KNOXVILLE, Tenn. (AP) _ The Tennessee Valley Authority received assurances Thursday it could implement plans to save $100 million in annual interest costs by refinancing old, long-term debts without jeopardizing access to short-term federal loans.

The U.S. Treasury agreed to give the giant utility a two-year, $2 billion line of credit from the Federal Financing Bank while it attempts to refinance $6.7 billion in high-interest FFB loans through the public markets.

The deal permits TVA ''to go after the very large savings that we've been trying to achieve the past four months through our refinancing,'' said William Malec, TVA's chief financial officer.

U.S. Rep. Don Sundquist, R-Tenn., said the agreement was reached Thursday after discussions between federal regulators in Washington and TVA Chairman Marvin Runyon.

Malec said TVA planned to go to market with its first debt refinancing issue - at least $1 billion of the $6.7 billion - in the second week of October and expected to have the remainder refinanced over the next six months.

TVA's objective is to cut its annual operating expenses by reducing the interest costs on its long-term debt. The average interest rate on the $6.7 billion TVA wants to refinance is 12.25 percent. TVA believes it can secure rates of around 9 percent from public credit markets.

Debt interest represents TVA's single largest annual expense - currently $1.8 billion a year or 34 cents for every dollar needed to operate the power system.

By refinancing the $6.7 billion in old debt at lower interest rates, the TVA expects to save $100 million a year.

Treasury officials suggested in May that TVA go to Wall Street to refinance its old FFB loans, but last month said if the utility proceeded with the plan it would prohibit TVA from seeking long-term loans from the FFB in the future.

The TVA did not want to lose this access, having borrowed $17.3 billion from the FFB since 1974.

Malec said Thursday's agreement does not address new long-term FFB borrowing, but does allow TVA to go to the FFB for up to $2 billion in short- term borrowing to meet operating obligations over the next two years.

TVA will lump any long-term financing needs into other public bond issues through 1991, he said. At the end of the two-year period, Treasury will review the agreement and decide whether it again will offer long-term financing.