ML19199A175
| ML19199A175 | |
| Person / Time | |
|---|---|
| Site: | WM-00064 |
| Issue date: | 10/11/2019 |
| From: | Douglas Mandeville Division of Decommissioning, Uranium Recovery and Waste Programs |
| To: | Janice Nguyen US Dept of Energy, Office of Legacy Management |
| Mandeville D | |
| References | |
| Download: ML19199A175 (5) | |
Text
October 11, 2019 Mr. Jason Nguyen U.S. Department of Energy Office of Legacy Management 2597 Legacy Way Grand Junction, CO 81503
SUBJECT:
LAKEVIEW ROCK DEGRADATION MONITORING PROGRAM
Dear Mr. Nguyen:
By letter dated March 2, 2015, the U.S. Department of Energy (DOE) submitted a request to the U.S. Nuclear Regulatory Commission (NRC) staff to discontinue rock degradation monitoring at the Lakeview, Oregon Uranium Mill Tailings Radiation Control Act (UMTRCA), Title I, Disposal Site (Agencywide Document Access and Management System [ADAMS] Accession No. ML15068A252). DOEs 2015 letter also provided responses to a series of comments from the NRC staff dated November 12, 2014 (ADAMS Accession No. ML14303A623).
As discussed in more detail below, the NRC staff has no additional comments or concerns about DOEs decision to cease rock degradation monitoring and modify its long-term surveillance plan (LTSP) to perform more focused visual inspections on the west side slope.
This letter also reviews past efforts related to understanding the rock degradation issue at the Lakeview site; further details the NRC staffs position on DOEs request to discontinue rock degradation monitoring; and provides suggestions for DOE to consider moving forward with regard to this issue.
History of Rock Degradation at Lakeview During construction of the west side slope of the disposal cell in 1988, it was acknowledged that weathering of rock on the side slope of the disposal cell (also referred to as Type B riprap) would likely occur and accelerate degradation of the cover system. This weathering results from the presence of clay minerology within the basalt rock chosen for use in the cover system.
Therefore, the LTSP for the Lakeview disposal cell included monitoring the particle size of the riprap (referred to as D50 monitoring in the remainder of this letter) to generally quantify the rate of rock degradation over time and to compare the measured D50 value to the design specification, which is a D50 of 2.7 inches to 3.9 inches. D50 monitoring is performed during the annual site inspection and the test results are included in DOEs annual inspection report.
In 2009, DOE initiated a rock durability monitoring procedure as part of the rock degradation monitoring program. DOE and NRC collaboratively developed the rock durability monitoring procedure. The purpose of durability monitoring was to identity the durability class of the rock present on the side slope. The durability classes range from durable, susceptible to near-term degradation, to nondurable (i.e. rocks that have already crumbled). This rock durability monitoring has been implemented at the D50 monitoring locations that are randomly selected
J. Nguyen prior to each monitoring event. This durability information along with the D50 data facilitated the documentation of the condition of the existing rock cover.
The results of the D50 monitoring in 2018 showed an average D50 of 2.53 inches, which was within the range of D50 values (2.26 to 2.88 inches) measured over the last 22 years. However, the average D50 of 2.53 inches is lower than the design specification D50 of 2.7 to 3.9 inches. In the 2018 annual monitoring report, DOE also performed a statistical analysis of the D50 values measured over the last 22 years. While DOEs analysis did not identify a statistically significant trend, the measured D50 values typically fall at or below the minimum D50 design specification of 2.7 inches. At the NRC staffs suggestion, DOE collected 10 samples of the riprap during the 2010 annual site inspection (ADAMS Accession No. ML110180360). DOE had an off-site laboratory perform a particle size analysis in accordance with ASTM method D5519. The purpose of this effort was to obtain the field D50 based on weight and not the number of rocks retained. The results indicated that the mean D50 was 2.24 inches. The D50 value obtained using ASTM D5519 is based on weight, which is how the D50 value is typically specified in an engineering design.
DOE Request to Discontinue Rock D50 and Durability Monitoring In its 2015 letter, DOE summarized its rock degradation monitoring experience and the current condition of the disposal cell at Lakeview. In its correspondence, DOE:
- Recognized the limitations of the rock D50 monitoring.
- Discussed the conservatism associated with the design D50 range of 2.7 to 3.9 inches.
- Acknowledged that the rock on the west side slope is degrading.
- Stated that the that Lakeview disposal cell continues to meet the requirements of 40 CFR Part 192. DOEs statement was based on observations that the erosion protection was intact and functioning properly during the 2014 annual inspection and the lack of water infiltration identified during the 2010 geoprobe field investigation.
For these reasons, DOE proposed replacing rock degradation monitoring with more focused visual inspections along the west side slope of the disposal cell. Specifically, DOE proposes to revise the inspection checklist in the LTSP to include visual documentation of any erosion rills that form along the west side slope. This would include taking photographs of the rills, mapping their location, inspecting the condition of the erosion protection downslope of a rill, and making repairs as warranted. Because this augmented inspection approach more directly focuses on the potential development of vulnerabilities on the side slope (including those associated with rock degradation), DOE proposes to discontinue the annual rock D50 and durability monitoring.
