Use of forward sensitivity analysis method to improve code scaling, applicability, and uncertainty (CSAU) methodology

Abstract Since the code scaling, applicability, and uncertainty (CSAU) methodology was proposed about two decades ago, it has been widely used for new reactor designs and existing LWRs power uprates. In spite of these huge successes, CSAU has been criticized for the need of further improvement, focusing on two main issues – lack of objectiveness and high cost. With the effort to develop next generation safety analysis codes, new opportunities appear to take advantage of new numerical methods, better physical models, and modern uncertainty qualification methods. Forward sensitivity (FS) analysis directly solves the partial differential equations for parameter sensitivities. Moreover, our work shows that time and space steps can be treated as special sensitivity parameters so that numerical errors can be directly compared with physical uncertainties. It should be noted that FS analysis is an intrusive uncertainty quantification method that requires the user of the method to be familiar with the simulation code structure including numerical spatial and temporal integration techniques. When the FS analysis is implemented in a new advanced system analysis code, CSAU could be significantly improved by quantifying numerical errors and allowing a quantitative PIRT (Q-PIRT) to reduce subjective judgment and improve efficiency. This paper will review the issues related to the current CSAU implementations, introduce FS analysis, show a simple example to perform FS analysis, and discuss potential improvements on CSAU with FS analysis. Finally, the general research direction and requirements to use FS analysis in an advanced system analysis code will be discussed.