Details On The Arkansas One Nuclear Accident

Over the weekend there was an incident on the “non-nuclear” side of the Arkansas One nuclear plant. Seven workers were hospitalized and released, one worker died after a generator stator fell from a temporary crane.

What we have been able to piece together based on various confirmed and unconfirmed reports:

• The 500 ton stator was on a temporary crane being used to move it out of the building on the unit 1 side of the turbine building. The temporary crane failed sending the piece of equipment falling.
• The generator stator fell onto the turbine deck and then approximately 30 feet onto the train bay floor.
• The stator broke through the wall of the machine shop when it fell. Platts confirms there is building damage at the plant
• An 8″ fire line pipe broke flooding the turbine building and machine shop with about 6 inches of water.
• A permanent overhead crane was knocked off its rail and some electrical conduit trays were damaged, the temporary crane for the stator was seen collapsed on top of the stator
• The falling stator made a 2 foot “space” in the turbine building floor
• The stator hitting the ground sounded like a lightning strike in the plant, there are reports it was felt and heard at nearby homes
• At least some of the workers involved were contractors working on the equipment change over
• Electrical equipment that operates cooling water intake pumps for unit 2 were damaged. A switchgear system for unit 2 was also damaged. Loss of the cooling water pumps for unit 2 caused the reactor to trip (turn off). Unit 2 is in hot shutdown mode (one level below full operation). The power loss for the cooling pumps is being cited as due to an explosion in the electrical bus cubicle.
• Unit 1 is completely without offsite power due to a damaged breaker but was in shutdown mode at the time of the accident. Unit 1 is operating on diesel power
• Unit 2 has partial grid power and is also using a generator. The partial offsite power loss was due to water from the broken fire main causing a short circuit
• Unit 2 was using emergency feed water and “convective cooling” as of the April 1st report. Decay heat is being removed via steam dumps. This seems to confirm loss of access to the ultimate heat sink (river water via the intakes)
• Contract workers may have begun leaving the plant site per reports from locals indicating that upgrade work will not resume at the plant any time soon
• Additional fire protection equipment is on site and a fire watch set up to check for potential problems

 

The big questions that come out of these initial reports are why unit 2’s electrical systems were able to be taken out by an incident on the unit 1 side.  This incident at unit 1 took out key systems for unit 2 including off site power, loss of coolant flow and loss of access to the ultimate heat sink. They are currently trying to make up 100,000 GPM of water cooling with convective flow, emergency water and steam dumps. There have been no updated reports on the plant regaining cooling water intake access or resolving the electrical failures. The NRC allowed them to go out of heightened condition without resolving either problem.

The crane seen in the damage photos below is a Biggie brand temporary rig. The company has experience with these types of generator replacements as noted in their report of similar work for the Cooper nuclear plant in Nebraska.

Some more technically specific information:

• The falling stator and crane components caused the supply breaker to Unit 2 reactor coolant pump B to open
  
• The loss of reactor coolant pump B resulted in a Unit 2 reactor trip, which had been operating at 100 percent power
• With the loss of offsite power to Unit 1, both Unit 1 emergency diesel generators (EDGs) started and loaded onto their electrical busses.
• At 0750 [CDT] on 3/31/2013, during movement of the Unit 1 Main Turbine Generator Stator (~500 tons), the Unit 1 turbine temporary lift device failed. This caused a loss of all off site power on Unit 1. The ANO Unit 1 #1 and #2 EDG [Emergency Diesel Generator] have started and are supplying A-3 4160V switchgear and A-4 4160V switchgear. P-4A Service Water pump and P-4C Service Water pump has been verified running. Unit 1 has entered [procedures] 1202.007 – Degraded Power, 1203.028 – Loss of Decay Heat, and 1203.050 – Spent Fuel Emergencies. Unit 1 is in MODE 6.
• ANO-1 entered TS 3.8.2 A, ‘One Required Offsite Circuit Inoperable’. All required actions are complete. The event caused a loss of decay heat removal on ANO Unit 1 which was restored in 3 minutes and 50 seconds.
• Unit 2 tripped and is in MODE 3. Emergency Feed Water was initiated on Unit 2 and Unit 2 was in [Technical Specification] 3.0.3 from 0817 [CDT] to 0848 [CDT] due to Emergency Feedwater. Unit 2 is being powered by off-site. Unit 2 Startup 3 [transformer] lock out at 0921 [CDT]. [Bus] 2A1 is on Start up 2 [transformer] and [bus] 2A3 is on #2 EDG.
•  At 9:22 a.m. (CDT), offsite power to Unit 2 from startup transformer 3 was lost because water from a fire main caused a short circuit. ANO Unit 2 EDG 2 started and energized the train B vital electrical bus, while train A vital and non-vital electrical busses were re-energized from startup transformer 2. The supply breaker from startup transformer 3 failed because of water intrusion stemming from damaged fire suppression system piping. Operators cooled down Unit 2 to hot shutdown.
•  At 10:44 a.m. (CDT), the licensee declared a Notification of Unusual Event because the failure of the supply breaker may have been caused by an explosion in the breaker cubicle. The event was terminated at 6:21 p.m. because the affected electrical bus was not energized and there was no other damage. The fire suppression system to ANO Unit 1 is shutdown due to the damage to the fire water system piping. Damaged portions of the ANO Unit 2 fire protection system have been isolated. Additional fire water pumps have been positioned to provide fire water if necessary. The licensee has established fire watches in the auxiliary buildings of both units.  (via Nucpros)

 

Photos from a local news crew show the extent of the damage.

Before the accident this is what it looked like

Additional source material:

Wiki Article
http://en.wikipedia.org/wiki/Arkansas_Nuclear_One

Reactor cooling system
http://www.nucleartourist.com/systems/rcs1.htm

Watts Bar – electrical systems
http://pbadupws.nrc.gov/docs/ML0935/ML093570500.pdf

Arkansas One
http://pbadupws.nrc.gov/docs/ML0328/ML032890506.pdf

State emergency document
http://www.healthy.arkansas.gov/programsServices/preparedness/Documents/EmergencyInstructions.pdf

Platts
http://platts.com/RSSFeedDetailedNews/RSSFeed/ElectricPower/21894074

Nucpros
http://www.nucpros.com/content/preliminary-notification-event-or-unusual-occurrence-pno-iv-13-002

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