ML20245C599

Forwards Revised Technical Evaluation Rept Which Documents NRC Evaluation Re Final Design & Remedial Action Plan for Stabilizing Tailings at Green River Site

ML20245C599
Person / Time
Issue date:	04/24/1989
From:	Tokar M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To:	Fliegel M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
References
REF-WM-68 NUDOCS 8904270287
Download: ML20245C599 (17)

Text

_

O c

. -a GRN RFTER MEMU 2 MEMORANDUM FOR: Myron Fliegel, Section Leader APR 2 41989 Operations Branch Division of Low-Level Waste Management and Decommissioning, NMSS FROM: Michael Tokar, Section Leader }

Technical Branch i Division of Low-Level Waste Management and Decommissioning, NMSS

SUBJECT:

FINAL TER FOR GREEN RIVER UNTRA PROJECT- REVISION 1

REFERENCES:

1. DOE, 1989, Remedial Action Plan and Final Design for Stabilization of Inactive Uranium Mill Tailingt, at Green River, Utah, Final, Vol. I, II, and III, January, 1989; UMTRA-DOE /AL 050510.GRNO.

2. Memo dated March 27, 1989, from M.Tokar to M. Fliegel; sub: Final TER for Green River UMTRA Project.

3. Memo dated April 6, 1989, from P. Lohaus to J. Greeves; sub: NRC/D0E Green River Meeting of April 5, 1989.

4. Memo dated April 18, 1989, from M. Fliegel to M. Tokar; sub: Green River Final TER The final design and Remedial Action Plan for stabilizing tailings at the Green River site were reviewed, and our report (TER) of March 27, 1989 documented our evaluation (Reference 2). The TER identified four items that needed to be resolved before concurring with the DOE's design. In order to expedite the resolution of these issues, the DOE committed to design changes documented in the NRC/D0E agreements (Reference 3). Your memo (Reference 4) provided the rationale for resolving the concern on the potential for migration of fines into the Type B riprap at the toe of the disposal cell. The other 1 three concerns on the radon barrier and materials within the disposal cell are considered to be resolved assuming that the DOE will comply with the confirmatory (agreement items 4, 5, and 6 of Reference 3) and conditional (agreement item 3) items of the agreement. The attached revised TER documents our evaluation, and our findings are predicated upon the DOE satisfactorily complying with all the changes agreed to in the NRC/D0E agreement (Reference 3). The staff expects to review all the information to be submitted by the DOE as per the agreement.

l f5 ylmdc0) n+

meow

- r-

. s ,

.f.- h;'>.3;.%

, .p

.t >

,y ~

,o

(

1 GRN RFTER MEMO.2-

2. ,

This-review was' performed by Banad'Jagannath; please contact him should-

'.you have any(questions.

Original Signed By Michael Tokar, Section Leader Technical Branch Division of Low-Level Waste Management-and Decommissioning, NMSS

Enclosure:

As' stated 1

Distribution: i Et sntreUfilesT M & W h JSurmeier, LLTB RBangart, LLWM  ;

NMSS r/f- LLTB r/f JGreeves, LLWM MBell, LLRB  !

JSurmeier, LLTB; PLohaus,-LLOB MTokar, LLTB BJagannath, LLTB l SWastler, LLOB  ;

)

PDR Yes:/ M PDR- No:/ / Reason: Proprietary / / or CF Only / /

ACNW Yes':/ g / No:/ / l l

SUBJECT' ABSTRACT:'  !

i 0FC :LLTIL :LLJR ?LLTB :LLWM :LLWM :NM55 :NM55 X I.

..../JEJtf......._____....__...........................___...... ,

TNAME:BJagannath/1j:MTokar  :  :  :  :  : l

DATE:4/S:/89  :// /s//89 : / /89 :' / /89: / /89 : / /89 : / /89  :

OFFICIAL RECORD COPY I 1

i l

p.

1 s

3.0 GE0 TECHNICAL STABILITY l

3.1' Introduction

.The'NRC staff revie'w'of the geotechnical engineering. aspects of the remedial I

< f 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

' remedialLactions would result,in the stabilized disposal cell complying with the,long-term' stability requirements of the EPA standards in 40 CFR Part 192.02 (a) Subpart A, from the geotechnical engineering perspective of slope

' stability, liquefaction, and settlement. The staff review of the related geological aspects such as. geologic, geomorphic, and seismic characterization of.the site is presented in Section 2 of this report. The staff review of the groundwater conditions at this site is presented in Section 5 of this report.

