Stress corrosion cracking (SCC) in the blade attachment region of low-pressure (LP) turbine rotors first emerged as a significant threat to the integrity of nuclear turbines. In the last decade, an increasing number of fossil LP turbines have been afflicted by this cracking problem. Supercritical fossil units and units with once-through boilers are most prone to blade attachment SCC. This paper provides a detailed discussion of an evaluation and repair of two turbine blade attachments in the LP turbine of an 1150 MW super-critical fossil unit operated by TVA. The affected stages were the second (L-3) and third (L-2) stages of a five stage double flow rotor. Each attachment had a different repair based on the extent of the damages revealed in the NDE inspection. A probabilistic method was used to evaluate risk and life of different repair options for the L-3 attachment. The L-2 attachment required a complete redesign of the attachment and blade replacement. The uncertainties associated with the many variables that govern the risk of blade attachment failure were modeled using a probabilistic approach. The approach provides a more realistic basis for making run/repair decisions than an overly conservative deterministic analysis. This approach was the basis for economic justification for the repairs and blade replacements. Repair and mitigation strategies to allow for continued operation of this unit are described in this paper. These repairs allowed for a complete restoration of turbine output without a significant extension to the outage cycle.
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