ML26027A170
| ML26027A170 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 03/07/2024 |
| From: | Ted Sullivan 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-24-002, Rev 0 | |
| Download: ML26027A170 (0) | |
Text
FC-24-002 Revision 0 Radionuclide Release Through Grouted Trenches Prepared By:
TM Sullivan 3/6/2024 Date Reviewed By:
Date Approved By:
Date David N Fauver 3/7/2024 Approved By:
Daniel Whisler 3/7/2024 Date
FC-24-002 Revision 0 Page 2 of 16 1.0 ISSUE STATEMENT Residual radioactive contamination will be left in the cement of trenches in the Fort Calhoun plant. These below-grade trenches will be backfilled with grout or concrete and remain when decommissioning is complete. The trench will be covered with additional grout to prevent the release of radionuclides to the groundwater above the grout. Estimates of the fractional release of these contaminants from the grout/concrete is required to verify that Derived Concentration Guideline Level (DCGL) values are conservative.
2.0 DEFINITIONS 2.1 Derived Concentration Guideline Level (DCGL): A radionuclide-specific term for a surface or volume residual radioactivity level that is related to a concentration or dose or risk criterion.
2.2 Initial Suite of Radionuclides is the group of radionuclides that may impact the predicted dose.
2.3 Radionuclides of Concern (ROC): A term to categorize the radionuclides that contribute a significant portion of the predicted dose. The ROC are a subset of the initial suite of radionuclides.
2.4 Grout in this report will refer to the material used to backfill the trenches. This could be grout or concrete with additives to improve long-term performance of the backfill.
2.5 Cumulative Fractional Release (CFR). The cumulative total amount of inventory released from the grout is divided by the initial inventory of the grout.
2.6 Diffusion coefficient. An experimentally determined parameter that reflects the rate of diffusion of a radionuclide through a media. In this document, the diffusion coefficient is the effective diffusion coefficient that accounts for porosity and tortuosity of the grout, and chemical interactions between the radionuclide and grout.
2.7 DUST-MS Disposal Unit Source Term Multiple Species is a computer code used to predict the migration of radionuclide contaminants from the waste zone through the contacting media (soil or grout).
3.0 METHODOLOGY 3.1 Develop Conceptual Model of Release from concrete through a grout cover based on trench characterization data.
FC-24-002 Revision 0 Page 3 of 16 3.2 Use literature values to determine a conservative upper bound for the effective diffusion coefficients for the simulation of release through different thicknesses of grout covers. Each radionuclide in the initial suite of radionuclides will be analyzed (FC 20-007, 2020).
3.3 Use the DUST-MS computer code to predict the cumulative fractional release of the initial suite of radionuclides through a grout cover.
4.0 ASSUMPTIONS 4.1 Two scenarios are simulated, a) all contamination in the floor, and b) the contamination is distributed throughout the grout uniformly. The trench will be covered by clean grout. Four grout cover thicknesses will be considered, 0, 15, 30, 45 cm. This information will be used to determine the grout thickness needed to reduce release of radionuclides to levels that will not exceed dose criteria.
The uniform contamination scenario is a conservative upper bound on release because characterization suggests that most of the contamination is at or near the bottom of the trench. Using the uniform contamination assumption puts more of the inventory near the grout surface where release to the groundwater occurs.
4.2 The diffusion coefficients represent conservative upper bounds for what might be expected in the grout.
4.3 The trench cover will be extended 30 cm from the edge of the trench in all directions.
5.0 CONCLUSION
The DUST-MS computer code was used to simulate the release of the initial suite of nuclides through a grout backfill of the trenches at Fort Calhoun. The majority of the contamination is expected to occur at the bottom of the trench and this scenario was simulated. To account for contamination of the walls, a scenario with uniform contamination throughout the backfill grout was simulated. The six scenarios simulated were:
- 1. All contamination at the bottom with 45 cm of clean grout on top
- 2. All contamination at the bottom with 90 cm of clean grout on top
- 3. Uniform contamination of 45 cm in the trench with no cover.
