[en] A study was undertaken to evaluate alternate criteria for creep fatigue evaluation of LMFBR steam generators constructed of 2 1/4 Cr.-1 Mo alloy steel. A review was conducted of the ASME Criteria for Design of Elevated Temperature Class 1 Components and the existing data available for 2 1/4 Cr.-1 Mo alloy steel. The current method in the Code for calculating creep-fatigue interaction damage using linear summation of fatigue damage and creep damage was employed on the existing test data. Alternately, the strain range partitioning method was considered. It was determined that the Code method is generally conservative for predicting life of the 2 1/4 Cr.-1 Mo test specimens, and the strain range partitioning method, as originally proposed, appeared to be unconservative when extrapolated to the low strain range long hold time service required by the design. A series of creep relaxation analyses employing the linear time fraction creep damage rule to evaluate the effect of plant thermal transients on the predicted design life and sensitivity of calculated creep damage to various material behavior parameters was conducted. Several creep equations and the effect of biaxiality were studied to determine their effect on stress relaxation and calculated creep damage. Calculated stress relaxation results from various creep equations were compared to measured relaxation from structural tests to aid in selecting an appropriate creep equation. In order to explore the sensitivity of calculated creep-fatigue damage to primary stress intensity, an infinite thick-cylinder was analyzed for variable pressure conditions using elastic-plastic-creep finite element techniques. In order to evaluate the effect of potential elastic follow-up effects on calculated creep damage, a built-in cylinder was also analyzed by finite element techniques