RELAP5 simulation of the ROSA-IV/LSTF SB-CL-18 test using countercurrent flow limitation modeling

The Wallis form of countercurrent flow limitation (CCFL) model was implemented in RELAP5/MOD2-B ampersand W, and the code was assessed against the ROSA-IV 5% cold-leg small-break loss-of-coolant accident (SBLOCA) test SB-CL-18. The objective of this study was to demonstrate that the code can predict system thermal-hydraulic responses, including CCFL considerations, during an SBLOCA in a pressurized water reactor (PWR). The CCFL correlation was added to RELAP5/MOD2-B ampersand W, similar to the implementation in RELAP5/MOD3. The RELAP5/MOD3 model was found to produce intermittent, nonphysical flow behavior. It was found that such behavior occurred when the momentum equations predicted countercurrent flow, and the CCFL correlation indicated cocurrent flow. RELAP5/MOD2-B ampersand W uses an interative approach to resolve this behavior. When inconsistent flow patterns are predicted by the momentum solution and CCFL correlation, the interphase drag term is progressively adjusted until both solutions predict consistent flow behavior