ML20011F368
| ML20011F368 | |
| Person / Time | |
|---|---|
| Issue date: | 02/22/1990 |
| 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 9003050215 | |
| Download: ML20011F368 (26) | |
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-BJ/MEMTK02.MEM FEB 2 2100 MEMORANDUM FOR: Myron Fliegel, Section Leader Operations Branch Division of Low-Level Waste Management and Decomissioning, NMSS FROM:
Michael Tokar, Section Leader Technical Branch Division of Low-Level Waste Management-and Decommissioning, NMSS
SUBJECT:
FINAL TER FOR GREEN RIVER UMTRA PROJECT
REFERENCES:
1.
Memo dated April 24,1989,from M. Tokar to M.Fliegel; Sub: Final TER for Green River UMTRA Project - Revision 1.
2.
Meno dated April 6,1989, from P. Lohaus to J.
Greeves; Sub: NRC/ DOE Green River Meeting of April 5, 1989.
3.
DOE, 1989, Remedial Action Plan and Final Design for Stabilization of Inactive Uranium Mill Tailings at Green River, Utah, Final VOL. I, IIA, III and two volumes of Supporting Calculations and Field Test Data, December 15, 1989; UMTRA-DOE /AL 050510.GRN.
The final design and Remedial Action Plan for stabilizing tailings at the Green River site were previously reviewed, and our report (TER) of April 24, 1989,-
- documented our evaluation (Ref.1). Our TER of April 24, 1989, was based on the DOE. satisfactorily complying with the comitments made to NRC (items No. 3.
4, 5, and 6 of NRC/ DOE-agreement, Ref. 2). The final RAP documents reviewed at this time (Ref. 3) include DOE's documentation of their compliance with the comitments.
The staff evaluation of DOE's compliance with the commitments identified in Reference 2 is as follows:
1.
Comitment No. 3 - To demonstrate by laboratory tests on as-compacted samples of infiltration / radon barrier that the saturated hydraulic g
conductivity is less than 2x10-8 cms /sec.
The DOE has perforned the required number of tests, and the results indicate that the average value of the saturated hydraulic conductivity of the as-com) acted samples of infiltration / radon barrier is 0.61x10-8
/ /
. cms /sec, witch is less than the required value.
The DOE has therefore complied with this comitment to demonstrate the saturated hydraulic conductivity of the infiltration / radon barrier material.
2.
Comitnent No. 4 - To place tailings and other contaminated materials in the disposal cell at a volumetric moisture content of 5 percent and 10.6 9003050215 900222 D
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percent for tailings and other contaminated materials respectively. Also, the DOE comitted to perform trial compaction of these materials to develop compaction procedures so that the materials can be compacted to the required densities at the desired moisture contents.
The DOE has submitted results of field density and moisture content tests to demonstrate compliance with this comitment.
The required density has been achieved, but the moisture content is slightly higher than the o
comitted values. This higher moisture content has no adverse effect on the geotechnical stability of the disposal cell.
The impact of this on the disposal: cell. complying with the EPA Groundwater Standards should be addressed by the hydrogeologist in Section 5 of this TER.
3.
Comitment No. 5 - To amend the infiltration / radon barrier material with 6 percent by weight of Sodium Bentonite and to compact the material to a density of 100 percent Proctor density and at a moisture content of 0 to 3 percent above the optimum moisture content.
The DOE h' as amended the specifications to reflect this comitment.
Results of tests performed on as-compacted samples from the.
J infiltratior./ radon barrier indicate that the desired density and moisture content have been achieved in the field.
The DOE has therefore complied with this comitment.
4.
Connitment No. 6 - To use material with a minimum of 70 percent fines for the first lift and material with a minimum of 50 percent fines for the subsequent lifts of the infiltration / radon barrier.
L The DOE has changed the specifications for the infiltration / radon barrier E
material to comply with this commitment. Documentation of compliance with this will be reviewed during the review of the project completion report,
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Excest for the slightly higher noisture content of the contaminated materials in t1e disposal cell, the DOE has met the geotechnical engineering related
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comitments.
The significance of the higher moisture content on the groundwater travel time should be addressed in Section 5 of the TER.
Because the final quantity of the contaminated material to be stabilized in the disposal cell was higher than the quantity used in the original design, the
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height of the disposal cell in the final design is approximately 16 feet higher than that analyzed in the original design.
DOE should submit an analysis to demonstrate the geotechnical slope stability of the disposal cell slope.
This is a confirmatory item in this TER and will be removed after DOE demonstrates tho'long-term stability of the disposal cell slope.
Except for the above confirmatory item, and the moisture content issue (which is referred to the hydrogeology review area) the other aspects of the geotechnical engineering review of the final RAP for the Green River project are acceptable and are documented in the attached TER.
