A time-lapse video released by TVA shows the Watts Bar Unit 2 reactor being assembled.

It shows the assembly of three pieces over a week-long period as TVA prepares for another phase of testing the reactor to bring the nation's first new nuclear of the 21st century online by December 2015.

The fully-assembled Watts Bar 2 reactor vessel is in place for the first time. It happened in three separate, very precise lifts between over the last several weeks.

The first piece, the core barrel, was placed Aug. 9. The barrel is more than 33 feet in height, and weighs 282,000 pounds. It holds the 193 fuel assemblies. This move took roughly 12 hours, as the barrel had to fit into a space with a clearance of less than a centimeter in any direction. If the sides had touched, the entire process would have had to begin again.

The upper assembly, which houses the control rods, was placed Aug. 12.

The final and heaviest piece, the reactor head, was placed Aug. 13. The head alone weighs 298,700 pounds. All the pieces were secured together over the next week.

The assembly is part of preparations for the next round of testing of key systems to demonstrate the readiness of the unit for commercial operation by December, 2015.

Click here for the video.