GE Hitachi Nuclear Energy said it had discovered extensive cracking and "material distortion," and likely would recommend that the boiling water reactors using its Marathon control rod blades replace them more frequently than they had been told to previously.
"The design life if not revised, could result in significant control blade cracking and could, if not corrected, create a substantial safety hazard and is considered a reportable condition," the company said in its report to the NRC.
Both David Lochbaum of the Union of Concerned Scientists and Arnold Gundersen, a former nuclear industry engineer who now frequently consults with groups critical of the industry, said the faulty blades could make affected control rods inoperable.
"It could either slow down or stop the control rod from inserting" when plant operators were trying to reduce power or shut a plant down, Lochbaum said.
Gundersen said control rods "are like the brakes on a nuclear reactor. It's almost like they have a 100,000 mile warranty on them and they need to be changed out at 40,000."
He added that the reactors also have an emergency brake: an "explosive valve" to be used in emergencies when operators are unable to gain control of the reaction by inserting control rods. The valve forces water containing high levels of boron, which slows and eventually stops the reaction by absorbing neutrons.
After using that measure, Gundersen said, "it takes months to clean up" while the plant is shut down. In their training reactor operators are "taught it's there, but you pray you never have to use it."
Signs of cracking in the blades would include increased levels of boron and tritium, a radioactive isotope of hydrogen, in the water used to cool the reactor, said Neil Sheehan, an NRC spokesman.
"As long as there is no significant increase in boron or tritium observed, the recommendation would be continue operation until the end of the operating cycle," Sheehan said in an e-mail.
Jim Klapproth, chief engineer with GE Hitachi Nuclear, said in a statement, "Customers with affected control blades can avoid the safety concern by monitoring the water chemistry of their plants and replacing affected blades prior to the end of the revised service life."
Commercial reactors in the U.S. are divided into two classes: boiling water reactors, the type of concern here, and pressurized water reactors. GE Hitachi listed the affected plants as follows:
Nine Mile Point, Unit 1 and Fitzpatrick plants near Oswego, N.Y.; Millstone, Unit 1, Waterford, Conn.; Pilgrim, Plymouth, Mass.; Vermont Yankee, Vernon, Vt.; Grand Gulf, Port Gibson, Miss.; River Bend, St. Francisville, La.; Clinton, near Clinton, Ill., Oyster Creek, Lacey Township, N.J.; Dresden, Unit 2; Dresden, Unit 3, near Morris, Ill.; Peach Bottom, Unit 2; Peach Bottom, Unit 3, Peach Bottom Township, Penn.; Quad Cities, Unit 1; Quad Cities, Unit 2, near Cordova, Ill.; Perry, Unit 1, North Perry, Ohio; Duane Arnold, near Palo, Iowa; Cooper; near Brownville, Neb.; Monticello, Monticello, Minn.; Brunswick, Unit 1; Brunswick, Unit 2, near Southport, N.C.; Hatch, Unit 1; Hatch, Unit 2, Browns Ferry, Unit 1; Browns Ferry, Unit 2; and Browns Ferry, Unit 3, Athens, Ala.