Significance of Refined Core Thermal-Hydraulics Nodalization in the MSLB Analysis

Improvements achievable by use of refined core thermal-hydraulics (T-H) nodalization are evaluated for the OECD Main Steam Line Break (MSLB) Benchmark Exercise III problem using the MARS/MASTER and RELAP5/PARCS codes. The k{sub eff} is lowered by the refined model because of the lowered importance of reactive nodes experiencing stronger feedback with the refined model. The two MARS/MASTER transient core power behaviors are slightly different in that the return-to-power occurs earlier and more strongly with the refined T-H model, whereas the two RELAP5/PARCS cases are almost identical. The peak linear power density is, however, significantly reduced by the refined model for both codes. The results also show that the departure from nucleate boiling ratio remains sufficiently high throughout the transient.