ML23221A278
| ML23221A278 | |
| Person / Time | |
|---|---|
| Site: | WM-00110 |
| Issue date: | 08/29/2023 |
| From: | Douglas Mandeville NRC/NMSS/DDUWP/URMDB |
| To: | Udovitsch M US Dept of Energy (DOE) |
| Mandeville, Doug, NMSS/DDUWP 415-0724 | |
| References | |
| Download: ML23221A278 (6) | |
Text
Matthew Udovitsch Acting Federal Cleanup Director Moab Uranium Mill Tailings Remedial Action Project U.S. Department of Energy 200 Grand Avenue, Suite 500 Grand Junction, CO 81501
SUBJECT:
UNITED STATES NUCLEAR REGULATORY COMMISSION STAFF OPTIONS FOR MOVING FORWARD ON UNITED STATES DEPARTMENT OF ENERGY MOAB URANIUM MILL TAILINGS REMEDIAL ACTION PROJECT ALTERNATIVE FINAL COVER SYSTEM 90 PERCENT DESIGN REPORT (DOCKET WM00110).
Dear Mr. Udovitsch:
In October 2022, the U.S. Nuclear Regulatory Commission (NRC) staff provided feedback on the U.S. Department of Energy, Office of Environmental Management (DOE-EM) 90 percent design for an evapotranspiration (ET) cover system at the Crescent Junction facility. The submittal is available in the NRCs Agencywide Documents Access and Management System (ADAMS) accession number ML22262A067. The NRC staff understands the DOE-EM is pursuing these changes to the cover system to minimize the amount of infiltration through the cover system and to reduce construction costs related to the amount of imported rock in the cover system. In December 2022, DOE-EM provided responses to the NRC staffs comments (ML22336A207).
The NRC has reviewed DOE-EMs December 2022 submittal and appreciates DOE-EMs consideration of our comments. After reviewing DOE-EMs responses, the NRC staff remains concerned about the ability of the proposed design to meet the regulatory requirements in 40 CFR Part 192. Specifically, the NRC staff remains concerned about the following aspects of the design.
The ability of the ET cover to resist erosion in the long-term. The regulations in 40 CFR 192.02(a) require the control of residual radioactive materials be effective for up to one thousand years, to the extent reasonably achievable, and in any case for at least 200 years. Additionally, 40 CFR 192.02(d) requires that the site is stabilized in a manner that minimizes the need for future maintenance. Based on the NRC staffs review, the proposed rock-soil admixture is susceptible to gully formation. Formation of gullies on the cover system may require future maintenance. NRC guidance in NUREG-1623 Design of Erosion Protection for Long-Term Stabilization was based on use angular rock with rock-to-rock contact. The NRC discussed in that document that a riprap layer with the rock voids filled with soil provides similar stability characteristics. However, the DOEs proposed rock-soil admixture results in rock to soil contact, which has not been similarly evaluated for stability. The flume studies that form the basis for the design approaches in NUREG-1623 were based on angular rock, which can have different friction characteristics than rounded rock that has typically been used for erosion resistance. The combination of reducing the amount of rock-to-rock contact and the use of rounded rock results in an approach that is outside of the considerations used in the development of NUREG-1623.
August 29, 2023
M. Udovitsch 2
The ability of the cover system to protect the radon barrier. The regulations in 40 CFR Part 192.02(b)(1) require that releases of radon be limited to 20 pCi/m2s. As currently proposed, the decrease in thickness of the cover system above the radon barrier increases the potential degradation of the radon barrier due to frost, root penetration, and desiccation. A portion of the radon barrier will be located above the frost penetration depth, resulting in the potential for cracking or damage to that layer. Additionally, the shallower depth of the radon barrier increases the potential for root penetration into the radon barrier. This could result in a drying effect within the radon barrier as well as create fast pathways once the roots decay, both of which could result in an increase in radon flux. Finally, the shallow depth of the radon barrier, especially in an arid climate, increases the risk of desiccation cracking. These degradation mechanisms can create cracking and fast pathways, as well as decrease the moisture content of the radon barrier. The NRC staff observes that the radon flux calculations are particularly sensitive to the moisture content of the radon barrier. Considered together, these aspects of the cover system make it challenging for the NRC staff to reach a reasonable assurance finding related to meeting the radon flux criteria.
