ML20247E552
| ML20247E552 | |
| Person / Time | |
|---|---|
| Issue date: | 03/27/1989 |
| From: | Tokar M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Fliegel M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-68 NUDOCS 8904030144 | |
| Download: ML20247E552 (26) | |
Text
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! MAR 2 71989 MEMORANDUM FOR: f(yron Fliegel, Section Leader Operations Branch Division of. Low-Level Waste Management and Decommissioning, NMSS FROM: Michael Tokar, Section Leader Technical Branch Division of Low-Level Waste Management l and Decommissioning, NMSS
SUBJECT:
FINAL TER FOR GREEN RIVER UMTRA PROJECT
REFERENCES:
- 1. 00E, 1989, Remedial Action Plan and Final Design for Stabilization of Inactive Uranium Mill Tailings at Green River, Utah, Final, Vol. I, II, and III, January,1989; UMTRA-DOE /AL 050510.GRNO.
The final design and Remedial Action Plan for stabilizing tailings at the Green River Site have been reviewed, and a report documenting our evaluation of these is attached. Since the DOE has significantly changed their approach to the design of the cover to meet the EPA standards, the review effort involved more than what is normally required at the final RAP stage. Although the design is satisfactory with regard to providing protection against freezing and complying with radon release standards, there are items yet to be resolved from a perspective of complying with the EPA groundwater standards.
In summary, we can not concur with the actions pro Design document and the Remedial Action Plan (RAP) until posed by of the resolution thethe-DOE in the Fi following issues: (1) verifying the validity of the h parameters used in the design for the radon barrier; (ydraulic conductivity
- 2) demonstrating that the tailings and other sandy materials can be placed to the specified densities at low moisture contents, as re the groundwater standards; (quired in
- 3) revising thethe DOE's strategy specifications forradon for the compliance with barrier material to represent the characteristics of the soil samples tested to arrive at the design parameters; and (4) addressing the potential for migration of fines from the Type D fill into the voids of the Type B riprap. Section 3.6 of this report addresses these issues in detail.
This review was performed by Banad Jagannath; please contact him should you have any questions.
(Original Signed by /j h
Michael Tokar, Section Leader Technical Branch Division of Low-Level Waste Management and Decommissioning, NMSS
Enclosure:
As stated %,
s904030144 890327 PDR WASTE p
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RBangart, R IV JGreeves,-LLWM MBell, LLRB JSurmeier, LLTB PLohaus, LLOB MTokar, LLTB BJagannath, LLTB SWastler, LLOB LJahandari,LLTB rf JJones, LLOB rf HMSS rf PDR YES- M /.
PDR NO / / Category: Proprietary / / or CF Only / /
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SUBJECT ABSTRACT: Geotech TER for Green River
- LLO :LLOB :LLWM :LLWM :NMSS :NMSS OFC :LLO> g NAME:BJagannath/jj :MTokar : : : : :
DATE:3 /Af /89 : / /89 : / /89 : / /89 : / /89 : / /89 : / /89 0FFICIAL RECORD COPY 4
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.4 GRNFTER3 3.0 GE0 TECHNICAL STABILITY
' 3.1 Introduction The NRC staff review of the geotechnical engineering aspects of the remedial actions at the Green River site is presented in this section. The review consisted primarily of evaluations of the site characterization and stability aspects of the stabilized tailings embankment (disposal cell), and cover design. The object of the review was to determine whether the proposed remedial actions would result in the stabilized disposal cell complying with the long-term stability requirements of the EPA standards in 40 CFR Part 192.02 (a) Subpart A, from the geotechnical engineering perspective of slope stability, liquefaction, and settlement. The staff review of the related geological aspects such as geologic, geomorphic, and seismic characterization of the site is presented in Section 2 of this report. The staff review of the groundwater conditions at this site is presented in Section 5 of this report.
At the Green River Uranium Mill site (presently an inactive site) the ore concentrate was shipped to a processing plant in. Rifle, Colorado, and thereby the tailings left at this site were predominantly sandy tailings with no slime.
The 48-acre area designated for remedial action censists of the tailings pile, former ore storage area, and abandoned structures and facilities associated with the uranium mill during its operation. In addition, tailings dispersed by wind and water erosion have contami~nated approximately 30 acres of adjoining area. The proposed remedial action of stabilization-on-site consists of placing all the contaminated material at the site (approximately 343,000 cyds) in to a single pile, which is called the disposal cell. The location of this disposal cell is approximately 500 ft. south and about 50 f t. higher in elevation than the existing tailings pile location. The disposal cell bottom (elevation 4098) is approximate'iy 42 feet below the existing ground surface and this requires approximately 26 feet of excavation in the bedrock. The top of the disposal cell is about 25 ft above the adjoining ground surface. The
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4 GRNFTER3 portion of the disposal cell above the existing ground surface (elevation 4140) rises to elevation 4160 at a gentle slope of 5 horizontal to 1 vertical (5H:1V) andtothecrownofthecell(elevation 4165)ataflatslopeof5 percent.
