ML20234D053
| ML20234D053 | |
| Person / Time | |
|---|---|
| Issue date: | 11/16/1987 |
| From: | NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| To: | |
| Shared Package | |
| ML20234D012 | List: |
| References | |
| REF-WM-48 NUDOCS 8801060425 | |
| Download: ML20234D053 (33) | |
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WM-48/SRG/87/11/12/0 p 161987 not been finalized, the NRC's finding of. compliance with ground-water standards must be deferred.
Staff review of additional information will be presented as a supplement to the TER and will include the NRC concurrence position on the proposed.
remedial action.
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Scott R. Grace, Project Manager Licensing Branch 1 Uranium Recuvery Field Office Approved by:
Edward F. Hawkins, Chief Licensing Branch-1 Uranium Recovery Field Office, Region IV
Attachment:
As stated Case Closed: 040WM048850E 8801060425 871127 PDR WASTE WM-48 PDR JM me
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NAME : : T01sen :TJohnson : PGarcia : EHawkins : :
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Final Technical Evaluation Report For The Proposed Remedial Action At The Durango Tailings Site Durango, Colorado I
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Prepared by Uranium Recovery Field Office !
U.S. Nuclear Regulatory Commission i i
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Table of Contents Section P, age 1.0 Introduction........... ............... .. ... ....... .. 1 !
2.0 Geology / Seismology....... ..................... ....... ... 2 2.1 Disposal Site Geologic Characterization. . .......... 2 2.2 Seismotectonic Site Characterization...... ...... .... 3 2.3 Seismic Design. .......... ........ .. .. ..... . 4 2.4 Conclusion...... . . .. ........ ... .. . . ...... 4 l 3. 0 Water Resources. . ... . . . .... .. . ..... . 4
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3.1 Surface-Water Characterization - Processing Site. . 4 i
3.1.1 Surface-Water Quality - Processing Site. . .. 4 3.1.2 Surface-Water Impacts and Restoration-Processing Site .... ... .. .... . .... ... . 5 3.2 Ground Water - Processing Site. . . . . . . . . . .... ... . 5 3.2.1 Ground-Water Characterization - Processing Site. 5 3.2.2 Ground-Water Quality - Processing Site. . . .. 6 l
3.2.3 Ground-Water Impacts and Restoration-Processing Site. .... ... .. ... ... . 6
- 3. 3 Surface Water - Disposal Site. . . . . .. ... 7 3.3.1 Surface-Water Characterization - Disposal Site.. 7 3.3.2 Geomorphic Considerations.... ..... ..... .... 7 3.3.3 Surface-Water Quality - Disposal Site. . ... 7 3.3.4 Surf ace-Water Impacts - Disposal Si te. . . . . . . . . .. 7 3.4 Ground Water - Disposal Site .......................... 8 3.4.1 Ground-Water Characterization - Disposal Site... 8 3.4.2 Ground-Water Quality - Disposal Site. . . . . . . . . . . . 9 3.4.3 Ground-Water Im 9
- 3. 5 Conclusions........... ...................
pacts - Disposal. Site. ....... ..
..... .... 11 3.5.1 Processing Site.... ...... ...... ... .... . 11 3.5.2 Disposal Site.. ............. .... .. . . ... 11 4.0 Surface-Water Hydrology and Erosion Protection..... ...... . 12 )
4.1 Flooding Determinations......... ................... .. 12 I 4.2 Upstream Dam Failures.................... ....... .... 13 4.3 Design of Erosion Protection...................... .. . 13 4.4 Conclusion........................................ .. 14
- 5. 0 Geotechnical Stability................................ ..... 15 ,
5.1 Geotechnical Site Characterization............. ....... 15 5.2 Soil Properties.. ............... .... . . ............ 15
- 5. 3 Slope Stability.. ..... ........... .... ............. 16 i
s Table of Contents (cont.)
Section- Page
- 5. 4 Settlement........................................ .. . 17
- 5. 5 Liquefaction....................... .... ... .... . ... 18 i 5.6 Construction Criteria.................. . .. .......... 18
- 5. 7 Conclusion........................ ..... .......... . . 19 6.0 Radon Attenuation and Site Cleanup........... ...... ..... 19 j 6.1 Characterization of Tailings and Cover Material. . ... 19 6.2 Radon Attenuation... .. ................ .... ...... 19
- 6. 3 Site Cleanup...... . . .... .. .. .. ... .... 20 6.4 Conclusion......... ....... ..... . .. . .. ... . .. 20
- 7. 0 S u mm a ry . . . . .. . . ......... .. ............. . .. ..... .. 20 REFERENCES BIBLIOGRAPHY
1.0 INTRODUCTION
The Durango site was designated as one of 24 sites to be reclaimed by the Department of Energy (00E) under the Uranium Mill Tailings Radiation Control Act (UMTRCA) of 1978. UMTRCA requires that the Nuclear Regulatory Commission (NRC) concur in the selection and performance of remedial actions at the 24 sites. The purpose of this report is to document the NRC staff technical review of DOE's proposed remedial action for the Durango site.
The Durango processing site is located in southwest Colorado, approximately one-half mile southwest of the city of Durango, on the west side of the Animas River and about 21 miles north of the Colorado-New Mexico border. The location is shown on Figure 1 which shows the 25-acre mill site (tailing piles) and 45-acre raffinate pond area.
The remedial action proposed by DOE consists of relocation of an estimated 96,700 cubic yards of off pile contaminated material and 2,403,000 cubic yards of tailings pile materials to the Bodo Canyon site located about 2 miles southwest of Durango, Colorado (Figure 2). The contaminated material will be placed behind an embankment and covered with a soil layer to attenuate radon and a rock layer to protect against erosion. Drainage ditches will be provided to direct runoff away from the site.
The proposed remedial action must comply with standards established by the Environmental Protection Agency (EPA). These primary design standards are summarized below:
(a) The reclamation is to be effective for 1000 years, to the extent practicable, but in any case for a minimum of 200 years.
(b) The radon flux from the contaminated materials must be reduced to less than 20 picocuries/ square meter /second.
(c) To determine necessity for cleanup, radium-226 concentrations in soil, averaged over 100 square meters, must be reduced to 5 picocuries/ gram above background in the top 15 centimeters of soil and to 15 picocuries/ gram above background in any subsequent 15 centimeter layer.
