India's third research reactor, Zerlina, reached criticality on 14 January 1961. This zero-energy experimental reactor was indigenously built but used U.S.-supplied heavy water as a moderator and coolant. Zerlina was used for studies of the spatial arrangement of fissionable and nonfissionable materials in uranium heavy water reactors. The tank-type reactor was the first of a series of critical assemblies built by BARC. The unsafeguarded reactor burned natural uranium fuel rods to produce a nominal power output of 100 watts. The reactor was decommissioned and dismantled in 1983. BARC scientists applied lessons learned from Zerlina in the design, construction, and operation of future reactors.

Sources:
[1] Andrew Koch, "Selected Indian Nuclear Facilities," Center for Nonproliferation Studies (CNS), 1999, http://cns.miis.edu;
[2] Bhabha Atomic Research Center (BARC); 2000 World Nuclear Industry Handbook (Wilmington, UK: Nuclear Engineering International, 2000), p. 198.