Final Report 

REPORT ON THE REVIEW OF METHOD 115 

TO MONITOR RADON EMISSIONS 

FROM URANIUM TAILINGS 

 

 Prepared by 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

Vienna, VA 22182 

 Under 

Contract Number EP-D-05-002 

Work Assignment No. 4-11, Task 6 

 

Prepared for 

U.S. Environmental Protection Agency 

Office of Radiation and Indoor Air 

1200 Pennsylvania Avenue, N.W. 

Washington, DC 20460 

Reid J. Rosnick 

Work Assignment Manager 

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

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

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

following individuals: 

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

 Harry Pettengill 

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

 Abe Zeitoun 

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

 Gregory Beronja 

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

 Stephen Ostrow 

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

1.0 Introduction......................................................................................................................... 1 

2.0 Objectives ........................................................................................................................... 1 

3.0 Measurement Frequency..................................................................................................... 2 

4.0 Conclusion .......................................................................................................................... 6 

5.0 References........................................................................................................................... 7 

WA 4-11, Task 6 – Method 115 Compliance ii SC&A – September 25, 2008
1.0 INTRODUCTION 

EPA promulgated its NESHAP for emissions from uranium tailings in 1989. Section 112(q) of 

the Clean Air Act (CAA), as amended, requires the EPA to review, and if appropriate, revise or 

update the Subpart W standard on a timely basis (10-year interval). EPA has determined that it 

is reasonable and appropriate to revisit this standard at this time, especially in view of renewed 

interest in uranium exploration and recovery. 

Currently, compliance with the existing emission standards for uranium tailings is achieved 

through the use of Method 115, as prescribed in Appendix B of 40 CFR 61.253. 

 

2.0 OBJECTIVES 

The purpose of this report is to evaluate whether Method 115 is still current and applicable to 

monitor radon emissions from uranium tailings. Secondarily, it evaluates what other methods 

using current technology may be employed to meet the emissions standard in lieu of dependence 

upon Method 115. 

Furthermore, this report reviews Method 115 as it was first designed and includes consideration 

of those variations in detection methodologies that could be employed to meet the requirements 

of Subpart W. 

The review also considers various aspects of Subpart W in the context of what is known, or has 

been learned since its promulgation, regarding radon diffusion through uranium tailings or 

improvements in radon detection capability. 

Subpart W of 40 CFR 61, National Emission Standards for Radon Emissions from Operating 

Mill Tailings, is basically a three-part standard (EPA 1991), summarized as follows: 

(1) Radon-222 emissions from an existing uranium mill tailings pile must not exceed 

20 pCi/m2

-sec (implicitly this includes background) 

(2) Tailings piles after December 15, 1989, have to be designed, constructed, and operated to 

meet one of two work practices: 

- Phased disposal in lined tailings impoundments that have an area of no more than 

40 acres with no more than two impoundments in operation (open) at any one time 

- Continuous disposal of tailings requires that they are dewatered and immediately 

disposed of with no more than 10 acres being uncovered at any time 

 

(3) All mill owners and operators must comply with 40 CFR 192.32(a) in the operation of 

tailing piles (design of impoundments) 

The above performance standards depend in part on certain definitions, such as dewatering, 

which is defined as removing moisture until the water content is less than 30% by weight. 

This definition can invoke other parameters, which can further influence radon flux from the 

tailings. For example, the radon flux from tailings with 6% moisture is actually 3.5 times 

WA 4-11, Task 6 – Method 115 Compliance 1 SC&A – September 25, 2008
that from tailings with 0.2% moisture (typical of the southwest) and slowly increases to 

saturation (NRC 1983). Thus, no matter whether the moisture content is 6%, 30%, or even 

60%, the radon flux will be about the same (NRC 1983). The 30% moisture definition 

appears somewhat arbitrary and does not really affect radon flux reduction considerations. 

Historically, compliance with the radon flux standard was met through the use of Method 

115 of Appendix B, “Test Methods:” 

§ 61.253 Determining compliance 

Compliance with the emission standard in this subpart shall be determined 

annually through the use of Method 115 of Appendix B. When measurements are 

to be made over a one year period, EPA shall be provided with a schedule of the 

measurement frequency to be used.

Method 115 in Appendix B, entitled “Monitoring for Radon-222 Emissions,” consists of 

numerous sections that discuss the frequency, distribution, number of measurements, and quality 

assurance from three types of uranium mining or recovery operations: 

(1) Radon-222 Emissions from Underground Uranium Mine Vents 

(2) Radon-222 Emissions from Uranium Mill Tailings Piles 

(3) Radon-222 Emissions from Phosphogypsum Stacks. 

The most important elements are measurement frequency and measurement methodologies 

employed to demonstrate compliance. 