Specific ground-water standards for the remedial actions will be established by EPA in the near future. Final decisions regarding the need for ground-water restoration activities at the processing site will be made following promulgation of the standard. Other requirements of the standards and UMTRACA include such things as provisions for monitoring and surveillance, minimization of the need for future
2 maintenance, ownership of the tailings and site by the Federal government, State participation in the development and construction of the remedial actions and licensing of the site by the NRC.
The staff review of the proposed remedial action included reviews of the following documents:
(a) Remedial Action Plan (RAP) - draft dated June, 1985 (Reference 1),
and preliminary final dated June, 1986 (Reference 2).
(b) Environmental Impact Statement - draft dated October, 1984 (Reference 3), and final dated October,1985 (Reference 4).
(c) Disposal Site Characterization Report (DSCR) - draf t dated May,1985 (Reference 5).
(d) Processing Site Characterization Report (PSCR) - draft dated June, 1984 (Reference 6).
(e) DOE responses dated March 21, 1986, to previous NRC comments (Reference 7).
(f) Design documents preliminary dated November 1985 (Reference 8);
final (Volumes I, II and III) dated April 1986 (Reference 9);
revision volume dated June 1986 (Reference 10) and Subcontract Documents, Final Design for Construction dated August 1986 (Reference 11).
The staff review of the proposed remedial action is based primarily on the Standard Review Plan (Reference 12) developed by the NRC.
2.0 GE0 LOGY / SEISMOLOGY 2.1 Disposal Site Geologic Characterization The Bodo Canyon disposal site is situated on a Hogback Monocline along the southwest boundary of the San Juan Basin. At Bodo Canyon, bedrock dips to the southeast at 5 to 15 degrees, but further to the ,
south, the dip angle increases to approximately 35 degrees.
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A northeast trending fault occurs along the south ridge of Bodo Canyon. Several smaller faults also occur which have displacements of 10 feet or less. The smaller faults are believed not to be associated with the fault that occurs along the south ridge. There is also a major fault which lies west of the Bodo Canyon site. This fault has a west trending displacement, but is located approximately l 2,000 feet west of the site. The fault occurrences in this area are l
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probably related to the Monoclinal structure which forms the ridges; in the area of the Bodo Canyon site. Bedrock formations that occur '
in the Bodo Canyon area include the Cliff-House Sandstone, Menefee Formation, Point Lookout Sandstone and Lewis Shale. -All of these formations are Upper Cretaceous in age.
The Cliff 9.use Sandstone, the first formation underlying the site, exhibits tuo distinct units. The lower unit is made up of sandstone beds up to 3 feet in thickneso which form the ridges north _of the site. The lower unit also exists as a bedrock slope along the south drainage at the' site. The upper unit acts as a cap' rock'along the ridges to the north and is made up of thinly bedded sandstoner sequences.
2.2 Seismotectonic Site Characterization In the Bodo Canyon area, faults have displaced Cretaceous age' geologic units. However, the age of.these faults has not been determined due to the complex structure and. lack.of exposures for mapping purposes. Mappable faults suggest that the movement' associated with these systems is small,'with little surface expression observed.
Aerial photographs of the Bodo Canyon area indicate linear features which are associated with structural trends. However, ground surveys do not show any displacement of Quaternary deposits to support structural movement. This would indicate that the potential for onsite fault rupture or ground motion due to-local earthquake activity associated with faults in the site vicinity is extremely small. Historical data shows that a seismogenic feature was measured at a magnitude of 7.5 near Dulce, New Mexico. From this measurement, the Maximum Credible Earthquake (MCE) was. calculated and found to be 0.09g.
Regional seismic hazard analysis is associated with undesignated faults within the Colorado Plateau / Western province transition. zone and Rio Grande Rift, known faults within the Colorado Plateau, faults in the Uncompaghre Uplift and salt anticlines; and the j unidentified seismic source in the Dulce,-New Mexico area. .No data is available to calculate recurrence relationships for these features. However, historical earthquake data was used to make a probabilistic analysis based on work by.Chaing and others (Reference 13). This approach indicated that for the 1000 year. time {
interval, an acceleration greater than 0.12 g is very unlikely or )
the probability is very low,'but is not zero.
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4 2.3 Seismic Design The DSCR (Reference 5) provided a detailed and conservative determination of seismic design parameters based on a detailed evaluation of best available regional and site specific active faulting data which includes the following: aerial photography, remote sensing, ground survey, field investigation, geologic information, NOAA earthquake data files, computer analysis, references, and published data.
2.4 Conclusion A review of geologic conditions existing at the site preclude the likelihood of poor foundation materials, ground settlement, and hazard due to slope instability or creep. A review of seismotectonic data indicates little or low probability that ground accelerations at the site could exceed the design earthquake. The staff concludes that impacts from local geologic and seismic hazards at the disposal site have been and will continue to be minimal.
3.0 WATER RESOURCES 3.1 Surface-Water Characterization - Processing Site Perennial surface waters in close proximity to the Durango processing site include the Animas River and its tributary, Lightner Creek. An ephemeral stream (possibly intermittent), South Creek, runs west to east across the southern end of the raffinate ponds area (South Creek is referred to as the North drainage in the disposal site area). The Animas River flows north to south within 100 feet of the tailings piles and raffinate ponds (Figure 1).
Lightner Creek flows across the northern part of the designated site boundary from the northeast to the east into the Animas River.
3.1.1 Surface-Water Quality -
Processing Site A 1 year water quality sampling program was conducted on the Animas River, South Creek, and the Bodo Canyon drainage. Sampling locations are shown on Figure 2. The results indicate (Reference 3) that for the Animas River that the water is a calcium-bicarbonate sulfate type and that concentrations of iron, manganese, lead, ammonia, radium, and gross alpha all exceeded either State or Federal drinking water standards at least once during the sampling period.
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5 3.1.2 Surface-Water Impacts and Restoration - Processing Site The Durango processing site does not significantly contribute to increases in water quality in the Animas River. There is no indication that there is a relationship between concentrations of constituents in the Animas River above and below the site.
l Therefore, no water quality evidence supports a conclusion of noticeable contributions of contaminants to the Animas River from i the Durango processing site. This is probably due in large part to I
the large volume of flow of the Animas River that effectively dilutes any contaminant seepage. Relocation of the tailings will effectively eliminate the source of any surface water contamination.
3.2 Ground Water - Processing Site 3.2.1 Ground-Water Characterization - Processing Site There are two northeast-trending faults in the area of the processing site that control the occurrence of the uppermost hydrostratigraphic units. One fault occurs in the middle of the raffinate ponds area and the other between the ponds and the tailings piles. The tailings and i inate ponds areas are underlain by up to 60 feet of alluvia , colluvium and fill.
