Following the CFD4NRS workshops held in Garching, Germany (September 2006), Grenoble, France (September 2008) and Washington D.C., USA (September 2010), this workshop, held in Daejeon, Korea on 10-12 September 2012, was intended to extend the forum created for numerical analysts and experimentalists to exchange information in the application of computational fluid dynamics (CFD) and computational multi-fluid dynamics (CMFD) to nuclear reactor safety (NRS) issues and in guiding nuclear reactor design thinking. The workshop included single-phase and multi-phase CFD applications, and offered the opportunity to present new experimental data for CFD validation.