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Abstract: The AP1000® plant is an advanced Pressurized Water Reactor designed and developed by Westinghouse Electric Company which relies on passive safety systems for core cooling, containment isolation and containment cooling, and maintenance of main control room emergency habitability. The AP1000 design obtained the Design Certification by NRC in January 2006, as Appendix D of 10 CFR Part 52, and it is being built in two locations in China. The AP1000 plant will be the first commercial nuclear power plant to rely on completely passive safety systems for core cooling and its licensing process requires the proper operation of these systems to be demonstrated through some pre-operational tests to be conducted on the real plant. The overall objective of the test program is to demonstrate that the plant has been constructed as designed, that the systems perform consistently with the plant design, and that activities culminating in operation at full licensed power including initial fuel load, initial criticality, and power increase to full load are performed in a controlled and safe manner. Within this framework, Westinghouse Electric Company and its partner Ansaldo Nucleare S.p.A. have strictly collaborated, being Ansaldo Nucleare S.p.A. in charge of the simulation of some pre-operational tests and supporting Westinghouse in the definition of tests procedures. This paper summarizes the work performed at Ansaldo Nucleare S.p.A. in collaboration with Westinghouse Electric Company for the Core Makeup Tank (CMT) tests, i.e. the CMTs hot recirculation test and the CMTs draindown test. The test procedure for the two selected tests has been defined and, in order to perform the pre-operational tests simulations, a fully detailed AP1000 Relap5 Mod. 3.3 model has been developed and validated against the available data. Such model has been used to simulate the selected pre-operational tests. The Relap5 simulations have demonstrated that the tests can be successfully conducted with the selected boundary and initial conditions and tests procedures: in fact CMTs are able to inject cold water in the Reactor Pressure Vessel (RPV) through the Direct Vessel Injection (DVI) lines, while they are heated up by the hot water entering from the cold legs pressure balance lines. The ability of CMTs to transition from water recirculation mode to draindown mode has been demonstrated through the simulation of the CMTs draindown test: when some significant void forms in the loop 2 cold legs (CMTs pressure balance lines are connected to loop 2 cold legs), the CMTs pressure balance lines void (they are fed by steam) starting the CMTs drain. [Copyright &y& Elsevier]