The HTS is laid out with the heat transport pumps and steam generators above the core to promote natural circulation of the primary coolant for accidents when the heat transport pumps are not operating (see Fig. II-4). This natural circulation flow allows the plant to recover from any trip without relying on the heat transport pumps. Commissioning tests have been done (on current generation of CANDU plants ) to validate the effectiveness of the design feature.

Core make-up tanks (CMTs, see Fig. II-2) are provided to passively limit the extent and duration of voiding in the HTS following events that cause a rapid depletion of the HTS inventory. The CMTs are located above the tops of the steam generator U tube bundles (i. e. at the highest point in the HTS) and are normally maintained at approximately the pressure and temperature of the reactor inlet headers. ‘Flashing’ of the CMT inventory when the HTS depressurizes to below the saturation pressure of the CMTs forces a rapid flow of coolant into the HTS, thus maintaining the HTS at a relatively high pressure and fully filled with water.

Keeping the HTS full and free of void ensures the thermosyphoning capability and allows operation of the LTC pumps (if available) in the shutdown-cooling mode (see Section 2) without the risk of void entrainment and consequential cavitation of the LTC pumps.

Thermosyphoning in the HTS is supported by provision of feedwater to the secondary side of the steam generators for heat removal. There are several feedwater options available in ACR-1000. The main feedwater system (using either the main or start-up feedwater pumps) and the four-quadrant EFW system are capable of supplying feedwater to the SGs at full pressure. If these active sources are not available, passive design feature of supply is provided by gravity from the RWS (see Fig. II-3) after auto-depressurization of the SGs.

With the HTS full and free of void, and with a continuous supply of feedwater to the secondary side, thermosyphoning can continue indefinitely to remove heat from the fuel.