Filtering contaminants out of thorium bred U-233 to make weapons grade fissile material is not rocket science. Oak Ridge National Laboratory (ORNL) created a process to do this. They kindly wrote about it in a history included in the ORNL Review publication (search the long page for the words “THOREX” or “Uranium-233″):
By 1954, the Laboratory’s chemical technologists had completed a pilot plant demonstrating the ability of the THOREX process to separate thorium, protactinium, and uranium-233 from fission products and from each other. This process could isolate uranium-233 for weapons development and also for use as fuel in the proposed thorium breeder reactors.