The disposal cell has a six-feet-thick buffer zone. consisting of select material placed at the bottom of the cell between the bedrock and tailings.
The disposal cell will be covered with (1) a three-feet-thick
- infiltration / radon barrier, (2) a six-inch-thick gravel bedding, and (3) .a 12-inch-thick rock layer (riprap). The cover is designed to ensure the following: (1) long-term stability of embankment and reduced radon emissions; (2) reduced infiltration; (3) protection of surface water quality; (4) protection against animal intrusion; (5) minimized plant root intrusion; (6) prevention of inadvertent human intrusion; and (7) prevention of material dispersion. (Reference 8). This section presents geotechnical engineering evaluation of the long-term stability and reduced radon emanation aspects of the proposed remecial actions.
3.2 Site Characterization 3.2.1 Site Description Section 1 of this report presents a description of the Green River project site.
3.2.2 Site Investigations Subsurface explorations at the site were ptirformed by the following investigators:
(1) Bendix Field Engineering Corporation to determine the extent of contamination. The investigations resulted in data from 105 bore-holes, 184 in situ Ra-226 measurements, and 139 soil samples.
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GRNFTER3 Addendum D1 to Appendix D in Reference 1 presents detailed information en this investigation. The results of this investigation were used in establishing the volume of contaminated material to be removed to comply with the EPA standards. This contaminated material is to be placed in the disposal cell.
(2) Jacobs Engineering Group, Inc. (1986, 1987, 1988) and fiorrison-Knudson Engineers, Inc. (1986-1987). The scope of the geotechnical investigations included borings from which soil samples and rock cores were obtained, test pits from which bulk samples were obtcined, and installation of monitoring wells. These investigations '
were performed to determine geotechnical characteristics of the site and to obtain samples of the soil and rock material to perform labcratory tests to determine their properties. Information to Bidders, Volumes I, II, and III of Reference 9, and Volume IIA Appendix D of final RAP dated January 1989 (Reference 8) present detailed information on site conditions and logs of these field investigations and laboratory test results.
3.2.3 Site Stratigraphy The elevation of the Green River project site varies from about 4050 to 4200 feet above mean sea level. Borings were obtained using standard ,
geotechnical drilling and sampling techniques. These methods included drilling with hollow stem augers, and sampling at near continuous intervals with Standard Penetration Tests (SPT). On occasion, a 2.5-inch inside-diameter, ring-lined, split-barrell sampler was used to sample the materials. The SPT tests were conducted according to ASTM D 1586 procedures. Figure 3.7 of RAP (Reference 9) shows locations of the borings and test pits. Section 2 of this report presents an an evaluation of the geologic, geomorphic, and seismic characteristics of the site.
4 GRt;FTER3 The overburden materials at the site consist of a alluvium deposit underlaid by a thin layer of gravel which in turn overlies the bedrock. The alluvium deposit consists of silty to clayey sand, with dense sand and gravel occurring at the bottom of this deposit. The alluvium deposit is in a loose to dense condition with the Standard Penetration Test resistance values ranging from 3 to 43, with an average of 18 blows /ft.. The sedimentary bedrock units at the site consist of a shale member of the Mancos shale, the Dakota sandstone, and the Cedar Mountain Formation. The upper portion of the bedrock is weathered and fractured. Refer to Section P of this report for detailed evaluation of the bedrock conditions at the site.
At the existing tailings pile area, the site stratigraphy consists of sand j
tailings overlying the alluvium deposit (silty sand- clayey sand) which in turn overlies the bedrock. Tailings and contaminated alluvium will be excavated for disposal in the disposal cell.
At the proposed disposal cell site, the bedrock units are the Dakota sandstone underlaid by the Cedar Mountain Formation consisting of shale and mudstone. The overburden soils at the proposed cell location consist of from 5 to 16 f t. of loose to dense alluvium (silty sand - clayey sand). Large lenses of clay are contained within this layer. Dense to very dense sand and gravel occur at the bottom of this deposit. Since the disposal cell is proposed to be founded on the bedrock, the overburden material will be excavated. This material will be selectively used as Select fill Type-A material for the six-feet-thick buffer zone placed at the bottom of the cell between the bedrock and the tailings.
The groundwater table at the proposed disposal cell location ic estimated to be at elevation 4085 f t, approximately 55 ft below the ground surface and 13 ft. below the bottom (elevaticn 4098 ft.) of the disposal cell. Rsfer to i Section 5 for detailed evaluation of the groundwater conditions at this site.
c GRNFTER3 Soil for the radon barrier cover and gravel for the bedding layer are proposed to be taken from Borrow Site 1. Figures 3.15 through 3.24 of the draf t RAP (Reference 9) show the location and stratigraphy of the proposed borrow area. A total of 24 test pits were dug to investigate the availability and suitability of the soils for the intended use. The stratigraphy at the borrow site consists of alluvial deposit with a surficial layer of silty-clayey sand, underlaid by low-plasticity clay. The clay layer is underlaid by a alluvial sand and gravel stratum. The test pits were terminated in the sand-gravel stratum. The low-plasticity clay is proposed to be used for the infiltration / radon barrier cover and the alluvial sand-gravel material will be processed to cbtain the gravel needed for the bedding layer.
