Utility engineers are looking at options ranging from rewriting procedure manuals to extensive plant modifications to deal with a problem that caused the shutdown Tuesday night of the Palo Verde nuclear plant west of Phoenix.
No decision had been made by Thursday night, and officials could not say how long the three reactors at the largest nuclear complex in the nation will be down.
The potential problem involving the reactor core emergency cooling system had been undetected since the plant began producing power in 1986, according to the Nuclear Regulatory Commission and Arizona Public Service, which is the plant operator.
The problem was discovered when engineers did an analysis of the cooling system after NRC inspectors raised questions during a detailed inspection last week. The NRC was following up to see if earlier cooling system problems had been fixed.
As engineers evaluated the situation, it was unclear if the cooling system would operate properly under a theoretical scenario involving a slow leak of water, said APS spokesman Mark Fallon.
The emergency systems in each of the three units are designed to replace the cooling water if there is a leak, but the analysis showed that pumps that provide emergency cooling water may not sense that a storage tank is getting low on water and switch to another source, said NRC spokesman Victor Dricks.
The plant’s operating license requires that engineers verify the backup system would work, and therefore the plant had to be shut down, Fallon said.
Under a worst-case scenario, the reactor core could melt down and radiation would be released into the atmosphere if the cooling system doesn’t work properly. However, such a scenario is unlikely because of the plant’s many redundant systems, experts said.
The potential solutions are to either make physical changes to the plant or to change the license and procedures, Fallon said.
"It’s an engineering analysis issue," he said. "They have to analyze the system as designed and determine if we change the licensing documents accordingly or we alter the set up of the plant in some way to meet the specifications. . . . No decision has been made as to what direction it might take."
Earlier this year, the NRC fined the plant operator $50,000 because of another problem in a different part of the same cooling system.
Fortunately, cooler temperatures are reducing consumer demand for electricity, making it easy for utilities that use Palo Verde power to find energy from other sources. Power use by customers of the two major Valley utilities is down more than 40 percent from their July peaks.
"Operationally, we are not having any problems meeting the load," said Scott Harelson, spokesman for Salt River Project, which derives about 14 percent of its electric supply from Palo Verde.
However, substitute supplies are more expensive, meaning customers could eventually pay more for electricity. The extent of the impact will depend on the length of the the shutdown, Fallon said.
Any price increases for APS customers would have to approved by the Arizona Corporation Commission, he said. SRP is not regulated by the commission, and a future rate increase would have to be approved by its board of directors.
The fact the latest problem took so long to be discovered should prompt the NRC to look at other nuclear plants and procedures, said David Lochbaum, a nuclear safety engineer for the Union of Concerned Scientists, a watchdog group.
Lochbaum said the Palo Verde plant has been a ‘‘stellar’’ performer until the past two years, when a series of problems have cropped up.
‘‘It’s a fairly subtle problem, and it was a good catch by the NRC,’’ Lochbaum said of the current issue. ‘‘It just would have been a great catch sooner.’’