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BJ/MENTK02.NEM i
i This review was performed by Banad Jagannath; please contact him should you have any questions.
i originaT Signed W
. Michael Tokar, Section Leader Technical Branch Division of Low-Level Waste Management
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and Deconmissioning, NMSS
Enclosure:
-As stated i
i Distribution:
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LLTB r/f
. JGreeves, LLWM MBoyle, LLRB PLohaus, LLOB MToker, LLTB BJagannath, LLTB SWastler, LLOB i
Reason:
Proprietary /
/
or CF Only /
/
ACNW Yes: N/
No: /
/
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SUBJECT ABSTRACT:
Final TER for Green River UMTRA project, Feb. 1990 0FC.:L lLLTB g
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3.0 GEOTECHNICAL STABILITY j
4 3.1 Introduction TheNRCstaffreviewofthegeotechnicalen[gineeringaspectsoftheremedial
-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 I
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)SubpartA,fromageotechnicalengineeringperspectiveofslopestability, I
liquefaction, and settlement.
The staff review of 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.
AttheGreenRiverUraniumMillsite(presentlyaninactivesite)theore concentrate was shipped to a processing plant in Rifle, Colorado, and thereby the-tailings left at this site were pmdominantly sandy tailings with no slime.
The remedial action of stabilization-on-site consisted of placing all the contaminated material at the site (approximately 382,000 cyds) into a single l.-
pile, which is called the disposal cell.
The location of this disposal cell is
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approximately 500 feet south and about 50 feet higher in elevation than the existing tailings pile location. Thedisposalcellbottom(elevation 4098 f
feet) is appmximately 42 feet below the existing ground surface (elevation 4140 feet)andthetopofthedisposalcell(elevation 4181 feet)isabout41 feet above the adjoining ground surface.
The construction of the portion of disposal cell below the ground surface required excavation of approximately 16 feet of overburden material and 26 feet of bedrock.
The portion of the disposal cell above the existing ground surface rises to the crown of the cell
-(elevation 4181 feet)'at a gentle slope of 5 horizontal to 1 vertical (SH:IV).
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The disposal cell design provided for placing a six-feet-thick layer of select
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material as a buffer zone at the bottom of the cell between the bedrock and the l
contaminated materials. The disposal cell has been covered, in the ascending
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order, with (1) a three-feet-thick infiltration / radon barrier, (2) a
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six-inch-thick gravel beading, and (3) a 12-inch-thick rock layer (riprap).
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The cover was designed to ensure the following: (1)long-termstabilityof embankment and reduced radon emissions; (2) reduced infiltration., (3) protectionofsurfacewaterquality;(4)protectionagainstanimalintrusion;
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(5)minimizedplantrootintrusion;(6)preventionofinadvertenthuman intrusion;and(7)preventionofmaterialdispersion(Ref.8).
This section presents the geotechnical engineering evaluation of the long-term stability aspects of the proposed remedial actions.
3.2 Site Characterization
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3.2.1 Site Description Section 1 of this report presents a description of the Green River project I
site.
3.2.2 Site Investigations Subsurface explorations at the site were performed by the following
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investigators:
(1) Bendix Field Engineering Corporation determined the extent of contamination.
The investigations resulted in data from 105 bore-holes, 184 in situ Ra-226 measurements, and 139 soil samples.
Addendum DI to Appendix D in reference 8 presents detailed information on this investigation.
The results of this investigation
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I were used in establishing the volume of contaminated material to be removed from its present location to comply with the EPA Standards.
This removed contaminated material was placed in the disposal cell.
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.l (2) Jacobs Engineering Group, Inc. (1986, 1987, 1988) and Morriso'n-Knudson Engineers Inc. (1986-1987). The scope of the geotechnical investigations included (1) borings from which soil samples and rock cores were obtained (2) test pits from which bulk samples were obtained, and (3) installation of monitoring wells.
l These investigations were performed to determine geotechnical characteristics of the site and to obtain samples of the soil and rock materials for laboratory determination of their properties.
Information to Bidders, Volumes I, II, and III of Reference 9, and j
Volume IIA Appendix D of the draft final RAP dated January 1989 (Ref.
8)presentdetailedinformationonsiteconditionsandlogsofthese 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 drilled 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-barrel sampler was used to sample the materials. The SPT tests were conducted according to ASTM D 1586 procedures.
Figure 3.7 of the RAP (Refs. 8
.and46)showslocationsoftheboringsandtestpits. Section 2 of this report presents an evaluation of the geologic, geomorphic, and seismic characteristics i
of the site.
L The overburden materials at the site consist of an alluvium deposit underlain by a thin layer of gravel which in turn overlies the bedrock.
