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Arkansas Nuclear One: Pictures of an Accident Fission Stories #181

Dave Lochbaum, director, Nuclear Safety Project

Union of Concerned Scientists Fission Stories #139 described the March 31, 2013, event at Arkansas Nuclear One where a heavy load dropped during a refueling outage on Unit 1 caused the automatic shut down of the Unit 2 reactor from full power. The accident killed one worker and injured several others. The damage to the plant by the dropped stator took weeks to repair. In response to a request under the Freedom of Information Act, the NRC recently released several dozen photographs showing the consequences from the dropped load and the steps taken to fix the damage it caused. This post updates the original description using several photographs released by the NRC. Links to the NRC’s FOIA documents containing these and many other photographs (more than 400 photographs in all) are provided towards the bottom of this post. The operators shut down the Unit 1 reactor on March 24, 2013, to enter a routine refueling outage. In addition to removing some of the irradiated fuel assemblies from the reactor core and replacing it with new fuel assemblies, workers planned to replace the main generator’s stator. Like all the nuclear power reactors currently operating in the US, the Unit 1 reactor at Arkansas Nuclear One used the heat produced from splitting atoms in the reactor core to boil water. The steam flowed into the main turbine to spin its blades. The turbine shaft was connected to the main generator’s rotor.

http://allthingsnuclear.org/fission-stories-139-arkansas-nuclear-one-fatal-event/

Source: Bob Lochbaum This photograph shows the rotor and the stator for the Unit 1 reactor at the Watts Bar nuclear plant in Tennessee. The rotor is in the foreground, partially inserted into the stator. The low pressure turbines can be seen in the upper left background behind the stator. The rapidly turning rotor within the stator generates electricity.

Source: NRC Flickr Gallery The two reactor containment buildings at Arkansas Nuclear One appear in the center of this picture. Unit 1 is on the left and Unit 2 is on the right. The turbine buildings appear in front of the containment building in this picture as one long, rectangular structure. A large door inside the red circle provides entrance to an area called the train bay. As implied by the name, the train bay has tracks for a railcar to enter. Heavy equipment can be offloaded from the railcar for use in the plant. Similarly, heavy equipment can be loaded into an empty railcar for removal offsite. Eighteen-wheel trailer truckers and other haulers also use the train bay for deliveries and pickups.

Source: NRC ADAMS ML003747839 The train bay is circled in red on this map of the Arkansas Nuclear One site. In addition to the main turbine and generator, the turbine building contains associated equipment like the condensers, condensate pumps, feedwater heaters, and feedwater pumps. Immediately to the left of the turbine building are the auxiliary buildings. To their left are the reactor containment buildings. The auxiliary buildings contain much of the safety equipment needed to mitigate reactor accidents. The containment buildings house the reactor pressure vessels, reactor coolant pumps, steam generators, pressurizer, and containment coolers.

Source: NRC FOIA Document ML13213A270 The plan called for workers to remove the rotor from the stator and unconnect the stator from its mounting on the turbine building floor. The stator weighed over 500 tons—too heavy for the turbine building’s permanent crane to handle. A temporary lifting device (including the dark object) was installed to pick up the stator and carry it over the turbine building floor to the train bay opening.

Source: NRC FOIA Document ML13213A270 The stator would then be rotated 90 degrees and slowly lowered down to a heavy duty hauler parked on the ground floor of the train bay. A truck rather than a train engine would tow the hauler and stator away.

Source: NRC FOIA Document ML14219A431

This view shows the Unit 1 turbine building floor and the opening occupied by the main generator stator before its removal. Several vertical pegs are visible that matched holes in alignment and mounting plates on the stator to aid installing it in the proper position. The train bay is located behind the camera. The low pressure and high pressure turbines are located behind the blue blocks.


This view through the train bay door shows the stator where it came to rest on the hauler after being dropped. The blue beam resting at an angle is part of the temporary lift device that broke to cause the drop.

The stator dropped onto the hauler and fell to the side. This view shows a corner of the stator gouging a masonry block wall inside the train bay. The dropping stator caused significantly more damage before this last hole.


