Part 6: SCRAM! When Disaster Strikes

On 23 occasions since 2000, the Columbia Generating Station faced emergency situations that forced the plant to “scram” – a term defined by the Nuclear Regulatory Commission as an unplanned automatic or manual shut  down, usually achieved by the insertion of control rods into the reactor. In each of the 23 instances, the Columbia plant scrammed in order to avoid an accident, repair an important broken part or correct a human error. To continue operating in those circumstances could have potentially  risked an accident or the health and safety of workers, the public and the environment.

Part 6
By Paul Koberstein

On another 50 occasions, the plant reduced power — but did not fully shut down — for the purpose of making a minor repair, such as a fixing leaky pipe or mending balky water pumps.

Scrams, also known as trips, are an emergency off-ramp available that any of the nation’s 100 nuclear power plants can take. A plant’s computers are programmed to shut the plant down automatically any time they receive signals from various electronic sensors of an impending great danger. Plant workers can also trip the plant manually.

The owners of nuclear power plants do not take scrams lightly. An emergency shut-down can cost the Columbia plant about $1.1 million per day in lost power sales, plus the cost of repairs, if there are any. Since 2000, the 23 emergency shut-downs at Columbia have ranged from 1 to 13 days in length, and averaged 6.3 days. The shutdowns cost the owner, Energy Northwest, nearly $7 million in lost power sales, for a total of about $160 million over the entire period. At other plants, emergency outages have lasted much longer. The Columbia Generating Station hasn’t had a scram since November 2009.

Since the turn of the century, no plants in the US have had more emergency shutdowns than Columbia Generating Station, according to NRC data. The Millstone 2 plant in Connecticut had the second most with 20 shutdowns, while the Indian Point reactor in upstate New York was third with 19, according to a Cascadia Times analysis of NRC. data The Columbia plant produces only about 1 percent of the nation’s nuclear power, but has been responsible for nearly 3 percent of the total number of emergency shutdowns at all plants nationwide since 2000.

The NRC says that only two of Columbia’s shutdowns averted accidents that had remote chances of damaging the reactor. The NRC calculated that each event created risks that were higher than those posed by normal operations, but in neither case were the risks very insignificant. Nevertheless, the NRC also determined that these two errors were among the 125 most dangerous situations in the history of the US nuclear power industry. The  most dangerous event was h 1979 accident at Three Mile Island.

In the first instance, the NRC said an error involved the mis-installation of 22 circuit breakers, including 16 in sensitive safety systems such as  power circuits for emergency diesel generators, standby service water pumps, and emergency core cooling system pumps.

Plant workers had not been trained on how to properly install the new breakers, and failed to follow instructions. The mis-installed breakers were in place at the plant for nine months in 2001 and 2002. The safety equipment in which they were installed malfunctioned four times, but each time workers failed to notice that the breakers had caused the malfunctions.

If an accident had occurred during the nine months, the safety systems in question may not have been able to respond adequately and end the danger. Fortunately, no accident occurred.

After the problem was finally detected, the plant was shut down for 9 days to fix the problem.

The second potentially fateful event occurred on June 23, 2005, when a pump in a vital reactor cooling system developed a crack. The cooling system pump keeps the nuclear reactor covered with water to prevent its overheating. A broken pump can be unable to deliver cooling water to the reactor, which can melt down if it gets too hot.

At the time, concerns were raised about the condition of a backup water pump because of its similar design and operating conditions. However, the plant waited for six months before inspecting the backup pump, which appeared to be performing adequately. During that time, plant officials kept the plant in operation while they waited for replacement parts to arrive. When the plant finally got around to inspecting the backup pump, it too was found to suffer from similar cracking and corrosion.

Some plant employees thought the pump failures posed serious safety risks. If the plant had needed to rely on the backup pump during an emergency, it might not have been able to prevent a meltdown. The risk of that happening, however, was very small. The NRC calculated that the error had a  1 in 100,000 chance of causing a meltdown. The plant was shut down for seven days and both pumps were fixed.

The NRC is seeking to reduce the frequency of scrams at plants nationwide because each scram places “unnecessary strain on plant components,” according to NRC spokesman Victor Dricks. In a scram, pumps stop delivering superhot water and steam to the turbines, allowing pipes and other components to cool. When the plant is restarted, they get hot again. A constant shrinking and expanding of metal parts can increase the amount of stress they must endure and accelerate their aging, causing them potentially to break down more easily.

