ML20236C327
| ML20236C327 | |
| Person / Time | |
|---|---|
| Issue date: | 10/16/1987 |
| From: | Lohaus P NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | James Anderson ENERGY, DEPT. OF |
| References | |
| REF-WM-54 NUDOCS 8710270100 | |
| Download: ML20236C327 (8) | |
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WM Reconf Fih
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E___ ' 'j LPOR James R. Anderson,' Project Manager WM
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Uranium Mill Tailings Project Office
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U.S. Department of Energy-
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Albuquerque Operations Office P.0; Box 5400 Albuquerque, NM 87115
Dear Mr. Anderson:
Enclosed are NRC Staff comments on the Phase'II Preliminary Design documents for Grand Junction, Colorado. ' Concern's raised in these comments should be resolved in development of the final design..
These comments. represent the staff's. review in all technical areas, with the exception of geotechnical engineering. Comments resulting from the staff's geotechnical review will: be sent to you by October 30, with a draft copyLfaxed to Karen Agogino as soon as practicable.-
1 Please contact Susan.Bilhorn at FTS 427-4145 if you have any questions regarding the Staff's comments.
l Sincerely,.
i
~ Paul H. Lohaus, Acting Chief Operations Branch-I Division of Low-Level Waste Management and Decommissioning, NMSS
Enclosure:
As stated DISTRIBUTION:
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TJohnson, LLTB DWidmayer, LLTB
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NAME:SBilho n :MFliegel :PLohaus DATE:87/10/ 6 ':87/10//r :87/10/k$
OFFICIAL RECORD COPY 8710270100 871016 PDR WASTE WM-54 ppg l.
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v NRC STAFF COMMENTS 0N GRAND JUNCTION PHASE II PRELIMINARY 10ESIGN DOCUMENTS SURFACE WATER HYDROLOGY Comment: SW/1 - Geomorphic Processes at the Cheney Reservoir Site In DOE's draft Remedial Action Plan for Cheney Reservoir, headward.and' lateral erosion of existing gt;11es'were identified as the greatest-geomorphic hazard to the. site' (page:E-49).' LAs t a result,. 00E: proposedi that -
the. tailings perimeter be set back.from existing: gullies to: assure
.long-term stability of the disposal ' area.
Selection.of the' proposed disposal location and.the phase II, design are based on an: assumption-that..
all surface-water impacts on the dispoi al 'will result'only. from 'sheetwash and uniform areal erosion. Therefore, 00E used the Uniform Soil Loss-Equation (USLE) (Israelson, 1980)'to calculate:the erosion protection criteria in the Phase II Design.-
NRC staff's analysis indicates?that pediments at Cheney Reservoir.may be in a state of. incipient landscape instability. and that-there is 'a.
1 potential for gully formation' unrelated to existing gullies. 'This is j
related to the channel gradient and area'of the drainage basin.
In general, a channel becomes unstable.and:is likely to. develop'a gully as-qo drainage area increases 'and/or the; gradient is steepened.. For~ analysis, the staff compared natural conditions at Cheney Reservoir'with other.
1 semiarid drainage basins in Colorado, Nebraska, and.New Mexico.
Pertinent data at Cheney Reservoir were' derived from the-dRAP and USGS-topographic maps:-
2 drainage area above proposed s'ite 1.25 km - (310 ' acres),
1 channel gradients - above proposed site.
0.038
- through proposed site 0.021-Morphometric conditions at Cheney Reservoir'were plotted with data derived.
from five western. watersheds (Wells 'andL Gardner,:1985) and are presented,
graphically in Figure:1.. The-discriminant functions'which separate gullied from ungullied valleys represent a statistical-determination of!
threshold conditio'ns.
In each case, however, a:few gullied channels plot below the threshold, and a-few ungullied channels-plot 'above.- Functions-
"a" through "d" represent channels. underlain by fine-grained-deposits, similar to those at Cheney _ Reservoir. : Function? '!e" represents coarse-grained alluvium.
