S02E19 Title: "A Nuclear Reactor and a Boy Called Lovey" Original Air Date: April 4, 2019
Sheldon decides to build a in the shed after getting inspired by a book about a "radioactive boy scout" who tried to build a breeder reactor. He needs a neutron source, so he tries to get radioactive material from household items (smoke detectors, etc.). His father, George Sr., initially supports his "science project" until he realizes Sheldon is actually attempting something dangerous involving radioactive isotopes. young sheldon s02e19 webrip
Here’s the (full synopsis) for that episode: S02E19 Title: "A Nuclear Reactor and a Boy