LIC-25-0008, Independent Spent Fuel Storage Installation - Response to Audit Report for Omaha Public Power District Request to Revise License Termination Plan Requirements and Request for Additional Information
| ML25211A290 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 07/28/2025 |
| From: | Pearson B Omaha Public Power District |
| To: | Office of Nuclear Material Safety and Safeguards, Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| LIC-25-0008, EPID L-2024-LLA-0095 | |
| Download: ML25211A290 (1) | |
Text
U. S. Nuclear Regulatory Commission LIC-25-0008 10 CFR 50.90 10 CFR 50.82 LIC-25-0008 July 28, 2025 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001 Fort Calhoun Station, Unit No. 1 Renewed Facility License No. DPR-40 NRC Docket No. 50-285 Fort Calhoun Station, Unit No. 1 Independent Spent Fuel Storage Installation NRC Docket No.72-054
Subject:
Response to Fort Calhoun Station, Unit 1 - Audit Report for Omaha Public Power District Request to Revise License Termination Plan Requirements and Request for Additional Information (EPID No. L-2024-LLA-0095)
References:
- 1. Letter from OPPD (A. Barker) to USNRC, License Amendment Request (LAR) To Revise License Termination Plan (LTP) (ML24177A132), dated June 18, 2024.
- 2. Letter from USNRC to OPPD (B. Pearson), Fort Calhoun Station, Unit No. 1 Acceptance of Requested Licensing Action RE: License Amendment Request to Revise the Termination Plan (L-2024-LLA-0095), dated November 18, 2024.
- 3. Email from USNRC to OPPD (B. Pearson), Omaha Public Power District - Regulatory Audit RE: Request for License Amendment to Revise License Termination Plan License No. DPR-40 (L-2024-LLA-0095), dated January 14, 2025.
- 4. Letter from USNRC to OPPD (B. Pearson), Fort Calhoun Station, Unit 1 - Audit Report for Omaha Public Power District Request to Revise License Termination Plan Requirements and Request for Additional Information (EPID No. L-2024-LLA-0095),
dated June 18, 2025.
On June 18, 2024 (Reference 1) OPPD submitted a License Amendment Request to revise the License Termination Plan to include the ability to utilize grout as a mitigation measure in the dose modeling for Final Status Surveys. On November 18, 2024 (Reference 2), following the NRCs review process, the NRC accepted the Licensing Action request to revise the License Termination Plan. By E-mail dated January 14, 2025 (Reference 1), NRC began a Regulatory Audit of the License Amendment Request of Revision 2 of the License Termination Plan. The results of the Audit are captured in the report dated June 18, 2025 (Reference 4). In the Audit report there was a Request for Additional Information (RAI). The enclosure attached to this letter documents the response to the RAI.
The required additional information is contained in the enclosure.
is the Request for Additional Information response.
U. S. Nuclear Regulatory Commission LIC-25-0008 There are no regulatory commitments contained within this letter.
If you should have any questions regarding this submittal or require additional information, please contact Mr. Benjamin P. Pearson - Regulatory Assurance & Emergency Planning Manager at (531) 226-7249.
Respectfully, Benjamin P. Pearson Regulatory Assurance and Emergency Planning Manager BPP/bpp c: M. V. Doell, NRC Senior Project Manager R. Wagner, NRC Project Manager
11 Pages Follow - Response to the Request for Additional Information
Page 1 of 11 NRC 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.
Request #1 FCS Response In the application of Equation 6-15, the mixture fraction value, for each radionuclide, is synonymous with, i.e., equal to, the concentration value of the given radionuclide, in units of pCi/m2. The mixture fraction set, including all initial suite radionuclides, is then equivalent to a complete hypothetical source term. Equating mixture fractions with concentrations to create a hypothetical source term is the foundation of the standard insignificant contributor dose calculation that has been performed for years.
