The nation's nuclear power plants have become safer and more efficient over the past four years, according to the head of the Nuclear Regulatory Commission.

The remarks by Chairman Lando W. Zech Jr. came at ceremonies Wednesday to dedicate an 18-story headquarters building for the NRC in suburban Rockville, Md."I am pleased to report that we are seeing evidence that overall performance is improving," Zech said.

Spokesmen for anti-nuclear groups challenged Zech's upbeat assessment, with one contending that the situation with power reactors amounted to a "mixed bag" of improvements and problem areas.

"From 1984 until now, there has been a clear and definite improvement," Zech said, noting that the NRC began an analysis in that year of safety indicators. Zech said there had been reductions in "significant operating events" at power reactors, unplanned automatic shutdowns, safety system activations, "precursor events" signaling the possibility of a major accident and exposure of workers to radiation.

"Along with this improvement in critical safety performance indicators, we have seen improvements in plant reliability and availability," Zech said. "Recent data show an increase in the average capacity factors for U.S. reactors from about 59 percent in 1986 to almost 67 percent in 1988."

The capacity factor is the actual output of a plant compared with what it would generate if it operated at full power all the time.

NRC spokesman John Kopeck later provided the following specific figures for the performance indicators cited by Zech:

-"Significant operating events," or major problems: 2.4 per operating reactor in 1985; 1.6 in 1986; and 0.8 in 1987.

-Unplanned automatic shutdowns: 5.24 per reactor in 1984; 5.04 in 1985; 3.96 in 1986; and 3.24 in 1987.

-Safety system activations: 2.76 per reactor in 1984; 2.6 in 1985; 2.04 in 1986; and 1.68 in 1987.

-Precursor events: a nationwide total of 17 in 1984; 10 in 1985; and 6 in 1986.

-Radiation exposure: 714 man-rem in 1984; 574 man-rem in 1985; 431 in 1986; and 425 in 1987.

Man-rem is a measure of radiation exposure for large numbers of people. For example, when 100 people receive an average exposure of 0.01 rem, that would result in a figure of 1 man-rem.

Spokesmen for anti-nuclear groups said they had questions about Zech's assessment of overall plant performance.

Kenneth Bossong of Public Citizen, a Washington-based public interest group founded by consumer advocate Ralph Nader, said: "It's a mixed bag. There are some positives and there are some negatives."