A new methodology using a free turbulent flow model to evaluate control room habitability is developed, and the theoretical model can be applied to the postulated event of rupture or line break of the on-site hazardous gas pressurized tank/system. Based on the conservation of mass law and momentum equations, correlations of the control room ventilation hazardous gas intake concentration and the control room buildup toxic concentration were established and can be used to evaluate control room habitability. Compared with current methodology widely used in the industry (introduced by NUREG-0570), the developed theoretical analysis methodology is applicable to events occurring without any constraint on the distance between the site of toxic gas release and the inlet of the control room fresh air intake or the control room. With a given amount of hazardous gas release source, the analysis results indicate that maximum control room toxic gas concentration will depend on the mass release rate or its break size, the density of the hazardous gas, and the distance between the site of the toxic release and the control room fresh air intake. The limiting case of the control room habitability analysis will occur at the break size resulting in the highest control room toxic gas concentration. The control room toxic gas transient concentration at the limiting break size can be predicted by the model and compared with its acceptance criteria of short-term exposure limit and time-weighted average to evaluate the control room habitability whether protection actions of the control room operators are required to prevent incapacitation or death due to the postulated events of toxic gas release.