The fact that mixture fractions are synonymous with concentrations is implied in the application of Equation 6-15 but not specifically described in the text or the equation. Equation 6-15 accurately calculates the relative dose, in a shorthand manner, which was the purpose of the calculation. But it was an omission to not specifically describe the relationship between mixture fraction and concentration and changing the units to pCi/m2. In addition, for clarity, the output of the revised Equation 6-15 is changed to total relative dose at year x (TRD) as opposed to relative dose at year x.
The original form of the Equation 6-15 is shown below.
Original Equation 6-15
,=
=0 where:
Dr,x = relative dose at year x from year 0 to 1000 fi = mixture fraction for initial suite radionuclide i DCGLi = BFM excavation DCGL for radionuclide i, at year x The revised version of Equation 6-15 is shown below resulting in a unitless value.
Revised Equation 6-15
=,
=0 where:
TRDx = total relative dose at year x Ci = hypothetical concentration (i.e., mixture fraction) for each initial suite radionuclide i, (pCi/m2)
DCGLi = BFM IU excavation DCGL for radionuclide i at year x (pCi/m2)
Revised Equation 6-15 produces TRDx, not actual dose, because the source term is hypothetical as described above. In addition, the numerical output from the Revised Equation 6-15 is identical to the numerical output from the Original Equation 6-15 and therefore the revision has no effect on the decision that the maximum TRDx for the IU excavation scenario occurs at year zero for all mixtures and DCGLs.
Page 2 of 11 Perhaps another way to view this is understand that Equation 6-15 is used in the denominator of the standard relative dose fraction equation used to select the IC dose as shown in Equation 6 of RSCS Technical Support Document FC-21-043, Radionuclides of Concern in Support of the Fort Calhoun License Termination Plan. The normalization requires the denominator to be the sum of the relative doses from each radionuclide in the mixture, i.e., the Total Relative Dose, at a given year. Because the results are normalized, the output of Equation 6 is unitless. Equation 6-15 does not include normalization which is the reason for requiring the mixture fraction to be converted to concentration in units of pCi/m2 units as described above. The issue with units is corrected in the Revised Equation 6-15.
Although the description of terms are somewhat different, Equation 6 from RSCS Technical Support Document FC-21-043, Radionuclides of Concern in Support of the Fort Calhoun License Termination Plan provides the equation for the relative dose fraction for each radionuclide.
,=
[
1
]
where:
RDFr,x = relative dose fraction for radionuclide i TRDx =
=0 from the Revised Equation 6-15 NRC Request #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.
Request #2 FCS Response The DCGLs in Table 6-23 are a compilation of the DCGLs developed for the Auxiliary Building in Revision 2 including Industrial Use in situ, Industrial Use Drilling Spoils, Industrial Use Excavation, and Resident Farmer Excavation. Two DCGLs that were not revised in LTP Rev 2, i.e., Resident Farmer in situ and Resident Farmer Drilling Spoils, are also included in Table 6-23 because they are used in the subsequent calculations.
Equation 6-13 is applied using the DCGLs in Table 6-23 and the diffusion factors for grout to the Resident Farmer excavation of upper walls ( 3 m from the ground surface) to calculate the DCGLs in Chapter 6 Table 6-24. Each step of the calculations is provided in the Excel spreadsheet submitted with Revision 2 of the LTP but the calculations were not described in the text of Chapter 6 Revision 2. Note that the Resident Farmer DCGLs in columns 2 and 3 of Table 6-23A originally had an adjustment factor of 0.95 to account for excavation of upper walls. In practice, this adjustment factor was found to be unnecessary and also caused Equation 6-17 to incorrectly produce a SOF greater than 1.0 when the actual SOF was equal to 1.0. This change resulted in a minor change to the wall/floor DCGLs in Table 6-24.
The intermediate set of calculations in the spreadsheet using Equation 6-13 were not specifically described in Chapter 6 Revision 2 text but the assumptions that serve as inputs to these calculations are listed in Section 6.13 of Chapter 6 Revision 2 and Table 6-22. The intermediate calculations are provided in one table in the spreadsheet that is reproduced below and labeled as Table 6-23A. As stated in Chapter 6 Revision 2, Equation 6-13 is used to calculate the values in each column of Table 6-23A which are then used as inputs to Table 6-24. The equations for each column of Table 6-23A are described below.