Underlying this upper layer at the tailings is about 1900 feet of Mancos Shale. Under the alluvium / colluvium at the raffinate ponds area is about 400 feet of the Point Lookout Sandstone on the western flank of the fault and about 250 feet of the Menefee Formation on the eastern (downdropped) flank of the fault.
The main aquifer identified beneath the processing site area is the shallow alluvial / colluvial aquifer. Recharge to this aquifer is from infiltration of precipitation and surface runoff as well as inflow from the surface water drainages during periods of high flow.
Discharge from the aquifer is primarily to the Animas River. The areal extent of this water table aquifer is limited, as its borders 1 are Lightner Creek to the north, South Creek to the south, the l Animas River to the east and Smelter Mountain on the west. The faults that occur between the tailings piles and raffinate ponds area, and within the raffinate ponds area, affect the flow of ground water in the alluvial system. However, there is insufficient information to determine to what degree the faults act as vertical and horizontal conduits of ground water.
Below the local alluvial / colluvial aquifer system is a deeper, more regional system. The deeper, regional aquifers are recharged by direct infiltration of precipitation in their outcrop areas and by 1
6 flow from the overlying alluvial aquifers as well as vertical flow via the fault systems. The direction of ground-water flow in the uppermost systems are reported to be to the south-southeast.
There are no known users of ground wate- from the shallow aquifer system downgradient of the processing site. However, there is inadequate information to assess users of ground water from the deeper aquifers.
3.2.2 Ground-Water Quality - Processing Site Due to the limited areal extent of the alluvial / colluvial aquifer system, no uncontaminated upgradient wells occur, and background water quality in the processing site area has not been determined.
However, there is evidence of contaminant increases downgradient of the processing site. There are elevated concentrations of uranium, selenium and sulfate downgradient of the small tailings pile.
Monitoring wells downgradient of the large pile show elevated concentrations of uranium, arsenic, molybdenum, vanadium and selenium. Downgradient of the raffinate ponds area, the shallow aquifer shows elevated concentrations of uranium, chromium, selenium, sulfate and vanadium.
Analysis of ground-water samples collected from wells completed in the bedrock beneath the raffinate ponds area indicates that contaminants have migrated downward into the fault zone. At depth in the fault zone, contamination by uranium, selenium, cobalt and sulfate has been documented (Reference 2).
3.2.3 Ground-Water Impacts and Restoration - Processing Site As described in the previous section, contamination of the alluvial ground water at the processing site has occurred in the vicinity of the tailings piles and raffinate ponds. Part of the alluvial ground water flows into the Animas River and is effectively diluted (see section 3.1.2). It is unknown to what extent contaminated ground water is flowing downward into the deeper, more regional aquifers either via the fault or other avenues. Even though most of the contamination is probably being carried to the Animas River via the shallow aquifer system and effectively diluted, there may be an exposure pathway for contaminants to human and environmental populations. Impacts, however, cannot be a s essed until such time as the geohydrology of the fault zone and lower aquifers is further addressed by DOE, and when final ground-water standards are promulgated by EPA.
7 3.3 Surface Water - Disposal Site-3.3.1 Surface-Water Characterization -
Disposal-Site The disposal site is located in a small sub-basin near the upper west end of the Bodo Canyon drainage that is subject to intermittent flow only. Maximum relief of the drainage is about 90 feet. The disposal site watershed drains north into the ephemeral drainage that runs eastward into the Animas River, just south.of the processing site. This drainage is referred to as the North drainage in the vicinity of the disposal site, but is referred to as South Creek in the vicinity of the processing site. The North drainage / South Creek generally contains' flow April through July and is subject to fluctuations in flow caused by in'ense rainstorms.
3.3.2 Geomorphic Considerations The geomorphic setting at tne site is relatively unstable. The slopes in the area are generally steep and gullied. There are several naturally occurring channels which have the potential to headcut or erode and will require erosion protection to prevent exposure and possible displacement of the reclaimed tailings.
3.3.3 Surface-Water Quality - Disposal Site Surface waters from the North drainage / South Creek and main Bodo Canyon drainages are classified as magnesium-sulfate to mixed cation-sulfate types. No surface water samples have been taken at the disposal site due to its ephemeral nature.
3.3.4 Surface-Water Impacts -
Disposal Site Due to the ephemeral nature of the disposal area drainages and the fact that the whole disposal site watershed will be covered by the-tailings cell, surface-water impacts will be minimal. Since the ;
tailings cell will cover the whole watershed, almost all precipitation will result in eventual surface water runoff. There-will be some infiltration into the filter bed below the riprap.
Most of the water should flow laterally and discharge to the surface water diversion ditche.s. However, some of the water infiltrating the filter bed will also infiltrate into the radon barrier cover and tailings.
It can be reasoned that surface-water _ quality would be better than when ground-water recharge occurred, as the increased volume of runoff would tend to dilute surface wuter. Once leaving the .
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disposal area watershed, it is not known if this surface discharge will remain as surface flow or recharge some shallow aquifer in the ephemeral stream valleys.
3.4 Ground Water - Disposal Site 3.4.1 Ground-Water Characterization - Disposal Site The uppermost strata exposed in the vicinity of the Bodo Canyon disposal site is the Cliff House Sandstone. The Cliff House consists of interbedded calcareous sandstone, siltstone and silty shale. The disposal site is directly underlain by sandy shale and siltstone strata of the Cliff House (thickness of about 300 feet).
Underlying the Clif f House is the Menef ee Formation. The Menefee consists of a complex assemblage of lenticular sandstone beds, shale, siltstone and coal beds. Tne Menefee ranges from 250 to 350 feet in thickness and contains the major coal resources of the area. Below the Menefee is the Point Lookout Sandstone, a thin-to-massive, 400-foot thick sandstone unit. Below the Point Lookout is the Mancos Shale.
The Bodo Canyon disposal site is located in an area of regional ground-water recharge and can be divided into several hydro-stratigraphic units. Of primary importance are the upper two units. The upper unit consists of sandy and silty clay alluvial and colluvial sediments and the upper fractured and weathered silty shale strata of the Cliff House Sandstone. The second hydro-stratigraphic unit occurs below the weathered Cliff House and into the Menefee Formation.
