Final Report 

Review of Existing and Proposed Tailings Impoundment Technologies 

 

 Prepared by 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

 Vienna, VA 22182 

 Under 

Contract Number EP-D-05-002 

Work Assignment No. 4-11, Task 5 

 

Prepared for 

U.S. Environmental Protection Agency 

Office of Radiation and Indoor Air 

1200 Pennsylvania Avenue, N.W. 

Washington, DC 20460 

Reid J. Rosnick 

 Work Assignment Manager 

September 25, 2008
In accordance with the Quality Assurance Project Plan: Technical and Regulatory Support to 

Develop a Rulemaking to Modify the NESHAP Subpart W Standard for Radon Emissions from 

Operating Uranium Mills (40 CFR 61.25), this document has been reviewed and approved by the 

following individuals: 

Work Assignment Task Manager: ________________________ Date: __09/25/2008__ 

 Harry Pettengill 

Project Manager: ________________________ Date: __09/25/2008__ 

 Abe Zeitoun 

Corporate Quality Assurance Mgr: ________________________ Date: __09/25/2008__ 

 Gregory Beronja 

Work Assignment QA Manager: ________________________ Date: __09/25/2008__

 Stephen Ostrow 

WA 4-11, Task 4 – NESHAPs History i SC&A – September 25, 2008
TABLE OF CONTENTS 

1.0 Historical Conventional Tailings Impoundments ............................................................... 1 

2.0 Profile of the Existing Industry........................................................................................... 2 

3.0 Anticipated Changes in the Industry Profile....................................................................... 3 

4.0 Comparison of Uranium Tailings Disposal Technology with the Requirements of 

RCRA, Subtitle C................................................................................................................ 4 

5.0 Discussion of Climate and Impoundment Size on Rn-222 Emissions ............................... 6 

6.0 In-Situ Leach Uranium Recovery Facilities ....................................................................... 7 

6.1 Scope and Background ........................................................................................... 7 

6.2 Uranium Recovery Process for ISL........................................................................ 9 

6.2.1 Uranium Mobilization................................................................................. 9 

6.2.2 Wells............................................................................................................ 9 

6.2.3 Uranium Processing .................................................................................... 9 

6.3 Radon Source Terms............................................................................................. 10 

6.3.1 Source Terms............................................................................................. 10 

6.3.2 Example In-Situ Recovery Radon Source Calculation ............................. 13 

6.3.3 Decommissioning of Well Fields and Evaporation Ponds ........................ 16 

6.4 Discussion............................................................................................................. 16 

6.5 Next Step............................................................................................................... 17 

7.0 References......................................................................................................................... 17 

LIST OF TABLES 

Table 1. Tailings Impoundments at Conventional Uranium Mills................................................. 2 

Table 2. Anticipated New Conventional Uranium Milling Facilities............................................ 3 

Table 3. Comparative Rn-222 Emissions over 70 Years................................................................ 6 

Table 4. Operating ISL Facilities................................................................................................... 7 

Table 5. Anticipated ISL Facilities................................................................................................. 7 

Table 6. Estimated Rn-222 Releases from an ISL Facility .......................................................... 17 

WA 4-11, Task 5 – Impoundment Technologies ii SC&A – September 25, 2008
1.0 HISTORICAL CONVENTIONAL TAILINGS IMPOUNDMENTS 

Although the recovery of uranium dates back to at least the 16th century, large-scale operations 

did not commence prior to the 1940s. As there was initially little appreciation of the radiological 

or chemical hazards associated with the uranium tailings, early disposal practices followed those 

used in other metal extractions processes. The tailings were often disposed of in the least costly, 

most expeditious manner. Specific practices included returning the tailings to a mined out pit, 

placing them on open ground adjacent to the mill, depositing them in natural depressions, 

depositing them in a valley behind a dam or dyke, or placing them in man-made impoundments 

known as ring dykes or turkey nest dams (IAEA 2004). Specific examples of such early 

practices include the following: 

 

• Grand Junction, Colorado, where tailings were deposited adjacent to the mill on open 

land bordering the mill or deposited in shallow depressions behind levees bordering a 

river 

• Uravan, Colorado, where tailings were deposited on an open bench on a canyon wall with 

nothing to prevent their movement to the canyon and river below 

• Grants, New Mexico, where a massive ring dyke embankment was created using the 

course fraction of the tailings sands 

Disposal of the tailings in water (either deep lake or river), which has occurred in other countries, 

has not been practiced in the United States. 

By the 1970s, the serious public health and ecological damage arising from the past disposal of 

tailings led to the passage of the Uranium Mill Tailings Radiation Control Act (UMTRCA). 

Pursuant to the regulations promulgated under UMTRCA (40 CFR 192), the Department of 

Energy (DOE) at Title I sites and NRC licensees at Title II sites have almost completed the 

restoration and stabilization of current tailings impoundments. For the most part, these 

reclamation activities have entailed retrieval of wind-blown tailings from adjacent land and 

properties, stabilization of the piles in place under thick earthen covers with gravel caps, and 

riprap shoulders to minimize erosion. In extreme cases, the tailings have been physically 

removed to a new location to minimize their vulnerability to flooding or intrusion. 

Impoundments at the few mills that remain operational and all future tailings impoundments 

must be built and operated to comply with the regulatory requirements of both the EPA and the 

NRC (or an NRC Agreement State). 

