The ACR-1000 incorporates multiple features utilizing natural circulation to prevent or mitigate accidents. The intrinsic passivity and inherent reliability of these features greatly strengthens the safety case for the ACR-1000.

— Thermosyphoning of the HTS primary coolant can remove decay heat from the fuel upon shutdown for normal maintenance outages and/or when forced circulation is not available.

— Natural circulation of the moderator in the calandria prevents formation of ‘hot spots’ when forced moderator circulation is unavailable. Natural circulation in the calandria can also limit damage to the fuel channels, thereby preventing severe accidents.

— Natural circulation airflows in containment prevent formation of regions of locally high temperature and dilution of hydrogen concentration following an accident.

The inclusion of features using passive natural circulation, in addition to the ‘traditional’ active safety systems used for active mitigation, is intended to improve the overall safety of the ACR design, by virtue of the simplicity and reliability inherent to passive designs.

Enhanced reliability through use of passive natural circulation features helps prevent events from progressing to the level of ‘severe’, or ‘beyond design basis’ accidents, and reduces severe core damage frequency for the ACR.

By utilizing natural circulation to remove heat from the HTS and the moderator, and to cool and circulate the containment atmosphere, the ACR-1000 design objectives are to improve overall reliability for the key safety functions, and to have a greatly enhanced safety case for postulated severe accidents.