Healthy discussion of the Arkansas Nuclear One accident

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ANO Stator 2

Early Easter Sunday morning, an industrial accident at the Arkansas Nuclear Power Plant killed one worker and injured 8 others and left the unit without offsite power.  At the time of the accident, workers were moving the Main Turbine Generator Stator out of the turbine building when the lift crane failed and dropped the load.  In the days after the event it was announced that the United States Occupational & Health Administration would investigate the incident and the NRC later announced that they would investigate the licensee’s response to the event.

While the event was immediately picked up by headlines around the world, very little information has been released in the meantime. Our readers have been sharing different opinions on the accident, describing two distinct scenarios which may have resulted in the accident.  We would like to share these two scenarios, not to imply which may or may not be the most likely, but to help communicate the difficulty of the operation being performed and encourage healthy discussion of safety-related accidents to ensure that lessons are learned.


ANO stator 3

[dropcap]1[/dropcap]In the first scenario, it was noted that the Bigge equipment did not look severely damaged, that the stator had made it from the turbine deck to the hatchway and was in the process of being turned correctly to be lowered to the transport below.  It appears that the front left leg of the gantry crane was positioned on a floor beam. That beam is bent. A rational hypothesis could be posed asking whether or not the floor which supported the front two legs of the gantry beams was strong enough, and if the under-beam buckled and the gantry came apart.  The turbine deck was strong enough to support the back two legs of the gantry, and may have had jack plates built into the foundation. The legs of gantry may have been positioned right above foundation columns under the turbine deck, while the front legs were on building floor. This could like the fatal flaw for this scenario, and even though the floor beam held for a while, it may have then deflected.


[dropcap]2[/dropcap]In the second scenario, it was noted that the end of the long travel beams over the hoist well were fully supported on two vertical beams from a beam in the rail bay with a horizontal beam at the turbine floor level and two further verticals to the main long travel beams (this could be a standard arrangement for Bigge systems). In this scenario, the accident could have been the result of the vertical supports tilting, stopping the load too quickly as it was being rotated to align with the hoist well and causing the whole rig to collapse.

ANO stator 4

ANO stator

Source: Efin Engineer

Source: Frank Bailey

Source: SteamShovel2002

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3 Comments

  1. While I think that either scenario could be likely, one thing that I noticed in the first picture is that it appears one side of the travel beams of the crane itself did not span the opening supported on the side under which the floor beam bent. There appears to be a wall that interferes with support of that beam on the on that side. One notices that the other side, the travel beam’s length is well past the opening. Did that travel beam just come apart with the collapse and that which was on the other side fall into the opening or was pushed beyond so that it’s out of the picture?

  2. In your top photo, which is looking southeast, the vertical support, which was on the northwest side, was the first to collapse from what I’ve been told. The stator had not been lowered at all. It fell the full distance into the train bay. The collapse of the vertical support resulted in the gantry falling to the west, blocking access to the control room extension. It fell toward where all of those too many people are standing around. If you’ll notice on the right of your picture, there is an orange man-lift. In the next image, it’s crushed under the gantry beam. Several people crawled out from under that area. The buckling of plant structural material is from impact from the stator and/or gantry beams still bearing the weight of the stator. I suspect the damage on the north side of the train bay is from the stator and the damage on the south is from the combined kinetic energy of the stator and the gantry beam.

    This accident was captured on video. My information comes from people who were there or who have watched the video multiple times combined with familiarity with the plant. I do not work for the utility and have not been inside the plant since 2007.

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