The refined core thermal-hydraulics (T-H) nodalization feature of the MARS/MASTER code is used to generate a high-fidelity solution to the OECD main steam line break benchmark problem and to investigate the effects of core T-H nodalization. The MARS/MASTER coupling scheme is introduced first that enables efficient refined node core T-H calculations via the COBRA-III module. The base solution is generated using a fine T-H nodalization consisting of fuel assembly-sized radial nodes. Sensitivity studies are performed on core T-H nodalization to examine the impacts on core reactivity, power distribution, and transient behavior. The results indicate that the error in the peak local power can be very large (up to 25%) with a coarse T-H nodalization because of the inability to incorporate detailed thermal feedback. A demonstrative departure from nucleate boiling (DNB) calculation shows no occurrence of DNB in this problem.