National Public Radio: There aren’t a lot of answers yet about what caused the catastrophic Dec. 22 spill of coal ash from a Tennessee Valley Authority plant near Knoxville. But the disaster has raised lots of questions about whether regulations of coal ash are strict enough. Coal ash is the stuff that’s left over after coal-fired power plants generate electricity and strip out pollutants. Plants produce about 130 million tons of it every year. “That’s enough to fill a line of railroad boxcars …