The Nuclear Regulatory Commission proposed a $100,000 fine yesterday against the Babcock & Wilcox Co., charging the builders of the Three Mile Island nuclear power plant with withholding information that could have prevented last year's accident there.

It was the first time the NRC has moved to fine a reactor supplier. The company has 20 days to protest before the fine takes effect. It can then appeal further in the courts. B & W yesterday said it had met all its obligations, and would file a protest.

Victor Stello, head of the NRC's inspection and enforcement office, said B&W had learned information from a September 1977 accident at the Davis Besse plant in Toledo, Ohio, that it should have passed on to the rest of the nuclear community through the NRC.

B&W spokesman Nelson Embrey responded from the company headquarters in Lynchburg, Va., that the NRC action was "unwarranted" and would be protested. "B&W believes it has met all applicable federal regulatory requirements," he said.

Stello cited four cases in which B&W had allegedly been aware of important safety information but had not told the NRC about it:

B&W engineers probing the Davis-Besse accident suggested instructions for operators dealing with similar incidents in the future, but the instructions were never passed on.

B&W ignored a November 1978 memorandum from the Tennessee Valley Authority that raised questions about a safety valve that had stuck open at Davis-Besse. The same valve later stuck open at Three Mile Island.

B&W began deliberating in November 1978 the issue of whether or not to leave certain pumps operating in an accident involving a small loss of coolant, as at Davis-Besse, but did not raise the question with the NRC. The issue was a factor at Three Mile Island.

B&W failed to anlyze properly the difficulties revealed at Davis-Besse in determining water level in the reactor's steam generator, another problem that recurred in Pennsylvania.