The upper hydro-stratigraphic unit occurs primarily within or beneath the thicker alluvial / colluvial deposits along the central drainage-way at the disposal site. The main water transmitting zone of the upper unit is probably the weathered sandy shale of the Cliff House which appears to be at least partially confined by overlying silts and clays. Flow directions of this upper hydro-stratigraphic unit are reported (Reference 5) to follow surface contours in the immediate area which is north-northeast toward the North drainage / South Creek. However, after review of the data supplied in the supporting documents (References 2, 3, 4 and 5), it is inconclusive as to which direction ground water of the upper unit flows. Seasonal fluctuations in water levels of the upper aquifer range from 9 to 28 feet with the lower water levels occurring in February / March and the high water levels occurring in May/ June.
Depth to water ranges from 19 to 68 feet.
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9 The second hydro-stratigraphic unit is a confined unit which occurs below the weathered bedrock, within the intermittent sandstone strata and fracture zones of the Cliff House Sandstone. Seasonal depth to water ranges from 31 to 75 feet with a seasonal fluctuation of up to 33 feet. Flow directions in the upper two hydro stratigraphic units are reported (Reference 5) to coincide with the surface drainage of the area. However, as discussed above, the supporting evidence is inconclusive.
Hydraulic conductivity values (from Table B.3.1, Reference 2) determined for the alluvium / colluvium of the upper aquifer system ranges from 2.8E-7 cm/s to 4.6E-4 cm/s and values from the fractured 6nd weathered shale of the Cliff House Sandstone range from 7E-6 cm/s to 2E-3 cm/s. Mean values are 8.6E-5 cm/s and 3.9E-4 cm/s, respectively. Values of hydraulic conductivity for the unweathered sandy shale of the Cliff House ranges from 1.6E-4 cm/s to IE-7 cm/s with a mean value of 1.7E-5 cm/s. The range of values for the sandstone strata and fracture zones within the Cliff House are 7E-6 cm/s to 1.1E-3 cm/s with a mean of 2.5E-4 cm/s.
There are no users of ground water within the disposal site and three domestic and stock wells within a 1-mile radius of the disposal site (Reference 2). However, the hydro-stratigraphic location of these wells is not related to the shallow aquifer systems of the disposal site.
3.4.2 Ground-Water Quality - Disposal Site The water of the upper hydro-stratigraphic unit, the alluvial colluvial /weatheced shale unit is primarily a calcium-sulfate / bicarbonate type. The water of the second I hydro-stratigraphic unit, the Cliff House Sandstone, is primarily a l
sodium-magnesium / bicarbonate type.
In the upper uait, water quality parameters that exceed Federal or State drinking water standards include iron, lead, manganese, chromium, barium, sulfate, combined radium-226 and -228, gross alpha and total dissolved solids. In the lower two units, water quality parameters which exceed Federal and State drinking water standards
, include iron, lead, manganese, chromium, barium, sulfate, ammonia, l total dissolved solids, combined radium-226 and -228, gross alpha l and gross beta.
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3.4.3 Ground-Water Impacts - Disposal Site The tailings disposal cell at Bodo Canyon is to be designed to hydrologically isolate the tailings from the ground water to the
4 10 extent practicable, including minimizing migration of contaminants from the pile to the ground water. The tailings cell is designed to have a 2-foot thick low permeability base and a 6-foot thick low permeability radon barrier cover. Flow through the cover, the tailings, the bottom clay liner and the strata above-the water table will be under unsaturated flow conditions.
Of initial concern is the time for infiltration of water through the cover as this will control the timing and amount of water flux through the tailings and to the ground water. The DOE is conducting analyses of the timing and rate of infiltration of the radon barrier cover due to the uncertainty of their analysis of infiltration in the RAP (Reference 2). The RAP analysis utilized a Darcian (saturated) flow equation to evaluate unsaturated flow. Although Darcian flow equations can be used for evaluation of such covers, there is considerable uncertainty, even when conservative data are used. The DOE is presently considering the use of a computer model to estimate the timing and rate of infiltration of the radon barrier cover. The one-dimensional unsaturated flow model is being evaluated to determine if it will adequately predict radon barrier infiltration. If this model is acceptable to DOE for use, they intend to re evaluate cover infiltration, to estimate the potential for leachate and the resultant discharges to the ground water.
The staff evaluation of time for infiltration and saturation of the 6-foot radon barrier cover, using Darcian infiltration, is calculated to be on the order of 500 years. The staff evaluation used the modified Millington-Quirk method (Reference 14), which uses the results of capillary moisture testing to calculate an equivalent saturated hydraulic conductivity which can then be used in Darcian analysis of flow. The staff evaluation used data from the RAP 1 (Reference 2), but a more conservative value of hydraulic conductivity. As discussed in the RAP, analyses of the clay.
material selected for use for the cover indicate unsaturated hydraulic conductivities values on the order of 1E-9 cm/s to 1E-15 cm/s, using the modified Millington-Quirk method. For greater conservatism, a value of IE-8 cm/s and a unit gradient of one was used to calculate a Darcian infiltration of 0.0022 cm/yr, for a total estimated infiltration and saturation time of greater than 500 years.
As discussed, there are uncertainties with the use of Darcian equations. However, with the amount of conservatism applied in the staff analysis, there is some confidence that significant infiltration and resulting seepage from the tailings cell to the ground water will not occur within a 500 year timeframe.
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11 Additionally, giving greater certainty to the staf f analysis is the fact that as a result of limiting recharge to the ground water below the tailings cell by covering the recharge basin with the cell, there will be a tendency for the lowering of the water table. As the water table lowers, the ground-water flow rates will also tend to decrease in the upper aquifer system. Although the characterization of the ground-water flow direction is inconclusive at the disposal site, the lowering water table will result in longer travel distances and greater time periods for contaminants that do seep from the tailings cell.
As previously mentioned, there is limited use of ground water in the vicinity of the disposal site. These users should have no adverse impact on their use of ground water as a result of the disposal of tailings at the Bodo Canyon disposal site. However, due to the uncertainties, DOE will provide the NRC with additional information on the potential for infiltration of the cover and subsequent saturation of the tailings.