Source: NRC FOIA Document ML14219A431 The blue beams in the center of this photograph are part of the temporary lift device. The Unit 2 main generator and turbine are in the background. The stator rests on the damaged hauler in the train bay below these beams. The masonry wall to the upper right of the beams shows signs of damage. Damage to the concrete floor and structural steel is partially visible behind the beams.

Source: NRC FOIA Document ML14219A431 This photograph shows the damage to the concrete floor and structural steel caused by the dropped stator striking the turbine building floor on the Unit 2 side of the train bay opening.


The dropped stator cause more extensive damage upon striking the turbine building floor on the Unit 1 side of the train bay opening. In additional to crumbing the concrete floor, the impact deformed and displaced a large structural steel support beam.


This photograph shows the structural steel beam displaced from its connection to the vertical I-beam illustrating the amount of force imparted by the dropped stator.


This photograph was taken from the turbine building floor looking down into the train bay at the stator resting on the damaged hauler. Damage to the Unit 2 side is evident on the left while more extensive damage to the Unit 1 side appears on the right side.

Source: NRC FOIA Document ML14218A607 The deformation of the turbine building floor on the Unit 1 side compressed the wall from the ground floor of the train bay up to the turbine building floor.

Source: NRC FOIA Document ML14219A433 The dropped stator (partially shown on the right) created a lot of debris before coming to rest on the hauler in the train bay.


“Coming to rest on the hauler” understates what happened to the hauler when the 500-plus ton stator arrived. The stator smashed and broke the hauler. This photograph shows the hydraulic jacks being used to lift the stator off the hauler so it can be removed and replaced with an undamaged hauler. Recall that this accident began when the temporary lift device transporting the stator in the turbine building broke. A temporary lift device was being used because the permanent crane lacked sufficient capacity to lift the stator. Without means to lift the stator from above, workers used hydraulic jacks to push the stator upward.

Source: NRC FOIA Document ML14219A428 Once the weight of the stator was removed using the hydraulic jacks, workers used a fork lift to tug the damaged hauler from the train bay.


The hauler had lots of wheels to distribute the load from heavy objects like the stator, but not all of the tires were properly inflated and pointing in the same direction after the stator fell on it.

Source: NRC FOIA Document ML14219A426 Rather than rolling the hauler out from beneath the jacked up stator, the fork life dragged it away.


As shown within the cyan circles in the photograph, the cylinders of the hydraulic jacks holding the stator off the train bay floor were nearly fully extended. Workers needed to raise the stator higher to allow the replacement hauler to move in underneath it. Workers used an iterative process to raise the stator to the desired height.


The cylinder of the hydraulic jacks had a finite length and couldn’t extend farther. So workers used stands (like the one circled in red on the right) to support the stator. With the stator supported on the stands, workers retracted the hydraulic jacks’ cylinders. As shown within the cyan circle to the left, wood beams were then placed under the hydraulic jacks. Workers extended the hydraulic jacks’ cylinders again, lifting the stator up a few inches. Workers repositioned the stands to the new height, too.

Source: NRC FOIA Document ML14219A426 Repeating this process, workers slowly and steadily raised the stator higher and higher off the train bay floor.

Source: NRC FOIA Document ML14219A426 The raised stator was now ready for the replacement hauler to be moved underneath it.


Not coincidentally, the replacement hauler was just outside ready to do its thing.

Source: NRC FOIA DocumentML14218A737

Workers didn’t need a fork lift to move the self-propelled replacement hauler into place beneath the stator.


Once the replacement hauler was in place, workers needed to remove the hydraulic jacks and their supporting beams. This photograph shows the turbine building crane about to be used to lift a support beam.

Source: NRC FOIA Document ML14218A737 And so the hydraulic jack support beams were removed.


The stator begins its overdue departure from the train bay on board the replacement hauler.


The stator continues its overdue departure from the train bay on board the replacement hauler.