The comparison.shows that the Cheney Reservoir site' possesses conditions-which occur on or beyond a threshold condition for channel instability'.-
The. staff's analysis suggests that '(1) the Cheney Reservoir area:is -
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- unstable, (2) gullies' may be expect'ed Lto form along any of. the area'.s a
i channels,.and.(3), adequate: protection from the effects of future erosion l
'may not be;provided by setting thel tailings back fromtthe existing gullies and assuming uniform soil: erosion will occur. A more ' conservative approach shoul.d be considered.'
]
- This issue was raised in the staff's coments.on the draf t; RAP.
It is-
.still:not apparentLthat the, impacts of new gully formation have been considered, nor-has DOE provided a rationale for theidesign criteria which d
z resolves this issue.
DOE should' address the potential effects offgullya formation in development of the final ~ design.
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~ Coment:.SW/2 - Design of Energ.y Dissipation Areas at Ditch Outlets NRC Staff _'s review of energy ~ dissipation areas' (EDA) proposed for the..
l-diversion' ditch outlets indicates'.that' the design may not be adequate to prevent long-term erosion.: The designs appear to include-several shortcomings:
A.
Depth of Rock Toe Placement.
The depth offrock toe placement at the-l downstream end of the EDA has been based on computations using the USLE. Depth of erosion was computed assuming removal of a'certain soil volume across the' EDA. Maximum predicted erosion. depth is 26 inches and is accommodated by placing-rock to a depth' of thr'ee ' feet.-
NRC Staff do not agree that use-of the USLE is appropriate for-computing erosion depths for the Cheney Reservoir site., The USLE i
assumes erosion from sheet flow, and bas little or no' applicability-to gully erosion..In all likelihood soil slopes: downstream of EDA's 4
will be eroded since the slope of the naturaliground'is 2.5% while j'
the slope of the EDA is about 0.4%.
The adequacy of transitioning.
from a 0.4% rock slope to an unprotected 2.5% earth slope'is-questionable.
Concentrated flow will eventually occur on the earth slope, and gully erosion ~ should be expected.
i The staff suggests DOE consider all of the following computational methods and assumptions to arrive at a reasonable and conservative
-estimate of gully depth for design of the rock: toe.
[
1.
Depths of naturally occuring gb111es existing adjacent ~to the-site should be detennined.
For example, southwest of the' site, natural gully depths exceed three feet.;. Correlations should be developed between depth and drainage area for gullies near' the site using readily available maps and field data 'so that the-potential depth of gullies can be estimated for the drainage area of the EDA in question.
ii.
It may.be appropriate'to use the USLE for order-of-magnitude.
estimates.of gully depths,.if DOE assumes that the total amount i
of soil eroded.is from a single V-shaped gully. Then, for DOE's assumed EDA width of 300 feet and 2-foot depth of erosion, a-Page2lof-7 4
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V-shaped gully section with a cross-sectional area of-1 4
approxima'tely 600 ft2(DOE'scomputedsoilloss)'couldpossibly
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be used for. design purposes. The side slopes of the -V-shaped V-shaped gully should be assumed equal to the angle of repose.
i for the soil.
i iii. For the; EDA on the east side;of the pile, DOE should consider.
potential lowering of base level and bank erosion in the adjacent stream.D It appears that the rock toe should be at.
least as deep as the adjacent stream in order to prevent undercutting (see'also part 8, below).
y B.
Protection of EDA Against Erosion from Adjacent Stream. LStaff's review of Drawing No. GRJ-DS-10-0220 indicats that the east EDA will-j be located relatively close to an existing natural stream channel.
i It appears that the 4-inch rock proposed for. placement..in the EDA may-not be capable of withstandingiflow velocities likely to;occure j
'during 'the 1000-year' design life, especially. if erosion of the -stream -
bank occurs. Riprap for the EDA should be designed to resist such velocities in the-stream, or analyses should be presented to
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demonstrate that the proposed rock is large enough. Consideration should be given to-the potential for erosion at the outside of channel bends, potential for hydraulic jumps or energy dissipation to:
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occur, potential lowering of stream, and potential.' lateral erosion or undercutting of the EDA by erosion in the stream (see also part A.iii,above).
