Members of the House Armed Services Committee Thursday urged President Reagan to take drastic steps to ensure the safe operation of the nuclear weapons manufacturing complex, particularly the Savannah River Plant, where reactors have been closed for safety reasons.

"This crisis has not arisen suddenly but over a long period, and it stems from inadequate attention to maintenance, safety and operating conditions," the 31 representatives said in a companion letter to Energy Secretary John S. Herrington.The letters, drafted by Rep. John Spratt, D-S.C., and signed by committee chairman Les Aspin, D-Wis., urged the administration to move quickly to appoint the five-member Nuclear Facilities Safety Board, which Congress ordered created this fall to oversee the troubled weapons complex.

In other developments, government documents obtained by The Associated Press show that the Energy Department gave a manager of the Savannah River Plant $59,750 in awards for excellence despite continuing problems there.

The manager, Robert Morgan, who retired early this year, is one of many individuals and organizations that received cash bonuses totaling millions of dollars despite a steady decline in the safety of the Savannah River nuclear weapons production complex run by the Energy Department for the Pentagon.

A department spokesman, Will Callicott, said he was not aware of any plans to review the system of awards.

Morgan received the bonuses in 1981-1987 despite growing safety problems which finally forced the department this year to close down all three nuclear reactors at Savannah River, according to Energy Department documents and Capitol Hill testimony. The three reactors went into operation in the early 1950's at the plant, in Aiken, S.C.

The department spokesman, Callicott, said he could not discuss the bonus program in detail.