The stator on the replacement hauler outside the train bay. The stator is resting partially on its top side. Running left to right across the middle of the photograph (or running right to left it you prefer) is a metal plate with numerous holes. The holes in this plate, along with a companion plate not visible on the opposite side, match up with the vertical metal pegs in the turbine building floor shown in an earlier picture.

Source: NRC FOIA Document ML14220A162 The stator on its way from the site. Like Elvis, the stator has left the building. But workers still had to repair the damage caused within the building on its way out.

Source: NRC FOIA Document ML14218A737 The most extensive repairs were needed to the damaged turbine building floor and associated structural supports on the Unit 1 side of the train bay. The photograph shows the crumbled and broken concrete removed from where the stator impacted the floor. The force overstressed and damaged other concrete and steel, requiring this area to be significantly enlarged.

Source: NRC FOIA Document ML1419AA424 Workers chipped away concrete and removed material damaged or potentially damaged by the dropped stator. This task had to be performed carefully as it was similar to cutting a tree limb while perched on the limb.

Source: NRC FOIA Document ML14220A092 This overhead view shows the turbine building floor under repair. The Unit 1 low pressure turbine appears in the upper center flanked on either side by moisture separator reheaters (these components receive the steam leaving the high pressure turbine and strive to remove water droplets before the steam enters the low pressure turbines.) Across the bottom of the photograph is the deformed and dislodged structural steel beam.

Source: NRC FOIA Document ML14219A425 With the suspect floor area removed, workers turned to removing the damaged structural steel beams. Note the worker tethered to an anchor spot for protection against falling into the train bay on the left.

Source: NRC FOIA Document ML14219A434 This overhead view shows the turbine building floor under repair. The Unit 1 low pressure turbine appears in the upper center flanked on either side by moisture separator reheaters. Note that the deformed and dislodged structural steel beam has been removed from its former location toward the bottom of the photograph. Also note that the cross beams have been removed.

Source: NRC FOIA Document ML14219A434 This photograph looks down into the train bay from the turbine building floor on the Unit 2 side. When the stator dropped onto the floor on the Unit 1 side, the force depressed the floor downward. In the center of this photograph is a vertical steel beam that supported the floor. The downward movement of the floor tore its connections to the vertical support beam and crumbled the masonry block wall below it. The damaged section of the train bay wall has been removed.

Source: NRC FOIA Document ML14219A425 This photograph was taken from the train bay’s floor. In the center of the photograph is the vertical support beam that was torn loose from the turbine building floor when it moved downward due to the stator’s impact. The damaged section of the masonry block wall lining the train bay has been removed.

Source: NRC FOIA Document ML14218A736 This 3D schematic shows the structural steel beams that were replaced during the repairs of the damaged caused when the stator dropped onto the turbine building floor. The Unit 1 turbine and generator would be to the lower right while the Unit 2 turbine and generators would be to the upper left. Using the grid system on the schematic, the train bay includes areas A-9 and A-10. When the temporary lift device failed, the stator fell while positioned over the train bay. The dropped stator struck the turbine building floor on both sides of the train bay opening, causing more extensive damaged on the Unit 1 side. The stator twisted and fell through the train bay opening onto the hauler. The stator caused significant damage to the hauler. The stator fell over to the side atop the hauler and further damaged the train bay’s walls before coming to a stop.

Our Takeaway Fission Stories #139 described this event. Our takeaway now is the same as expressed then:

Gravity never takes a minute off. Neither can vigilance to safety or tragedy can occur.

Links to the documents in NRC’s ADAMS online electronic library containing the photographs:

ML003747839 ML13213A270 ML14218A605 ML14218A607 ML14218A736 ML14218A737 ML14219A424 ML14219A425 ML14219A426 ML14219A427 ML14219A431 ML14219A433 ML14219A434 ML14220A092 ML14220A162

http://pbadupws.nrc.gov/docs/ML0037/ML003747839.pdf

http://pbadupws.nrc.gov/docs/ML1321/ML13213A270.pdf














http://allthingsnuclear.org/fission-stories-139-arkansas-nuclear-one-fatal-event/

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