NEW YORK - STP Nuclear Operating Co's 1,280-megawatt Unit 1 at the South Texas nuclear power station in Texas shut by early Thursday from 92 percent early Wednesday, the U.S. Nuclear Regulatory Commission said in a report.

Electricity traders guessed the unit shut for a planned five week refueling and maintenance outage expected to start in early October.

The unit last shut for refueling from about March 29-April 28, 2008. It is on an 18 month refueling cycle.

The 2,560 MW South Texas station is located in Bay City in Matagorda County about 80 miles south of Houston. There are two 1,280 MW units at the station, which entered service in 1988 and 1989.

Unit 2 meanwhile continued to operate at full power.

One MW powers about 500 homes in Texas.

South Texas Project Nuclear Operating Co operates the station for its owners: NRG Energy Inc (44 percent), the City of San Antonio's CPS Energy (40 percent) and the City of Austin's Austin Energy (16 percent).

NRG, which has partnered with Toshiba Corp to form Nuclear Innovation North America LLC, wants to build two modified versions of the General Electric Co designed 1,350 MW Advanced Boiling Water Reactors at the station. Toshiba has already built modified versions of the ABWR in Japan.

In 2007, NRG filed with the NRC for permission to build the new reactors. The company filed a modified application in September to reflect the modifications of the ABWRs built in Japan. The companies expect the 1,350 MW Units 3 and 4 to cost more than $7 billion and enter service in 2015-2016.

The new units would be owned by Nuclear Innovation North America and others. Toshiba will own a 12 percent equity stake in Nuclear Innovation North America.

NRG, of Princeton, New Jersey, owns and operates more than 24,000 MW of generation in the United States and provides power to more than 1.7 million customers in Texas. (Reporting by Scott DiSavino; Editing by John Picinich)