The staff has reviewed the details of the borings and test pits as well as the scope of the overall geotechnical exploration program. The staff concludes that the geotechnical investigations conducted at the Green River site have adequately established the stratigraphy and soil conditions to support assessment of the geotechnical stability of the stabilized tailings and contaminated material in the disposal cell. Further, the geotechnical explorations are in general conformance with applicable provisions of Chapter 2 of the f:RC Stanaard Review Plan (SRP) for UMTRCA Title I Mill Tailings Remedial Action Plans (Reference 5).
3.2.4 Testing Program The staff has reviewed the geotechnical engineering testing program for the Green River site. The testing program included physical properties tests, compaction tests, triaxial shear strength tests, permeability tests, and dispersion tests on samples of tailings and borrow materials intended for use in the disposal cell. The staff finds that the testing program employed to define the material properties was appropriate for the support of necessary engineering analyses described in the following sections. Further, the scope of the testing program and the utilization of the resulting data to define the
GRNFTER3 material properties are in general agreement with applicable provisions of the SRP (Reference 5).
However, the DOE has not submitted all the test data (.for example, capillary-moisture relationship ) for the infiltration / radon barrier soil, which is the silty clay from the borrow site amended with 3 weight percent sodium bentcnite. The additional geotechnical data presented in the final RAP (Reference 8) is for the tailings and buffer Zone material, and there is no new data on the infiltration / radon barrier material in the final RAP. Because of the DOE's new approach to complying with the EPA Groundwater Standards, additional test data on the radon barrier material will be required to demonstrate validity of the assumptions made in the design. Section 3.3.4 of this report presents details on this item.
3.3 Geotechnical Engineering Evaluation 3.3.1 Stability Evaluation The evaluation of the geotechnical stability of the slopes of the proposed stabilized tailings pile is presented in this section. The staff has reviewed the exploration data, test results, critical slope characteristics and methods of analyses pertinent to the slope stability aspects of the remedial action plan (References 10 & 11). The analyzed cross section with the 5 horizontal to 1 vertical slope has been compared with the exploratory records and design details. The staff finds that the characteristics of the slope have been properly represented and that the most critical slope section has been considered for the stability analysis.
Soil parameters for the various materials in the stabilized embankment slope have been adequately established by appropriate testing of representative material. Values of soil parameters have been assigned to other layers (riprap, gravel bedding, bedrock etc.) on the basis of data obtained from '
O GRNFTER3 geotechnical explorations at the site and data published in the literature.
The staff finds that the determination of these parameters for slope stability follow conventional geotechnical engineering practice, and~ ore also in compliance with the applicable provisions of Chapter 2 of the SRP. The staff also finds that an appropriate method of stability analysis (Bishop method) has been employed and has addressed the likely adverse conditions to which the slope might be subjected. Factors of safety against failure of the slope for seismic loading conditions have been evaluated for both the short term (end-of-construction) state and long-term state. Factors of safety for the static loading conditions were not evaluated because the seismic loading condition is more critical and results in lower factors of safety than those for the static loading condition. The seismic stability of the slope was investigated by the pseudo-static method of analysis using horizontal seismic coefficients of 0.1 for the end-of-construction case and 0.14 for the long-term case. The values of the seismic coefficients were calculated as per the guidance in the SRP and are ecceptable to the staff. The staff finds the pseudo-static method of analysis to be acceptable considering the degree of conservatism in the soil parameter values and the flatness of the slopes (5H:1V). The minimum factors of safety against failure of the slope were 2.3 and 1.67 for the ena-of-construction and long-term conditions, compared to a required minimum of 1.1 for both conditions. The details of the cover in the final RAP are different from the one analysed in the draft RAP stage. However, f rom a geotechnical slope stability perspective, the change, viz, thicker radon barrier and elimination of frost protection layer, has no significant impact on the stability of slopes. Therefore, the results of the analyses presented in the draft RAP are applicable to the final RAP design. In addition, the strength parameters for the cover materials used in the stability analysis are conservative.
l The :taff concludes that the proposed slopes of the disposal cell will be stable under both short-term and long-term conditions. Therefore, from a geotechnical engineering long-term slope stability perspective, the disposal
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d GRNFTER3 cell will cen: ply with the EPA standard (40 CFR Part 192.02(a)) for long-term stability.
