` 

U.S. NUCLEAR REGULATORY COMMISSION November 2008

Revision 3 

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 3.11 

(Draft was issued as DG-3032, dated January 2008) 

DESIGN, CONSTRUCTION, AND INSPECTION OF 

EMBANKMENT RETENTION SYSTEMS AT 

URANIUM RECOVERY FACILITIES 

A. INTRODUCTION 

This guide describes some engineering practices and methods generally considered by the 

U.S. Nuclear Regulatory Commission (NRC) to be satisfactory for the design, construction, and 

inspection of embankment retention systems used for retaining liquid and solid wastes from uranium 

recovery operations. These practices and methods are the result of NRC review and action on a number 

of specific cases, and they reflect the latest general engineering approaches that are acceptable to the NRC 

staff. If future information results in alternative methods, the NRC staff will review such methods to 

determine their acceptability. Separate guidance (Refs. 1–3) addresses the closure of retention systems. 

Licensees who process or refine uranium ores in a milling operation are required by Title 10, 

Section 20.1101, “Radiation Protection Programs,” of the Code of Federal Regulations (10 CFR 20.1101) 

to use, to the extent practical, procedures and engineering controls based on sound radiation protection 

principles, to maintain occupational radiation exposure and radiation exposure to members of the public 

that are as low as is reasonably achievable (ALARA). In addition, Subpart B, “Environmental Standards 

for the Uranium Fuel Cycle,” of 40 CFR Part 190, “Environmental Radiation Protection Standards for 

Nuclear Power Operations,” requires that the annual dose equivalent not exceed 25x10-5 sieverts (Sv) 

(25 millirem (mrem)) to the whole body, 75x10-5 Sv (75 mrem) to the thyroid, and 25x10-5 Sv (25 mrem) 

to any other organ of any member of the public as the result of exposures to radiation (radon and its 

daughters excepted) from uranium fuel cycle operations, including planned discharges of radioactive 

The NRC issues regulatory guides to describe and make available to the public methods that the NRC staff considers acceptable

for use in implementing specific parts of the agency’s regulations, techniques that the staff uses in evaluating specific problems 

or postulated accidents, and data that the staff needs in reviewing applications for permits and licenses. Regulatory guides are 

not substitutes for regulations, and compliance with them is not required. Methods and solutions that differ from those set forth

in regulatory guides will be deemed acceptable if they provide a basis for the findings required for the issuance or continuance 

of a permit or license by the Commission. 

This guide was issued after consideration of comments received from the public. 

Regulatory guides are issued in 10 broad divisions—1, Power Reactors; 2, Research and Test Reactors; 3, Fuels and Materials 

Facilities; 4, Environmental and Siting; 5, Materials and Plant Protection; 6, Products; 7, Transportation; 8, Occupational 

Health; 9, Antitrust and Financial Review; and 10, General. 

Electronic copies of this guide and other recently issued guides are available through the NRC’s public Web site under the 

Regulatory Guides document collection of the NRC’s Electronic Reading Room at http://www.nrc.gov/reading-rm/doccollections/ and through the NRC’s Agencywide Documents Access and Management System (ADAMS) at 

http://www.nrc.gov/reading-rm/adams.html under ADAMS Accession No. ML082380144. 
Rev. 3 of RG 3.11, Page 2 

materials to the general environment. Liquid and solid wastes generated in uranium recovery operations 

typically contain radioactive materials in excess of the discharge limits and are generally confined by an 

embankment retention system. 

This regulatory guide contains information collection requirements covered by 10 CFR Part 20 

and 10 CFR Part 40 and that the Office of Management and Budget (OMB) approved under OMB control 

numbers 3150-0014 and 3150-0020, respectively. The NRC may neither conduct nor sponsor, and a 

person is not required to respond to, an information collection request or requirement unless the 

requesting document displays a currently valid OMB control number. 
Rev. 3 of RG 3.11, Page 3 

Table of Contents 

A. INTRODUCTION ..................................................................................................................................1

B. DISCUSSION .........................................................................................................................................4

Background......................................................................................................................................4

1. General Planning, Siting, and Design Considerations ........................................................4

1.1 Site Evaluation.......................................................................................................5

1.2 Field Exploration ...................................................................................................6

1.3 Laboratory Tests ....................................................................................................6

2. Design Analysis ..................................................................................................................6

2.1 Stability and Failure Analyses ...............................................................................7

2.2 Water Control and Management..........................................................................12

3. Construction Considerations.............................................................................................15

4. Inspection and Maintenance .............................................................................................17

4.1 Engineering Data Compilation ............................................................................18

4.2 Inspection Programs ............................................................................................18

4.3 Technical Evaluation ...........................................................................................20

4.4 Inspection Reporting............................................................................................20

4.5 Inspection Personnel............................................................................................20

C. REGULATORY POSITION.................................................................................................................21

1. Basic Design Criteria........................................................................................................21

2. Methods of Analysis .........................................................................................................21

3. Construction Methods.......................................................................................................22

4. Inspection and Maintenance .............................................................................................22

D. IMPLEMENTATION...........................................................................................................................23

REFERENCES ...........................................................................................................................................24
Rev. 3 of RG 3.11, Page 4 

B. DISCUSSION 

Background 

The milling of uranium ores results in the production of large volumes of liquid and solid wastes 

(tailings). These tailings are usually stored behind manmade retaining structures, following the practice 

of the nonuranium mining industry. In addition, other liquid wastes from operations and ground-water 

corrective action activities at uranium recovery facilities are often retained behind evaporation pond 

embankments. The design and construction of early tailings retention structures were based largely on 

mining experience, with little use of design concepts. These empirical approaches resulted in various 

mining dam mishaps and failures (Refs. 4 and 5). The 1972 failure of Buffalo Creek Dam in 

West Virginia resulted in the U.S. Congress passing a national dam safety law affecting all waterimpounding structures in excess of either 7.62 meters (25 feet) in height or 61,674 cubic meters (50 acrefeet) in impoundment capacity (Ref. 6). 

Wastes from uranium recovery operations, unlike most nonuranium mine wastes, contain 

concentrations of radioactive materials in excess of allowable discharge limits (Ref. 7). Furthermore, the 

most significant radioactive element in the wastes is radium-226, which has a half-life of about 1600 

years (Ref. 8). Therefore, it is necessary to confine such wastes to prevent or control their release to the 

environment, not only during the operation of the uranium recovery facility, but also for generations after 

operations have ceased. The embankments, foundation, and any abutments need to be stable under all 

conditions to prevent the uncontrolled release of the retained liquid or semifluid wastes. Seepage from 

the tailings cell or evaporation pond, which contains dissolved radium and other toxic substances (Ref. 8), 

needs to be controlled under normal and severe operating conditions to prevent the possibility of 

unacceptable contamination of the ground water or nearby streams. The impoundment and embankments 

need to be designed to prevent wind and water erosion during operation, and after facility closure in the 

case of reclaimed tailings impoundments. 

Factors pertaining to safety, contamination, and environmental damage determine the basic 

requirements in planning and constructing retention systems. To achieve the basic requirements, the 

design must be based on a thorough understanding of both the geotechnical and hydrological problems 

involved and the requirements of the uranium recovery operation. 

The latest advances in geotechnical engineering, together with engineering experience and 

knowledge in the field of water storage dams and retention structures, can be used in the design and 

construction of uranium recovery retention systems. The basic concepts of conventional water storage 

impoundments can be suitably modified to produce economical designs that will ensure the stability of 

the retention system and minimal contamination. 

