My understanding, as an engineer, is that existing nuclear plants were designed to last for 40 years of operation, and that extension of licensing beyond that lifetime was done without an engineering design review to justify the safety of that extension. Reason 13 should be that the design lifetime has been exceeded, and like the space shuttle these plants need to be either re-engineered, with associated rebuilding as found necessary, or shut down. Safety can be assured only if all elements of the plant, down to the smallest part, have been reviewed to determine whether the new licensing timeline is within the acceptable safe design limits of the part or system, given the hostile environment including high radiation and salt spray; otherwise the part must be replaced. Note that many parts may be prohibitively expensive to replace. Designs should also have been thoroughly reviewed with respect to current knowledge vs design baseline, and upgraded as needed to assure safety, prior to re-licensing. The NRC's failure to take a scientific and engineered approach to re-licensing does not take that off the table as a valid reason to shut down the plant.