Uranium-233 is important as seed-fissile for dual-fluid “Pure Cycle” Thorium Molten-Salt Reactors. U-233 does the fissioning in the core-salt (FLiBe salts containing U-233), and basically operates like ORNL’s MSRE seen in this video…

The fissioning of Uranium-233 uses it up. Replacement U-233 is created in a “breeder blanket” of blanket-salt (FLiBe salts containing Thorium FLiTh) where Thorium absorbs a neutron and beings changing into Uranium-233. This U-233 is moved from blanket-salt into core-salt, so core-salt can continue fissioning.

America has a unique stockpile of Uranium-233, which can be used to pilot a Th-MSR. Without U-233, a Th-MSR must be seeded with an alternate fissile such as Uranium-235. U-233 is preferable because the Th-MSR isn’t truly operating in “pure cycle” until all the seed-fissile has fissioned, and been replaced with U-233… a process which inches towards 100% but never truly reaches it. During that journey, Plutonium is created in the core-salt. Less and less over time, but the existence of Plutonium means modelling and testing would then need to anticipate the chemistry and neutronics of fuel-salt evolving across that journey.

Unfortunately America’s U-233 is being destroyed. Thorium Energy Security Act has been introduced to save the U-233. Here is an exchange shortly after the legislation was introduced…

…and here’s additional context from right-before the legislation was introduced.

The legislation itself can be read here:
https://www.congress.gov/bill/117th-congress/senate-bill/4242/text

If you live in The United States and would like to see Thorium Reactors developed domestically, rather than only in China and India, then please contact your representative to explain your support for this bill.