370101-4

30 August 2010

Energy Fuels Resources Corporation

44 Union Boulevard, Suite 600

Lakewood, CO 80128

Attention: Frank Filas, P.E., Environmental Manager

Re: Evaporation Pond Radon Flux Analysis, Piñon Ridge Mill Project, Montrose 

County, Colorado

Dear Frank,

This letter is to address Task 1 of our proposal of 16 June 2010 relative to radon flux from 

evaporation ponds, namely:

Task 1: Estimate the radon flux from the proposed Piñon Ridge Mill evaporation ponds 

for both an initial 40 acres of ponds and a potential increase to 80 acres. In addition, the 

effect of spraying to enhance evaporation has been considered. 

Task 1 is directed towards providing information to the U.S. Environmental Protection Agency 

(EPA) relative to their current Subpart W NESHAP rulemaking. We understand that this 

information will be provided as a courtesy to the EPA since Energy Fuels does not believe that 

the ponds are within EPA’s regulatory mandate.

As shown below, using a model from Nielson and Rogers for water-covered uranium tailings, 

[whose work has been the primary basis for NRC and EPA radon emission models from uranium 

tailings impoundments], it can be shown that the radon flux from the evaporation ponds at the 

Piñon Ridge Mill site is expected to be well within the range of pre-operational background 

radon flux rates measured at the proposed tailings locations. The basis for this conclusion is 

presented in subsequent sections of this letter.
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 2

We would be pleased to answer any questions you may have concerning our evaluation. In my 

absence, please communicate with my colleague Dr. Douglas Chambers.

Yours very truly,

For SENES Consultants Limited

Steven H. Brown, CHP

cc: Dr. Douglas Chambers, dchambers@senes.ca
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30 August 2010

Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 3

ATTACHMENT

1.0 INTRODUCTION

1.1 Background

Energy Fuels Resources Corporation (EFRC) is in the process of completing designs for a 

uranium mill, termed the Piñon Ridge Project, located in Montrose County, Colorado. The mill 

is designed for start-up operations at 500 tons per day (tpd) (Phase I), with a potential to expand 

to 1,000 tpd (Phase II). The design raffinate flows from the process circuit are in excess of that 

needed for re-circulation to the mill; therefore, the design of the mill requires construction of 

10 evaporation ponds (Phase I) and another 10 evaporation ponds (Phase II) for the disposal of 

the excess raffinate solution from the milling operation. The evaporation ponds are expected to

be very small sources of radon emissions to the ambient air. To confirm this assumption, and in 

the interest of demonstrating that radon emissions to unrestricted areas from operation of the 

Piñon Ridge Mill will be maintained ALARA, these potential radon emissions are the subject of 

this assessment.

The emission of radon from uranium tailings has been studied and modeled for many years. For 

example, the U.S. Nuclear Regulatory Commission (NRC) makes use of their 1984 handbook for 

uranium tailings cover design (Rogers et al. 1984). 

Because of the very low diffusion of radon through water (as compared to partially air-filled 

unsaturated tailings pores), the diffusion of radon through water-covered tailings has been argued 

to be effectively zero (e.g. Chambers 2009). The EPA has previously assumed zero radon 

emissions from ponded areas of uranium tailings impoundments (e.g. EPA 1986). This is based 

on the assumption of no or low measurable radium concentrations in water covering the tailings; 

that is, the source of radon-222 (radon) is primarily the radon from the radium-226 (Ra-226) in 

the tailings. However, during a recent presentation at the annual National Mining Association 

/Nuclear Regulatory Commission (NMA/NRC) Uranium Workshop in Denver, Colorado, 

representatives of the United States Environmental Protection Agency (EPA) stated that the work 

practice standards in its 40 CFR Part 61, Subpart W National Emissions Standards for Radon 

Emissions from Operating Mill Tailings apply to evaporation ponds at conventional and in situ 

uranium recovery (ISR) sites licensed by NRC or its Agreement States.

A presentation by Baker and Cox (2010) at the most recent NMA/NRC workshop in Denver 

considers the situation where appreciable concentrations of radon are present in the ponded 

water, as may arise for example from elevated levels of Ra-226 dissolved in the pond water. 

