The owners of the Three Mile Island nuclear power plant filed a $500 million lawsuit yesterday charging that "gross negligence" by the Babcock & Wilcox Co. in building the reactor caused the Pennsylvania accident a year ago this week.

The General Public Utilities Inc. suit, filed at U.S. District Court in New York, alleged that B&W provided "incorrect, incomplete and inappropriate" training to the reactor operators and so led them into mishandling the events of last March 28.

At the same time, electric utility leaders reported to the White House that a year of evaluation shows the nation needs nuclear power now more than ever. Antinuclear critics disagreed, saying the past year has changed very little and that the nuclear industry still ought to be shut down.

The GPU suit was a jarring break in what had been a united industry front on nuclear power questions. Previously the line had been one of shared responsibility, assigning partial blame to suppliers, utilities, operators and regulators alike.

But GPU President William G. Kuhns charged that B&W operating procedures and training were "a critical and proximate cause" of the accident, in which small amounts of radiation were released over the Pennsylvania countryside.

The reactor supplier had studied incidents at the Davis-Besse plant in Toledo, Ohio, and at other B&W plants that foreshadowed the events at Three Mile Island, the GPU suit charged, but had failed to tell its other customers about either the events or the studies.

GPU asked the court to restore $500 million in damages and replacement power costs spent so far, and also asked for future damages: cleanup costs now estimated at $400 million; capital and operating costs of the shut plants which are running $10 million per month and future power replacement costs that now reach $25 million monthly.

Total damages asked are well over $2 billion in future costs alone. A spokesman for J. Ray McDermott & Co. Inc., the New Orleans-based parent company of B&W, said the firm has "no material legal liability" in the Three Mile Island events and will "vigorously defend" itself in court.

At a simultaneous press conference, consumer advocate Ralph Nader led several environmental and antinuclear groups in denouncing the industry and its regulators for "a year of paper-filling, buckpassing and general paralysis."

"Despite Three Mile Island, Congress has yet to mandate a single fundamental reform," he said. "The industry seems to be waiting for a full-class nuclear catastrophe to convince them" that the nation's 70 operating reactors ought to be shut down, he added.

David Brower, head of Friends of the Earth, equated nuclear defense weaponry and nuclear power generation. "The genie is out of the bottle, and peaceful genie or wartime genie, they are not separable," he said. "This is a dead end technology."

Robert Pollard of the Union of Concerned Scientists conceded that there have been some improvements in reactor operation over the last year but said they had been inadequate.

Four electric utility organizations asserted just the opposite. Reporting to President Carter as the Nuclear Oversight Committee, they called Three Mile Island "a serious but not highly dangerous accident" and said, "Many of the fears at the time were based largely on misinformation."

The group, headed by Floyd Lewis, board chairman of Middle South Utilities, said the industry had "responded quickly and forcefully" to the accident, revising its training and procedures and setting up several new organizations to study and deal with longstanding problems.