At the Gree'n River Uranium Mill site (presently an inactive site) the ore concentrate was . shipped to a processing plant -in Rifle, Colorado, and thereby the tailings left at this site were predominantly sandy tailings with no slime.

The 48-acre area ~ designated for remedial action consists of the tailings pile, former ore storage area, and abandoned structures and facilities' associated with'the' uranium mill during its operation. In addition, tailings dispersed by wind and water erosion have contaminated approximately 30 acres-of adjoining area. -De proposed' remedial action of stabilization-on-site consists of placing all the contaminated material at the site (approximately 343,000 cyds) in to a single pile, which is called the disposal cell. The location of this disposal cell is approximately 500 ft. south and about 50 ft. higher in elevation'than the existing tailings pile location.

The disposal cell bottom

~

(elevation 4098) is approximately 42 feet below the existing ground surface and this requires approximately 26 feet of excavation in the bedrock. The top of the disposal cell is about 25 ft. above the adjoining ground surface. The portion of the disposal cell above the' existing ground surface (elevation 4140) rises to elevation 4160 at a gentle slope of 5 horizontal to 1 vertical (5H:1V) and'to the crown of the cell (elevation 4165) at a flat slope of 5 percent.

The disposal cell has a six-feet-thick buffer zone consisting of select material placed at the bottom of the cell between the bedrock and tailings.

The disposal cell will be covered with (1) a three-feet-thick infiltration / radon barrier, (2) a six-inch-thick gravel bedding, and (3) a 12-inch-thick rock layer (riprap). The cover is designed to ensure the

, .following: (1) long-term stability of embankment and reduced radon emissions; i L (2) reduced infiltration; (3) protection of surface water quality; (4) i protection against animal intrusion; (5) minimized plant root intrusion; (6) l prevention of inadvertent human intrusion; and (7) prevention of material dispersion. (Reference 8). This section presents geotechnical engineering )

L evaluation of the inng-term stability and reduced radon emanation aspects of

the proposed remedial act'ons.

t

-i t >; . y I ' - . F i

{, _

y .c -

gg x; , ,,

p '

4: , =2 ll

! i P '

3.2 Site' Characterization

< 3.2.15SiteDesbription 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)i.Bendix Field Engineering Corporation to determine the extent of contamination. The investigations resulted in data from'105 bore-

- holes,.184 in. situ-_Ra-226 measurements, and 139 soil samples.

Addendum D1.to Appendix D in Reference 1 presents detailed f information on this investigation. The results of this investigation were used in establishing the volume of, contaminated material to'be removed to comply with the. EPA standards. 'This conta'minated material is to be 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 borings from which soil samples

and rock cores were:obtained, test pits from which bulk samples were 3

obtained, and installation'of monitoring wells. These investigations were performed to determine geotechnical characteristics of the site-and to obtain samples of the soil and rock materials for laboratory

~

M. determination of their properties. Information to Bidders, Volumes 1, II, and III of Reference 9, and Volume IIA Appendix D of the final RAP dated' January' 1989 (Reference '8) present detailed information on site' conditions and logs of these field investigations and laboratory test.. resul ts.

3.2.3 ' Site Stratigraphy The elevation of the Green River project site varies from about 4050 to 4200 feet above'mean sea level. Borings-were obtained using standard geotechnical

drilling and; sampling techniques. These methods included drilling with hollow stem augers, and sampling at near continuous intervals with Standard Penetration Tests (SPT). On occasion, a 2.5-inch inside-diameter, ring-lined split-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 (Reference 9) 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.

)

c .

3 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 deposit consists of silty to clayey sand, with dense sand and gravel occurring at the bottom of this deposit. The alluvium deposit is in a loose to dense condition, with the Standard Penetration Test resistance values ranging from 3 to 43 and averaging 18 blows /ft. The sedimentary bedrock units at the site consist of a shale member of the Mancos shale, the Dakota sandstone, and the Cedar Mountain Formation. The upper portion of the bedrock is weathered and fractured. 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 contaminated alluvium will be excavated for disposal in the disposal cell.

