Performance of Diablo Canyon annulus steel structure

This paper describes portions of the review effort that were undertaken to license the Diablo Canyon nuclear power plant. A rather extensive process was undertaken because of the severe seismic environment and because several errors were uncovered which cast doubt on the adequacy of the quality assurance program that was implemented by the designers during the seismic analysis of the plant. An IDVP was established which reviewed a large portion of the safety related seismic analyses. The function of this review was to identify specific errors in design and to determine whether these errors had generic implications for other portions of the design. The principal sources of errors which were found resulted from poor communication across design interfaces. Sufficient controls had not been established to insure that the latest design data was available as needed. The NRC performed independent analyses of the vertical and horizontal seismic response of the annulus steel structure. Complete three dimensional models of the structure were used for these analyses. A comparison of the floor response spectra generated from these models with the simpler models used by the utility showed significant differences. The differences generally were found to result from the ommision of structural flexibilities in the models used by the utility. Special care must be taken when using simple structural models to insure that all important modes of response are included. (orig.)