ML20012D634
| ML20012D634 | |
| Person / Time | |
|---|---|
| Issue date: | 03/15/1990 |
| 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 9003280157 | |
| Download: ML20012D634 (19) | |
Text
o p
i e
r
=..
A:\\BJ-FLIEG.MEM MAR 15 W
-MEMORANDUM FOR:
Myron Fliegel, Section Leader 1
Operations Branch Division of Low-Level Waste Management and Decommissioning, NMSS
-FROM:;
Michael Tokar, Section Leader Technical Branch Division of-Low-Level Waste Management and Decommissioning, NMSS
SUBJECT:
REVISED FINAL TER FOR GREEN RIVER UMTRA PROJECT
REFERENCES:
1.
Memo dated February 22, 1990, from M. Tokar-to M.
Fliegel; Submitting Final TER for
' Green River-UMTRA Project.
~
2.
Letter dated February 23, 1990, from M.
Matthews of DOE to P. Lohaus of NRC; transmitting information requested by NRC on items that were not presented in the final RAP of December 1989.
The Technical Evaluation Report (TER), presenting the staff evaluation'of the geotechnical engineering aspects of the design-presented in the final RAP for the Green River UMTRA project, identified a confirmatory item pertaining to the evaluation of the. stability _of the disposal cell slope-(Ref. 1).
The DOE has submitted an analysis of the stability of the disposal. cell slope (Ref. 2).
The staff has evaluated the information presented by the DOE and concludes.that there is reasonable assurance that the disposal-cell-slopes will be stable in the long-term.
From a
.geotechnical engineering perspective there are no open-items in this TER.
The TER attached to this memo is a revised version incorporating the staff findings on the stability of the disposal cell slopes and an evaluation of the design presented in the final RAP of December 1989.
However, there is a inconsistency in the elevation of the top of the disposal cell as presented in Volume l'and Appendix F of the i
RAP documents.
This has been brought to the attention of the L
DOE, and DOE has consented to rectify this (Ref. 2) in the final published version of Appendix F of the final RAP.
The moisture content issue identified in the TER (Ref. 1) is a hydrogeology issue, and therefore, is not addressed here.
g 9003280157 900015 PDR WASTE
/ V,
ge s
u c
7, l:
i.;
l r
i -
l:.
(.
This review was performed by Banad Jagannath; please contact him should you have any questions.
Original Signed W Michael Tokar, Section Leader
' Technical Branch Division of Low-Level Waste Management and Decommissioning, NMSS
Enclosure:
As stated Distribution:
C7CohEEIl?Fi1Eif 2
M 68h LLTB r/f.
ENMSS'r/f-BJagannath MTokar RBoyle
.JSurmeier PLohaus JGreeves RBangart SWastler.
PDR Yes:/Y/
PDR-No:/
/
Reason:
Proprietary /
/ or CF Only /
/
ACNW Yes:/ M /
No:/__
/
- r SUBJECT ABSTRACT:
SEE SUBJECT OF MEMORANDUM OFC :LLT
- LLTB
- LLTB
/
- LLWM 4
=
=
=====
NAME:BJagannath/lj:MTokar
- JSu eier
=====================================================
DATE:p/19/90
- 9 //p/90
- / //y/90
/
/90 OFFICIAL RECORD COPY l
l I
l l
l' I
r '
1 3.0 GE0 TECHNICAL STABILITY l
3.1 Introduction s
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 a geotechnical engineering perspective of slope stability, 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.
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 remedial action of stabilization-on-site consisted of placing all the contaminated material at the site (approximately 382,000 cyds) into a single pile, which is called the disposal cell.
The location of this disposal cell is approximately 500 feet south and about 50 feet higher in elevation than the existing tailings pile location.
The disposal cell bottom (elevation 4098 feet) is approximately 42 feet below the existing ground surface (elevation 4140 feet), and the top of the disposal cell (elevation 4181 feet) is about 41 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).
The disposal cell design provided for placing a six-feet-thick layer of select material as a buffer zone at the bottom of the cell between the bedrock and the contaminated materials. The disposal cell has been covered, in the ascending order, 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 was 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 (Ref. 8).
This section presents the geotechnical engineering evaluation of the long-term stability aspects of the proposed remedial actions.
nn 4 -
1 2.
i 3.' 2 Site Characterization 1
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 performed by the following 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 sofi samples.
Addendum DI to Appendix D in reference 8 presents detailed information on this investigation.
The results of this investigation 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.
(2) Jacobs Engineering Group, Inc. (1986, 1987, 1988) and Morrison-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.
