Montgomery County planners now are urging only one country site-Silver Spring-for the proposed new headquarters of the 2,300-employe Nuclear Regulatory Commission (NRC).

Three county locations and two in downtown Washington are being considered by the General Services Administration, but the Montgomery County Planning Board apparently is backing off from the other two proposed county sites, in central Bethesda and at Nicholson Lane and Rockville Pike.

Employees at NRC, which regulates commercial use of nuclear power, now work in nine scattered office buildings, eight in Montgomery County and one in the District. About 75 percent of them now live in Montgomery County.

Royce Hanson, Chairman of the planning board, said Monday night at a GSA public hearing that of all the proposed sites "Silver Spring best meets the crucial criteria for relocation of federal agencies."

The criteria include President Carter's August Executive order that federl facilities should be located in cities or "centralized community business areas and adjacent areas of similar character."

Previously the planning board had endorsed equally all three county sites, which the County Council and county executive still do, according to James F. Giegerich, director of the county office of economic development.

A spokesman for GSA said after the hearing that the Silver Spring site (but apparently not Bethesda or Nicholson Lane Sites) meets President Carter's definition of "a centralized community business area." Both downtown District sites, in urban renewal areas near Union Station, also meet that definition.

The controverst over the NRC will locate began more than a year ago when GSA proposed a downtown urabn renewal site. A House of Representatives committee apprved that location but after Maryland congressman objected, a Senate committee asked GSA to reconsider and to look at several Montgomery County locations. The hearing is part of an environmental impact statement GSA is preparing on the five sites.

Two House committee chairmen, Reps. Henry S. Reuss (D-Wis) and Morris K. Udall (D-Ariz), issued a joint press release this week urging GSA to "reject affluent Montgomery County as the site for a proposed NRC" headquarters and to choose "the only sites that make sense-areas that stand in need of revitalization in downtown Washington." Reuss heads the committee on banking, finance and urban affairs and its subcommittee on cities, and Udall heads the House committee on interior and insular affairs.

The congressmen said the county sites would "fly in the face" of the President's executive order on locating federal facilities in urban areas.

They also cited a GSA study that says the county sites would be more expensive to commute to and cause more air pollution-even though most present employes live chose to those sites-because commuting would be primarily by car. The majority of NRC employes would ride in the subway to work if a downtown site were chosen, the draft of the GSA environmental impact statement says.

Rep. Newton I. Steers (R-Md.) and Sens. Charles McC. Mathias (R-Md). and Pau S. Sarbanes (D-Md) appeared at the Gsa hearing or sent representatives criticizing the draft GSA environmental study and urging that Nrc locate in Montgomery County.

About two dozen NRC employes, including spokesman for the National Treasury Union which represents them, spoke at the hearing, criticizing the GSA draft environmental study as "biased." One speaker said it is a document "prepared to justify a decision already made" by GSA.

The study estimates that most NRC employes would commute to Washington by subway, which would cost them up to $3.55 a week less than the cost of driving to a headquarters in the county. It would tkake most of them about 15 minutes more a day to commute downtown, the study says. The GSA estimates assume a 9 percent annual turnover of NRC employes and that new employes and some present workers will move close to a District site if one is chosen.

A similar county study reached an opposite conclusion, which NRC emploes at the hearing said was obvious-that commuting a longer distance to downtown Washington would cost employes more as well as take longer.

The position of the Montgomery County Planning Board backing only a Silver Spring location for NRC was adopted unanimously last week, Hanson said. Previously the planning board had endorsed all three county sites.