Technical bases for the use of the Square Root of the Sum of Squares (SRSS) method for combining dynamic loads for Mark II plants

This report describes the methods and bases for the use of Square Root of the Sum of the Squares (SRSS) rule for combining co-directional components of dynamic response for Mark II plant. Numerical computations were conducted for a total of cases of load combinations for actual Mark II systems, structures and components using the dynamic loading combinations specified in the DFFR for earthquake, LOCA and suppression pool dynamic loadings. The Square Root of the Sum of the Squares (SRSS) rule has been extensively used in dynamic analysis of nuclear power plants. The justification for the use of the SRSS rule is the observed low probability for the peak dynamic responses occurring simultaneously even though the various components of the same event (e.g., earthquake) or different events (e.g., earthquake and S/RV discharge) occur at the same time. The reasoning behind use of the SRSS rule is quantified through the use of probability theory in this study.