WA 4-11, Task 5 – Impoundment Technologies 1 SC&A – September 25, 2008
WA 4-11, Task 5 – Impoundment Technologies 2 SC&A – September 25, 2008

2.0 PROFILE OF THE EXISTING INDUSTRY 

At the time the NESHAP for Operating Uranium Mill Tailings Impoundments was promulgated 

(40 CFR 61, Subpart W, December 15, 1989 [FR 1989]), there were 11 conventional mills that 

were operating or on standby status. Of these mills, seven had unlined impoundments, while 

four had impoundments with synthetic liners. As the NESHAP revoked the exemption to the 

liner requirement of 40 CFR 192.32(a), the mills with unlined impoundments had to close them 

and move towards final reclamation and stabilization. Given the economics of uranium recovery 

throughout the ensuing years, all seven of the mills with unlined impoundments and one of the 

mills with a lined impoundment decided to dismantle the mills and move to final 

decommissioning and license termination. As a result, as of June 2008, the conventional 

uranium milling industry in the United States is comprised of only three facilities: 

• The Sweetwater mill, with a capacity of 2,700 tonnes/day, owned by Kennecott Uranium 

Company 

• The White Mesa mill, with a capacity of 1,100 tonnes/day, owned by UMETCO 

• The Canon City mill, with a capacity of 1,800 tonnes/day, owned by the Cotter 

Corporation 

The Sweetwater mill is located approximately 40 mills northwest of Rawlings, Wyoming; the 

White Mesa mill is near Blanding, Utah; and the Canon City mill is near Canon City, Colorado. 

All three mills employ acid-leaching to extract uranium from the ground ore and dispose of the 

tailings in synthetically lined, partially below grade impoundments with earthen dams. Of the 

three, only the Canon City mill is believed to be currently processing ore; Sweetwater and White 

Mesa are being maintained in standby mode. 

Only limited new data have been developed on the current status of the impoundments. Based 

on the data developed during the 1989 NESHAPs rulemaking (EPA 1989), the impoundments are 

as shown in Table 1. 

Table 1. Tailings Impoundments at Conventional Uranium Mills 

Mill Total Acres Ponded Wet Dry Radium Content 

(pCi/g) 

Sweetwater 37 30 0 2 280 

White Mesa 130 55 70 5 961 

Canon City 130 128 2 0 400 

The impoundments at the Sweetwater and Canon City mills are both single-cell impoundments. 

The White Mesa mill has a four-cell impoundment. Based on information reported in a 1st

Quarter 2005 Inspection Report (UDRC 2005), one cell holds processing liquids, two cells hold 

tailings, and the fourth cell, previously used to hold liquid wastes, has a torn liner. The liner and 

waste materials in cell four are being removed to cell two for disposal. The first of the two cells 

that holds tailings is filled and covered with random fill to control radon. The second of the two 

cells has an earthen cover over about 40% of the surface, with tailings and ponded areas covering 

the rest.
3.0 ANTICIPATED CHANGES IN THE INDUSTRY PROFILE 

The rapid increase in energy costs, increased concerns about global warming, and the 

tremendous world-wide surge in energy use have all led to considerable new interest in uranium 

recovery. At the spring 2008 joint National Mining Association/Nuclear Regulatory Commission 

(NMA/NRC) Uranium Recovery Workshop, the NRC identified 29 projects that have or are 

anticipated to file applications for new licenses, expansions of existing operations, or restarts of 

existing operations by the end of fiscal year 2011 (NMA/NRC 2008a). The vast majority of the 

new projects involve in-situ leach operations (discussed below), one is a new heap leach facility, 

and six are new conventional uranium mills. Of the six anticipated new mills, one is in Arizona, 

two are in Wyoming, and three are in New Mexico. Table 2 identifies each of these facilities. 

Table 2. Anticipated New Conventional Uranium Milling Facilities 

Owner Site State Expected Filing Date 

Strathmore Minerals Corp. Rocha Honda NM April 2009 

Uranerz Energy Corp Mt. Taylor NM August 2009 

Concentric Yavapai County AZ November 2008 

Neutron Energy Marquez NM April 2010 

Strathmore Minerals Corp. Gas Hills WY February 2011 

Wildhorse Energy Sweetwater WY May 2011 

It should be noted that the identified facilities are only those that the NRC is aware of. It is 

possible that additional facilities will be proposed in the Agreement States of Colorado, Utah, 

and/or Texas. 

None of the conventional mills have filed anything more than a letter of intent at this time, so 

there is no information available on the types of tailings impoundments that they propose to 

utilize. In order to limit the potential radon that could be emitted from the tailings 

impoundments, Subpart W requires that the tailings be disposed of in a phased disposal system 

with disposal cells no larger than 40 acres, or by continuous disposal in which not more than 10 

acres of undisposed tailings may accumulate at any time. Regardless of whether the new mills 

opt for phased- or continuous-disposal, they will all have to also demonstrate that their proposed 

tailings impoundment systems meet the design criteria given in Appendix A of 10 CFR 40. 

Criterion 3 of Appendix A identifies below-grade disposal, either in mines or excavated cells, as 

the “prime” candidate for tailings management. However, other methods may be approved if the 

applicant demonstrates that below-grade disposal is not feasible and that the proposed method 

will provide equivalent protection of persons and the environment. Additionally, every applicant 

will have to demonstrate that the proposed facility's operations will meet the NRC's ALARA (as 

low as reasonably achievable) requirement. The ALARA requirement almost certainly dooms 

any application that seeks to construct an above-grade impoundment with a ring dyke 

constructed with tailings sand. It also means that some method will have to be used at all 

facilities to minimize the potential for wind to disperse exposed tailings. The usual methods to 

minimize tailings dusting include using sprinklers to keep the tailings saturated, applying a 

sealant, or simply covering them with a clay or earthen cap.

WA 4-11, Task 5 – Impoundment Technologies 3 SC&A – September 25, 2008
4.0 COMPARISON OF URANIUM TAILINGS DISPOSAL TECHNOLOGY WITH 

THE REQUIREMENTS OF RCRA, SUBTITLE C 

The comparison of tailings facilities at current or future uranium mills and the final reclaimed 

configuration with RCRA subtitle C surface impoundments and landfills is not directly 

comparable; however, there are many salient similarities in construction methods. There are 

significant similarities in the construction, even though the underlying “basis of design” of each 

is different. 

