TPNC regime occurs as a consequence of coolant loss from the primary system. Owing to this, both driving and resistant forces increase when decreasing mass inventory of primary system. Assigned the typical geometrical layout of PWR, the former effect, i. e. increase of driving forces, is prevalent at small decreases of mass inventories. The opposite occurs for larger decreases of mass inventories. The net result is a ‘peak’ in core mass flowrate versus primary system inventory (when primary mass flowrate decreases), as can be observed in Fig. 1. Forced convection, subcooled and saturated heat transfer regimes occur in the core. Condensation occurs inside the U-Tubes of SG. The average core void fraction is typically
less than 30%, whereas at the outlet values around 50% can be reached without occurrence of thermal crisis in the considered pressure range.