Radiation Protection

Subpart W: National Emission Standards for Radon Emissions From Operating Mill Tailings

Rule Summary

Subpart W protects the public and the environment from the emission of radon-222 from uranium mills and their associated tailings. Since uranium ore generally contains less than one percent uranium, conventional uranium milling produces large quantities of tailings. These tailings are collected in impoundments that vary in size from 20 to 40 acres (“conventional impoundments” in the rule). The tailings contain radium and can emit radon gas.

Subpart W requires operating uranium recovery facilities to employ specific management practices or generally available control technology (GACT) to control radon emissions.  “Operating” means that an impoundment is being used for the continued placement of uranium byproduct material or tailings or is in standby status. In-situ leach (ISL) is now more common than conventional milling. ISL primarily produces liquid wastes, which can also emit radon.

Requirements for Conventional Impoundments

Impoundments built before December 15, 1989,  are subject to monitoring requirements and must meet a radon-222 emissions standard of 20 picocuries per square meter per second (pCi/m2-sec).

Impoundments built after December 15, 1989 must meet one of the two following work practice standards:

  • There are two impoundments in operation at any time (including existing impoundments), each of which cannot be larger than 40 acres. Tailings management and disposal is by phased disposal.
  • Tailings are immediately dewatered and disposed of with no more than 10 acres uncovered at any time. Operators must also follow applicable requirements in 40 CFR Part 192.

Requirements for Non-Conventional Impoundments

“Non-conventional” impoundments (commonly known as evaporation or holding ponds) hold uranium byproduct material or tailings in ponds suspended in or covered by liquids. Radon emissions from non-conventional impoundments must be controlled by keeping the solid uranium byproduct material or tailings in the ponds saturated with liquid at all times. No solid material may be visible above the liquid level.

Requirements for Heap Leach Piles

Some facilities may use an extraction method known as heap leaching. In some instances uranium ore is of such low grade, or the geology of the ore body is such that it is not cost effective to remove the uranium via conventional milling or through ISL. EPA is requiring heap leach piles that have completed processing but not entered closure to be managed such that there are no more than two such piles, with each no larger than 40 acres.

Construction Requirements for All Impoundments

Subpart W references other regulations that require impoundments to be designed, constructed and installed in a way that protects adjacent soils and waters. These requirements apply to all types of uranium recovery facilities.

Recordkeeping Requirements

Under the final regulations, uranium recovery facilities must maintain records to demonstrate compliance with requirements for impoundment construction and liquid coverage of ponds. Digital photographs are required to demonstrate liquid levels in non-conventional impoundments, and photographs are to be electronically submitted to EPA.

Rule History

In accordance with the Clean Air Act Amendments of 1990, EPA formed a workgroup to review the standards at Subpart W. EPA concluded that revisions were necessary.  On January 17, 2017, EPA issued revisions to Subpart W.

View supporting documents related to Subpart W Rulemaking Activity.

Additional Resources

View a fact sheet on the 2016 revisions to Subpart W(2 pp, 114 K, About PDF).


All facilities subject to any NESHAP must meet 40 CFR Part 61, Subpart A: General Provisions.