The basis of the design of the reclaimed tailings area is to prevent the release of radioactive 

material into the atmosphere, primarily radon gas, and secondarily radioactive particulates. The 

prevention of radioactive material from leaching into the groundwater is also important via the 

Safe Drinking Water Act. Furthermore, all current and future uranium tailings impoundments are 

expected to be in arid to semi-arid areas in the western U.S., particularly in the states of Arizona, 

Utah, Colorado, New Mexico, Texas, Wyoming, Nebraska, and possibly Washington (the high 

desert area), with one possible exception in Virginia. Thus, on completion of the operating 

period, the reclamation of the tailings favors keeping water out of the tailings, i.e., total yearly 

evaporation is greater than total precipitation. This is in contrast to an RCRA subtitle C landfill 

cover with multiple layers of protection specifically designed to keep moisture away from the 

waste. 

The cover or final cap on a tailings pile is designed to limit the radon flux from the surface to 

less than 20 pCi/m2

-sec. While this is called a “radon barrier,” the nomenclature does not reflect 

the physics of the situation. Rather than a barrier, the cap is of sufficient thickness and density 

(actually tortuosity) that radon atoms diffusing from the surface of the tails (higher 

concentration) to the surface of the cap (lower concentration) must travel a long enough path that 

most will decay before reaching the surface. This is molecular diffusion through the soil. 

Obviously, soils such as clay or shale have much lesser porosity then sand or gravel and, 

therefore, require the radon atom to use more time to diffuse through the material to the surface. 

As the half-life of radon is 2.8 days, a few feet of clay or shale soil is sufficient to reduce the 

radon flux. 

As briefly noted above, the typical RCRA subtitle C surface impoundment and landfill bottom 

liner systems have as a design basis the prevention of moisture from mixing with the waste and 

leaching hazardous material in the groundwater. The landfills are sited throughout the U.S. in 

areas where precipitation is likely to be greater than evaporation. Thus additional protection, 

such as double confinement with a leachate collection system in between, is the standard. The 

confinement layer is also a composite composed of clay and a High Density Polyethylene 

(HDPE) liner. This is the typical base layer of the landfill. The caps can be equally complex 

with topsoil often used as a drainage layer (to remove precipitation), and immediately below that 

is a liner which has been placed over compacted fill. Their primary function is not to delay the 

diffusion of gas coming from the waste, but to prevent moisture from getting in and contacting 

the waste. 

The explicit similarities are more inherent to the technologies being applied. The use of 

synthetic liners, clay caps, and erosion barriers are common to both and are used to achieve the 

WA 4-11, Task 5 – Impoundment Technologies 4 SC&A – September 25, 2008
WA 4-11, Task 5 – Impoundment Technologies 5 SC&A – September 25, 2008

regulatory goal of isolation of wastes contained therein from release to the environs (air and 

groundwater) over extended timeframes. 
5.0 DISCUSSION OF CLIMATE AND IMPOUNDMENT SIZE ON RN-222 

EMISSIONS 

A number of factors influence the emission of Rn-222 from tailings impoundments, including the 

climate and the size of the impoundment. For a given concentration of radium in the tailings and 

a given grain size, the moisture content of the tailings will control the radon emanation rate; the 

higher the moisture content the lower the emanation rate. In the arid and semi-arid areas of the 

country where these impoundments are located or proposed, the annual evaporation rate is quite 

high. As a result, the exposed tailings (absent controls like sprinkling) dry rapidly. In its 

previous assessments, the EPA has explicitly taken the fact of rapid drying into account by using 

a Rn-222 flux rate of 1 pCi/m2 -s/pCi/g Ra-226 to estimate the Rn-222 source term from the dry 

areas of the impoundments (EPA 1984, EPA 1989). (Note: The estimated source terms from the 

ponded and saturated areas of the impoundments are zero, reflecting the complete attenuation of 

the Rn-222.) 

Climate also effects the source term from the impoundments once they are filled. The 

emplacement of the thick earthen covers that are necessary to assure the long-term stability of 

the reclaimed impoundments cannot be initiated until the tailings have dried sufficiently to allow 

heavy earth-moving equipment to be operated on the surface. For impoundments in arid or semiarid regions, the drying period will be shorter than in areas where annual evaporation does not 

exceed annual precipitation. In its 1989 evaluation of mills for the Subpart W rulemaking 

(EPA 1989), the EPA assumed that a 5-year drying period would be required. In practice, a 

number of facilities have been able to start cover operations earlier than that. For example, the 

White Mesa mill has a partial earthen cover on the disposal cell that is still open to accept 

additional tailings. 

The size of the impoundment has a direct linear correlation with the Rn-222 source term. Again, 

assuming the same Ra-226 concentration and grain sizes in the tailings, a 100-acre dry 

impoundment will emit 10 times the radon of a 10-acre dry impoundment. This linear 

relationship between size and Rn-222 source term is one of the main reasons that the Subpart W 

NESHAP imposed size restrictions on all future impoundments (40 acres per cell if phased 

disposal is chosen and 10 acres undisposed if continuous disposal is chosen). 

Using the sizes of the three existing conventional impoundments, and assuming identical grain 

sizes and a Ra-226 concentration of 400 pCi/g, the effect of impoundment size on Rn-222 

emissions can be illustrated for the operational period, the drying period, and the reclamation 

period as shown in Table 3. 

