Spent Fuel Damage: Pool Criticality Accident

, former director, Nuclear Safety Project

Disaster by Design/Safety by Intent #29

Disaster by Design

Disaster by Design/Safety by Intent #26 described a progression leading to overheating and damage to a reactor core, often labeled a meltdown. Disaster by Design/Safety by Intent #27 described the damage to a reactor core that can result from reactivity excursions. Disaster by Design/Safety by Intent #28 and #29 mirror those commentaries by describing how irradiated fuel stored in spent fuel pools can experience damage from overheating and reactivity excursions. Read more >

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Nuclear Autopsies

, former director, Nuclear Safety Project

Nuclear Energy Activist Toolkit #59

Medical professionals diagnose the health of their patients using non-destructive techniques (keeping with the Hippocratic Oath to do no harm) like taking temperatures and pulses and, when necessary, peering inside bodies using X-rays and Cat-scans.

Medical coroners, on the other hand, can use destructive techniques on persons when determining the causes of their deaths. Chest cavities and skulls are cut open to enable organs to be assessed. Tissue samples may be cut from bodies for microscopic examinations.

Insights gained during autopsies supplement the information gathered from treatments and examinations of living patients to broaden the knowledge of human health, allowing medical professionals and researchers to make health care advances.

Nuclear autopsies have been undertaken, and more are planned, for similar purposes—to supplement information gathered during tests and inspections conducted during reactor operation and support technological advances.

For example, the NRC outlined “harvesting” (their use, my re-use) several components during the decommissioning of permanently shut down nuclear power reactors.

Samples from the Zorita nuclear plant

Samples from the baffle plate and core barrel were harvested from the José Cabrera Nuclear Power Station (also known as the Zorita nuclear plant) in Spain as part of the Zorita Internals Research Project.

Fig. 1.

Fig. 1. (click to enlarge)  (Source: Nuclear Regulatory Commission)

Zorita was a pressurized water reactor similar to about two-thirds of the reactors currently operating in the United States. The baffle plate and core barrel formed a metal enclosure around the nuclear fuel within the reactor pressure vessel. They function to direct cooling water flow through the reactor core. Metal specimens attached to the core barrel and other internal components are periodically removed during refueling outages over the reactor’s lifetime for laboratory analysis of radiation effects.

The Zorita samples supplement those insights. Among other things, the results from the examinations will factor into the NRC’s decision-making about subsequent license renewal (SLR)—operating reactors beyond 60 years.

Additionally, there are plans to for samples from concrete structures surrounding the reactor pressure vessel at Zorita to be harvested. Previous concrete samples have been harvested from the containment building at Crystal River 3, from the shield building at Davis-Besse, and from the auxiliary building at Seabrook. The  samples at Zorita will come from concrete structures within the containment building and closer to the high temperature and high radiation emitted from the reactor pressure vessel.

Fig.. 2. (click to enlarge)

Fig.. 2. (click to enlarge) (Source: Nuclear Regulatory Commission)

Samples from the Zion nuclear plant

There are plans to harvest neutron absorber materials from the spent fuel pool at the Zion nuclear plant in Illinois, which permanently shut down a decade ago.

Fig. 3. (click to enlarge)

Fig. 3. (click to enlarge) (Source: Nuclear Regulatory Commission)

Spent fuel pools originally relied on geometry to prevent a nuclear chain reaction—the fuel assemblies in the storage racks were far enough apart to preclude a critical mass from being formed. When reprocessing was eliminated and the repository delayed, spent fuel pools were re-racked to store more fuel assemblies. The fuel assemblies within storage racks are typically closer together than when they are in the reactor core.

To guard against nuclear chain reactions, the walls of the storage racks contain neutron absorbing material. The objective of this material is to “soak” up neutrons emitted by the decay of unstable radioactive byproducts in the nuclear fuel and thus prevent them from interacting with uranium and plutonium atoms to cause fissions. But experience with neutron absorbing materials has not been stellar. There were fabrication problems, degradation problems, leaching problems, analytical problems, and expansion problems to list some of the many problems.

The Zion harvests may complement the humongous trial and error database.

Electrical cables will be harvested from both the Zion nuclear plant in Illinois and Crystal River 3 in Florida. Cables consist of metal wires that carry electricity throughout the nuclear plant wrapped in insulating material that guards against the electricity touching things along the way. The focus is on the cable insulation because it tends to age (degrade) faster at higher temperatures.

Fig. 4. (click to enlarge)

Fig. 4. (click to enlarge)  (Source: Nuclear Regulatory Commission)

Bottom Line

Harvesting of materials to enable fuller insights from their destructive examinations (i.e., nuclear autopsies) has not waited until nuclear power reactors have permanently shut down. As parts have worn out and been replaced (e.g., steam generators, reactor vessel heads, eroded pipes, etc.), specimens have been harvested and analyzed.

The nuclear industry and the NRC seek to expand those insights by harvesting materials from previously un-examined plant areas. These collections permit real data to replace positions established by extrapolating from other real data and/or by computer analyses. Hopefully, the real data reinforces previous positions. Either way, the real data supports better decision-making in the future.

