Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons. Uranium-233 is produced by the neutron irradiation of thorium-232. Thorium-232 absorbs a neutron, becomes thorium-233, then quickly decays into protactinium-233. Protactinium-233 has a half-life of 27 days and before decaying into uranium-233.This protactinium has a large cross-section and can absorb neutrons needed to sustain fission. Because uranium-233 releases so few neutrons in thermal-spectrum, and because 2 neutrons are needed to sustain a chain reaction, existence of protactinium would stop fission.Protactinium-233 is a challenge unique to thorium reactors.Low breeding ratio is a challenge unique to breeder reactors fueled by thorium, which operate in thermal-spectrum.The 5,000 tpy figure of Thorium assumes a 50,000 tpy Rare Earths facility that primarily utilizes monazite as its feed-stock.Thorium is a companion element to Monazite. Monazite runs at +50% REE and about 7% Th. So if you processed 50,000 tons of monazite you would get about 3,500 tpy of Th. However, monazite would not be the only feed-stock. You would use many other mineralizations. like apatite running at 3% REE and .002% Th (but with lots of heavy REE). So it would be a mix and tend toward the 5,000 tpy range.