Table 3. Comparative Rn-222 Emissions over 70 Years 

(Assuming 400 pCi/g Ra-226) 

Operating Drying Reclaimed Total 

Mill Total 

Acres Ponded Wet Dry 15 yrs 

Ci 

5 yrs 

Ci 

50 yrs 

Ci 

70 yrs 

Ci 

Sweetwater 37 30 0 7 5.4E+3 9.4E+3 4.7E+3 2.0E+4 

White Mesa 130 55 70 5 3.8E+3 3.2E+4 1.7E+4 5.0E+4 

Canon City 130 128 2 0 0 3.2E+4 1.7E+4 5.0E+4

WA 4-11, Task 5 – Impoundment Technologies 6 SC&A – September 25, 2008
6.0 IN-SITU LEACH URANIUM RECOVERY FACILITIES 

6.1 Scope and Background 

Currently, in-situ leach (ISL) [also called in-situ recovery (ISR)] has become the predominant 

method for uranium recovery in the United States. Table 4 shows the ISL facilities that are 

currently in operation under NRC or State of Texas control. As was made clear at the 2008 

National Mining Association/Nuclear Regulatory Commission’s 2008 Workshop on Uranium 

Recovery, the number of ISL facilities is expected to increase substantially in the coming years. 

The National Mining Association estimated some 26 license/license amendment applications for 

uranium recovery facilities will be submitted by the end of 2009 (NMA/NRC 2008a). This may 

be an underestimate, as the NRC at the same conference indicated it anticipated some 29 

applications for facility restarts, expansions, or new licenses for uranium recovery by the end of 

fiscal year 2011 (NMA/NRC 2008b). Of these applications, 23 are for ISL facilities (14 new, 

7 expansions, 1 combined, and 1 restart). Table 5 shows the company, site, type of project, and 

anticipated application data for these projects. As with conventional mills, this list includes only 

facilities established or to be developed in non-Agreement States, e.g., Wyoming, and those 

agreement states that do not regulate uranium recovery, e.g., Nebraska. The list does not include 

ISL or conventional facilities in major uranium-producing Agreement States, such as Colorado, 

Utah, and Texas. There are at least three proposed facilities in Texas. 

Table 4. Operating ISL Facilities 

Company Site State 

Cameco Smith Ranch – Highland WY 

Cameco Crow Butte NE 

Hydro Resources Crown Point NM 

Hydro Resources Church Rock NM 

Mestena Alta Mesa 1,2,3 TX 

Uranium Resources Kingsville Dome 1,3 TX 

Uranium Resources Vaquez 1,2 TX 

Table 5. Anticipated ISL Facilities 

Owner Site Scope State Anticipated 

Application Date 

Cogema Christensen Ranch Restart WY Rec'd April 2007 

Cameco North Trend Expansion NE Rec'd June 2007 

Cameco Plant Upgrade Expansion NE Rec'd October 2006 

Lost Creek ISR Lost Creek New ISL WY Rec'd October 2007 

Uranez Energy Hank & Nichols New ISL WY Rec'd December 2007 

Uranium One Moore Ranch New ISL WY Rec'd October 2007 

Uranium One Jab & Antelope New ISL WY July 2008 

Cameco Gas Hills Expansion WY July 2008 

Kennecott Uranium Sweetwater Expand Resin WY January 2009 

Cameco Three Crow Expand ISL NE February 2008 

WA 4-11, Task 5 – Impoundment Technologies 7 SC&A – September 25, 2008
Table 5. Anticipated ISL Facilities 

Owner Site Scope State Anticipated 

Application Date 

Lost Creek ISR Lost Creek Expand ISL WY October 2008 

PowerTech Uranium Dewey Burdock New ISL SD October 2008 

Cameco Smith Ranch Expand CPP WY May 2009 

Cameco North Butte Expand ISL WY May 2009 

Strathmore Minerals Sky New ISL WY April 2009 

UR-Energy Lost Soldier NEW ISL WY January 2009 

Uranez Energy Collins Draw New ISL WY July 2009 

Uranium One Ludeman New ISL WY October 2008 

Uranium One Allemand-Ross New ISL WY August 2009 

Wildhorse Energy West Alkali Creek New ISL WY April 2009 

Cameco Ruby Ranch New ISL WY October 2009 

Strathmore Minerals Reno Creek New ISL WY December 2009 

Wildhorse Energy Sweetwater New ISL WY May 2011 

In-situ leach uranium mining is defined as the leaching or recovery of uranium from the host 

rock (typically sandstone) by chemicals, followed by recovery of uranium at the surface (IAEA 

2005). Leaching, or more correctly the re-mobilization of uranium into solution, is accomplished 

through the injection into the ore body of a lixiviant. The injection of a lixiviant essentially 

reverses the geochemical reactions that deposited the uranium in the first place, and assures that 

the dissolved uranium remains in solution until it is pumped out of the ground and the uranium 

recovered. Two types of lixiviant systems can be used, loosely defined as acid or alkaline 

systems. In the U.S., the geology and geochemistry of the majority of the uranium ore bodies 

favor the use of an “alkaline” lixiviant or bicarbonate-carbonate lixiviant and oxygen. Other 

factors in the choice of the lixiviant are the uranium recovery efficiencies, low operating costs, 

and the ability to achieve satisfactory groundwater restoration. The acid systems once used in 

the United States are still used in Eastern Europe and Asia and recently in Australia on ore bodies 

in saline aquifers (IAEA 2005). 

There are four major types of uranium deposits in the United States; strata bound, solution 

breccia pipes, vein, and phosphatic deposits (EPA 1995). Of these, in-situ leach is the uranium 

recovery technique used mostly on strata-bound ore deposits. Strata-bound ore deposits are ore 

deposits that are contained within a single layer of sedimentary rock. They account for more 

than 90% of the recoverable uranium and vanadium in the United States and are found in three 

major geographic areas—the Wyoming Basin (Wyoming and Nebraska), Colorado Plateau or 

Four Corners area (northwestern New Mexico, western Colorado, eastern Utah, and north eastern 

Arizona), and south Texas. A discussion of the origin of the uranium ore, including ore body 

formation and geochemistry, may be found in the reference, Technical Resource Document 

Extraction and Beneficiation of Ores and Minerals - Volume 5 Uranium (EPA 1995). Much of 

the recoverable uranium in these regions lends itself to in-situ leach or recovery, based on the 

physical and geochemical properties of their ore bodies. 

