Uranium-233 ($\mathbf{^{233}U}$) is a man-made, fissile isotope of uranium, meaning it can sustain a nuclear fission chain reaction when struck by low-energy (thermal) neutrons. Unlike naturally occurring uranium isotopes like $\mathbf{^{235}U}$ and $\mathbf{^{238}U}$, $\mathbf{^{233}U}$ is not found in significant quantities in nature and must be bred artificially.
The concept originated from the discovery of its fissile properties in 1940 at the University of California, Berkeley, by scientists like Glenn Seaborg and Emilio Segrè, following experiments irradiating thorium with neutrons. It was investigated for use in both nuclear weapons and as a reactor fuel, with the United States producing approximately 2 metric tons during the Cold War.
It is created through the Thorium fuel cycle by irradiating Thorium-232 ($\mathbf{^{232}Th}$), which is a fertile material. When $\mathbf{^{232}Th}$ absorbs a neutron, it becomes $\mathbf{^{233}Th}$, which then undergoes two successive beta decays—first to Protactinium-233 ($\mathbf{^{233}Pa}$) and then to $\mathbf{^{233}U}$. This process is crucial because $\mathbf{^{233}U}$ has a superior neutron economy compared to $\mathbf{^{235}U}$ and Plutonium-239 ($\mathbf{^{239}Pu}$), making it highly efficient for breeding more fuel.
$\mathbf{^{233}U}$ is central to India's three-stage nuclear power programme, which was designed to leverage the country's vast reserves of thorium. The first two stages use natural uranium and $\mathbf{^{239}Pu}$ to generate power and breed fissile material. In Stage 2, Fast Breeder Reactors (FBRs) like the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam are designed to use $\mathbf{^{239}Pu}$ fuel while simultaneously converting $\mathbf{^{232}Th}$ in the reactor blanket into $\mathbf{^{233}U}$. This bred $\mathbf{^{233}U}$ will then be used as the fuel for the Stage 3 thorium-based reactors, which will secure India's long-term energy independence. India has already demonstrated its capability with $\mathbf{^{233}U}$, including the detonation of an experimental device, Shakti V, during the Pokhran-II tests in 1998.