Watts Bar Unit 2 Reactor Goes Critical (and That’s a Good Thing)

The Tennessee Valley Authority’s (TVA’s) newest nuclear plant, Watts Bar Unit 2, achieved initial criticality on May 23 at 2:16 a.m.

Although that may sound ominous to anyone untrained in nuclear terminology, initial criticality simply means that Watts Bar Unit 2 reached a state in which the reactor’s neutron population remained steady from one generation to the next and the nuclear fission chain reaction was self-sustaining for the first time (Figure 1).




1. Initial criticality.
Operators in this image practice taking the reactor critical from the Watts Bar control room. Source: TVA

“This milestone is the result of the hard work by Watts Bar employees supported by the entire TVA nuclear team,” said the TVA’s Chief Nuclear Officer Joe Grimes. “While this achievement is important, safety remains our top priority and we will now move forward with fully integrating the seventh unit into the fleet with that focus in mind.”

According to the TVA, the reactor is currently operating in a stable condition at low power. During the coming weeks, power will be slowly increased as part of scheduled power ascension testing and the unit will begin producing electricity. Plant systems and controls will be monitored and tested at various power levels up to 100%. The tests will be repeated multiple times to ensure the entire system operates safely, as designed.

No timeline was given for when the unit might enter commercial operation. For more on Watts Bar Unit 2, see “Watts Bar Unit 2: A ‘Deferred Nuclear Plant’ Gets Back into the Game,” “New U.S. Nuclear Plant, Watts Bar Unit 2 Is One Step Closer to Startup,” and “NRC Issues Watts Bar 2 Nuclear Power Plant Operating License.”

Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)