At the proposed disposal cell site, the bedrock units are the Dakota sandstone underlain by the shales and mudstones of the Cedar Mountain Formation. The overburden soils at the proposed cell location consist of from 5 to 16 ft. 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 deposit. Since the disposal cell is proposed to be founded on the bedrock, the overburden material will be excavated. This alluvium material will be selectively used as Select Fill Type-A material for the six-feet-thick buffer zone placed at the bottom of the cell between the bedrock and the tailings.

The groundwater table at the proposed disposal cell location is estimated to be  :

4083-4085 ft in elevation, approximately 55 ft. below the ground surface and 13 ft. below the bottom (elevation 4098 ft.) of the disposal cell. Section 5 of this report presents a detailed evaluation of the groundwater conditions at the site.

Soil for the radon barrier cover and gravel for the bedding layer are proposed to be taken from Borrow Site 1. Figures 3.15 through 3.24 of the RAP (Reference 9) show the location and stratigraphy of the proposed borrow area.

A total of 24 test pits were dug to investigate the availability and suitability of the soils for the intended use. The stratigraphy at the borrow site consists of 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 is proposed to be used for the infiltration / radon barrier cover and the alluvial sand gravel material vill be 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 i that the geotechnical investigations conducted at the Green River site have adequately established the stratigraphy and soil conditions to support

4 assessment of the geotechnical stability of the stabilized tailings and contaminated material in the disposal cell. Further, the geotechnical-explorations are in general conformance with applicable provisions of Chapter 2 of the NRC Standard Review Plan (SRP) for UMTRCA Title I Mill Tailings Remedial Action Plans (Reference 5).

3.2.4 Testing Program The staff has reviewed the geotechnical engineering testing program for the Green River site. The testing program included physical properties tests,

, compaction tests, triaxial shear strength tests, permeability tests, and dispersion tests on samples of tailings and borrow materials intended for use in the disposal cell. The staff finds that the testing program employed to define the material properties was appropriate for the support of necessary engineering analyses described in the following sections. Further, the scope of the testing program and the utilization of the resulting data to define the material properties are in general agreement with applicable provisions of the SRP (Reference 5).

However, the DOE has not submitted all the test data (for example, capillary-moisture relationship) for the infiltration / radon barrier soil. The additional geotechnical data presented in the final RAP (Reference 8) is for '

the tailings and buffer zone material, and there are no new data on the infiltration / radon barrier material in the final RAP. Because of the DOE's new approach to complying with the EPA Groundwater Standards, additional test data on the radon barrier material will be required to demonstrate validity of the assumptions made in the design. Section 3.3.4 of this report presents details on this item.

3.3 Geotechnical Engineering Evaluation ,

3.3.1 Stability Evaluation The evaluation of the geotechnical stability of the slopes of the proposed stabilized tailings pile is presented in this section. The staff has reviewed the exploration data, test results, critical slope characteristics and methods of analyses pertinent to the slope stability aspects of the remedial action ,

plan (References 10 & 11). The analyzed cross section with the 5 horizontal to  !

1 vertical slope has been compared with the exploratory records and design details. The staff finds that the characteristics of the slope have been  ;

properly represented and that the most critical slope section has been considered for the stability analysis.

Soil parameters for the various materials in the stabilized embankment slope have been adequately established by appropriate testing of representative  !

material. Values of soil parameters have been assigned to other layers (riprap, gravel bedding, bedrock etc.) on the basis of data obtained from geotechnical explorations at the site and data published in the literature.

The staff finds that the determination of these parameters for slope stability l 1

l l

i 1

l. _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ . _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _

p l

5 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 employed and has addressed the likely adverse conditions to which the slope might be subjected. Factors or cafety against failure of the slope for seismic loading conditions have beer de1 ermined for both the short term (end-of-construction) state and long-ta m state. Facto of safety for the static loading conditions were not detecmined because the seismic loading condition is more critical and results il lower factors of safety than those for the static loading condition. The oismic stability of the slope was ,

investigated by the pseudo-static methcd of analysis using horizontal seismic t coefficients of 0.1' for the end-of-construction case and 0.14 for the long-term case. The values of the seismic coefficients were calculated as per the guidance in the SRP and are 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 (5H:1V). The minimum factors of safety against failure of the slope were 2.3 and 1.67 for the end-of-construction and long-term conditions, compared to a required minimum of 1.1 for both conditions. The details of the cover in the final RAP are different from the one analysed in the draft RAP stage. However, from a geotechnical slope stability perspective, the change, (viz, thicker radon barrier and elimination of frost protection layer) has no significant impact on stability of the slopes. Therefore, the results of the analyses presented in the draft RAP are applicable to the final RAP design. In addition, the strength parameters for the cover materials used in the stability analysis are conservative.

