Of the 104 operating power reactors in the United States, the youngest entered service in 1996.

The four reactors to be built are the only survivors in what had been envisioned as a bigger field of new plants that narrowed over the last three years as investors ran into financial and other obstacles.

In fact, it is not clear whether ground will be broken on any additional reactors soon; industry experts say the biggest obstacle is that the price of natural gas remains quite low, making it difficult to produce electricity from a reactor at a price competitive with electricity from a gas-burning plant.

Congress has approved $18.5 billion in loan guarantees for new reactors and there is considerable support for even more, but it is not clear that borrowers will emerge.

Among other design improvements, the Westinghouse AP1000 is supposed to shut down safely in the event of a loss of all electrical power, which is what befell the Fukushima Daiichi plant in Japan after the earthquake and tsunami in March.

Westinghouse says that a combination of automatic systems and design features would keep the reactor safe for three days without human intervention and that its core could be kept from melting indefinitely with only minimal operator effort.

The regulatory commission approved an earlier version of the AP1000 in 2006, but the design was later ruled out for American utilities when the agency adopted a rule in 2008 requiring newly constructed reactors to be able to withstand the impact of a crashing aircraft.

China is in advanced stages of constructing four units of an earlier version of the AP1000. The first unit is scheduled to go online in 2013, about three years before the first one would begin operating in the United States.