3.5 CONCLUSION
S 3.5.1 Processing Site Due to inadequate characterization of the processing site, an assessment of the proposed action cannot be made for the protection of water resources in the vicinity of the processing site. However, since the source of the contamination, the tailings, are being relocated, additional contamination of the ground water will not occur. Because the rate, direction and areal extent of contaminated I ground-water flow in the deeper aquifers and fault zone has not been j fully characterized, the human health and environmental impacts of I any existing contamination cannot be evaluated at this time. I Additionally, since the EPA standards for ground-water protection are not yet finalized, a finding of compliance with the standards must be deferred.
i 3.5.2 Disposal Site DOE plans to re-evaluate potential infiltration and subsequent j saturation of the tailings and cover. Therefore, the staff are i deferring their final conclusions on ground water at the disposal I site until such time that DOE re evaluates infiltration of the cover ;
and the tailings. This re evaluation will provide the information to assess seepage from the tailings cell to the ground water with j greater certainty. Additionally, since the EPA standards for
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ground water protection are not yet finalized, a finding of ;
compliance with the standards must be deferred. However, the staff j l
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t 12 concludes that if the proposed cover does need modification to further minimize infiltration, the modifications will probably not be major enough to significantly affect the overall cell design.
Accordingly, construction could commence prior to final concurrence of the proposed cover design.
4.0 SURFACE-WATER HYDROLOGY AND EROSION PROTECTION 4.1 Flooding Determinations In order to determine site impacts from flooding, DOE analyzed flooding in various on-site drainage channels to determine peak flows and velocities and to evaluate the need for erosion protection features. DOE estimated the PMF peaks in the channels resulting from an occurrence of the PMP over the limited areas draining into the channels. These design events meet the criteria outlined in the Standard Review Plan (Reference 12) and are, therefore, acceptable.
Our review of the design indicates that the PMP and PMF were appropriately derived and applied.
The details of the flood computations analyzed by the NRC staff are as follows:
A. Probable Maximum Precipitation (PMP)
A PMP rainfall depth of approximately 10.6 inches in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> was used by DOE to compute the PMF for the small drainage areas at the site. This rainfall estimate was developed by DOE using Hydrometeorological Report (HMR) 49 (Reference 15). Based on a check of the rainfall computations,_we conclude that the PMP was acceptably derived for this site.
B. Infiltration Losses DOE assumed that no infiltration losses would occur. Based on a review of the computations, we conclude that this is a very conservative assumption, and is acceptable.
C. Time of Concentration Various times of concentration (tc) for the ditches and embankments were estimated by DOE using procedures discussed in Reference 3. Based on our review, we conclude that the procedures used for computing tc are representative of the small, steep drainage areas present at the site. DOE utilized tc's as low as 2 minutes, which is generally considered to be conservative and, therefore, acceptable.
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13 D. PMP Rainfall Distributions DOE derived rainfall distributions and intensities from HMR 49 (Reference 15), which is acceptable. In the determination of peak flood flows in ditches, where the actual times of concentration were much shorter than 5 minutes, rainfall intensities were extrapolated to the appropriate time of concentration (or to a minimum of 2 minutes) in accordance with Reference 16. Based on our review, the staff concludes that the rainfall intensities were acceptably derived.
E. Computation of PMF DOE utilized the Rational Formula (Reference 17) to compute the peak sheet flows down the slopes and PMF flows in the ditches, given the input parameters discussed above. Based on our review of the calculations presented, we conclude that this method of computation is acceptable.
4.2 Upstream Dam Failures There are no dams whose failure could affect the site. The site is not located near any surface water impoundments.
4.3 Design of Erosion Protection A. Perimeter Ditches DOE proposes that the erosion protection in the perimeter drainage ditches will be designed for an occurrence of a-local PMP. This design basis meets the criteria outlined in the SRP (Reference 12) and is, therefore, acceptable.
Additionally, at those locations where the proposed diversion ditches transition into existing channels and gullies, the riprap protection will be keyed into bedrock and designed so that erosive velocities at the outlets do not affect the tailings. Energy dissipation areas will be provided at those locations where these transitions occur. The riprap in these areas will be designed for the PMF and will be keyed into bedrock.
B. Top and Sides of Piles The rock covers, which will be used to protect the soil cover from wind and water erosion, are designed for an occurrence of the local PMP. For the top of the pile (3 % slopes), DOE
14 proposes to provide a 12-inch layer of rock with an average diameter (D50) of about 2 inches. For the sides of the pile (20 percent slopes), DOE proposes a 12-inch layer of rock with a 050 of about 4 inches. Each of the rock layers will be placed on a 12-inch bedding layer. The Safety Factors Method (Reference 18) was used to determine required rock sizes for the top slopes of the pile. The Stephenson Method (Reference 19) was used for the steeper side slopes.
The rock to be placed in the energy dissipation areas was designed using the Safety Factors Method. This rock will have a minimum 050 of about 12 inches and will be keyed into bedrock at a depth equal to the existing depth of gullies in the site area. Based on our review, we find this design acceptable.
Based on a review of the calculations provided, we conclude that the proposed rock for the top and sides of the pile have been conservatively estimated.
C. Rock Durability For the rock to be placed in the ditches and on the pile, gradation and rock durability criteria were presented. Based on a comparison of the data with the criteria provided in Reference 20, we conclude that the rock durability criteria proposed are adequate to assure that rock of acceptable quality will be provided at this site.
Our review indicates that the rock will generally meet the criteria for " intermediate" quality rock (as defined in Reference 20) and will not need to be oversized. We find the rock quality to be acceptable.
4.4 Conclusion Based on our review of the information submitted by DOE, we conclude that the site design will meet EPA requirements as stated in 40 CFR 192 with regard to flood design measures and erosion protection. We conclude that an adequate hydraulic design has been provided to reasonably assure stability of the contaminated material at Bodo Canyon for a period of up to 1000 years in accordance with the acceptance criteria set for in the SRP (Reference 12).
15 5.0 GE0 TECHNICAL STABILITY 5.1 Geotechnical Site Characterization The subsurface stratigraphy at the Bodo Canyon site was determined by drilling 16 borings to depths ranging from 35 to 255 feet and digging 12 test pits. Standard penetration tests were run during the drilling program. Both disturbed and undisturbed samples were obtained from the borings and test pits for further testing. The locations of the borings are shown on Figure 3, and the boring logs are provided in Appendix C to the OSCR (Reference 5).
The boring logs and test pits show the foundation soils to consist of clay and silt soils which vary in depth at the site from one to 65 feet. The overburden soils are underlain by sandy shale bedrock.
The upper five to 35 feet of the bedrock is fractured and unweathered to moderately weathered. The unweathered, unfractured sandy shale underlying the upper layer extends to a depth of at least 145 feet.
The staff concludes that the field exploration program was conducted in accordance with standard engineering practice. Further, the extent of the program was adequate to geotechnically characterize the site. Accordingly, the investigation meets the acceptance criteria set forth in the SRP (Reference 12).