1. General Planning, Siting, and Design Considerations 

Because the prime functions of the retention system are to store radioactive solids and/or to 

provide temporary storage of contaminated water for clarification and evaporation, the system must be 

designed and constructed to remain stable for its intended life. It must provide the required storage at any 

given time, and it must provide sufficient control of seepage to prevent unacceptable contamination of 

adjacent land, waterways, and ground waters. It must also be designed to be resistant to wind and water 

erosion during and after facility operations. 
Rev. 3 of RG 3.11, Page 5 

The planning, siting, and design of uranium recovery retention systems need to ensure that such 

systems meet any other regulatory and permitting requirements for a proposed impoundment that exist 

outside NRC’s regulations (10 CFR Part 40, Appendix A) and regulatory process. 

The siting and design should consider the requirements of the U.S. Environmental Protection 

Agency’s (EPA) national emission regulations at Subpart W, “National Emissions Standards for Radon 

Emissions from Operating Mill Tailings,” of 40 CFR Part 61, “National Emission Standards for 

Hazardous Air Pollutants.” Subpart W requires that “... no new tailings impoundment can be built unless 

it is designed, constructed, and operated to meet one of the two following work practices: (1) Phased 

disposal in lined tailings impoundments that are no more than 40 acres in area and meet the requirements 

of 40 CFR 192.32(a) as determined by the Nuclear Regulatory Commission. The owner or operator shall 

have no more than two impoundments, including existing impoundments, in operation at any one time. 

(2) Continuous disposal of tailings such that tailings are dewatered and immediately disposed with no 

more than 10 acres uncovered at any time and operated in accordance with 40 CFR 192.32(a) as 

determined by the Nuclear Regulatory Commission.” Furthermore, the design should consider the 

requirements of 40 CFR 264.221 addressing surface impoundment design and operation. An applicant or 

licensee should consider these EPA regulations during the preliminary design and planning stages of the 

tailings retention system. 

1.1 Site Evaluation

The goal in the siting and design of uranium recovery retention systems is to achieve permanent 

isolation of tailings and associated contaminants by minimizing disturbance and dispersion by natural 

forces and to do so without ongoing maintenance. In the selection of alternative tailings disposal sites or 

in the evaluation of the adequacy of existing tailings sites, all site features that will contribute to this goal 

should be considered, including (1) remoteness from populated areas, (2) hydrologic and other natural 

conditions as they contribute to continued immobilization and isolation of contaminants from groundwater sources, and (3) potential for minimizing erosion, disturbance, and dispersion by natural forces over 

the long term. In the selection of disposal sites, primary emphasis must be given to the isolation of 

tailings or wastes, a matter having long-term impacts, as opposed to the consideration of only short-term 

convenience or benefits, such as minimization of transportation or land acquisition costs. While isolation 

of tailings will be a function of both site conditions and engineering design, siting features are the primary 

consideration for ensuring permanent isolation of wastes given the long-term nature of the tailings 

hazards. 

To the extent possible, sites should be selected that are at or near the top of local drainage divides 

and/or are not located in floodplains or flood-prone areas. Such site selection will avoid the need for 

diversion channels or extensive riprap protection to prevent erosion of the toes and slopes of the 

embankments. 

The “prime option” for the disposal of tailings is placement below grade. Where full below-grade 

burial is not feasible, the size of retention structures, as well as the size and steepness of slopes associated 

with exposed embankments, must be minimized by excavation to the maximum extent reasonably 

achievable given the geologic and hydrologic conditions at a site. In these cases, it must be demonstrated 

that an above-grade disposal program will provide equivalent isolation of the tailings from natural erosion 

forces. 

In selecting any site for uranium recovery waste retention, detailed local conditions, including 

climate, ground-water and surface-water hydrology, geology, and seismology, need to be assessed and 

their impacts evaluated. 
Rev. 3 of RG 3.11, Page 6 

1.2 Field Exploration

Subsurface investigations at the site of the retention system and at possible borrow areas need to 

be of adequate scope to determine the suitability of the foundation and the availability and characteristics 

of embankment materials. Borings should be drilled along the axis of the retention structure and at 

critical locations perpendicular to the axis to establish geologic sections and ground-water conditions. 

Generally, borings should extend to a depth in the natural soils at least equal to the height of the planned 

embankment section. A minimum of 4.57 meters (15 feet) into natural soils is required for small 

retention structures. The investigations should cover classification, physical and chemical properties, 

location, and extent of soil and rock strata, and variations in ground-water conditions. Evaluation of 

ground water should be focused on the uppermost aquifer. The field exploration should identify this 

aquifer, its flow direction, and the distance from the impoundment to potential down-gradient users. In 

addition, the background ground-water quality of the uppermost aquifer should be obtained in accordance 

with the preoperational guidance in Regulatory Guide 4.14, Revision 1, “Radiological Effluent and 

Environmental Monitoring at Uranium Mills,” issued April 1980. Observation of ground-water 

conditions should be recorded over a sufficient period to permit the ground-water depths and range of 

seasonal fluctuations to be established. 

The foundation conditions must be determined to assess the ability of subsurface materials to 

support the embankments without failure and without excessive total or differential settlement. The 

permeability of foundation soils and rocks must be ascertained to estimate the seepage potential. The 

availability of suitable borrow material for retention system construction must be assessed with 

consideration of the construction sequence and schedule. 

Information is needed on the characteristics of the underlying soil and geologic formations, 

particularly as they will control transport of contaminants and solutions. This includes detailed 

information concerning extent, thickness, uniformity, shape, and orientation of underlying strata. 

Hydraulic gradients and conductivities of the various formations must be determined. This information 

must be gathered from borings and field survey methods taken within the proposed impoundment area 

and in surrounding areas where contaminants might migrate to ground water. Hydrologic parameters 

such as permeability may not be determined on the basis of laboratory analysis of samples alone; a 

sufficient amount of field testing (e.g., pump tests) must be conducted to ensure an adequate 

understanding of actual field properties. 

1.3 Laboratory Tests

Testing soil samples of foundation and embankment materials from the field investigation should 

result in detailed knowledge of such physical and mechanical properties as classification, gradation, shear 

strength, consolidation, permeability, sedimentation, compaction, piping and cracking susceptibility, and 

wind-water erosion characteristics. 

2. Design Analysis 

Design analysis should consider stability, settlement, seepage, hydrologic analyses, liner stability, 

and liner compatibility. Specifically, the design must ensure that retention system failure will not occur. 

Historical records (Refs. 9–12) indicate that most failures associated with tailings retention systems have 

been caused by overtopping by flood waters, erosion, piping in the retention embankment or the 

foundation, foundation failure, slope failure, or liquefaction. 
Rev. 3 of RG 3.11, Page 7 

2.1 Stability and Failure Analyses

2.1.1 Slope Stability 

Slope failure occurs when an outer portion of an embankment slides downward and outward with 

respect to the remaining part of the embankment. The slide generally occurs along a fairly well-defined 

slip surface. 

2.1.1.1 Methods of Analyses

Stability analyses involve comparing the shearing stresses along potential failure surfaces with 

the available shearing resistance along those surfaces. The factor of safety is the ratio of the available 

shear strength to the developed maximum shear stress. 

A number of computer programs can be used to perform slope stability analyses. Computer 

programs provide an easier means to (1) consider complex slope geometries and subsurface soil layering, 

(2) use a number of different types of soil in the analysis, (3) search for circular, wedge, and noncircular 

failure surfaces, (4) consider different models to represent soil strength, and (5) consider different loading 

conditions. When used properly, these programs can allow a designer to consider a significant number of 

potential slip surfaces. Commercially available programs also may allow calculation of the factor of 

safety using several of the methods identified below. Despite these advantages, the output from a 

computer program needs to be checked carefully to verify that the critical surface with the lowest factor 

of safety has been identified, the critical surface represents a possible or realistic scenario, and 

computational problems resulting from the parameters used are minimized. For complicated situations, it 

may be prudent to verify the analysis with a second computer program or a hand calculation. 

