Design of rock covers for reclaimed uranium mill tailings impoundments: A regulatory perspective

Public Law 95-604, the Uranium Mill Tailings Radiation Control Act of 1978, provides the Department of Energy with authority to perform remedial actions at designated inactive uranium mill sites. The Environmental Protection Agency promulgated radiological and non-radiological standards (40 CFR 192) for remedial actions at inactive uranium mill sites. All remedial actions require the concurrence of the Nuclear Regulatory Commission (NRC). The standards set forth in 40 CFR 192 require that remedial action designs provide reasonable assurance that tailings will be controlled for 1000 years (to the extent reasonably achievable) and, in any case, for at least 200 years. Implementation of this criteria requires the use of sound engineering practice combined with engineering judgment in many analytical areas. It has been the experience of the NRC staff that the design of a rock cover can be significantly affected by the selection of the design flood or design precipitation event. Preliminary reviews conducted by the NRC staff for various remedial action sites have indicated that a range of design assumptions and strategies are used in the determination of design basis floods and precipitation events for rock cover designs. The NRC staff has concluded that EPA criteria are met if the designs for erosion protection covers are based on the concepts of the Probable Maximum Flood (PMF) and the Probable Maximum Precipitaton (PMP), and that EPA criteria may not be met by the use of statistically-derived flood estimates. In addition, dam failures need to be analyzed at those sites where upstream dams are not designed for a PMF