Heat transfer during boiling of a liquid metal during emergency cooldown of a fast-neutron reactor

Thermal hydraulic considerations of nuclear reactor systems: Past, present and future challenges

“…During the consideration of severe faults in this type of nuclear reactor system such as the unprotected loss of flow, loss of piping integrity, loss of heat sink, anticipated transient without scram and subassembly blockage, the boiling of sodium may appear, leading to dryout and even the melting of fuel bundles. Compared with the boiling of water in LWR or PWR technology, the boiling of liquid metal according to Sorokin et al (1999) can be characterised in the following: the complex interaction of the internal factors in the system makes it difficult to accurately determine the incipient boiling superheat of liquid sodium under actual conditions; large vapor bubbles are formed in liquid sodium at several nucleation sites and the formation time of most vapor bubbles is within the waiting period; the growth of liquid sodium vapor bubbles can be explosive at a rate of about 10 m/s; the major two-phase flow patterns of liquid sodium are the same as that of conventional fluids and dispersed annular flow pattern dominates around the barometric pressure; the phase change of dispersed annular flow of liquid sodium in pipe is realized through the evaporation of liquid film rather than the formation of vapour bubbles (formed by boiling) on the wall surface; and the heat transfer coefficient can be very large.…”