The Diablo Canyon nuclear power plant in California halted its controversial start-up operation yesterday but not because of anything related to antinuclear protesters who have been laying siege to the site.

It was more embarrassing than that.

The plant superintendent discovered late Sunday that drawings used to label certain crucial pipes in Unit One of the plant as safe from earthquakes actually were drawings of pipes in Unit Two.

The foulup means that the plant cannot operate until the Unit One pipes are checked and approved, which could take several weeks, according to a spokesman for the plant's owner, Pacific Gas and Electric Co.

Earthquake readiness has been the major issue in the battle of Diablo Canyon in San Luis Obispo, 150 miles northwest of Los Angeles, where more than 1,600 demonstrators have been arrested in the last two weeks. The plant is built over an old earthquake fault.

Critics of the plant began arguing in 1977 that it was not being built to withstand a major earthquake, forcing PG&E to supply bales of documents and studies on the issue to the Nuclear Regulatory Commission. After prolonged hearings that PG&E complained were unnecessary and after additional delays for changes ordered after the Three Mile Island accident in 1979, Diablo Canyon received preliminary operating clearance Sept. 15.

The demonstrators, calling themselves the Abalone Alliance, summoned nuclear protesters nationwide to come to Diablo Canyon and make it a symbol for all that they abhor about the nuclear era. Even though far fewer people showed up than the alliance had hoped, police have been busy at the plant.

"We are thrilled by any type of delay at all," Abalone spokesman Raye Fleming said. "We're not surprised there are technical problems with the reactor. We've been raising those concerns for years." She said demonstrations would continue.

The $2.3 billion facility received further clearance last Tuesday to load fuel and begin low-power testing and operation. Spokesman Richard Davin said some loading had begun but has been halted until the piping is cleared.

The problem involves the so-called residual heat removal system, which cools the hot reactor core after it is shut down for any reason and is central to reactor operation. Its piping occupies a 20-foot space between the reactor containment wall and an outer shield designed to protect the site against missiles.

The systems for Units One and Two are similar but not identical. "Consequently, the licensee PG&E has concluded that the system (and possibly other piping in this area) has not been properly analyzed and that the RHR system is inoperable at this time," the official Nuclear Regulatory Commission description of the problem said.

"It's not like it's broken or anything," Davin said. "It's just that in the event of an accident, we couldn't guarantee its operability. It's not a major problem. We just have to go back and do the analysis."