Austenitic stainless steels, especially AISI 304 and 316, form the core internals that frame and support the cores of pressurized water reactors. In some reactors alloys such as Inconel 600 or 718 were also used for selected in-core applications such as guide tubes and springs. The nearcore portion of the baffle-former assembly receives relatively high neutron exposures (40-100 dpa) over a 40 year lifetime. Extending the lifetime of these components to 60 or 80 years will lead to

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... higher damage exposures. Within the 40 year licensing period a number of issues of first-order importance have been addressed, especially embrittlement, IASCC and irradiation creep. A number of second-order issues have been recognized but not considered to be life-limiting or deleterious. However, the non-linear nature of a number of second-order processes gives cause to worry that they might become first-order with extension of PWR lifetimes. The first of these second-order process is the void swelling phenomenon which is clearly a non-linear and non-saturable process. The second of these second-order processes is the progressive promotion of the five naturally occurring isotopes of nickel to higher atomic weight via transmutation. One product of this transmutation sequence is the production of Ni-59, a nonnaturally occurring isotope that continues to increase in concentration until a thermal neutron fluence of about 4 x 1022 n/cm2 has been reached. The consequences of the Ni-59 production are a very high and continuously accelerating production of helium and hydrogen, concurrent with a generally unrecognized increasing rate of atomic displacement and increased nuclear heating, the latter two consequences arising from the highly exothermic nature of the Ni-59 (n, α) and (n, p) reactions. These processes become increasingly important for higher nickel alloys such as Inconel 600 and 718. The third process is the now well-known tendency of hydrogen to be stored in heliumnucleated bubbles and voids, and for the increasing storage to promote formation of very high densities of bubbles not only in the matrix but also on grain boundaries. Cracks moving along grain boundaries or through the matrix will constantly be intersecting cavities filled with hydrogen with possible consequences on accelerated cracking. The fourth process is the generally unrecognized but progressive radiation-induced movement of 300 series steels toward stress-induced martensitic instability, especially at very high damage levels. These four processes will be shown in this paper to have already begun to reveal their growing presence. Based on the known parametric dependencies of these four processes it is expected that they will most likely interact in a very synergistic manner. These processes should be studied proactively before they become truly first-order in importance.