Under what conditions would polonium 210 (Po-210) start a nuclear reaction?

Your best bet would be to work in a vacuum away from electromagnetic interference. You wouldn’t be getting the same kind of performance as you see in nuclear weapons and power plants, though.

Polonium 210 was historically used an an initiator in nuclear bombs. It is a powerful alpha particle emitter. When the polonium was crushed with a beryllium sphere, the alpha particles caused the beryllium to emit a large number of neutrons, which activated the plutonium explosive.

As for starting a nuclear reaction on its own, polonium 210 decays straight into lead 206, a stable element. A single gram of 210 Po produced about 140 watts in alpha radiation, but it is not nuclear fission

I suppose the most famous nuclear reaction polonium is known for is when it is ingested, then it works it’s nuclear magic on living tissue. Not by definition a nuclear reaction, but a reaction to it ‘nuclearness’ (radioactivity). It only takes a wee bit to mess up your whole week, or life as it were.

@majorrich Pertinent point, considering today’s revelations about Yasser Arafat.

On a similar note to the above posts, I’ve heard that plutonium is perhaps the most poisonous substance known to man. If you get some on your skin, it’s not a big deal. If you inhale it, however, the atoms will sit there and emit ionizing radiation (of all three types) for the next 10,000 years. According to Wikipedia, the inhalation risk of plutonium is 23,000 times that of weapons-grade uranium. One pound of inhaled plutonium is enough to kill 2 million people.