Implementation and assessment of a stability-enhancing and high-resolution numerical scheme in RELAP5

Background: RELAP5 has been widely used in the field of nuclear thermal-hydraulics and safety analysis. However, improvement issues in terms of numerical stability and accuracy exist due to its basic single-pressure two-fluid model. Purpose: This study aims to enhance the numerical stability and numerical accuracy of RELAP5 by modifying the mathematical structure of RELAP5 basic model. Methods: Firstly, the default mathematical expression of the virtual mass force was modified in the RELAP5 code. Secondly, an interface pressure term was incorporated in the code, and a total variation diminish (TVD)-type flux limiter method was implemented in the RELAP5 code in place of the default 1st-order upwind scheme for the convective terms in mass and energy conversation equations. Results: In a numerical test where the evolvements of void fraction perturbation were concerned, the modified RELAP5 predicted no growth of the perturbation amplitude in contrast to a rapidly developing divergence for the default RELAP5. In addition, for the phase segmentation problem, convergence was accomplished on finer mesh by the modified RELAP5, while non-physical distribution of the void fraction was rendered by the default RELAP5 especially for finer mesh. When applied to the Ransom water faucet numerical test and Marviken CFT 15 experiments, the improved RELAP5 with 2nd-order of accuracy was more reliable in that the predictions were closer to the experimental results. Conclusion: Compared with the original RELAP5, the modified or improved RELAP5 showed evident numerical stability and accuracy during the assessment in this work.