Baker and Cox, reporting on a stagnant film model and some measurement data, suggest a radon 

flux of the order of 1 pCi m

-2

s

-1

per 100 pCi/L of dissolved radon in the ponded water. 
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 4

1.2 Published Models and Regulatory Context

As previously noted, much of the work on radon emission models undertaken by Rogers and 

Associates has been adopted by the U.S. NRC and is widely used by the NRC and others in 

assessing radon releases from uranium tailings. Besides the NRC use of the Rogers et al models 

in the NRC 1984 handbook for uranium tailings cover design (Rogers et al 1984), their work is 

also the basis for NRC’s Regulatory Guide 3.64 on radon attenuation by earthen tailings covers 

(NRC 1989).

In some earlier work, Nielson and Rogers (1986) examined the issue of surface water 

considerations in predicting radon emissions from water-covered uranium tailings 

impoundments. They suggest that radon emissions from water-covered tailings can be non-zero. 

These authors attribute this to advective mixing, as opposed to straight radon diffusion 

mechanisms. Although there are various papers on the diffusion and transport of radon available 

from published literature, the 1986 work of Rogers and Nielson is of direct relevance to the 

present study of radon emissions from evaporation ponds, and was adopted for application here.

The Nielson and Rogers model (1986) makes use of equations based on the well-known first 

Fick’s Law of gaseous diffusion (non-reactive) through media in order to estimate surface radon 

flux rates. They considered that mixing leading to non-diffusive radon emissions could take 

place in the top 1 m of water cover within tailings impoundments. In their analysis, they divided 

radon releases into three components:

radon originating from tailings covered with < 1 m of water;

radon originating from tailings covered with > 1 m of water; and

radon originating from dissolved radium in the pond water.

In this approach used by Nielson and Rogers, the emission of radon from shallow waters is 

controlled by the emanation of radon and diffusion through the pore water. However, the radon 

emission from tailings covered with deep water (>1 m) is controlled by the diffusion of radon 

through the water column. As noted further below, the assumption of complete release in the top 

1 m is conservative in that the mixing advective layer would likely be less than 1 m deep. 

Regulatory Context

It is beyond the scope of this analysis to comment on the applicability of the Subpart W

NESHAP limits on radon emissions from operating mill tailings (EPA 1989), other than to note 

that the applicability of the rule to evaporation ponds has been questioned. For present purposes, 

to provide a context for the estimates of radon flux described below, a reference value of 

20 pCi m

-2

s

-1

has been assumed for uranium tailings impoundments. This flux rate, which 

represents the regulatory limit in Subpart W for tailings impoundments constructed prior to 

December 1989, was typical of tailings impoundments operating in the 1980s.
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 5

The baseline radon flux for the site represents a second point of reference. The Piñon Ridge site 

has been used historically for grazing cattle and has not been impacted by uranium mining or 

milling activities. Radon emission rates were measured at nine locations within the proposed 

tailings areas on three separate occasions (fall, spring, and summer). The radon-measuring 

canisters were analyzed using EPA Test Method 115, Monitoring for Radon-222 Emissions. The 

background radon flux rates ranged from 0.41 to 3.78 pCi m

-2

s

-1

and averaged 1.7 pCi m

-2

s

-1

(ERG 2009).

1.3 Approach to the Present Problem

In this study, we examined the effect of various wind speeds on the radon emission rates from

the evaporation ponds based on the Nielson and Rogers model. In order to improve performance 

of the evaporation pond system (i.e., enhance the evaporative capabilities), the design of the 

ponds includes implementation of a sprinkler system. The sprinklers will be placed and sized to 

maximize evaporation and minimize the potential for wind drift beyond the extent of the lined 

evaporation pond area. In this assessment, the emission of radon from sprinkler systems was 

also estimated.

While the Nielson and Rogers model can theoretically be used for estimation of radon emissions

from the evaporation ponds, the following potential limitations should be noted. The tailings are 

relatively thick and a significant radon concentration gradient may be developed across the 

thickness of the tailings due to diffusional movement of radon towards the solid-liquid interface. 

In the evaporation ponds, however, the precipitate layer may be much thinner and the 

concentration of radon in the pore water can be assumed to be uniform across the thickness of 

the precipitate layer.

The Nielson and Rogers model provides a conservative estimate of radon emissions based on the 

assumption of complete mixing in the top 1 m layer of the water covering the tailings. Nielson 

and Rogers based their model on observed wave action at the air-water interface. On a smaller 

scale, such as over evaporation ponds, the magnitude of the wave dimensions may not be large 

enough to induce complete mixing in the top 1 m of the water column. 

