The Energy Department told Congress in a report it is concerned the three aging nuclear reactors at South Carolina's Savannah River Plant may never be able to produce enough tritium for the nation's nuclear weapons, The New York Times reported Tuesday.

The department also said the safety and modernization program needed to restart the reactors will not be completed until at least the end of 1989, according to the report.The Savannah River Plant, in Aiken, S.C., is the nation's sole producer of tritium, a radioactive gas used to build modern thermonuclear warheads.

Last spring, structural problems forced the closing of the three reactors at the plant. And before the closing, the plant's manager - E.I. du Pont de Nemours & Co. - had reduced the power under which the reactors could operate, resulting in reduced production of tritium.

Even if the Energy Department successfully completes the improvements program, the report said, it may not be enough to ensure that sufficient tritium can be produced to maintain the nation's supply of 22,000 to 24,000 warheads.

"In spite of these improvements and increasingly extensive maintenance, the reactors may not be able to achieve acceptable producton efficiencies," the report said.