The analysis adds, “Evaluation of the contributing failure modes identified only one practical root cause subject to control and that was inadequate scope/sequence of tendon de-tensioning used in the [steam-generator replacement] outage.”

The unit's design added to its risk, according to PII. Furthermore, the tendons were de-tensioned in sequence, rather than through another approach that would have distributed the stress more evenly, says the analysis.

The U.S. Nuclear Regulatory Commission reviewed the damage and the PII report and found no violations at the unit.

Progress Energy CEO Vincent Dolan defended the repairs in a June statement, claiming they were justified by the plant’s low cost of operation and fuel use.

However, the utility says its repairs are not contingent upon receiving PSC’s permission to charge its customers for the costs, according to Rob Sumner, a spokesman. A portion of the damage is covered by insurance on the unit.

According to the St. Petersburg publication, other utilities had tended to hire San Francisco-based Bechtel Corp. or SGT,  a nuclear-engineering joint venture of Areva North America and URS Corp., to manage work on previous nuclear steam-generator replacement projects. The publication quoted Sandia National Laboratory scientist Kenneth Bergeron as saying that those companies have proprietary methods of replacing steam generators in nuclear units.

Progress Energy hired Bechtel to do some work on the initial replacement projects and still retains the firm, Sumner says.

Michelle Michael, a spokeswoman for Bechtel, would not comment on the contractor’s scope of work for Progress Energy, but says that its methods were not proprietary.

“While our experience in [steam-generator replacement] is extensive, with 35 [steam-generator replacements] in 30 years, our methods are not proprietary," she told ENR. "Rather, they are based on our engineering talent and time-tested processes and procedures.”

The PSC does not expect to issue a final ruling until next year, with some portions of the case expected to continue until early 2014, according to its procedural schedule.