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Criticality Accident Mock-up

Learning from the Slotin Accident.
July 29, 2019
Criticality Accident Mock-up

Criticality Accident Mock-up.


  • Stacy Baker
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Slotin didn’t know that standing just two feet further away from the experiment might save his life.

Looking through a retrospective lens, it’s easy to identify particularly influential moments in time, those moments that dramatically and irrevocably altered the course of history.  One such moment for Los Alamos was the Slotin Accident. While experimenting with the exposed core of a nuclear weapon, the so-called  “demon core,” in May of 1946, Canadian scientist Louis Slotin was involved in a criticality accident that took his life and reshaped operations at Los Alamos forever. Slotin’s accident became the final “hands-on” experiment of its kind and the catalyst for dramatic changes in weapons-materials related protocols.

The first change in operations due to this tragic accident was an immediate end to all hands-on critical assembly work. Robotic equipment became the standard for work with these dangerous materials, a standard that continues today. Another crucial lesson Los Alamos scientists took away from the accident was the gravity of proximity. Slotin, standing only inches away from the core, instantaneously received a fatal dose of radiation. Al Graves, on the other hand, was standing roughly behind Slotin. Just a few additional inches away, Graves received a much smaller dose and survived the accident.  A third result of the Slotin Accident is the Lab’s current safety culture, which includes an unwavering dedication to situational training, which in turn led to the Museum’s accession of its latest artifact, the Criticality Accident Mock-up.

The Criticality Accident Mock-up was commissioned and created in 2018 as a teaching tool for workers at the Lab. The artifact is a striking visual and spatial aid to remind workers that tiny actions can have massive consequences and that not knowing what you don’t know can be the greatest threat.  Slotin didn't know that standing just two feet further away from the experiment might save his life, but workers standing next to this model get a very personal, physical sense of how distance matters in their day-to-day operations.

So why is this incredible training tool now part of the Bradbury collections? If it’s important and so effective, why put it in the Museum? For two reasons:


The Bradbury exists to tell the Lab’s story, to share its history and extend a welcome to those wishing to learn more about the 75 years of Lab research and about how that research affects their world, their communities, and their families. In order to tell the most germane on behalf of our Lab exhibit partners, the Museum design team seeks out the most representative artifacts from its collections and pairs them with illustrative, informational graphics on a physical framework that fits the flavor of the exhibit. Because the Museum now hosts public tours to TA-18, site of the Slotin Accident, the Museum felt it timely to display the Critically Accident Mock-up in its History Gallery. Eventually, we hope to see the mock-up installed at TA-18 in a larger exhibit of the building as it was in the 1940s. 

Safe Keeping

Part of the Bradbury’s role is safeguarding the Lab’s historically significant artifacts. After all, today’s groundbreaking science is tomorrow’s history and the Bradbury is uniquely qualified to accession, catalogue, and care for these priceless pieces from the past, whether from the 1900s or from yesterday. Our collections specialist has created a best practice process for cultivating our collection that allows individuals and organizations, internal to the Lab or from the public, to formally donate artifacts to us. They can also lend them to us, as well as borrow items from us. The flexibility built into our formal collections process allows us to bring items such as the Criticality Accident Mock-up into our collections and onto our gallery floor even though its original owners will indeed request to use it in the future.

The Bradbury Science Museum invites you to experience for yourself what it might have been like to stand so close to to Slotin’s experiment on that fateful day and reflect on the change seven decades have brought to some of the Lab’s most sensitive work.


For more information on our collections process, please visit the Bradbury Science Museum’s webpage.