2.0 RADON EMISSION MECHANISMS

2.1 Overview of Radon Emanation Mechanisms

Radon occurs in raffinate and precipitates in the evaporation ponds naturally as the decay 

product of the dissolved radium in pond water and radium in the precipitates. Figure 1 shows 

various mechanisms involved in the release of radon from the evaporation ponds.
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 6

Figure 1: Mechanisms Involved in the Release of Radon from the Evaporation Ponds
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30 August 2010 

Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 7 

Radon produced in water diffuses toward the direction of its decreasing concentration in water 

(in most cases toward the air-water interface where it is released to the ambient air). In stagnant 

water columns, the diffusion of radon can be described by Fick's Law, which states that the flux 

density of the diffusing radon is linearly proportional to its concentration gradient and its 

diffusion coefficient in water. In water columns, the diffusion of radon is enhanced by various 

mechanisms such as natural convection in the water column and wind action. As a result, the 

Fick’s Law is expressed in term of effective diffusion coefficient. Typically, the gas transport 

across the air-water interface is expressed using the overall mass transfer coefficient. This 

coefficient is very sensitive to the thickness of the boundary layers in both sides of the interface 

and the wind speed over the water surface. 

In addition to the above transfer mechanisms, the transport of radon produced inside the solid 

particles is also influenced by the diffusion of radon within the solid particle. After being 

generated, the radon atoms tend to move away from their original location toward the pore 

spaces in the medium. Consequently, depending on their original location within the solid phase, 

the pore distribution, and the moisture content of the solid particles, the newly created radon 

atoms may end up within the same solid particle in which they were created, or within the pore 

of the medium. Table 1 shows nominal diffusion coefficients for radon in water and in air as 

reported by Drago (1998). 

Table 1: Diffusion Coefficient of Radon in Water and Air at 20°C 

Source: Drago (1998) 

Medium Value Unit 

water 1.2x10-5 cm

2

/s 

air 0.12 cm2/s 

Since the diffusion coefficient of radon in air is approximately 10,000 times larger than its 

diffusion coefficient in water, the migration of radon in saturated solids is much different than its 

migration in unsaturated solids. The fraction of the total amount of radon produced by radium 

decay that escapes from the solids particles and gets into the pores of the medium is referred to 

as the radon emanation coefficient or emanation fraction (often written as E). The radon 

emanation coefficient is strongly influenced by the moisture content of the medium, particularly 

within the range of low water saturation. 

A clear change in trend of the data, separating these two regimes, occurs at u=3 m/s as shown in 

Figure 2. Ocampo-Torres et al (1994) note that this critical wind speed corresponds to the lowest 

value of u at which waves are observed. In another study, Kanwisher (1963) observed a sudden 

increase in the rate of CO2 outgassing in a wind/wave tunnel, at a wind velocity of about 3 m/s 

where the waves begin to emerge. It is suggested that the random surface drift velocities 

observed at the surface of water may act to generate some degree of bulk mixing. 
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 8

Figure 2: Typical Plot of Mass Transfer Coefficient K versus Wind Speed in a Wind/Wave 

Tunnel. Source: Saylor and Handler (1997)

The wave dimensions versus wind speed depend on the geometry and size of the ponds and 

could not be easily established for this study. However based on the information available from 

the pond design documents regarding the depth of the freeboard (1 ft), it is expected that the 

maximum wave depth would be less than 1 ft for the maximum wind speed (18.1 miles/hour or 

8.1 m/s) for the site1. The average wind speed for the site is considerably less at 6.2 miles/hour 

or 2.8 m/s. According to the above observations, no significant wave action is expected to occur 

at the surface of the ponds at the average wind speed.

3.0 PHYSICAL PARAMETERS

3.1 Description of Evaporation Ponds 

The design flow rates of raffinate associated with the start-up (500 ton per day) and ultimate 

production rates (1,000 ton per day) are 63 and 126 gallons per minute (gpm), respectively. The 

average volumetric flow rate to the evaporation ponds for the 1,000 tpd scenario is somewhat 

less at 117 gpm (7.4 L/s) (Golder 2008). The evaporation pond system is designed for 

construction in two phases. Phase I includes 10 ponds (or cells), each with a surface dimension 

of 300 feet by 600 feet (i.e., 91 m by 182 m), designed to evaporate the inflows associated with 

the 500 tpd production schedule. Similarly, Phase II includes an additional 10 ponds with the 

 

1 Maximum average daily wind speed recorded over 730-day period. From Pinon Ridge Meteorological Data Base 

maintained by Energy Fuels Resources Corporation (2010).
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 9

same dimensions designed to evaporate the flows associated with the 1,000 tpd production 

schedule Golder 2008). 

