Financial terms of the detail between the Canoga Park manufacturer and Solar System Resources Corp. in Krakow were not disclosed.
The investment by US Nuclear follows by a month the announcement that it would act as the sales agent for Solar System Resources to market and sell helium-3 and other rare elements taken from the moon and asteroids.
US Nuclear will cooperate with Solar System Resources and other partners to build a value chain starting with mining and selling valuable helium-3, lanthanide metals and other vital or precious materials from space deposits, the company said in a release.
Helium-3 is used to detect neutrons created when building a nuclear bomb and can also be used as an accelerant or booster for fusion generators to power spacecraft.
Helium-3 is extremely rare to find on earth but is in abundance on the moon, where it gets deposited by solar winds. In the U.S. there are about 29 kilograms of helium-3 available, with another 187 kilograms mixed in with stored natural gas, the company said.
Solar System is expected to deliver 500 kilograms, or about a half ton, of the helium-3 isotope by 2028 to 2032.
Shares of US Nuclear (UCLE) closed Wednesday down 11 cents, or more than 11 percent, to 88 cents on the over-the-counter market.