3.3.2 Liquefaction Based on review of results of the geotechnical investigations, including boring logs, test data, soil profiles, and disposal cell design, the NRC staff concludes that the 00E has adequately assessed the potential for liquefaction at the Green River site. Because the compacted dry density-of the tailings and other contaminated materials in the disposal cell will be a minimum of 90 percent of the maximum dry density by the ASTN D-698 test, and the design requires these materials to be in an unsaturated condition, these materials are not susceptible to liquefaction. The disposal cell is founded on bedrock, which is also .ot susceptible to liquefaction. The groundwater table at the site is estimated to be approximately 13 ft. below the foundation of the disposal cell. Considering the placement density and absence of free moisture l in the disposal cell, the materials in the disposal cell are judged to be not susceptible to liquefaction.
The staff concludes that the stabilized tailings and other contaminated materials in the disposal cell are not susceptible to liquefaction.
1 3.3.3 Settlement Long-term settlement of materials in the disposal cell, which could result in either local depressions on top of the cover or cracks in the cover, is of concern. The depressions in the cover could initiate erosion gully pathways, and a severe deep erosion of the cover might expose the tailings materials. A crack in the cover might open up a pathway for surface water to infiltrate into or through the tailings materials. Since the tailings and contaminated materials in the disposal cell are sandy materials ccmpacted to 90 percent
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.g-standard proctor density at a moisture content of minimum 3 percent less than the optimum moisture content, a major portion of the settlement will be instantaneous and will take place during construction. Thus, any potential adverse effects of the instantaneous settlement of these sandy materials will be compensated for before completion of the construction. Therefore, instantaneous settlement will not adversely effect the long-term performance of the disposal cell. Any time dependent or delayed settlement is expected to be minimal-or insignificant and is not expected to result in any differential settlement cracks in the cover.
The staff concludes that the long-term settlements of the sandy materails in the disposal cell will be minimal and will not have any adverse impact on the performance of the disposal cell cover. Therefore, from a long-term settlement perspective, there is reasonable assurance that there will be no adverse ef fects on the ability of the disposal cell to meet the EPA standards.
3.3.4 Cover Design The proposed design for the disposal cell cover consists of the following, in descending order from the top: (1) one-foot-thick, Type-A riprap; (2) 6-in.-thick gravel bedding; and (3) three-foot-thick radon / infiltration barrier layer (Reference 8).
The riprap and its bedding layer is designed to protect the radon / infiltration barrier in the long-term. Section 4 of this report presents the staff evaluation of this erosion protection layer. The radon / infiltration barrier layer is designed to attenuate the emanation of radon from the tailings and other contaminated materials placed in the disposal cell to comply with the EPA standards on radon release for UMTRCA projects. The staff evaluation of the adequacy of the thickness of the radon barrier to attenuate the release of radon to comply with the EPA standards is addressed in Section 6 of this report. The staff evaluation of the adequacy of the infiltration barrier as
r GRNFTER3 part of the DOE's design to comply with the the EPA Groundwater Standards is addressed in Section 5 of this report.
The DOE has performed an evaluation of the freezing conditions at the Green River project site and has :oncluded that the freezing depth at the site is 39 inches. The DOE has used 200-year weather data for the Green River site and a computer code developed by U.S Arnly Cold Regions Research and Engineering Laboratory for the modified Bergren Solution to calculate the depth of frost penetration. As part of the oesign, the DOE has performed a sensitivity analyses to arrive at the recommended frost penetration depth of 39 inches.
The statf h6s reviewed the values of the input parameters and the range of parameters investigated in the sensitivity analyses and concurs with the DOE's analyses and reconunendations. As an independent verification, the depth of frost penetration indicated in Figure 7.1-42 of Reference 37, prepared by the U.S. Arnty Corps of Engineers, is 36 inches for the project site region.
Therefore, the staff agrees with the DOE's estimation of the frost penetration depth of 39 inches at the site. A 39-inch frost penetration will result in the freezing of the upper 39 inches of the cover, while the lower 15 inches of radon / infiltration barrier layer will be in the unfrozen or intact condition.
This 15 inches of radon / infiltration barrier is estimated to be acequate to reduce the radon emanation from the disposal cell to comply with the EPA standards (see Section 6.2) and to control the infiltration into the disposal cell to a flux of 1x10 E-09 cm3/cm2.sec (see Section 5).
The radon / infiltration barrier design assumes that the bentonite anended silty clay material, intended for the radon barrier, can be compacted to result in a material with a saturated hydraulic conductivity of 2x10E-8 cm/sec.
Further, the design against infiltration assumes that the long-term moisture conditions will be unsaturated, and therefore, the unsaturated hydraulic conductivity will be 1x10E-9 cm/sec or lower. This permanent unsaturated condition will result in an infiltration rate or flux of 1x10E-9 cm3/cm2.sec or less through the cover. This infiltration rate is the critical parameter value in the design of the disposal cell in assuring compliance with the EPA
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- l GRNFTER3 Groundwater standards for UMTRCA projects. Section 5 of this report presents details on the design approach to satisfy the EPA Groundwater standards.
