The U. S. Nuclear Regulatory Commission (NRC) is amending its regulations governing the disposal of high-level radioactive wastes in a potential geologic repository at Yucca Mountain, Nevada, to define the term "unlikely" in quantitative terms. NRC regulations now specify a range of numerical values for use in determining whether a feature, event or process, or a sequence of events and processes, should be excluded from certain required assessments. NRC is taking this action to clarify how it plans to implement two of the environmental standards for Yucca Mountain issued by the U.S. Environmental Protection Agency (EPA). Specifically, EPA's standards require the exclusion of "unlikely" features, events or processes, or sequences of events and processes, from the required assessments for the human-intrusion and ground-water protection standards. In accordance with the Energy Policy Act of 1992, NRC has adopted EPA's standards in its recently published technical requirements for a potential geologic repository at Yucca Mountain.