nbsp   Why do CANDU reactors have a “positive void coefficient”?
[A. CANDU Technology] [B. The Industry] [C. Cost/Benefit] [D. Safety/Liability] [E. Waste] [F. Security/Non-Proliferation] [G. Uranium] [H. Research Reactors] [I. Other R&D] [J. Further Info] In any type of reactor the void coefficient refers to the reactivity feedback (the increase or decrease of the fission rate, and thus the rate of heat generation) that occurs if coolant in the core turns to steam or otherwise disappears. In PWR’s and BWR’s this coefficient is negative because a decrease in coolant density leads directly to a decrease in overall neutron moderation (the slowing down of neutrons by scattering) – the higher the degree of coolant voiding, the more negative the reactivity feedback (decreasing the fission rate). In CANDU reactors [1],[2] the coolant and moderator are separate heavy water circuits, with the coolant making only a minor contribution to overall neutron moderation. Therefore, a loss of coolant (or a decrease in coolant density) would not be expected to affect