Both phases of construction are designed with an additional one foot of freeboard (above the 

required design capacities). The water depth in each pond will be similar, maximizing the 

evaporative surface area. In order to improve performance of the evaporation pond system (i.e., 

enhance the evaporative capabilities), the design includes implementation of a sprinkler system. 

The sprinklers will be placed and sized to maximize evaporation and minimize the potential for 

wind-drift beyond the extents of the lined evaporation pond area.

3.2 Parameters Used for the Modeling

Table 2 shows the physical parameters used for the current evaluation. These parameters were 

compiled from data provided by Energy Fuels based on their current designs for the evaporation 

ponds (Golder 2008). In addition, Energy Fuels contracted with J.E. Litz and Associates to 

undertake bench-scale studies to characterize the raffinate (Energy Fuels 2010). Measurement 

data on radium concentrations in precipitate and pond water were available from the studies. 

Relative to the pond water, the measured radium concentrations ranged from 59 to 600 pCi/L, 

with an average of 241 pCi/L. For this analysis, the maximum value of 600 pCi/L was 

conservatively assumed.

Table 2: Physical Parameters Used for the Current Evaluation

Parameter Value Unit Reference

Concentration of radium in 

precipitates 7.9 pCi/g Energy Fuels (2010)

Concentration of radium in pond 

water 600 pCi/L Energy Fuels (2010)

Ambient air radon concentration 270 pCi/m3 Estimate*

Bulk density of precipitates 2 g/cm3 Estimate

Radon emanation coefficient 0.35 - Nielson and Rogers 1986

Radon decay constant 2.1x10-61/s Nielson and Rogers 1986

Radon diffusion coefficient in water 1.2x10-5cm2/s Drago 1998

Radon diffusion coefficient in air 0.12 cm

2

/s Drago 1998

Effective diffusion coefficient of 

radon in deep water** 0.003 cm2

/s Nielson and Rogers 1986

Radon Henry’s Constant 4.08 dimensionless Drago 1998

Pond width 91 m Golder 2008

Pond length 182 m Golder 2008

Number of ponds 10 (P I) and 20 (PII) - Golder 2008

Total evaporation rate 117 GPM Raffinate flow rate, 

Golder 2008

Average evaporation rate 4.4x10-5 L m-2s

-1 Estimated based on data 

in Golder 2008

* Based on a generic value in the order of 10 Bq/m3for background radon (UNSCEAR 2009, NCRP 2009).

** Effective diffusion coefficient incorporates the effect of natural convection and other mixing in water 

column on the diffusional transport of radon in water column toward the surface.
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 10

4.0 ESTIMATION OF THE RADON FLUX 

4.1 Diffusive and Advective Radon Emission

Estimates based on Nielson and Rogers

The information in previous sections was used in the Nielson and Rogers (1986) model as 

implemented by SENES to estimate the radon emissions from the ponds at high surface 

turbulence conditions. This model considers both diffusion and turbulence at the air-surface 

interface by assuming that the radon in the top 1-m layer of water is released to air instantly.

Table 3 shows the results of the emission estimation for two depth scenarios. The input and 

output values, as well as the equations used for calculations, are provided in Appendix A.

Table 3 summarizes the results (Appendix A) of the emission estimates for two water cover 

depth scenarios. For comparison purposes, the same parameters shown in Table 2 were used to 

estimate the radon emissions using the on-line program that is available from the World 

Information Services on Energy (WISE) website, also attributed to the Nielson and Rogers 

(1986) model. It is not known if or to what degree the on-line program has been independently 

verified; however, the program produced the same results as generated by the model and 

scenarios used in this assessment (see http://www.wise-uranium.org/ctb.html). [Appendix B 

provides details on the parameter values used in the calculations with the on-line model.] It 

should be noted that unlike the edges of tailings impoundments, the precipitates in the 

evaporation ponds would be covered by water (i.e., submerged) at all times.

Table 3: The Results of the Emission Estimation for Two Depth Scenarios

Depth Scenario Radon Flux

(pCi m

-2

s

-1

)

Total Radon 

Emission (Phase I) 

(pCi/s)

Total Radon 

Emission (Phase II) 

(pCi/s)

Water cover less than 1 m deep 0.91 1.52x1053.04x105

Water cover 3 m deep 1.27 2.12x1054.24x105

The results of calculations indicate that, for the current situation, as the depth of water increases, 

the radon emissions increase. According to the Neilson and Rogers model (1986), this is 

because the radium concentration in the water column becomes a major contributor of the total 

radon flux from the ponds.

