PHYSICIST IS NAMED AS SCIENCE ADVISOR

President Reagan today selected Dr. George A. Keyworth, a nuclear physicist, as his science adviser. The 41-year-old scientist, who heads the physics division at the Los Alamos National Laboratory in Los Alamos, N.M., is the first Presidential science adviser drawn from outside the ranks of a small scientific elite.

Those who know Dr. Keyworth well call him a first-rate scientist with an engaging personality who can be expected to learn his job quickly, although he has little experience in science policy and few ties to the scientific establishment. He was offered the post after a number of better known scientists spurned it.

After graduating from Yale University, Dr. Keyworth earned his Ph.D. at Duke University in 1968. He has been at Los Alamos since 1968 and was in charge of 500 people. Hawkish Views on Defense

The nomination of Dr. Keyworth, which had been expected for several weeks, stirred unease among some scientists, both for his lack of the usual credentials and, for some, his hawkish views on military matters.

In an interview two weeks ago, Dr. Keyworth expressed backing for a strong nuclear deterrent and said he expected to play a role in military policy if nominated. His nomination was urged by, among others, Dr. Edward Teller, the nuclear physicist. Dr. Teller favors an American buildup of nuclear weapons.

The nomination, technically as director of the Office of Science and Technology, requires Senate confirmation. Some scientists have also expressed doubts about how much influence Dr. Keyworth will have, either with the President or with other scientists. The new science adviser will have a sharply reduced staff and budget and is unlikely to have the access to the Oval Office that some of his predecessors had. Outstanding in Fusion Physics

It remains unclear how Dr. Keyworth will fit into the White House structure, but he expects to work with both the national security and domestic policy staffs.

While largely unknown to many scientists, Dr. Keyworth has a good reputation in his fields of nuclear structure and fusion physics. He was ''one of the outstanding graduates'' at Yale, recalls Dr. D. Allan Bromley, director of the A.W. Wright Nuclear Structure Laboratory at Yale.

Dr. Bromley, who, as president of the American Association for the Advancement of Science, has criticized the Reagan budget cuts, said of Dr. Keyworth, ''Intellectually and by nature and personality, he'll move very rapidly and effectively into the job.'' But, with Mr. Reagan's proposed budget for the fiscal year 1982 largely in place, he said he doubted that the science adviser could do much more than ''affect the margins.''