From my research, neutrons from the sun irradiating thorium a small portion would turn into uranium 233.
Thorium: the wonder fuel that wasn't | Bulletin of the Atomic Scientists
Early thorium optimism. The energy potential of the element thorium was discovered in 1940 at the University of California at Berkeley, during the very early days of the US nuclear weapons program. Although thorium atoms do not split, researchers found that they will absorb neutrons when irradiated. After that a small fraction of the thorium then transmutes into a fissionable material—uranium 233—that does undergo fission and can therefore be used in a reactor or bomb.