The algorithm for the transient containment analysis code (PCCSAC) developed to analyze the AC600 passive containment cooling system (PCCS) is described. The AC600 reactor uses passive external spray from an elevated tank. The PCCSAC code has been developed with a number of unique modeling capabilities for the AC600 PCCS. The unique feature of the PCCSAC code is the nine-equation model that classifies the fluid in the containment as steam, noncondensable air, and liquid water. The model includes the k- turbulence model and a diffusion model for gas flow. The code considers nonuniform spatial factors. It can analyze most physical phenomena in the AC600 PCCS. The code was validated by comparing the present results with those of the COMMIX code for the analysis of a postulated small-scale model of AP600. The PCCS response of AC600 during design-basis accidents is analyzed.