A Nuclear Regulatory Commission color-coded performance grading system assigned a “yellow” to Browns Ferry Unit 1 reactor late Monday as the result of five unplanned “scrams” or shutdowns in the first seven months after restart, according to spokesman Craig Beasley.

The NRC uses color-coded inspection findings and performance indicators to assess nuclear plant performance. The colors start with “green” and then increase to “white,” “yellow” or “red,” depending on the safety significance of the issues involved.

“TVA has high standards of performance and we want to once again be in the ‘green’ category,” said Beasley, who retired soon after the Unit 1 restart but has returned as a part-time contract worker.

Beasley said the NRC cannot do an assessment and assign a color-coded performance indicator until a reactor has attain enough “critical” operational hours – 7,000—which Unit 1 attained at the end of December.

The reactor has automatically shut down five times.

“We’ve already taken a lot of hard looks at the shutdowns,” said Beasley. “Safety is paramount.”

The Tennessee Valley Authority restarted Unit 1 May 22, ending a 22-year shutdown prompted by safety concerns and after a five-year, $1.8 billion renovation. The entire three-reactor plant was idled in 1985 out of concerns over plant safety and management.

Units 2 and 3 were restarted in the 1990s. Unit 1’s restart marked the first time all three Browns Ferry reactors have been operating at the same time in about 27 years.

Beasley said engineers and operators have determined that two of the scrams resulted from fittings in the electric hydraulic control system that “were not up to TVA standards.” He said TVA has found “across the board” responsibility for the substandard fittings.

Two other scrams were the result of instrumentation errors on moisture separators. “That was an engineering issue,” said Beasley. “It caused false signals.”

He said one scram resulted from false signals to the neutron monitor, which controls nuclear reaction in the reactor’s core.

“A sensing line came loose and that was a workmanship issue,” said Beasley.

Tuesday, Unit 1 was operating at 80 percent. The 1,200-megawatt atomic generator is capable of lighting 650,000 homes at full power.

“As operation continues we’ll be improving operations, work management and maintenance,” said Beasley. “We’ll make sure that we’re on top of our game. Our goal is to meet consistently the highest standards.”

Beasley emphasized that the “yellow” performance rating is not an indication of substandard performance of the entire three-reactor Browns Ferry site.