<p>The deployment of nuclear reactors of various designs in large numbers is inevitable for meeting the net zero emissions target. Initiatives have been taken at the international level to design and promote new generation of nuclear reactors. Bhabha Atomic Research Centre (BARC) is also pursuing evaluation of such innovative reactor systems. These reactor systems are (1) lead bismuth eutectic (LBE)-cooled compact high temperature reactor (CHTR), (2) gas-cooled reactor (GCR) and (3) molten fuel salt-based Indian molten salt breeder reactor (IMSBR). Being non-water-cooled reactors, new technology development for designing such reactors is necessary. The design of the heat transfer system is one of the major challenges for realising these reactor concepts. Developmental activities on the thermal hydraulics of all these coolants are underway in BARC. Experimental natural circulation loops with LBE were developed and operated up to 1000°C to understand the thermal hydraulics of CHTR. In addition, in-house-developed computer codes and commercial computational fluid dynamics (CFD) codes have been validated with these experimental results. For molten salt technology development, nitrate salt and fluoride salt experimental facilities have been developed and operated up to 750°C for thermal hydraulic evaluation, material compatibility, component testing and development of high-temperature instrumentation. A supercritical pressure natural circulation loop (SPNCL) has also been set up and tests were conducted with supercritical water and carbon dioxide fluids. The experimental results of the thermal hydraulic studies provided a good insight of the heat transfer phenomenon of the new coolant systems relevant to the advanced reactor concepts. This paper highlights the challenges encountered during the development and operation of the test facilities for these high-temperature coolants, as well as important results obtained from the experimental and theoretical analyses.</p>