Irregularities have been found in X-ray inspections of the welds in the containment vessel at the Three Mile Island nuclear power plant's "sister" reactor, the Los Angeles Times has learned.

This "sister" reactor, Unit 1 at Three Mile Island, has one of the best safely records in the nuclear industry. But the new inspection problesm-and an unpublicized mishap at Unit 1 exactly a week before the major accident at adjacent Unit 2 March 28 - have raised new question about its safety.

In an eerie parallel to "The China Syndrome" movie. Metropolitan Edison co., which operates the reactors at Three Mile Island, discovered problems last month among 63 inspection reports by a subcontractor on welds in Unit 1.

Company officials would not say Monday what the irregularities in the routine inspection reports were, pending a meeting with the subcontractor. Nuclear Energy Services declined to comment.

Met Ed had asked NES to inspect the welds - including studs and nuts on the pressure vessel and other elements in the containment vesel complex housing the Unit 1 reactor - during normal refueling operations in February and March.

The inspection reports did not have to be submitted to the Nuclear Regulation Commission until the end of the year, a Met Ed spokesman said. The NES inspection reports may have been flagged because of a misunderstanding about new inspection regulations, he added.

But the Met Ed spokesman said his company would not have asked for an extensive repeat of the inspections if it had not found something seriously wrong with the initial reports.

"Some of our people determined that some of the testing procedures on the radiographs (Xrays) did not look too accurate on the welds in the reactor vessel," the spokeman said. "So they asked for a re-do. Then when they got the re-do back, they felt it was too short a period of time for them (NES) to have gone ahead and redone all those inspections."

Met Ed was in the process of reviewing the inspection reports when the accident occurred at Unit 2 on March 28, delaying indefinitely the reactivation of both Unit 1 and Unit 2 at Three Mile Island.

If reports on Unit 1 were inaccurrate, that would violate Nuclear Regulatory Commission rules, according to NRC engineer Tom Elsasser.

"Obviously if cracks and are developing as the thing ages and they're not finding them, then they do have potential safety problems." he said.

NES and its affiliated companies, Conam Inspection Co. and Automation Industries, have done much of the X-ray inspection work at Unit 1 since it underwent construction in 1969, according to Med Ed officials and NRC records. The Med Ed spokeman said he did not know whether these earlier inspection reports have been reevaluated in light of the new problem.

Also, a minor mishap at Unit 1 on March 22, blamed on a closed valve and an instrument deficiency, foreshadowed the far more serious events a week later in Unit 2, The Times has learned.

In the earlier mishap, a key instrument failed to tell control room operators the level of water in the Unit 1 reactor. A similar problem a week later in Unit 2 led to an uncovering of the reactor core and subsequent radiadent in U.S. commercial nuclear history.

In both cases the instrument - called a pressurizer level indicator - correctly read the water level in the reactor's pressurizer, but not the water level in the reactor itself. When these instruments were designed, it was thought that the pressurizer always would reflect the same level as the reactor.

In the Unit 1 mishap, the mechanical maintenance workers had stepped off an elevator into the Unit 1 reactor area to perform the final venting of the reactor prior to starting it up again after its month-long refueling process.

The workers found several hundred gallons of water pouring out of a small vent at the top of the reactor, with spray shooting many feet into the air. But when they called the control room to report the problem they were told, "That's impossible. I don't have any readings" (that would indicate the water overlow), sources said.

"In both cases the instrument told us exactly what was going on in the pressurizer, but it didn't tell us the whole story. It fooled us about what was going on on the primary system," said the Med Ed official.

Fortunately, the water coming off the Unit I reactor that day was "fairly clear" of radiation since the reactor was not in operation, the official said.