5.2 Soil Properties Geotechnical properties of the foundation soils were determined by performing various laboratory tests. The tests included moisture content, gradation, Atterberg Limits, specific gravity, consolidation, triaxial shear, and dispersivity tests. The results of the testing program are discussed in detail in Section 7 of the OSCR (Reference 5) and the data is provided in Appendix 0 of the OSCR and Volume I of DOE's Design Calculations (Reference 9).
The laboratory tests show the foundation soils to consist of very low to medium plasticity clayey silts and sandy to silty clays (CL, CH, ML, and ML-CL). The soils are generally medium dense with dry unit weights ranging from 98 to 114 pounds per cubic foot and moisture contents ranging from 8.6 to 24.4 percent.
The 00E proposes to use onsite low plasticity clays (CL) for constructing the radon barrier. Initial tests performed on a small number of samples from a potential borrow source indicated the soil had a high potential for swell and dispersivity. Subsequent correspondence from the DOE stated this borrow source would not be l
1
- )
. I 1
ll 1
16 utilized and provided a limited number of test results from other pctential borrow sources which did not indicate a high potential for swell and dispersivity. Due to the importance of utilizing soils with good engineering characteristics as radon barrier soils, the suitability of soils used for the radon barrier will be made a conditional concurrence item.
The DOE was afforded only limited access to the processing site during the design phase, and therefore only limited laboratory testing of tailings samples has been conducted. However, the proposed plans for placing the tailings at the Bodo Canyon site call for mixing the sand and slime tailings prior to placement. The 00E has therefore classified the mixed tailings as a silty sand (SM).
All tests were conducted in accordance with applicable ASTM standards. The staff concludes that the extent of soils testing was adequate to characterize the soils and tailings and support the soil parameters used in the stability and settlement analyses performed in accordance with the acceptance criteria set forth in the SRP (Reference 12).
- 5. 3 Slope Stability The 00E performed slope stability analyses to evaluate the stability of the reclaimed pile. Static and dynamic loading conditions were evaluated using the computer program " ICES-SLOPE," which utilizes the modified Bishop method (Reference 21), and infinite slope stability analyses. Sliding wedge stability analyses were performed using a version of the computer program "SWASE" (Reference 22).
A typical cross-section of the reclaimed pile is shown on Figure 4.
The cross-section consists of the following layers: rock erosion protection, radon barrier, tailings, recompacted clay soils, and natural foundation soils. The top of the pile will be graded to a slope of approximately 3 percent, while the embankment outslope will consist of a 5H:1V slope.
Parameters used as input in the stability analyses were determined during the field and laboratory testing programs described previously. Strength parameters for recompacted soils for the period immediately following construction were obtained by performing unconsolidated-undrained (UU) triaxial tests, while long-term strengths were obtained from consolidated-undrained (CU) tests. Standard penetration test (SPT) blow count values as well as the results of CV and UU triaxial tests performed on relatively undisturbed samples were used to estimate soil strengths for the foundation soils. A summary of the strength values used is provided on Table B.2.4 of the RAP (Reference 2).
_ - _ - _ _ - _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . - _ _ _ _ _ _ _ = _ _ _ _ _ _ _ _ _ _ _
17 The results of the static stability analyses showed minimum factors of safety of 1.3 for the end of construction stage and 2.3 for the
~
long-term stability. These values conservatively meet the minimum factors of safety of 1.3 and 1.5, respectively, recommended in Regulatory Guide 3.11, " Design, Construction, and Inspection of Embankment Retention Systems for Uranium Mills," Rev. 2, dated December 1977 (Reference 23).
The long-term pseudo-static stability analysis performed utilized a seismic coefficient of 0.16g. The procedure used for determining the seismic coefficient is in accordance with the acceptance criteria set forth in the SRP (Reference 12). The long-term pseudo-static stability analysis performed resulted in a minimum factor of safety of 1.25. The minimum value specified in Regulatory Guide 3.11 is 1.0.
The stability analyses conducted by DOE indicate that the proposed design meets or exceeds the minimum factors of safety recommended in Regulatory Guide 3.11 (Reference 23). In addition, the staff conclude that the stability analyses performed by the DOE utilized methods which are widely used in engineering practice and are therefore acceptable, in accordance with the acceptance criteria set forth in the SRP (Reference 12).
5.4 Settlement An analysis of the settlement expected during and following construction activities at the disposal site was performed to evaluate the potential for disruption of the radon barrier and erosion protection layers. The settlement analysis was performed using the results of laboratory consolidation tests on remolded samples of radon barrier material, a combination of empirical correlations and consolidation tests on relatively undisturbed samples of foundation soils, and the results of laboratory tests performed on remolded samples of tailings from the Grand Junction, Gunnison, and Riverton VMTRAP sites.
The results of the analysis indicate that the total settlement for '
the embankment and foundation soils is estimated to be approximately '
2.33 feet, with the majority of the settlement occurring during the i 2.5 year construction period. The DOE therefore concludes that l cracking of the radon barrier layer will not be a problem.
The staff review, in accordance with the acceptance criteria set forth in in the SRP (Reference 12), of the settlement analysis performed by the DOE indicates that the analyses utilized commonly
r s
18 accepted procedures and reasonably conservative soil parameters.
The staff therefore concurs with the settlement analysis performed by the DOE.
5.5 Liquefaction Liquefaction is the phenomenon whereby fine grained, cohesionless soils may undergo loss of strength as a result of seismically-induced increases in pore water pressure. There are two factors considered necessary for liquefaction of fine grained, cohesionless soils: (1) saturation of the soils, and (2) a low relative density (soils with a relative density exceeding 70% are generally not considered susceptible to liquef action).
The foundation soils at the site consist of cohesive clayey soils.
These types of soils are not considered susceptible to liquefaction.
The tailings, however, consist of cohesionless sands and silts which are considered susceptible to liquefaction. The recompacted tailings will not exhibit either of the factors considered necessary for liquefaction. The tailings will be placed at moisture contents well short of saturation, and the placement of the low permeability radon barrier soils will inhibit infiltration / saturation of the tailings (see discussion in section 3.4.3). Additionally, the tailings will be compacted to 90 percent Standard Proctor (ASTM D698) which should assure relative densities exceeding 70 percent.
The staff therefore concludes that an acceptable analysis of liquefaction potential was conducted, in accordance with the acceptance criteria set forth in the SRP (Reference 12), and that the site should not be subject to liquefaction.
