in the mixture at year zero, it will contain some amount of Np-237 in future years due to ingrowth from Am-241 and thus will have a contribution to dose (i.e., following ingrowth, the Np-237 mixture fraction would not be zero). This example suggests that changes to the mixture fractions for all radionuclides, including daughter products, over time could impact the relative dose calculation and ultimately which IU concrete excavation DCGLs are selected.
Request:
1.
Clarify how dividing the mixture fraction (unitless) by a DCGL (pCi/m2) results in a relative dose in units of mrem/yr that is used as the basis for selecting the year of peak dose for purposes of selecting the IU concrete excavation DCGLs 2.
Confirm (e.g., with RESRAD output files) that the concrete excavation DCGLs derived using the spreadsheet calculation correspond to a dose of 25 mrem/yr.
3.
Provide the basis for only using current (i.e., year zero) wall/floor and trench mixture fractions to assess relative doses for future years. Specifically, given the concerns regarding Np-237 as a possible ROC, clarify the role of the Np-237 mixture fraction for walls and floors in determining the relative dose year and, ultimately, the selection of IU concrete excavation DCGLs.