Comment: SW/3 - Design of Diversion Ditches From design documents, the staff note that the ditches proposed for.
diversion of flood flows from upstrecm of the pile will be constructed on slopes ranging from 0.4% to a maximum of 1.3%.
However. proposed recontouring of the area immediately upstream will produce a ground surface with a slope of up to 3.3%.
Because of the limited ability.of a relatively flat ditch to " flush" sediment received from steep slopes, it appears that the ditches could be frequently clogged with sediment and debris. These clogs could form at critical areas in ditches, resulting in flows and potential erosion adjacent to the remediated embankment, therefore maintenance may be required. Hence,;the EPA long-term' stability, criteria (40 CFR 192) may not be met by this design.-
Based on examination of the site and of information provided by geomorphic analyses, it appears that gu11ying of natural slopes could occur;-
therefore a potential' exists for concentration of runoff into' diversion ditches at one or more points.- This concentration would occur where the-gullies discharge into the ditch.
It is important that erosion protection i
in the ditches be designed to resist-forces associated with concentrated -
flows discharging into the ditch and ener the point where such discharge may occur.gy dissipation'in the ditch at Accordingly, the ditch design should be revised.to account for the above phenomena or justification' should be provided to show that sedimentation and flow concentrations will not cause significant problems.
The basis for all assumptions and calculations should be provided.
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i Coniment: SW/4
' Design of Rock Apron j
o NRC staff note that the rock apron surrounding the pile. has been designed 1
to resist headcutting'and erosion to a depth of three feet., However, the 1
basis and rationale for this design.have not been provided.. DOE should' provide the rationale for design 'of the vertical extent of the rock. apron.
This: rationale should include analyses of the rock-apron design in regard ~
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to the design criteria' suggested in NRC Comment: SW-2.
H Comment: SW/5 - Rock Durability Criteria NRC staff note that D0E intends to use rock acquired 'from onsite.
excavation as' riprap erosion protection for the pile and diversion I
ditches.
Furtherr: ore, due to it's marginal quality, some of the. rock will j
need to be oversized to meet rock durability requirements.
In general,'NRC staff conclude that gcod-quality _ rock should probably be.
used at UMTRAP sites,'if practicable.. Marginal. quality rock is acceptable:
in those cases where DOE can justify it's use, based on the difficulties associated with obtaining good rock and 00E can show reasonable assurance, of meeting at least the 200-year minimum of the longevity standard, i
However, if use of good-quality rock is practicable, marginal-quality rock can be used only if DOE'can show reasonable assurance of meeting'the 1000-year design standard, a much more difficult task with marginal-quality rock, i
DOE should document the search for sources of good-quality rock 'and should I
justify the use of the rock source (s) selected. This documentation.should include analyses and discussions regarding the location durability, and costs associated with the most practical source of good quality rock and/or the difficulties and costs associated.with its use.
In addition, our review of durability test data'provided by DOE show
" borderline" results regarding durability scores for several. samples tested. 00E should perform additional durability tests to better enhance the scoring methodology. The staff suggest that tests such as the Schmidt Impact Hammer test, Tensile Strength test, and Slake-Abrasion' test, be-performed in order to better define the rock quality and to better l
determine the rock-quality scores. As an alternative, DOE should: propose additional ~ tests that could be used to better define the quality of this specific rock.
t REFERENCES CITED israelson,1980, Erosion control during highway construction: U.S Federal Highway Administration, NCHRP Report Number 221.
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Wells, S.G. and Gardner, T.W.,1985, Geomorphic criteria for selecting stable uranium tailings disposal sites in New Mexico: New Mexico Energy Research and Development Institute, NMERDI 2-69-1112, v. 1, p. 353.
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GROUND-WATER HYDROLOGY I
Comment: GW/1 - Dewatering j
j DOE performed two calculations on dewatering in the Preliminary Phase II' design document, calculation numbers 05-622-01-00 and 05-654-01-01.
DOE i
states that the Theim method was used on data from the five test pit pump test to determine hydraulic conductivity, however this method may have been inappropriate. The Theim method assumes steady state conditions.
DOE did not indicate that steady state existed prior to taking the final measurements, therefore the values taken during the test may not accurately reflect equilibrium conditions. Also, the test pits were l
constructed in alluvial material along the perimeter of the tailing pile and may not adequately represent conditions in the tailings.
DOE should provide measurements to indicate that equilibrium was reached in the testpits 1 sed in calculation No. 05-622-01-000.
In addition, DOE should show that che test pits used adequately represent the actual tailings pile.
Comment: GW/2 - Hydrogeology - Dewatering, Calculation No. 05-654-01-01 DOE calculated the rate of discharge needed to maintain unsaturated conditions in the zone of excavation during removal of the tailings.
For this calculation the source of water was divided into four discrete portions (1) water flowing through the slurry wall, (2) water flowing under the slurry wall, (3) water entering the excavation zone from sides not bounded by the slurry wall and (4) drainage of water from the saturated tailings.
The fourth water source, denoted Q4, was calculated based on assumptions that appear to be nonconservative.
00E assumed that the specific yield cf the tailings (slime and sand / slime mixture) was 0.03.
However, available literature indicates that the specific yield is much higher, possibly ranging from from 0.20 to 0.33 (Mercer, 1982). Using Mercer's estimates of specific yield in DOE's calculations, almost an order of magnitude more drainage can be expected from the tailings.
This would greatly increase the volume of water that would need to be discharged to dewater the tailings.
NRC staff suggests that DOE recalculate the drainage volumes (04) using more conservative estimates of specific yield, or demonstrate that values used in the phase II preliminary design documents are appropriate.
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' Comment: GW/3 - Hydrogeology - Dewatering, CalcNiation-No.Yd5-654-01-01' 00E calculated the'effect of ground-w'ater discharge from.the saturated zone 'in the excavation area lon, the quality of water in thelColorado' River.-
DOE concluded that no impact on surface' water quality?is expected.'.NRC
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staff reviewed:these calculations.and considers the results to be.
nonconservative. 00E ' calculated.the degradation of 'surfaceNater! quality '
.by diluting.the maximum constituent concentration:of.sthe ground water,-
times the maximum discharge rate, with the' low; flow discharge rate from-the. river. Howdver, the maximum concentrations 4used for.both NHg (393; r
mg/1)andTDS-(6000mg/1)are'lowerLthanconcentrationslistedin:the, Final = Environmental Impact Statement,~pages'F86, F97,.F167', F182', F187JandL F188.-
DOE.should. consider.using these'more' conservative concentrations 1to calculate the' potential. effect of ground-water discharget r
l Comment: GW/4 - Radon Barrier Design, Calculation No. IO5-670-01-00!L DOE considered only radon attenuation when theyJcalculated thicknessiof the radon -barrier for the embankment at the' Cheney Reservoir disposal:
site. Apparently: no consideration was given as to whether.the proposed.,
thickness would; limit infiltration'into the embankment =enough to-adequately protect ground-water resources'.
This; aspect of the remedial action must be. considered-so that both radon exhalation and ground-water protection are. acceptable and adequate. The two foot. thickness' proposed in the preliminary' design may be adequate to= inhibit infiltration rates and protect. ground water,' but no calculations were;provided.-
DOE should consider infiltration rates in-design of the cover thick' ness,;
l and should include supporting calculations lin future design documents.
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REFERENCES CITED Mercer, J.W., et al.,1982, Parameters and Variables Appearing in Repository Siting Models, prepared for the U.S. Nuclear Regulatory 1
Commission,NUPL/CR-3066.
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