Bananas contain high levels of potassium. Radioactive K-40 has an isotopic abundance of 0.01% and a half-life of 1.25 billion years. The average banana contains around 450 mg of potassium and will experience about 14 decays each second.
It's nothing to worry about though. Your body already contains potassium. It's actually necessary for digestion, and the human body can handle low levels of radioactivity.