- 4. Uniform contamination of 45 cm in the trench with 15 cm of clean grout
- 5. Uniform contamination of 45 cm in the trench with 30 cm of clean grout
- 6. Uniform contamination of 45 cm in the trench with 45 cm of clean grout A detailed discussion of the modeled scenarios and input parameters is in Section 9,.
The major findings are:
For all contamination at the bottom of the trench, the grout cover is an effective barrier to release. Only H-3 had a cumulative release greater than 1% of the initial
FC-24-002 Revision 0 Page 4 of 16 inventory (H-3 8.7%). For most nuclides the cumulative fractional release is < 1E-
- 30. Table 5.1 provides the results for this scenario.
For uniform contamination of the backfill grout, CFRs are effectively controlled by adding a grout cover. Without a cover, releases range from 22% (H-3), 15.7% (Ni-
- 59) to 15% (Tc-99) with most nuclides releasing about 2 to 3% of their inventory.
Adding a clean cover decreased the fractional release substantially. For a 45 cm clean cover, the maximum release was 2.4% for H-3 and Tc-99 had a cumulative fractional release of 1.3E-7%. Most radionuclides had a cumulative fractional release of less than 1E-30 with a 45 cm clean cover. Table 5.2 presents the results for this scenario.
For the scenarios with a clean cover above the trench, to prevent migration out of the side of the clean cover, the grout cover will be extended a distance equal to the clean cover height in each direction. This will ensure that the CFR for the clean grout cover scenario will be less than the values in Table 5.2. More detailed calculations may be able to reduce this restriction.
FC-24-002 Revision 0 Page 5 of 16 Table 5.1 Cumulative Fractional Release from bottom contamination migrating through 45 or 90 cm of clean cover with an initial inventory of 1 Ci.
CFR CFR Selected 45 cm 90 cm D (cm2/s)
Reference Am-241
<1E-30
<1E-30 5.00E-13 Serne, 1992 C-14
<1E-30
<1E-30 1.00E-11 Serne, 1992 Ce-144
<1E-30
<1E-30 1.00E-12 Serne, 1992 Cm-243
<1E-30
<1E-30 5.00E-13 Serne, 1992 Cm-244
<1E-30
<1E-30 5.00E-13 Serne, 1992 Co-58
<1E-30
<1E-30 4.10E-11 Muurinen, 1982 Co-60
<1E-30
<1E-30 4.10E-11 Muurinen, 1982 Cs-134
<1E-30
<1E-30 3.00E-09 Atkinson, 1986 Cs-137 5.60E-10 1.20E-19 3.00E-09 Atkinson, 1986 Eu-152
<1E-30
<1E-30 5.00E-11 Serne, 1992 Eu-154
<1E-30
<1E-30 5.00E-11 Serne, 1992 Eu-155
<1E-30
<1E-30 5.00E-11 Serne, 1992 Fe-55
<1E-30
<1E-30 5.00E-11 Serne, 1992 H-3 0.087 0.0093 5.00E-07 Szantos, 2002 Ni-59 2.6E-7 5.0E-24 1.10E-09 Jakob, 1999 Ni-63 3.6E-10 5.0-27 1.10E-09 Jakob, 1999 Np-237
<1E-30
<1E-30 1.00E-11 Serne, 1992 Pu-238
<1E-30
<1E-30 5.00E-13 Serne, 1992 Pu-239
<1E-30
<1E-30 5.00E-13 Serne, 1992 Pu-240
<1E-30
<1E-30 5.00E-13 Serne, 1992 Pu-241
<1E-30
<1E-30 5.00E-13 Serne, 1992 Sb-125
<1E-30
<1E-30 1.00E-10 Conservative Estimate Sr-90 5.60E-23
<1E-30 5.20E-10 Sullivan, 1989 Tc-99 5.9E-8 8.0E-26 1.00E-09 Serne, 1992
FC-24-002 Revision 0 Page 6 of 16 Table 5.2 Cumulative Fractional Release from uniformly contaminated grout of 45 cm with various thicknesses of clean cover.
