WATCH: 185,000 gallons pour into TVA nuclear reactor in this time-lapse video

photo Watts Bar Nuclear Plant Unit 2's core barrel will encase the nuclear core.

TVA filled the new reactor vessel in the Watts Bar Nuclear Plant reactor being built near Spring City, Tenn., this week with 185,000 gallons of water in just over 23 minutes.

The flow filled 18 feet of water into a space 60 feet long and 22 feet wide, and was part of key tests to show that major safety systems will work as designed when the unit comes online in December 2015.

"This was the first time the systems were tested at once, and results showed the systems performed as designed," TVA spokesman Scott Brooks said.

"This full flow open vessel testing happens only once in the life of a nuclear plant."

Open vessel testing is the first of several major milestones completed toward the goal of commercial operation by December, 2015.

Watts Bar 2 is expected to be the first American nuclear unit to come online in the 21st century.