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.