NASA's safety program, overhauled after the 1986 Challenger disaster, is monitoring many problems but so far has seen no "blockbuster" of a problem that would force major new changes in the space shuttle's flight hardware or schedule, the safety chief said yesterday.

Employees now may voice their concerns anonymously, by phone or in writing, through an independent contractor, if they feel NASA's internal reporting system is failing.

"We get two or three reports under that {anonymous} system each week," said George A. Rodney, a former test pilot and aerospace executive who was appointed in July 1986 to head the new Office of Safety, Reliability, Maintainability and Quality Assurance.

But the reports deal mostly with issues that should have been dealt with by the internal system, such as "not enough light in the work area, or work platforms that are deficient, . . . not significant flight hardware-type problems," he said.

Rodney, in a briefing, said his top safety concerns are the redesigned shuttle boosters, the redesigned propellant valves that feed fuel between the external fuel tank and the orbiter, and the shuttle's complex main engines.

A part in the booster nozzle assembly failed in a test firing in December and forced the postponement of the shuttle flight until at least August. NASA officials met yesterday for an update on the problems. They could set a formal flight date next week.

Rodney and others said the nozzle problems may prompt the agency to require three additional test firings of the boosters, instead of the two now considered mandatory. NASA's astronaut chief, Daniel Brandenstein, reportedly has asked that a third test, already in the schedule but not required before the flight, be mandatory.

Requiring the third test probably would not delay the schedule, officials said, but some managers are described as reluctant to make it a formal requirement.

Two years ago this month, the Challenger exploded shortly after liftoff, killing its crew of seven. Although the direct cause of the explosion was a failure in a poorly-designed booster joint, a presidential investigating commission also blamed NASA's "silent safety program." One of the chief shortcomings cited was a failure to "get critical information to the proper levels of management."

Since then NASA has created the new safety office under Rodney, who reports directly to NASA Administrator James C. Fletcher. It has also increased the safety staff and budget, promoted open communication of problems, set up a system for tracking and assessing "close calls," included astronauts in safety planning and taken a number of other measures to improve the operation, officials said.

For example, James R. Thompson, director of Marshall Space Flight Center in Huntsville, Ala., has an "open door policy" under which anyone can walk in and "he'll give them 15 minutes of his time," Rodney said.

Noel W. Hinners, NASA's third-ranking official and chief scientist, said, "I'm not so concerned about STS-26 {the next shuttle flight, the 26th in the series}. I'm concerned about STS-47." It will be difficult to maintain a focus on safety once the shuttles are flying again, and the system will have lost many experienced shuttle workers to retirement, he said.

An independent panel of the National Research Council, set up to monitor NASA's handling of risk, has found shortcomings in the system, as has an internal ad hoc committee that reviewed the operation a year ago. NASA officials say their recommendations are being followed.

While some critics have charged that NASA still pays too little attention to safety, others argue that the agency has become so cautious that the next shuttle flight could be postponed indefinitely.

NASA officials frequently restate their intention to put safety first, but they also emphasize that space flight will never be risk free.