T 1/2 (years)
No Cover 15 cm cover 30 cm cover 45 cm cover D (cm2/s)
Am-241 4.32E+02 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 C-14 5.73E+03 0.028 2.87E-22
<1E-30
<1E-30 1.00E-11 Ce-144 7.79E-01 0.022
<1E-30
<1E-30
<1E-30 1.00E-12 Cm-243 2.85E+01 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 Cm-244 1.81E+01 0.021
<1E-30
<1E-30
<1E-30 5.00E-13 Co-58 1.94E-01 0.015
<1E-30
<1E-30
<1E-30 4.10E-11 Co-60 5.27E+00 0.021
<1E-30
<1E-30
<1E-30 4.10E-11 Cs-134 2.06E+00 0.024 2.71E-13 3.11E-24
<1E-30 3.00E-09 Cs-137 3.02E+01 0.054 3.62E-05 2.44E-08 1.66E-11 3.00E-09 Eu-152 1.34E+01 0.022 1.69E-25
<1E-30
<1E-30 5.00E-11 Eu-154 8.20E+00 0.021 1.45E-28
<1E-30
<1E-30 5.00E-11 Eu-155 4.76E+00 0.021
<1E-30
<1E-30
<1E-30 5.00E-11 Fe-55 2.70E+00 0.019
<1E-30
<1E-30
<1E-30 5.00E-11 H-3 1.23E+01 0.22 0.098 0.049 0.024 5.00E-07 Ni-59 8.00E+04 0.157 5.84E-03 2.05E-05 5.24E-09 1.10E-09 Ni-63 1.00E+02 0.06 7.62E-05 5.64E-08 8.2E-12 1.10E-09 Np-237 2.14E+06 0.03 3.2E-22
<1E-30
<1E-30 1.00E-11 Pu-238 8.77E+01 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 Pu-239 2.41E+04 0.023
<1E-30
<1E-30
<1E-30 5.00E-13 Pu-240 6.54E+03 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 Pu-241 1.47E+01 0.021
<1E-30
<1E-30
<1E-30 5.00E-13 Sb-125 2.73E+00 0.0204
<1E-30
<1E-30
<1E-30 1.00E-10 Sr-90 2.90E+01 0.031 1.12E-09 1.78E-15 8.16E-25 5.20E-10 Tc-99 2.13E+05 0.150 4.71E-03 1.00E-05 1.30E-09 1.00E-09 6.0 REFERENCES 7.1. California Water Board https://www.waterboards.ca.gov/water_issues/programs/swamp/docs/cwt/guidanc e/3130en.pdf, Accessed February 15, 2024.
7.2.
Calhoun Station Technical Basis Document, 2020.
7.3.
Jakob, A., F.-A. Sarott and P. Spieler, "Diffusion and sorption on hardened cement pastes - experiments and modeling results", Paul Scherer Institute. PSI-Bericht Nr. 99-05 ISSN 1019-0643, August 1999.
FC-24-002 Revision 0 Page 7 of 16 7.4.
1100.
7.5.
Muurinnen, A, Research Meeting, 1982.
7.6.
to Support Performance
-287, 1992.
7.7.
Sullivan, T. M., and C. J. Suen, "Low-Level Waste Shallow Land Disposal Source Term Model: Data Input Guides,", NUREG/CR-5387, BNL-NUREG-52206, July 1989.
7.8.
Sullivan, T.M., "DUST - Disposal Unit Source Term: Data Input Guide."
NUREG/CR-6041, BNL-NUREG-52375, 1993.
7.9.
Sullivan, T.M., "DUSTMS_D - Disposal Unit Source Term Multiple Species BNL-75554-2006, Brookhaven National Laboratory, Upton, NY, 11973, January 2006.
