The Environmental Protection Agency said Tuesday that the proposed Yucca Mountain nuclear waste dump in Nevada must be designed to ensure against excessive radiation exposure to nearby residents for up to a million years.

The agency announced a rule providing for an annual radiation exposure limit from the dump of no more than 15 millirems a year — about the amount from an average X-ray — for the first 10,000 years.

And it said that releases from the planned installation must be no more than 100 millirems beyond that, up to a million years. People receive 360 millirems of radiation annually from all background sources.

The government last June asked the Nuclear Regulatory Commission for a license to build the underground dump 90 miles from Las Vegas. The government hopes the site will be ready to take nuclear waste from commercial reactors by 2020.