UPDATE 1-Entergy reduces Vermont Yankee reactor for rod exchange

(Updates with company comment)

NEW YORK, March 12 (Reuters) - Entergy Corp (ETR.N: Quote, Profile, Research) reduced the 506-megawatt Vermont Yankee nuclear power station in Vermont by early Wednesday for a planned quarterly rod pattern exchange, a spokesman for the plant said.

Electricity traders guessed the unit would return to full output in a day or so.

Nuclear power operators adjust the pattern of the control rods to maximize unit efficiency.

A control rod is made of chemical elements capable of absorbing neutrons that control the speed of the fission of uranium.

On Monday, the unit was operating at full power.

Vermont Yankee, which entered service in 1972, is located in Vernon in Windham County about 80 miles north of Hartford, Connecticut.

Entergy in January 2006 filed for a 20-year extension of the unit's original 40-year operating license. In August 2007, however, the NRC revised the schedule for issuing a safety evaluation after the company provided additional information.

At that time, the NRC said its staff would take at least two months to go over the additional information and determine if the agency requires more data.