The NRC tries to limit the number of scrams a plant has each year. A plant that incurs more than three scrams during 7,000 hours of operations (10 months) is issued a “white citation, a category of violation that the NRC deems to be major. If plant has 6 or more scrams within 7,000 hours of operation, it is issued the more serious “yellow” citation. If a plant has 25 scrams during a 7,000-hour period, it is issued a “red” citation and will be forced to shut down. The Columbia Generating Station has received two White violations for an excessive number of scrams– once in 2006 (for five scrams in 2004–2005) and again in 2010 (for four scrams in 2008–2009).

According to legend, the word “scram” was a quaint acronym coined in the 1940s that meant “safety control rod axe man.” The legend said that the way to quickly shut down a nuclear reactor in those early days involved cutting a rope that restrained control rods from entering the reactor and halting further fission. With one swing of an axe, the axe man would cut the rope, saving the day.

Many people believe the axeman legend to be true. If you Google the terms “scram” and “axeman,” you get 1.2 million hits.

But the NRC suggests the term scram may be a “backronym,” or the invention of an acronym to match an existing word.  NRC historian Tom Willeck writes in his blog that he has found no truth to the story. He says the term “scram” is just slang for what people working at a nuclear power plant will do when something goes terribly wrong.

In this view, plant workers and not the reactor, must run for their lives. “You scram … out of here.”

The following section provides details of all emergency outages announced by the Columbia Generating Station from 2000–2011. The list includes the cause and length of each outage, and the estimated value of revenue that was lost as a result.

 

1.            June 26, 2000

Cause: Electrical problems caused the turbine to trip and the reactor to scram.

Length of Outage: 8 days

Lost Revenue: $8.8 million

Enforcement Action: None.

Notes: The root cause of the trip was determined to be an electrical short. Over time, normal plant vibrations led to a breakdown of wire insulation, causing wires to short. The plant was automatically scrammed and the damaged wiring was repaired.

 

2. September 1, 2000

Cause: The plant was shut down manually to replace a seal on the reactor recirculation system pump.

Length of Outage: 5 days

Lost Revenue: $5.5 million

Enforcement Action: None

Notes: Pump seals at the plant had been a problem since August 12, 2000, when one of two seals failed on a recirculation pump.

 

3.            September 18, 2000

Cause: The plant was manually scrammed in response to a hole in a condenser pipe. The problem was traced to an improper weld in conducted in 1986.

Length of Outage: 5 days

Lost Revenue: $5.9 million

Enforcement Action: None

Notes: The condenser (described in Chapter 2) is the main source of radioactive air emissions at the plant. To minimize emissions, the condenser operates at a vacuum. On Sept. 18, 2000,  an alarm sound indicating an increase in the amount of gases emitted by the condenser. Plant workers manually scrammed the reactor and the hole was repaired.

 

4. June 29, 2001

Cause: A severe steam leak appeared in a feedwater valve, causing plant workers to manually shut down the plant

Length of Outage: 2 days

Lost Revenue: $2.4 million

Enforcement Action: During an outage, mechanics had failed to properly pack a valve and set a relay. This human performance error resulted in a plant transient, which increased the potential for a plant scram. A transient is a change in the reactor coolant system temperature, pressure, or both, attributed to a change in the reactor’s power output. Transients can be caused by adding or removing neutron poisons, increasing or decreasing electrical load on the turbine generator, or accident conditions. The licensee also did not address the radiological consequences associated with not surveying an area after completely draining a component. Although Energy Northwest originally estimated that plant workers would receive 45 mrem, the workers actually received 109 mrem.

Notes: After the leak was fixed, operators restarted the reactor on June 30 and increased power to 89 percent.

 

5. July 26, 2001

Cause: On July 26, operators shut down the reactor to repair a degraded reactor recirculation pump seal. Operators restarted the reactor on July 31, and achieved full power on August 3.

Length of Outage: 5 days

Lost Revenue: $6.2 million

Enforcement Action: Inspectors found that the issue was of very low safety significance (Green) based on the finding that it did not: (1) contribute to the likelihood of a primary or secondary system loss of coolant accident, (2) contribute to both a reactor scram and the unavailability of mitigating systems, (3) increase the likelihood of fire or flooding, or (4) result in an open pathway to containment.

Notes: The recirculation pump seal had leaked several times previously.

 

6. February 14, 2002

Cause: Technical Specifications required a forced shutdown because of uncorrected issues associated with the Division II emergency diesel generator output breaker. The licensee completed corrective maintenance and restarted the reactor on February 23, 2002.

Length of Outage: 10 days

Lost Revenue: $12 million

Enforcement Action: 1 white violation: low to moderate safety significance.