As mentioned before, the results shown in Table 3 (based on the Neilson and Rogers model) 

represent highly turbulent surface conditions on the ponds. This will provide a conservative 

estimate of the emissions as in the majority of time, the surface of the ponds will be relatively 

calm and free of significant waves.
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 11

Effect of Wind speed

As shown on Figure 2 in Section 2.2, the mass transfer coefficient, K, is small and only weakly 

dependent on wind speed, u, when u is below 3 m/s. K is much larger and more sensitive to u at 

higher wind speeds.

Considering that the transfer rate at the air-water interface has a linear relationship with the mass 

transfer coefficient, the emission rate is expected to be much less at average wind speeds

(6.2 miles/hour or 2.8 m/s) compared with the emission rates at the maximum average wind 

speed (18.1 miles/hour or 8.1 m/s) where waves of less than 1 ft (0.3 m) are expected to form at 

the surface of the ponds. Table 4 shows the effect of various wind speeds on radon emissions

from the ponds. [The mass transfer coefficients (K) in Table 4 were estimated from Figure 2.] 

Appendix A provides additional details of the radon flux calculations.

Table 4: The Effect of Wind Speed on the Radon Flux*

Wind Speed, m/s

Mass Transfer 

Coefficient (K) 

(cm/s)

Radon Flux (pCi m

-2 s-1

)

Water Depth: 3 m Water Depth: 1 m

8.1 0.0055 1.27 0.91

6.0 0.003 0.69 0.50

4.0 0.002 0.46 0.33

2.8 0.0014 0.32 0.23

2.4 0.001 0.23 0.17

*Sample calculation (3 m depth): 

Maximum flux at maximum wind speed (8.1 m/s) = 1.27 pCi m

-2

s

-1

(at 3 m depth from Table 3)

Mass transfer coefficient at maximum wind speed = 0.0055 cm/s (Figure 2)

Mass transfer coefficient at 2.8 m/s wind speed = 0.0014 cm/s (estimated from Figure 2)

Radon flux at 2.8 m/s wind speed = 1.27 pCi m

-2

s

-1

* 0.0014 / 0.0055 = 0.32 pCi m

-2

s

-1

(assuming 

linearity)

4.2 Evaporative Radon Emission

Radon produced in the water column could be released to ambient air via evaporation. It is 

expected that the entire radon content dissolved in the portion of the water evaporated is 

released. As a thin film at the interface is being evaporated all the time, the concentration of 

radon in the water right at the interface can be estimated. This was done using the Henry’s Law 

constant for radon and the concentration of radon in ambient air provided in Table 2. The 

concentration of radon at the interface was estimated at 1.1 pCi/L as shown below:

Ambient air radon concentration = 270 pCi/m3(Table 2)

Henry’s constant for radon = 4.08 (Table 2)

Water activity concentration = Henry’s constant * Air activity concentration = 

4.08 * 270 pCi/m3= 1102 pCi/m3= 1.1 pCi/L
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 12

The evaporation rate of water was estimated using the average flow of raffinate to the ponds 

(117 gpm or 7.4 L/s) over the total surface area of the ponds (1.67x105 m

2

) which is

4.4x10-5L m

-2

s

-1

:

Evaporation rate = 7.4 L/s / (1.67x105 m

2

) = 4.4x10-5L m-2s

-1

. 

The radon release was estimated as follows:

Radon release = Interface water concentration * Evaporation rate

= 1.1 pCi/L * 4.4x10-5L/m2/s = 4.9x10-5pCi m

-2

s

-1

 

This value is extremely insignificant compared to the diffusional release of radon.

4.3 Radon Emission from the sprinklers.

Rost (1981) demonstrated the ability of spray aeration to remove radon from well water at 

private homes in Maine. One-stage aeration system achieved 75.7% radon removal efficiency. It 

was assumed that the rate of removal of radon from sprinkler systems is similar to the removal 

rate of radon from spray aeration system used by Rost (1981).

The radium concentration in raffinate, the raffinate average flow rate, and the removal efficiency 

of sprinkler were used to estimate the radon release from the sprinkler systems. It was assumed 

that the sprayed water is in contact with air for 10 seconds and sprinklers are one-stage systems 

with the removal efficiency of 75%.

