Four U.S. power reactors shut & NYC sweats during heat wave

July 18 (Reuters) - Several nuclear plants on the U.S. East Coast shut down by early W ednesday and New York's Consolidated Edison power company reduced the voltage in parts of Manhattan as the obsessive heat wave stressed the region's power system.

Despite the shutdown of four giant nuclear reactors in New York, Pennsylvania, Maryland and South Carolina, the power systems delivered the juice needed by the regions' homes and businesses to keep air conditioners humming on the projected last day of a brutal heat wave.

Temperatures in New York City -- the biggest metropolitan region in the United States -- hit 91 degrees Fahrenheit (33 Celsius) on Monday, 96 F on Tuesday, and were expected to reach 96 F again on Wednesday before thunderstorms Wednesday night were likely to drive temperatures back to near-normal levels in the 80s F on Thursday, according to AccuWeather.com.

In New York, Wednesday's voltage reduction was the second for Con Edison during this week's heat wave, aimed at easing the load on the grid to allow workers to fix heat-stressed equipment in the affected neighborhoods.

Customers do not lose power in a voltage reduction, but incandescent lights, for example, glow more dimly, hot water heaters take longer to heat water and some motors run more slowly.

Con Edison, which sells power to 3.2 million customers in the New York City area, said its system was working well despite the reduction and only 92 customers were without power Wednesday morning, which is low for a utility of its size at any time of year.

U.S. natural gas futures meanwhile jumped 8 percent to $3.02 per million British thermal units (mmBtu) due to the heat and nuclear outages.

Although the heat makes it more difficult to use the warmer river water to cool power plants and can stress power lines due to high usage, the reactors did not necessarily shut due to the heat.

REACTORS DOWN

Constellation Nuclear Energy Group's 630-megawatt Nine Mile Point 1 nuclear reactor in New York automatically shut on Tuesday due to high neutron flux -- meaning neutrons are not equally spread around the reactor core. Power traders guessed it could have been a faulty sensor and the unit could be back soon.

Constellation Nuclear is a venture between French power company Electricite de France SA (EDF) and Chicago power company Exelon Corp.

A unit at Exelon's Limerick nuclear plant in Pennsylvania shut early Wednesday, according to power traders. Officials at the company and the U.S. Nuclear Regulatory Commission could not confirm the Limerick shutdown.

Constellation Nuclear 855-MW took the Unit 1 at the Calvert Cliffs nuclear plant in Maryland offline by early Wednesday due to a small leak in an instrument line. The company said it had already fixed the plant and was ramping up the unit.

North Carolina-based Duke Energy's 846-MW Unit 1 at the Oconee nuclear plant in South Carolina also shut by early Wednesday. Details about the Oconee shutdown were not immediately available to comment.