The Energy Department on Friday selected a Bechtel Corp. subsidiary to design the entire disposal system for highly radioactive waste from the nation's nuclear power stations.

Separately, the department's Oak Ridge, Tenn., office said it will enter a five-year extension of existing contracts with a Martin-Marietta Corp. subsidiary to operate four nuclear weapons plants and a laboratory in Ohio, Kentucky and Tennessee.Under a 10-year contract for about $100 million a year, a team headed by Bechtel Systems Management Inc. will design the waste packaging, transportation system, temporary "monitored retrievable storage" depot, if one is built, and the ultimate underground permanent storage installation currently planned for Yucca Mountain, Nev.

Other companies on Bechtel's team are Westinghouse Electric Corp.; Batelle Memorial Institute; Science Applications International Corp.; Parsons, Brinckerhoff, Quade and Douglas; Dames and Moore; Shannon and Wilson; and Los Alamos Technical Associaties.

Losing bidders in competition for the contract were TRW Environmental Safety Systems Inc. and Systems Engineering and Management Corp., a group formed by Ralph M. Parsons Corp. of Pasadena, Calif. and Stone & Webster Engineering Corp. of Boston.

The contracts with Martin Marietta Energy Systems Inc. will be worth about $10 billion over five years, the Oak Ridge office said.

The two contracts cover the gaseous diffision plants at Oak Ridge, Paducah, Ky. and Portsmouth, Ohio; the Oak Ridge National Laboratory; and the Oak Ridge Y-12 weapons component plant. The Oak Ridge gaseous diffusion plant has been shut down, but employees there support the other plants.

The Portsmouth plant has been in a separate contract and the other four units have been lumped together, but the department said it was now putting the Oak Ridge installations in one contract and the Paducah and Portsmouth plants in another.

This will "focus management attention on the uranium enrichment enterprise," the department said.

The current contracts expire next Sept. 30. About 18,800 people work in the five installations.

Bechtel is based in San Francisco and Martin-Marietta, in Bethesda, Md.