WASHINGTON - Federico Pena may not be an expert on Department of Energy matters, but President Clinton's nominee for Energy secretary has some experience with nuclear cleanup problems.

He had one of DOE's most polluted nuclear sites - Rocky Flats -in his back yard while he was mayor of Denver.

Although the state of Colorado played the major role in negotiating the cleanup, Pena said he was well aware of the hazards it presented.

"I have great memories of Rocky Flats outside of Denver," was all he would say Friday when asked by reporters what he knew about DOE and its massive cleanup programs. He declined further comment.

While Rocky Flats may not be as polluted as the Hanford Nuclear Reservation, by some estimates it could cost $25 billion to clean up. For years, Rocky Flats was used to produce plutonium triggers for nuclear weapons, and at least 14 metric tons of plutonium and highly enriched uranium still is stored there.

The most serious cleanup problems involve ductwork filled with plutonium.

Rocky Flats has one other similarity to Hanford. Its cleanup is covered by a consent agreement among DOE, the state and the federal Environmental Protection Agency. In Colorado, DOE has had repeated problems meeting its deadlines.

Washington state lawmakers said Friday they weren't troubled by Pena's lack of detailed knowledge on DOE matters and pointed out President Clinton clearly picked him for two reasons - to add Hispanic representation to the cabinet and because of Pena's reputation as a solid administrator while he was secretary of transportation.

"Even though this is clearly a diversity thing, he is bright enough, skilled enough and a strong enough manager to do a great job," said Rep. Norm Dicks, D-Wash.

Pena's nomination came as a last-minute surprise after Clinton had tentatively settled on Elizabeth Moler, chairwoman of the Federal Energy Regulatory Commission. But late Thursday, after realizing none of the 14 members of his cabinet would be Hispanic, Clinton and his staff turned to Pena.

After last month's election, Pena told Chief of Staff Leon Panetta he did not want to serve a second term as transportation secretary and had put his house up for sale. But Pena's name had been mentioned for other possible cabinet slots, though not Energy secretary.

In picking Pena, Clinton focused on Pena's managerial skills and made clear a top priority at DOE will be to continue the reorganization begun under outgoing Secretary Hazel O'Leary.

"He will continue to streamline and reinvent the Energy Department," Clinton said in announcing Pena's nomination. "He will build on the unprecedented commitment to openness. He will oversee the urgent cleanup of our nuclear stockpiles. And, he will work with the energy industry to create economic opportunity by using energy in way that does not hurt our environment."

During three years at the Transportation Department, Pena reduced the size of the department by about 11,000 employees and won congressional approval of his plan to overhaul the Federal Aviation Administra tion. While mayor of Denver, Pena was actively involved in various transportation and public works projects, including the much-criticized new Denver airport.

Dicks and Sen. Patty Murray, D-Wash., said they had developed solid working relationships with Pena while he has been at the Transportation Department.

"He's smart, bright, engaging and someone we know," Dicks said. "As long as we can teach him to spell F-F-T-F, we will be OK."

Dicks was referring to the Fast Flux Test Facility at Hanford, an experimental reactor he and others have urged DOE to keep operating.

Murray said she was ready immediately to start educating Pena on Hanford issues .

"He's been very responsive to the state's needs on transportation," she said. "I will work closely with him to make sure he understands Hanford's needs. I know he will do a good job."