For these reasons, the NRC staff is highly unlikely to be able to accept the proposed changes to the remedial action plan in its current form. The NRC staff recognizes DOE-EMs interest in ET covers and offers the following suggestions or approaches to DOE-EM. One of, or a combination of these suggestions, could be incorporated into the design to facilitate NRC acceptance. Note that the NRC staffs suggestions are based on the hydrogeology, site layout, and waste characteristics at the Crescent Junction disposal cell. At Crescent Junction, the disposal cell is underlain by a thick clay layer and there are no nearby groundwater users (ML082060216). The Mancos shale was estimated by DOE to be approximately 2,400 feet thick beneath the site.
Localized pockets of water do occur within the Mancos shale however they are not potable due to very high concentrations of total dissolved solids ranging from 23,000 mg/L to 42,000 mg/L.
Additionally, the NRC staff observes that the residual radioactive material (RRM) excavated at Moab is dried and mixed before it is placed at Crescent Junction. DOE-EM has developed rigorous specifications for placement of RRM. The drying, mixing, and placement specifications results in a more homogeneous material. As a result, there is less potential for differential settlement of the cover system than is typically encountered at mill tailings sites. These site features mean the potential for erosion of the cover system and the release of radon are the more risk significant performance factors at Crescent Junction.
With these site-specific features in mind, the staff offers eight suggestions for DOE-EMs consideration. The first five suggestions are focused on changes to the design to facilitate NRC acceptance. The last three suggestions can be viewed as potential changes to reduce construction costs. The remaining suggestions are focused on the radon barrier.
Suggestion 1 - Increase quantity of rock within the ET cover DOE-EM could consider increasing the ratio of rock to soil in the admixture from a 30:70 mix by volume to a 50:50 mix (evenly split by volume), or even 70:30 mix (with rock being the predominant feature by volume). If DOE-EM follows the rocky soil slope approach outlined in NUREG-1623, the calculation should be based on the grain size analysis of the rock and soil when they are combined. Note that under this approach, minor channeling, rilling, or gully formation may occur.
Suggestion 2 - Concentrate the rock within the ET cover closer to the surface As currently designed, rock will be present throughout the 24-inch depth of the admixture. DOE-EM could consider a design change to concentrate more of the rock within the upper 6 to 12 inches of the admixture. The remaining thickness of the cover above the radon barrier could remain as soil.
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Note that while suggestions 1 and 2 are offered independently, they may be particularly effective if they are combined as they would provide more rock-to-rock contact at and near the surface of the cover system.
Suggestion 3 - Flatten the slope of the ET cover If DOE-EM continues to follow the rocky soil slope approach to verify erosion resistance, maintaining the proposed 30:70 rock to soil ratio in the admixture, but decreasing the slope angle may result in an acceptable design. Similar to suggestion 1, if the rocky soil slope approach is used, it should be based on the grain size analysis of the rock and soil when they are combined.
As discussed above, the potential for differential settlement at Crescent Junction is minimized through mixing and placement of RRM. Given the lower potential for differential settlement, the slope of the ET cover would not necessarily need to be as steep as is currently proposed in the design. This type of change may prompt a re-evaluation of the rock size on the cover system side slopes. The NRC staff recognizes that a flatter slope may result in more infiltration through the cover system, which may not be acceptable at other sites with more groundwater resources.
DOE may also consider field and/or laboratory studies, as well as analog sites, evaluating the erosion resistance of rock-soil admixtures that were not within the envelope of materials considered within NUREG-1623.
Suggestion 4 - Increase thickness of cover The NRC staffs 2008 technical evaluation of the remedial action plan for the site, considered a frost penetration depth of 45 inches (ML082060216). Keeping the radon barrier below the frost penetration depth is one way to provide adequate protection against freeze/thaw effects within the radon barrier. DOE-EM could consider increasing the overall thickness of the cover above the radon barrier. Note that the increase in thickness does not necessarily need to be the rock-soil admixture. The extra thickness can result from adding soil between the top of the radon barrier and the bottom of the admixture layer. Increasing the thickness of the cover system above the radon barrier would provide additional soil water storage capacity and rooting media for vegetation.