The staff concludes that the proposed 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 i l

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 will be a minimum of 90 percent of the maximum dry density by the ASTM D-698 test, and the design requires these materials to be in an unsaturated condition, these materials are not susceptible to liquefaction. The disposal cell is founded on bedrock, which is also not susceptible to liquefaction. The groundwater table at the <

site is estimated to be approximately 13 ft. below the foundation of the i 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.

)

4 I

6 ,

c -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 90 percent of.. standard proctor density at a moisture content of minimum 3 percent less.than the optimum, a major portion of the settlement will be instantaneous and will take' place during construction. Any potential adverse effects of the instantaneous settlement:of thes'e sandy materials will be compensated for before completion of the construction and therefore, will not adversely affect the long-term performance of. the disposal cell. Any time

. dependent or delayed settlement is expected to be minimal or insignificant and is not expected to result in any differential settlement cracks in the cover.

The staff concludes that the long-term settlements of the sandy 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 proposed cover design for the disposal cell consists of the following, in descending order 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:in_ filtration / radon barrier composed of a three-foot-thick layer silty-clay amended with Bentonite (Reference 8).

The staff's evaluation of the cover design considered the design adequacy with  !

regard to erosion protection, radon' attenuation, frost penetration and 1

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. {

l erosion protection layer and its ability to comply with 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 -f Groundwater Standards, is addressed in Section 5 of this report. The staff's i evaluation of the adequacy of the thickness of the radon barrier to attenuate I' the release of radon to comply with the EPA standards is addressed in Section 6 of this report.

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 i site and a computer code developed by U.S Army Cold Regions Research and f Engineering Laboratory for the modified Bergren Solution to calculate the depth j l

h'  !

q 7

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 D0E's analyses and recommendations. As an independent verification, the depth of' frost penetration indicated in Figure 7.1-42 of' Reference 42, prepared by the U.S. Army Corps of Engineers, is 36 inches for the project site region.

Therefore, the staff agrees with the DOE's estimation of the frost penetration depth'of 39 inches at'the site. A 39-inch frost penetration will result in the-

. freezing of the upper 39 inches'of the cover, while the lower 15 inches of radon / infiltration barrier layer will be in the unfrozen or intact condition.

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 are 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, intended for the radon barrier, can be compacted to result in a material whose saturated hydraulic conductivity does not exceed 2E-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 1E-9 cm/sec or lower. This permanent unsaturated condition will result in an infiltration rate or flux of IE-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 in assuring compliance with.the EPA Groundwater standards for UMTRCA projects. Section 5 of this report presents details on the adequacy of the lower 15 inches of the radon barrier to satisfy the EPA Groundwater standards.

The laboratory test data (Table D.4.4 of RAP, Reference 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 2E-8, 1.5E-8, and 3.4E-8 cm/sec with an average value of 2.3E-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 2.4E-8 to a high of 8.5E-5 cm/sec. The staff i

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 IE-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 fines in the soil, the staff believes that the DOE has not adequately established with reasonable i

, assurance that the silty clay amended with 3% bentonite (radon barrier) will have a saturated hydraulic conductivity of 2E-8 cm/sec. In addition, mixing silty clay with three percent of Bentonite in the field to achieve a uniform mixture could be difficult to accomplish, and could result in a nonhomogeneous or heterogeneous soil-bentonite mixture, which in turn may not have the desired average hydraulic conductivity as determined from tests on laboratory compacted l

au .

8 samplss. Increasing the bentonite content to 6 percent is likely to result in the soil-bentonite mixture being relatively uniform. This mixture, when

.. compacted to 100% standard Proctor density, will provide greater assurance of

achieving;the desired averags hydraulic conductivity; viz.,'in the range of the values determined from laboratcry testing.

Th'e'00E has committed to the following changes in the RAP, and this evaluation is predicated upon' full compliance with the following commitments (Reference 44). ,

1. 00E commits to.using a minimum gradation specification for the radon l L barrier. material of greater than 70% of the material passing the no. 200 sieve for the_first lift'and 50% of the material passing the no. 200 sieve for_the remaining lifts.

