Giant cooling towers at Constellation Energy's Three Mile Island nuclear plant in Pennsylvania have sat dormant for so long that grass has sprung up in the towers' hollowed-out bases and wildlife roam inside.

Labor, training and inspections are just some of the variables as TMI prepares to go back online, something no other nuclear plant has done after shutting down.

Constellation Energy has placed an order for a main power transformer valued at $100m, advancing the revival of the Three Mile Island nuclear reactor in the US state of Pennsylvania, Reuters has reported.

Constellation Energy has ordered a main power transformer for the Three Mile Island nuclear reactor it is trying to reopen in Pennsylvania, pushing ahead with work critical to the plant's revival, Reuters learned on a tour of the site on Wednesday.

Constellation Energy will revive the half-century-old nuclear power plant to supply electricity for Microsoft's data centers.

Water, oil and workers is all it will take to get the shuttered Three Mile Island nuclear plant back up and running, according to the plant’s owners.