Evaporation rate = 117 gpm = 7.4 L/s

Radium concentration in water = 600 pCi/L (Table 2)

Total radium loading in 10 seconds = 7.4 L/s * 10 s * 600 pCi/L = 44,400 pCi

Radon production = Total radium * Radon decay constant = 

44,400 pCi * 2.1x10-6s

-1

= 0.0.093 pCi/s

Radon release (75% efficiency) = 0.093 pCi/s * 0.75 = 0.07 pCi/s

The calculations indicate that the total radon emission is approximately 0.07 pCi/s for the 

sprinkler system. This value is extremely insignificant compared to the estimated value based on 

the Neilson and Rogers model (diffusion and wave action from the evaporation ponds).

5.0 ESTIMATION OF THE AIR CONCENTRATION

The method used to calculate the concentration of radon in the atmosphere over the evaporation 

ponds was based on the box model developed by Schiager (1974) to calculate the atmospheric 

radon concentrations near the uranium mill tailings piles. Assuming that 5 ponds were placed 

end to end, the following equation was used for calculations (Schiager 1974):

CRn = X/u z
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 13

where:

CRn = radon concentration in atmosphere (pCi/m3) 

= radon emission flux (0.32 pCi m

-2

s

-1

for the average wind speed of 2.8 m/s)

X = maximum length of the evaporation ponds in the direction of the wind (5 ponds 

* 182 = 910 m)

u = average wind speed (2.8 m/s)

z = vertical dispersion coefficient (14 m from Figure 3 based on a conservative 

assumption of F stability class, i.e. the least atmospheric dispersion)

The resulting radon concentration is:

CRn = 0.32 pCi m

-2

s

-1

* 910 m / (2.8 m/s * 14 m) = 7.4 pCi/m

3

Figure 3: Vertical Dispersion Coefficient by Atmospheric Stability Class (Schiager 1974)
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 14

The above estimate of 7.4 pCi/m3is the incremental (above background) air concentration due to 

the emission of radon from the evaporation ponds. This value is very small (3%) compared to 

the assumed background atmospheric radon concentration of 270 pCi/m

3

(Table 2).

6.0 DISCUSSION AND CONCLUSION

Conservative estimates of radon flux indicates that the emissions are low and less than or similar 

to the pre-operational average background radon flux of 1.7 pCi m

-2

s

-1

observed at various 

locations within the proposed tailings areas on the site. The estimated radon flux levels from the 

evaporation ponds is also a small fraction (less than 10%) of the 20 pCi m

-2

s

-1

limit for pre-1989 

uranium tailings that has been assumed here for context. This conservative estimate was based 

on the Nielson and Rogers model. The model assumes that the emission rates are enhanced by 

the turbulence at the top layer of the water column where all the radon in the top one-meter of 

water is assumed to be released to air instantaneously. For comparison purposes, the same 

parameters were used to estimate the radon emissions using an on-line program that is available 

on the World Information Services on Energy (WISE) website. The on-line model, which is

attributed to the Rogers and Nielson model, produced identical results.

The results of this assessment also indicated that the radon emissions associated with the 

evaporation of the raffinate solution and the emissions due to the operation of sprinkler system 

are extremely low and insignificant compared to the radon flux from the ponds due to diffusional 

and turbulence processes.

Finally, the calculations indicated that the incremental air concentration due to the emission of 

radon from the evaporation ponds is very small (on the order of 3%) relative to the assumed 

background radon concentration.

7.0 REFERENCES

Baker, K.R. and A.D. Cox 2010. Radon Flux from Evaporation Ponds. Presented at National 

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

Workshop 2010, Denver, CO, May 26-27.

Chambers, D.B. 2009. Radon Emissions from Tailings Ponds. Presented at National Mining 

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

Workshop 2009, Denver, CO, July 2.

Drago, J. A. 1998. Critical Assessment of Radon Removal Systems for Drinking Water Supplied, 

Chapter 3, Table 3.2. American Water Work Association.

Energy Fuels Resources Corporation (2010). Raffinate Characterization, Piñon Ridge Mill,

Montrose County, Colorado. Prepared for U.S. Environmental Protection Agency, 

Region 8, August.
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 15

Environmental Protection Agency (EPA) 1989. Subpart W – National Emission Standards for 

Radon Emissions from Operating Mill Tailings. 40 CFR Part 61, Federal Register, 

15 December.

Environmental Protection Agency (EPA) 1986. Final Rule for Radon-222 Emissions from 

Licensed Uranium Mill Tailings – Background Information Document. 

