The design and construction rules for nuclear components at elevated temperatures are currently in the ASME BPV Code, Section III, Division 1, Subsection NH, and will soon be incorporated in the new Section III, Division 5. The current allowable stresses are limited to 300,000 hours, which is equivalent to a 40-year design life. As a part of the Next Generation Nuclear Plant (NGNP) initiative, the allowable stress values are being revised to include higher temperatures and longer lifetimes. This is expected to support the design and construction of high temperature reactors for sixty-year (500,000-hour) design lifetimes. The technical bases for the revised temperature and time dependent stress allowable values for Type 304 and Type 316 stainless steels are presented in this paper. Since the allowable stresses at elevated temperatures are controlled by creep, the revised allowable stresses are based on Larson-Miller parametric analyses of creep data from Oak Ridge and NIMS, some of which was not available when the current allowable stresses were computed. The time-dependent stress intensity values (St) are based on ASME rules covering time to rupture, time to reach 1% strain and time to reach the onset of tertiary creep. The effect of the tertiary creep criterion on St at long times and high temperatures is significant. The tertiary creep criterion applied to the current Oak Ridge and NIMS data results in lower and more conservative high temperature and long time allowable stress values than those currently in Section III, NH. The potential reasons for the limiting tertiary creep effects are discussed, with the conclusion that the ASME graphical method specified to determine the onset of tertiary creep from the creep data is not appropriate for Type 316 stainless steel.

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