The Tennessee Valley Authority's $12.3 billion investment in nuclear power has produced a network of facilities in the South dangerously marred by structural defects and a managerial elite that has bent safety standards to accommodate poor-quality construction, TVA employes and consultants told a House subcommittee yesterday.

All five nuclear-power plants operated by the TVA in Tennessee and Alabama have been closed since last year, reflecting what TVA nuclear engineer Jerry D. Smith called "the ineffective or nonexistent" program of quality assurance.

"Basically, the nuclear program is in shambles," said Owen L. Thero, a nuclear consultant. "It's hard to see that TVA places any priority on safety."

The harsh criticism of the nation's largest electric utility came at a hearing of the House Energy and Commerce subcommittee on oversight and investigations, the latest example of intense congressional scrutiny of the U.S. nuclear industry since the Chernobyl nuclear accident in the Soviet Union.

Senior TVA officials, fending off sharp attacks by subcommittee members who accused them of everything from incompetence to criminal negligence, blamed the problems on a managerial system lacking in centralized control and accountability.

"I'm not here to defend what I have. What I have is plenty of problems," said retired Navy admiral Steven A. White, the utility's new, $355,000-a-year manager of nuclear power.

Among technical and managerial problems cited in testimony by a panel of consultants and whistle-blowing TVA nuclear engineers, who claim they have been harassed because of their outspoken views, were:

*Structural welding, which connects key components of nuclear plants, does not meet industrial standards and is of "indeterminate quality." The Watts Bar plant, almost completed near Knoxville, Tenn., was built with 18 tons of welding material rejected years earlier by TVA safety officials.

When deficiencies were detected during welding inspections there, TVA managers directed inspectors to check the welds after they had been painted, in violation of licensing requirements. In cases where the welding failed to meet standards, officials ordered the standards lowered.

*Electronic cables that carry signals to key safety systems were improperly installed in TVA plants, hampering their ability to function in an emergency. TVA officials have known of the problem since 1979 and failed to correct it.

Seismic supports intended to cushion the reactor during an earthquake are "deficient," leaving "the safety of the entire system indeterminate."

Employe interviews at Watts Bar by Quality Technology Co., a Kansas-based nuclear consultant hired by TVA last year, revealed 1,868 safety concerns about the plant, which TVA had described in February 1985 as ready to operate.

The consultant sampled 230 concerns and found 79 percent of them valid. But TVA officials, advised of the findings, spent more time "trying to discredit and invalidate the findings than it would have taken to fix a given problem," said Thero, president of the consulting firm.

Quality Technology's investigation found 300 allegations of employe wrongdoing, including drug and alcohol abuse during work hours. Plant supervisors were accused of trying to sell personal insurance to workers.

Equipment thefts were reported. In one case, seven new welding machines were said to have been lifted over a fence as a plant guard watched, apparently "to make sure that the wrong people didn't steal it," according to Thero.

A safety system at Watts Bar that supplies cooling water for the reactor was erected on a type of soil that turns to quicksand during an earthquake. The plant is located 15 miles from a geological fault.

The Tennessee facility, according to Thero, is so riddled with defects that, putting it in service, "you'd get a chance to check out the Watts Bar evacuation plan."

Nuclear power accounts for one-sixth of the electricity generated by the TVA for seven southern states. The utility is operated by federal government employes but financed by customers.

TVA Chairman Charles H. Dean, asked repeatedly why the utility requested a permit from the Nuclear Regulatory Commission in February 1985 to begin loading fuel at Watts Bar, said officials were "unaware" at the time that the facility had so many safety problems.

Of the large number of employe safety concerns voiced to Quality Technology, he said, "a concern is not necessarily a validated problem."

"Either you and the TVA board are incompetent or you've committed a criminal act," subcommittee Chairman John D. Dingell (D-Mich.) said, referring to the utility's request for the NRC permit in which it claimed that Watts Bar met safety standards.

"I don't think either supports your continued employment on the federal payroll," Dingell told Dean.