These investigations were performed to determine geotechnical characteristics of the site and to obtain samples of the soil and rock materials for laboratcry determination of their properties.
Information to Bidders, Volumes I, II, and III of Reference 9,.and' Volume IIA Appendix D of the draft final RAP dated January 1989 (Ref.
- 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 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 and'46) shows locations of the borings and test pits.
Section 2 of this report presents an evaluation of.the geologic, geomorphic, and seismic characteristics of the site.
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 3
1 deposit consists of silty to claycy 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 and averaging 18 blows / foot.
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.
Section 2 of this report presents 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.
The overburden soil at the disposal cell location consists of from 5 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 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 l
shale /mudstone down to an elevation of 4098 feet.
The groundwater table at the 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.
1 Soil for the radon barrier cover and gravel for the bedding layer were taken l
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 intended use.
The stratigraphy at the borrow site consists of an alluvial deposit'with a surficial layer of silty-clayey sand, underlain by low plasticity clay. The clay layer is underlain by a alluvial sand and gravel stratum.
The test pits were terminated in the sand gravel stratum.
The low plasticity clay was used for the infiltration / radon 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 I
adequately established the stratigraphy and soil conditions to support an l
assessment of the geotechnical stability of the stabilized tailings and
4 l
contaminated material in the disposal cell.
Further, the geotechnical i
explorations are in general conformance with applicable provisions of Chapter 2 of the NRC Standard Review Plan (SRP) for UMTRCA Title I Mill Tailings Remedial Action Plans (Reference 5),
3.2.4 Testing Program 1
-The staff has reviewed the geotechnical engineering testing program for the l
Green River site..The testing program included physical properties tests, compaction tests, triaxial shear strength tests, permeability tests, and i
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 tests) for the infiltration / radon barrier material in the draft final RAP (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 final RAP (Ref.-46)', and conclude that the DOE has met the commitments in terms of providing the data to support their design.
Details of the relevance of this data to the design is cddressed 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 appropriaie 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 material properties are in general agreement with applicable provisions of the SRP (Reference 5).
3.3 Geotechnical Enaineerina Evaluation i
3.3.1 Stability Evaluation The evaluation of the geotechnical stability of the slopes of the disposal cell containing stabilized tailings and other contaminated soils is presented in this section.
The staff has reviewed the exploration data, test results, slope characteristics and methods of analyses pertinent to the slope stability aspects of the remedial action plan (Refs. 10 & 11).
The analyzed cross 1.
L
F 5
section with a 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 stability analysis.
Soil parameters for various materials in the disposal cell slope have been adequately established by appropriate testing of representative materials.
Values of soil parameters have been assigned to other layers (riprap, gravel bedding, bedrock etc.) on the basis of data obtained from geotechnical explorations at the site and data published in the literature.
The staff finds that the determination of these parameters for slope stability evaluation follow conventional geotechnical engineering practice, and are 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 employeo 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 determined for both chort-term (end-of-construction) state and long-term state.
Factors of safety for the static loading conditions were not determined 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 end-of-construction case and 0.14 for the long-term case. The values of the seismic coefficients were calculated as per guidance in the SRP and are acceptable 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 (SH:1V).
The minimum factors of safety against failure of the slopes were 2.3 and 1.67 for the end-of-construction seismic and long-term seismic conditions, compared to a required minimum of 1.1 for both seismic conditions.
The above analysis was performed for the slopes of the disposal cell that was proposed in the draft final RAP.
However, during the actual construction more windblown material was encountered and disposing it in the disposal cell resulted in increasing the height of the disposal cell.
In response to a staff y
request, the 00E submitted a conservative analysis (infinite slope analysis) to support their assertion on the stability of the' final slope of the disposal l
cell.
The resulting factors of safety are 2.44 and 1.42 respectively for l-long-term static and long-term seismic loading conditions.
These factors of safety are higher than the required minimum of 3.5 and 1.1 respectively.
The l-staff has reviewed the DOE's analysis and agrees with the results from this analysis after (1) reviewing the conservatism in the properties of materials in L
the potential failure zone determined in the rigorous analysis performed during L
the final draft RAP stage, (2) considering the flatness of the slope (5H:1V),
L and (3) assessing the margins in the computed factors of safety compared to the required minimum values.
l
4
)
6 1
1 The staff concludes that the slopes of the disposal cell will be stable under both short-term and long-term conditions from a geotechnical engineering slope stability perspective and this aspect of the design will comply 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 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 0-698 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 susceptible to liquefaction.
The groundwater table at the site is estimated to be approximately 13 feet below the foundetion of the disposal cell.
