Polonium crystal structure, computer model. This is the alpha (cubic) form of solid polonium. Polonium is a rare radioactive element. It is most often found in nature in uranium ores. There are 25 known isotopes of polonium, with polonium-210 being the most common. Polonium can be produced artificially by bombarding bismuth or lead with neutrons. It has been used as an energy source to power thermoelectric cells in satellites and Soviet moon rovers. Polonium is highly toxic, and only trace amounts are needed for poisoning to be fatal. Polonium-210 was used to kill the former KGB agent Alexander Litvinenko in London, UK, in November 2006.