Notes: During a refuelling outage in May and June 2000, incorrect types of circuit breaker were installed 22 times in critical safety systems. Energy Northwest missed multiple opportunities to identify and fix the breaker problem, such as when equipment malfunctioned on June 29, 2001, November 19, 2001, January 17, 2002, and February 13, 2002. The NRC determined that the error created one of the 50 most dangerous situations in the history of the US nuclear power industry and resulted in a greater-than-normal risk of a major accident or meltdown. The NRC said Energy Northwest missed several other opportunities to identify and fix the problems. On January 15, 1998, the NRC informed licensees of the need to perform appropriate breaker maintenance. The NRC found that plant engineers failed to do adequate maintenance because felt pressure to minimize plant maintenance, and said their problem identification and resolution efforts were too narrowly focused on superficial symptoms and lacked rigor.

 

7. February 27, 2003

Cause: On February 27, 2003, operators initiated a Technical Specification required plant shutdown, due to an inoperable Division I emergency diesel generator. Operators manually scrammed the reactor (per the normal shutdown procedure) and achieved cold shutdown on February 28. Following generator repairs the reactor achieved criticality on March 13.

Length of Outage: 14 days

Lost Revenue: $16.8 million

Enforcement Action: None

Notes: On February 13, CGS declared the diesel generator inoperable and on February 27 shut down the reactor to make repairs. Plant vibration data indicated that a generator bearing had been degrading since 1995. Inspections in February 2003 described the bearing having a degraded by 95 percent to 99 percent in less than 700 hours of run time.

 

8. May 4, 2003

Cause: Columbia was manually shut down due to degradation in water chemistry in the main condenser.

Length of Outage: 6 days

Lost Revenue: $7.2 million

Enforcement Action: None

Notes: A hole was detected in the condenser, which allowed mineral-rich water from one side of the condenser to leak into the other side, which holds the crystalline water of the reactor water system. The increase of minerals in the extremely pure reactor water caused operators to manually shut down the plant.

 

9.            June 30, 2003

Cause: Electrical problems caused the main turbine to trip, which in turn caused an automatic scram.

Length of Outage: 3 days

Lost Revenue: $3.6 million

Enforcement Action: 1 Green violation: very low safety significance. The Green finding was for the failure to take effective corrective measures in response to a similar scram in June 2000.

Notes: The difference between the two events was that the first event originated in Main Transformer 3 circuitry while the recent event initiated in Main Transformer 2 circuitry.26 Subsequent to the June 2000 event, Energy Northwest made repairs to the affected wire, but failed to properly consider the extent of condition and did not specify similar repairs to other wires vulnerable to the same failure mechanism. The failure to perform additional corrective measures contributed to this initiating event.27

 

10. July 30, 2004

Cause: The reactor automatically scrammed when the reactor protection system (RPS) received trip signals from three out of four reactor steam pressure instruments.

Length of Outage: 15 days

Lost Revenue: $18.0 million

Enforcement Action: 1 green violation: very low safety significance.

Notes: During the scram, two control rods were not fully inserted. Energy Northwest declared an alert on July 30, 2004, but failed to activate the Emergency Response Data System within 60 minutes in accordance with NRC rules. By failing to do so within the required time, Energy Northwest hindered the NRC’s ability to verify plant conditions to ensure the appropriateness of emergency response actions.

 

11. August 15, 2004

Cause: The plant was manually tripped due to a loss of feedwater which occurred when the main condenser hotwell overflowed.

Length of Outage: 1 day

Lost Revenue: $1.2 million

Enforcement Action: 1 green violation: very low safety significance

Notes: The plant had been operating for less than 24 hours when the second scram in three weeks occurred. The plant was not yet reconnected to the Bonneville Power Administration’s grid. The NRC said Energy Northwest’s failure to adequately monitor the condenser hotwell level was the cause and was reasonably within Energy Northwest’s ability to foresee and correct and could have been prevented.

 

12. August 17, 2004

Cause: The reactor was manually scrammed when a feedwater heater was filled too quickly.

Length of Outage: 1 day

Lost Revenue: $1.2 million

Enforcement Action: 1 green violation: very low safety significance

Notes: The plant had been operating for less than 24 hours when the third scram in three weeks occurred. This time, the plant had been connected to the grid for less than 2 hours. An equipment operator’s failure to fill and vent condensate heat exchangers was the cause and was reasonably within Energy Northwest’s ability to foresee and correct and could have been prevented, the NRC said.

 

13. June 15, 2005

Cause: An automatic reactor scram caused by a false low suction pressure signal resulted when a technician touched the wrong termination point with a meter during maintenance.