3.0 MEASUREMENT FREQUENCY 

These were assumed based on reviews of historic tailings piles constructed prior to the standard. 

In other words, peripheral discharges of tailings from one or more spigots would have 

eventually resulted in sands depositing close to the spigot (the dam or beach area) and slimes 

(material less than 200 mesh) depositing further downgrade forming a “pond.” According to 

Sears et al , the slimes typically contained up to 80% of the radioactivity (ORNL 1975) and were 

often allowed to dry naturally before being covered or mixed with the sands. The size of these 

piles varied considerably, but 100 acres was common [NRC used 124 acres (50 hectares) in 

Regulatory Guide 3.59 (NRC 1985)]. On the other hand, the ORNL (1975) report used 100 

acres in the adopted model mills. 

Section 2.1.3 of Method 115 requires the following measurements: 

(1) Water covered area – no measurements required, as radon flux is assumed to be zero 

(2) Water saturated beaches – 100 radon flux measurements 

(3) Loose and dry top surface – 100 radon flux measurements 

(4) Sides – 100 radon flux measurements, except where earthen material is used in dam 

construction 

WA 4-11, Task 6 – Method 115 Compliance 2 SC&A – September 25, 2008
Thus a minimum of 300 radon flux measurements are required. 

The question then becomes whether this required frequency (300 measurements) is necessary 

under the new standard, which is requiring much smaller ponds. For example, under the new 

standard, particularly continuous disposal with only 10 acres uncovered at one time, a possible 

configuration is a reclaimed area and a 10-acre uncovered area with a water saturated beach, and 

dry top surface and sides. The 300 flux measurements on a 10-acre area translate to one 

measurement every 1,500 square ft or one every 40 ft. According to NUREG/CR-3166 (NRC 

1983), the number of locations a parameter must be measured to determine its average value 

within a precision of 25% at the 95% confidence level is given by the following: 

Number = 45 (coefficient of variation)1

Measurements of the coefficient of variation on covered and uncovered tailings piles results in a 

range for the Number of 25 to 32 radon flux measurements for older ponds (NRC 1983). The 

recommended minimum number is 30 radon flux measurements for these older and much larger 

tailings areas (typically 100 acres or more) (NRC 1983). This is an order of magnitude less than 

the strict interpretation of the current requirements (requiring 300 measurements). It is 

recommended that the basis for the number of measurements required in Method 115 be 

provided and perhaps revised. In addition, the definition of water saturated beach and dry top 

surface should be provided in terms of percent (%) saturation. 

The measurement of radon flux has been reviewed and discussed by many authors since the early 

1960s (see, for example, Tanner 1964 and Tanner 1978). In the 1970s, the subject was further 

investigated as regulators began investigating uranium mining and milling (NRC 1983). In all 

these investigations, the basic technology remained the same. As the direct measurement of 

radon flux is extremely difficult and very likely could disturb the actual flux with the entry of 

measuring devices into the flux, an accumulation (can) method was used. Cans, drums, etc., 

with one end opened are turned over and the open end sealed into the ground to prevent leakage. 

Radon is allowed to accumulate into the can when, after a period of time, an aliquot is sampled 

and measured. From knowledge of the can volume, area of the face, time and measured radon 

concentration, the average radon flux over that time period can be determined. The sampling 

device can be charcoal in a canister, loose charcoal, a plastic chip (alpha track or track etch 

systems), or thermoluminescent dosimeters (TLDs). Equivalently if the accumulator can has a 

valve, a sample can be bled off and measured in a scintillation cell (Lucas cell), or directly 

measured by solid state devices or other “real time” detectors. The challenge is to determine the 

optimal measuring system. 

The device described in Method 115 is a Large-Area Activated Charcoal Canister (LAACC) 

discussed in Appendix A of Radon Flux Measurements on Gardinier and Royster 

Phosphogypsum Piles near Tampa and Mulberry, Florida (EPA 1985). The method is basically 

a large charcoal canister composed of a 10-in diameter PVC end cap with open end, retainer 

spring, 1⁄2-in thick scrubber pad, 1⁄2-in thick charcoal support grid, 1⁄2-in thick scrubber pad, 1-in 

 1

 The coefficient of variation (CV) or relative standard deviation (RSD) is a measure of precision calculated 

as the standard deviation value divided by the average of a set of values. For a population where σ is the standard 

deviation of a population and μ is the mean, the CV= σ/μ for a population. 

WA 4-11, Task 6 – Method 115 Compliance 3 SC&A – September 25, 2008
thick scrubber pad, 1⁄4-in hole (vent) in the top. A close examination of the appendix indicates a 

number of potential problems with the design, including the following: 

(1) The specifications of the scrubber materials are not defined. For example, is a scrubber 

pad made of steel wool? 

(2) There are two scrubber pads adjacent to each other. Are they made of different 

materials? 

