On November 22, 2012, workers at the Browns Ferry nuclear plant in Alabama were testing excess flow check valves on the Unit 1 reactor. Excess flow check valves are used in piping and tubing less than an inch in diameter to protect against ruptures. For example, tubing from sensors inside the containment building often penetrates the several feet thick containment walls to reach instruments in adjacent buildings. If the tubing outside containment breaks, it creates a pathway for radioactivity to escape.
One of the excess flow check valves failed its test. Workers replaced it. The new valve had been installed exactly like the one it replaced. When tested, the new valve failed also.
Workers investigated and discovered that the excess flow check valve was installed backwards. It would not have stopped excessive flow in event of a broken tube just as it failed to do during the tests. When installed in the proper orientation, the excess flow check valve passed the test.
The excess flow check valve had been installed – backwards it turns out – on October 15, 2006. Browns Ferry Unit 1 had been shut down since March 1985 at the time, but workers were preparing it for restart.
On March 19, 2007, workers tested the excess flow check valve. It failed the test. Workers retested it and it failed a second time. Despite the failed tests, the valve was deemed acceptable as-is and the reactor was restarted.
The plant’s owner reported the backwards excess flow check valve to the NRC. The owner informed the NRC the excess flow check valve had been installed backwards because the installation procedure lacked adequate guidance on how on proper orientation. The NRC was also informed that even if other excess flow check valves had been installed incorrectly using this inadequate procedure, testing would identify any problems.
Two wrongs must make a right, at least when it comes to testing safety equipment at Browns Ferry. An excess flow check valve failed a routine test, and then failed a re-test. Based on the two failures, the test engineer concluded that the excess flow check valve was still acceptable.
Why even bother testing safety equipment when passes and failures are treated equally?
Because the March 2007 test failures were handled wrong, the excess flow check valve remained installed backwards for five and a half more years.
Workers should have gone back through past test results to see if any other excess flow check valves also failed tests but were deemed acceptable anyway.
And the big gaping hole in the testing regime at Browns Ferry that tolerates failures on safety tests needs to be patched, too.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.