It is estimated that between one-half to two-thirds of the value of all U.S. currency in circulation is outside of the U.S. In 1934, the $100,000 Gold Certificate became the highest denomination ever issued. It was never intended for public use. Instead, it was meant solely for official transactions between Federal Reserve Banks.
Enriched uranium. Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight. 235U is the only nuclide existing in nature that is fissile with thermal neutrons.
However, because of the tiny amounts found in nature, uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor.