U.S. Department of Energy Office of Civilian Radioactive Waste Management Geochemistry of Dust in...

U.S. Department of EnergyOffice of Civilian Radioactive Waste Management

Geochemistry of Dust in the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada2004 Annual Meeting of the Geological Society of America

Zell E. Peterman, Leonid A. Neymark, and James B. Paces U.S. Geological Survey, Denver CO

November 7, 2004Denver CO

2

Yucca Mountain is a Ridge of Rhyolitic Tuff in Southern Nevada (Looking South)

3

ESF Tunnel (Yellow), Cross Drift (Red), and Conceptual Emplacement Drifts (Blue)

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Construction of Underground Space Is a Dirty Business

• Dust is the inevitable product of underground mining and construction

• Fine dust is a health hazard for workers in the underground and must be controlled thru-

– Ventilation

– Filtration

– Generation reduction by wetting at production sites

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Why is Dust Composition Important in a Nuclear Waste Repository?

• Dust will accumulate on waste canisters and drip shields during and after emplacement

• Dust salts may dissolve in water that drips on canisters or deliquesce in high humidity environments to form brine droplets or films

• These salts or saline waters may accelerate corrosion of the canisters

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Sources of Underground Dust

• Mining (TBM, Alpine miner, drill and blast, muck haulage)

– comminution of rock including fracture minerals, vapor-phase minerals, alteration minerals (clays and zeolites)

• Particulates from diesel exhaust

• Salts from evaporated water

– Construction water (tagged with LiBr)

– Pore water migrates to tunnel walls and evaporates leaving a salt halo

• Abraded rubber and fiber from conveyor belts

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Sources of Underground Dust (continued)

• Aerosols from lubricating oil, diesel oil, grease, hydraulic fluids, etc.

• Ferrous metals from variety of sources

• Concrete particles from emplacement and abrasion of inverts

• Salts from human effluents

• Dust from the surface transported into the Exploratory Studies Facility (ESF) on materials and by the supply air

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Dust Collection

• Dust was vacuumed from the tunnel wall, trapped by a cyclone, and deposited in a 250 mL sample bottle attached to the cyclone

• Several square meters of surface were vacuumed to yield 200 to 400 grams of dust

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Methods

• Chemistry of bulk dust size fractions by standard rock analysis methods (XRF, ICPMS, gravimetric, titration, combustion)

• Chemistry of leachates by ion chromatography and ICPMS

• Mineralogy of soluble salt fraction

– Scanning electron microscope (SEM)

– XRD analyses of evaporated leachates

– Calculation of normative minerals using SALT NORM (Bodine and Jones, 1986)

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Particle Size Distribution of ESF Dust(shown in mesh size)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

N.Portal

2+24 6+13 8+61 12+63 22+07 30+07 40+13 54+05 65+15 70+50 76+22

ESF Station Location

We

igh

t P

erc

en

t

60-100

100-200

200-325

-325

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Major and Minor Elements in Dust Size Fractions Relative to Host Rock

0.1

1

10

100

1000N

orm

aliz

ed t

o T

op

op

ah S

pri

ng

Rh

yolit

e

60-200 Mesh200-325 Mesh-325 Mesh

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Trace Elements in Dust Size FractionsRelative to Host Rock

0.1

1

10

100

As

Ba

Be Bi

Cd

Co Cr

Cs

Ga Li

Mo Ni

Pb

Rb

Sb

Sc Sr

Th Tl U V Y Z

n La

Ce Pr

Nd S

Eu

Tb

Gd

Dy

Ho Er

Tm Yb

No

rmal

ized

to

Rep

osi

tory

Un

its

60-200 Mesh200-325 Mesh<325 Mesh

Rare Earth Elements

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Major and Trace Element Enrichments in Dust Relative to Rhyolite

• Major Elements

– FeO (introduced as metallic iron), CO2 (from calcite veins) , Organic C and Cl (neoprene abraded from conveyor belt etc.), and Cl (from water)

• Trace Elements

– Bi, Cd, Co, Cr, Mo, Ni, Sb, V, Zn (metallic elements associated with construction and materials introduced during construction)

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Soluble Fractions of Surface and Underground Dust

• Underground dust contains a small amount of soluble salts

– Dust in main tunnel contains 0.37±0.18 (1 σ) weight percent (n=66)

– Dust in cross drift contains 0.17±0.17 (1 σ) weight percent (n=18)

• Surface dust contains a much larger amount of soluble salts (Reheis, 2003)

– Atmospheric dust collected at and near Yucca Mountain contains 13.3±8.1 (1 σ) weight percent (n=51)

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Identification of Salt Minerals in Underground Dust

• SEM examination for S and Cl in dust mounts

– halite, sylvite, gypsum, natroaulnite (also pyrite, molybdenite, native sulfur identified)

– No CaCl2 was identified

• XRD analyses of dried leachates

– halite, sylvite, calcite, gypsum, and bassanite (2CaSO4·H2O)

– salammoniac NH4Cl

– mascagnite (NH4)SO4

– biphosphammite (NH4,K)H2PO4

– weddellite CaC2O4•2H2O

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Identification of Salt Minerals in Underground Dust (cont’d)

• Estimation of soluble minerals from water leachates using SALT NORM (Bodine and Jones, 1986)

– Carbonates (calcite, dolomite, pirsonnite)

– Nitrates (niter, soda niter, ammonia niter)

– Sulfates (glauberite, aphthitalite, thenardite, anhydrite, syngenite, mascagnite)

– Chlorides (halite, sylvite, salammoniac)

– Fluorides (fluorite, villiaumite)

– Phosphates (fluorapatite, hydroxyapatite, wagnerite)

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Salt Minerals Expected from Atmospheric Dust Intrusion into Repository (Chemistry from NADP Red

Rocks Site; Minerals from SALT NORM)

Ammonia Niter

GlauberiteCalcite

Dolomite

NiterSyngenite

Salammoniac

Anhydrite

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Salt Minerals Expected from Atmospheric Dust Intrusion into Repository

Mineral FormulaAmmonia Niter NH4NO3 32.7 48.5Glauberite CaNa2(SO4)2 8.4 20.7Calcite CaCO3 10.3 21.9Dolomite CaMg(CO3)2 9.3 18.9Salammoniac NH4Cl 3.9 6.0Anhydrite CaSO4 2.5 13.3Syngenite CaK2(SO4)2·H2O 0.0 1.8Niter KNO3 0.3 1.6

Range (wt percent)

SNORM Minerals from Red Rocks Precipitation

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Nitrate-Chloride Ratios

• Ratio of soluble nitrate-to-chloride is an important parameter for corrosion

• Critical weight ratio (NO3/Cl) is approximately 0.9

• Soluble salts in dust typically have NO3/Cl ratios greater than 0.9

• Pore waters typically have NO3/Cl ratios less than 0.9

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Nitrate-to-Chloride Weight Ratios in Dust Salts and Pore water

0.1

1

10

100

1000

1 10 100 1000Cl (mg/Kg)

NO

3 (m

g/k

g)

Pore WaterDust

NO3/Cl

0.9

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Conclusions

• Most of the underground dust is finely comminuted rhyolite

– Fine silicate particulates will neutralize acids formed in the near-field environment (D. Langmuir, 2004)

• Soluble salts are more relevant to corrosion issue

– Soluble salts average less than 1 percent of the underground dust

– Atmospheric dust contains much larger amounts of soluble salts (average=13.3 weight percent)

– Nitrate-chloride ratios of both underground and atmospheric dust are favorable (greater than 0.9)

– No CaCl2 was identified (none expected)