Fission Stores #64: Nuclear-Sized Sink Stopper

November 1, 2011
Dave Lochbaum
Former contributor

On December 28, 1994, a core shroud head bolt dropped into the Unit 1 spent fuel pool at the Edwin I. Hatch Nuclear Plant in Georgia. Workers using an overhead crane were carrying this bolt about one foot above the water surface when the sling holding the bolt broke.

The bolt, 17 feet long by three inches in diameter and weighing 365 pounds, glanced off the side wall and fell to the bottom of the spent fuel pool without hitting the storage racks or irradiated fuel assemblies in them. The bolt tore a three-inch long gash in the 3/16 inch thick stainless steel liner. Approximately 2,000 gallons leaked through the hole and through a drain line to the radwaste system before workers closed valves in the drain line to stop the leak.

The spent fuel pool water level dropped nearly two inches in 23 minutes, causing the fuel pool cooling system pumps to shut down due to low suction pressure. Operators restored the water level after the leakage path was isolated, then returned the fuel pool cooling system to service. Workers removed the bolt and placed a large rubber mat (i.e., a nuclear-sized sink stopper) over the hole to limit leakage until underwater welding repairs were completed.

The Hatch incident occurred less than a year after a screwdriver dropped into the spent fuel pool at a foreign nuclear power plant with similar results. On January 31, 1994, workers at Tricastin Unit 1 in France were removing the control rod cluster guide tube from a spent fuel assembly. A 15-foot long screwdriver weighing 44 pounds fell into the spent fuel pool and punctured the stainless steel liner. The water level in the spent fuel pool dropped nearly four inches. A stainless steel plate was welded over the hole.

Our Takeaway

Spent fuel pools are used to store and cool irradiated fuel bundles discharged from the reactor core. The water in the pools serves two purposes: (1) to remove the heat being generated by the radioactive decay of fission products within the fuel pellets inside spent fuel rods, and (2) to attenuate the radiation being emitted by the spent fuel rods and therefore shield workers on the refueling floor.

Obviously, these two purposes are best met when the water remains inside the spent fuel pool instead of wandering away.

Among the recommendations made by the NRC’s Fukushima task force is to install reliable instrumentation to allow the operators to monitor the level and temperature of the water inside spent fuel pools. Many nuclear reactors operating in the U.S. lack such instrumentation, leaving operators clueless, or blissfully ignorant, of these parameters during accidents and events like station blackout.

The NRC’s task force recommended a bliss reduction program, but the NRC’s Commissioners are still pondering its merits. The Commissioners need to stop pondering and start protecting Americans.

“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.