The NRC hydrogeologist recently conducted an audit of the data developed by the DOE in support of the unsaturated conditions assumed in the radon barrier and tailings material in the design (Reference 36). The NRC team concluded that (1) the radon barrier is likely to be unsaturated, (2) the low hydraulic conductivity expected has not been established by adequate testing, and (3) the DOE has not demonstrated the validity of the unsaturated condition assumption for the long-term as the data used to arrive at this conclusion is only for a two-year period. Hence, the DOE still needs to demonstrate that the saturated hydraulic conductivity of the bentonite amended silty clay material, compacted at a density of 100 percent of the maximum in ASTil D 698 test (Proctor test) and at a moisture content of zero to three percent above the optimum moisture content, will be 2x10E-8 cm/sec.
The reasons for NRC's conclusions regarding the radon barrier are as follows. The laboratory test data (Table D.4.4 of RAP, Reference 9) presented in support of this consists of only three tests on silty clay amended with 3 percent of Bentonite resulting in saturated hydraulic conductivities of 2x10E-8, 1x10E-8, and 3.4x10E-8 cm/sec with an average value of 2.3x10E-8 !
cm/sec. However, the data presented in Table D.4.4 show the hydraulic conductivity parameter to range from a low of 1x10E-8 to a high of 2.1x10E-5 cm/sec; the low value is for the soil amended with 6 percent of Bentonite.
Considering the range of the hydraulic conductivity values presented in Table D.4.4, and the sensitivity of this parameter to percent fines in the soil and compaction density and moisture, the staff believes that the DOE has not adequately established that the radon barrier will have a saturated hydraulic 1 conductivity of 2x10E-8 cm/sec. The staff notes that in the data presented in l Table D.4.4 there were two tests on soil amended with 6 percent of Bentonite, I and both yielded hydraulic conductivity of 1x10E-8 cm/sec. However, mixing l silty clay with three percent of Bentonite in the field to achieve a uniform mixture could be difficult to accomplish, and could result in a nonhomogeneous
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or heterogeneous soil-bentonite mixture, which in turn may not have the desired '
average hydraulic conductivity. The DOE should consider increasing the 1 bentonite content to 6 percent to ensure that the soil-bentonite mixture will be relatively uniform, as this may result in the average hydraulic conductivity to be within the desired values. The DOE should demonstrate statistical reliability or reproducibility of this critical parameter by additional testing in the laboratory and by laboratory tests on actual field compacted samples taken during the initial stage of racon barrier placement.
The design of the cover, from a perspective of providing protection against freezing of the radon / infiltration barrier and reducing the radon emanation from tailings and other contaminated materials, to comply with the EPA Standards (40 CFR Part 192.02 (b)), is satisfactory (see Section 6.2 of this report). The evaluation of the disposal cell and cover regarding compliance with the EPA Groundwater Standards is addressed in Section 5 of this report. The evaluation findings in Section 5 of this report, frcm a groundwater protection perspective, are predicated upon the expectation that the DOE will satisfactorily demonstrate the validity of the assumed hydraulic conductivity of the radon / infiltration barrier. Therefore, the DOE must develop and submit tc NRC additional data from laboratory tests that demonstrate that the design assumptions of 2x10E-8 cm/sec saturated hydraulic conductivity and 1x10E-9 cm/sec unsaturated hydraulic conductivity for the radon barrier material can be successfully achieved. Laboratory tests on samples taken during construction that demonstrate that the desired hydraulic conductivity has been achieved must also be conducted and documented (see Section 3.4 below).
l 3.4 Gcotechnical Construction Criteria The radon barrier material is proposed to be mixed with 3 percent by weight of sodium bentonite and compacted to 100 percent Proctor density to result in a hydraulic conductivity of 2x10E-8 cm/sec. In support of this, the
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00E quotes their experience of field testing at the Tuba city site. The hRC staff would like to point out (1) that at the Tuba city test the DOE initially experienced difficulty in achieving 100 percent Proctor compaction, (2) the soil in the Tuba city test is more plastic than the soil at the Green River site as evidenced by comparing the Atterberg limits values for these soils, and (3) the specifications for mixing bentonite (Section 3.5.C, page 02200-22 of specifications, Reference 8) have no statement on the weight percent of the bentonite to be added to the radon barrier soil. The weight percent of bentonite to be added should be stated in the specifications. Since the assumed hydraulic conductivity of the radon barrier is critical to the success of the DOE's strategy to comply with the EPA groundwater standards, the staff requires demonstration of having achieved this assumed hydraulic conductivity in the field by performing laboratory tests on actual field compacted samples taken during construction of the cover.
Part of the DOE's strategy to meet the groundwater standards is to ensure that the tailings and other contaminated materials in the disposal cell are at their equilibrium or steady state moisture content, so that there is no free water that would be migrating towards the bottom of the disposal cell. There is no statement in the RAP on the estimated values of these moisture contents.
