[en] Circulation and primary gas concentration in an HTGR containment during a PCRV depressurization accident have been studied to analyze layering and complete mixing models. If layering occurs, the problem of potential buildup or local concentration of combustible gas becomes important. The study was performed for a typical 3000 MW(t) system assuming that the clean-up system failed. Because of the difficulty and complexity in calculation, no obstacles were considered. The CIRC code was developed for this analysis because it offers advantages over other codes such as SOLA or KACHINA for easier application to HTGRs. While the typical HTGR containment has a semi-spherical dome, CIRC is limited to rectangular geometry in x-y and cylindrical dimensions, and to simulate three-dimensional phenomena, several cases were calculated