Abstract Under very low probability postulated accident like LOCA with unavailability of ECCS, the moderator heat sink maintains the channel integrity by providing adequate cooling. As the cooling is external to channel, the circumferential as well axial temperature gradient exists within the channel internals. An experimental set-up was fabricated to simulate the boil-off condition during a postulated accident in NPP. The experiments were conducted at average fuel bundle temperature 600 °C, 800 °C and 1100 °C with superheated mixture of steam and argon. The minimum and maximum temperature in CT, PT and fuel bundle were diametrically opposite to each other; the circumferential as well axial temperature gradient increased with an increase in bundle temperature. The rate of moderator heat absorption increases with increase in bundle temperature. The moderator was successfully able to prevent a breach in the channel; it proved to be the ultimate heat sink during a postulated LOCA with unavailability of ECCS.