TVA has released the most recent inspection report of the fly ash containment ponds. This is ominous:

Since last year’s inspection, ash was dredged into Cells 1, 2, and 3 to such levels that the divider dikes for Cell 3 were buried and now there are only two large cells (1 and 2). Dredging was stopped in mid-November 2007 based on recommendations from EDS and Geosyntec Consultants, Inc. This preventative measure was taken to reduce water levels in the dredge cell through the winter months in an attempt to avoid another blow out.

I don't understand all the engineering lingo, but it sounds as though TVA was aware of problems and taking steps to address them but it was too late.

And if I understand it correctly, this 2002 environmental impact report on the installation of NOx scrubbers at Kingston also hints that the fly ash storage system was nearing capacity and that TVA was looking at alternatives because of new contaminates (ammonia) that would be introduced:

Ash is periodically dredged to either of the two active dredge cells on the north side of the ash pond. This is estimated to provide capacity for ash storage until 2012; however, the actual closure date will be affected by both ash production and utilization. The TDEC issued a solid waste disposal permit for the ash pond and dredge facility in September 2000.

[..]

TVA is currently looking at the option of redirecting the ash pond discharge (DSN 001) to the CCW discharge channel. In doing so, discharging any potential ammonia nitrogen from NOXTech and/or high dust SCR operation to the plant intake could be avoided. Flow distribution to the ash pond for this configuration would remain as they currently are today as reflected in Table 3-10.

Other options include a dry fly ash handling system which would replace wet fly ash sluicing. By eliminating the wet sluicing with a dry stacking configuration, any ammoniated ash would be isolated. The potential for ammonia releases would be managed by maintaining the exposed area to 10 acres or less thus reducing runoff and infiltration.

UPDATE: KNS: TVA official says language in inspection report 'misleading'

TVA spokesman Gil Francis said engineers and consultants describing minor failures in the walls used the term "blowout," a word he said sounds worse than the actual conditions.

"It's a matter of perspective," Francis said. "An engineer knows it's not as bad as it sounds."