UPDATE 1-Dominion Va. North Anna 1 reactor shut

(Updates with company comment to NRC)

NEW YORK, Oct 26 (Reuters) - Dominion Resources Inc's (D.N) 903-megawatt Unit 1 at the North Anna nuclear power station in Virginia shut by early Monday from full power early Friday, the U.S. Nuclear Regulatory Commission said in a report.

In a report to the NRC, Dominion said on Oct. 23 it planned to shut the unit due to excess letdown heat exchanger tube leak and an after the fact unusual event.

The unusual event - the lowest of four levels of emergency classification - was due to an unidentified or pressure boundary leakage greater than 10 gallons per minute.

By the time the company realized the leak was greater than 10 gallons per minute, the leak was already below that amount.

Electricity traders guessed the unit would return within a week.

The 1,806 MW North Anna station is located in Mineral Louisa County about 50 miles northwest of Richmond. There are two 903 MW Units 1 and 2 at the station, which entered service in 1978 and 1980.

Unit 2 continued to operate at full power.

One MW powers about 700 homes in Virginia.

In 2007, the NRC approved an early site permit for North Anna that will allow Dominion to build one or more new reactors.

Also in 2007, Dominion filed with the NRC to build one of General Electric Co (GE.N)/Hitachi Ltd's (6501.T) 1,550 MW Economic Simplified Boiling Water Reactors (ESBWR) at the site.

In January 2009, Dominion said it had not reached an agreement with GE/Hitachi for the new reactor, though the utility said it would continue to pursue the license based on the GE/Hitachi design.

Ultimately, if Dominion said if it decides to move forward with the project, the new reactor could enter service in 2015.

Using an industry estimate of $4,000 per kilowatt, the new reactor could cost about $6.2 billion.

Dominion operates the existing station for its owners, Dominion (88.4 percent) and Old Dominion Electric Co-op (11.6 percent).

Dominion, of Richmond, Virginia, owns and operates about 27,000 MW of generating capacity, markets energy commodities and transmits and distributes electricity to 2.4 million customers in Virginia and North Carolina. (Reporting by Scott DiSavino; Editing by John Picinich)