Nuclear reactor

According to Sheldon's mother:

"One summer when he was 13, he built a small nuclear reactor in the shed. Told everyone he was going to provide free electricity for the whole only problem was he had no what you call fissionable materials. Anyway when he went on the internets to get some, a man from the government came by and sat him down real gentle and told him it’s against the law to have yellow cake uranium in a shed."

Apparently, Sheldon didn't take this too well and in response built a sonic death ray to use on the neighbour kids, but all it did was annoy the family dog.