Abstract The decommissioning of a nuclear power plant/site is a very complex process that is necessary to be performed at the end of its useful life to allow its release from regulatory control and a new use of the site by the society. Typically, a decommissioning project requires the management of several tasks and resources. In case of multiple reactors site, this management becomes harder since the interdependencies between each plant could influence the way that the tasks could be executed as well as its schedules. In literature is possible to find different kinds of tools for planning and management purposes, some of them specific for nuclear installations decommissioning projects. This is the case of tools based on the ISDC. However, these tools do not allow to consider rightly the interdependencies and previous learning. Notwithstanding, most part of existent tools/methods requires a large quantity of data/information to produce consistent results. However, this data often is unknown, requiring assumptions to justify some values, which enhances the uncertainties of the results. To overcome these difficulties, the present work will introduce a management tool and detail its mathematical model. This mathematical model aims to cost estimate the decommissioning cost for budget/bid purposes. The main differential of the model is that it requires a significantly less data compared with models available in literature. As part of the work objectives, a novel method for attribute values to the difficulty values and contingencies is presented. In addition, a validation simulation is presented as well as a sensibility analysis. The validation was performed considering a Brazilian nuclear power plant as case study, which had its decommissioning cost estimated in about US$761.7 million. The sensibility analysis indicates that is interesting to postpone the beginning of decommissioning some years after plant shutdown since it allows not only some radiological results but also a cost reduction.