The mining term for underground, beneath the earth’s surface. Underground describes areas not reached by daylight, in contrast to above ground. Drill holes, which explore areas from the earth’s surface, are above ground. If, however, the exploration work is performed from a mine, it is underground.

Natural radioactive element. Naturally occurring isotopes are the fissile uranium-235, uranium-238 which cannot be split with thermal neutrons, and uranium-234.

Physical procedure with which the percentage of the fissile isotope Uranium-235 (U-235) can be increased beyond the content of 0.72 % of natural uranium. In the Federal Republic of Germany an uranium enrichment plant is operated for this purpose by URENCO Deutschland GmbH in Gronau where uranium - in the form of the volatile chemical compound uranium hexafluoride (UF₆) - is enriched with the help of a gas centrifugal system on a large technical scale.