(3) What type of charcoal was used and how was the system calibrated? 

The device is homemade and was developed and constructed by Pacific Northwest Laboratory 

[PNL, now known as Pacific National Northwest Laboratory (PNNL)] whose machine shops, 

etc., constructed 50 of these LAACC devices. Obviously, the resources of other groups may be 

much more limited than PNNL. 

Variations of the LAACC have also been used. One researcher used the basic technique of the 

LAACC, but instead of the loose charcoal, a commercial charcoal canister was secured inside the 

PVC holder (CoPhysics 2008). 

A paper by Andreas C. George presented at the Natural Radiation Environment Conference VIII 

in Buzios, Brazil (George 2007), and also published in the November 2007 Radon Bulletin of the 

CRCPD, is particularly relevant in terms of radon measuring devices: 

In the last 20 years, new instruments and methods were developed to measure 

radon by using grab, integrating, and continuous modes of sampling. The most 

common are scintillation cell monitors, activated carbon collectors, electrets, ion 

chambers, alpha track detectors, pulse and current ionization chambers, and 

solid state alpha detectors. 

The author breaks down radon detection into two large groupings: 

I. Passive integrating radon measurements

(1) Activated carbon collectors of the open face (OF) or diffusion barrier (DB) type. 

Charcoal canisters often employ a gamma spectrometer to count the radon daughters 

as surrogates (Bi-214, for example). Liquid scintillation vials also use alpha and beta 

counting. About 70% of radon measurements in the United States are canister type. 

(2) Electret ion chambers are being used for 2–7 days duration to measure the voltage 

reduction (drop). The voltage drop on the electrets is proportional to the radon 

concentration. About 10%–15% of radon measurements use this methodology. 

 

(3) Alpha track detectors are used for long-term measurements. Alphas from radon 

penetrate a plastic lattice, which is etched with acid, and the resulting tracks are 

counted. There is some use in the United States, but this is more popular in Europe. 

WA 4-11, Task 6 – Method 115 Compliance 4 SC&A – September 25, 2008
II. Passive or Active Continuous radon measurements 

(1) Scintillation cell monitors mostly include the flow through type 

(2) Current and pulse ionization chambers (mostly passive) 

(3) Solid State devices are either passive or active if they use a pump to move air 

through the sensitive volume of the monitor like the RAD 7, which uses a solid state 

alpha detector (passive implanted planar silicon (PIPS) detector) 

He further reviewed the groupings and compiled sensitivities of a number of monitors using data 

from manufacturers or cited from published literature. He presents his results in three extensive 

tables, which were modified and summarized below in Table 1. Table 1 presets the sensitivity 

range for various types of commercially available OF or DB charcoal canisters, followed by the 

range of sensitivities of various types of continuous radon monitors (CRMs) utilizing solid state 

detectors or pulse ionization chambers. The last group is the scintillation cells, which are 

typically cylinders coated with zinc sulphide or other scintillation materials that respond to the 

alpha decay of radon (also known as Lucas cells). In the first series, the exposed canisters are 

counted and the radon is determined from the concentration of the Bi-214 or other daughter 

surrogate. 

For the second group, the first set of sensitivities range from 0.18 to 0.36 cpm/4.0 pCi/L). These 

detectors run continuously, which compensates for their lesser sensitivity. Other solid state 

monitors use PIPS as the detector or ionization chambers with somewhat higher sensitivities. 

The last group includes the scintillation cells, which vary in size and sensitivity. 

Table 1. Sensitivity of Radon Monitoring Instruments 

Instrument Detection Principle Sensitivity Range cpm/(4 pCi/L) 

DB canister (various types) gamma 0.80 to 4.2 (range) 

OF canister gamma 250 

Db (2g) canister alpha 54 

Continuous radon monitor (CRM) Solid state 0.18–0.36 

Other CRM (e.g., PIPS) Solid state 2.80 

 Pulse or continuous ionization Pulse ion 1.2–3.0 

Pylon CRM others Scintillation Cell 5.7 to 24 (Lucas Cell or equivalent) 

The paper also presents the results from various track etch detectors and electret ion chambers 

with different units for the sensitivity. The sensitivity of various track etch units ranges from 

1.70 tracks per cm2/4 pCi/L-day to 23.4 tracks per cm2/4 pCi/L, while the sensitivity for the 

electrets are 8 volts per 4 pCi/L-day for short-term measurements to 0.7 volts per 4 pCi/L-day for 

long-term measurements. 

