On April 20, 1995, workers at the Clinton nuclear plant in Illinois applied a freeze seal on a drain line from the reactor vessel so they could repair some valves. Freeze seals use liquid nitrogen at very low temperature flowing through a special blanket wrapped around a pipe to freeze the water within that section of pipe.

The freeze seal at Clinton did not form properly because the high temperature of the water in the reactor vessel kept thawing the ice plug. Without consulting anyone, the workers applied a second freeze seal close to the first one. This stopped the water flow, and they repaired the valves and removed the freeze seals.On February 24, 1997, workers at Clinton reviewed a report about an ice escapade at the Diablo Canyon nuclear plant in California. Workers at Diablo Canyon applied three freeze seals in a row on a section of piping. Ice formed by the freeze seals compressed the water between the seals, which created high stresses that deformed the piping.

After reading this report, workers at Clinton inspected the reactor vessel drain line piping where they had applied the two freeze seals and discovered it had deformed. The deformed piping was replaced.

Clinton’s owners promised the NRC that they’d bring in contractors who knew how to perform freeze seals in the future rather than try it themselves.

The Clinton plant operated for over a year with a damaged drain line off the reactor vessel. Had that weakened pipe broken while the plant was running, the consequences would have been extremely severe. There would have been no way to plug the broken line. Emergency pumps could have replaced the reactor water escaping through the break, but the water would have flooded the containment.

Our Takeaway

Fission Story #51 covered freeze seal problems at River Bend. Fission Story #60 covered problems with double fixes at Browns Ferry. Here, these topics are combined with problems caused by double freeze seals.

As in the Browns Ferry case, the workers at Clinton wandered into trouble when they departed from their approved plan. That plan had been prepared and reviewed by many people to establish what to do and how to do it to ensure that safety margins were maintained. When that plan did not yield the expected outcome (i.e., a reliable freeze seal), the workers should have stepped back and sought a formal revision to the plan. Instead, they opted to wing it and apply an on-the-spot backup plan. It solved their immediate problem, but at the expense of introducing a larger safety problem through the weakened and degraded piping it caused. Two is still not always better than one.

There are no short-cuts to nuclear safety. Short-cuts frequently lead to undesired destinations.

“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.