2.1.1.2 Static Stability Analysis

Limit Equilibrium 

Conventional limit equilibrium methods of slope stability analysis evaluate the equilibrium of a 

soil mass tending to move down slope under the influence of gravity. Many methods use the limit 

equilibrium approach. Various publications (Refs. 13–15) offer detailed discussions. The following 

provides a brief overview of several of these methods: 

• Friction Circle Method—This method considers the entire sliding block as a rigid free body and 

makes assumptions regarding the distribution of normal stresses along the failure surface. It can 

be used only to evaluate failure surfaces that are circles or single straight lines and is most suited 

to homogeneous soil conditions. The Logarithmic Spiral Method is a different version of this 

method. 

• Method of Slices—This method divides the free body into many vertical slices, and the 

equilibrium of each slice is considered. The best known and most widely used versions of this 

method are the Ordinary Method of Slices, the Swedish Circle Method, Modified Swedish 

Method, Simplified Bishop Method, Spencer’s Method, and Morgenstern-Price Method. 

Although this method can be used to analyze wedge and noncircular slip surfaces, certain 

methods, such as the Ordinary Method of Slices and Simplified Bishop Method, require a circular 

slip surface. The analyses should consider both shallow slip surfaces that run through the 

embankment as well as deep slip surfaces that run beneath the embankment (Refs. 16 and 17). 
Rev. 3 of RG 3.11, Page 8 

• Wedge Method—This method is used whenever the failure surface can be satisfactorily 

approximated by a series of straight lines (usually two or three). 

• Special Cases 

- Infinite Slope—This analysis is suited to cases in which a slip surface may form parallel 

to the face of the slope. This type of failure can be approximated using a circular surface 

with a large radius. However, a simple approach has been developed based on 

equilibrium of forces. Several publications (Refs. 15–18) discuss this situation in detail. 

- Geomembranes—If a geomembrane is to be used as a component of a retention system, a 

stability analysis of a layer of cover soil on top of the geomembrane may be needed. This 

type of analysis can be sensitive to the slope angle and the interface friction between the 

cover soil and the geomembrane (Refs. 19 and 20). 

Deterministic versus Probabilistic Analyses 

In traditional deterministic approaches to slope stability analysis, the foundation soil properties, 

pore pressure, geometry, and loading conditions are held constant, and one of the analysis techniques 

identified above is used to calculate the factor of safety. In this approach, the numerical values used in 

the analysis do not account for the inherent variability of material properties, pore pressures, and loading 

conditions that can exist within a given slope. Probabilistic methods make it possible for a slope stability 

analysis to consider the variability of these parameters. While the use of probabilistic methods is not 

required when designing an embankment or retention system, it can aid in the interpretation of an 

analysis. Reference 21 further discusses probabilistic methods in slope stability analysis. 

Finite Element Method 

The finite element method (FEM) is most useful when calculating the stresses, pore pressures, 

and deformation of a slope. FEM can be used to calculate a factor of safety in a slope stability analysis, 

but this requires additional steps beyond the typical FEM output. FEM usually requires information 

about the stress-strain behavior of soil to provide a reasonable answer. This can require complex 

laboratory testing. Because of the considerable time and effort required to develop an accurate and 

representative understanding of the soil conditions, FEM is best suited to aiding the interpretation of 

difficult slope stability problems. Examples of situations in which FEM may be applicable include 

(1) pore pressure dissipation and corresponding strength gain in tailings slimes, (2) settlement of an 

embankment as it is raised, (3) consolidation of soft soils beneath an embankment, and (4) identifying 

areas of displacement near critical structures. References 15 and 22 further discuss FEM. 

2.1.1.3 Dynamic (Seismic) Stability Analysis 

In areas where embankments are subjected to seismic disturbances, seismic stability analyses 

should be performed. Seismic vibrations can cause liquefaction of saturated or nearly saturated loose 

sands and sensitive silts (Ref. 4). The dynamic shearing stresses induced during the seismic events can 

cause excessive deformation, distortion, or even shear failure of the embankment (Refs. 23 and 24). 

Seismic stability analyses of embankments are conventionally made using pseudostatic methods 

(Ref. 25). In this approach, the stability of a potential sliding mass is determined assuming static loading 

conditions, and the computation accounts for the effects of an earthquake by including an equivalent 

horizontal force acting on the potential sliding mass. The horizontal force representing earthquake effects 

is expressed as the product of the weight of the sliding mass and a seismic coefficient. The value of the 
Rev. 3 of RG 3.11, Page 9 

seismic coefficient is normally selected on the basis of the seismicity of the region in which the 

embankment is to be constructed. 

During earthquakes, large cyclic inertia forces are induced in embankments. In certain zones of 

an embankment, the inertia forces may be sufficiently large and may occur often enough to cause 

permanent displacements. Newmark (Ref. 26), Goodman and Seed (Ref. 23), and Makdisi and Seed 

(Refs. 27 and 28) have proposed procedures for estimating the magnitude of these displacements. These 

approaches are more involved than conventional methods and have been used successfully to predict the 

surface displacements of embankments. Other approaches may be used; however, good engineering 

judgment should be exercised in the selection of soil characteristics, the application of the approach to the 

soil type and saturation conditions, and the evaluation of the results obtained. 

In dealing with saturated, cohesionless soils, the dynamic analysis procedures developed by Seed 

et al. (Ref. 29) provide a basis for assessing the stability and deformation of the embankment during 

earthquakes. This type of analysis may be used to predict the development of the liquefaction zone and 

the anticipated movements, deformation, and stability of the embankment and its foundation. However, 

good engineering judgment based on adequate data must be exercised in the selection of soil 

characteristics for use in the analyses, the detailed steps followed to conduct the analyses, and the 

evaluation of the results obtained. 

Reference 30 contains a detailed discussion and applicable guidelines for seismic analysis and 

design of tailings embankments. 

2.1.1.4 Loading Conditions and Factors of Safety

A tailings embankment and its foundation are subjected to shear stresses imposed by the weight 

of the embankment, the filling of the impoundment, seepage, or earthquake forces. The cases for which 

stability analyses are necessary include the following: 

• End of construction—Analyses of the upstream and downstream slopes are needed for the end-ofconstruction conditions if the embankment and its foundation are partially or entirely composed 

of impervious soils. The unconsolidated undrained shear strength should be used in the analyses 

for slow-draining soils, while consolidated drained shear strength should be used for free-draining 

soils in which excess pore pressures would not develop. 

• Partial pool with steady seepage—Analyses of the upstream slope are needed for several 

intermediate pool stages with corresponding steady seepage conditions. The analyses account for 

a reduction in effective normal stresses when pore water pressure that developed during 

construction or filling did not dissipate before the subsequent partial pool condition. The lower 

strength from either the consolidated undrained shear test or consolidated drained shear test is 

used in the analyses. The minimum factor of safety should be determined as a function of pool 

elevations. 

• Maximum storage pool with steady seepage—This condition may develop and may be critical to 

downstream slope stability. A flow net would be helpful in determining the phreatic line and 

seepage forces. Shear strength selection should be the same as for the partial pool with steady 

seepage condition. 

