Unit 2 at the US Tennessee Valley Authority’s (TVA’s) Watts Bar NPP achieved first criticality on 24 May. The 1,165MWe (net) pressurized water reactor is the first nuclear unit to start up in the USA since Watts Bar 1 in 1996.

Construction began in 1972, but work was suspended in 1985 when the unit was about 55% complete. TVA decided to resume work on the unit in 2007, awarding an engineering, procurement and construction contract to Bechtel.

TVA received an operating licence for the unit from the Nuclear Regulatory Commission (NRC) in October 2015 and fuel loading was completed in December. TVA said that the unit was operating in a stable condition at low power levels that will slowly be increased over the coming weeks as it moves towards commercial operation. Plant systems and controls will be monitored and tested at various power levels up to 100%. The unit is expected to enter commercial operation later this summer.

Watts Bar 2 has been upgraded in response to the 2011 Fukushima Daiichi accident in line with NRC requirements. Enhancements include a new hardened FLEX equipment storage building designed to withstand earthquakes, floods and other incidents. This houses emergency backup diesel generators, pumps and other equipment needed to keep the reactor containment cooled in the event that onsite power is lost.

The total estimated project cost of $4.7bn was approved by TVA’s board in February. Orders related to post-Fukushima regulatory requirements and cyber security issues have increased by $125m TVA’s original $4.5bn estimate.