~ DOE commits to mixing' homogeneously no less than 6% by weight sodium

~

2.

bentonite into radon barrier' material'and compacting the radon barrier to

' 100% of standard Proctor density within 0 to +3% of Optimum Moisture Content.

3. DOE commits _to perform representative testing of the radon barrier to ensure that its as-built saturated hydraulic conductivity does not exceed 2E-8 cm/sec and to assess.its unsaturated hydraulic characteristics (tension as a function'of moisture content; hydraulic conductivity as a-

- function of moisture content). Samples for hydraulic conductivity tests should be taken at a frequency of at least one per 2,000 cyd of as-compacted radon barrier material, which is estimated to total 28,000 cyd. Sample locations should be distributed evenly over the cover,

. provided that at least 50%_of the samples are collected from the slopes of the disposal unit.-_ At least 13 of the _14 samples (approximately 90%) must exhibit saturated hydraulic conductivities less than or equal to 2E-8 cm/sec. Standard geotechnical parameters, such as physical properties, should also be determined for.the samples.

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 sodium bentonite and compacted to 100% standard Proctor density, had a saturated hydraulic conductivity in the ~

range of IE-8 cm/sec. Also, silty clay with 70% fines passing no. 200 sieve ara amended with 3% of sodium bentonite and compacted to 100% standard Proctor density had an average saturated hydraulic conductivity of 2.3E-8 cm/sec.

Therefore, the silty clay material mixed with 6% by weight of bentonite and complying with the above committed gradation and compacted to 100% standard Proctor density at a moisture content 0 to +3% of optimum is expected to have a l saturated hydraulic conductivity not exceeding 2E-8 cm/sec.

Full compliance by the DOE with the above 1 and 2 commitments, which are confirmatory items, is an adequate basis for the staff to reach a conclusion l that there is a reasonable assurance that the radon barrier can be constructed to the requirement that the as-built saturated hydraulic conductivity does not ,

_______________________.__m__ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ ___ _ _ _ _ _

9 exceed 2E-8 cm/sec. However, as per commitment no. 3, the DOE will demonstrate by tests on radon barrier that the as-built saturated hydraulic conductivity does not exceed 2E-8 cm/sec. The above mentioned items 1 and 2 are confirmatory, which the DOE can comply to by revising the RAP. Item 3 is conditional, to which the DOE should demonstrate compliance af ter constructing the cover. Based on the above commitments by the DOE, the staff concludes that there is a reasonable assurance that the radon barrier can be constructed to the requirement that the saturated hydraulic conductivity does not exceed 2E-8 cm/sec.

The design of the cover, from a perspective of providing protection against freezing of the radon / infiltration barrier is satisfactory. The evaluation of the disposal cell and cover regarding compliance with the EPA Grount. water Standards is addressed in Section 5 of this report. The evaluation findings in Section 5 of this report, from a groundwater protection perspective, are predicated upon the expectation that the DOE will satisfactorily comply with the'above mentioned commitments and demonstrate that the hydraulic conductivity of the as-built radon / infiltration barrier does not exceed 2E-8 cm/sec. The staff concludes with reasonable assurance that the radon barrier can be constructed to have a saturated hydraulic conductivity of 2E-8 cm/sec, and therefore the radon barrier design from the perspective of controlling infiltration is satisfactory. This conclusion is predicated upon the DOE L satisfactorily complying with the confirmatory and conditional commitments made in Reference 44.

3.4 Geotechnical Construction Criteria Part of the 00E's strategy to meet the groundwater standards is to ensure that the tailings and other contaminated materials in the disposal cell are at their L equilibrium or steady state moisture content, so that no free water migrate i towards the bottom of the disposal cell. The specifications state that these materials should be compacted at a minimum of 3 percent dry of 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 D.5.22 of Reference 8) for tailings and approximately 6 to 9 percent for buffer zone material, The optimum moisture content for these materials range from 10 to 16 percent for tailings and 10 to 13 percent for buffer zone material (natural material at the disposal site). The flux calculations, using the SUTRA code, indicate that the required steady state moisture contents (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 will result in the material being placed close to its steady state moisture. There are no Proctor compaction data for the windblown material, and in the absence of that no judgement can be made on whether the placement moisture for this material will be close to its steady state moisture content. Placing the tailings at its in situ moisture content will result in the placement moisture content close to '

the required steady state moisture content (3% to 9%) indicated in the i

w A

10 analysis. If necessary, DOE will dry the tailings to achieve the 3 to 5 percent placement moisture content and the specified density. The DOE has to present data on the in situ moisture content and compaction characteristics of the windblown material. It is reiterated that the moisture contents mentioned

.above are all weight percent moisture contents used by geotechnical engineers and not volumetric moisture. contents used by hydrogeologists.