Considering the placement density and absence of free moisture in the disposal cell, the materials in the disposal cell are judged to be not susceptible to liquefaction.
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, has been 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 instantaneous and will take place during construction.
Any potentially adverse effects of the instantaneous settlement of these sandy 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
7 Tne 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 infiltration / radon barrier composed of a three-foot-thick layer silty-clay amended with Bentonite (Refs. 8 and 46).
The staff's evaluation of the cover design considered the design adequacy with regard to erosion protection, radon attenuation, frost penetration and 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 aspects 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 I
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.
t The DOE 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 site and a computer code developed by U.S Army Cold 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 Raference 42, prepared by tho U.S. Army Corps of Engineers, is 36 inches for the goject 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 1
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 emanation from the disposal cell to comply with the EPA Standards is addressed in Sections 5.0 and 6.2, respectively, of this report.
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 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.,1x10-9 cm/sec or lower) than the saturated hydraulic conductivity.
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
8 disposal cell cover to comply with the EPA Groundwater Standards for UMTRCA 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 0.4.4 of draf t final RAP, Ref. 9) presented in support of the saturated hydraulic c0nductivity consists of three tests on i
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 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 i
hydraulic conductivity values presented in Table 0.4.4, and the sensitivity of this parameter to compaction density, Bentonite content, moisture, and percent 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 nave 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 i
heterogeneous soil-Bentonite mixture, which in turn may not have the desired 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 to 100% Standard Proctor density, achieved the desired average hydraulic conductivity; viz., in the range of the values determined from laboratory 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% Stu dard Proctor density, had a saturated hydraulic 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 naterial mixed with 6% by weight of 8entonite and complying with the above gradation.and compacted to 100% Standard Proctor density at a moisture content of 0 to 3% higher than the optimum is expected to result in a saturated hydraulic conductivity not exceeding 2x10-8 cm/sec.
As a result of a NRC/00E meeting on the results of staf f evaluation of the draf t final RAP, the DOE cominitted to three changes in the RAP (Ref. 44).
The final RAP (Ref. 46) presents the information to support that the DOE has made i
i the changes and developed the required additional information to support the l
design.
The following is a listing of the DOE commitments and NRC's evaluation l
of the DOE's fulfilment of the commitments.
l l
\\
t 9
1 i
1.
The DOE committed to constructing the first lift of the infiltration /rsden barrier with material that has greater than 70 percent i
of the material passing the No. 200 sieve and matarial for the other lifts having 50 percent passing the No. 200 sieve.
i The basis for this requirement was that the radon barrier material should be similar to that tested in the laboratory, and the toil samples used in hydraulic conductivity tests perforrred in the laboratory had an average fines (passing No. 200 sieve) contoat of 70 percent.
Although the infiltration / radon barrier is three feet thick, the lower 12-inch portion of the cover is adequate to fulfill its function, and therefore, this requirement was imposed on the first lift.
The DOE has changed tho subcontract specifications to include this condition (Section 02200 PART 2-B.3, pg.02200 - 9, Appendix F of Ref.
46), and proper implementation of this specification will satisfy the commitment.
2.
The DOE connitted to mix ne less than six percent by weight of Bentonite into the radon barrier material.
The basis for this requirement was that the hydraulic conductivity test 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 will satisfy the commitment.
3.
The DOE committed to perform moisture content and hydraulic conductivity testing of the radon barrier to demonstrate that the as-built saturated hydraulic conductivity does not exceed 2x10-8 cms /sec.
The testing to be at a frequency of at least one test per 2,000 cubic yards of radon / infiltration barrier material.
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 analysis it was determined that a saturated hydraulic conductivity of 2x10-8 cms /sec. for the radon / infiltration barrier would result in a cover s
that would meet tne desired groundwater travel time through the disposal cell; (details of this aspect of the design are addressed in Section 5 of
10 this report).
Therefore, the above requirement of saturated hydraulic conductivity of the as-constructed radon / infiltration barrier was imposed.
The hydraulic conductivity was to be demonstrated by laboratory tests performed on as-compacted block samples of the 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 /see with an average of 0.61x10-8 cms /sec.
The hydraulic conductivity average value compared favorably with the required value of less than 2x10-8 cms /sec.
All the samples had been compacted to a dry density of 100 percent Proctor i
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.
Full compliance by the DOE with the above commitments 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 i
freezing of the radon / infiltration barrier is satisfactory.
The staff concludes with reasonable assurance that the radon barrier has been constructed l
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, 3.4 Geotechnical Construction Criteria L
The DOE's strategy to meet the EPA Groundwater Standards includes ensuring that i
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 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 (Table 0.5.22 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
+
11 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 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 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 content as possible, and all the materials placed in the disposal cell were granular material, there was potential of not being able to compact the relatively dry granular material to the desired density.