The alluvium l
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deposit consistr of silty to clayey sand, with dense sand and gravel occurring 1
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 j
to 43 and averaging 18 blows / foot. The sedimentary bedrock units at the site consi'st of a shale member of the Mancos shale, the Dakota sandstone,.and the Cedar Mountain Formation.
The upper portion ',f the bedrock is weathered and fractured. Section 2 of this report presenti a detailed evaluation of the bedrock conditions at the site.
At the existing tailings pile area, the site Stratigraphy consists of sand tailings overlying the alluvium deposit (silty sand-clayey sand) which in turn overlies the bedrock. Tailings and underlying contaminated alluvium materials were excavated from their present location and placed in the disposal cell.
j The overburden soils at the proposed disposal cell location consist of from 5 j
to 16 feet of loose to dense alluvium (silty sand - clayey sand).
Thick lenses of clay are contained within this layer.
Dense to very dense sand and gravel occur at the bottom of this alluvium deposit.
Since the disposal cell was founded on the bedrock, the overburden material was excavated.
This alluvium material was selectively used as Select Fill Type-A material, for the L
six-feet-thick buffer zone placed at the bottom of the cell between the bedrock and the tailings.
The disposal cell excavation extended to a depth of approximately 26 feet into the bedrock.
This excavation resulted in removing the entire Mancos shale and Dakota sandstone stratum, and part of the Cedar Mountain Formation shale /mudstone down to an elevation of 4098 feet.
The groundwater table at the proposed disposal cell location is estimated to be 4083-4085 feet in elevation, approximately 55 feet below the ground surface and 13 feet below the bottom (elevation 4098 feet) of the disposal cell. Section 5 of this report presents a detailed evaluation of the groundwater conditions at the site.
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5 Soil for the radon barrier cover and gravel for the bedding layer were taken-
-from Borrow Site 1.
Figures 3.15 through 3.24 of the RAP (References 9 and 46) show the location and stratigraphy of the borrow area. A total of 24 test pits were dug to investigate the availability and suitability of the soils for the
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intended "se.
The stratigraphy at the borrow site consists of an alluvial 1
deposit with a surficial layer of silty-claby sand, underlain by low-plasticity clay. The clay layer is underlab by a alluvial sand and gravel 1
stratum.
The' test pits were terminated in the sano gravel stratum. The low-plasticity clay was used for the infiltration /ravon barrier cover and the alluvial sand-gravel material was processed to obtain 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 J
adequately established the stratigraphy and soil conditions to support an J
assessment of the geotechnical stability of the stabilized tailings and i
contaminated material in the disposal cell.
Further, the geotechnical explorations are in general conformance with applicable provisions of Chapter 2 of the NRC Standard Review Plan (SRP) for UMTRCA Title ! Mill Tailings Remedial ActicaPlans(Reference 5).
3.2.4 Testing Program The staff has reviewed the geotechnical engineering testing program for the
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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.
However, the DOE had not submitted all the test data (for example, capillary-moisture relationship, adequate number of hydraulic conductivity
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(Reference 8). As a result of an NRC/ DOE meeting to discuss the staff evaluation of the draft final RAP, the DOE committed to provide the following data to support the design in the RAP (Ref. 44).
Hydraulic conductivity test results for field-compacted samples of infiltration / radon barrier material.
Density and moisture content of tailings and contaminated materials as-placed in the disposal cell.
The staff has reviewed the above test data submitted by the DOE along with the finalRAP(Ref.46),andconcludethattheD02hasmetthecommitmentsinterms i
of providing the data to support their design.
Details of the relevance of this data to the design is addressed in the staff evaluation of the cover properties and geotechnical stability of 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
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and the utilization of the resulting data to define the material properties are in general agreement with applicable provisions of the SRP (Reference 5).
l' 3.3 Geotechnical Engineering Evaluation f
3.3.1 Stability Evaluation The evaluation of the geotechnical stability of the slopes of the proposed disposal cell containing stabilized tailings and other contaminated soils 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 (Refs.10 & 11). The
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analyzed cross section with a 5 horizontal to I vertical slope has been-compared with the exploratory records and design details. The height of the slope analyzed in the draft final RAP document was approximately 25 feet above the adjoining ground surface. The staff finds that the height of the disposal l
cell has increased by approximately 16 feet, as per the final RAP (Reference 46).-becauseofanincreaseinquantityofthecontaminatedmaterialstobe f
stabilized in the disposal cell.
The final RAP does not present any analyses i
of the long-term stability of the disposal cell slope for the final height of approximately 41 feet.
Because of the revision in the height of the slope, the slope stability analyses presented in the RAP documents is not for the most I
critical slope, and therefore is not acceptable to the staff.