Page 3 of 11 Equation Table 6-23A Column 2 Resident Farmer Scenario Wall/Floor
()=
1 1
+
1
where:
DCGL(RF)wf = Resident Farmer DCGL for walls and floors DCGLi = Resident Farmer insitu DCGL from Table 6-23 DCGLds = Resident Farmer drilling spoils DCGL from Table 6-23 Equation Table 6-23 Column 3 Resident Farmer Scenario Trench with Grout
()=
1 1
+
1
where:
DCGL(RF)t = Resident Farmer DCGL for Trench with grout DCGLi = Resident Farmer insitu DCGL from Table 6-23 df = diffusion factor for grout in trench (radionuclide dependent)
DCGLds = Resident Farmer drilling spoils DCGL from Table 6-23 Equation Table 6-23A Column 4 Industrial Use Excavation Wall/Floor and Trench with Grout
()=
1 1
where:
DCGL(IU)wf = Industrial Use DCGL for walls and floors DCGLe = Industrial Use Excavation DCGL from Table 6-23 (assumes excavation of all Auxiliary Building concrete)
Equation Table 6-23A Column 5 Industrial Use Wall/Floor in situ and Drilling Spoils (No Excavation)
()=
1 1
+
1
where:
DCGL(IU)wf = Industrial Use DCGL for walls and floors DCGLi = Industrial Use insitu DCGL from Table 6-23 DCGLds = Industrial Use drilling spoils DCGL from Table 6-23
Page 4 of 11 Equation Table 6-23A Column 6 Industrial Use Trench with Grout (No Excavation)
()=
1 1
+
1
where:
DCGL(IU)t = Industrial Use DCGL for Trench with grout DCGLi = Industrial Use insitu DCGL from Table 6-23 df = diffusion factor for grout in trench (radionuclide dependent)
DCGLds = Industrial Use drilling spoils DCGL from Table 6-23 The DCGLs in Table 6-23A apply to either the walls/floors or the trench as indicated in the headers of Table 6-23A. The DCGLs listed in Table 6-24 of Revision 2 for Wall/Floor are the lowest values from columns 2, 4, and 5 of Table 6-23A. The DCGLs listed in Table 6-24 of Revision 2 for Trench are the lowest values from columns 3, 4, and 6 of Table 6-23A.
Page 5 of 11 Table 6-23 (LTP Rev 2)
Auxiliary Building Basement BFM Scenario DCGLs for Resi dent Farmer and Industrial Use Resident Farmer in situ DCGLi Industrial Use in situ DCGLi Resident Farmer Drilling Spoils DCGLds Industrial Use Drilling Spoils DCGLi Resident Farmer Concrete Excavation DCGL (DCGLe,c)
Industrial Use Concrete Excavation DCGL DCGLec pCi/m2 pCi/m2 pCi/m2 pCi/m2 pCi/m2 Am-241 4.642E+06 8.435E+06 2.687E+10 1.450E+11 7.544E+07 3.674E+09 C-14 2.341E+07 8.639E+07 1.587E+14 7.878E+15 6.133E+08 6.823E+12 Ce-144 1.526E+09 3.253E+09 2.139E+10 6.243E+10 3.298E+08 2.603E+09 Cm-243 1.330E+07 2.629E+07 5.957E+09 1.868E+10 1.056E+08 8.553E+08 Cm-244 1.664E+07 3.288E+07 8.336E+10 1.111E+12 1.384E+08 9.511E+09 Co-58 7.422E+08 4.366E+09 2.875E+09 8.387E+09 4.395E+07 3.468E+08 Co-60 2.868E+07 1.686E+08 3.358E+08 9.798E+08 4.681E+06 3.693E+07 