A moderator to thermalize the neutrons is highly desirable to maximise the probability of fission and transmutation while minimising the inventory of fissile and fertile materials in the reactor. A possible moderator for a molten-salt reactor may be colloidal graphite in molten salt.
Such a 3-fluid reactor would feature a graphite moderator that is unstressed, dynamically tuneable, refreshable and not in contact with either the fuel or the blanket fluids. The void coefficient becomes more sharply negative in a liquid than a solid, improving load response. Heat developed in, and transferred into the moderator is also removable by external heat exchange.