Using Code Adequacy Methodologies in Confomance with ASME Standards for Nuclear Power Plant Analysis Evaluation Models

The development of the systems analysis codes in use today was a very challenging task, stemming from the interplay of multiple physical phenomena, special components and control systems, and particularly the wide thermodynamic state envelope for a typical design basis accident scenario that includes single and two-phase behavior of the water working fluid within both the reactor vessel and the steam generator for the indirect cycle pressurized water reactor systems and also for the direct cycle boiling water reactor systems. The major developmental work leading to the current systems analysis codes was performed between the 1970s through the 1990s—and today these analysis tools are used throughout the world by organizations that design, submit their designs for licensing reviews, build, and operate light water reactor nuclear power plants. Differences in form of the discretized equations and closure relationships used within the systems analysis codes versus those in higher-fidelity computational fluid dynamics (CFD) codes lead to correspondingly different techniques to verify and validate (V&V) the equations in these two classes of codes. Systems analysis codes use a fundamental approach which has been developed over the years and which has been approved by the regulatory agencies whereas the CFD codes use high-fidelity V&V techniques as described in the ASME V&V standards for computational fluid mechanics and heat transfer codes. Because of the wide usage of high-fidelity CFD codes together with systems analysis codes, it is advisable to normalize the techniques for verifying, validating, and performing code adequacy assessments of these tools within the methodology that is presently available in the U.S. Nuclear Regulatory Commission’s Regulatory Guide 1.203. A strategy to begin closing the gap between the fundamental approach used to V&V systems analysis codes and the high-fidelity techniques used for modern CFD codes is outlined. It is postulated that the gap can be closed to the extent that some of the “high-fidelity” techniques may be used for systems analysis codes and thus enhance the quality of the code adequacy determination process for systems analysis codes.