Yugoslav technicians began constructing the RB reactor, the country's first, in 1956. Most of the reactor was manufactured in the workshops and laboratories of the Institute of Nuclear Sciences, Vinča. However, the Soviet Union provided RB's uranium fuel elements and heavy water moderator. The reactor went critical in 1958, and remains operational. RB originally was configured with 2% enriched uranium. However, in 1976, the Soviet Union introduced 80% enriched uranium fuel elements to the core. RB reactor is now configured with a mixed 80% enriched and 2% enriched fuel load. The reactor was modified several times (1960, 1976, 1988), and was eventually converted to a flexible heavy water reflected experimental reactor with 1 W nominal power, operable up to 50 W (0.050 kW).

Sources:
[1] 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), pp. 345-367.
[2] "Research Reactor Details – RB," IAEA Research Reactor Database, updated 2002, www.iaea.org.
[3] M.P. Pesic, S.T. Cupac, Z.S. Vukadin, "Management of Ageing Research Reactors," IAEA-SM-360/42P, Symposium on Reactor Reactor Utilization, Safety and Management, Portugal, 6-10 September 1999.
[4] "RB Reactor," Center for Nuclear Technologies and Research (NTI), updated 2003, www.vin.bg.ac.yu.