7.10. Szanto, Zs, Svingor, M. Molnir, L. Palcsu, I. Futo, Z. Szucs. "Diffusion of 3H, 99Tc, 125I, 36Cl, and 85Sr in granite, concrete and bentonite," Journal of Radioanalytical and Nuclear Chemistry, Vol. 252, No. 1 (2002) 133-138.
7.0 TOOLS & EQUIPMENT 7.1 DUST-MS computer code.
8.0 ATTACHMENTS 8.1 : Conceptual Model and DUST-MS Calculation Details and Results 8.1.1 CONCEPTUAL MODEL The current plan at Fort Calhoun includes backfilling approximately 50 trenches with grout or concrete. In this report, the term grout will refer to the final backfill material whether it is concrete or grout. The floors and to a lesser extent, the walls of these trenches are contaminated. There is a concern over radioactive contamination migrating through the grout, reaching the surface of the grout where it will enter the groundwater. To minimize this, the trenches will be covered with clean grout with a thickness of 15 to 45 cm. The trenches range in depth from 1.5 to 5 feet. The median depth is 2.79 feet, and the mean depth is 3 feet. To examine the potential release from this pathway into the groundwater, the maximum release from this condition will be simulated by placing the entire inventory
FC-24-002 Revision 0 Page 8 of 16 of the existing cement at the interface with the grout in the trench (initially uncontaminated zone) and simulating the diffusion up to the edge of the trench (i.e., no additional cover),
Figure 8.1. Two cases are simulated, 45 cm (~1.5 feet, the minimum initial grout thickness) and 90 cm (mean depth of the trenches.
Initial characterization indicates there will is some residual contamination in the walls.
However, most of the contamination is at the bottom surface of the trench. To accommodate the possibility of wall contamination, it is assumed that the contamination along the walls causes the grout/concrete to be uniformly contaminated. This is a conservative assumption based on the initial characterization data. The contamination will be covered by clean grout and three cover thicknesses are simulated, 15, 30, and 45 cm. As the distribution of contamination along the walls and at the bottom of the trench becomes better defined, a distribution of contamination in the grout based on characterization data can be simulated, Figure 8.2.
The output of this model is the fractional release of contamination at the surface of the grout/concrete in contact with the groundwater. In both simulated distributions of contamination, it is assumed that the total inventory is 1 Ci. This assumed inventory can be scaled to the measured inventory when predicting dose impacts.
Figure 8.1 Conceptual model for diffusion release through uncontaminated grout.
FC-24-002 Revision 0 Page 9 of 16 Figure 8.2 Conceptual model for diffusion release through variably contaminated grout.
In summary five cases are simulated.
- 1. All contamination at the bottom with 45 cm of clean grout on top
- 2. All contamination at the bottom with 90 cm of clean grout on top
- 3. Uniform contamination of 45 cm in the trench with 15 cm of clean grout
- 4. Uniform contamination of 45 cm in the trench with 30 cm of clean grout
- 5. Uniform contamination of 45 cm in the trench with 45 cm of clean grout The initial site of radionuclides (FC-007-20, 2020) were modeled using the DUST-MS computer code (Sullivan, 1993, Sullivan, 2006). This code was developed under the sponsorship of the Nuclear Regulatory Commission and has been used in decontamination and decommissioning studies at Maine Yankee, Connecticut Yankee, Zion, and LaCrosse. Application of diffusion modeling through grout or concrete was performed for each of these sites.
8.2 Input Parameters computational cell in the DUST-MS model for the case with initially clean grout. This computational cell is at the boundary and the boundary condition specifies zero mass flux out of the boundary. This will force all the contamination to migrate up through the initially as a waste form with instantaneous release. For the uniform concentration simulation, each cell is given the same initial concentration such that the total inventory in the grout is 1 Ci.
Mass release: All nuclides were simulated to begin with a total of 1 Ci of inventory. This is used as a scaling value. The actual release should be scaled to the actual inventory.
For this reason, the results are presented in terms of the cumulative fractional release of the inventory.
