The first full-scale emergency exercise at Calvert Cliffs, the nuclear plant closest to Washington, was "tainted," federal investigators found, because copies of the supposedly secret scenario for the exercise were obtained in advance by two of the three participating counties.

In a report released yesterday, the Federal Emergency Management Agency rated the Nov. 17 exercise "more than adequate" on a scale of "adequate" or "failing," but stated that county emergency officials in both Dorchester and St. Mary's counties had copies of the scenario before the drill. In addition, participants in Dorchester County were briefed on exact details and times of the plan five days before the exercise, according to Steve Adukaitis, an official who was in charge of the review.

"The credibility of the performance of these counties will justifiably be questioned because of this fact," Adukaitis wrote, but added that the counties' good performance could not have been achieved simply by knowing the scenario.

The drill was held in Calvert County, where the plant is located, St. Mary's, to the north, and in Dorchester, across Chesapeake Bay.

Joseph Langer, director of the Maryland Civil Defense and Disaster Preparedness Agency, said his agency had not determined how the secret plan was leaked. "We haven't been able to figure it out," he said. "But I object to the word 'taint.' Even if you had all the details, you can't do a successful performance like we had."

The scenario -- which called for a woman terrorist to explode a pipe bomb, causing a 10-mile evacuation -- was written by Langer's training aide and distributed early to "15 to 20" county and state officials who engineered the event.

The exercise was a vast improvement over the previous year's mini-drill, which "was a disaster within a disaster," according to one emergency official.

But this fall's exercise was far from perfect. Federal monitors found instances of sirens failing, too few rescue operators knowledgeable about radiation hazards and poor security at some emergency operations centers. In all, federal inspectors made 43 recommendations.

Specifically, investigators found:

* Security was so lax at one emergency center near the plant that one observer, who lacked proper identification, "talked his way" past unarmed guards. In Dorchester County, security at its emergency center "was physically nonexistent."

* Technical information on the nuclear plant given to state emergency coordinators was so general "it was essentially useless."

* The plant's operator, Baltimore Gas and Electric, and state officials disagreed -- in some cases dramatically -- on the amount of radiation that would be released.

* At one point, a monitoring team was sent into an area of potentially high radiation without being warned.

* The potassium iodide pills supposed to be given to emergency workers as a temporary antidote for radiation's effects on the thyroid were not distributed.