The nation's biggest business, in terms of dollars, is Social Security: it has 115 million contributors, 36 million beneficiaries and estimated outlays of $172 billion in fiscal 1983, not including Medicare. Some 87,000 employes feed its computers, place folders in musty files, send out checks and staff its 1,300 field offices.

The man who runs it all is John A. Svahn, known to everyone as Jack, a big man -- 6 feet 4 inches and 215 pounds -- who serves as commissioner of the Social Security Administration.

Besides Social Security, Svahn's domain extends to the nation's two largest cash welfare programs, Aid to Families with Dependent Children (AFDC) and Supplemental Security income (SSI) for the aged and disabled, adding about 15 million more people to his province.

Svahn, 39, has been around social service and welfare programs most of his life, either in government or as a consultant. After graduation from the University of Washington and spending two years in the Air Force, he went to work for the state of California in 1968, eventually becoming director of the state's social welfare department.

His official biography says he was a principal architect of then-California Gov. Ronald Reagan's welfare reform program, which clamped down hard on alleged cheaters, sought to install a major "workfare" experiment and tried to improve the administration of the programs.

Svahn joined the Nixon administration in 1973 as acting commissioner of the Community Services Administration, and later was named head of a number of sub-agencies of the Health, Education and Welfare Department that were involved with welfare programs.

Now Svahn is head of the whole federal welfare apparatus, a job he said presents him with several major problems.

First, he said, "The Social Security financing problem has to be solved." The system is going to run out of money over the next few years unless someone figures out a way to cure its shortfall.

Svahn is concerned that people have lost confidence in the system because of the talk of bankruptcy and because service sometimes is poor. But he said he wants to "restore the confidence of the 87,000 employes of the system," whose pride in their work has been eroded by administrative problems and those same public fears.

Svahn said the agency must bring the Social Security system "up to the standards of the late 20th century" by modernizing the agency's ancient computer system. The system is a generation or more behind the times and still requires the agency to keep tens of millions of paper records. Svahn recently announced a five-year, $500 million plan to improve the computers.