FC-24-002 Revision 0 Page 10 of 16 Diffusion Coefficients: The diffusion coefficients were selected based on conservative literature values. These values are the effective diffusion coefficients and are measured values. The effective diffusion coefficient includes the effects of tortuosity and sorption.
For this reason, sorption is not simulated in the model and all partition coefficient values are set to 0.
PNNL conducted extensive tests of diffusion through four different grout formulations under consideration for use at the Hanford site (Serne, 1992). Other studies have been conducted. Table 8.1 provides a conservative estimate of the diffusion coefficient from these studies. There is a range of values for diffusion coefficients of the more commonly studied radionuclides. Consideration is being given to adding bentonite or other sorptive materials to the grout. This will cause the diffusion coefficients to be reduced.
This simulation is a diffusion problem and water flow are set to zero.
The moisture content was set to 0.25 in the grout. The peak concentrations are inversely proportional to moisture content. Thus, if the moisture content was 0.125, the predicted peak concentration would double. However, the total mass released is not impacted by moisture content. For example, if the actual moisture content was 0.125 the peak concentration would double, but there would be 50% less water to migrate through. Thus, the total amount released would remain the same.
The bulk density of the grout does not impact the results because sorption is not modeled directly. Sorption is accounted for through using an effective diffusion coefficient that incorporates sorption into the value. A nominal value of 1.5 g/cm3 was used. If concrete is used for the backfill a value of 2.2 g/cm3 would be more appropriate.
FC-24-002 Revision 0 Page 11 of 16 Table 8-1 Typical diffusion coefficients in cement for radionuclides of concern Range Serne, 1992 Literature Values Selected Nuclide T 1/2 (years)
D (cm2/s)
D (cm2/s)
D (cm2/s)
Reference Comment Am-241 4.32E+02 5E 2E-13 5.00E-13 Serne, 1992 C-14 5.73E+03 5E 1E-11 1.00E-11 Serne, 1992 Ce-144 7.79E-01 5.00E-11 1.00E-12 Serne, 1992 Sr Analog Cm-243 2.85E+01 5.00E-11 5.00E-13 Serne, 1992 Am analog Cm-244 1.81E+01 5.00E-11 5.00E-13 Serne, 1992 Am Analog Co-58 1.94E-01 1E-13 - 1E-11 4.10E-11 4.10E-11 Muurinen, 1982 Co-60 5.27E+00 1E-13 - 1E-11 4.10E-11 4.10E-11 Muurinen, 1982 Cs-134 2.06E+00 1E 3E-10 3.00E-09 3.00E-09 Atkinson, 1986 Cs-137 3.02E+01 1E 3E-10 3.00E-09 3.00E-09 Atkinson, 1986 Eu-152 1.34E+01 5.00E-11 5.00E-11 Serne, 1992 Sr Analog Eu-154 8.20E+00 5.00E-11 5.00E-11 Serne, 1992 Sr Analog Eu-155 4.76E+00 5.00E-11 5.00E-11 Serne, 1992 Sr Analog Fe-55 2.70E+00 5.00E-11 5.00E-11 Serne, 1992 Conservative Estimate H-3 1.23E+01 5.00E-08 5.00E-07 5.00E-07 Szantos, 2002 Ni-59 8.00E+04 4.30E-10 1.10E-09 1.10E-09 Jakob, 1999 Ni-63 1.00E+02 4.30E-10 1.10E-09 1.10E-09 Jakob, 1999 Np-237 2.14E+06 1E 1E-15 5.00E-13 Serne, 1992 Conservative estimate Pu-238 8.77E+01 1E 1E-15 5.00E-13 Serne, 1992 Pu-239 2.41E+04 1E 1E-15 5.00E-13 Serne, 1992 Pu-240 6.54E+03 1E 1E-15 5.00E-13 Serne, 1992 Pu-241 1.47E+01 1E 1E-15 5.00E-13 Serne, 1992 Sb-125 2.73E+00 1.00E-10 Serne, 1992 Conservative Estimate Sr-90 2.90E+01 2E 1E-10 5.20E-10 5.20E-10 Sullivan, 1989 Tc-99 2.13E+05 1E 2E-9 1.00E-09 Serne, 1992 Conservative Estimate 8.3 Results The simulation was conducted for 1000 years. Table 8.2 presents the cumulative fractional release (total mass released divided by initial inventory) for 45, and 90 cm of grout with all the contamination at the bottom. Table 8.3 presents the cumulative fractional release for uniform contamination of 45 cm of grout with three clean grout cover thicknesses, 15, 30, and 45 cm. Simulation of 90 cm of contaminated grout would lead to lower cumulative fractional release because the use of a unit inventory will dilute the concentrations, and therefore release by a factor of 2 as compared to the 45 cm case.
