8.19. In the design of various parts of the Nuclear Steam Supply System (NSSS), the effect of one parameter on another must be considered. For example, in a PWR, the fuel rod diameter and spacing affect both the moderator/fuel ratio and the coolant pressure loss through the core (§9.110). Throughout the NSSS, there are numerous design parameters which form an interrelated matrix which must be addressed systematically during the design process in order to develop specifications. A simplified indication of the need for integrating various parametric contributions in the design process is shown in Fig. 8.1. To simplify the figure, we have omitted the equally important areas of safety design and economics-related design, which also interrelate with the three areas indicated.

Sensitivity Analysis and Design Parameter Interactions

8.20. After the system has been modeled and intersystem dependencies identified, it is desirable to carry out a sensitivity analysis. Assume that the system in question is described by input parameters that may be varied and by output functions which characterize the behavior of the system and might, in turn, serve as input to another system. We would like to know how sensitive the output functions are to adjustments to the input param­eters. Should some parameters have little effect on the output, the model could be simplified. Also, should one or more input parameters be found to have an overriding effect, such input parameters would be identified as being critical to the design. At any rate, the sensitivity analysis step provides insight to the workings of the system and is a decision tool.