WA 4-11, Task 5 – Impoundment Technologies 8 SC&A – September 25, 2008
Other important physical or geologic properties also include the location of the ore body. Ore 

bodies amenable to ISL are usually within an aquifer (EPA 1995). The water quality within the 

aquifer can also vary, depending on the presence of and boundary between oxidizing and 

reducing groundwaters (EPA 1995). Additional factors important in the selection of the 

production method (lixiviant and well pattern) include chemical constituents, ore grade, 

permeability, and surrounding material. Ideally, the ore body should be confined by 

impermeable strata above and below the deposit. This assures better hydrogeologic control of 

the lixiviant and facilitates restoration of the groundwater following completion of the mining 

(IAEA 2005). These hydrogeologic controls include preventing contamination of adjacent 

aquifers by excursions (leaks of lixiviant and other material from the ore body area). 

6.2 Uranium Recovery Process for ISL 

The following discussions are taken in part from EPA 530-R-94-032 (EPA 1995) and 

NUREG/CR-6733 (NRC 2001). The operational steps for the recovery of uranium are 

straightforward and consist of the three primary operations of uranium mobilization, uranium 

processing, and aquifer restoration. 

6.2.1 Uranium Mobilization 

First, barren extraction solution (lixiviant) usually composed of groundwater enhanced by an 

oxidant and carbonate/bicarbonate is injected through wells into the ore zone. The lixiviant 

oxidizes and dissolves uranium. Carbon dioxide in the lixiviant reacts with water, forming 

carbonic acid, which complexes with the solubilized uraniferous ions forming uranyl carbonates 

keeping the uranium in solution. The resulting solution is referred to as “pregnant lixiviant.” 

This process essentially reverses the geochemical reactions that initially caused deposition of the 

ore body particularly in “roll front” deposits. The solubilized uranyl carbonates and gangue 

minerals remain in solution as the pregnant lixiviant is pumped to the surface through production 

(recovery) wells. The pregnant lixiviant includes both radium and dissolved radon. 

6.2.2 Wells 

Three types of wells are used in the uranium recovery process; injection wells, recovery wells, 

and monitoring wells. The pattern of the injection and recovery wells is usually the 5-spot 

pattern or 7-spot pattern. In a 5-spot pattern, the injection well is in the center of a square with 

four recovery wells at the corners. The 7-spot pattern uses a hexagon for the recovery wells. 

The spacing between production and recovery wells can range from 20 to 200 feet. Pumping 

rates are also variable, and at one facility ranged from 2 gallons per minute (gpm) to 30 gpm for 

injection wells and 5 to 40 gpm for recovery wells. 

6.2.3 Uranium Processing 

The pregnant lixiviant is pumped from the production wellheads through sand filters to remove 

any large particulates and then pumped to the processing operations. Processing generally 

consists of three operations; ion exchange (IX), elution, and precipitation and drying. These are 

no different than that found in conventional mills, and indeed some ISL operations are co-located 

WA 4-11, Task 5 – Impoundment Technologies 9 SC&A – September 25, 2008
with conventional mills or truck the pregnant lixiviant (liquor) to a conventional processing 

facility. 

The pregnant lixiviant is either stored in a surge tank or sent directly to the IX resins. In the IX 

exchange resins, uranium is adsorbed onto the resin beads. The barren lixiviant is recharged and 

sent back to the production well fields. When the IX resins become saturated with uranium, they 

are taken offline and washed with a concentrated brine solution (the elution step). The uraniumrich solution, typically containing 8 to 20 grams/liter (g/l) uranium, is normally discharged to a 

holding tank. After a sufficient amount of pregnant eluant is obtained, it is moved to the 

precipitation and drying circuit. 

In the precipitation and drying circuit, acid is added to the pregnant eluant to destroy the uranyl 

carbonated complex and is precipitated out with the addition of hydrogen peroxide. A base is 

added to neutralize any excess acid. The resultant slurry is sent to a thickener, where it is settled, 

filtered, and dewatered. The thickened slurry may be transported to a uranium processing plant 

to produce yellow cake (U3O8), or it may be dried and packaged (NRC 2001). 

In terms of aquifer restoration, the objective is to provide extended flow through the mine field 

by injection, recovery, and bleed circuits, until it has been demonstrated that uranium is no 

longer being oxidized and released in the ore zone, and that confinement has stabilized within the 

aquifer. 

6.3 Radon Source Terms 

The above presents a capsule summary of the operations at an ISL facility. In this section, the 

potential radon source terms from these operations are identified and estimated. 

6.3.1 Source Terms 

The focus of this investigation is radon gas (radon-222, Rn-222), one of the principal 

contaminants in the uranium ore body and pregnant lixiviant. Radon in groundwater is well 

known from drinking water and home radon studies throughout the United States. These studies 

have found a range of radon concentrations from background to 105 to 106 pCi/l. While high, it 

is noted that the theoretical solubility limit for radon in liquid phase at atmospheric pressure is 

estimated to be about 3 × 1016 pCi/L [8.59 × 10-3 mol/L (NRC 2001)], considerably higher than 

what has been measured. Thus, concentrations of radon in water greater than that presented in 

the literature are possible. 

To estimate the quantity of radon released during the operation of an ISL, the methodology 

presented in Appendix D of NUREG-1569 was used (NRC 2003). The authors of this document 

estimated not only radon, but also releases of radium-226 (Ra-226) and some of its daughter 

products, and uranium-238 (U-238). They considered effluent from the following: 

(1) The drilling operation at new well fields 

(2) Uranium extraction operations at production well fields 

(3) Drying and packaging of yellowcake 

WA 4-11, Task 5 – Impoundment Technologies 10 SC&A – September 25, 2008
(4) Restoration operations of old well fields 

(5) Land application areas (broadcast of holding pond water) 

(6) Decommissioning of well fields and evaporation ponds 

Of all these, the third is not relevant for radon. To provide some quantitative framework for a 

discussion of the radon released from ISL, the following was taken from Appendix D and 

modified to focus only on the radon issue. The parameters were also taken from the same 

reference. 

