The Tennessee Valley Authority’s Watts Bar Nuclear Plant Unit 2 automatically shut down for a little less than a day-and-a-half last week because of low steam levels, the authority confirmed this week.
The quasi governmental company confirmed the outage to the Exchange Monitor after the Nuclear Regulatory Commission on Monday reported that Unit 2, which was running at 100 percent power on Aug. 4, “automatically tripped due to number 2 steam generator low low level.”
“Following the Watts Bar Unit 2 trip Friday evening, operators performed as trained in accordance with plant procedures and safely shutdown the unit,” a Tennessee Valley Authority (TVA) spokesperson told The Exchange Monitor. “After the team safely made repairs to the equipment, the unit was returned to service early Sunday morning.”
The reactor was down for a total 32 hours, 39 minutes, TVA said. The National Nuclear Security Administration confirmed that timetable, to the minute. Unit 2 was back up and running at 100 percent power as of this week, the authority’s spokesperson said, without identifying the root cause of the low steam levels.
The NRC’s resident inspector was notified and was investigating the cause of the low level in the number 2 steam generator. “There was no impact on the health and safety of the public or plant personnel,” the NRC report said.
Unit 2 is the second of two reactors that irradiates rods that help the National Nuclear Security Administration produce tritium for U.S. nuclear weapons. The other, Unit 1, was not affected by the automatic trip and maintained 100 percent power while Unit 2 was down, the TVA spokesperson said.
An NNSA spokesperson said there were no impacts to tritium production because of this reactor trip. NNSA wants the two reactors to produce 2,800 grams of tritium per cycle starting in fiscal year 2025, which begins Oct. 1, 2024. That’s substantially more than forecast in 2017, when a focus group convened at the Pacific Northwest National Laboratory made an early assessment of the modernization program’s needs.
“The trip was not complex, with all systems responding normally post-trip,” according to an NRC report published Monday about the Watts Bar 2 hiccup. “Operations responded and stabilized the plant. Decay heat is being removed by using the auxiliary feedwater and steam dump systems.”
“The tritium program takes these nominal reactor trips into account when estimating tritium production,” the NNSA said in an email. “ TVA is responsible for the operations of their reactors. TVA is very proactive in keeping their reactors operating safely.”
All four of Unit 2’s 67-foot, 80,000-pound steam generators were replaced in 2022. The generators use heat generated by the reactor to create steam that drives turbines, which turn a generator to create electricity that’s sent to the power grid. The original steam generators on Watts Bar Unit 1 were replaced in 2006.
Watts Bar’s Unit 1 reactor returned to operation in May after a scheduled month-long shutdown for maintenance and refueling that occurs about every 18 months. Irradiated tritium-producing burnable absorber rods (TPBAR), the source of weapons tritium, come out of the Watts Bar reactors during some refueling outages and unirradiated TPBARs are inserted.
TPBARs are specially designed rods that are inserted into nuclear fuel assemblies in substitution for normal nuclear fuel rods. Watts Bar 1 has irradiated tritium since nearly 2003. Unit 2 started producing tritium in late 2020.
TPBAR irradiated in the TVA reactors are shipped to South Carolina for tritium harvesting by Department of Energy contractors at the agency’s Savannah River Site. From there, full tritium reservoirs go to the Pantex Plant in Amarillo, Texas, to be packed into nuclear weapons.
Westinghouse Government Services manufactures TPBAR at the Columbia Fuel Fabrication Facility in Hopkins, S.C., under a long term NNSA contract awarded in 2000 to the company then known as WesDyne. The Pacific Northwest National Laboratory designed the current generation of TPBAR.
The tritium gets transferred to reservoirs that are installed in nuclear weapons. Modern thermonuclear weapons use tritium to increase the efficiency of nuclear explosions. Radioactive tritium decays relatively quickly and so must be produced regularly for as long as a nation wishes to maintain an arsenal of thermonuclear weapons.
