[leonidasrup]

"Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as Th-231), which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, Th-232 absorbs neutrons to produce U-233. Depending on the design of the reactor and fuel cycle, the generated U-233 either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel."

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

Many of specific issue around design nuclear weapons based on U-233 are classified. But:

"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. It was claimed that if the existing weapons were based on uranium-233 instead of plutonium-239, Livermore would not be interested in switching to plutonium.

The co-presence of uranium-232 can complicate the manufacture and use of uranium-233, though the Livermore memo indicates a likelihood that this complication can be worked around."

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