The staff has requested the DOE-to provide an appropriate analyses of the long-term stability of the disposal cell slopes. The geotechnical slope. stability of the disposal I
cell is considered to be a confirmatory item, pending submittal of a revised i
analyses by the DOE and an evaluation by the staff.
l 3.3.2 Liquefaction Based on review of results of the geotechnical investigations, including boring i
logs, test data, soil profiles, and disposal cell design, the NRC staff concludes that the DOE 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 are a minimum of 90 percent of the maximum dry density by the ASTM D-698 test, and by design these materials are in an unsaturated condition, these materials are not susceptible to liquefaction.
The disposal cell is founded on bedrock, which is also not 1
susceptible to liquefaction.
The groundwater table at the site is estimated to be approximately 13 feet below the foundation of the disposal cell.
Considering the placement density and absence of free moisture in the disposal l
cell, the materials in the disposal cell are judged to be not susceptible to liquefaction.
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3.3.3 Settlement Long-term settlement of materials in the disposal cell, which could result in
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either local depressions on top of the cover or cracks in the cover, has been j
adequately addressed in the RAP. If depressions in the cover were to form they could initiate erosion gully pathways followed by a potential exposure of 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 compacted to a minimum of 90 percent of Standard Proctor density at a moisture content of 3 or more percent below the optimum moisture content, a major portion of the settlement will be instantareous and will take place during construction.
Any potentially adverse effects of the instantaneous settlement of these san @
materials was rectified before completion of the construction and therefore, will not adversely affect the long-term performance of the disposal cell.
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 materials 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 effects on~ the ability of the disposal cell to meet the EPA Standards.
3.3.4 Cover Design The cover for the disposal cell consisted of the following, in descending order I
from the top: (1) an erosion protection feature composed of a one-foot-thick, Type-A riprap; (2) a 6-in.-thick gravel bedding layer; and (3) an I
infiltration /raden barrier composed of a three-foot-thick layer silty-clay amended with Bentonite (Refs 8 and 46).
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- The staff's evaluation of the cover design considered the design adequacy with I
regard to erosion protection, radon attenuation, frost penetration and I
' infiltration. The riprap and its bedding layer is designed to protect the
' radon / infiltration barrier in the long-term.
The staff's evaluation of the erosion protection layer and its ability to comply with the long-term stability l
u pects of the EPA Standards is presented in Section 4 of this report.
The staff evaluation of the adequacy of the infiltration barrier, as part of the DOE's design to comply with the EPA Groundwater Standards, is addressed in Section 5 of this report.
The staff's evaluation of the adequacy of the thickness of the radon barrier to attenuate the release of radon to comp 1.y with the EPA standards is addressed in Section 6 of this report.
i The D0E.has' performed an evaluation of the freezing conditions at the Green River project site and has concluded that the maximum freezing depth at the site is 39 inches.
The DOE has used 200-year weather data for the Green River l
site and a computer code developed by U.S Ary C:ld Regions Research and Engineering Laboratory for the' modified Bergren Solution to calculate the depth of frost penetration.
As part of the design, the DOE has performed a sensitivity analysis to arrive at the recommended frost penetration depth of 39 inches. The staff has 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 recommendations. As an independent verification, the depth of frost penetration indicated in Figure 7.1-42 of Reference 42, prepared 7
by the U.S. Arg 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 54-inch thick cover, while the lower 15 inches of radon / infiltration barrier layer will be in the unfrozen or intact condition. Therefore, the DOE's design of the radon barrier for freezing / frost condition is satisfactory.
The adequacy of this lower 15 inches of radon / infiltration barrier to control the infiltration into the disposal cell and to reduce the radon amanation from the disposal cell to comply with the EPA i
Standards are addressed in Sections 5.0 and 6.2, respectively, of this report.
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The radon / infiltration barrier design assumes that the Bentonite amended silty clay material, used for the radon barrier, can be compacted to result in a 1
material.whose saturated hydraulic conductivity does not exceed 2x10-8 cm/sec.
-Further, the design against infiltration assumes that the long-term moisture saturation condition of the radon barrier will be unsaturated, and therefore, the unsaturated hydraulic conductivity will'be approximately an order of magnitude lower (i.e., Ix10-9 cm/sec or lower) than the saturated hydraulic cond'uctivity. This permanent unsaturated condition will result in an infiltration rate or flux of 1x10-9 cm3/cm2.sec or less through the cover.
This infiltration rate value is the critical parameter in the design of the disposal cell cover to comply with the EPA Groundwater Standards for_UNTRCA projects. Section-5 of this report presents details on the evaluation of the disposal cell cover to satisfy the EPA Groundwater Standards.
The laboratory test data (Table D.4.4 of draft final RAP, Ref. 9) presented in support of the saturated hydraulic conductivity consists of three tests on silty clay amended with 3 percent of Bentonite resulting in saturated hydraulic conductivities of 2x10-8, 1.5x10-8, and 3.4x10E-8 cm/sec with an average value of 2.3x10-8 cm/sec.
