The National Capital Planning Commission is expected to urge today that the Nuclear Regulatory Commission locate its headquarters and 3,100 employes in downtown Washington, rather than Montgomery County.

The staff of the Federal planning agency is recommending that a District site be chosen by the General Services Administration because it would "maximize the use of mass transit" and reduce the amount of automobile commuting and air pollution associated with any of the three Montgomery County sites being considered by GSA.

The nuclear agency, which oversees civilian use of nuclear energy, presently leases office space in nine buildings, eight of them in Montgomery County, where about 75 percent of NRC's current employes live. Most employes now commute by car, and an environmental impact statement being prepared by GSA estimates the vast majority of future employes -- 68 percent -- also would commute by car if the NRC headquarters is in Montgomery. About half -- 48 percent -- would ride buses and the subway if a downtown site is chosen.

Commuting to either of the two proposed downtown areas also would be cheaper for employes because mass transit is cheaper than commuting by car, but would take somewhat longer, according to the GSA study, expected to be released in its final form early this spring. The federal agency in charge of locating government offices will then recommend a site to Congress. GSA originally proposed a downtown site, which was approved by a House subcommittee but blocked by a Senate subcommittee. The Senate subcommittee asked that GSA study possible sites in Montogomery County. NRC would either lease or build an office building in the area chosen.

This is the first time the planning commission, the federal planning agency for the Washington area, has reviewed the NRC consolidation because GSA has refused to submit major leasing proposals here for commission review, according to planning commission staff. Commission review today is necessary under the environmental impacet statement process.

The NCPC staff also urges selection of a downtown site for the NRC because it would be "a major stimulus to the revitalization of the historic downtown areas of Washington between teh White House and the Capitol," and becaue it would best comply with President Carter's executive order last summer that calls for locating future federal offices in urban areas.

The Montgomery County Planning Board two weeks ago announced that it was now backing only one county site, Silver Spring, instead of two proposed Bethesda locations. GSA has since said that Silver Spring, long the county's largest commercial area, meets the executive order definition of an urban area although the Bethesda sites do not.