EPA 520/1-86-009, August.

Environmental Restoration Group, Inc (ERG) 2009. Baseline Investigation Radiological Report. 

Rev. 1. Submitted to Radiation Management Unit, Colorado Department of Public Health 

and Environment, 5 October.

Golder Associates Inc. 2008. Evaporation Pond Design Report Piñon Ridge Project Montrose 

County, Colorado. Report prepared for Energy Fuels Resources Corporation, Lakewood 

Colorado.

Kanwisher, J. 1963. On the Exchange of Gases between the Atmosphere and the Sea. Deep-Sea 

Res. 10, 195, 1963.

National Council on Radiation Protection and Measurements (NCRP) 2009. Ionizing Radiation 

Exposure of the Population of the United States. NCRP Report No. 160.

Nielson, K.K. and V.C. Rogers 1986. Surface Water Hydrology Considerations in Predicting 

Radon Releases from Water-Covered Areas of Uranium Tailings Ponds. Proc. Eighth 

Annual Symposium on Geotechnical & Hydrological Aspects of Waste Management, 

Geotechnical Engineering Program, Colorado State University & A.A. Balkema, Fort 

Collins, CO, USA, February 507, PP:215-222.

Nuclear Regulatory Commission (NRC) 1989. Calculation of Radon Flux Attenuation by 

Earthen Uranium Mill Tailings Covers. Regulatory Guide 3.64, June.

Ocampo-Torres, F. J., and M. A. Donelan, N. Merzi, and F. Jia 1994. Laboratory Measurements 

of Mass Transfer of Carbon Dioxide and Water Vapour for Smooth and Rough Flow 

Conditions. Tellus 46B, A Journal Published by the International Meteorological 

Institute in Stockholm.

Rogers, V.C., K.K. Nielson and D.R. Kalkwarf 1984. Radon Attenuation Handbook for 

Uranium Mill Tailings Cover Design. Prepared for the U.S. Nuclear Regulatory 

Commission, NUREG/CR-3533. April.

Saylora, J. R., and R. A. Handler 1997. Gas Transport Across an Air/Water Interface Populated 

with Capillary Waves. Phys. Fluids 9 (9). 
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 16

Schiager, K. J., 1974. Analysis of Radiation Exposures on or near Uranium Mill Tailings Piles. 

Radiation Data and Reports 15(7): 411-425. Publisher: U. S. Environmental Protection 

Agency, Office of Radiation Programs, Washington D.C.

Shwarzenbach, R.P., P.M. Gshwend and D.M. Imboden 2002. Environmental Organic 

Chemistry, Second edition, Wiley-Interscience. Published by John Wiley & Sons, Inc., 

Hoboken, NJ.

United Nations Scientific Committee on the Effects of Atomic Radiation 2009. Effects of 

Ionizing Radiation. UNSCEAR 2006 Report to the General Assembly with Scientific 

Annexes. Volume II, Annexes C, D and E. Annex E: Sources-to-Effects Assessment for 

Radon in Homes and Workplaces.
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Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 17

APPENDIX A: INPUT PARAMETERS FOR NIELSON AND ROGERS MODEL

The input parameters for Nielson and Rogers (1986) model for a 3 m deep water column:

Ra tailings Ra content 7.9 pCi/g

Raw Raffinate Ra content 0.6 pCi/cm3

dens bulk density of tailings 2 g/cm3

E Rn emanation coeff 0.35 - 

lambda Rn decay constant 2.1E-06 1/s

D diff. coeff. in tailings 1.2E-05 cm2/s

Fs fraction of pond with < 1 m depth 0 - 

Dtr eff. stagnant water transport coeff 0.003 cm2/s

Xp avg. pond depth for areas > 1 m depth 250 cm

The output from Nielson and Rogers (1986) model for 3 m deep water column:

At attenuation factor for deep water 1.89E-02

J Radon flux 1.27E+00 pCi m-2 s

-1

The input parameters for Nielson and Rogers (1986) model for a 1 m deep water column:

Ra tailings Ra content 7.9 pCi/g

Raw Raffinate Ra content 0.6 pCi/cm3

dens bulk density of tailings 2 g/cm3

E Rn emanation coeff 0.35 - 

lambda Rn decay constant 2.1E-06 1/s

D diff. coeff. in tailings 1.2E-05 cm2/s

Fs fraction of pond with < 1 m depth 1 - 

Dtr eff. stagnant water transport coeff 0.003 cm2/s

Xp avg. pond depth for areas > 1 m depth Not Applicable cm

The output from the Nielson and Rogers (1986) model for a 1 m deep water column:

