Sensitivity analysis of a complex, proposed geologic waste disposal systemusing the Fourier Amplitude Sensitivity Test method

The Fourier Amplitude Sensitivity Test (FAST) method has been used to perfo rm a sensitivity analysis of a computer model developed for conducting tota l system performance assessment of the proposed high-level nuclear waste re pository at Yucca Mountain, Nevada, USA. The computer model has a large num ber of random input parameters with assigned probability density functions, which may or may not be uniform, for representing data uncertainty. The FA ST method, which was previously applied to models with parameters represent ed by the uniform probability distribution function only, has been modified to be applied to models with nonuniform probability distribution functions . Using an example problem with a small input parameter set, several aspect s of the FAST method, such as the effects of integer frequency sets and ran dom phase shifts in the functional transformations, and the number of discr ete sampling points (equivalent to the number of model executions) on the r anking of the input parameters have been investigated. Because the number o f input parameters of the computer model under investigation is too large t o be handled by the FAST method, less important input parameters were first screened out using the Morris method. The FAST method was then used to ran k the remaining parameters. The validity of the parameter ranking by the FA ST method was verified using the conditional complementary cumulative distr ibution function (CCDF) of the output. The CCDF results revealed that the i ntroduction of random phase shifts into the functional transformations, pro posed by previous investigators to disrupt the repetitiveness of search cur ves, does not necessarily improve the sensitivity analysis results because it destroys the orthogonality of the trigonometric functions, which is requ ired for Fourier analysis. (C) 2001 Elsevier Science Ltd. All rights reserv ed.