Cs-134 2.009E+07 2.897E+08 5.739E+08 1.675E+09 8.850E+06 6.983E+07 Cs-137 2.531E+07 3.648E+08 1.361E+09 3.973E+09 2.110E+07 1.665E+08 Eu-152 9.173E+08 1.710E+09 7.108E+08 2.073E+09 1.040E+07 8.212E+07 Eu-154 6.307E+08 1.176E+09 6.675E+08 1.948E+09 9.631E+06 7.599E+07 Eu-155 4.059E+09 7.565E+09 1.859E+10 5.426E+10 4.108E+08 3.242E+09 Fe-55 2.695E+10 6.837E+10 2.349E+15 5.712E+16 3.260E+11 3.696E+13 H-3 2.727E+08 6.732E+08 1.153E+14 6.689E+14 1.227E+09 3.442E+11 Ni-59 2.011E+09 1.515E+10 4.005E+14 3.498E+16 5.646E+09 9.391E+13 Ni-63 7.344E+08 5.533E+09 1.474E+14 3.153E+16 2.063E+09 3.433E+13 Np-237 8.454E+04 1.491E+05 3.186E+09 1.046E+10 9.505E+05 4.541E+08 Pu-238 3.176E+06 5.587E+06 5.200E+10 6.966E+11 8.603E+07 5.916E+09 Pu-239 2.861E+06 5.030E+06 4.728E+10 6.324E+11 7.746E+07 5.325E+09 Pu-240 2.861E+06 5.030E+06 4.735E+10 6.357E+11 7.748E+07 5.332E+09 Pu-241 1.445E+08 2.590E+08 1.084E+12 5.547E+12 2.480E+09 2.645E+11 Sb-125 1.225E+08 4.911E+07 2.068E+09 6.034E+09 3.272E+07 2.579E+08 Sr-90 1.383E+06 5.201E+06 7.062E+10 5.314E+11 3.818E+06 1.961E+10 Tc-99 8.256E+06 2.686E+07 1.003E+12 1.562E+13 3.357E+07 4.356E+10
Page 6 of 11 Table 6-23A Wall/Floor and Trench DCGLs for all Scenarios Accounting for Diffusion through Grout Applies to Walls/Floors Applies to Trench Applies to Trench and Walls/Floors Applies to Walls/Floors Applies to Trench Radionuclide Resident Farmer BFM DCGL 1 All Floors and Walls LTP Chapter 6 Equation 6-13 Resident Farmer Trench BFM DCGL1 LTP Chapter 6 Equation 6-13 Industrial Use BFM Excavation Only no in situ no Drilling Spoils LTP Chapter 6 Equation 6-13 Industrial Use BFM in situ and Drilling Spoils only no excavation LTP Chapter 6 Equation 6-13 Industrial Use Trench BFM DCGL in situ and Drilling Spoils only no excavation LTP Chapter 6 Equation 6-13 pCi/m2 pCi/m2 pCi/m2 pCi/m2 pCi/m2 Am-241 4.641E+06 2.687E+10 3.674E+09 8.435E+06 1.450E+11 C-14 2.341E+07 1.587E+14 6.823E+12 8.639E+07 7.878E+15 Ce-144 1.424E+09 2.139E+10 2.603E+09 3.092E+09 6.243E+10 Cm-243 1.327E+07 5.957E+09 8.553E+08 2.625E+07 1.868E+10 Cm-244 1.664E+07 8.336E+10 9.511E+09 3.288E+07 1.111E+12 Co-58 5.899E+08 2.875E+09 3.468E+08 2.871E+09 8.387E+09 Co-60 2.642E+07 3.358E+08 3.693E+07 1.439E+08 9.798E+08 Cs-134 1.941E+07 5.739E+08 6.983E+07 2.470E+08 1.675E+09 Cs-137 2.484E+07 1.361E+09 1.665E+08 3.342E+08 3.973E+09 Eu-152 4.005E+08 7.108E+08 8.212E+07 9.372E+08 2.073E+09 Eu-154 3.243E+08 6.675E+08 7.599E+07 7.333E+08 1.948E+09 Eu-155 3.332E+09 1.859E+10 3.242E+09 6.639E+09 5.426E+10 Fe-55 2.695E+10 2.349E+15 3.696E+13 6.837E+10 5.712E+16 H-3 2.727E+08 1.855E+09 3.442E+11 6.732E+08 4.580E+09 Ni-59 2.011E+09 3.024E+13 9.391E+13 1.515E+10 2.446E+14 Ni-63 7.344E+08 