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Riverton Mill Site
Fremont County, Wyoming
| Years of Operation |
Status of Mill
or Plant Site |
Uranium Ore
Processed
(Million Short Tons) |
Production
(Million Pounds U3O8) |
| 1958-1963, 1964-1966 |
Decommissioned |
0.91 |
3.40 |
Mill/Plant Area
(Acres) |
Disposal Cell
Area
(Acres) |
Disposal Cell
Radioactive
Waste
Volume
(Million Cubic Yards) |
Disposal Cell
Total Radioactivity
(Ci, 226Ra) |
Disposal Cell
Average Tailings
Radioactivity
(pCi/g, 226Ra) |
UMTRA Project
Final Cost
(Million Dollars) |
| 140 |
A |
1.79 |
A |
292 |
49.66 |
Location: The site of the former Riverton uranium-vanadium ore processing mill is located about one mile southwest of the city of Riverton, Fremont County, Wyoming. The mill site and tailings pile were situated on privately owned land that is located within the boundaries of the Wind River Indian Reservation.
Background: The Wyoming Basins region is a part of the Rocky Mountain province and consists geologically of high mountain ranges separated by synclinal basins that form broad valleys between the ranges. Most of the uranium deposits in the Wyoming basins are hosted by sedimentary rocks of Tertiary age. The mountain ranges are mainly breached anticlinal structures where Precambrian granitic-rock cores were exposed by erosion during the waning stages of the Laramide orogeny.
Uranium occurrences are widely scattered across the region. During World War I, uranium ore was mined for radium from a vein deposit at the Silver Cliff mine in the Hartville uplift area near Lusk, Wyoming. Though other deposits were reportedly known in the region, none was exploited before the mid 1950s. In 1951 while conducting field work, J. D. Love, U.S. Geological Survey, discovered uranium minerals on outcrops near the Pumpkin Buttes area of the Powder River Basin, Wyoming. In 1953, Neil McNiece, prospecting along the Beaver Rim in the Gas Hills area east of Riverton, Wyoming, found uranium on outcrops of the Wind River Formation (early Eocene) above the Lucky Mc claims. These developments kindled further exploration efforts that soon led to other discoveries. Subsequent airborne radiometric surveys by the U.S. Atomic Energy Commission (AEC) and follow-on surface drilling by private industry resulted in the discovery and development of ore deposits in the Gas Hills district. Exploration rapidly spread to areas in other basins, and by 1954 uranium occurrences had been found in all areas of the Wyoming Basins region. By 1956, the deeper-lying, large unoxidized uranium ore deposits had been discovered in the central Gas Hills area in the coarse arkosic sandstone and conglomerate beds in the upper part of the Wind River Formation. Of the 70 mine properties that had been developed in the district through 1990, 16 had each produced more than one million pounds of U3O8 and six properties had each produced more than one million tons of ore. The Gas Hills district is located along the south central edge of the Wind River Basin near the northern border of the Granite Mountains-Sweetwater Arch.
Uranium was discovered in the Big Pryor Mountains area of southern Montana in September 1955 and in the Little Mountain area of Wyoming in early 1956. The deposits occur in the Madison Limestone (Missippian age) which in the upper 200 feet contains an extensive system of solution caverns. Some caves are filled with insoluble breccia and silty debris. Locally, the cave-filling and permeable zones within the limestone contain uranium and vanadium mineralization. The caves vary widely in size and most are barren. Ore produced from 20 mines averaged 0.36 percent U3O8 and 0.38 percent V2O5. The median mine yield was about 250 tons of ore, but two mines produced more than 5,000 tons of ore. The ore's erratic character, excessive distance to the closest mill, milling penalties due to the ore's high lime content (usually above 70 percent CaCO3), and the declining grades of mined ore over time caused operations to close by 1966.
In November 1954, the U.S. Atomic Energy Commission (AEC) leased a site southwest of Riverton, Wyoming, from the Chicago & North Western Railway Company for a uranium-ore buying station. AEC operated the buying station from March 1, 1955, to late 1957. Some 152,700 tons of ore averaging 0.23 percent U3O8 were purchased and stockpiled at the site during that period.
In December 1957, the AEC entered into a contract for the purchase of uranium concentrate from a mill at Riverton, Wyoming. The initial contract was with Fremont Minerals, Inc., a subsidiary of Susquehanna-Western, Inc. (SWI). In January 1961 the Fremont contract was replaced when the AEC and SWI entered a new contract for which the term extended through December 31, 1966, with final concentrate deliveries scheduled February 1967. The prime purpose of the Riverton mill was to provide a market for ore and uranium from the increasing number of uranium mining operations then emerging in the Gas Hills, Pryor-Little Mountains, and Wind River Basin areas. Most of the ore treated at the Riverton mill, which was operated as a "custom" mill from November 1958 until May 1963, came from mines in the Gas Hills area. After the mill was shut down in June 1963, SWI shipped uranium ore from company mines and ore purchased from independent miners to the Federal-American Partners' uranium mill in the Gas Hills district. The AEC continued until July 1965 to purchase the uranium concentrate produced under the SWI-Federal toll-milling arrangement.
