President Reagan, who has frequently described his Strategic Defense Initiative as a "non-nuclear" shield against nuclear missiles, wants to double spending on SDI nuclear weapons research and testing next year, according to budget documents.

The Energy Department is seeking $603 million for nuclear power and nuclear weapons research related to SDI in fiscal 1987, up from $288 million this fiscal year. At least $250 million -- also more than twice this year's spending level -- would pay for underground test explosions in the Nevada desert, according to John Pike, a critic of the "Star Wars" program who has studied the documents for the Federation of American Scientists.

The Energy Department's request comes on top of the Defense Department's $4.8 billion request for SDI, an increase over this year's $2.7 billion Pentagon spending level. The military has not released a detailed breakdown of its proposal, but about $50 million is expected to go toward additional nuclear weapons research.

The sharp increase in nuclear weapons development and testing for the missile defense program, the Energy Department said, is "a hedge against the failure of non-nuclear defensive weapons to meet performance requirements." The research also will help the United States understand what types of weapons the Soviet Union may be developing, officials said.

The funds would pay for development of what the military calls "third-generation" nuclear weapons -- the next step beyond atomic bombs and hydrogen bombs.

The technologies envision exploding a bomb, perhaps in outer space, and then channeling its phenomenal force into some kind of destructive directed energy -- lasers, microwaves or a cloud of "hypervelocity pellets," according to official documents.

"In the past we relied on what I would call a brute force approach," Richard L. Wagner Jr., assistant to the secretary of defense for atomic energy, told Congress last year. "This new idea is different because it focuses energy on the target in a direct way."

Wagner also explained why the administration is inventing new nuclear weapons for SDI, which is intended to defend the nation against Soviet attack.

"I think the president truly has . . . an objective of eliminating nuclear weapons from this defense-dominated world that he is committed to," Wagner said. "But the first stages of the SDI program, which as you say may last decades, I believe, and the Defense Department believes, will have this nuclear component."

Several members of the House Armed Services procurement subcommittee responded that an emphasis on nuclear weapons in the defense shield would make the program difficult to promote. Rep. Nicholas Mavroules (D-Mass.) said Wagner's explanation was "confusing."

"It helps, of course it helps, but trying to explain that to 435 colleagues is going to be very difficult," Mavroules said.

"Just say it's non-nuclear," Rep. Marjorie S. Holt (R-Md.) responded.

Reagan, who has used the non-nuclear description many times, said that during the Geneva summit meeting he explained to Soviet leader Mikhail Gorbachev that "we are investigating non-nuclear defensive systems that would only threaten offensive missiles, not people." A White House spokesman did not respond to requests for comment.

Rep. Samuel S. Stratton (D-N.Y.), chairman of the procurement subcommittee, said yesterday that the third-generation weapons being developed are not really "nuclear weapons."

"We're all big boys," he said. "I think we can understand the difference between a nuclear explosion that is near the ground that has a fallout and an explosion way out in space where nobody's going to be affected."

But Stratton said budget pressures may force the committee to scale back the requested increase for Star Wars, which accounts for almost all the growth in the Energy Department's nuclear weapons research and testing budget. The department is in charge of producing all nuclear warheads for the military.

Until now, the administration has openly discussed only one "nuclear-driven directed energy weapon" (NDEW) -- the X-ray laser favored by physicist Edward Teller that could theoretically burn a hole through a missile. Although documents this year list some other "precisely tailored" effects that atomic bombs might produce, a Pentagon spokesman said no one would discuss anything except the names of the systems.