However, the data presented in Table D.4.4 for the radon 1
barrier material only (without Bentonite) show the hydraulic conductivity parameter to range from a low of 2.4x10-8 to a high of 8.5x10-5 cm/sec.
The staff notes that in the data presented in Table D.4.4 there were two tests on soil amended with 6 percent of Bentonite, and both yielded-hydraulic conductivity in the range of 1x10-8 cm/sec. Considering the range of the hydraulic conductivity values presented in Table D.4.4, and the sensitivity of this parameter to compaction density, Bentonite content, moisture, and percent
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fines in the soil, the staff believed that in the draft final RAP document the DOE had.not adequately established with reasonable assurance that the silty clay amended with 3% Bentonite (for radon barrier) would have a saturated hydraulic conductivity of 2x10-8 cm/sec.
In addition, mixing silty clay with three percent by weight of Bentonite in the field to achieve a uniform mixture could be difficult to accomplish, and could result in a nonhomogeneous or heterogeneous soil-Bentonite mixture, which in turn may not have the desired
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4 average hydraulic conductivity as determined from tests on laboratory compacted.
samples.
Increasing the Bentonite content to 6 percent resulted in the soil-Bentonite mixture being relatively uniform.
This mixture, when compacted l
- to 100% Standard Proctor density, achieved the desired average hydraulic l
conductivity; viz., in the range of the values determined from laboratory
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testing.
Data in Table D.4.4 shows that silty clay material, with 55.to 60 percent fines passing No. 200 sieve and amended with 6% of Bentonite and compacted to 100% Standard Proctor density, had a saturated hydraulic 1
conductivity in the. range of 1x10-8 cm/sec.
Also, silty clay with-70% fines passing No '200 sieve and amended with 3% of Bentonite and compacted to 100%
Standard Proctor density had an average saturated hydraulic conductivity of 2.3x10-8 cm/sec. Therefore, the silty clay material mixed with 6% by weight ofc Bentonite and-complying with the above gradation and compacted to 100% Standard Proctor density at a moisturo content of 0 to 3% higher than the optimum resulted in a saturated hydraulic conductivity not exceeding 2x10-8 cm/sec.
As a result of a NRC/D0E meeting on the results of staff evaluation of the draft' final RAP, the DOE committed to three changes in the RAP (Ref. 44).
The
.finalRAP(Ref._46)presentstheinformationtosupportthattheDOEhas.made.
the changes and developed the required additional information to support the design.
The following is a listing of the DOE conritments and NRC's evaluation of the DOE's fulfilment of the connitments.
1.
The DOE connitted to constructing the first lift of the infiltration / radon barrier with material that has greater than 70 percent of the material passing the No. 200 sieve and material for the other lifts having 50 percent passing the No. 200 sieve.
The basis for this requirement was that the radon barrier material should be similar to that tested in the laboratory, and the soil samples used in hydraulic conductivity tests performed in the laboratory had an average fines (passing No. 200 sieve) content of 70 percent. Although the
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infiltration / radon barrier is th'ree feet thick, the lower 12-inch portion of the~ cover is adequate to fulfill its function, and therefore, this.
requirement was imposed 'n the first lift.
o The DOE has changed the subcontract specifications to include this i
condition (Section 02200 PART 2-8.3, pg.02200 - 9, Appendix F of Ref.
~ 46), and proper implementation of this specification will satisfy the -
commitment.
2.-
The DOE committed to mix no less than six percent by weight of Bentonite into the radon barrier material.
The basis for this requirement was that the hydraulic conductivity test
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results presented in the draft final RAP scattered over a wide range, and only samples with six percent Bentonite consistently met the hydraulic conductivity requirement of 2x10-8 cms /sec... Because the hydraulic conductivity of-the radon / infiltration barrier was a critical' parameter in the design of the cover to comply with the EPA Groundwater Standards, a conservative approach was taken in requiring six percent by weight' Bentonite in the radon barrier soil.
The DOE has changed the subcontract specifications to include this requirement (Section 02200 PART 3 - Section 3.5. C.2, pg 02200 -22, Appendix F of Ref. 46), and proper implementation of this specification met the comitment.
I; 3.
The DOE committed to perform moisture content and hydraulic conductivity testing of the radon barrier to ensure that the as-built saturated i
hydraulic conductivity does not exceed 2x10-8 cms /sec. The testing had a frequency of at least one test per 2,000 cubic yards of radon / infiltration barrier material.
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The design of the cover to satisfy the EPA Groundwater Standards was' not finalized at the time of the draft final RAP review, and based on a simple analyses it was determined that a saturated hydraulic conductivity of 2x10-8 cms /sec. for the radon / infiltration barrier would result in a cover that would meet the desired groundwater. travel time through the disposal cell. Details of this aspect of the design is aodressed in Section 5 of this report. Therefore, the above requirement of saturated hydraulic conductivity of the as-constructed radon / infiltration barrier was imposed.
