(WBIR - Rhea County) The parts are in place and ready to assemble the new reactor at the TVA Watts Bar nuclear power plant.

TVA expects assembly of the Watts Bar 2 reactor to begin in the next few days. The assembly is a combination of extremely heavy equipment that requires delicate surgical precision.

WBIR toured the construction site and vessel that will house the reactor on Thursday morning. TVA says 90 percent of total construction of the project is now complete.

When it is finished, Watts Bar 2 will be the first nuclear reactor to go online in the United States in the 21st century. Right now TVA estimates the reactor will be operational by December 2015.

The last reactor to go online in the U.S. is the reactor that is already operational at Watts Bar. The Watts Bar 1 reactor went online in 1996.

The utility has spent decades getting to this point with several delays and a budget that ballooned to twice the original estimate. As of now TVA says the project will cost around $4.2 billion, although the utility issues that estimate with the caveat that the price could range anywhere between $4 billion and $4.5 billion.

Once the reactor is assembled, it will go through test after test to ensure there are no problems with what amounts to a $4 billion nuclear pressure cooker.

"Nuclear power construction and testing is very complex. It's not something that we cut corners on," said Michael Skaggs, the Senior Vice President for TVA who oversees Watts Bar operations and construction.

Skaggs said the tests will be done with water to pressurize the primary systems to 110 percent of the normal operation pressure. Water is used because no actual nuclear fuel can be loaded into the reactor until the NRC issues an operating license for the reactor. Earlier this month, TVA filled the new reactor vessel with 185,000 gallons of water in just over 23 minutes.

TVA said the new reactor will benefit from lessons learned during operation of Watts Bar Unit 1. The project has also implemented lessons learned from the Fukushima nuclear crisis in Japan.

TVA built a huge 47-ton concrete building that is waterproof, tornado-proof, and earthquake-proof to house and protect all of the backup generators that provide emergency electricity at the Watts Bar Nuclear Power Plant.