Kalna was the first uranium mine established in Yugoslavia. In 1963, the Nuclear Energy Commission began operating the mine and mill. Between 1964-1966, the staff at Kalna extracted and produced an estimated 900 kilograms of UO2 and 400 kg of uranium metal. The Kalna ore was of poor quality, containing very low uranium content, which required higher-cost mining and refining methods.

Sources:
[1] Andrew Koch, "Yugoslavia's Nuclear Legacy: Should We Worry?" The Nonproliferation Review, Spring/Summer 1997, http://cns.miis.edu.
[2] James P. Nichol and Gordon L. McDaniel, "Yugoslavia," in Nuclear Power in Developing Countries, James Everett Katz and Onkar S. Marwah (eds.), (Lexington: D.C. Heath and Company, 1982), p. 354.