Nuclear autopsies yield insights that cannot be obtained by other means.


The UCS Nuclear Energy Activist Toolkit (NEAT) is a series of post intended to help citizens understand nuclear technology and the Nuclear Regulatory Commission’s processes for overseeing nuclear plant safety.

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Fission Stories #97: His Left Foot

, former director, Nuclear Safety Project

On May 17, 1998, a security guard at the Zion nuclear plant outside Chicago, Illinois was rushed to the hospital with a bullet wound to his left foot. The guard said he had been injured while repelling an intrusion by unauthorized individuals. The two reactors at Zion, permanently closed by their owner in January 1998, still had irradiated fuel assemblies onsite that represented a radiological threat to the health of plant workers and the public.

The following day, the injured security guard recanted his story, or at least parts of it. His left foot was indeed injured, but not by terrorists or saboteurs. The wound was self-inflicted. The official report indicated that the security guard (or former security guard) emptied his gun of all six bullets. The report did not specify if he hit his foot with the first shot or if he missed his foot with the first five shots because it kept moving.

Our Takeaway

Some children had imaginary friends while growing up. This security guard apparently had imaginary enemies.

The morale of this story – don’t shoot at imaginary enemies, or friends, with real guns.

“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.

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Fission Stories #79: Crossing the Line at Zion

, former director, Nuclear Safety Project

On February 21, 1997, the operators were in the process of shutting down the Unit 1 reactor at the Zion nuclear power plant outside Chicago due to a broken emergency pump. In parallel, workers struggled to repair the pump. If the pump could be fixed in time, the plant would not have to shut down and could return to full power.

With time running out, the control room supervisor instructed a control room operator to reduce the reactor’s power level to about one percent. The operator inserted control rods to lower the power level, but went too far. The nuclear chain reaction had been interrupted. Upon realizing the error, the operator simply began withdrawing the control rods that had just been inserted. By procedure, the operator should have stopped, evaluated reactor conditions, and obtained authorization to withdraw control rods and re-establish a nuclear chain reaction.

The operator was far from alone in the control room. At the time, there were 39 folks in the control room with 15 individuals crowding the little area slightly larger than a phone booth where the operator was working. It was, according to the NRC, “a loud and disruptive environment.” Repeat, a LOUD and DiSruPTiVe eNVirOnMenT.

Time ran out on Unit 1. The operators depressed two pushbuttons that caused all of the control rods to fully insert into reactor core within seconds.

On September 2, 1997, the NRC fined the owner $330,000 for the control room antics and two other problems. In January 1998, the owner announced it was permanently closing the two units at Zion.

Our Takeaway

“Beat the Clock” might be an entertaining game show, but it is a nuclear safety trap. The moment you start playing the game, you have one eye on the clock and, at most, one eye on safety. You may have one eye on the clock and the other eye on whatever problem started the clock ticking.

The control room operating crew cannot be fully isolated from the rest of the plant’s workers. But when situations like this one arise, they must be protected as much as possible. Even the best juggler in the world can get too many balls into the air. Giving operators more tasks to do while compressing the time allotted to complete them is setting the stage for their making mistakes. The NRC found that the company replaced the professional decorum a control room deserves with a circus atmosphere. And the company paid more than peanuts for it.

“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.

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Fission Stories #28: Navy (or Naïve) Nuke

, former director, Nuclear Safety Project

In the early morning hours of Sunday, May 11, 1997, an intoxicated trainee climbed a fence thinking he was returning to his barracks at the Great Lakes Naval Base near Chicago, Illinois. Apparently, this navy base does not have a gate, forcing people to scale fences to enter and leave. Or, perhaps the trainee went to the trouble of climbing the fence to avoid disturbing the MPs at the gate in the middle of the night. Or, maybe the trainee could not find the gate in his inebriated condition.

In any case, the trainee had the right state but the wrong facility. He scaled the fence only to find himself in the isolation zone between the two perimeter security fences at the Zion nuclear plant. The trainee activated the plant’s security system by climbing over the fence. Armed guards responded and apprehended him. He was taken by ambulance to a hospital for the treatment of lacerations received while climbing and then falling from the outer fence. Security personnel interviewed the individual at the hospital and determined that he had no malevolent intent (other than to himself). The man was turned over to military authorities upon release from the medical facility.

This event occurred on the 109th anniversary of the birth of famed songwriter Irving Berlin. Berlin, who composed such standards as God Bless America and White Christmas, wrote a song that may have been this Navy Nuke’s favorite—Oh! How I Hate to Get Up in the Morning.

Our Takeaway

This episode, however entertaining, provides insight into the challenges faced by security officers at nuclear plants. Alarms, such as this perimeter fence one, could indicate a drunken sailor being off course, an animal hitting the fence, a false signal, or the opening gambit in a legitimate attack with life and death at stake. Maintaining vigilance through many false alarms is the key to being prepared to thwart the real thing should it occur.

Just as security personnel must take every alarm seriously, plant owners must take the security function seriously. Security personnel must be viewed, and treated, as priceless defenders of company assets and public health. Security personnel defending America’s infrastructure perform as vital a role as military personnel defending America’s interests in foreign countries.

“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.

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