A nuclear reactor malfunction, including some of the same elements that caused the near-disaster at Three Mile Island in Pennsylvania, caused the shut-down of a New Jersey power plant Wednesday afternoon, officials of the Nuclear Regulatory Commission announced yesterday.

A combination of erroneously closed valves, conflicting instrument readings and signals that went ignored resulted in a "triple low" water level over the radioactive fuel core of the nuclear reactor at the Oyster Creek plant near Toms River, N.J., the NRC was told at a staff briefing.

The holding company that owns Oyster Creek is the same one that owns Three Mile Island.

Beginning at 1:53 p.m. Wednesday the water was no higher than 4 feet 8 inches above the top of the core for at least 30 minutes, and may have fallen lower than that, the officials said, possibly exposing the core.

However, no radiation was found in any of several water and air samples taken at the site, indicating that there was no damage to the core. The reactor was cooled and depressurized to cold shutdown by 10:20 p.m. Wednesday, and will remain shut until the incident is investigated, according to Norman Moseley, chief of the NRC's reactor operations inspection branch.

The Oyster Creek reactor was built by the General Electric Co. in 1969 for Jersey Central Power & Lights Co., which is owned by General Public Utilities Co., the same holding company that owns Three Mile Island's utility. The plant produces about 25 percent of Jersey Central's peak power load to some 677,500 customers.

NRC officials gave this description of the event:

The Oyster Creek reactor is of the "boiling water" type, in which the cooling water that circulates around the core boils directly into steam, which then drives a turbine to produce electricity. As the water passes through the turbine, it condenses back to water and recirculates to be heated again at the core. The system at any time is part water and part steam.

Maintenance work being done on the plant somehow caused a "spurious high pressure signal" to go off, which caused the actor to "scram," or shut down, at 1:50 p.m. Wednesday. Residual heat kept the water boiling away, but for some reason five valves in the system had been shut so water could not flow back from the turbines.

One had been shut for maintenance, two were shut for unknown reasons, and the last two were inexplicably closed by the operators during the incident, the NRC was told.

The water level over the core kept dropping, but at the time of the "triple low," which was the limit of safety," according to one official, "the operator was looking at . . . water-level indicators that were telling him the water level was all right."

At that point, automatic bypass lines opened around the shut valves and brought cooling water back to the core. Normal circulation was reestablished 36 minutes into the incident, and the reactor was gradually brought to cold shutdown by 10:20 p.m.

The NRC regional office was notified of the event at 3:20 p.m., but nobody at Oyster Creek was apparently aware that there had been a triple-low water level until sometime in the evening. Darrel Eisenhut, deputy director of the NRC's operating reactors division, said he learned of the low level at 11:40 p.m. at home.

This was apparently because the computerized event recorder, which automatically lists the changes in any instruments, had been shut off by the operators sometime during the incident. Officials said this may have been because the operators thought there was no further event to record or simply because "it was spewing paper all over the floor."