Since the design requirement is to place all the materials in the disposal cell at as low a moisture content as possible, and all-the materials to be placed in the disposal cell are granular material, there is a potential of not being able

.to compact the relatively dry granular material to the desired density.

Under-compaction of relatively dry granular material would reselt in the caterial being placed in a loose condition, which in turn wou'.u result in low strength and,a potential for volume change during a seismic event. The specifications provide for the first 1,000 cyds of the fill material to be placed under controlled conditions to aevelop compaction procedures that would ensure the specified density. The staff concludes that this trial compaction should be extended to at least four lifts and that the desired density should be achieved for the full depth of compaction i.e. four lifts.

The DOE has committed to the following (Reference 44). DOE commits to placing and maintaining contaminated materials in the disposal unit at the specified densities.and at average moisture contents that are less than their average steady-state moisture contents presented in the RAP and, in any case, less than 5% by volume for the tailings and less than 10.6% by volume for the windblown and other vicinity property contaminated materials. DOE will place and test at least four lifts of contaminated materials during the trial compaction (first 1,000 cyd of material), which is intended to develop procedures to ensure compaction of the materials in accordance with material specifications. DOE will submit physical properties and compaction data on windblown material 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.

In summary, the DOE will cubmit to NRC additional data, on in situ moisture content ind compaction characteristics of windblown materials and tailings, that will be reviewed to ensure (1) the proposed placement at in situ moisture content will result in the material being placed at a moisture content close to the steady state moisture, and (2) that relatively dry granular materials can be successfully compacted in the field to the density that is assumed in the design.

3.5 Conclusion Based on a review of the design for the Green. River site as presented in the

- remedial action plan (References 8, 9, and 10) and DOE commitments (Reference

. 44), the NRC staff-concludes that the design will comply with the long-term stability aspects of the EPA standards (40 CFR Part 192-02 (a)). This conclusion is predicated upon the DOE satisfactorily complying with the L__ u______ _____._______________________________._1._ u___._ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

l.

11 i

l l

confirmatory and conditional comrritments made in Reference 44, which are as follows:

1. DOE commits to using a minimum gradation specification for the radon L barrier material of greater than 70% of the material passing the no.

l- 200 sieve for the first lift and 50% of the material passing the no. 200 seive for the remaining lifts.

2. 00E commits to mixing homogeneously no less than 6% by weight sodium bentonite into'the radon barrier material and compacting the radon barrier to 100% of standard Proctor density within 0 to +3% of Optimum Moisture Content.

3. DOE commits to performing representative testing of the radon barrier to ensure that its as-built saturated hudraulic conductivity does not exceed 2E-8 cm/sec and to assess its unsaturated hydraulic characteristics (tension as a function of moisture content; hydraulic conductivity as a function of moisture content). Samples for hydraulic conductivity tests should be taken at a frequency of at least one per 2,000 cyd of as-compacted radon barrier material, which is estimated to be 28,000 cyd.

Sample locations should be distributed evenly over the cover, provided that at least 50% of the samples are collected from the side slopes of the disposal unit. At least 13 of the 14 samples (approximately 90%) must exhibit saturated hydraulic conductivities less than or equal to 2E-8 cm/s. Standard geotechnical parameters, such as physical properties, should also be determined for the samples.

4. DOE commits to placing and maintaining contaminated materials in the

. disposal unit at the specified densities and at average moisture contents that are less than their average steady-state moisture contents presented in the RAP and, in any case, less than 5% by volume for the tailings and less than 10.6% by volume for the windblown and other vicinity property contaminated materials. D02 will place and test at least four lifts of contaminated materials during the trial compaction (first 1,000 cyd of material), which is intendeo to develop procedures to ensure compaction of the materials in accordance with material specifications. As part of the submission required under Condition 1 of this agreement, DOE will submit physical properties and compaction data on windblown material 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.

i

,, a

+ 3;, , ,

t-V m '

I L , ,

12 g ,

a

• A i

6.0 . RADON ATTENUATION AND SOIL CLEANUP

<. 6.1 Introduction s This:section of the TER documents the staff evaluation of the radon attenuation design and the radiation' survey. plan to assure compliance with'the EPA standard.

