5.1. Decay heat removal

All designs use the in-vessel steam generators or dedicated in-vessel heat exchangers for decay heat removal(DHR). Steam generators require valve movements to use them for this purpose. In all cases there is an external heat exchanger to transfer the heat to the atmosphere or to water tanks. This arrangement permits the use of a passive system based on natural circulation. Use of an independent heat exchanger in a dedicated circuit is claimed to give added reliability at the expense of extra cost. Since the primary coolant feed and bleed pipes as well as the CVCS pipes are small in diameter, the possibility of a large LOCA is eliminated. If the water level drops below the steam generator/heat exchanger level or drops to a level where natural circulation within the vessel is prevented, heat continues to be removed through steam/water being released through the break and cool water returned to the vessel by various systems or by make up from the inventory maintenance system. In the low pressure NHR, such a system is unnecessary and is not provided since heat can be removed by condensation on the surface of the heat exchanger.