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The hydraulic conductivity was to be demonstrated by laboratory tests performed on as-compacted block samples of radon / infiltration barrier layer taken from the field.
The DOE has completed.the placement of the radon / infiltration barrier layer and has submitted the results of hydraulic conductivity tests performed on as-built or field compacted block samples of radon barrier layer taken during construction.
The field-compacted samples were tested-in-the laboratory and'the saturated-hydraulic conductivity of 14 samples ranged from a low of 0.17x10-8 cms /sec to a high of 1.5x10-8 cms /sec with an. average of 0.61x10-8 cms /sec.
The hydraulic conductivity average value compared favorably with the required value of less tha'n 2x10-8 cms /sec.
All the samples had been compacted to a dry density of'100 percent Proctor density and a moisture content of 0 to 3 percent higher than the optimum, as per the specifications (Section 02200 PART 3, Section 3.8, pg 02200 -28 of Appendix F of Ref. 46).
The DOE has demonstrated compliance with the commitment on the hydraulic conductivity of the radon barrier.
I' Full compliance by the DOE with the above connitments is an adequate basis for the staff to reach a conclusion that there is a reasonable assurance that the radon barrier has been constructed to ensure that the as-built saturated hydraulic conductivity not exceed 2x10-8 cm/sec.
The design of the cover, from a perspective of providing protection against freezing of the radon / infiltration barrier is satisfactory.
The staff
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4 concludes with' reasonable assurance that the radon barrier has been constructed to have a saturated hydraulic conductivity of 2x10-8 cm/sec or lower.
The evaluation of the. disposal cell and cover regarding compliance with'the EPA
. Groundwater Standards is addressed in Section 5 of this report.
L.
.3.4 - Geotechnical Construction Criteria
- The DOE's strategy to meet'the EPA Groundwater Standards includes ensuring that
~ the tailings and other contaminated materials in the disposal cell are at their equilibrium or steady state moisture content, i.e. at unsaturated condition.
This unsaturated condition will slowdown the migration of any moisture towards.
i the bottom of the disposal cell. The specifications. state that these materials should be compacted at a minimum of 3 percent less than the optimum' moisture content determined by ASTM D 698 test (Standard Proctor test).
The in situ moisture ~ contents range from a low of 1.2 percent to a high of 15.5 percent
.i
- (Table D.5.22 of Ref. 8) for. tailings and approximately 6 to 9. percent for buffer zone material (overburden material at the disposal site). The optimum moisture content for these materials range,from 10 to 16 percent for tailings and 10 to 13 percent for buffer zone material, and_11 percent for'the windblown material. The flux calculations; using the SUTRA code to calculate the-groundwater travel time through the disposal cell, indicate that the required steady state moisture contents (weight percent of values presented as volume percent in Table E-3-5 of Reference 8) are approximately 9% for buffer zone material, 6% to 9% for windblown material,.and 3% to 9% for tailings material.
Placing the buffer zone material at the in situ moisture content would result i
p in the material being placed close to its steady state moisture. The proposed placement moisture content-for the windblown material was close to the desired
- state of approximately 3 percent drier than its optimum moisture content.
Placing the tailings at its in situ moisture content resulted in the placement moisture content close to the steady state moisture content (3% to 9%)
indicated in the analysis.
It is reiterated that the moisture contents mentioned above are all weight percent moisture contents used by geotechnical 4
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engineers and not volumetric moisture contents used by hydrogeologists. Since the design' objective was to place all the materials in the disposal cell at as low a moisture l content as-possible, and all the materials placed in the disposal cell were granular material, there was potential of not being able to'
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- compact the relatively dry granular material to the desired censity.
The j
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 indicated 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 i.e. four lifts.
Since compacting at such dry state was not originally contemplated in the draft RAP design, the staff-asked the DOE to demonstrate that these materials can be placed'in the l
L disposal cell at the densities and moisture contents assumed in the final design.
L i
As--a result of a NRC/00E meeting on the results of staff evaluation of the draft final RAP, the DOE comitted to the following (Reference 44).
L The DOE comitted to placing and maintaining contaminated materials in the disposal cell at the specified densities and at average moisture contents that are less than their average steady-state moisture contents and, in i
any case, less than 5% by volume (3% by weight) for the tailings and less
.than 10.6% by volume (5.5% by weight) for the windblown and other vicinity 1
property contaminated materials. The DOE comitted to place and test at least four lifts of-contaminated materials during the trial compaction (first 1,000 cyd of material), which was intended to develop procedures to ensure compaction of the materials ~in accordance with material specifications. The DOE also comitted to submit physical properties and compaction-data on windblown materials and any other data to support compliance with the condition that contaminated materials will be placed 1.
and maintained at the specified densities and moisture contents.