6.2. Radon Attenuation

'The review of the cover design for the radon attenuation included evaluation of-the pertinent design parameters for both the tailings and the radon ~ barrier

-soils,- and the calculations of the radon barrier (earth cover) thickness

'(References 9, 10, and 38).

The design parameters for the tailings and earth cover materials' evaluated for acceptability include:'long-term moisture content, material thickness, bulk'-

fdensity, 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 will be covered with an earthen cover (radon barriar). The radon barrier reduces the effluence of Ra-222 by reducing the diffusion rate to acceptable quantities. The thickness

-of the barrier depends on the properties of the barr er and tailings. For the i

earthen cover for radon attenuation, the DOE proposes to use-sil_ty clay from a borrow site and mix it with 3 percent by weight-of sodium Bentonite.

~

The material: properties and radiological. parameters useo in the design of the radon barrier for the stabilized tailings pile at the Green River site have been reviewed.

The radon barrier will'be compacted at a moisture content of 0 to 3 percent

• above the. optimum moisture content. This will result in the average placement moisture content of-approximately.17 percent. The average in situ moisture.

content for this material is approximately 5.5 percent. The staff has calculated.the long-term moisture content using Rawls' (Reference 5) method (a very conservative method) to be 9 percent. The DOE calculation uses a long-term moisture content of 11.9 percent based on data from a capillary-moisture test. - Considering the presence of a one-foot-thick rip rap and a

'6-inch-thick gravel bed on top of the radon barrier, and that only the bottom 15 inches of the three-feet thick radon barrier is designated for protection against radon emanation, the staff concludes that the lower portion of the radon barrier will retain most of its placement moisture in the long-term. The

-- .__.m__U._m.m._ _ _ . - __ .__m. _ _ _ _ _ _

2 - 3 1

13 staff, therefore,~ concurs with the DOE's estimation of 11.9 percent long-term moisture content for- the radon barrier material.

The tailings materials will be :.ompacted to the specified density at its present in situ moisture content of 3 to 5 percent.- The average long-term moisture content at 15 bar capillary pressure, determined from moisture-capillary-tests-for the" sandy material at this site, is approximately 2.7 percent; The 00E has-used a long-term moisture content'of 10 percent for tailings in theLdesign calculation. As the 00E is. proposing to place the

't'ailings material in the : disposal cell at its in situ moisture content of 3 to.

5 percent.and. compact it to the design specifications, the long-term twisture content;of 10 percent for~the tailings used in the. calculations is high. The.

staff estimates the. tailings long-term moisture content to be in the range of 5 c percent, which is.close to;the steady state moisture stipulated in the DOE's analysis for complianc~e with ground water standards. The effect of this lower.

moisture content-on the thickness of the radon barrier is discussed in Section

' 6. 2. 2 c , r The material thicknesses (layers)~used in DOE's. analysis are based on the conceptual' design of the remedial action plan and' data available from field investigations..-The tailings and the contaminated wind blown materials will be

.placed in the' disposal cell, and there is no layering or preferred placement of these materials within the disposal cell. The design assumes uniform, average properties for these materials. The material thickness (44 feet for tailings and contaminated materials) used in the analysis for the, radon barrier thickness calculation is a reasonable representation of the field conditions.

The staff is aware that the propesties 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 in 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 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 i saturation data from both field and laboratory measurements of soil samples that are representative of the condition in the stabilized pile. The diffusion

14 l

coefficient for the radon barrier material is 0.00247 cm2/sec for the estimated long-term moisture content of 11.9 percent. The diffusion coefficient for the tailings material is estimated to be 0.021 cm2/sec for the long-term moisture a 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 Reference 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 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 43) 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 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 measurej. The average radium content to be used in the analysis was determined by weighted averaging with depth in a measurement hole and then averaging over an area at any given depth. The weighted average value of the radium content for the entire pile was calculated to be 74 pCi/gm. However, the average radium content will be verified by field measurements on the stabilized tailings pile before placing the radon barrier earth cover, and the radon barrier design will be reassessed at that time to ensure that the radium content used in the design is 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 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 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

15 I

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 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 having 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 conclud s 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) l l

l l

l ,