Thorium in itself can't fission, but when you stimulate with a little polonium or uranium you get uranium 233 which then is fissioned. That's why it's safe, you just need to pull off polonium and reaction is stopped. The main problem is the inexperience with this fuel. I don't think there is any operating plants besides the research ones now, though China plans to launch thorium plants for commercial use towards 2030. Btw it is a byproduct of an ore. I remember there was hidden thorium rings on aliexpress for being that cheap and unused.