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The basis for this requirement was that DOE should demonstrate that the-densities and low moisture condition assumed in the design can be achieved in the field.
L In response to this commitment, the DOE has submitted results and analyses p
of field tests performed to determine the actual placement density and moisture content.
Becausc of difficulty in complying with the dust I
control requirements, the DOE could not place the materials at the desired moisture contents.
Some water had to be added to control the dust and this resulted in the placement moisture content being slightly higher than I
the' desired values. All the materials were compacted to the specified densities. The average percent compaction for the materials placed in the disposal cell is 95.6, 94.67 and 97.38 percent' Proctor compaction for tailings, contaminated materials and buffer zone materials respectively, whereas the specifications. required a minimum of 90 percent Proctor L
compaction. Therefore, the materials in the disposal cell have been placed to comply with the density specifications.
The placement moisture l
content _1's slightly higher than the values comitted to by the DOE,
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because of adding water to control the dust. The placement moisture-content was 7.2 % percent and 10.2 percent by volume for tailings and I
windblown and other vicinity property-contaminated material, respectively.
The corresponding moisture contents by weight are 4.6 and 5.5 percent, respectively, for tailings and windblown and other vicinity property-
'E contaminated material.
The DOE-comitted placement moisture contents are 5 and 10.6 percent by volume for tailings and windblown and other vicinity property-contaminated materials, respectively.
The effect of slightly higher moisture content of the contaminated materials placed in the disposal cell on the remedial action at the site (disposal cell) complying with the EPA Groundwater Standards for UMTRCA projects is addressed in Section 5 of this report. From a perspective of geotechnical stability of the disposal cell, the contaminated materials have been placed at specified density and the as-placed moisture content of the contaminated l
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materials in the disposal cell has no adverse impact.on the'geotechnical-
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stability'of the disposal cell.
3.5 Conclusion q
'The staff concludes that the slope stability analyses presented in support of L
the design in the draft RAP:is not appropriate for-the. final conditions of'the~
. disposal cell, and therefore a'. revised analyses is warranted.
Pending satisfactory resolution of this item, the staff' considers the long-term-stability aspect of the proposed remedial action to' be a confirmatory item.
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'6.0 RADON ATTENUATION AND S0ll CLEANUP i
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 L
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 (Refs. 9,10, and 38).
I The design parameters for the tailings and earth cover materials evaluated for l;
. acceptability include the following: long-term moisture content. material thickness,
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bulk density, porosity,. and radon diffusion coefficient.
In addition, radium content and radon emanation coefficient parameters were evaluated for.the tailings
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' materials only.
The computer code RAECOM was used to calculate the radon barrier cover thickness, and the input included the above parameters, v
6.2.1 Evaluation of Parameters l
To meet-the EPA standards for limiting release of Radon-222 from residual radioactive material to the atmosphere, the tailings pile was covered with an earthencover(radonbarrier).
The radon barrier reduces the effluence of Ra-222 by reducing the diffusion rate to acceptable quantities.
The required thickness'of the
. radon barrier depends on the properties of the barrier material and tailings.
For
.the earthen cover for radon attenuation, the DOE used silty clay from a borrow site and' mixed it with 6 percent by weight of Bentonite.
The material properties and radiological parameters used in the design of the radon barrier for the stabilized
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tailings disposal cell at the Green River site have been reviewed.
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The radon barrier material was compacted at a moisture content of 0 to 3 percent above the optimum moisture content. This resulted in an average placement moisture
.j content of approximately 16 percent. The staff has calculated the long-term moisture content using Rawls' (Ref. 5) method (a very conservative method) to be 9
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. 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 L
designated for protection against radon emanation, the staff concludes that the lower portion of the radon barrier will retain most of its placement moisture in.the long-term. The staff, therefore, concurs with the DOE's estimation of 11.9 percent long-term moisture content for the radon barrier material.
The tailings and other contaminated materials were to be compacted to the specified density at their in situ moisture contents of 3 to 5 percent.
But the DOE has placed the tailings material and other contaminated materials in the disposal cell at average placement moisture content of 4.6 and 5.5 percent respectively.
The average as-compacted moisture content for both the tailings and other contaminated materials is 5 percent. However, the DOE has used a long-term moisture content of 10 percent for tailings in the design calculation.
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.
However, the tailings and other contaminated materials were placed in the disposal h
cell without any layering or preferred placement of these materials within the
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-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 for the radon barrier thickness calculation 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 have very little or no impact on the calculated thickness of the of radon barrier.
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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 of these parameters are not expected to have any significant impact on the calculated thickness of the radon barrier.
The staff has reviewed the geotechnical parameters used in the design computations and concludes that the above values of the
. parameters are a reasonable representation of the average site conditions.