Length of Outage: 7 days

Lost Revenue: $8.4 million

Enforcement Action: 1 green violation: very low safety significance

Notes: With this scram, the plant was given a white violation for having more than 3 scrams in 7000 hours. The previous 3 scrams were in July and August 2004. The plant went on to have a 5th scram just eight days later. All five unplanned scrams involved equipment failures. Two were caused by failures in the turbine digital electro-hydraulic control system, which does not have a backup system capable of providing continued control of the main turbine if such a failure occurs. The other three were caused by a low reactor pressure vessel water level condition following the loss of an operating reactor feed pump.

 

14. June 23, 2005

Cause: The plant was shut down following an automatic reactor scram due to an inadvertent closure of the main turbine throttle valves when a circuit control card failed.

Length of Outage: 7 days

Lost Revenue: $8.4 million

Enforcement Action: 1 white violation: low to moderate safety significance.

Notes: Maintenance technicians failed to follow system operating procedures. A pump in the system that feeds cooling water to the reactor failed due to a crack in its shaft. CGS neglected to inspect the backup pump when it discovered the crack in the first pump. If it had, they would have discovered that the backup pump had the same problem. Again, if CGS had needed the backup during an emergency, it might not have been able to prevent a major accident.

 

15. October 31, 2006

Cause: The plant was shut down following an automatic reactor scram due to a digital electrohydraulic (DEH) control system card failure.

Length of Outage: 7 days

Lost Revenue: $8.4 million

Enforcement Action: None

Notes: The DEH system card was replaced but the plant remained shut down an additional five days to perform other planned outage work, most notably the replacement of the shaft assembly in high pressure core spray pump 2. Repairs were also made to the source range monitors, a main steam isolation valve relay, an intermediate range monitor cable and several valves.

 

16. April 9, 2007

Cause: On April 7, 2007, the station declared an Alert due to a fire in a transformer located in a safe shutdown building.

Length of Outage: 3 days

Lost Revenue: $3.6 million

Enforcement Action: None

Notes: As a result of subsequent repair activities on the affected transformer, Columbia Generating Station conducted a technical specification required shutdown on April 9,2007, and entered forced outage FO-07–01.•

 

17. June 28, 2007

Cause: The plant was shutdown following an automatic scram which occurred due to lowering reactor vessel water level.

Length of Outage: 2 days

Lost Revenue: $2.4 million

Enforcement Action: None

Notes: A condensate booster pump was inadvertently tripped while switching oil filters resulting in a trip of both reactor feedwater pumps.

 

18. August 21, 2008

Cause: A failed main turbine digital electro-hydraulic system pressure fitting.

Length of Outage: 2 days

Lost Revenue: $2.4 million

Enforcement Action: None

Notes: The reactor scrammed after 383 consecutive days on line. It would be the first of a series of six scrams over the next 15 months.

 

19. February 9, 2009

Cause: The reactor was shut down following an automatic reactor scram due to a malfunction of the main turbine digital electro-hydraulic system.

Length of Outage: 5 days

Lost Revenue: $6.0 million

Enforcement Action: None

Notes: This was the second malfunction of the digital electro-hydraulic system since the installation of a new DEH system at Columbia in 2007.

 

20. May 8, 2009

Cause: Loss of pressure in the main generator occurred during testing of a back up pump

Length of Outage: 5 days

Lost Revenue: $6.0 million

Enforcement Action: 1 green violation: very low safety significance

Notes: A burst of accumulated corrosion crud from the back up oil supply line and biological growth from the heat exchanger plugged a filter. The filter, which represents a single point vulnerability as a reactor trip sensitive component, was previously approved for upgrade to a high efficiency duplex filter assembly, but was not implemented prior to the event. This was the third scram in 2009, a violation

 

21. June 26, 2009

Cause: The station initiated a manual reactor scram following a fire near the main turbine.

Length of Outage: 6 days

Lost Revenue: $7.2 million

Enforcement Action: None

Notes: The fire occurred when leaking lube oil came into contact with a hot pipe causing the oil to flash.

 

22. August 5, 2009

Cause: The plant experienced an automatic scram due to a fire in a non-segmented electrical bus.

Length of Outage: 30 days

Lost Revenue: $36.0 million

Enforcement Action: 1 white violation: low to moderate safety significance.

Notes: The white violation was for having too many scrams in one year. This was the fifth scram in 12 months. The NRC said this series of scrams was a “repetitive condition” of the series of scrams that occurred between July 2004 and June 2005.

 

23. November 9, 2009

Cause: Power operators initiated a manual scram due to a leak in the digital electro-hydraulic control system

Length of Outage: 5 days

Lost Revenue: $6.0 million

Enforcement Action: None

Notes: This was the sixth scram in 15 months. Three of these scrams involved problems with the digital electro-hydraulic control system, which has been a source of continuing problems.

NEXT: Cancer and a Nuclear Plant’s Radioactive Pollution 

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