Fission Stories #46: Powerless Nuclear Power Plants

June 28, 2011 | 6:00 am
Dave Lochbaum
Former Contributor

Did the station blackout event at Vogtle described in Fission Stories #45 shock the nuclear industry into taking actions to prevent recurrence? Hardly.Almost exactly one year later, on March 7, 1991, the boom of a mobile crane neared, but did not touch, a 500,000 volt overhead power line connecting the main transformer at Diablo Canyon Unit 1in California to the offsite electrical power grid. Plant procedures required mobile cranes to be kept at least 27 feet away from overhead power lines. The boom of the mobile crane in question ventured to within 2 or 3 feet of the 500,000 volt power lines. Electrical arcing (i.e., nuclear-sized sparks) between the boom and the transmission lines caused an electrical fluctuation that tripped the main transformer. Since the backup transformer was out of service for maintenance at the time, Unit 1 lost all offsite power. The three emergency diesel generators automatical1y started and supplied power to essential equipment. Power to the rest of Unit 1’s electrical equipment was restored about five hours later by cross-tying connections to a Unit 2 transformer (i.e., putting all the eggs in one basket).

Okay, some snoozed through the Vogtle wakeup call. But surely the Diablo Canyon event triggered actions to prevent power plants from becoming powerless. Guess again.

On March 13, 1991, six days after the Diablo Canyon miscue, the Unit 4 reactor at the Turkey Point nuclear plant south of Miami, Florida experienced a loss of offsite power at a time when all its emergency diesel generators were unavailable.

Turkey Point Unit 4 along with Unit 3 (Units 1 and 2 are fossil-fired generators), had been shut down the previous November for a lengthy outage to fix safety problems. All the irradiated fuel had been transferred from the reactor core to the spent fue1 pools. As long as one irradiated fuel assembly resides in the reactor core, at least one emergency diesel generator (EDG) must be available. But when that last irradiated fuel assembly is relocated to the spent fuel pool (which now contains ALL the irradiated fuel assemblies), none of the EDGs is required to be available—despite the fact that they supply backup power to the spent fuel pools.

When the event began, the startup transformer was connected to the offsite electrical grid. It was supplying power from the grid to equipment throughout the plant. Due to an electrical disturbance, the startup transformer was automatically disconnected from the grid and prevented from reconnecting.

Workers inspected the startup transformer and associated circuit breakers and found no electrical fault indications. About an hour after the incident began, the operators re-energized the startup transformer. In the next hour, power was restored to the spent fuel pool cooling pumps and forced cooling to the spent fuel pool was resumed.

Did the Turkey Point event finally deliver the safety warning that the Vogtle and Diablo Canyon events failed to send? Maybe, but apparently not to folks in Arizona. On November 15, 1991, workers at Palo Verde Unit 3 were replacing the “A” phase bushing on the main transformer. The bushing was the connection between the transformer and the power transmission line carrying electricity to customers far and wide. Lightning damaged the bushing a day earlier. Workers used a crane to lift the damaged bushing from the transformer. Prior to installing the new bushing, the crane operator left the cab to discuss the process with other maintenance personnel.

A gust of wind, perhaps called Mariah, caused the crane’s boom to rotate and contact one of the energized phases of the 13,800 volt overhead power line. This line was transmitting power to various vital and non-vital loads in the plant. The electrical fault current should have actuated protective devices to limit the extent of the electrical problems. But the crane had not been properly grounded when the work began. Consequently, the overhead power line remained energized as electrical current flowed down the boom, through the truck, and into the ground causing the asphalt around the crane’s front outrigger pads to catch on fire.

The foreman of the maintenance crew reported the electrical short and ensuing fire to the shift supervisor in the control room. The foreman mistakenly reported that the electrical problem affected the good electrical circuit. The shift supervisor opened circuit breakers and cut off power for the good electrical circuit. Power to two of the four large pumps circulating water through the reactor core was cut off. An emergency diesel generator automatically started to provide power to essential safety equipment.

When the miscommunication was corrected, the operators opened circuit breakers for the bad electrical circuit. Power to the remaining two large pumps circulating water through the reactor core was cut off. The reactor core was left with no cooling pumps working for about a half hour until one pump was restarted. It took workers nearly three hours to restore power to all plant equipment.

Our Takeaway

Power outages can be dangerous—the disaster at Fukushima Dai-Ichi was primarily caused by an extended power outage. While in the cases described above workers were able to restore power in time to avoid serious problems, the wrong combination of incidents like these can lead to disaster.

The key factors in power outages can be represented by the three spinning wheels on a casino slot machine: (1) the odds that the offsite electrical power grid becomes unavailable, (2) the odds that the onsite emergency diesel generators become unavailable, and (3) the odds that power from either of these sources is not restored before the batteries are depleted. The goal should be to lower the odds of failure on each wheel, which can significantly lower the odds of all three wheels someday coming up as failures.

The NRC and the nuclear industry must not hide behind the smoke screen of “tsunamis cannot happen here.” Non-tsunami things can and do happen here that cause electrical grids and emergency diesel generators to become unavailable. Steps must aggressively be taken to reduce the frequency of such events in order to lessen the odds that we someday lose at the nuclear casino, too.

“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.