Although non-zero numbers are calculated for this simulation, if the value was less than 1E-30 it is reported as <1E-30 in the tables that follow.
FC-24-002 Revision 0 Page 12 of 16 Table 8.2 Cumulative Fractional Release from bottom contamination migrating through 45 or 90 cm of clean cover with an initial inventory of 1 Ci.
CFR CFR Selected 45 cm 90 cm D (cm2/s)
Reference Am-241
<1E-30
<1E-30 5.00E-13 Serne, 1992 C-14
<1E-30
<1E-30 1.00E-11 Serne, 1992 Ce-144
<1E-30
<1E-30 1.00E-12 Serne, 1992 Cm-243
<1E-30
<1E-30 5.00E-13 Serne, 1992 Cm-244
<1E-30
<1E-30 5.00E-13 Serne, 1992 Co-58
<1E-30
<1E-30 4.10E-11 Muurinen, 1982 Co-60
<1E-30
<1E-30 4.10E-11 Muurinen, 1982 Cs-134
<1E-30
<1E-30 3.00E-09 Atkinson, 1986 Cs-137 5.60E-10 1.20E-19 3.00E-09 Atkinson, 1986 Eu-152
<1E-30
<1E-30 5.00E-11 Serne, 1992 Eu-154
<1E-30
<1E-30 5.00E-11 Serne, 1992 Eu-155
<1E-30
<1E-30 5.00E-11 Serne, 1992 Fe-55
<1E-30
<1E-30 5.00E-11 Serne, 1992 H-3 0.087 0.0093 5.00E-07 Szantos, 2002 Ni-59 2.6E-7 5.60E-24 1.10E-09 Jakob, 1999 Ni-63 3.62E-10 5.00E-27 1.10E-09 Jakob, 1999 Np-237
<1E-30
<1E-30 1.00E-11 Serne, 1992 Pu-238
<1E-30
<1E-30 5.00E-13 Serne, 1992 Pu-239
<1E-30
<1E-30 5.00E-13 Serne, 1992 Pu-240
<1E-30
<1E-30 5.00E-13 Serne, 1992 Pu-241
<1E-30
<1E-30 5.00E-13 Serne, 1992 Sb-125
<1E-30
<1E-30 1.00E-10 Conservative Estimate Sr-90 5.60E-23
<1E-30 5.20E-10 Sullivan, 1989 Tc-99 5.9E-8 8.0E-26 1.00E-09 Serne, 1992
FC-24-002 Revision 0 Page 13 of 16 Table 8.3 Cumulative Fractional Release from uniformly contaminated grout of 45 cm with various thicknesses of clean cover.