The specifications state that these materials should be compacted at a minimum of 3 percent dry of the optimum moisture content determined by l'TM D 698 test (Proctortest). The in situ moisture contents for tailings and other contaminated materials range from a low of 1.2 percent to a high of 15.5 percent (Table D.5.22 of Reference 8), whereas the optimum moisture content for these materials range from 10 to 16 percent for tailings and 10 to 13 percent for buffer zone material (natural material at the disposal site). The flux calculations, using the SUTRA code, indicate that the required steady state moisture contents are approximately 9% for buffer zone material, 6% to 9% for ,
winablown material, and 3% to 9% for tailings material. Placing the buffer l zone material at 3 to 5 percent less than the optimum moisture content will result in the material being placed close to its steady state moisture. There are no Proctor compaction data for the windblown material, and in the absence
GRNFTER3 l of that no judgement can be made on whether the placement moisture for this material will be close to its steady state moisture content. Placing the tailings at a minimum of 3 percent dry of optimum moisture content will result in the placement moisture content (7% to 13%) being higher than the required steady state moisture content (3% to 9%) indicated in the analysis. The DOE needs to develop data on the windblown material to determine if it can be placed at a moisture content close to the steady state moisture content for that material. It is reiterated that the moisture contents mentioned above are all weight percent moisture contents used by geotechnical engineers and not volumetric moisture contents used by hydrogeologists.
Since the design requirement is to place all the materials in the disposal cell at as low a moisture content as possible, and all the materials to be placed in the disposal cell are granular material, there is a potential of not being able to compact the relatively dry granular material to the desired density. Under-compaction of relatively dry granular material would result in the material being placed in a loose condition, which in turn would result in low strength and a potential for volume change d ring a seismic event. The specifications provide for the first 1,000 cyds of the fill material to be placed under controlled conditions to develop compaction procedures that would ensure the specified density. The staff concludes that this trial compaction should be extended to at least four lifts and that the desired density should be achieved for the full depth of compaction ie. four lifts. This requirement must be met because 1,000 cyd of material spread over the base of the disposal cell (400 ft. by 400 ft.) would result in a thin layer, and trial compaction of a thin layer will not yield information on compaction procedures that would be applicable for actual earth work.
The specifications for the radon barrier material (Section 2.1.B, page 02200-9 of Reference 8) are very general and require that the material shall be comprised of soils with a minimum of 50 percent by weight passing a No. 200 sieve and a maximum of 10 percent by weight retained cn a No. 4 sieve. The restriction on the percent passing No.200 sieve is not conservative from a
o GRNFTER3 hydraulic conductivity perspective. Most of the hyoraulic conductivity tests performed on this material had in excess of 65 to 70 percent passing a No. 200 sieve (Table D.4.4 of Reference 9), and few tests on samples with 55 to 60 percent passing the No. 200 sieve indicated hydraulic conductivities in 10E-5 to 10E-7 cm/sec range. As the hydraulic conductivity parameter is dependent on the percent fines content of the soil, the specifications should be revised to i ensure that the percent fines content of the material placed is very similar to the percent fines content of the materials tested to arrive at the design parameter. Also, the clay content of the fines in a soil is critical and the specifications should include a provision on the Atterberg limits for the radon barrier soil to be similar to the Atterberg limits for the materials tested to develop the design parameters. The specifications for the radon barrier materials should be revised to address the above concern and also to include the percent by weight of the bentonite to be added.
In summary, the DOE should develop and submit to NRC additional data that would ensure (1) the placement moisture content (minimum of 3 percent cry of opcimum moisture content) is appropriate to result in the material being placed at a moisture content close to the steady state moisture, and (2) that relatively dry granular materials can be successfully compacted in the field to the density that is assumed in the design. The specifications for the radon barrier material should be revised to address the above mentioned concerns.
3.5 Site Design The area at the toe of the disposal cell slope is filled with riprap.
This riprap placement is to ensure that the toe will not be eroded in the event erosion reaches the toe of the slope. Type B riprap with a D50 size of 1.5 ft.
is used in this area (See Drawing No.GRN-PS-10-0517, Typical Riprap Toe Protection Detail, Reference 8). The Select Fill Type-B soil, proposed to be filled above the riprap, is excavated from the disposal cell foundation area, and the specifications for this material have no special gradation
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I GRNFTER3 requirements. The staff is concerned that the fine particles of this overlying ,
fill will gradually migrate with time into the large voids in the riprap. A combination of gravity and infiltrating water could initiate this particle movement, and will become noticeable in the long-term when this migration )
results in surficial subsidence. This will require maintenance in the long-term. This is not expected to affect the stability of the disposal cell slope, but the area adjoining the toe of the disposal cell slope will require maintenance. The DOE should either address this concern in the design or submit adequately documented information to support a conclusion that the above postulated scenario will not occur.
