Seismic Proving Test of Ultimate Piping Strength: Safety Margin of Seismic Design Code for Piping

The proving test of a large-scale piping system and elasto-plastic analyses were conducted to assure safety margins of current and newly proposed seismic design codes for piping in Japanese nuclear power plants. Two test models were used, one for the design method confirmation test and one for the ultimate strength test. The allowable acceleration safety margin (M_A) was defined as the ratio of A_f/A_S2, where A_f is the maximum input acceleration corresponding to piping failure in a single seismic input, and A_S2 is the maximum allowable input acceleration for the S₂ seismic wave according to Japanese seismic design codes for Class 1 piping components. When the current code was applied to the design method confirmation test model, associated with the 10% broadened response spectrum of the S₂ seismic wave for a PWR building, M_A was revealed to be 9.6 for a frequency ratio (R_W) of 0.7. Without the response spectrum broadening, M_A decreases to 4.9. Redefined as the ratio in terms of the displacement and cyclic peak stress of the highest stressed component, the safety margins (M_D, M_FC) tend to change to 7.7 and 3.7, respectively. The newly proposed code includes the following modifications from the current code rules: 1) only the allowable stress limits to prevent fatigue failure is ruled for the “S₂” seismic wave, 2) a new diagram of the K_e factor in the fatigue design analysis is implemented. The safety margin M_A of the proposed code with the broadened spectrum was revealed to be still as high as 6.6 for R_W = 0.7. Analytical evaluation was conducted to discuss the tendency of the safety margins for seismic input conditions that differ from R_W = 0.7, and the degree of conservatism or margin included in design analysis methods, whose degree is related to the safety margins.