After a nuclear facility (e.g. nuclear power plant, research reactor, etc.) is finally shut down, it is generally decommissioned. 

In the technical sense, the term "decommissioning" refers to the entire process from the granting of the decommissioning licence to the dismantling of the plant. This usually means removing all parts of the building and restoring the site to its original natural state in the form of the so-called "greenfield site".