"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."
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."
I think the larger concern is containment breach via sabotage and the resulting material release, definitely less than ideal if these things are put under random buildings where you have little perimeter control.