Suggestion 5 - Reliance on some form of maintenance The requirements in 40 CFR Part 192.02(d) states that each site on which disposal occurs shall be designed and stabilized in a manner that minimizes the need for future maintenance. DOE-EM could propose relying on a limited form of maintenance to maintain the function of the cover system. An example would be repairing gullies once they reach a certain depth.
Suggestion 6 - Using biointrusion layer to prevent deep gully formation The currently approved design calls for inclusion of a 6-inch thick rock layer buried 4-ft below the final surface to serve as a biointrusion layer. DOE-EMs proposed 90 percent design does not include a biointrusion layer. If the biointrusion layer is included in the design moving forward, DOE-EM could evaluate the ability of the buried rock layer to stop or cut off gully formation using the methodology in NUREG-1623, Appendix D, Section 3.2.2. The NRC staff recognizes the drainage area estimation used in DOE-EMs 90 percent design is based on conservative assumptions and is therefore acceptable. Calculations related to flow velocities and shear stresses within the gully should be based on peak velocities. If it can be demonstrated that a buried rock layer can cut off gully formation, any cover material present above the buried rock
M. Udovitsch 4
layer could be viewed as sacrificial and not risk significant. This approach could allow for much smaller rock sizing, or potentially the elimination of rock in the upper portions of the cover. Note that this approach may result in the presence of deep gullies on the cover system. While the gullies could be visually alarming, they would be an expected performance feature under this approach.
Suggestion 7 - Alter design approach Alternatively, DOE-EM could alter the design approach and follow the methodology for an unprotected soil slope in Section 2.2.1 and Section 2.4.1 of NUREG-1623. In following this approach, DOE-EM may be able to reduce the amount and size of rock present in the ET cover.
Given the long slope lengths at Crescent Junction, the NRC staff recognizes that this approach may result in a nearly flat slope. As mentioned in suggestion 3, this approach could warrant a re-evaluation of the rock size on the cover system side slopes.
Suggestion 8 - Varying admixture design based on slope length As currently designed, the admixture assumes one rock size over its entire slope length. The calculations are based on the longest slope length of approximately 1300 ft. The NRC staff recognizes this approach is conservative, especially in areas of the cover with shorter slope lengths. DOE-EM could consider re-evaluating the admixture design for the north facing slope of the ET cover, which has a maximum slope length of approximately 566 ft. Alternatively, DOE-EM could consider re-evaluating the admixture design for a set distance on the upper portion of any slope. For example, the admixture on the upper 200 ft of slope length could be re-evaluated and the admixture on the lower portion of the slope could remain as proposed in the 90 percent design.
The NRC staff is available to clarify its comments, if necessary. While these suggestions are largely focused on addressing erosion resistance of the ET cover, any changes DOE-EM pursues should consider and balance the overall performance objectives of the site. The NRC staff looks forward to working with DOE on this revision to the remedial action plan for the Moab project.
A copy of this letter will be available electronically for public inspection in the NRC Public Document Room or from the Publicly Available Records System component of the NRC's ADAMS. ADAMS is accessible from the NRC Web site at http://www.nrc.gov/reading-rm/adams.html.
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If you have any questions regarding this letter, please contact me at (301) 415-0724, or by e-mail at douglas.mandeville@nrc.gov.
Sincerely, Douglas Mandeville, Sr. Project Manager Uranium Recovery and Materials Decommissioning Branch Division of Decommissioning, Uranium Recovery, and Waste Projects Office of Nuclear Material Safety and Safeguards Docket No.: WM00110 cc: C. Pulskamp (DOE) Moab Mill ListServ Signed by Mandeville, Douglas on 08/29/23
Ltr ML23221A278 OFFICE NMSS/DUWP/URMDB NMSS/DUWP
/URMDB NMSS/DUWP/URMDB NAME DMandeville RVon DMandeville DATE Aug 21, 2023 Aug 22, 2023 Aug 29, 2023