New Well Fields 

New well fields are commonly investigated and established using conventional rotary drill rigs. 

Investigation drill holes assist in mapping the ore body and determining the establishment of the 

injection and production wells. Because all exploration drill holes are sealed with bentonite to 

maintain aquifer isolations, no radiological particulates are expected to be released from this 

operation. The only source of radioactivity is the radon (Rn-222) from the radium in the drill 

cuttings, which are temporarily stored in a mud pit. During storage, Rn-222 is generated from 

the decay of Ra-226 and released to the atmosphere. The amount of Rn-222 available for release 

on a yearly basis as a result of Ra-226 decay from stored cuttings can be estimated from the 

classic radon release equation: 

 Rnnw = 10-12 E L [Ra] T M N (equation 1) 

 where: 

Rnnw = Rn-222 release rate from the new well field (Ci/yr) 

10-12

= unit conversion factor (Ci/pCi) 

E = emanating power or coefficient (dimensionless) 

L = Rn-222 decay constant (0.181/d) (ln2 ÷ half-life of Rn-222) 

[Ra] = Ra-226 concentration in the ore zone (pCi/g) 

T = storage time in mud pit (days) 

M = average mass of material in mud pit 

N = number of mud pits generated per year 

Production Well Fields 

No particulates are released from the production well field, because its process streams from 

production and injection wells to IX columns are all in a closed-loop circuit. The primary 

radioactive emission from the process streams is Rn-222. During the process, radon released 

from the ore body is readily removed by the lixiviant (process water) moving through the well 

field by injection and production wells. The 3.8 day half-life of Rn-222 allows it to circulate 

along with the process water in the well field over a long period of time before it decays (NRC 

2001). Not all the Rn-222 is dissolved in the process water. The portion or fraction that remains 

is trapped in the ore zone. Also, a portion of the water in the ore zone is periodically purged 

(called the bleed) to maintain a cone of depression around the well field to prevent leakage of the 

mining solution outside the production zone (minimize excursions). 

WA 4-11, Task 5 – Impoundment Technologies 11 SC&A – September 25, 2008
The radon source term in the production ore can be estimated from the following: 

 

 S=106

 L E[Ra] A D P (equation 2) 

 where: 

106

= unit conversion (cm3/m3) 

E = Active ore zone emanating power (fraction of radon released) 

[Ra] = Ra-226 concentration in the ore zone (pCi/g) 

A = active area of the ore zone m3

D = average thickness of the ore Zone (m) 

P = bulk density of the ore material (g/cm3

) 

In the IX circuit, the water discharge from resin unloading, Fi can be calculated from the 

following: 

 Fi =Ni Vi Pi 

 where: 

Vi = Volume content of the IX column (L) 

Ni = number of IX column unloading per day 

Pi = porosity of the resin material 

Under steady state conditions, the Rn-222 concentration in the process water CRn can be 

calculated from the following: 

 CRn (equation 3) 

 where: 

f = fraction of radon source carried by circulation water (dimensionless) 

v = rate of radon venting from piping and values during circulation (1/d) 

V = volume of water in circulation (L) 

Fp = “purge” rate of treated water (L/d) 

Fi = water discharge rate from resin unloading of IX columns (L/d) 

When pressure is reduced during purging or when water is aerated during irrigation, radon is 

released to the atmosphere. The amount of Rn-222 available for release from the “purge” is 

dependent on the water volume purge rate, Fp, and on the Rn-222 concentration in the purged 

water CRn. Assuming conservatively that all available Rn-222 in the purge water is released, the 

annual Rn-222 emission is as follows: 

 Rnw =3.65 × 10-10 CRn Fp (equation 4) 

 where: 

3.65 × 10-10

= unit conversion factor (Ci/pCi) (d/yr) 

Rnw = Rn-222 release rate from purge water (Ci/yr) 

 

WA 4-11, Task 5 – Impoundment Technologies 12 SC&A – September 25, 2008
The annual Rn-222 release from occasional venting from wellheads and leaking transport piping 

can be calculated from the following: 

 Rnv = 3.65 × 10-10 v CRn V (equation 5) 

 where Rnv is the annual Rn-222 Release from Venting (Ci/yr). 

The annual Rn-222 discharge from the unloading of the IX column content is as follows: 

 RnX = 3.65 × 10-10 CRn Fi (equation 6) 

 where RnX is the annual Rn-222 release from unloading of IX column content (Ci/yr). 

 

The total annual Rn-222 release from the production well field is the sum of the radon releases, 

Rnw+Rnv+RnX. 

Restoration Well Field 

The basic operation of the restoration well field is the same as that of the production well field. 

Groundwater in the ore body is restored to its pre-mining level by flushing with fresh or treated 

water injections (groundwater sweep). Again, the primary contaminant is the release of Rn-222 

in the process water circulating within and discharged from the restoration operations. The 

annual Rn-222 releases from the restoration well field can therefore be calculated from equations 

2 and 3. 

6.3.2 Example In-Situ Recovery Radon Source Calculation 

The following presents an estimation of the radon source term at a “typical” ISL operation. For 

brevity, only those facilities expected to release radon are considered and include the following: 

• One well field under development 

• Two production well fields 

• One restoration well field 

• Main processing facility 

• A holding pond 

• Two radium-settling ponds 

A satellite facility and irrigation plot is not considered. It is noted that the irrigation plot is also a 

source of radon; however, the concentration of Ra-226 in the water is limited. The example 

provided in Appendix D (NRC 2003) results in a surface soil concentration of 1.3 pCi/g above 

background or total Ra-226 concentration of 2.3 pCi/g (background concentration of 1.0 pCi/g of 

Ra-226), which would give rise to radon flux of about 2.3 pCi/m2-sec. 

