The promise of thorium is that it requires external energy to be added to maintain the reaction. The theory is that it is safer because of this as it's far less likely that you get a runaway or out-of-control reaction.

The reality is more complex [1].

Molten salt reactors are another active area of research but they have been for decades as well.

[1]: https://www.youtube.com/watch?v=2IqcRl849R0&t=1652s