Nuclear criticality accidents over the past 60+ years

| December 13, 2008

I’m trying to figure out where I ran across the link for this report (PDF, 3.7 MB); it was sometime in the past few weeks and was most likely from one of the science blogs I review daily, but I just can’t remember where.

The report (“A Review of Criticality Accidents”) details sixty (60) criticality accidents, that is, accidents where radioactive material that was being processed achieve critical mass sufficient to discharge a burst of radiation. These date from 1945 to 1999 and include America, Soviet/Russian, Japanese and British incidents.

The earliest is one of the most famous. It occurred at Los Alamos on August 21, 1945. From the report:

In the first accident, a critical assembly was being created by hand stacking 4.4 kg tungsten carbide bricks around the plutonium core. Figure 41 shows a reenactment* of the configuration with about half of the tungsten blocks in place. The lone experimenter [Harry Daghlian] was moving the final brick over the assembly for a total reflector of 236 kg when he noticed from the nearby neutron counters that the addition of this brick would make the assembly supercritical. As he withdrew his hand, the brick slipped and fell onto the center of the assembly, adding sufficient reflection to make the system superprompt critical. A power excursion occurred. He quickly pushed off the final brick and proceeded to unstack the assembly. His dose was estimated as 510 rem from a yield of 1016 fissions. He died 28 days later.

An almost-identical accident involving the exact same plutonium core occurred at Los Alamos the following year, exposing several people. The person working with the plutonium core, Louis Slotin, died nine days later. And, as I’ve just discovered, I have a family connection to that core, which became known as the “Demon core“. It was used in the Able bomb in the Crossroads (Bikini Atoll) nuclear tests — where my father, all of 22 years old, was one of the sailors who went on board the target ships after the blasts (Able and Baker) to survey damage.

Here’s another criticality accident, this one in Russia in 1958:

After each experiment was completed, written procedures called for the solution to be drained through a line to favorable geometry 6 liter bottles. This process was to be repeated until the entire experiment vessel had been drained. After filling some of these 6 liter bottles, the experimenters judged the remaining solution volume to be highly subcritical. It was then decided to circumvent the routine, tedious draining process and manually pour the remaining solution of 418 g U(90)/ l from the vessel (there are no records of the molarity of the solution). To accomplish this, the neutron source and its guide tube were removed and then the vessel was unbolted from its stand. Then three of the experimenters manually lifted the vessel and began to move it (in order to directly pour the contents into containers) when the excursion occurred.

They immediately noticed a flash (due to Cherenkov radiation), and simultaneously, fissile solution was violently ejected, reaching the ceiling about 5 m above. The three experimenters dropped the vessel and, along with a fourth experimenter who was located about 2.5 m away from the excursion, went immediately to the change room, showered, and were transported to the hospital. The combination of additional reflection from the three experimenters and the change in the geometry of the solution volume was sufficient to cause the system to exceed prompt critical. The small neutron background, estimated at only 100 neutrons per second, apparently also contributed to delayed initiation and thus to increased excursion energetics.

Based on fission product activity in the solution, the single–pulse yield was evaluated to be approximately 2 × 1017 fissions. Total neutron and gamma absorbed doses were estimated at 6,000 ± 2,000 rad for the three who lifted the tank and 600 rad for the coworker at 2.5 m. The three massively exposed workers died in five to six days. The fourth experimenter survived but had acute radiation sickness, followed by continuing health problems. She developed cataracts and lost sight in both eyes some years later. Due to the severe consequences of this accident, the experimental apparatus was disassembled and the critical experiment program at the plant was terminated.

Something to keep in mind the next time you watch “The Simpsons”.  ..bruce w..

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Category: History, Main, Science

About the Author ()

Webster is Principal and Founder at Bruce F. Webster & Associates, as well as an Adjunct Professor of Computer Science at Brigham Young University. He works with organizations to help them with troubled or failed information technology (IT) projects. He has also worked in several dozen legal cases as a consultant and as a testifying expert, both in the United States and Japan. He can be reached at, or you can follow him on Twitter as @bfwebster.

Comments (2)

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  1. Larry says:

    I have no idea where I read it, but I have for years believed the first on happened at the stadium in Chicago.

  2. Larry says:

    I’m still too lazy mto work through the myriad books in my library to find it, but mI did find this on-line reference to the incident in 1942: