Fusion: 50 years away… and always will be. (Can’t remember who I’m quoting here). The difficulties involved are almost unimaginable, and not worth discussing in an auto forum. Not entirely clean, either.
The byproduct of Thorium reactors can indeed be used to build nuclear bombs - I know of three ways around the 232U contamination issue. The most promising technologies I’ve read about propose to denature 233U with 238U, making it at least a more difficult proliferation path than simply enriching natural uranium (like South Africa and Iran).
I just read about a theorized fast neutron mixed fuel (mostly thorium) breeder idea, but I lean toward the inherent safety advantages of thermal neutron reactors and low (even <1) fuel replacement ratios. If you need to replace 0.1% of the fuel charge every year, you can be made to let someone check your plant to ensure that you aren’t skimming off weapons material.
Also not that relevant to auto mechanics…