Extremely accurate sequential verification of RELAP5-3D

Large computer programs like RELAP5-3D solve complex systems of governing, closure, and special process equations to model the underlying physics of thermal-hydraulic systems and include specialized physics for the modeling of nuclear power plants. Further, these programs incorporate other mechanisms for selecting optional code physics, input, output, data management, user interaction, and post-processing. Before being released to users, software quality assurance requires verification and validation. RELAP5-3D verification and validation are focused toward nuclear power plant applications. Verification ensures that the program is built right by checking that it meets its design specifications, comparing coding algorithms to equations, comparing calculations against analytical solutions, and the method of manufactured solutions. Sequential verification performs these comparisons initially, but thereafter only compares code calculations between consecutive code versions to demonstrate that no unintended changes have been introduced. An automated, highly accurate sequential verification method, based on previous work by Aumiller, has been developed for RELAP5-3D. It provides the ability to test that no unintended consequences result from code development. Moreover, it provides the means to test the following code capabilities: repeated time-step advancement, runs continued from a restart file, and performance of coupled analyses using the R5EXEC executive program. Analyses of the adequacy of the checks used in these comparisons are provided.