Some text from a different, non-broken site: "Protactinium-233 is a precursor to uranium-233, where uranium-233 is an accountable nuclear material under international nuclear safeguards. Currently, there are no conceptual approaches for monitoring and verifying protactinium-233 during thorium irradiation and spent fuel reprocessing."
I should add that the half life for decay into 233U is only about 3 weeks. The critical mass of 233U is also lower, so less material is needed to make a weapon.