Despite marked improvements in safety, the Nuclear Regulatory Commission decided yesterday to keep the Calvert Cliffs power plant on its national list of problem plants.

Calvert Cliffs, which has been on the list for more than two years, was one of four across the nation the NRC said needed special attention to ensure it is safely operated.

NRC officials said they wanted to see the utility operate both 825-megawatt reactors over a period of months before they would agree to take the plant off the list. Since the two reactors in Lusby were shut down in May 1989, they have produced energy for only two weeks in May.

In addition, the NRC said three incidents this spring made them question whether the procedural and management improvements the utility has made in the past couple years.

"We are disappointed, obviously,"said Arthur J. Slusark, public information director at Baltimore Gas andElectric Co. At full capacity, Calvert Cliffs accounts for about 40 percent of all electricity BG&E; produces.

However, Mr. Slusark said, the company is not surprised that the plant remains on the "watch list."

Richard I. McLean, the state's administrator for nuclear evaluations, said he agrees with the NRC decision. "We want to see six months to a year of reliable performance," he said.

The plant, once considered one of the best-run nuclear reactors in the country, was found to be problem-ridden in late 1988. After a management reorganization and other changes, the NRC gave the utility higher marks in its annual report card last year.

Also yesterday, the NRC named Three Mile Island in Pennsylvania, the symbol of the nation's nuclear power problems in the 1980s, to another list: one of three outstanding plants in the nation.