Fission Stories #76: Whodunnit?

January 24, 2012
Dave Lochbaum
Former contributor

Agatha Christie was born in England on September 15, 1890. This author received international acclaim for whodunnit mystery novels featuring sleuths like Miss Marple and Hercule Poirot. One hundred years later, a mystery at Beaver Valley Unit 2 outside Pittsburgh, PA was a less entertaining whodunnit.

Workers were filling the reactor cavity with water in preparation for refueling activities. The reactor cavity is a large volume inside the containment above and around the reactor vessel (see the figure). It is flooded with water during refueling to allow irradiated fuel assemblies to be removed from the reactor vessel and moved through a horizontal transfer tube (labeled (3) in the figure) to the spent fuel pool in the adjacent fuel handling building. At Beaver Valley that day, the transfer tube between the reactor cavity and the spent fuel pool was open. In other words, these two volumes were cross-connected to form one large volume.

During the filling evolution, the containment building’s ventilation system inadvertently shut down. As the reactor cavity filled with water, the displaced air had no place to go. The ventilation system usually pulled this air out, but the system had stopped running. The air pressure above the reactor cavity increased as more and more water poured in. The downward pressure on the reactor cavity water forced water through the open fuel transfer tube into the spent fuel pool. The spent fue1 pool level increased until it overflowed.

The report on this event specified what happened and when it happened. But whodunit remains a mystery.

After overfilling the Unit 2 spent fue1 pool in 1990, workers at the Beaver Valley nuclear plant experimented the next year with lowering the spent fuel pool water level. During Unit 1 maintenance activities on May 30, 1991, a worker stepped on a piece of wood. This particular piece of wood covered the air hose to an inflatable bladder sealing around the concrete gate between the spent fuel pool and the transfer canal area. The air hose separated at a quick disconnect fitting linking two hose sections. The air bladder deflated, allowing water to leak from the spent fuel pool into the transfer canal area. The spent fuel pool level dropped nearly 20 inches before equaling the transfer canal area water level.

Luckily, the nearly two feet drop in spent fuel pool water level did not cause fuel damage. Workers restored the proper level.

Our Takeaway:

Had Goldilocks visited Beaver Valley, she’d not want too much water in the spent fuel pools or too little water, but just the right amount. The Nuclear Regulatory Commission wants just the right amount of water, too. One of the recommendations made by the NRC’s Fukushima task force was to require plant owners to install instrumentation to provide workers with reliable information on the level and temperature of the water inside the spent fuel pools. It’s really hard to determine whether the level is too high or too low when no level information is available. The NRC is right to seek to swiftly remedy this safety shortcoming.

“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.