Thorium dioxide was used to help increase the refractive index of elements (usually 1) in older designs before newer designs and glass formulations came about. It won't harm you and any yellowing of surrounding elements (this is ionization) is easily reversed with a black light on the lens for 7-30 days (depending on where the thoriated element is [usually in the rear group]).

Some older Nikkors were radioactive (my coveted 35/1.4 for instance), older 58mm Rokkors (58/1.2), Takumars, and the Kodak Aero-Ektars. Plenty of others too.

Many older designs utilizing radioactive elements are also, when designed well, kick ass lenses. Nothing to do with the thorium though.