Nuclear fission is a powerful phenomenon. When the conditions are right, atomic nuclei split, releasing neutrons that then split other nuclei in an ongoing chain reaction that releases enormous amounts of energy. This is how nuclear weapons work. In a more stable and controlled fashion, it’s how our nuclear reactors work too.
However, these chain reactions can also happen accidentally—with terrifying results. Though rare, criticality incidents – events where an accidental self-sustaining nuclear chain reaction occurs – serve as sobering reminders of the immense and unwieldy forces we attempt to harness when playing with nuclear materials.
A criticality incident is when a nuclear fission chain reaction is caused by accident. The cause is usually quite simple. When it comes to fissionable material, like radioactive isotopes of uranium, there is a certain critical mass at which a chain reaction will occur. At this point, the natural radioactive decay of the material will release enough neutrons such that one might strike and split another atom. This then releases further neutrons, which split more atoms, and the chain reaction continues.
Calling it critical mass is a simplified way of saying it. More realistically, the critical mass depends on more factors—the shape of the radioactive material plays a role, too. As does the presence of any neutron reflectors that could bounce neutrons back towards more atoms to split.
Long story short, if you put too much fissionable material in one place, you’re asking for trouble. If it gets to that critical point and the chain reaction starts, it’s going to release a ton of radiation in a split second.
The Slotin experiment is one of the most well-known criticality incidents. Credit: Los Alamos National Laboratory
The most famous example of a criticality incident occurred when Louis Slotin was working with the Demon Core at Los Alamos back in 1946. The story has been told many a time, including on these hallowed pages. Start there if you’re curious, before we look at some more recent disasters.
America’s nuclear program hasn’t just had one awkward mistake like this. It’s had a few. One of the most serious criticality accidents in history occurred on December 30, 1958, once again at the Los Alamos National Laboratory in New Mexico. Chemical operator Cecil Kelley was processing plutonium-containing liquids in a large mixing tank as per his regular duties.
The mixing tank which Kelley was operating was filled with a concentration of uranium 200 times higher than expected. Credit: Los Alamos National Laboratory
The tank was used for recovering and reusing plutonium solutions from various experiments, and was expected at that time to contain less than 0.1 grams of plutonium per liter of solution. Unbeknownst to Kelley, the tank actually held a far greater quantity of plutonium—over 3 kilograms—due to improper transfers of waste materials to the tank. The fluid in the tank wasn’t homogenous, either—there was a denser layer of aqueous solution at the bottom, topped with a lighter layer of organic solution which contained more of the plutonium.
The tank was already close to a critical state at rest. When Kelley switched on the mixer inside, the blades formed a vortex, pushing the dense aqueous layer of fluid outwards. In turn, the more plutonium-rich organic fluid was drawn to the center of the vessel, where it promptly went critical.
As Kelley stood on a ladder viewing the mixing tank, there was a sudden bright flash of blue light. A huge surge of neutron and gamma radiation flooded the room, delivering Kelley a lethal dose in a split second. His death was harrowing, and he passed away just 35 hours after the accident. While investigations were undertaken into the matter, there has never been a public explanation for how the excessively high concentration of plutonium ended up in the tank.
When the mixer was turned on, the plutonium-rich layer of solution was brought closely together to the point where a criticality incident occurred. Credit: KDS4444, CC BY 3.0
Fast forward to 1999, when carelessness caused a similar incident in Tokaimura, Japan. At a uranium processing facility, technicians were tasked with preparing a batch of fuel. Official regulations mandated that a uranyl nitrate solution be stored in a buffer tank, and added to a precipitation tank in controlled increments. However, as per a company operations manual that was unapproved by regulators, technicians were mixing chemicals in stainless steel buckets instead, rather than using the buffer tank that was designed to prevent criticality incidents. The crew were pouring the liquid directly into the precipitation tank, which had a cylindrical geometry that was favorable for inducing criticality.
The tank soon ended up with over 16 kg of uranium inside, well over the 2.4 kg limit set by regulators. As the seventh bucket was added, the tank went critical with a bright blue flash. Radiation alarms wailed as neutron levels shot up to 15,000 times normal. Three technicians received extreme radiation doses with severe ill effects; two of the three later died. The facility was irradiated, with residents in surrounding areas having to evacuate in the immediate aftermath.
Much like the Los Alamos event, the cause of the problem was simple. The technicians simply combined too much fissile material in one place.
CRITICALITY (1969) is a British documentary on the danger of criticality incidents, and how to avoid them. If you work with nuclear materials, you’ve ideally been educated with something more up-to-date. Still, the basic physics was well-understood back then, and the lessons here largely ring true today.
If you see someone arranging nuclear materials like this for a quick photo, you’d be well advised to tell them to stop. Credit: Los Alamos National Laboratory, Department of Energy
Los Alamos suffered an embarrassing incident in more recent times, too, though thankfully a near miss. Back in 2011, technicians had arranged a number of plutonium rods on a table in order to take a photo—the intent being to celebrate their successful production. A supervisor returning to the area noticed the close assembly of the rods and quickly instructed they be separated, lest a criticality incident occur. Disaster was averted before the dreaded blue flash occurred, but it was yet another harrowing example where fundamental safety rules around criticality had been ignored.
Lessons
So what can these unfortunate incidents teach us? Strict limits and controls on fissionable materials are key. Standard procedures that control the flow of fissionable material are important to achieve this. The Tokaimura incident showed how bypassing these protocols even briefly can be disastrous. Beyond that, it’s important that those working with these materials are cognicent of the risks at all times. Even something as simple as bringing together a few rods to take a photo could cause a major incident through carelessness.
But perhaps the biggest lesson is respecting the sheer power of fission itself. When a chain reaction starts, things go wrong fast. By the time the blue flash has told you something’s happened, it’s all too late. Radiation levels have spiked through the roof and the damage is done. There is no early warning sign in these cases. Proper procedure is the only real way to avoid disaster.
Fssion remains a fickle phenomenon that is not to be trifled with. When we do trifle with it, either by honest accident or gross negligence, the results can be swift and brutal. Each of these criticality incidents was a stern reminder to humanity to maintain the utmost vigilance and safeguards when working with fissionable materials. Failure to do so always ends up the same way.
This articles is written by : Nermeen Nabil Khear Abdelmalak
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By Nermeen Nabil Khear 