3.6 Conclusion Based on a review of the design for the Green River site as presented in the remedial action plan (References 8,9, and 10), the NRC staff concludes that the design will comply with the long-term stability aspects of the EPA standards (40 CFR Part 192.02(a)) provided the following items can be successfully resolved. For reasons provided in Sections 3.3.4, 3.4, and 3.5, the DOE must: (1) demonstrate that a saturated hydraulic conductivity of 2x10E-8 cm/sec and a unsaturated hydraulic conductivity of 1x10E-9 cms /sec is achievable for the amended radon barrier material when placed at 100 percent standard proctor density at a moisture content of 0 to 3 percent above the optimum moisture content; (2) confirm during construction that the desired hydraulic conductivity is being achieved by performing laboratory tests on field-compacted radon barrier samples; (3) demonstrate that the tailings and other contaminated granular materials designated to be placed in the disposal cell in a relatively dry condition can be compacted to the desired density at a moisture content of 3 to 4 percent less than the optimum moisture content and that the placement moisture content is close to the steady state moisture .
content for these materials; (4) revise the specifications for the radon barrier material to reflect the actual percent fines and Atterberg limits of the samples tested to demonstrate the design hydraulic conductivity; (5)
GRNFTER3 resolve the issue of the potential for migration of Select fill Type B into the voids of Type B riprap in the area adjoining the toe of the disposal cell slope by either modifying the design or submitting aaequately documented information to support a conclusion that migration will not occur.
d GRNFTER3 6.0 RADON ATTEf:UATION AND S0Il CLEANUP 6.1 Introduction This section of the TER documents the staff evaluation of the radon attenuation design and the radiation survey plan to assure compliance with the EPA standard.
6.2 Radon Attenuation The review of the cover design for the radon attenuation included evaluation of the pertinent design parameters for both the tailings and the radon barrier soils, and the calculations of the radon barrier (earth cover) thickness (References 9, 10, and 38).
The design parameters for the tailings and earth cover materials evaluated for acceptability include: long-term moisture content, material thickness, bulk censity, porosity, and radon diffusion coefficient. In addition, radium content and radon emanation coefficient parameters were evaluated for the tailings materials only. The computer code RAECOM was used to calculate the radon barrier cover thickness, and the input included the above parameters.
6.2.1 Evaluation of Parameters To meet the EPA standards for limiting release of Radon-222 from residual i radioactive material to the atmosphere, the tailings pile will be covered with an earthen cover (radon barrier). The radon barrier reduces the effluence of Ra-222 by reducing the diffusion rate to acceptable quantities. The thickness l of the barrier depends on the properties of the barrier and tailings. For the earthen cover for radon attenuation, the DOE proposes to use silty clay from a borrow site and mix it with 3 percent by weight of Sodium Bentonite. The material properties and radiological parameters used in the design of the radon l l l
s D GRNFTER3 barrier for the stabilized tailings pile at the Green River site have been reviewed.
The radon barrier will be compacted at a moisture content of 0 to 3 percent above the optimum moisture content. This will result in the average placement moisture content of approximately 17 percent. The average in situ moisture content for this material is approximately 5.5 percent. The staff has calculated the long-term moisture content using Rawls' (Reference 5) method (a very conservative method) to be 9 percent. The DOE calculation uses a long-term moisture content of 11.9 percent based on data from a capillary-moisture test. Considering the presence of a one-foot-thick rip rap and a 6-inch-thick gravel bed on top of the radon barrier, and that only the bottom 15 inches of the three-feet thick radon barrier is designated for protection against radon emanation, the staff concludes that the lower portion of the radon barrier will retair, most of its placement moisture in the long-term. The staff, therefore, concurs with the D0E's estimation of 11.9 percent long-term moisture content for the radon barrier material.
The tailings materials will be compacted at a moisture content of 3 percent below the optimum moisture centent. This will result in an average placement moisture content of 9 to 10 percent. The average in situ moisture centent for this material is approximately 3.5 percent. The average long-term moisture content at a 15 bar capillary pressure, determined from moisture-capillary tests for the sandy material at this site, is approximately 2.7 percent. The DOE has used a long-term moisture content of 10 percent in the design calculation. As the DOE is proposing to place the materials in the disposal cell at the least possible moisture (minimum of 3 percent belcw the optimum moisture content) that would permit compaction to the design specifications, the DOE's estimation of 10 percent long-term moisture content for the tailings is high. The staff estimates the tailings long-term moisture content to be in the range of 6 to 7 percent, which is close to the steady state moisture stipulated in the DOE's analysis for compliance with ground
o GRNFTER3 water standards. The effect of this lower moisture content on the thickness of the radon barrier is discussed in Section 6.2.2.
The material thicknesses (layers) used in DOE's analysis are based on the conceptual design of the remedial action plan and data available from field investigations. The tailings and the contaminated wind blown materials will be placed in the disposal cell, and there is no layering or preferred placement of these materials within the disposal cell. The design assumes uniform, average properties for these materials. The material thickness (44 feet for tailings and contaminated materials) used in the analysis is a reasonable representation of the field conditions. The staff is aware that the properties of materials below a depth of 10 to 15 feet beneath the radon barrier will have very little or no impact on the results of radon barrier thickness design.