1.425E+14 3.433E+13 5.533E+09 1.605E+16 Np-237 8.454E+04 3.186E+09 4.541E+08 1.491E+05 1.046E+10 Pu-238 3.175E+06 5.200E+10 5.916E+09 5.587E+06 6.966E+11 Pu-239 2.861E+06 4.728E+10 5.325E+09 5.030E+06 6.324E+11 Pu-240 2.861E+06 4.735E+10 5.332E+09 5.030E+06 6.357E+11 Pu-241 1.445E+08 1.084E+12 2.645E+11 2.590E+08 5.547E+12 Sb-125 1.156E+08 2.068E+09 2.579E+08 4.871E+07 6.034E+09 Sr-90 1.383E+06 7.062E+10 1.961E+10 5.201E+06 5.314E+11 Tc-99 8.256E+06 2.160E+11 4.356E+10 2.686E+07 8.467E+11
Page 7 of 11 Table 6-24 (LTP Rev 2 Revised)
Auxiliary Building Basement BFM Wall/Floor and Trench DCGLs Radionuclide Wall/Floor DCGL pCi/m2 Trench DCGL pCi/m2 Am-241 4.641E+06 3.674E+09 C-14 2.341E+07 6.823E+12 Ce-144 1.424E+09 2.603E+09 Cm-243 1.327E+07 8.553E+08 Cm-244 1.664E+07 9.511E+09 Co-58 3.468E+08 3.468E+08 Co-60 2.642E+07 3.693E+07 Cs-134 1.941E+07 6.983E+07 Cs-137 2.484E+07 1.665E+08 Eu-152 8.212E+07 8.212E+07 Eu-154 7.599E+07 7.599E+07 Eu-155 3.242E+09 3.242E+09 Fe-55 2.695E+10 3.696E+13 H-3 2.727E+08 1.855E+09 Ni-59 2.011E+09 3.024E+13 Ni-63 7.344E+08 3.433E+13 Np-237 8.454E+04 4.541E+08 Pu-238 3.175E+06 5.916E+09 Pu-239 2.861E+06 5.325E+09 Pu-240 2.861E+06 5.332E+09 Pu-241 1.445E+08 2.645E+11 Sb-125 4.871E+07 2.579E+08 Sr-90 1.383E+06 1.961E+10 Tc-99 8.256E+06 4.356E+10 NRC Request #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.
Request #3 FCS Response The mixture fraction of 0 for Np-237 in the wall/floor mixture has no impact on the relative dose calculation for walls/floors other than causing the relative dose from Np-237 to be 0 for all years. The relationship between the mixture fractions, DCGLs, and relative doses is discussed in detail below and in the response to RAI Request #1 which should provide clarification regarding the role of the Np-237 mixture fraction. This response will also explain why applying the year 0 mixture to calculate relative doses at all years is the only mixture that is compatible with the RESRAD calculation of DCGLs.
RESRAD provides a DCGL for individual radionuclides assuming that remediation is completed prior to license termination (beginning of year 0, assumed to be October 5, 2026, for the LTP) and that exposure occurs at some year in the future (0-1000 years). For example, the DCGL for 100 years is calculated under the assumption that there will be no occupancy of the site until year 100. The code was designed in this way by the Department of Energy (DOE) to take credit for periods of institutional control. In this case
Page 8 of 11 taking credit means being able to apply DCGLs with higher concentrations during site cleanup at year 0 using year 0 mixture fractions. The DCGL includes the dose from daughter products.