• Earthquake—In areas subject to seismic shocks, appropriate earthquake forces need to be added 

onto the previous loading conditions in the stability analyses. 
Rev. 3 of RG 3.11, Page 10 

The use of a factor of safety in stability analyses should allow sufficient margin for variations 

between the parameters used in the design and those existing in the field, as well as consideration of the 

limits of strains. Many soils undergo relatively large plastic strains as the applied shear stresses approach 

the shear strength of the soil. 

When choosing the factor of safety, the analyst needs to consider the consequence of a failure, the 

tolerable limits of strains, and the degree of confidence in engineering parameters used in the analyses. 

The minimum factor of safety suggested in the regulatory position of this guide presumes that the stability 

analysis has been sufficient to locate the critical failure surface and that parameters used in the analysis 

are known, with reasonable certainty, to represent actual conditions of the dam and its foundation. 

Otherwise, higher factors of safety would be required. 

2.1.2 Liquefaction

Liquefaction impacts on stability need to be considered, if potentially liquefiable soils exist below 

the site of a retention system. Evaluation of liquefaction potential should include laboratory testing, in 

situ testing, and comparisons to similar soil deposits. The following five screening criteria should be used 

to determine whether there are potentially liquefiable soils at a site (Ref. 31): 

(1) Geologic age and origin—If a soil layer is a fluvial, lacustrine, or aeolian deposit of Holocene 

age, a greater potential for liquefaction exists than for till, residual deposits, or older deposits. 

(2) Fines content and plasticity index—Liquefaction potential in a soil layer increases with 

decreasing fines content and plasticity of the soil. Cohesionless soils having less than 15 percent 

(by weight) of particles smaller than 0.005 millimeters, a liquid limit less than 35 percent, and an 

in situ water content greater than 0.9 times the liquid limit may be susceptible to liquefaction 

(Ref. 32). 

(3) Saturation—Although soils with low water content have been reported to liquefy, at least 80- to 

85-percent saturation is generally deemed to be a necessary condition for soil liquefaction. The 

highest anticipated temporal phreatic surface elevations should be considered when evaluating 

saturation. 

(4) Depth below ground surface—If a soil layer is within 15.24 meters (50 feet) of the ground 

surface, it is more likely to liquefy than deeper layers. 

(5) Soil penetration resistance—Seed et al. (Ref. 33) state that soil layers with a normalized standard 

penetration test (SPT) blowcount [(N1)60] less than 22 have been known to liquefy. Marcuson et 

al. (Ref. 34) suggest an SPT value of [(N1)60] less than 30 as the threshold to use for suspecting 

liquefaction potential. Liquefaction also has been shown to occur if the normalized cone 

penetration test cone resistance is less than 1.59 megapascals (157 tons per square foot) (Ref. 35). 

If three or more of the above criteria indicate that liquefaction is not likely, the potential for 

liquefaction can be dismissed. Otherwise, a more rigorous analysis of the liquefaction potential at a 

facility is required. However, even if three or more of the liquefaction evaluation criteria indicate that 

liquefaction is unlikely, historical evidence of past liquefaction or sample testing data collected during the 

subsurface investigation may raise enough of a concern that a full liquefaction analysis still should be 

done. 

If liquefaction potential exists at a retention system site, additional subsurface investigation may 

be necessary. Once all testing is complete, a factor of safety against liquefaction should be calculated for 
Rev. 3 of RG 3.11, Page 11 

each critical layer that may liquefy. Seed and Idriss (Ref. 36) outline one procedure for evaluating 

liquefaction potential. A liquefaction analysis should, at a minimum, include the following: 

• development of a detailed understanding of site conditions, the soil stratigraphy, material 

properties and their variability, and the areal extent of potential critical layers 

• development of simplified cross-sections amenable to analysis 

• calculation of the force required to liquefy the critical zones (resisting force), based on the 

characteristics of the critical zone(s) (e.g., fines content, normalized standardized blow count, 

overburden stresses, level of saturation) 

• calculation of the design earthquake effect (driving force) on each potentially liquefiable layer 

using the site-specific, in situ soil data and an understanding of the earthquake magnitude 

potential for the facility 

• computation of the factor of safety against liquefaction (resisting force divided by driving force) 

for each liquefaction susceptible critical layer 

2.1.3 Settlement

If the foundation beneath an embankment retention system consists of layers of compressible 

soils or weathered rock, or if the bedrock profile is very irregular, differential settlements could result 

from uneven loading or variable thicknesses in the compressible soils. Total settlement and differential 

settlements may cause cracking and/or excessive strain in the embankments or other retention system 

components that could lead to system failure. 

The magnitude of the anticipated settlement can be estimated from the results of laboratory 

consolidation tests on samples recovered from the compressible foundation strata and remolded 

embankment materials. The rate of settlement also can be estimated. However, the potential error in 

estimating the time for settlement to occur is significant, since settlement is influenced by soil drainage, 

which is controlled by minute geological details that may not be detected during the geotechnical 

investigation. Predictions based on laboratory data can be modified by actual measurements to provide 

reasonably accurate long-term estimates. Settlement should be calculated along as many cross-sections as 

are necessary to ensure that the expected amounts of overall and differential settlement that the 

engineered components of the facility will experience have been adequately estimated. 

After total and differential settlement analyses have been performed, the engineered components 

of the waste retention system, such as geotextiles, geomembranes, clay liners, drainage layers, leachate 

collection piping, and waste piping, should be analyzed for tensile strain. The analysis should verify that 

the components can maintain their integrity when subjected to the induced strain associated with the 

settlement determined in the total and differential settlement analyses. If analysis indicates that total and 

differential settlement along any cross-section is likely to damage an engineered component, or to cause 

the engineered component to be unable to meet the minimum design criteria, then the retention system 

must be redesigned to eliminate the adverse effects of total and differential settlement. Methods such as 

overbuilding, surcharging, removal of the material causing the problem, or engineered reinforcement can 

be used to mitigate the effects of settlement. 
Rev. 3 of RG 3.11, Page 12 

2.2 Water Control and Management

2.2.1 Impoundment Storage Capacity

Some catchment area will always contribute runoff into the tailings retention system. This 

generally will be the area of the system itself, given requirements for below-grade impoundments, but 

might, in some cases, be a larger area incorporating the drainage area of streams entering a valley in 

which a retention embankment is constructed. Substantial runoff volumes and flows can result from 

heavy precipitation or snowmelt over relatively small catchment areas. 

Because the probability of occurrence of large floods on small drainage basins in arid regions is 

very small and onsite personnel should be available to repair any minor damage that could occur, the staff 

may accept less conservative options for determining the design-basis flood. For small retention systems 

built in isolated areas where failure would neither jeopardize human life nor create damage to property or 

the environment beyond the licensee’s legal liabilities and financial capabilities, the design need not use 

extremely conservative flood design criteria. However, the selection of the design flood needs to be at 

least compatible with the hazard category guidelines set forth by the U.S. Army Corps of Engineers 

(Ref. 37). If impoundments are designed to contain only direct precipitation that falls into the reservoir 

area, a single occurrence of the 6 hour probable maximum precipitation (PMP) may be used to determine 

storage capacity and freeboard requirements. If the tailings retention system has some external drainage 

area, and hydraulic structures (such as diversion channels) are needed to safely divert the probable 

maximum flood (PMF), the peak PMF inflows and runoff used to design such structures should be 

determined in accordance with the suggested flood design criteria in NUREG-1623, “Design of Erosion 

Protection for Long-Term Stabilization,” (Ref. 2). 

