Bananas are radioactive because they contain potassium-40, which is pretty unstable. But you don't have to worry about gulping down the yellowish-radioactive-fruit, because it takes a whopping 37,290,000,000 bananas to kill you, and still, it won't qualify you for the Xavier Institute for Higher Learning.
The amount of energy released, however, is minute, around 0.0000000000000931 J/s, roughly 0.000000000107 Gray. It takes around 1 Gray to cause radiation sickness in an adult, and it takes 4 times that to have a good chance of killing someone. The simple multiplication that ensues gives 37,290,000,000 bananas.