President Reagan's plan to resume production of nerve gas weapons for the first time since 1969 barely survived yesterday in the Republican-controlled Senate as Vice President Bush cast the Senate's first tie-breaker in six years to save it.

As it worked late into the night on a $200 billion defense authorization bill, the Senate reversed the effect of an earlier vote by stripping the bill of $50 million for production of a nuclear shell that can be made into a neutron weapon. It voted to use the money for conventional weapons.

In a surprise move, Sens. Mark O. Hatfield (R-Ore.) and Edward M. Kennedy (D-Mass.) introduced a nuclear-freeze proposal as an amendment to the defense bill in an apparent attempt to force a prompt up-or-down vote on the volatile issue.

On nerve gas weaponry, the Senate split 49 to 49 before Bush, summoned in advance by a nervous Senate GOP leadership, cast the deciding vote and produced a final tally of 50 to 49 in favor of going ahead with the new generation of chemical weapons.

That shaky embrace of nerve gas weaponry, which followed a 216-to-202 House vote last month to scuttle the program, was Reagan's closest call so far on a major weapons system in the Senate.

The issue will have to be decided in a House-Senate conference after the Senate completes action on its version of the defense authorization bill for next year, which includes $130 million for nerve gas artillery shells and production equipment for the Bigeye nerve gas bomb.

Sen. David H. Pryor (D-Ark.), who led the fight to drop the $130 million allocation for nerve gas weapons, indicated afterward that the odds may favor Reagan in conference. "Whoever the conferees are will determine the outcome . . . . The administration may once again prevail," he told reporters.

Senate Republican leaders, fearing close votes on several controversial weapons programs, arranged to have Bush on hand for what turned out to be one of the Senate's more suspenseful moments in recent years.

As the Senate divided on a motion to table, or kill, Pryor's proposal, Majority Leader Howard H. Baker Jr. (R-Tenn.) approached the presiding officer's chair and told Bush that his vote would be required. Predictably, Bush voted against Pryor, meaning production funds stayed in the bill.

Not only was it Bush's first tie-breaking vote, but Baker told the Senate it was the first time since Nov. 4, 1977, that the vice president was called upon to vote, as the Constitution provides in case of a tie.

In that instance, Walter F. Mondale, then vice president, broke a 41-to-41 stalemate in favor of raising Social Security taxes.

In yesterday's vote, both parties split deeply over the nerve gas issue, with 17 Republicans breaking with Reagan and 14 Democrats joining him. Virginia's senators voted for the new weapons; Maryland's senators against them. The two absentees were Republicans Barry Goldwater (Ariz.) and Frank H. Murkowski (Alaska).

After the nerve gas vote, the Senate went on to reject, 68 to 30, a proposal by Democratic Sens. Kennedy, Dale Bumpers (Ark.) and Ernest F. Hollings (S.C.) to drop $7 billion for production of the B1, which Kennedy derided as a "supersonic albatross in the sky." But Armed Services Committee Chairman John G. Tower (R-Tex.) called the B1 "a bird in the hand and one we should keep hold of" as he rejected contentions by B1 foes that the Pentagon should concentrate instead on speeding up the Stealth bomber program.

The B1 vote was never in doubt. "There's part of the B1 produced in every state in the country," a leadership aide observed.

The Senate also rejected on budget grounds, 52 to 46, a proposal to create in 1987 a new GI Bill for veterans' education that critics charged could wind up as an expensive new entitlement program costing more than $1 billion a year.

Early this morning, the Senate rejected, 53 to 41, a proposal by Slade Gorton (R-Wash.) to send the whole defense bill back to committee for deletion of a $2.1 billion inflation adjustment.

Still facing the Senate as it neared the end of its third day on the bill was a potentially long fight over the MX missile, which Sen. Gary Hart (D-Colo.) has vowed to oppose "as long as I can hold the floor."

Hart, a Democratic presidential contender, has attempted to make the MX a focal point of the defense debate. But, with shaky support for a filibuster even among his Democratic colleagues, he has stopped short of pledging a fight-to-the-finish against the $4.6 billion allocation for initial MX production. Senate Republican leaders, appearing confident of retaining the funding, are pressing to wind up action on the bill by late today.

In debate over nerve gas weapons, supporters argued that a new, beefed-up chemical weapons arsenal is needed to force the Soviet Union to agree to reduce its stockpile, while opponents said it is unnecessary, wasteful and immoral.

"We cannot negotiate a ban without having a store of chemical weapons ourselves," argued Tower, contending that the American stockpile is obsolete.

Opponents said the U.S. arsenal is sufficient and argued that Soviet superiority stems from a better defensive posture, not weapons superiority. They also attacked it on moral grounds, with Hatfield decrying nerve gas as "grotesque and barbaric" and Pryor warning it would "mark the beginning of a new kind of arms race."

On the neutron shell issue, the Senate approved the shift to conventional weapons by voice after voting, 67 to 30, against a leadership move to table the proposal, offered by Sen. Sam Nunn (D-Ga.).

Nunn argued that Europe, where the shell is scheduled for deployment, can better be defended by conventional weaponry than nuclear devices. Tower strongly opposed Nunn's position, contending that the Senate was attempting to "make a major tactical decision that I don't think we ought to make in haste on the Senate floor."

Only the day before, the Senate had given what appeared to be tacit approval to move ahead with production facilities for the neutron weapon in stripping it of an earlier provision specifying that none of the money could be spent until a NATO nation agrees publicly to accept the weapon on its soil.