WA 4-11, Task 5 – Impoundment Technologies 13 SC&A – September 25, 2008
Generic parameters applicable to the entire facility: 

• Yellow cake production rate = 520 metric tons per years (MT/yr) 

• Average ore activity of U-238 sand each progeny in secular equilibrium = 280 pCi/g 

• Ore porosity = 0.28 

• Ore density = 1.8 g/cm3

Considering the different wells fields separately leads to the following: 

One Well Field Under Development

• The parameters selected for this well field are: 

• Number of new wells peak year = 600 

• Each well “covers” a 10 m × 10 m area = 100 m2

• Number of new wells per mud pit =12 

• Number of mud pits =50 

• 1.6 × 104

 L/d 

• Drill hole diameter = 8 in 

• Average ore material per well = 2.9 × 105 g 

• Total ore material in mud pit per year = 3.5 × 106

 g 

• Average storage time of ore grade waste in mud pits = 12 d 

• Radon emanating power = 0.25 

Using equation 1 (Rnnw = 10-12 E L [Ra] T M N) and substituting leads to the following: 

Rnnw = 0.027 Ci/yr 

The annual radon flux is estimated by dividing the total emission rate by the area under 

development: 

60,000 m2

 = 0.0143 pCi/m2-sec 

Two Production Well Fields

 The parameters selected for the hypothetical production well fields are as follows: 

Operating Parameter Well Field 1 Well Field 2 

 Operating days per year 365 d 365 d 

Active ore body dimensions: 

 Peak area per year to be mined 50,000m2 55,000m2

 Ave thickness of ore body 3 m 5 m 

 Total flow volume in circulation 4.2 × 107

 L 7.7 × 107 L 

Facility (IX) parameters: 

 Dimension/capacity of resin column 3,500 gal 3,500 gal 

 Resin porosity 0.4 0.4 

 Number loaded resin unloading per day 3 3 

WA 4-11, Task 5 – Impoundment Technologies 14 SC&A – September 25, 2008
Operating Parameter Well Field 1 Well Field 2 

Water discharge rate from unloading: 

 IX column 1.6 × 104 L/d 1.6 × 104 L/d 

 Total wastewater purge rate 5.5 × 105

 L/d (100gpm) 5.5 × 105 L/d (100gpm)

 Fraction radon in circulating water 0.8 0.8 

 Rate of radon venting during circulating 0.01/d 0.01/d 

 

The radon concentration in the circulating water is calculated using equation 3: 

CRn 

Substituting both sets of parameters leads to the following: 

For well field 1, CRn = 3.2 × 105

 pCi/L; for well field 2, CRn = 3.3 × 105 pCi/L 

Substituting these values in equations 4, 5 and 6 leads to the following radon emission 

estimates 

Radon Emission Contributions Well Field 1 Well Field 2 

Rnw –Release rate from “purge” (settling pond) 64 Ci/yr 66 Ci/yr 

Rnv-Release rate from gas venting and leakage 

during circulation 49 Ci/yr 93 Ci/yr 

Rnx-Release from IX unloading 1.9 Ci/yr 1.9 Ci/yr 

Total release rate 115 Ci/yr 161 Ci/yr 

The parameters selected for the hypothetical restoration well field are as follows: 

Restoration Well Field 

Expected restoration operation time = 7years 

Operating days per year = 240 d 

Area per year to be restored = 100,000 m2

Average thickness of ore body = 5 m 

Total flow volume in circulation = 100000 × 5 × 0.28 = 1.4 × 108

 L 

Total treated water purge rates = 1.1 × 106

 L/d 

Fraction of radon source carried by circulating water = 0.8 

Rate of radon venting during circulation = 0.01/d 

The radon concentration in the circulating water is as follows: 

CRn 

 =3.3 × 105 pCi/L 

WA 4-11, Task 5 – Impoundment Technologies 15 SC&A – September 25, 2008
The radon released rate from purge water into the settling pond is as follows: 

Rnw =3.65 × 10-10 CRn Fp = 87 Ci/yr 

The radon release rate form gas venting and leaking during circulation is as follows: 

Rnv = 3.65 × 10-10 v CRn V = 197 Ci/yr 

The total radon released from the restoration field is 197 Ci/yr. 

The radon flux from the irrigation plot is straightforward and was estimated as 

1.3 pCi/m2

-sec, not much over background. 

6.3.3 Decommissioning of Well Fields and Evaporation Ponds 

Evaporation ponds are typically lined temporary ponds with areas of 3 to 5 acres (10,000 to 

20,000 m2). They are designed with sufficient freeboard to contain the waste water from the 

process and any natural precipitation. As they are wet, they do not represent a source term for 

either radon or radioactive particulates. Radon sources are also a function of the thickness of the 

soils generating the gas and containing the parent radium. The quantity of sediment (thickness) 

that eventually is deposited on the bottom of the pond is much less than that in tailings pond, and 

the generated radon is less and is covered by water. However, in many uranium production 

areas, the water balance favors evaporation. Should the ponds begin to dry out, they are sprayed 

to keep the surface wet, eliminating particulates and minimizing radon. On closure of the pond, 

the material is typically excavated and disposed of as by-product 11(e02) materials (NRC 2003). 

Upon completion of groundwater restoration, the operator is required to decommission all well 

field piping and distribution systems, processing equipment, waste processing, and handling 

systems (including evaporation pond liners and resides) and to prepare it for appropriate 

disposal. Currently, under NRC guidelines, all residues and waste materials are treated as being 

uranium byproducts and disposed in licensed tailings impoundments. Radon emissions from 

well field piping and distribution networks and evaporation ponds have not been demonstrated to 

add significantly to the Rn-222 source term. 

