An Arizona nuclear power plant, still under construction, experienced a sharp drop in the water pressure of a cooling system earlier this week apparently after someone "tampered with" four sets of water valves, utility officials and law enforcement authorities said yesterday.

"It appears that the tampering was deliberately done, but why it was done we certainly don't know," said Dan Canady, a spokesman for plant owner Arizona Nuclear Power Project.

After the incident, plant officials notified local authorities, the Federal Bureau of Investigation and the Nuclear Regulatory Commission. No injuries were reported.

The local sheriff's office said it is handling the incident "as a criminal investigation."

The two-reactor Palo Verde plant is about 50 miles west of Phoenix.

NRC officials in Washington and California reported that the problem occurred at Unit 2 of the plant Monday afternoon while the facility was undergoing testing. No uranium fuel has been loaded at the plant, which is scheduled to receive an operating license by the end of the year.

After operators spotted the problem, they restored the pressure, and no further damage was discovered, officials said.

Greg Cook, an NRC official in Walnut Creek, Calif., said it is unlikely that "just anybody" manipulated the valves. "It would seem that whoever was involved in this incident was someone who had some degree of knowledge of the plant," he said.

Local law authorities detected fingerprints on the valves, which are located outside of the control room, according to Canady. Since the plant is under construction, hundreds of workers apparently have access to the valves.

Canady said he does not know if the tampering is related to two incidents reported in February 1984, when electrical cables were cut at the plant, causing damage of about $150,000. That problem has not been resolved.

He also reported that Unit 1 of the facility was suffering from a leak discovered early yesterday morning.

The leak of "mildly irradiated water," at a pace of 1.3 gallons per minute, continued through yesterday afternoon, Canady said, while its cause remained unknown.

The facility began reducing its power yesterday afternoon in response to NRC regulations requiring plants with unidentified leaks to cool down until the source of the problem is discovered. At the time of the leak, the reactor was generating about 50 percent of its capacity.

Officials said the plant, which began generating power in May, must be shut down if the source of the leak is not discovered.