There are many claims that Thorium fuel cycle is nuclear proliferation safe, it is safer but not perfectly safe. Uranium-233 bread during the Thorium fuel cycle can used to construct nuclear weapons

https://en.wikipedia.org/wiki/Thorium_fuel_cycle

https://en.wikipedia.org/wiki/Uranium-233#Weapon_material

"A declassified 1966 memo from the US nuclear program stated that uranium-233 has been shown to be highly satisfactory as a weapons material, though it was only superior to plutonium in rare circumstances."

Many nuclear energy opponents require absolute nuclear proliferation safety.