Abstract: In case of a severe accident, the lower head of the reactor pressure vessel will be melted when the corium cannot be effectively cooled after melting. Under the EVR (ex-vessel corium retention) strategy, the molten corium will be released into the core catcher, where it can be sufficiently cooled, so that a large release of radioactive materials to the environment, after the molten materials melting through the basement of the containment, can be avoided. A twin crucible type core catcher with cooling tubes embedded in the molten pool to enhance cooling efficiency was conceived for a pressurized water reactor. As an EVR strategy, the core catcher first cools the molten pool through both the side walls of the inner crucible and the embedded cooling tubes under natural circulation of cooling water supplemented from a lower tank in IRWST (internal refueling water storage tank), before the top flooding triggered, so that all the melt is surrounded by water and thus can be retained and cooled for a long time. It is evident that the coolability of the core catcher before the top flooding might be the predominant challenge for this conceptual design. In order to study the 3D transient coolability of the conceptual twin crucible type core catcher design, from the initiation of the water injection into the cooling channels to the establishments of first a two-phase natural circulation and then a thermal equilibrium, under given heat flux distributions from the wall, a 3D full scale distributed parameter model was built and analyzed with GOTHIC (generation of thermal-hydraulic information for containments) 8.3 (QA) code. By means of multiple blockages built inside a subdivided volume, the irregular shape flow channels of the twin crucible, can be modeled. With the mapped cell spanning method and the built-in heat transfer correlation package, covering the portion of the boiling curve spans single phase to pre-CHF (critical heat flux), of GOTHIC8.3 (QA), the transient investigation of a 3D flow and two phase heat transfer can be realized. Through the simulation and analysis, post processing of the calculation results and discussions of related models, a sound heat removal capability of the conceptual core catcher design was proved. Under postulated conditions, the heat removal power can be improved to 13.4 MW. The results also show meaningful suggestions for the design and optimization on the transient two-phase natural circulation characteristics of the conceptual core catcher, and the capability of GOTHIC 3D model for three-dimensional thermal-hydraulic phenomena.