In the APEX (Advanced Power Extraction) studies, the conventional first solid wall facing the plasma is replaced with a fast flowing layer of a thin liquid wall. The concept of a free-surface first liquid wall concept is a revolutionary concept. The first liquid wall flows very fast and detains charged particles, and is followed by the thick liquid wall (blanket) which flows slowly and absorbs generated energy and converts it to heat. In the study, the flowing molten salt (i.e., first wall and blanket) is composed of Flibe (Li2BeF4) and is considered the main constituent mixed with different mole fractions (0-12%) of heavy metal salt (ThF4 or UF4) to increase the energy multiplication. Self-sufficient tritium breeding ratio (TBR 1.05) was taken into account to determine the upper limit of the fraction of heavy metal salt in the mixture. Design and calculations of APEX were carried out as 3-D torus by using MCNP-4B computer code. Copyright (c) 2011 John Wiley & Sons, Ltd.