5.6 Construction Criteria The RAP (Reference 2) states that the tailings materials will be pla:ed at a minimum of 90 percent of the maximum dry density as
" te e . M by the Standard Proctor test (ASTM D698). The radon
'eier maa ial will be placed at a minimum of 95 percent of the me. + m nr unsity as determined by the Standard Proctor test (ASTM l D69e> anu at a moisture content zero to three percent above optimum moisture content. The low permeability layer will be compacted to 90 percent of Standard Proctor at a moisture content below one percent of optimum moisture. 4 To assure that the above construction criteria are achieved in the field, the DOE will implement a quality control program which as a minimum will consist of the following:
l
4' i
19 (a) -In place density and moisture tests: one test per 500 cubic yards of fill placed.
(b) Classification tests: one test per 2,000 cubic yards of fill placed (excluding tailings).
(c) Gradation tests: one test per 2,000 cubic yards of fill placed (excluding tailings).
The staff review of the construction criteria and quality control program provided in the RAP indicates that the criteria are consistent with standard engineering practice, in accordance with the acceptance criteria set forth in in the SRP (Reference 12),~and are, therefore, acceptable.
5.7 Conclusion The staff concurs that the proposed remedial action should meet the EPA criteria with regard to geotechnical stability. However, a review of swell and dispersivity test results for radon barrier soils as well as "as-built" drawings will be.necessary before the staff can conclude that the completed remedial action meets applicable standards and the acceptance criteria of the SRP (Reference 12).
6.0 RA90N ATTENUATION AND SITE CLEANUP 6.1 Characterization of Tailings and Cover Material As stated in Section B.2.2.5 of the RAP (
Reference:
2), site access restrictions prevented collection of data for Ra-226 concentrations, Rn-222 emanating fractions, and diffusion coefficients on both tailings and subpile material. The associated ancillary values which accompany these measurements (i.e. , dry unit' density, porosity and particle size distribution) are also not available. The absence of these values precluded verification of the long-term moisture content of the pile or the proposed cover thickness. Bulk density, porosity, moisture content and diffusion coefficient for the cover were based on site-specific data from the Bodo Canyon disposal site.
6.2 Radon Attenuation Based on estimated pile parameters and measured cover parameters, l
DOE estimates a required cover thickness of about 6 feet of compacted soil, based on calculations using the computer program RAECOM (Reference 24). For design purposes, a range of 4 to 7 feet
20 was utilizec. Voce actual site specific tailings characterization data becomes a.o;1able, calculation of the actual required thickness will be performed by DOE and independently verified by NRC.
The long-term moisture content utilized in the cover thickness determination for the Bodo Canyon cover soil must be equivalent to
-15 bar suction. The RAP specifies a long-term moisture content of l 15 percent based on what appears to be three separate soil samples.
The associated diffusion coefficient of 1.4E2 cm2/sec must also be determined for the -15 bar suction moisture value. Review of the Bodo Canyon cover soil data indicates that additional samples should be characterized and the -15 bar suction moisture value determined.
The diffusion coefficient should oe chosen such that it corresponds with the -15 bar suction value.
6.3 Site Cleanup The processing site will be characterized at a future date. All material contaminated in excess of the EPA standards will be removed. Verification gamma surveys and soil samples will be taken to ensure that the EPA standards are met. The proposed procedures for surveying and sampling have been reviewed in accordance with the acceptance criteria set forth in in the SRP (Reference 12), and are acceptable.
l 6.4 Conclusion In order for the NRC to concur on the radon barrier design, several parameters must be measured or estimated by 00E. For both the tailings and subpile materials these parameters are Ra-226 l
concentration, Rn-222 emanating fractions, diffusion coefficient, dry unit density, porosity and particle size distribution. In addition, a representative number of Bodo Canyon cover soils need to be moisture tested at the -15 bar suction value and this number used to determine cover soi'l diffusion coefficient. The resultant radon barrier design must also be submitted to NRC for review and approval.
- 7. 0
SUMMARY
This Technical Evaluation Report summarizes the NRC staff review of the proposed remedial action for the Durango tailings site. Additional information is needed prior to unconditional concurrence by NRC. The deficient areas have been noted in the text and additional information requested from 00E. Staff review of the additional information will be presented as a supplement to this report and will include the NRC concurrence position on the proposed remedial action.
F REFERENCES
- 1. U.S. Department of Energy, " Preliminary Draft Remedial Action Plan and Site Design for Stabilization of the Inactive Uranium Mill lailings Site at Durango, Colorado," UMTRA-00E/AL 050503.0000, Docket WM-48, June 1985. Available in the NRC PDR for inspection and copying for a fee.
- 2. U.S. Department of Energy, " Preliminary Final Remedial Action Plan and Site Design for Stabilization of the Inactive Uranium Mill Tailings Site at Durango, Colorado," UMTRA-DOE /AL 050503.0000, Docket WM-48, June 1986. Available in the NRC PDR for inspection and copying for a fee.
- 3. U.S. Department of Energy, " Draft Environmental Impact Statement, Remedial Actions at the Former Vanadium Corporation of America Uranium Mill Site, Durango, La Plata County, Colorado", Docket WM-48, October 1984. Available in the NRC PDR for inspection and copying for a fee.
- 4. U.S. Department of Energy, " Final Environmental Impact Statement, Remedial Actions at the former Vanadium Corporation of America Uranium Mill Site, Durango, La Plata County, Colorado,"
00E/EIS-0111F), Docket WM-48, October 1985. Available in the i NRC PDR for inspection and copying for a fee.
l l S. U.S. Department of Energy, " Draft Disposal Site Characterization Report for the Alternate Uranium Mill Tailings Disposal Site in Bodo Canyon near Durango, Colorado," UMTRA-00E/AL 050103.0001, Docket WM-48, May 1985. Available in the NRC PDR for inspection and copying for a fee.
- 6. U.S. Department of Energy, " Processing Site Characterization Report for the Uranium Mill Tailings Site at Durango, Colorado,"
i UMTRA-00E/AL-050103.0000, Decket WM-48, June 1984. Available l in the NRC Public Document koom for inspection and copying, for a fee.
l
- 7. Letter from J. G. Themelis, Department of Energy, to D. Smith, Nuclear Regulatory Commission,
Subject:
DOE responses to NRC comment numbers 8 through 21 on Docket WM-48, dated March 21, 1986. Available in NRC PDR for inspection and copying, for a fee.
- 8. U.S. Department of Energy, " Preliminary Design Documents," Docket VM-48, November 1985. Available in the NRC PDR for inspection and copying for a fee.