If decant or other reclaim systems have not been designed specifically to handle the design flood, 

other measures need to be taken. Those other measures may be one or a combination of the following: 

• The whole volume of flood runoff is stored. Sufficient freeboard should always be available to 

provide the necessary storage capacity without overtopping the embankment, as well as adequate 

protection against wave runup. 

• Diversion channels are provided to convey runoff water safely past the retention system. 

Determination of the freeboard necessary at any time to store flood runoff will require 

information on pond storage versus elevation, anticipated embankment settlement versus time, and the 

expected runup of wind-generated waves. Reference 38 presents procedures for determining wave runup. 

It is important that the embankment construction schedule ensure that the required freeboard is always 

available. 

Adequate slope protection is needed to guard the embankment against wind and water erosion, 

weathering, and ice damage. Methods for protecting slopes include dumped riprap, precast and cast-inplace concrete pavements, bituminous pavement, soil cement, tailings beaches, sodding, and planting. 

The necessary upstream slope protection depends on the expected wind velocity and duration and the size 

and configuration of the reservoir at the water-surface elevation. Reference 38 provides methods and 

criteria for the selection and design of slope protections. If the toe of the embankment is subject to 

flooding or erosion from nearby streams or arroyos, it may be necessary to provide erosion protection for 

the toes and exterior side slopes. NUREG-1623 (Ref. 2) provides guidance for determining design floods 

and erosion protection. 
Rev. 3 of RG 3.11, Page 13 

2.2.2 Diversion Channel Design

Any channels that are needed to protect against flooding and erosion of embankments or tailings 

should be designed to safely pass a PMF with minimal, if any, damage to the channel. The essential 

criterion is that no release of tailings or contaminated materials should occur during a PMF, with the 

recognition that onsite personnel can repair minor damage within a short period of time. For example, a 

channel could be designed to pass only a 100-year flood, so long as the PMF does not result in the release 

of contaminated material. 

2.2.3 Seepage and Hydrostatic Uplift Analyses

Since regulations require retention systems to be lined, seepage analysis for embankment stability 

purposes is unnecessary. However, special design features, such as impervious liners and collection 

systems, are needed to maintain the quality and quantity of seepage from the retention system within 

tolerable limits of water supply and pollution control requirements. Section 2.2.4 of this regulatory guide 

details seepage control considerations. 

Hydrostatic uplift may affect the subbase or engineered components of a waste containment 

facility anytime ground water exists at a facility. When an excavation or a portion of a waste containment 

facility is to be constructed at a depth at which a phreatic surface of ground water or piezometric 

pressures are present, the potential adverse effects on the waste containment facility need to be taken into 

account. An unstable condition caused by hydrostatic uplift may develop when the hydrostatic uplift 

force overcomes the downward force created by the weight of the overlying soil. If the area acted on by 

the hydrostatic force is sufficiently great, excess water pressure may cause overlying soil to rise, creating 

a failure known as “heave.” Although heave can take place in any soil, it will most likely occur at an 

interface between a relatively impervious layer (such as a clay liner) and a saturated, relatively pervious 

base. 

2.2.4 Seepage Control

The potential for seepage at an embankment retention system can be controlled through two 

means. The first is to employ a system to provide a method to dewater tailings after they are placed in the 

retention system. The other means is to install a liner system. 

Regulations focus on using synthetic liners for the retention systems. However, a design should 

consider that, with an impervious bottom, process liquids and/or infiltration into the impoundment can 

result in excessive buildup of liquids after closure. The minimization of this potential for “bathtubbing” 

should be addressed through discussion of mitigative design aspects, including plans for operational 

dewatering (see 2.2.4.1 below) and future construction of an infiltration barrier in the closure cover. 

2.2.4.1 Dewatering

Regulations require that new tailings impoundment retention systems be dewatered by a drainage 

system installed at the bottom of the impoundment. The goal of the drainage system is to lower the 

phreatic surface within the waste materials to reduce the head acting on the liner system. This can be 

accomplished by several methods. One method is to include a highly permeable layer immediately above 

the liner system and slope the liner system to a low point. A pump or gravity drain system can then be 

used to remove the collected liquids from this low point. An alternative method would be to pump liquids 

out of vertical wells within the tailings. In either case, the potential clogging of the drainage materials 

should be addressed. The U.S. Department of Agriculture (USDA) Engineering Handbook (Ref. 39) 
Rev. 3 of RG 3.11, Page 14 

offers examples of design methods for soil filters. References 40 and 41 outline methods for designing 

synthetic filters. 

2.2.4.2 Liners

An embankment retention system for uranium recovery wastes is required to have a liner to 

prevent the migration of wastes to surrounding soil, ground water, or surface water during its operation 

and closure period. The design of a liner system should consider subgrade material, type of liner system, 

liner system protection, and leak detection. A complete liner system design also should address 

anticipated installation techniques and operating practices. Sections 3 and 4 of this regulatory guide 

present specific items related to construction and operation respectively. 

Subgrade 

Proper design and understanding of the subgrade soils is very important to the success of a liner 

system. Design of the subgrade should consider the available soils, focusing on their gradation and 

moisture/density relationships. The subgrade surface needs to be competent and able to withstand the 

anticipated construction traffic. As previously mentioned in Section 2.1.3, a settlement analysis should be 

performed on the subgrade soils. The purpose of this analysis is to demonstrate that the anticipated 

settlement of the subgrade will not damage the liner system. The amount of settlement will depend on 

several factors, including the soil type, subgrade drainage condition, the depth and weight of the material 

that will be placed on the liner, and the rate of placement of the material on the liner system. 

As discussed in Section 2.2.3, the subgrade design should consider the location and potential 

changes in the ground-water table. If a retention system is located in an area where the water table could 

rise above the bottom of the liner system, an underdrain may be required to prevent the development of 

upward water pressure on the liner. 

Liner System Selection 

The choice of the liner system should consider several factors. A key factor is the liner material’s 

physical and chemical inertness when exposed to the waste materials within the retention system. The 

chemical qualities of the tailings, slurry, and/or liquid wastes must be assessed to determine the impacts 

on liners and/or the environment, if contamination resulting from seepage or surface water runoff occurs. 

One issue specific to earthen liner layers is the potential for the hydraulic conductivity to increase 

with time (Ref. 42). Excessive settlement or desiccation can lead to the development of cracks within an 

earthen layer. This increases the hydraulic conductivity which in turn decreases the effectiveness of the 

liner system. The subgrade design should address settlement, and desiccation should be handled through 

an understanding of the subgrade soil conditions and an identification of the proper moisture content 

range during design. 

The advantages of a synthetic liner system include a significantly reduced thickness, a greater 

resistance to cracking, and a much lower hydraulic conductivity (typically several orders of magnitude 

lower than an earthen liner system). The design of a synthetic liner system should consider the method of 

placement, the seaming techniques, and the puncture resistance. Lupo and Morrison (Refs. 43 and 44) 

outline current design approaches using synthetic materials for mining applications. Theory and design 

methods to evaluate puncture resistance have been developed and can be used to evaluate the puncture 

resistance of synthetics for different conditions (Refs. 45–47). 
Rev. 3 of RG 3.11, Page 15 

Protection of the Liner System 

Ultraviolet radiation, wave action, surface runoff, foot traffic, animals, ice, wind, and 

construction equipment may damage a liner system. Therefore, a liner system design should consider 

various protective measures to prevent damage. Protective measures may be particularly important when 

a synthetic liner is used. Possible protection methods include soil covers, sand bags, game-proof fences, 

and access restrictions. 