WA 4-11, Task 6 – Method 115 Compliance 5 SC&A – September 25, 2008
WA 4-11, Task 6 – Method 115 Compliance 6 SC&A – September 25, 2008

In summary, it appears that while no one type of detector is “better” than another type, it 

invariably depends more on the physical circumstances and the time required for the 

measurement. For radon fluxes over a few days, a series of charcoal canisters can be used or a 

series of accumulator cans could be deployed, with samples taken periodically over a few days to 

get an average. It is important to note that there is no inherent advantage to fabricating and using 

an LAACC device, except for looking at a larger area. The device was first designed to collect 

radon over a larger area, which, as mentioned, could be accomplished using a similar “shell” and 

a canister. Note that the amount of charcoal in the LAACC was 400 ml or, assuming a density of 

0.4 gm/cc, leads to a mass of 160 gm of activated charcoal. The EPA-recommended 4-in 

canister used in the home (the size of a tuna can) has a volume of about 66% of the LAACC and 

likely holds about half as much charcoal. The canisters, however, are easier to use and more 

standardized, and more importantly, they are commercially available. While these “off the shelf” 

devices could be used instead of the LAACC, they would have to be evaluated against a 

specification to assure accuracy, precision, and importantly reproducibility. 

It should be noted that the direct measurement of the radon flux from uranium tailings is not 

required by current guidance from the Nuclear Regulatory Commission (NRC). NRC 

requirements are presented in Regulatory Guide 4.14, Revision 1, Radiological Effluent and 

Environmental Monitoring at Uranium Mills, April 25, 1980 (NRC 1980) and 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 (NRC 2000). The regulatory guide calls for 

pre-operational radon flux measurements to establish background flux, but does not require 

operational flux measurements in this guide. This Regulatory Guide is expected to be updated 

by December 2009, as it is currently a phase 3 draft guide. The Standard Review Plan indirectly 

monitors radon emissions through calculations based on measurements or published values for 

the parameters governing radon diffusion, such as radium-226 concentration, soil type, diffusion 

coefficient moisture, and so on. 

4.0 CONCLUSION 

This review of Method 115 demonstrates that its use can still be considered current for use to 

monitor radon flux from uranium tailings. It is important to note, however, that the specific 

design protocols were developed with larger tailings impoundments in mind. Alternatively, 

numerous commercial enhancements to that design are widely available and in use today. Other 

forms of passive detectors, as well active measurement detectors, are also acceptable alternatives 

to demonstrate conformance with the standard. In addition, the method as currently written has a 

number of elements and requirements that should be reviewed and possibly revised, particularly 

the location and the frequency of measurement. These would be better based on statistical 

considerations or some other technical basis.
5.0 REFERENCES 

CoPhysics 2008. Private communication between Ted Rahon (CoPhysics Corporation) and Les 

Skoski (SC&A, Inc.) www.cophysics.com, July 2008. 

EPA 1985. “Radon flux Measurements on Gardinier and Royster Phosphogypsum Piles near 

Tampa and Mulberry Florida,” EPA 520/5-85-029, Hartley, J.N, and H.D. Freeman. September 

1985. 

EPA 1991. “Guidance on Implementing the Radionuclide NESHAPS,” U.S. Environmental 

Protection Agency. July 1991. 

George, A.C., 2007. “World History of Radon Research and Measurement from the Early 1900s 

to Today,” Natural Radiation Environment Conference VIII (NRE VIII) in Buzios, Brazil, 

reprinted in November 2007 Radon Bulletin of the CRCPD. 

NRC 1980. Regulatory Guide 4.14, Revision 1 “Radiological Effluent and Environmental 

Monitoring at Uranium Mills,” April 25, 1980. 

NRC 1983. “Recommended Procedures for Measuring Radon Fluxes from Disposal Sites of 

Residual Radioactive Materials,” NUREG/CR-3166, Young, Y.A., V.W. Thomas, and P.O. 

Jackson. March 1983. 

NRC 1985. “Methods for Estimating Radioactive and Toxic Airborne Source Terms for 

Uranium Milling Operations,” Regulatory Guide 3.59. March 1987. 

NRC 2000. “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,” NUREG-1620, May 2000. 

ORNL 1975. “Correlation of Radioactive Waste Treatment Costs and the Environmental Impact 

of Waste Effluents in the Nuclear Fuel Cycle for Use in Establishing ‘as Low as Practicable’ 

Guides-Milling of Uranium Ore,” ORNL-TM-4903 Volume 1, Sears, M.B., R.E. Blanco, R.C. 

Dahiman, G.S. Hill, A.D. Ryan, and J.P. Witherspoon. May 1975. 

Tanner, A.B., 1978. “Radon Migration in the Ground: A Supplementary Review,” Natural 

Radiation Environment III (NRE III). U.S. Department of Energy 51 Symposium Series. 

Tanner, A.B., 1964. “Radon Migration in the Ground: A Review” in The Natural Radiation 

Environment, William Marsh Rice University, Library Congress CCN 64-12256. 

WA 4-11, Task 6 – Method 115 Compliance 7 SC&A – September 25, 2008