Thorium–Uranium-233 Fuel Cycle

Thorium-232 is a fertile isotope: it cannot readily sustain fission with thermal neutrons, but it can be transformed into fissile uranium-233 after absorbing a neutron. The transformation proceeds through neutron capture followed by two successive beta decays.

Nuclear conversion sequence

Inside the reactor, thorium-232 is irradiated by neutrons and follows this chain: thorium-232 + neutron → thorium-233 → protactinium-233 → uranium-233.

• Thorium-232 first captures a neutron to form radioactive thorium-233.
• Thorium-233 undergoes beta decay, in about 22 minutes, to form protactinium-233.
• Protactinium-233 undergoes another beta decay, in about 27 days, to produce uranium-233.
• The atomic number rises through beta decay, while the mass number remains 233.

How the reactor sustains the process

Because thorium-232 is not itself fissile, an initial fissile driver such as uranium-235, plutonium-239 or uranium-233 must provide the neutrons. The uranium-233 subsequently produced can absorb a thermal neutron and undergo fission, releasing energy and additional neutrons.

• Some released neutrons sustain fission, while others convert more thorium-232 into uranium-233.
• The process is called conversion; it becomes breeding when more fissile material is produced than is consumed.
• Neutron losses and unwanted neutron capture by protactinium-233 can reduce uranium-233 production.

Energy significance for India

The thorium cycle can convert a fertile domestic resource into usable nuclear fuel and forms the basis of the third stage of India’s three-stage nuclear power programme. Its deployment requires suitable reactor designs, reprocessing, remote fuel fabrication and careful management of radioactive materials.

• India’s programme envisages using uranium-233 bred from thorium-232 to support long-term nuclear energy generation.
• The cycle does not make thorium a direct fuel; neutron irradiation and fuel-cycle facilities remain essential.