Soil covers can protect against ultraviolet radiation, wave action, animals, wind, and construction 

equipment. The design should address certain aspects of soil covers, including sloughing during heavy 

precipitation or during rapid drawdown of the liquid within the retention system and erosion during highprecipitation events. The stability of soil covers placed over synthetic liners may need to be analyzed. A 

series of properly arranged sandbags can be an effective method of protecting a synthetic liner from wind 

damage. The sandbags need to have an appropriate weight, spacing, and anchoring system to provide the 

required resistance to wind forces. Use of sandbags as protection has the added benefit of preserving 

access to the liner for visual inspection and repair. A game-proof fence may need to be installed around 

the perimeter of the embankment system. The fence should be designed to prevent entry of sharp-hoofed 

animals such as antelope, deer, and cattle. The fence should be of sufficient height and strength to 

preclude entry of species known to be in the area. 

Leak Detection 

A leak detection system is required with a synthetic liner. The leak detection system should be 

designed to identify the approximate locations of leaks so repairs can be made and to isolate leaks so that 

they can be controlled. The leak detection system generally consists of either a highly permeable soil or 

synthetic material such as a geonet located immediately beneath the synthetic liner. This highly 

permeable layer should be designed to drain to sumps where the leakage can be monitored. Consideration 

should be given to developing a contoured grading plan that has a series of peaks and valleys for the liner 

and leak detection system to identify the approximate location of any leak. The design of a leak detection 

system also should establish an allowable leakage rate (ALR). The ALR should take into account 

anticipated defect rates in the synthetic layer, hydraulic head conditions on the liner system, and flow 

rates within the detection layer. If leakage is found in the detection system at a rate greater than the ALR, 

remedial action is necessary. 

3. Construction Considerations 

Construction approaches for impoundments are closely related to the specific site and operational 

conditions. As discussed in Section 1.1 of this guidance, the prime option identified in Criterion 3 of 

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,” to 10 CFR Part 40, “Domestic Licensing of Source Material,” is to 

locate an impoundment below grade. However, certain geologic or hydrogeologic features at a site may 

make it impractical to locate an impoundment entirely below grade. Given the flexibility provided by 

Criterion 3, three possible construction scenarios exist: 

(1) Full excavation—In this scenario, the impoundment would be constructed by excavation to a 

depth sufficient to accommodate the tailings volume. Depending on the topography of the site, a 

small perimeter embankment may be required to prevent storm-water runoff from entering the 

impoundment. This scenario would result in excess soil that would have to be disposed of or 

used elsewhere. 
Rev. 3 of RG 3.11, Page 16 

(2) Partial excavation—Under this scenario, the site conditions prevent full excavation of the 

impoundment. A portion of the retention system would be excavated and an embankment would 

be built around the excavated cell to create the required disposal capacity. In this scenario, the 

depth of excavation and height of the perimeter embankment will dictate whether there will be a 

balanced soil cut and fill, some excess soil to use elsewhere, or a need for some additional borrow 

material. 

(3) No excavation—This scenario should be considered only when adverse site conditions exist, such 

as a high ground-water table or bedrock near the ground surface. The impoundment would be 

created by constructing an embankment around its perimeter. This scenario would require that a 

borrow source for embankment material be located in the general vicinity of the impoundment. 

The construction of embankment retention systems generally involves both excavation and filling 

in some specified order. For any of the construction scenarios identified above, successful embankment 

retention system construction requires understanding the moisture/density relationships of the soils, 

providing adequate compaction, and preventing poor-quality soils from being incorporated into the 

embankment retention system fill materials. Much additional information on the characteristics of 

foundations is obtained during clearing and stripping operations, which may confirm or contradict design 

assumptions based on earlier geotechnical investigations. Weather and ground-water conditions during 

construction may significantly alter water contents of proposed fill material or create seepage and/or 

hydraulic conditions, necessitating modifications in design. Projects must be evaluated and 

“reengineered” continuously during construction to ensure that the final design is compatible with 

conditions encountered during construction. Construction supervision, management, and monitoring of 

the embankment and associated structures are a critical part of the overall project management plan. 

Once the facility is placed into operation, observations, surveillance, inspections, and continuing 

evaluation are required to ensure the satisfactory performance of the retention system (see Section 4 of 

this regulatory guide). 

Installation of a synthetic liner system should focus on minimizing liner damage. Damage can 

occur in the form of wrinkles, improper seaming techniques, poor synthetic panel orientation, and 

punctures caused by construction equipment. The potential for wrinkle development can be minimized 

by orienting panels properly, seaming within the allowable range of temperatures, and compacting the 

subgrade properly. Synthetic liner manufacturers often provide specific guidance on proper techniques 

for minimizing wrinkles. Seams typically constitute the weakest portion of a synthetic liner system. 

Therefore, the layout of the synthetic panels should minimize the location of seams in high-stress areas. 

Punctures can be minimized by following manufacturer recommendations for allowable ground pressures 

and minimum protective cover requirements for construction equipment working on a synthetic liner. 

Quality assurance practices during synthetic liner installation need to be rigorous, and a leak location 

survey after synthetic liner installation may be beneficial. 

The construction plans should include construction specifications for excavation, embankment 

construction, subgrade preparation, liner placement, and the like. The general construction considerations 

for earthwork listed below should be considered as minimum guidelines, with the understanding that 

additional or more stringent specifications may be required depending on individual site conditions: 

• A geotechnical or construction inspector should be on site during embankment construction. 

• Fill material should be taken from an approved, designated borrow area. It should be free of 

roots, stumps, wood, rubbish, stones greater than 6 inches, and frozen or other objectionable 

materials. 
Rev. 3 of RG 3.11, Page 17 

• Areas on which fill is to be placed should be scarified before its placement. 

• The compaction requirements for the fill material should include the percent of maximum dry 

density for the specified density standard, allowable range of moisture content, and maximum 

loose lift thickness. 

• Fill material should be compacted with appropriate compaction equipment such as a sheepsfoot, 

rubber-tired, or vibratory roller. The number of required passes by the compaction equipment 

over the fill material may vary with soil conditions. 

• Fill material should contain sufficient moisture to allow the required degree of compaction to be 

obtained with the equipment used. 

• Field density tests should be performed regularly throughout the embankment construction. 

Many factors influence the frequency and location of control tests. Typically, a routine control 

test should be performed for every 764.5 to 2293.6 cubic meters (1000 to 3000 cubic yards) of 

compacted material or as directed by the geotechnical engineer. 

• Proper subgrade preparation during construction is necessary for the installation of a liner system. 

The site of the retention system should be cleared of all debris, vegetation, and potential root 

systems. The surface should be graded so that it is smooth and free of protruding rock particles. 

The soil may need to be moisture conditioned to prevent it from drying out before the liner is put 

into use. 

• To the extent possible, synthetic liner seams should run up and down and not across a slope. 

They should not be located near the crest of a slope. Seams should be tested for integrity along 

their entire length using methods recommended by the manufacturer. Seaming should be 

performed only under the supervision of experienced personnel. 

In general, widely accepted construction standards and specifications for embankments, such as 

those developed by the USDA Soil Conservation Service or the U.S. Army Corps of Engineers, should be 

followed (Refs. 48 and 49). 

4. Inspection and Maintenance 

Different conditions can develop throughout the active life of the retention system. Such changes 

can significantly affect the conditions governing the stability of a retention system. Therefore, a 

continuous program of inspection of the retention system is needed, beginning with the start of 

construction, through the waste disposal, and, in the case of tailings disposal areas, continuing after 

reclamation. Each site and structure has its own characteristics and its own susceptibilities to problems, 

and the inspection program should be tailored to consider these. Thorough physical examination is an 

essential part of the inspection program. The optimal frequency of inspections depends on the size and 

condition of the facilities, the character of the foundation, the regional geological setting, and the 

consequences of failure in jeopardizing human health and safety and inflicting property and 

environmental damage. Monitoring and analysis of performance data are necessary to ensure detection of 

adverse conditions. 

