Of the 104 nuclear reactors operating in the U.S., 89 made the Nuclear Regulatory Commission’s (NRC’s) highest performance category last year, said the federal body on Tuesday.

The agency rates reactor performance at five levels based on a variety of indicators, including safety system availability and reliability, control of radiation exposure and unplanned shutdowns. Levels range from “fully meeting all safety cornerstone objectives” (highest level) to “unacceptable performance” (lowest level). Announcing that it had issued its annual assessment letters to the nation’s operating reactors, the NRC said 98 reactors performed at the two highest levels.

Reactors rated at the top level were STP Nuclear Operating Co.’s units in Texas—as well as Entergy’s controversial Vermont Yankee unit. Vermont’s Senate last year voted against re-licensing the reactor after 2012, citing radioactive tritium leaks and a 2007 cooling tower collapse, among other problems. The NRC is considering an application to renew the reactor’s operation license, which is set to expire in March 2012. Sources say the agency is close to issuing a new 20-year license for the plant.

Nine nuclear reactors performed at the second-highest level, showing the need to resolve one or two items of low safety significance. For this performance level, regulatory oversight includes additional inspection and attention to follow up on corrective actions. These plants were: Brunswick 1 and 2 (N.C.), Calvert Cliffs 2 (Md.), Farley 1 (Ala.), Ginna (N.Y.), North Anna 2 (Va.), Susquehanna 1 (Pa.), and Turkey Point 3 and 4 (Fla.)

Six nuclear reactors were at the third level of performance, with one degraded safety cornerstone. “For this performance level, regulatory oversight includes more NRC inspections, senior management attention, and oversight focused on the cause of the degraded performance,” the agency said. These plants were: Oconee 1, 2, 3 (S.C.); Fort Calhoun (Neb.); H. B. Robinson 2 (S.C.); and Wolf Creek 1 (Kan.).

Sources: POWERnews, NRC