CHATTANOOGA, Tenn. (AP) — The failure of a reactor coolant valve at the Tennessee Valley Authority's Browns Ferry Nuclear Plant months ago has raised questions about apparent violations of Nuclear Regulatory Commission requirements.NRC Atlanta region spokesman Joey Ledford said Thursday that the valve was stuck shut. Although there is a separate reactor cooling system, there are scenarios when "this particular system would need to be operable," he said.An NRC statement said TVA has been called to a Monday meeting in Atlanta to explain the safety significance of the valve failure that TVA and the NRC discovered last fall during a shutdown of the Unit 1 reactor at the Browns Ferry Plant near Athens, Ala. The statement said the valve "would have been unable to fulfill its safety function if it had been needed."David Lochbaum, nuclear program director for the Union of Concerned Scientists, said Thursday that for most accidents the valve problem was inconsequential but not in a fire."Had there been a fire at the plant there would have been an inability to cool the core," Lochbaum said in a telephone interview.