The specifications provide for the first 1,000 cyds of the fili 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 lif ts.
Since compacting at such dry state was not originally contemplated in the draf t RAP design, the staff asked the DOE to demonstrate that these materials can be placed in the disposal cell at the densities and moisture contents assumed in the final design.
As a result of a NRC/ DOE meeting on the results of staff evaluation of the draft final RAP, the DOE committed to the following (Reference 44):
The DOE committed to placing and maintaining centaminated 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 property contaminated materials.
The 00E committed to place and test at L
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 00E also committed 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 and maintained at the specified densities and moisture contents.
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.
In response to this commitment, the DOE has submitted results and analyses l
of field tests performed to determine the actual placement density and
t 12 i
t moisture content.
Because of difficulty in complying with the dust 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 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 compaction.
Therefore, the materials in the disposal cell have been placed to comply with the density specifications.
The placement moisture content is slightly higher than the values committed to by the DOE, 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 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-contaminated material.
The DOE-committed 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 l
the disposal cell, the contaminated materials have been placed at specified density and the as placed moisture content of the contaminated materials in the disposal cell has no adverse impact on the geotechnical stability of the disposal cell.
3.5 Conclusion i
Based on a review of the design for the Green River site as presented in the remedial action plan (Refs. 8, 9, 10, 44, and 46), the NRC staff concludes that from a geotechnical engineering perspective the remedial action will comply with the long-term stability aspects of the EPA standards (40 CFR Part 192-02 (a)).
1 I
l
4 i
l 1
i 6.0 RADON ATTENUATION AND SOIL 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 I
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).
The design parameters for the tailings and earth cover materials evaluated for acceptability include the following: long-term moisture content,' material thickness, bulk density, 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 radioactive material to the atmosphere, the tailings pile was 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 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 tailings disposal cell at the Green River site have been reviewed.
The radon barrier material was compacted at a moisture content of 0 to 3 l
percent above the optimum moisture content.
This resulted in an average L
placement moisture 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 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 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 i
for' the radon barrier material, i
4 i
2 i
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 cont 9minated materials in the disposal cell at average placement moisture content of 4.6 and 5.5 percent i
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 cell without any layering or preferred placement of these materials within the disposal cell.
The design assumes uniform, averarp 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.
i 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.
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 thatarerepresentativeoftheconditioninthestabilizgdpile.
The diffusion coefficient for the radon barrier material is 0.00247 cm /see 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
3 determining the diffusion coefficient value for the radon barrier material and
+
judges it to be reasonable.
The staff does not agree with the valut 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 because it is higher than the placement moisture content of close to 5 percent.
Based on review of the field data and placement moisture content specified in the RAP, an appropriate value for this parameter for a long-term moisture content of 5%
is 0.028 cm2/sec.
The required 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 required thickness of radon barrier is not significant because the impact of the diffusion coefficient of material at dapth of ten feet and below the cover is not very significant on the required thickness of the radon barrier.
Section 6.2.2 of this report discusses the relevance of this thickness change by compsring 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 averaging with depth in a meast.rement 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 was verified by field measurements on the stabilized tailings pile before placing the radon barrier earth cover, and the radon barrier design was reassessed at that time to ensure that the radium content used in the design is l
a reasonable representation of actual measured values.
The staff concurs with the methodology used by the DOE to measure the radium content and the average values used in the design, The radon emanation coefficient was measured in the laboratory on samples i
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 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 square meter per second.
The current cover design l
l l
4 1
i 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-thaw conditions that could alter its as-compacted condition in terms of possibly initiating minor openings or cracks.
Therefore, the upper 21 inches of the radon barrier is not given credit for contributing to the radon diffusion function of the radon barrier, and only the lower 15 inches (38 cms) of the radon barrier is designated to be functional in reducing the radon release.
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 with the EPA standards.
However, the required radon barrier thickness using a lower 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 subsurface boundary of the tailings pile, as well as, the depth and area of the former mill yards, ore storage, and windblown contaminated areas.
Radiometric surveys and sampling were also conducted in the buildings at the site.
The results 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 to the clean-up of the processing site and mill buildings in accordance with the EPA standard (40 CFR 192 Subpart B).
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 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 has no objection to DOE's utilization of the NRC proposed
L 5
L limits for removable alpha contamination, the DOE should comply with their own more stringent standards as provided in the UMTRA Project Environmental, Health and Safety Plan (UMTRA-DOE /AL-150224).
6.4 Conclusions (by 7thers)
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.
!