T 1/2 (years)
No Cover 15 cm cover 30 cm cover 45 cm cover D (cm2/s)
Am-241 4.32E+02 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 C-14 5.73E+03 0.028 2.87E-22
<1E-30
<1E-30 1.00E-11 Ce-144 7.79E-01 0.022
<1E-30
<1E-30
<1E-30 1.00E-12 Cm-243 2.85E+01 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 Cm-244 1.81E+01 0.021
<1E-30
<1E-30
<1E-30 5.00E-13 Co-58 1.94E-01 0.015
<1E-30
<1E-30
<1E-30 4.10E-11 Co-60 5.27E+00 0.021
<1E-30
<1E-30
<1E-30 4.10E-11 Cs-134 2.06E+00 0.024 2.71E-13 3.11E-24
<1E-30 3.00E-09 Cs-137 3.02E+01 0.054 3.62E-05 2.44E-08 1.66E-11 3.00E-09 Eu-152 1.34E+01 0.022 1.69E-25
<1E-30
<1E-30 5.00E-11 Eu-154 8.20E+00 0.021 1.45E-28
<1E-30
<1E-30 5.00E-11 Eu-155 4.76E+00 0.021
<1E-30
<1E-30
<1E-30 5.00E-11 Fe-55 2.70E+00 0.019
<1E-30
<1E-30
<1E-30 5.00E-11 H-3 1.23E+01 0.22 0.098 0.049 0.024 5.00E-07 Ni-59 8.00E+04 0.157 5.84E-03 2.05E-05 5.24E-09 1.10E-09 Ni-63 1.00E+02 0.06 7.62E-05 5.64E-08 8.2E-12 1.10E-09 Np-237 2.14E+06 0.03 3.2E-22
<1E-30
<1E-30 1.00E-11 Pu-238 8.77E+01 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 Pu-239 2.41E+04 0.023
<1E-30
<1E-30
<1E-30 5.00E-13 Pu-240 6.54E+03 0.022
<1E-30
<1E-30
<1E-30 5.00E-13 Pu-241 1.47E+01 0.021
<1E-30
<1E-30
<1E-30 5.00E-13 Sb-125 2.73E+00 0.0204
<1E-30
<1E-30
<1E-30 1.00E-10 Sr-90 2.90E+01 0.031 1.12E-09 1.78E-15 8.16E-25 5.20E-10 Tc-99 2.13E+05 0.15 4.71E-03 1.00E-05 1.30E-09 1.00E-09 8.4 DISCUSSION For the simulations with all the contamination at the bottom and the minimum trench depth of 45 cm (1.5 feet) the cumulative fractional release is very low. For most radionuclides the release is near zero (<1E-30). There are only six nuclides that have cumulative fractional release higher than 1E-30. They are H-3, Tc-99, Ni-59, Ni-63, Cs-137, and Sr-90. For the more mobile nuclides the cumulative fractional release ranges from 0.087 (H-3), 0.0022 (Tc-99), down to 5.6E-23 (Sr-90). Figure 8.3 shows the release of H-3, Tc-99, Cs-137, and Ni-63 during the 1000-year simulation period for the 30 cm grout cover scenario.
FC-24-002 Revision 0 Page 14 of 16 Figure 8.3 Cumulative Fractional Release for C-14, H-3, and Tc-99 for the scenario with all contamination at the bottom of the 45 cm trench Figure 8.3 shows the impacts of diffusion coefficient and half-life on release. H-3 has the highest effective diffusion coefficient and cumulative fractional release through 45 cm of clean cover takes roughly 10 years to reach 1%. The release is effectively complete after 70 years due to radioactive decay. For Ni-59 and Tc-99, the fractional release continues to increase over the 1000-year period. Ni-59 and Tc-99 have long half-lives and lower diffusion coefficients than H-3 and it takes about 1000 years before approximately 1E-5% to be released. The actinides and C-14 with lower diffusion coefficients do not release much (cumulative fractional release < 1E-30).
The second scenario, uniform contamination, has contamination at the interface of the backfill grout and the groundwater with a clean cover. If there is no cover, the distance the contaminant needs to travel before entering the groundwater is effectively zero.
Without a cover, 22% of the H-3 and 15% of the Tc-99 would be released. This supports the need for a clean cover to substantially reduce the release if uniform contamination was an accurate depiction of the contamination.