At attenuation factor for deep water 1.89E-02

J Radon flux 9.08E-01 pCi m-2 s

-1
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30 August 2010

Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 18

The following equation was used to calculate the attenuation factor (At):

At = exp [-sqrt ( /Dtr) * (Xp -100)] (Nielson and Rogers 1986)

where:

= radon decay constant (2.1x10-6s

-1

)

Dtr = effective diffusion coefficient in water column (cm2/s)

Xp = average pond depth for areas greater than 1 meter deep (cm)

The following equation was used to calculate the radon flux (J) from the evaporation ponds:

J = 104 R E * sqrt( D)[fS + (1-fS)*At] + 106 Rw Sd(1-0.5fS) (Nielson and Rogers 1986)

where:

J = radon flux from the exposed pond surface (pCi m-2s

-1

)

R = solids radium content (pCi/g)

Rw = water radium content (pCi/cm3)

= bulk solid density (g/cm3)

E = radon emanation coefficient for solids (dimensionless)

D = radon diffusion coefficient in pore water (cm2/s)

fS = fraction of pond area with less than 1 m deep

Sd = depth of surface layer from which all radon is assumed to be released = 1 m

The 104factor converts the flux units from pCi cm

-2

s

-1

to pCi m-2s

-1

. The 106factor converts 

the water radium content (Rw) from units of pCi/cm3to pCi/m3. For consistency of units, the Sd

parameter, not explicitly shown in Nielson and Rogers (1986), was included here.
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30 August 2010

Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 19

APPENDIX B: PARAMETER VALUES USED IN ON-LINE MODEL

Input Parameters: 3 m deep water column

Parameter Parameter Values Used

in On-Line Model Units

Ra-226 Activity Concentration in tailings 7.9 pCi/g

Ra-226 Activity Ratio in slimes vs. sand 4 (default value) -

Rn-222 Emanation Fraction in slimes 0.35 -

Rn-222 Emanation Fraction in sand 0.15 (default value)

Fraction Passing #200 Mesh (75 μm) 0.4 (default value) -

Fraction of pond area with less than 1 m depth 0

Average pond depth for areas greater than 1 m 

deep: 2.5 m

Ra-226 Activity Concentration in ponding water 600 pCi/L

Ratio of radium in solution to radium in tailings 

solids - g/cm3

Effective stagnant water transport coefficient 3E-7 m

2

/s

Tailings Zone Submerged Saturated Unsaturated

Surface Area [m2] 1.0E5 - -

Bulk Density [g/cm3] 2.0 - -

Porosity 0.41(default value) - -

Moisture Contents

[dry wt %] (Saturation) - -

Fraction Passing #200

Mesh (75 μm) 0.5 (default value) - -

Rn-222 Eff. Diffusion

Coefficient [m2/s] 1.2E-9 - -

Output: Radon flux = 1.27 pCi m

-2

s

-1
370101-4

30 August 2010

Letter to Mr. F. Filas - Radon Flux from Surface of Evaporation Ponds (Continued) Page 20

Input parameters: 1m deep water column

Parameter Parameter Values Used

in On-Line Model Units

Ra-226 Activity Concentration in tailings 7.9 pCi/g

Ra-226 Activity Ratio in slimes vs. sand 4 (default value) -

Rn-222 Emanation Fraction in slimes 0.35 -

Rn-222 Emanation Fraction in sand 0.15 (default value)

Fraction Passing #200 Mesh (75 μm) 0.4 (default value) -

Fraction of pond area with less than 1 m depth 1

Average pond depth for areas greater than 1 m 

deep: Not applicable m

Ra-226 Activity Concentration in ponding water 600 pCi/L

Ratio of radium in solution to radium in tailings 

solids - g/cm3

Effective stagnant water transport coefficient 3E-7 m

2

/s

Tailings Zone Submerged Saturated Unsaturated

Surface Area [m2] 1.0E5 - -

Bulk Density [g/cm3] 2.0 - -

Porosity 0.41(default value) - -

Moisture Contents

[dry wt %] (Saturation) - -

Fraction Passing #200

Mesh (75 μm) 0.5 (default value) - -

Rn-222 Eff. Diffusion

Coefficient [m2/s] 1.2E-9 - -

Output: Radon flux = 0.91 pCi m

-2

s

-1