The San Onofre Nuclear Power Plant has embarked on a series of upgrades that will allow the plant to increase its power output by about 2% over it’s original design. The Nuclear Regulatory Commission (NRC) refers to this as a “power uprate” of the reactors.

Two new high-pressure steam turbines will replace existing ones. They will generate about 48 more megawatts of power — enough to support about 31,000 average-sized homes.  The first will be installed later this month when the Unit 2 reactor is shut down for refueling.

This should bring increased profits to Southern California Edison, but will it be at the risk of our safety?

In an uprated reactor, more neutrons bombard the core, increasing stress on its steel shell. Core temperatures are higher, lengthening the time to cool it during a shutdown. Water and steam flow at higher pressures, increasing corrosion of pipes, valves and other parts.

“This trend is, in principle, detrimental to the stability characteristics of the reactor, inasmuch as it increases the probability of instability events and increases the severity of such events, if they were to occur,” the NRC Advisory Committee on Reactor Safeguards, which is mandated by Congress to advise the NRC, has warned.

According to the NRC, some uprated reactors have had serious safety problems.

“The greatest concern is loose parts that you can’t find,” John Sieber, a nuclear engineer on the NRC advisory committee, said during a 2004 meeting. “Are they in the bottom of the reactor vessel? …. Is it floating around where it can damage internal parts of the core?”

The NRC allows companies to include increased containment vessel pressure in their safety calculations. But factoring in 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 NRC.

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. However, this alternative would require plant modifications so costly that companies claim it would not make economic sense to uprate.

For the U.S. nuclear industry, uprating is one of the cheapest ways to add power to the grid.

For more information see NRC Problems.