A federal appeals court has struck down the Environmental Protection Agency's regulations for disposal of high-level nuclear wastes in a ruling that is likely to mean additional delays in finding a final burial ground for thousands of tons of radioactive garbage.

In a decision released Friday, the 1st Circuit Court of Appeals in Boston said the agency's rules would allow drinking water near a nuclear dump to be contaminated with radioactivity at levels higher than the agency allows under the Safe Drinking Water Act. The three-judge panel ordered the agency to rewrite the rules or explain the "apparent inconsistency and irrationality."

The EPA rules, released in November 1985, were designed to provide the regulatory underpinning for selection of the nation's first high-level nuclear dump, which is supposed to be secure enough to prevent environmental contamination for at least 10,000 years.

The panel ruled, however, that the agency established "without a word of explanation, different and more relaxed criteria" than exist under the Safe Drinking Water Act. According to the ruling, the regulations will permit persons living near the nuclear dump to be exposed to greater levels of radioactivity than the EPA permits for public water supplies under the federal drinking-water law.

EPA and Justice Department officials said yesterday they have not decided whether to ask for a rehearing by the full court.

The ruling is the latest setback for the nuclear dump program, which has been mired in controversy since the Energy Department narrowed its list to three western sites last year.

Critics contend that the selection process is being guided by politics rather than science, and the three candidate states -- Texas, Nevada and Washington state -- have challenged DOE's procedures in court. The rejection of EPA's rules could put the entire selection process back to square one, according to Dan W. Reicher, who argued the case for the Natural Resources Defense Council.

"These standards form the basis for the siting guidelines that DOE uses to pick the repository and that the Nuclear Regulatory Commission will use to license it," he said. Should the EPA be required to tighten its rules to protect water supplies, Reicher said, "some of the sites that DOE is looking at would be in jeopardy."

The Texas site, in Deaf Smith County, sits above a massive aquifer that provides irrigation and drinking water for much of the Midwest. The Washington site, on the federal nuclear reservation near Richland, is adjacent to the Columbia River.