Defense Secretary Caspar W. Weinberger last year rejected a proposal to put the newest system for preventing the accidental or unauthorized release of nuclear weapons in the B1B bomber fleet because the added $200 million would shove the bombers' price tag above the $20.5 billion ceiling that Congress was promised, according to Pentagon sources.

The rejected system, known as permissive action link/command disable (PAL/CD), involves an electronic lock inside each nuclear weapon. The lock can only be opened, and the weapon armed for nuclear explosion, by insertion of an electronic code sent by the aircraft pilot, who must be authorized in a special message from the president through the Pentagon's national command authority.

The system also has an electronically activated explosive system that permits the weapon's internal mechanisms to be destroyed so it cannot trigger a nuclear detonation if seized by unauthorized persons.

Pentagon sources said Weinberger's decision does not mean the B1B's nuclear payload will be less safe, since an older safety system built into the bomber will be used. But rejection of the new feature indicates the lengths the Defense Department is willing to go to avoid exceeding the price ceiling, the sources added.

An important part of Weinberger's pitch in selling the aircraft to Congress was a vow to avoid cost overruns.

The Strategic Air Command (SAC) and top Defense Department nuclear weapons officials wanted the PAL/CD system made standard on all B1B aircraft, the Air Force's newest intercontinental bomber. It has already been placed in SAC's two older strategic aircraft, the B52 and the F111.

According to one source, the PAL/CD system had not been specified in the so-called "baseline," or original B1B program, because it was overlooked among other required changes

Instead, the bomber was given a less costly system, known as a coded switch, used in SAC's other bombers. This system puts the electronic lock in the aircraft rather than in individual weapons. The nuclear weapons are loaded on the airplane, but a lock interrupts the electronic current necessary to begin the arming sequence.

As with PALs, under the older system the pilot must get presidential authorization for release but the electronic code he receives unlocks the power from the aircraft to the weapons.

One expert in the nuclear weapons field said yesterday that both systems meet the required command and control needs but added, "I favor the PAL approach because it involves each weapon and gives just that much additional control."

Another expert said the coded switch approach did not include the command disable feature, preventing unauthorized detonation, "but it could be added if someone wanted to."

Air Force officials would not comment on the differences between the two systems.

In late 1981, to hold down the B1B costs, Weinberger ordered that no major changes could be made in the initial production of the 100 aircraft without approval of the deputy secretary of defense. The controversy over the nuclear weapons control device was the only modification to the plane requiring Weinberger's decision.

Weinberger rejected the PAL/CD system "because it was not part of the baseline configuration and was not a valid requirement," according to an Air Force presentation to the House Appropriations defense subcommittee.

Ironically, Weinberger's decision may end up costing the Pentagon much more than $200 million because "the Strategic Air Command wants to go to the PAL system in the future;" backfitting it into the B1B would be costly, according to one official associated with the program.

At that time, however, the cost will come out of modernization funds and not the original "baseline" B1B program, thus avoiding cost overruns.