Before the start of waste disposal, records of ground-water levels (including seasonal 

fluctuations), ground-water quality, ground elevations, and background radioactivities at the site should 

be compiled and compared with the operational conditions of the impoundment. Data gathered in 

accordance with Regulatory Guide 4.14 will be useful in these comparisons. As soon as waste disposal 
Rev. 3 of RG 3.11, Page 18 

begins, inspection should be performed at regular intervals to check the condition of the retention systems 

and associated facilities and to evaluate their structural safety and operational adequacy. A detailed, 

systematic inspection program should consist of, but not necessarily be limited to, the elements described 

in the sections below. 

4.1 Engineering Data Compilation

Engineering data related to the design, construction, and operation of the retention systems should 

be compiled and, to the extent practicable, included in the initial inspection report. Most engineering data 

are readily available in documents filed for the license application. A detailed reference to the original 

documents kept at the project site should be adequate. These data should include the following items, as 

available and applicable: 

• general project data, including regional vicinity maps showing the project location and the 

upstream and downstream drainage areas, and as-built drawings and photographs of the retention 

system 

• hydrologic and hydraulic data, including drainage area and basin characteristics, waste storage 

volume, surcharge capacity for floods, rate of waste inflow, elevation of the maximum design 

pool, and freeboard height 

• foundation data and geological features, including boring logs, geological maps, profiles, and 

cross-sections 

• properties of embankment and foundation materials, including results of laboratory and field 

tests, and assumed design material properties 

• principal design assumptions and analyses, including hydrologic and hydraulic analyses, stability 

and stress analyses, and seepage and settlement analyses 

• pertinent construction photographs and records, including construction control tests, construction 

problems and modifications, and maintenance repairs 

4.2 Inspection Programs

The retention system inspection program should be established and conducted systematically to 

minimize the possibility of overlooking any significant features. A detailed checklist should be 

developed and followed to document the observations of each significant feature. Photographs for 

comparison of previous and present conditions should be used as a part of the inspection program. The 

inspection program should include, but not be limited to, the following as appropriate: 

• Daily Inspection 

- Pond water elevations should be examined and recorded to ensure that minimum 

freeboard is maintained. 

- The slurry transport system should be examined for any evidence of obstruction of the 

pipes or pumps caused by waste clogging or ice accumulation. The pipe couplings 

should be examined for leakage of waste, and any flow rate sensor should be tested. 
Rev. 3 of RG 3.11, Page 19 

- The retention embankments should be visually inspected for signs of erosion, cracking, 

slumping, movement, or concentration of seepage. 

- The liner system should be inspected to identify any damage to the liner and any 

operating practices that may contribute to liner damage. The inspection should include a 

visual check of the presence of animals and the accumulation of water in the leak 

detection system. 

• Monthly Inspection 

- Slurry transport pipes should be examined using an ultrasonic device at locations where 

pipes cross streams or other natural water courses or where a rupture of the pipe could be 

expected to affect the stability of the retention system. 

- Channels should be examined for channel bank erosion, bed aggradation or degradation 

and siltation, obstruction to flow, undesirable vegetation, or any unusual or inadequate 

operational behavior. 

• Quarterly Inspection 

- The top of the embankment and downstream toe areas should be examined and surveyed, 

if necessary, for any evidence of unusual localized or overall settlement or depressions. 

- Embankment slopes should be examined for irregularities in alignment and variance from 

originally constructed slopes, unusual changes from original crest alignment and 

elevation, evidence of movement at or beyond the toe, erosion, and surface cracks that 

indicate movement. 

- The downstream embankment slopes and toes, and other downstream areas, should be 

examined for evidence of existing or past seepage, springs, and wet or boggy areas. 

- The slope protection should be examined for erosion-formed gullies and wave-formed 

notches and benches. The adequacy of slope protection against waves and surface runoff 

that may occur at the site should be evaluated. The condition of vegetation or any other 

types of protective covers should be evaluated, when pertinent. 

- Any installed instrumentation, such as survey monuments, settlement plates or gauges, 

and/or piezometers, should be examined and tested for proper functioning. The available 

records and readings of these instruments should be reviewed to detect any unusual 

performance or distress of the structure. Immediately following installation or the 

discovery of an unusual condition, all instrumentation readings should be taken more 

frequently than once a quarter (e.g., daily or weekly) until the patterns of the structural 

behaviors are stabilized. 

- The maintenance of operating facilities and features (such as pumps and valves) that 

pertain to the safety of the retention system should be examined to determine the 

adequacy and quality of the maintenance procedures followed in maintaining the 

retention system in a safe operating condition. 

- The general long-term performance of the liner, such as its resistance to degradation, 

should be examined. 
Rev. 3 of RG 3.11, Page 20 

• Special Inspections 

- Unscheduled inspections should be performed after the occurrence of significant 

earthquakes, tornadoes, floods, intense local rainfalls, or other unusual events. 

- The NRC’s implementation of the National Dam Safety Program and its associated 

guidelines may require special inspections of any uranium recovery site embankments 

that fall within the scope of the program. The Federal Emergency Management Agency 

guidelines for dam safety (Ref. 50) specifically include tailings dams in its program and 

define a dam in the following manner: 

Any artificial barrier, including appurtenant works, which 

impounds or diverts water, and which (1) is twenty-five feet or 

more in height from the natural bed of the stream or watercourse 

measured at the downstream toe of the barrier or from the lowest 

elevation of the outside limit of the barrier if it is not across a 

stream channel or watercourse, to the maximum water storage 

elevation or (2) has an impounding capacity at maximum water 

storage elevation of fifty acre-feet or more. 

4.3 Technical Evaluation

The existing conditions of the retention system should be evaluated annually unless changing 

conditions dictate a shorter period. This evaluation should include an assessment of the hydraulic and 

hydrologic capacities, water quality, and structural stability and should take into account both existing 

conditions and any changing conditions. In addition, surface-water and ground-water sampling data 

collected in accordance with Regulatory Guide 4.14 should be examined at the time of the technical 

evaluation to detect any patterns that could be a sign of failure of seepage control measures or foundation 

distress. 

4.4 Inspection Reporting

A report should be prepared to present the results of each technical evaluation and the inspection 

data accumulated since the last report. These documents should be kept at the project site for reference 

purposes, available for inspection by regulatory authorities, and retired only upon termination of the 

project. Any abnormal hazardous conditions observed during the inspection should be reported 

immediately to the NRC staff. 

4.5 Inspection Personnel

An experienced professional who is thoroughly familiar with the investigation, design, 

construction, and operation of these types of facilities should direct the planning and conduct of the 

inspections and evaluations. At each facility, this individual should ensure that all field inspectors are 

trained to recognize and assess signs of possible distress or abnormality. 
Rev. 3 of RG 3.11, Page 21 

C. REGULATORY POSITION 

Basic design criteria generally are drawn from 10 CFR Part 40, Appendix A, and describe the 

latest approaches approved by the NRC for compliance with the applicable regulations. Information 

related to the investigation, engineering design, proposed construction, inspection, and performance of a 

uranium recovery waste retention system should address all applicable areas discussed in Section B of 

this regulatory guide. If an applicant proposes the use of an alternative method or new information that 

may be developed in the future, the NRC will review the proposal and, if acceptable, approve its use. 