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Riverton mill was built with both an acid-leach and a carbonate-leach circuit to provide the capability of processing ore from widely differing geological environments and the large number of shippers. A sulfuric acid plant (100 tons-per-day) was built at the Riverton mill site to make the acid needed by the Riverton mill and also later to supply acid for other milling operations. The acid manufacturing process used sulfur made from sour gas.
The Riverton mill had a nominal ore-throughput capacity rating of 500 tons of ore per day (TPD), though the acid-leach capacity was later upgraded to treat up to about 600 TPD and the carbonate-leach circuit up to about 300 TPD. Over its operating life, the mill averaged close to 600 TPD. Most ore was processed through the acid-leach circuit, as the quantity of high lime ore eventually shipped to the mill was much lower than originally anticipated. Over its life, some 909,200 tons of ore averaging 0.20 percent U3O8 were treated at Riverton. Overall, the mill recovered 92 percent of the uranium from ore processed. The mill produced about 3.4 million pounds of uranium concentrate, all of which was delivered to the AEC under two procurement contracts.
Ore received at the mill was crushed, chemically analyzed to choose acid- or carbonate-leaching, and stored in fine-ore bins that could feed either the acid- or carbonate-leaching ore circuit. In the acid-leach circuit, ground ore was mechanically agitated while being leached with sulfuric acid and sodium chlorate. Pregnant solution was recovered in a four-stage liquid-solid separation circuit that used a parallel arrangement of cyclones (for sand collection) and thickeners (for slimes collection). Uranium was recovered from solution by solvent extraction. In the carbonate-leach circuit, ground ore was leached for 54 hours at 180°F with sodium carbonate-bicarbonate in open, steam-heated Pachuca tanks.1 The ore residue was separated from the pregnant solution using drum filters arranged in a three-stage circuit. Uranium was precipitated by addition of caustic. The barren solution was recarbonated using waste CO2 boiler-flue gases prior to being recirculated. The uranium concentrate product was filtered, dried, and packed for shipment.
Final uranium production at the Riverton mill site was in May 1963. The mill was dismantled soon thereafter, and most of the mill equipment and part of the main mill building and several other structures were removed from the site. In early 1981, the mill site covered about115 acres of which 72 acres were covered by an estimated 900,000 tons of mill tailings material. The tailings pile was built on slightly elevated ground and had an engineered base sealed with about 94,000 cubic yards of clay. Prior to 1981, the pile was stabilized by an earthen cover (sand and gravel) to guard against wind and water erosion. A ditch and berm system trapped rain runoff from the pile and also protected the pile against potential damage during flooding of the Wind River and Little Wind River.
UMTRA Surface Remediation: Contamination at the former SWI Riverton mill site resulted from milling of uranium ores. At the start of the UMTRA clean up project, radioactively contaminated materials at the site included uranium and vanadium mill tailings, radium, thorium, and uranium residues mixed locally with soil, and debris remaining from the prior mill demolition work. Prior to site remediation, the mill site and tailings pile were acquired in 1987 by the State of Wyoming. Remediation began in May 1988 and was completed in September 1990. After decontamination of the former mill site, the cleaned-up areas were backfilled with clean soil and graded to elevations compatible with the surrounding land and drainage. The areas were then revegetated with native species. Ownership of the remediated site will eventually revert from the State of Wyoming to industrial/commercial use. The U.S. Department of Energy, in order to fulfill requirements of the UMTRA program for groundwater restoration, will retain control of the property until all phases of the UMTRA Riverton Project are complete.
Disposal Area: The mill tailings and contaminated soils and debris from the Riverton mill site and contaminated materials from 42 nearby vicinity properties were transported to the UMETCO Gas Hills Disposal Site. That Title II disposal site is located in the Gas Hill uranium mining district about 45 miles east of the city of Riverton and is licensed by the U.S. Nuclear Regulatory Commission.
Responsibility for Remediation: U.S. Department of Energy, 100 percent.
Stewardship: The U.S. Department of Energy (DOE) is responsible for long-term stewardship tasks at the former Riverton mill site, including groundwater monitoring and implementation of water-use restrictions. For the remediated site, DOE maintains data relating to site characterization, design parameters for site restoration, and current and historical site monitoring data.
Groundwater Program: A plume of contaminated groundwater is present in the shallow alluvial and bedrock aquifers beneath the former mill site. The plume covers an area of about 160 acres and contains about 1.9 million cubic yards of groundwater contaminated with chromium, molybdenum, selenium, radium, uranium, and net gross alpha. These contaminants have not degraded the water in the adjacent flowing streams, the Wind River and Little Wind River. A permanent municipal water supply was engineered and installed to assure that the groundwater contamination does not threaten the local population.
Annual monitoring of groundwater at the former mill site will be used for a ten-year period following completion of surface remediation to demonstrate that natural flushing of contaminants is working as predicted. Passive remediation through natural flushing without active restoration intervention is anticipated to achieve compliance with the approved standards within the U.S. Environmental Protection Agency's (EPA) limit of 100 years. Verification monitoring of groundwater conditions will continue through 2004, when the site will be transferred to the Long-Term Surveillance and Maintenance Program. After the NRC certifies compliance with the EPA standards, the former mill site will be released to its owner for unlimited use.
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