It is obvious that remediation can only occur before license termination. Therefore, using the year 0 source term, which is defined by the year 0 mixture fractions, is the appropriate initial condition. While remediation can only occur at year 0, the 25 mrem/yr dose criterion must be met at all future years (0-1000 years). When selecting a DCGL for cleanup, the year of first exposure must be selected and applied to all radionuclides in the mixture with no exceptions. For example, if exposure is assumed to begin at year 100, the year 100 DCGLs would apply but it has to be taken as a fact that human occupancy and exposure will not occur before year 100. If exposure is assumed to begin at year 0, which is typical, the year 0 DCGLs provide the most accurate estimates of dose. Note that for both years of exposure, year 0 or year 100, the source term is based on the radionuclide mixture that exists at year 0. However, there is a minor complication in the use of year 0 DCGLs in that the maximum dose (minimum DCGLs) for certain radionuclides with long half-lives and low Kds, such as Np-237, occurs at some time after the assumed year 0 exposure time due to the time it takes to migrate to groundwater.
Using Np-237 as the example of this issue, the most conservative Np-237 DCGL occurs at year 341. As discussed above, the DCGLs at year 341 are calculated by RESRAD assuming that the cleanup occurs at year 0, using the year 0 mixture, but exposure does not begin until year 341. While the Np-237 DCGL at year 341 is lower, the DCGLs for the vast majority of radionuclides at year 341 are higher by orders of magnitude due to decay. The question to be answered is whether the increased dose from Np-237 at year 341, when combined with the decreased dose from all other radionuclides at year 341, exceeds the dose using the year 0 DCGLs assuming that the same source term is applied in both cases. To answer this question, the relative dose must be calculated using the year 0 DCGLs and year 0 mixture and then compared to the relative dose using the year 341 DCGLs. The comparison can be made using a hypothetical source term developed by direct comparison to the radionuclide mixture at year 0 or an actual source term that represents 25 mrem/yr when the year 0 DCGLs are used with the year 0 mixture.
The calculation in Chapter 6 Rev 2 uses the Revised Equation 6-15 and hypothetical source term. The year 0 mixture must always be used to develop the cleanup criteria, regardless of occupancy time, because year 0 is when the cleanup occurs. As stated above, this is a fundamental assumption in the RESRAD calculation of cleanup criteria (DCGLs). The objective is to demonstrate that the TRD is maximized using the year 0 DCGLs which then justifies the use of the full suite of DCGLs from year 0, including Np-237, and eliminating the excessive conservatism of applying the year 341 DCGL with the year 0 DCGL set.
A key point here is that the exposure to the lower Np-237 DCGLs does not begin until year 341 and that the full set of year 341-year DCGLs, including all radionuclides, must be used for a dose assessment. A human cannot begin site occupancy at year 0 and year 341 simultaneously. In addition, the decommissioning rule states that the dose cannot exceed 25 mrem in any year, over the span of 1000 years, indicating that the dose in a given year is consistent with the rule. The year of occupancy must be selected and the DCGLs from that year used as the cleanup criteria using the year 0 mixture. There are no other options when selecting cleanup criteria (i.e., DCGLs).
To reduce complexity, this question has been addressed in the past by using the minimum DCGL regardless of the year in which it occurs. This approach is obviously conservative because human exposure cannot occur in different years simultaneously but eliminates the need to calculate the relative dose at each year to confirm that the maximum relative dose occurs in year 0 and that the year 0 Np-237 DCGL can be applied. The added conservatism of using the lower Np-237 DCGLs from year 341 was deemed acceptable in past applications because the IC dose fraction for Np-237 was sufficiently low due to the very low Np-237 mixture fraction and different DCGLs. The low relative dose contribution resulted
Page 9 of 11 in Np-237 not being designated as an ROC. During the development of the trench mixture fractions and DCGLs, preliminary calculations indicated that the Np-237 dose fraction would exceed the IC dose threshold and Np-237 would be designated as an ROC if the year 341 Np-237 DCGL were applied.
Designating Np-237 as an ROC would have caused significant challenges in meeting the required scan MDC using hand-held instrumentation thereby requiring the use ISOCS over all surfaces. The additional time required by the use of ISOCS given the challenging trench geometries would have resulted in not completing the trench survey and grouting before the US Army Corp of Engineers increased the flow of water in the Missouri River. Increasing the river water level was known to cause in-leakage through the damaged trench concrete and basement flooding. The management of basement flood water would have a significant impact on cost and schedule. This led to the decision to perform the additional calculation to confirm that year 0 Np-237 DCGLs could be used by calculating the relative dose at each year, including all of the radionuclides, which would eliminate the need to apply the year 341 Np-237 DCGL.
