Moderator system
Materials of the Heavy Water Systems of CANDU Reactors

The moderator is an independent heavy-water system operating essentially at atmospheric pressure. The circulation system of type 304 stainless steel piping is illustrated schematically in figure 10. The take-offs from the bottom of the reactor vessel (calandria) are pumped through two loops to Alloy 800-tubed heat exchangers that are cooled with the plant recirculating cooling water (RCW) before the D₂O is returned to the calandria via inlets at the circumferential shell. In a CANDU-6 about 5.4 % of the fission power is dispelled to the moderator and the temperature must be kept below the regulatory limit of 60 °C (at these temperatures and with the rather benign chemistry...