HARRISBURG - Naturally occurring radon and a power glitch caused radiation monitors to sound false alarms Monday night and yesterday morning at the Three Mile Island nuclear plant - and caused a fresh round of criticism for the plant's operators from Gov. Rendell.

The false alarms were in the same Unit 1 containment building where a small contamination incident occurred on Saturday, Exelon Corp. officials said.

Though tests showed no abnormal radiation, Rendell said Exelon had again failed to quickly notify state emergency-management officials. He said the company did not tell the Pennsylvania Emergency Management Agency (PEMA) until about 9:30 a.m. yesterday - nearly 13 hours after the first alarm.

The alarms sounded in the containment building and were not audible in communities near the plant, which is on the Susquehanna River south of here.

All TMI officials had to do was telephone PEMA and say, "Hey, this is a false alarm . . . and we will get back to you as soon as we know that," said Rendell, who called such an effort "nothing more than common courtesy."

"What Exelon folks, at least the Exelon folks at TMI, are not understanding is that the people of Central Pennsylvania, even though this is 30 years old, understandably are very apprehensive and jumpy about all this," Rendell told reporters, "and there is no appropriate reason for us not to be notified about this."

He was referring to the partial meltdown in 1979 at the plant's Unit 2 reactor, which changed the course of the nation's commercial nuclear industry.

Exelon responded quickly yesterday to Rendell's latest comments. "We value the governor's view on this issue," said Beth Archer, a company spokeswoman, "and that's why we are committed to sitting down with [state] and local officials to review lessons learned and to ensure we have open lines of communication."

About 150 workers at Unit 1's containment building were sent home Saturday afternoon after monitors detected radiation. On Monday, the company said all the employees had returned to work.

No radiation escaped from the building on Saturday, and the public was not endangered, according to the company and federal inspectors.

The unit has been shut down since late last month for refueling and installation of massive generators.

More than five hours passed before state officials were made aware of the Saturday incident. Its severity, however, did not approach the level that would have required Exelon, under federal rules, to notify state emergency officials within 15 minutes.

Nonetheless, Rendell on Monday sent a strongly worded letter to John Rowe, Exelon's chief executive. Rendell criticized the delay in reporting Saturday's event.

Yesterday, the governor told reporters he had also spoken with Rowe on Monday, and that the head of the Chicago-based utility assured him that changes would be made.

Then, Rendell said, it happened again.

The first monitor sounded at 8:45 Monday night. Between 4:30 and 5 a.m., three others on two separate floors went off.

In each case, workers were told to leave the area as crews took air samples. Those tests indicated no abnormal radiation, said Archer, the Exelon spokeswoman.

An electrical-supply problem caused the first alarm, and the company believes the other monitors were set off by fluctuations in naturally occurring radon levels in the building, Archer said.

The U.S. Nuclear Regulatory Commission said yesterday that it was reviewing the cause of the alarms.