A lesson in critical mass is massively fun

LANL scientist presented to kids at Nuclear Science Week

November 2, 2023

Favorite at The Fleet
Image caption: Jeff Favorite (in the star-spangled tie) shares a critical mass demo at the Fleet Science Center Alissa Matthews, LLC

Scientist, musician, actor, and a Challenge Ambassador with our Challenge Tomorrow program — Jeff Favorite is a man of many talents. These were on full display at a Nuclear Science Week celebration in San Diego, California, where Favorite gave a hands-on presentation to help students understand critical mass and nuclear chain reactions.

Sponsored by the National Museum of Nuclear Science and History, Nuclear Science Week is an international weeklong celebration to generate interest on all aspects of nuclear science. Events occur all over the United States, but this year’s big celebration in October was in California.

Tossing up ideas

Representing Los Alamos National Laboratory, Favorite held court with several middle and high school students at the Fleet Science Center. Favorite explains the set-up: “The group sits in a square array of chairs. Everyone has three small plastic balls. I explain that the people are ‘nuclear fuel’ and the balls are ‘neutrons.’ If you catch a ball, you split in half and throw two of yours.

“The floor is a ‘neutron absorber,’ so it’s important to catch the balls. I throw in a ‘starter neutron’ and we see how long we can go. At first the chairs are far apart, and it doesn’t take long before people realize that if we ‘compress the fuel’ it will be easier to keep the balls from dropping.”

The kids get creative. “People often propose other ideas that we can experiment with. Some work — like changing the fuel so that it starts with more neutrons – and some don’t – like arranging the fuel in a line. Sometimes there are only a few people and they realize that it will last longer if we add more fuel!

“I also grab bystanders and make them reflectors that toss stray balls back in the pile. That often goes so long that I have to become a control rod and wade into the middle to catch and keep balls myself. All of these are ways to demonstrate (inexactly!) how a nuclear reactor works.”

That’s entertainment

It wasn’t just the students who had fun at the Fleet. Says Favorite, “I loved every part of this event! Presenting an interactive demonstration like this is a performance. I have my starting lines but once it gets going there is a lot of improv. And it was a kid gig, which is my favorite thing to do. It was a chance to teach science to young people and also entertain them. And they make me laugh, too.”

Want to try your own critical mass demonstration? The American Nuclear Society shares instructions for this and other cool K-12 nuclear science activities.