6.4 Discussion 

As indicated in the prior calculations, the amount of radon released from the production well 

fields (115 Ci/yr and 161 Ci/yr) is greater than from the new production facility (0.027 Ci/yr) and 

comparable to that from the restoration well field (197 Ci/yr). The flux can be found by 

assuming the “foot print” of the area releasing radon is approximately equal to the area to be 

mined per year for the production well field and the area to be restored for the restoration well 

field. It is also assumed that the ponds in both cases have an area which is a few percent of the 

area to be mined or restored and can be ignored during the operational life of the facility. At the 

time of closure, waste from the evaporation ponds must be disposed of as if it were tailings 

waste. The NRC's preference is that it be disposed of at existing conventional tailings 

WA 4-11, Task 5 – Impoundment Technologies 16 SC&A – September 25, 2008
impoundments, but where that is not feasible it may be disposed of on-site. The fluxes are just 

the total radon release per year divided by the area. They are presented in Table 6. 

Table 6. Estimated Rn-222 Releases from an ISL Facility 

Process Being Considered Area (m2) Annual Radon Release 

(Ci/yr) 

Average Radon Flux 

pCi/m2

-sec 

New Well Field 60000 0.027 0.0143 

Production Well Field 1 50000 115 73 

Production Well Field 2 55000 161 93 

Restoration Well Field 100000 197 63 

While these radon fluxes are three to four times greater than that allowed in conventional tailing 

facilities (20 pCi/m2

-sec), they are conservative. For the production well fields, they do not 

include the area of the ponds (purge water source term) or the area of the processing facility (IX 

unloading source term). Also, in the case of the restoration well field, they are transient releases 

and revert to background on completion of the restoration. 

6.5 Next Step 

The prior discussion indicates some emission of radon is estimated to be released from in-situ 

leach operations primarily during the operations period and during restoration. These releases 

could have an impact on the surrounding population. The estimated radon release is based on a 

number of parameters, many of which are assumed to be “best estimates” in and of themselves. 

As a start, it is suggested that a critical review of the parameters be undertaken to determine 

which are most susceptible to large variations and of these, which can be documented by actual 

or proposed but designed operations. 

7.0 REFERENCES 

10 CFR 40, Title 10, Part 40: Domestic Licensing of Source Materials, Appendix A: Criteria 

Relating to the Operation of Uranium Mills and the Disposition of Tailings or Wastes Produced 

by the Extraction or Concentration of Source Material from Ores Processed Primarily for Their 

Source Material Content.

40 CFR 61, Title 40, Part 61: National Emission Standards for Hazardous Air Pollutants, 

Subpart W: National Emissions Standards for Radon Emissions from Operating Mill Tailings. 

40 CFR 192, Title 40, Part 192: Health and Environmental Protections Standards for Uranium 

and Thorium Mill Tailings. 

EPA (Environmental Protection Agency) 1984. Final Background Information Document 

Proposed Standards for Radionuclides, EPA 520/1-84-022-1, Office of Radiation Programs, 

Environmental Protection Agency, Washington, DC. October 1984. 

EPA (Environmental Protection Agency) 1989. Risk Assessment Methodology Environmental 

Impact Statement NESHAPS for Radionuclides Background Information Document, Volumes 1–

WA 4-11, Task 5 – Impoundment Technologies 17 SC&A – September 25, 2008
WA 4-11, Task 5 – Impoundment Technologies 18 SC&A – September 25, 2008

3, EPA 520/1-89-005, Office of Radiation Programs, Environmental Protection Agency, 

Washington, DC. September 1989. 

EPA (U.S. Environmental Protection Agency) 1995. Office of Solid Waste, “Technical Resource 

Document, Extraction and Beneficiation of Ores and Minerals, Volume 5 URANIUM,” EPA 

530-R-94-032, January 1995. 

FR (Federal Register) 1989. Final rulemaking package for Subpart W. Volume 54, page 51679. 

December 15, 1989. 

IAEA (International Atomic Energy Agency) 2004. Long-Term Stabilization of Uranium Mill 

Tailings, Final Report of a Co-Ordinated Research Project, International Atomic Energy Agency. 

Vienna, Austria. August 2004. 

IAEA (International Atomic Energy Agency) 2005. “Guidebook on Environmental Impact 

Assessment for In-Situ Leach Mining Projects,” IAEA-TECDOC-1428, May 2005. 

NMA/NRC 2008a. “Uranium Recovery Licensing Activities,” Presentations at the 2008 

National Mining Association/Nuclear Regulatory Commission (NMA/NRC) Uranium Recovery 

Workshop. Camper, Larry W., Accession Number ML081430061. 

NMA/NRC 2008b. “Generic Environmental Report for In Situ Uranium Recovery Facilities,” 

Presentations at the 2008 National Mining Association/Nuclear Regulatory Commission 

(NMA/NRC) Uranium Recovery Workshop. Thompson, Anthony J. and Christopher S. Pugsley. 

Accession Number ML081430176. 

NRC (U.S. Nuclear Regulatory Commission) 2001. “A Baseline Risk-Informed, PerformanceBased Approach for In Situ Leach Uranium Extraction Licensees,” Nuclear Regulatory 

Commission, Office of Nuclear Material Safety and Safeguards, NUREG/CR-6733, September 

2001. 

NRC (U.S. Nuclear Regulatory Commission) 2003. “Standard Review Plan for In Situ Leach 

Uranium Extraction License Applications (NUREG-1569),” Appendix D, “MILDOS_AREA: 

An Update with Incorporation of In Situ Leach Uranium Recovery Technology,” May 1997. 

Nuclear Regulatory Commission, Office of Nuclear Material Safety and Safeguards, 2003. 

UDRC 2005. Utah Department of Radiation Control 1st Quarter 2005 Inspection Report.