The calculation of relative dose in the LTP Rev 2, now Total Relative Dose (TRD) using the Revised Equation 6-15 (see response to RAI request #1), used the hypothetical source term based directly as the radionuclide mixture. To hopefully provide additional clarity, in this discussion the hypothetical source term was converted to the actual source term, in units of pCi/m2, that would result in 25 mrem/yr using the year 0 IU Excavation DCGLs and year 0 trench mixture. The results are provided in Table 1.
Page 10 of 11 Table 1 Source Term That Produces 25 mrem/yr Using the IU Excavation Scenario DCGLs and Year 0 Mixture Fraction Radionuclide 25 mrem/yr Source Term pCi/m2 Am-241 1.60E+05 C-14 1.86E+07 Ce-144 1.04E+04 Cm-243/244 5.95E+05 Co-58 3.39E+00 Co-60 1.87E+07 Cs-134 1.65E+06 Cs-137 7.07E+07 Eu-152 5.22E+04 Eu-154 3.03E+06 Eu-155 2.07E+04 Fe-55 3.32E+06 H-3 1.80E+08 Ni-59 8.44E+05 Ni-63 7.96E+07 Np-237 1.04E+05 Pu-238 2.55E+06 Pu-239/240 8.04E+03 Pu-241 7.98E+05 Sb-125 3.02E+04 Sr-90 1.93E+06 Tc-99 6.73E+07 The results of the TRD calculation using the Revised Equation 6-15 and the 25 mrem/yr source term is provided Table 2. Although the TRD calculation results are unitless they are converted to dose for the purpose of the presentation. The unitless TRD value can be interpreted in the same manner as a typical sum of fractions (SOF) value used for assessment of FSS data. The SOF is unitless and dose is calculated by multiplying the SOF by 25 mrem/yr. In the same manner, the unitless TRD values is multiplied by 25 mrem/yr to derive the dose values in Table 2. The relative differences between the TRDs when the hypothetical and 25 mrem/yr source terms are used are identical but using the 25 mrem/yr source term provides a more direct perspective on the actual dose implications of applying the year 0 source term in subsequent years. As seen in Table 2, the dose using the year 341 DCGLs, when Np-237 is maximized, is 4.9 mrem/yr. The dose using the year 0 DCGLs is much higher at 25 mrem/yr. The maximum dose occurs when the year 0 DCGLs are applied. This demonstrates that it is unnecessary to take on the burden of additional conservatism when the year 341 Np-237 DCGL is combined with the year 0 DCGLs for other radionuclides. Further, this calculation demonstrates that using the year 0 DCGLs provides a more accurate estimate of dose from the IC Excavation scenario in the calculation of the trench DCGL.
Page 11 of 11 Table 2 Comparison of TRD when the Hypothetical Source Term is Converted to a 25 mrem/yr Source Term Year TRD 25 mrem/yr Source Term 0
25.00 1
23.11 2.1 21.33 10 13.32 39.8 5.78 100 1.31 341.4 0.54 1000 0.01 Note that this discussion has centered around Np-237 because it was the radionuclide of primary concern.
For completeness, the TRD values in Table 2 include each year where a radionuclide has a lower DCGL in a year other than year 0. This is done by setting the RESRAD calculation times to the year when the minimum DCGLs occur. Table 2 lists the TRDs for years 2.1, 39.8, 341.4 and 1000 which are the times when minimum DCGLs occur for H-3, Tc-99, Np-237, Ni-59, and Pu-239, respectively (both Ni-59 and Pu-239 have minimum DCGLs at year 1000). Year 100 is included as an example of year when all DCGLs are minimized at year 0. The calculations reported in Table 2 were performed using the trench mixture. The same calculations were done for the wall/floor mixture and the 971 wall/floor mixtures with the same conclusion, i.e., the maximum TRD is found with the year 0 mixture.