- 9. U.S. Department of Energy, " Final Design Documents," Volumes I, 11 and III, Docket WM-48, April 1986. Available in the NRC PDR for inspection and copying for a fee.
- 10. U.S. Department of Energy, " Revisions to Design Documents, Docket WM-48, 1986. Available in the NRC PDR for inspection and copying for a fee.
- 11. U.S. Department of Energy, " Subcontract Documents, Final Design for Construction" Docket WM-48, August 1986. Available in NRC PDR for inspection and copying, for a fee.
- 12. U.S. Nuclear Regulatory Commission," Standard Review Plan for UMTRCA Title I Mill Tailings Remedial Action Plans," USNRC, Division of Waste Management Report, October 1985. Available from NRC's Division of Low-Level Waste Management and Decommissioning.
- 13. Chiang, W., G.A. Guidi, C.P. Mortgat, C.C. Schocf, and H.C. Shah,
" Computer Programs for Seismic Hazard Analysis, A User Manual",
John A. Blume Earthquake Engineering Center, Dept. of Civil Engineering, Stanford University, Report No. 62, 1984.
- 14. Green, R.E., and J.C. Corey, " Calculation of Hydraulic Conductivity: a Further Evaluation of Some Predictive Methods, in Soil Science of America Proceedings, 35, 1971.
- 15. U.S. Army Corps of Engineers, " Probable Maximum Precipitation Estimates, Colorado River and Great Basin Drainages," Hydro-meteorological Report No. 49, 1977.
- 16. U.S. Nuclear Regulatory Commission, " Methodologies for Evaluating Long-Term Stabilization Designs of Uranium Mill Tailings Impoundments," NRC Report NUREG/CR-4620, p. 10, June 1986.
Available for purchase from National Technical Information Service, Springfield, Virginia 22161.
- 17. Chow, V. T., Open Channel Hydraulics, McGraw-Hill Book Company, New York, 1959.
- 18. Simons, D. B., and F. Senturk, " Sediment Transport Technology", Fort Collins, Colorado, 1976.
- 19. Stephenson, D, "Rockfill Hydraulic Engineering Developments" in Geotechnical Engineering I, 27, Elsevier Scientific Publishing Company, 1979.
l'
- 20. Nelson, J. D. , et. al. , " Methodologies for Evaluating Long-Term Stabilization Designs of Uranium Mill lailings impoundments,"
USNRC Report NUREG/CR-4620, 1986. Available for purchase from National Technical Information Service, Springfield, Virginia 22161.
- 21. McDonnell Douglas Automation Company, " ICES-SLOPE Computer l Program and User Manual,". St. Louis, Missouri, circa 1980.
- 22. Morrison Knudsen Engineers," Computer Program SWASE", Stability Analysis of Earth Slopes, Y. H. Huang, Van Nostrand Reinhold Co., New York, 1983.
- 23. U.S. Nuclear Regulatory Commission, Regulatory Guide 3.11, " Design, Construction, and Inspection of Embankment Retention Systems for Uranium Mills," December 1977. Copies are available from U.S. Government Printing Office, Washington, D.C. 20402, Attn:
Regulatory Guide Account. (16)
- 24. U.S. Nuclear Regulatory Commission, " Radon Attenuation Handbook for Uranium Mill Tailings Cover Design," NRC Report NUREG/CR-3533, April 1984. Available for purchase from National Technical Information Service, Springfield, Virginia 22161.
l' l
l l
l BIBLIOGRAPHY l
l Attwell, P. B. and I. W. Farmer, " Principles of Engineering Geology,"
Chapman and Hall, 1986.
Campbell, K. W. , "Near-Source Attenuation of Peak Horizontal Acceleration," Bulletin of the Seismological Society of America, 71, 2039-2070 (1981).
Crippen, J. R. and C.D. Bue," Maximum Floodflows in the Conterminous United States," USGS Water Supply Paper 1887, 1977.
Rahn, P. H., Engineering Geology. Elsevier, 1986.
Seed and Idriss, " Ground Motions and Soil Liquef action During l
Earthquakes, Earthquake Engineering Research Institute, Berkeley, Califorria, 1982.
Slemmons, D. B., P. O. O'Malley, R. A. Whitney, D. H. Chung, and D. L.
Bernreuter, " Assessment of Active Faults for Maximum Credible Earthquakes of the Southern California - Northern Baja Region,"
University of California, Lawrence Livermore National Laboratory Publication No. UCID 19125, 48 p., 1982.
U.S. Army Corps of Engineers, " Engineering and Design - Standard Project Flood Determinations," EM 1110-2-1411, 1965.
U.S. Army Corps of Engineers, " Probable Maximum Precipitation Northwest States," Hydrometeorological Report No. 43, 1066.
U.S. Army Corps of Engineers, " Hydraulic Design of Flood Control Channels," EM 1110-2-1601,1970.
U.S. Army Corps of Engineers, " Additional Guidance for Riprap Channel Protection," ETL 1110-2-120, 1971.
U.S. Army Corps of Engineers, " Water Surface Profiles, HEC-2," The Hydrologic Engineering Center, 1982. 1 U.S. Army Corps of Engineers, " Flood Hydrograph Package, HEC-1," The Hydrologic Engineering Center,1985.
U.S. Bureau of Reclamation, " Design of Small Dams, 1973.
U.S. Department of the Navy, " Soil Mechanics," NAYFAC DM-7.2, 1982.
1 I
l l
/
l U.S. Nuclear Regulatory Commission, Regulatory Guide 1.59, " Design Basis Floods for Nuclear Power Plants," January 1983. Copies are available f rom U.S. Government Printing Of fice, Washington, D.C. 20402, Attn: I Regulatory Guide Account.
U.S. Nuclear Regulatory Commission, " Radon Attenuation Handbook for Uranium Mill Tailings Cover Design," USNRC Report NUREG/CR-3533, ,
April 1983. Available for purchase from National Technical Information Service, Springfield, Virginia 22161.
U.S. Nuclear Regulatory Commission, " Hydrologic Design Criteria for Tailings Retention Systems," Division of Waste Management Staff Technical Position WM-8201, January 1983. Available f rom NRC's Division of Low-Level Vaste Management and Decommissioning.
U.S. Nuclear Regulatory Commission, " Design of Rock Armor for Uranium Mill Tailings Embankments," USNRC, Division of Waste Management Draf t Report, February 1985. Available f rom NRC's Division of Low-Level Waste Management and Decommissioning.
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