Radon diffusion coefficients for the cover material and tailings were 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 coefficient for the radon barrier material is 2
0.00247 cm /sec for the estimated long-term moisture content of 11.9 percent.
The diffusion coefficient for the tailings material used in the design is 0.021 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 moisture content will be in the range of 5.0 percent and the corresponding diffusion coefficient (Figure B.2.1 of Ref. 9) would be in the range of 0.028 cm2/sec.
The staff has reviewed the information used in determining the diffusion coefficient value for the radon barrier material and judges it to be
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. reasonable.
The staff does not agree with the value of the diffusion coefficient for the tailings material (.021 cm2/sec for a long-term moisture content of 10%,
Reference 3) used in the DOE's design, and an appropriate value for this parameter for a long-term moisture content of 5% is 0.028 cm2/sec.
The thickness of the radon barrier, calculated using the RAECOM code and higher diffusion coefficient of 0.028 cm2/sec for the tailings material. is 12 cm. compared to the thickness of 11 cm.
calculated by DOE for a diffusion coefficient of 0.0210 cm2/sec.
This change in the
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a required thickness.of radon barrier is not significant.
Section 6.2.2 of this report discusses the relevance of this thickness change by comparing it to the as-designed thickness of the radon barrier.
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 I
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 pC1/gm. However, the average radium content was verified by 3
field measurements on the stabilized tailings pile before placing the radon barrier i
earth cover, and the' radon barrier design was reassessed at that time to ensure that
'the radium content used in the design is a reasonable representation of actual.
j measured values.
The staff-concurs with the methodology used by-the DOE to measure
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the radium content and'the average values used in the design.
'l 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.
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6.2.2 Evaluation of Radon Barrier l
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 radon 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
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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 protection against freezing.
In a worst case scenario, the top 21 inches of the radon barrier will be subjected to freeze-tfiaw conditions that could alter its as-compacted condition in terms of possibly having minor openings or cracks.
Therefore, the upper 21 inches of the radon barrier is not given credit for l
contributing to the radon diffusion function of the radon barrier, and only the i
lower 15 inches (38 cms) of the radon barrier is designated to be functional 'in reducing the radon release.
i The DOE design (Reference 43) estimates that only 11,cm. (4.3 in.) thickness of radon barrier is required to reduce the radon release to a value in compliance L
with the EPA standards.
However, the required radon barrier thickness using a lower L
long-term moisture content for the tailings is estimated to be 12 cm. (4.7 in.) or about the same as DOE's estimate.
The radon barrier as designed is 36 inches. thick and the lower 15 inches of that, which will be below the frost depth, is designated to be functional as a radon barrier.
Considering the built in conservatism in the current design thickness of the radon barrier, 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 (by others)
Site characterization surveys have been conducted at the site to identify the subsurf&ce boundary of the tailings pile, as well as, the depth and area of the fomer mill yards, ore storage, and windblown contaminated areas.
Radiometric j
surveys and sampling were also conducted in the buildings at the site.
The results I
of the site characterization survey are being used to plan the control monitoring for the excavation and the building decontamination, as well as the final radiological verification survey for the land and the buildings. DOE has committed I
to the clean-up of the processing site and mill buildings in L
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accordance with-the EPA standard (40 CFR 192 Subpart B).
lP In addition to the EPA standards for the buildings DOE proposes that removable surface alpha contamination shall not exceed 1000 dpm/100 cm, and the average over I
one square meter total-non-removable alpha contamination shall not exceed 5000
^
dpm/100 cm.
DOE proposes an absolute maximum limit for total alpha contamination of
-15,000 dpm/100 cm.
These limits are in compliance with NRC Regulatory Guide 8.30
" Health Physics Surveys in Uranium Mills".
As a result of DOE's compliance with the EPA standard and NRC Regulatory Guide 8.30 with regard to removable alpha contamination, the NRC is prepared to concur with the DOE's radiological survey plan. Although it should be pointed out that while NRC h6 no objection to DOE's utilization of the NRC proposed limits for removable alpha contamination, the DOE should comply with their own more stringent standards as
'l provided in the UMTRA Project Environmental, Health and Safety Plan (UMTRA-00E/AL-150224).
6.4 Conclusions (by others)
With regard to the site clean-up, the DOE has committed to clean-up the processing
' site and mill buildings in accordance with the EPA standards and NRC Regulatory Guide 8.30.
Therefore, the NRC finds the proposed site clean-up to be acceptable.
REFERENCE
- 46. DOE, 1989, Remedial Action Plan and Final Design for Stabilization of the.
Inactive Uranium Mill Tailings at Green River, Utah, Final, Vol. I, II A, III, and tu volumes of Supporting Calculations and Field Data, December 15, 1989, UMTRA-00E/AL 050510.GRN0.
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