Such a scenario is extremely unlikely, because the accident itself would create a pressure buildup in the surrounding containment vessel that would ensure that the pumps kept working. The system would risk failure only if the containment vessel itself were breached, allowing outside air to rush in.
The NRC allows companies to include that containment vessel pressure in their safety calculations. It has been used in 17 uprate approvals, according to NRC spokesman Scott Burnell.
But factoring in the pressure buildup "represents a decrease in the safety margin available to deal with a phenomenon subject to large uncertainties," the agency's safety advisory committee wrote in a March 18, 2009, letter to the agency. Forcing regulators to show that the safety system would work without the pressure buildup would offer an extra layer of protection against "potential melting of the core," the letter said.
The alternative would be requiring plant modifications so costly that companies say it would no longer make economic sense to uprate.
For the U.S. nuclear industry, which supplies a fifth of the nation's electricity, uprating is attractive because it is one of the cheapest ways to add power to the grid.
The 1979 partial meltdown at Three Mile Island eroded public confidence in nuclear power. Construction proceeded on many reactors that had already been approved — the last one went into operation in 1996 — but the industry was forced to look for ways to get more out of existing plants.
The biggest gains have been achieved by running reactors more efficiently — less downtime for fuel changes, for example.
But uprates have played an important role, adding the equivalent output of nearly five average-sized reactors since 1996. Regulators say they expect to approve boosts totaling 3 1/2 more reactors over the next four years.
Exelon, the nation's top nuclear provider, plans to spend $3.65 billion on power boosts equivalent to one new nuclear reactor over the next eight years, according to its filings for investors.
"They would come at half the cost of a new plant and with less risk because of the opportunity to defer expansion if power prices do not support it," its annual report says.
