9.87. The contribution of high molecular conductivity results in heat transfer coefficients for a sodium-cooled core rod bundle that may be larger than those for a pressurized water reactor by a factor of 2 or 3. Uncertainty in prediction methods can be tolerated by the designer since such high values result in low temperature differences between the heat transfer surface and the bulk of the flowing fluid. A number of semiempirical correlations of experimental data are described in the literature [9].

9.88. Sodium-cooled fast reactors have a power density several times larger than that for a PWR. Also, sodium boiling in the core increases reactivity and must be avoided (§5.129). Hence, the designer needs to consider a number of heat-transfer problems that are only indirectly related to the heat-transfer coefficient based on average conditions. Attention must be given to local effects around a rod’s periphery, local flow restrictions, and a variety of safety-related questions arising from the possibility of accidental loss of coolant flow.