US Nuclear Corp. said Wednesday that it has taken a 5 percent stake in a Polish company that will engage in mining of space resources.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.