Figure 8.4 shows the cumulative fractional release for the three highest radionuclide releases, H-3, Ni-59, and Tc-99 when the 45 cm grout backfill is uniformly contaminated with 45 cm of clean grout on top of the trench. In this case, the H-3 release is effectively completed after 50 years, and the Ni-59 and Tc-99 release continues to increase. This is due to the time it takes to diffuse through 45 cm of grout for these nuclides that do not exhibit substantial radioactive decay over 1000 years. However, H-3 release is reduced to about 2% of the total inventory and Ni-59 and Tc-99 are less than 1E-5% of the total inventory.
1.0E-20 1.0E-18 1.0E-16 1.0E-14 1.0E-12 1.0E-10 1.0E-08 1.0E-06 1.0E-04 1.0E-02 0
100 200 300 400 500 600 700 800 900 1,000 Time (years)
Bottom Contamination 45 cm cover Cumulative Fractional Release Ni-59 H-3 Tc-99
FC-24-002 Revision 0 Page 15 of 16 Figure 8.4 Cumulative Fractional Release for a 45 foot uniformly contaminated grout with a 45 cm clean grout cover.
An important point for the uniformly contaminated grout without a cover is that for most nuclides the release is from the first few centimeters of the grout. Thus a grout cover will be effective at minimizing releases. For example, as a rough approximation, for most nuclides the inventory released came from within 3 cm of the surface. This is based on a maximum release of 9% for Ni-63 and 9% of the inventory in a 30 cm cover would be contained in 2.7 cm. This approximation does not hold for H-3, Tc-99, or Ni-59.
For the scenarios with a clean grout cover above the trench, as the contamination moves out of the trench region into the clean cover, the contamination could be released out the side of the cover. To ensure that the values in Table 8.3 are conservative, the minimum travel distance needs to be at least as thick as the clean cover. Therefore, to use the values in Table 8.3 for a 30 cm cover thickness, the cover would have to extend 30 cm beyond the trench.
8.5 CONCLUSION
S The DUST-MS computer code was used to simulate the release of the initial suite of nuclides through a grout backfill of the trenches at Fort Calhoun. Most of the contamination is expected to occur at the bottom of the trench and this scenario was simulated. To account for contamination of the walls, a scenario with uniform contamination throughout the backfill grout was simulated. The six scenarios simulated were:
- 1. All contamination at the bottom with 45 cm of clean grout on top
- 2. All contamination at the bottom with 90 cm of clean grout on top 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0
100 200 300 400 500 600 700 800 900 1,000 Time (years)
Uniform Contamination 45 cm Cover Cumulative Fractional Release Tc-99 H-3 Ni-59
FC-24-002 Revision 0 Page 16 of 16
- 3. Uniform contamination of 45 cm in the trench with no cover.
- 4. Uniform contamination of 45 cm in the trench with 15 cm of clean grout
- 5. Uniform contamination of 45 cm in the trench with 30 cm of clean grout
- 6. Uniform contamination of 45 cm in the trench with 45 cm of clean grout The major findings are:
For all contamination at the bottom of the trench, the grout cover is an effective barrier to release. Only H-3 had a cumulative release greater than 1% of the initial inventory (H-3 8.7%). For most nuclides the cumulative fractional release is < 1E-
- 30. Table 5.1 provides the results for this scenario.
For uniform contamination of the backfill grout, CFRs are effectively controlled by adding a grout cover. Without a cover, releases range from 22% (H-3), 15.7% (Ni-
- 59) to 15% (Tc-99) with most nuclides releasing about 2 to 3% of their inventory.
Adding a clean cover decreased the fractional release substantially. For a 45 cm clean cover, the maximum release was 2.4% for H-3 and Tc-99 had a cumulative fractional release of 1.3E-7%. Most radionuclides had a cumulative fractional release of less than 1E-30 with a 45 cm clean cover. Table 5.2 presents the results for this scenario.
For the scenarios with a clean cover, Table 8.3, to prevent migration out of the side of the clean cover, the grout cover will be extended a distance equal to the clean cover height in each direction. This will ensure that the CFR for the clean grout cover scenario will be less than the values in Table 8.3. More detailed calculations may be able to reduce this restriction.