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RICHMOND, Va., Sept 12 (Reuters) - Dominion (NYSE:D - News), in a move to ensure the safety of its North Anna and Surry nuclear power stations in Virginia, ordered four new reactor vessel heads for the twin reactor units, the company said on Thursday.

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The $175 million order was placed with Mitsubishi Heavy Industries America Inc., a unit of Japan's Mitsubishi Heavy Industries Ltd. (Tokyo:7011.T - News), and Westinghouse Electric Co., part of Britain's government-owned Nuclear Utilities Business Group of British Nuclear Fuels (BNFL).

Three of the new heads, all made of 6-inch (15 cm) thick carbon steel, will be installed during scheduled refueling outages in 2004 and the fourth head will be installed during refueling in the spring of 2005, Dominion said.

The company said the units could be safely operated until then.

The replacement heads aim to avoid the serious safety concerns seen at First Energy's (NYSE:FE - News) Davis-Besse nuclear power plant in Ohio, which was shut in February after severe corrosion was discovered in its reactor head.

Davis-Besse's design and certain elements of its construction are shared by several of the nation's 103 nuclear reactors, including the North Anna and Surry facilities.

Problems with the heavy, carbon steel heads capping some pressurized water reactors stem from boric acid seeping out around any of the dozens of holes through which the reactor's control rods pass.

The acid, used to help control the nuclear fission process in the reactor, eats away at the top of the steel lid, raising the threat of a radioactive leak if not repaired.

Dominion said inspections last year found "apparent" boric acid seepage in the heads bolted down on its Surry 1 and North Anna 2 units, prompting interim repairs until new heads could be ordered for all four Surry and North Anna units.

The North Anna station's reactors each generate 921 megawatts when at full power, while the Surry reactors are rated at about 812 megawatts each. One megawatt is roughly enough electricity to run 1,000 homes at any given moment.

The Surry reactors began commercial operation in 1972 and 1973, while the North Anna reactors entered service in 1978 and 1980, according to the U.S. Energy Information Administration.