1. Basic Design Criteria 

a. The “prime option” for disposal of tailings is placement below grade. Where full below-grade 

burial is not practicable, the size of retention structures and the size and steepness of slopes 

associated with exposed embankments must be minimized by excavation to the maximum extent 

reasonably achievable or appropriate, given the geologic and hydrologic conditions at a site 

(10 CFR Part 40, Appendix A, Criterion 3). 

b. Stability of the retention system should be ensured under all conditions of construction and 

operation. In ensuring structural integrity, it must not be presumed that the liner system will 

function without leakage during the active life of the impoundment (10 CFR Part 40, 

Appendix A, Criterion 5A(5)). 

c. The magnitude of total and differential settlement should be within tolerable limits that will not 

result in harmful cracking and embankment instability. 

d. Unless exempted under the regulations in Criterion 5A(3) of Appendix A to 10 CFR Part 40, 

liners and leak detection systems need to be included in the design of retention systems per 10 

CFR Part 40, Appendix A, Criteria 5A(1), 5A(2), and 5E(1), and considering EPA requirements 

in 40 CFR 264.221. 

e. Freeboard must be sufficient at all times to prevent overtopping by flood inflows and windgenerated waves and should include an allowance for settlement of the foundation and 

embankments (10 CFR Part 40, Appendix A, Criterion 5A(4)). Adequate slope protection should 

be provided for the embankment against wind and water erosion, weathering, and ice damage. 

 

f. Upstream rainfall catchment areas must be minimized to decrease erosion potential and the size 

of the floods that could erode or wash out sections of the tailings retention system 

(10 CFR Part 40, Appendix A, Criterion 4(a)). The surcharge capacity of the retention system 

must be adequate to store a PMF, calculated using the 6-hour PMP. 

2. Methods of Analysis 

a. The PMF should be based on the 6-hour PMP and should be developed in accordance with 

procedures provided in NUREG-1623 (Ref. 2). 

b. Wave runup may be determined using procedures discussed in the U.S. Army Corps of Engineers 

“Coastal Engineering Manual,” issued April 2002 (Ref. 38). 

c. The static stability of the embankment should be analyzed using commonly accepted detailed 

stability methods. The analysis should use appropriate static soil and rock properties established 
Rev. 3 of RG 3.11, Page 22 

on tested representative samples over anticipated in situ and placement conditions. Results of a 

manual check of the computer stability analysis outcome should be presented to illustrate adopted 

design procedures and criteria. 

d. Conventional pseudostatic analysis may be considered acceptable if the seismic coefficient 

appropriately reflects the geologic and seismologic conditions of the site and if the materials are 

not subject to significant loss of strength under dynamic loads. Liquefaction potential and the 

dynamic stability of the tailing dam and foundation should be assessed using appropriate state-ofthe-art methods. Reference 30 will be used to determine the extent of the required dynamic 

analyses. The analyses should employ appropriate dynamic material properties established on 

representative materials through adequate field and laboratory testing. 

e. The loading conditions to be evaluated in embankment stability analyses and corresponding 

minimum factors of safety are as follows: 

Loading Condition Minimum Factor 

of Safety 

End of construction 1.3 

Partial pool with steady seepage 1.5 

Maximum pool with steady seepage 1.5 

Earthquake (in combination with the above conditions) 1.0 

f. Evaluation of liquefaction potential should include laboratory testing, in situ testing, and 

comparisons to similar soil deposits. Screening criteria should be used to determine whether 

there are potentially liquefiable soils at a site. The factor of safety for liquefaction potential 

should be greater than 1.0. 

g. Appropriate laboratory test results should be used to estimate the rate and magnitude of 

settlement. 

3. Construction Methods 

a. Mill tailings embankment retention systems should use conventional acceptable engineering 

practices of construction control for water retention dams (e.g., controls on foundation 

preparation, suitability of materials, proper placement, field moisture, and density). 

b. Installation of a synthetic liner system should focus on minimizing liner damage. Damage can 

occur in the form of wrinkles, improper seaming techniques, poor synthetic panel orientation, and 

punctures caused by construction equipment. 

4. Inspection and Maintenance 

a. A detailed, systematic inspection and maintenance program should be established to detect and 

repair damage that might lessen the integrity of the retention system. Generally, visual 

inspections performed on a regular basis and supplemented by adequate instrumentation are 

acceptable. A detailed checklist should be developed and followed to document the observations 

of each significant feature. The inspection program should use photographs to compare previous 

and present conditions. In addition, the program should include radiometric and water quality 

surveys. 
Rev. 3 of RG 3.11, Page 23 

b. Daily inspections of tailings or waste retention systems should be planned, conducted, evaluated, 

and documented under the direction of an experienced professional who is thoroughly familiar 

with the investigation, design, construction, and operation of these types of facilities. The 

licensee should retain documentation (i.e., a record) of each daily inspection for 3 years after the 

documentation is made. The NRC must be immediately notified of any failure in a tailings or 

waste retention system that results in a release of tailings or waste into unrestricted areas or of 

any unusual conditions (conditions not contemplated in the design of the retention system) that, if 

not corrected, could indicate the potential for, or lead to, failure of the system and result in a 

release of tailings or waste into unrestricted areas. 

c. Unscheduled inspections should be performed after the occurrence of significant earthquakes, 

tornadoes, floods, intense local rainfalls, or other unusual events. The NRC’s implementation of 

the National Dam Safety Program and its associated guidelines may require special inspections of 

any uranium recovery site embankments that fall within the scope of the program. 

d. The inspection and maintenance program should start at the beginning of construction and 

continue at least through the operation of the facility. 

D. IMPLEMENTATION 

The purpose of this section is to provide information to applicants and licensees regarding the 

NRC’s plans for using this regulatory guide. The NRC does not intend or approve any imposition or 

backfit in connection with its issuance. 

In some cases, applicants or licensees may propose or use a previously established acceptable 

alternative method for complying with specified portions of the NRC’s regulations. Otherwise, the 

methods described in this guide will be used in evaluating compliance with the applicable regulations for 

license applications, license amendment applications, and amendment requests. 
Rev. 3 of RG 3.11, Page 24 

REFERENCES 

1. NUREG-1620, “Standard Review Plan for the Review of a Reclamation Plan for Mill Tailings 

Sites Under Title II of the Uranium Mill Tailings Radiation Control Act of 1978,” U.S. Nuclear 

Regulatory Commission, Washington, DC, June 2003. 

2. NUREG-1623, “Design of Erosion Protection for Long-Term Stabilization,” U.S. Nuclear 

Regulatory Commission, Washington, DC, September 2002. 

3. Regulatory Guide 3.64, “Calculation of Radon Flux Attenuation by Earthen Uranium Mill 

Tailings Covers,” U.S. Nuclear Regulatory Commission, Washington, DC, June 1989. 

4. R. Dobrey and L. Alvarez, “Seismic Failures of Chilean Tailings Dams,” Journal of the Soil 

Mechanics and Foundations Division, American Society of Civil Engineers, November 1967. 

5. W.A. Wahler and D.P. Schlick, “Mine Refuse Impoundments in the U.S.,” Proceedings of the 

International Congress on Large Dams, Mexico City, Mexico, 1976. 

6. Public Law No. 92-367, National Dam Safety Act, August 8, 1972. 

7. S.R. Borrowman and P.T. Brooks, “Radium Removal from Uranium Ores and Mill Tailings,” 

U.S. Bureau of Mines, U.S. Department of the Interior, Washington, DC, 1975. 

8. “Use of Uranium Mill Tailings for Construction Purposes,” Hearings before the Subcommittee on 

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