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