PITTSBURGH—A frigid arctic air mass that brought record low temperatures to Pennsylvania, closing schools, courts and even some ski slopes is also being investigated as the possible cause of a nuclear reactor shutdown.

The dense mass of ultra-cold air began moving into the state on Monday afternoon, which is when one of two reactors shut down at FirstEnergy Corp.'s Beaver Valley Nuclear Power Station in Shippingport, about 25 miles northwest of Pittsburgh.

"There are many potential reasons this could happen and the cold weather is one of them," FirstEnergy spokeswoman Jennifer Young said. "But there are hundreds of other causes we're considering."

Western Pennsylvania temperatures were in the mid-20s Monday before dipping precipitously after noon.

The temperature in Pittsburgh fell to minus-7 degrees late Monday night—a record low for that date—and continued falling to minus-9 on Tuesday, another record. The previous record low for Jan. 7 was minus-5, in 1884. (The all-time low of 22 degrees below zero was set on Jan. 19, 1994.)

The reactor shut down about 5 p.m. Monday when the National Weather Service said temperatures had fallen to about zero.

Nuclear Regulatory Commission spokesman Neil Sheehan called the shutdown "uneventful" and safe, but noted a fire suppression system was activated, dousing an electrical transformer with water. The transformer remained frozen Tuesday.

The transformer converts power created by the reactor to electrical voltage.

The transformer's failure prompted the reactor to shut down. The reactor remained shut Tuesday as an investigation into the cause continues, Young said.

The NRC has an inspector at the plant, who will oversee FirstEnergy's investigation, any repairs and the reactor's eventual return to service, Young said. That could take several days, if the transformer must be replaced, Sheehan said.

By mid-afternoon Tuesday, temperatures in and around Pittsburgh hit zero and were forecast to climb into the low single digits by nightfall, with a high in the mid-20s expected Wednesday..

Most schools, courts and many government offices closed in western Pennsylvania, where wind chills reached minus-30 to minus-40 during the coldest morning hours, and were still expected to be in the minus-teens Tuesday night.

In Philadelphia and the rest of southeastern Pennsylvania, the cold was less severe, with low temperatures hovering around zero and wind chills in the minus-teens.

The cold was oppressive enough that some ski resorts closed or limited activities.

Blue Knob Resort, about 80 miles east of Pittsburgh, closed until Wednesday morning. Hidden Valley and Seven Springs, sister resorts about 50 miles southeast of the city, opened some slopes but kept others closed to take advantage of ideal snow-making temperatures. Some slopes at both resorts were to remain closed Wednesday, so grooming machines can distribute the man-made snow.

The cold weather also caused scattered power outages, as utilities scrambled to restore service even while urging folks to conserve electricity to keep from overloading the grid. By daybreak Tuesday, West Penn Power reported more than 5,000 customers without electricity in Armstrong, Beaver, Butler, Fayette and Washington counties, though service was restored to nearly all those customers by the afternoon.

Officials with PJM Interconnection, which manages the wholesale power supply for all or part of 13 states, said there was record winter demand Tuesday morning, and said another record may be set Tuesday afternoon. PJM, based in Valley Forge, asked customers to conserve energy if possible between 3 p.m. and 7 p.m. Tuesday and 6 a.m. to 9 a.m. Wednesday.

Parts of Erie and Crawford counties in the northwestern corner of the state were hit with up to 15 inches of snow.

The two major newspapers in that area, the Erie Times-News and the Meadville Tribune, both alerted readers that papers might not be delivered in some areas Tuesday because of the weather. As a result, both newspapers made their online versions free for the day.

There were no immediate reports of deaths from the extreme cold, though doctors at University of Pittsburgh Medical Center hospitals reported treating at least five people for frostbite.