Some of the safety features included in the ACR-1000, which are discussed in this article, are:

— Two fast-acting, fully capable, diverse, and separate shutdown systems, which are physically and functionally independent of each other and also from the reactor regulating system;

— Emergency core cooling (ECC) system, comprised of:

o Emergency coolant injection (ECI) system to refill the HTS and cool the fuel following a loss of coolant accident (LOCA);

o Long term cooling (LTC) system, a four-quadrant system capable of operating in either ‘shutdown cooling mode’, taking suction from the reactor outlet headers and returning it to the inlet headers via heat exchangers for heat removal, or in post-LOCA ‘recovery mode’, drawing water from dedicated grade level tanks and the reactor building (RB) sumps to cool the fuel in the long term, and;

o Core make-up tanks (CMTs) (see Fig. II-2) that provide make-up to limit the extent and duration of voiding in the intact HTS following a rapid cooldown event that depletes HTS inventory.

— Containment system (steel-lined containment structures with low leakage, containment isolation system, containment heat removal system, etc.);

— Containment cooling system, comprised of local air coolers (LACs) for active air circulation and heat removal, a containment cooling spray for post-accident pressure and temperature suppression, and provisions to interconnect the major volumes of the RB to establish an air flow path for natural circulation;

— Reserve water system (RWS, Fig. II-3), which provides an emergency source of water by gravity to steam generators, moderator system, and HTS, if required;

— Emergency feedwater system (EFW), a four-quadrant system that provides make-up feedwater to the steam generators from reserve feedwater tanks when the main feedwater system is unavailable;

— Primary coolant system (heat transport system), laid out with 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, and;

— Moderator system, a low pressure and temperature heavy water system contained in a Calandria vessel, which moderates nuclear fission and acts as a heat sink following postulated accidents. Natural circulation of the moderator in the Calandria following an accident prevents localized heat-up of the moderator, and can prevent a severe accident even if all normal means of fuel cooling and decay heat removal are not available.

Natural circulation features in ACR are grouped into three generalized categories; cooling of the fuel in the primary heat transport system, cooling of the moderator in the Calandria, and circulation and cooling of the atmosphere inside containment.