The NRC staff reviewed DOEs letter, the past D50 and durability monitoring results, DOEs responses to previous NRC staff comments and the LTSP for the Lakeview disposal cell. For the rock durability monitoring, the NRC staff recognizes that this inspection activity was never incorporated into the LTSP. The NRC staff reviewed the rock durability data and concludes that the results have been helpful in documenting the rock durability class present at the site. Given the sampling methodology, the NRC staff recognizes that collection of additional rock durability data would not significantly enhance the understanding of rock durability at the site. Therefore, the NRC staff has no further comments or concerns with DOEs decision to terminate this activity during the annual site inspection.
J. Nguyen For the rock D50 monitoring, the NRC staff recognizes the limitations of the current approach.
The procedure used to measure the D50 is based on a count of the number of rocks retained on each sieve. This is different from the approach used in laboratory testing, such as the ASTM D5519 procedure, where the material retained on each sieve is weighed. The NRC staff performed its own statistical analysis of the data presented in the 2018 monitoring report and agrees that the data do not suggest a statistically significant trend in the D50 values. While the lack of a trend does not indicate a stable D50 value, it does indicate that the D50 values have not decreased by a statistically significant amount in the past two decades. However, the NRC staff notes that the current procedure, measuring the D50 value at different locations every year is potentially better suited to documenting the D50 values across the side slope than the estimating the rate of degradation.
In its November 12, 2014 letter, the NRC staff suggested that DOE consider mapping the D50 results to identify potential areas of the cover that may not be protective. In its March 2, 2015 response, DOE stated that its position that the original gradation and durability monitoring data were not intended to be used in this way, and such use could result in magnifying the data limitations identified in this letter. Thus, especially given the lack of agreement on how to use the rock D50 data, the NRC staff has no further comments or concerns with DOEs proposal to terminate the D50 measurements, concurrent with its proposed visual inspections, and the observations below.
The NRC staff believes that a focused visual inspection will allow for the identification of problems with the west-side slope cover. Additionally, the NRC staff understands that the LTSP states that DOE will provide an assessment of the development of any erosion rills or gullies on the cover system within 60 days of their identification, in addition to the obligations associated with DOEs annual inspection program and performing any necessary repairs.
Cover Observation in the Future While the NRC staff does not have any additional comments on DOEs decision to discontinue rock D50 and durability monitoring at Lakeview, the cessation of these activities does not change or mitigate our concerns regarding the presence of poor-quality rock at Lakeview. The NRC staff recognizes that DOEs design procedures and decision to double the rock thickness on the side slope likely resulted in a sufficiently robust cover in the near-term. However, the D50 monitoring has shown that the in-place D50 on the side slope is frequently smaller than the value specified in the design specification. Thus, it is possible that the existing conditions are not as robust as was envisioned at the time of construction of the cover system.
In its March 2, 2015, letter, DOE states that the Lakeview disposal cell continues to meet the criteria in 40 CFR 192. DOE basis its statement on the observations made during the annual inspections that the erosion protection is intact and functioning properly. The NRC staff recognizes that the past annual inspections have verified adequate performance of the cover system to date. However, these inspections and observations do not reflect the impacts of ongoing degradation and potential future events. The NRC staffs concern going forward remains that the current cover has not been demonstrated to be sufficient over the timeframes identified in 40 CFR Part 192. To provide the NRC staff with continuing assurance that the cover will be effective for the required timeframes, DOE should consider one or more of the following approaches:
- Calculate the minimum D50 required on the west side slope to meet the required timeframes in 40 CFR Part 192, based on the current conditions on the top slope of the
J. Nguyen disposal cell, while considering newer analytical techniques for calculation of the D50 and determination of the appropriate probable maximum precipitation (PMP) event. If DOE decides to perform this calculation, the potential for flow concentrations should be taken into consideration as well. This is discussed further in our December 3, 2009 letter and technical evaluation report (ADAMS Accession Nos. ML093220639 and ML093220669).
- Construct a rock apron at the intersection of the side slope and top slope to reduce flow concentrations.
- Place additional riprap on the side slope of the disposal cell.
In accordance with 10 CFR 2.390 of the NRCs Agency Rules of Practice and Procedure, a copy of this letter will be available electronically for public inspection in the NRC Public Document Room or from the Publicly Available Records component of NRCs Agencywide Documents Access and Management System (ADAMS). ADAMS is accessible from the NRC Web site at http://www.nrc.gov/reading-rm/adams.html.
If you have any questions, please contact me at (301) 415-0724, or by e-mail, at douglas.mandeville@nrc.gov.
Sincerely,
//RA//
Douglas T. Mandeville, Project Manager Uranium Recovery and Materials Decommissioning Branch Division of Decommissioning, Uranium Recovery and Waste Programs Office of Nuclear Material Safety and Safeguards Docket No.: WM-64