In a CANDU reactor the fuel is contained within pressure tubes that run through a vessel called the Calandria. Low pressure and temperature heavy water contained in a Calandria vessel moderates nuclear fission. The pressure tubes are contained within another set of tubes called calandria tubes, separated from the pressure tubes by an annulus gap filled with carbon dioxide. This keeps the high pressure, high temperature coolant on the primary side separated and thermally isolated from the low pressure, low temperature moderator.

The moderator system consists of a closed heavy water recirculating loop that serves to cool and circulate the heavy water moderator through the calandria (see Fig. II-5). If the moderator pumps fail, natural circulation flows prevent formation of hot spots inside the Calandria vessel.

For accidents when all means of normal fuel cooling and long term decay heat removal are not available, the fuel and fuel channels will heat up until the pressure tubes contact the Calandria tubes, resulting in direct conduction of heat to the moderator. If the forced circulation moderator cooling system not available to remove heat, natural circulation inside the Calandria prevents further damage to the fuel channels and keeps the accident from progressing to a severe accident.

Without cooling, the heat transferred to the moderator ultimately exceeds the latent heat of vaporization, and steam is released to the RB atmosphere. Passive make-up water supply from the RWS (by gravity) will keep the Calandria full (see Fig. II-3). Adequate natural circulation flow will be maintained indefinitely for long term decay heat removal.

The LTC system design allows the system to recover and cool water from the RB sumps and pump it back to the RWS. Alternatively, the RWS inventory can be made up by supply from the firewater system. The replenished RWS inventory can then be directed to the moderator. This permits thermosyphoning in the moderator to be sustained, thereby preventing severe core damage, for an unlimited period of time.