[en] This study was performed to estimate the cost of decontaminating and decommissioning Z-Plant. All of the buildings in the Z-Plant exclusion area except Building 2736-Z, the plutonium storage vault, are included in the study. The study also excludes all underground facilities within the exclusion area which are not contained within a building and all Z-Plant related facilities outside the perimeter fence. The contamination in Z-Plant is primarily ²³⁹Pu which has a half-life of 24,360 years. Because of the long half-life of ²³⁹Pu, it is not practical to consider the isolation of the facility to await reduction of the contamination level by natural decay. Therefore, this study analyzes the costs, risk and benefit of decontaminating Z-Plant to four different levels of residual contamination. The three principle criteria used in the analysis are cost, the risk of offsite dose to the public, and the occupational exposure to onsite personnel