The Senate Armed Services Committee will undertake an investigation into the safety and effectiveness of the aging Titan II intercontinental ballistic missile system, according to committee sources.

The nation's 54 Titan II missiles have been based in Kansas, Arkansas and Arizona since 1963. Together they represent one-third of the total explosive nuclear power of the entire 1,054-missile land-based force.

Each Titan II has a nine megaton warhead, equal to 9 million tons of TNT, and thus 750 times bigger than the Hiroshima bomb.

The 17-year-old, liquid-fueled system's critics on Capitol Hill and within the administration say that because of leaks of highly toxic fuel, the Titans may be more dangerous to their U.S. crews than they are to the Russians.

The inquiry will be based on a just-completed Air Force study, which concluded that the physical conditions of the Titan II system is "good" and "considered by many to be better now than when it was new."

They study details the extensive modifications and improvements that have been made in the 17-year-old liquid-fueled missile to keep it operational well past its original 1971 retirement date.

One congressional source who has read the study said yesterday "more drastic" recommendations than those presented in the Air Force study were needed. The only change sought by the study was congressional approval of incentive pay for the men who handled the highly toxic fuel used to power the rocket engines.

"We may start from the premise we ought to get rid of the Titan," one Senate staffer said yesterday, "and put the burden of proof on the Department of Defense" to argue for keeping it.

The Titan II has become increasingly controversial since a fuel leak in 1978 killed two airmen and injured 29 others. It was of 125 major and minor leaks from the Titan IIs over the past five years. The latest was two weeks ago at another Kansas site.

In the study, the Air Force argued that the Titan II system had a lower accident rate than U.S. industry and particularly the chemical and allied products areas.

In its six-page study summary, the Air Force recognized that some hardware on the Titan II will "continue to age," but it said the service would develop the methods necessary to correct deficiencies.

Detailed appendices to the study outlined how severe some of the problems may be.

The launch control system, according to the study, is currently reliable but "the amount of support required to maintain the system is increasing and the electronic and electro-mechical parts . . . are becoming difficult and expensive to obtain." The proposed solution is to replace the old system with a new one, built or adapted to the other missile.

Since the United States stopped purchasing Titan IIs in 1967, it had to stop flight-testing programs for the rockets in 1969 before it ran out of spare missiles.

Periodic testing through 1976, according to the study, has left the program with just three extras. Since 1969 a reliability and aging surveillance program has been undertaken which features static testing of the engines of two missiles each year and a thorough inspection of its parts and airframe.

According to the study, repairs have been required in 24 percent of the airframes studied.

As of last August, "there were a total of 19 Titan II missiles which have one or more tank patches and a total of 29 patches in the force," the study says.

The study notes that such "repair of structural airframe assemblies is a common and effective practice in airplanes."

"Airframe corrosion," the study adds, "is a continuing problem."