2. Steady State Reactor Thermal Hydraulics

As discussed in Section 1.3.1, one of the desired features of an “ideal” reactor coolant would be “well-defined phase state” or no phase change during steady state or transient operation. Only gaseous coolants such as helium or carbon dioxide meet this criterion. Liquid metal cooled reactors such as Sodium-cooled fast reactors (SFR) and Molten salt reactors (MSR) are designed to avoid phase change or boiling in the reactor core so that the coolant remains as a liquid for all practical purposes. Determination of cladding temperature for these cases is simple once the surface heat flux, coolant bulk temperature and the local heat transfer coefficients are known (see Eq. 1-3). Well-established single-phase heat transfer correlations such as Dittus-Boelter correlation or those due to Gnielinski are used for gases and liquids (e.g., water) with Prandtl number on the order of unity. For liquid sodium with very low Prandtl number (on the order of 0.01 or lower) different heat transfer correlations based on experimental data are used.