Material properties such as bulk density and specific gravity were determined by field and laboratory tests, and the corresponding porosity was calculated. The bulk density and porosity for the tailings material are 1.52 gm/c.c and 0.430 respectively. The corresponding properties for the radon barrier soil (virgin soil, not mixed with bentonite) were 1.87 gm/c.c and 0.306 respectively. Though the DOE has not provided these parameters for the amended soil, they are not expected to be very different from the values for the virgin soil, and any minor variations in these parameters are not expected to have any impact on the radon barrier design. The staff has reviewed the geotechnical data and concludes that the above values of the parameters are a reasonable representation of the average conditions that should be used in the design computations.
Radon diffusion coefficients for the cover material and tailings were i derived from a correlation curve of moisture saturation versus radon diffusion coefficients based on the estimated moisture for the long-term for the materials. This curve was developed using diffusion coefficient and moisture )
saturation data from both field and laboratory measurements of soil samples that are representative of the condition in the stabilized pile. The diffusion
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coefficient for the radon barrier material is 0.00247 cm2/sec for the estimated long-term moisture content of 11.9 percent. The diffusion coefficient for the tailings material is estimated to be 0.02100 cm2/sec for the long-term moisture content of 10 percent. However, because of the DOE's approach of compacting tailings at as dry a condition as possible, the staff estimates the long-term rr.oisture content will te in the range of 6.5 percent and the corresponding diffusion coefficient (Figure B.2.1 of Reference 9) would be in the range of 0.023cm2/sec. The staff has reviewed the information used in determining the diffusion coefficient value for the radon barrier material and judges it to be reasonable. The staff does not agree with the value of the diffusion coefficient for the tailings material used in the DOE's design (Reference 38).
However, the staff has evaluated the effect of this change in diffusion coefficient on the thickness of radon barrier required to comply with the EPA i standards. The thickness of the radon barrier, calculated using the RAECOM code and higher diffusion coefficient (0.023 for a moisture content of 6.5%)
for the tailings traterial, did not show any significant increase (increased by 1 cm.) compared to the thickness of 11 cm. calculated for a diffusion coefficient of 0.0210 cm2/sec used in the DOE's calculations. Section 6.2.2 of this report discusses this item.
The radium content (Ra-226) of several materials at the site was measured.
The average radium content to be used in the analysis was determined by weighted averaging with depth in a measurement hole and then averaging over an area at any given depth. The weighted average value of the radium content for the entire pile was calculated to be 74 pCi/gm. However, the average radium content will be verified by field measurements on the stabilized tailings pile before placing the radon barrier earth cover. Although the radon barrier design will be reassessed at that time, the DOE has committed not to reduce the thickness of the radon barrier layer even if the assessment indicates that the thickness could be reduced. The staff concurs with the value and methodology used in design for establishing the average radium content .
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GRNFTER3 The radon emanation coefficient was measured in the laboratory on samples representative of field conditions. An emanating coefficient of 0.28 was conservatively used in design for the tailings material. Based on the values of this parameter determined for similar materials at other UMTRAP sites, the staff considers this value to be reasonable and acceptable.
The ambient air radon concentration was measured to be 2 pCi/1. The technique used to measure the radon concentration has been previously approved by NRC and the result is acceptable to the NRC staff. This parameter is an input for the RAECOM modeling calculation used in designing the thickness of the radon barrier cover.
6.2.2 Evaluation of Radon Barrier P The radon barrier (earth cover) thickness necessary to comply with the radon efflux limit was calculated using the RAECOM computer code. For a given assumed thickness of the redon barrier, the RAECOM code calculates the radon gas release rate. The EPA standard requires that the release of radon-222 from residual radioactive material to the atmosphere not exceed an average release rate of 20 picocuries per square meter per second. The current cover design has a three-foot-thick radon barrier beneath the riprap and gravel bedding. As discussed in Section 3.3.4 of this report, the upper 39 inches of the cover consisting of 12-inch-thick riprap, 6-inch-thick gravel bed, and top 21 inches of the radon barrier will provide provide protection against freezing. In a worst case scenario, the top 21 inches of the radon barrier will be subjected to freeze-thaw conditions anc would have lost its as-compacted condition in terms of density and possibly have minor openings or cracks. Therefore, the upper 21 inches of the radon barrier is not expected to contribute to the radon diffusion function of the radon barrier, and only the lower 15 inches (38 cms) of the radon barrier are designated to be functional in reducing the radon release.
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GRNFTER3 The DOE design (Reference 38) estimates that only 11 cm. (4.3 in.) of radon barrier is sufficient to reduce the radon release to a value in compliance with the EPA standards, and therefore, the 15-inches provided in the current design is more than adequate to fulfill the radon barrier function.
The staff estimated the required radon barrier thickness using a lower long-term moisture content for the tailings to be 12 cm. (4.7 in.) or about the same as DOE's estimate. Considering the built in conservatism in the current cesign thickness of the radon barrier required to meet the EPA standards, the staff concludes that the DOE design is satisfactory and that the disposal cell cover will comply with the radon release requirements of EPA (40 CFR Part 192.02 (b), Subpart A).
6.3 Site Cleanup
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